Nanochemistry_and_nanomaterials_for_photovoltaics

Cite this:Chem.Soc.Rev.,2013,42,8304

Nanochemistry and nanomaterials for photovoltaics

Guanying Chen,ab Jangwon Seo,w b Chunhui Yang*a and Paras N.Prasad*bc

Nanochemistry and nanomaterials provide numerous opportunities for a new generation of photo-voltaics with high solar energy conversion e?ciencies at low fabrication cost.Quantum-confined nanomaterials and polymer–inorganic nanocomposites can be tailored to harvest sun light over a broad range of the spectrum,while plasmonic structures offer effective ways to reduce the thickness of light-absorbing layers.Multiple exciton generation,singlet exciton fission,photon down-conversion,and photon up-conversion realized in nanostructures,create significant interest for harvesting underutilized ultraviolet and currently unutilized infrared photons.Nanochemical interface engineering of nanoparticle surfaces and junction-interfaces enable enhanced charge separation and collection.In this review,we

survey these recent advances employed to introduce new concepts for improving the solar energy conver-sion efficiency,and reduce the device fabrication cost in photovoltaic technologies.The review concludes with a summary of contributions already made by nanochemistry.It then describes the challenges and opportunities in photovoltaics where the chemical community can play a vital role.

A.Background

A1.

Introduction

Meeting ever-growing energy needs is one of the important challenges of the twenty-first century.Fossil fuels (coal,oil and natural gas)provide energy sources for our needs now;how-ever,they will run out of stock considering the high consump-tion rate.1,2Power generation through burning fossil fuels also raises a significant concern of damage to environment,as large amounts of carbon dioxide and sulfur dioxide are released in the burning process.1,2A quest for new alternative renewable energy sources is urgent and necessary.

There are about one hundred and twenty thousand terawatts of solar power irradiating earth.Globally,humans consume only fifteen terawatts.3Harnessing solar energy through photo-voltaic (PV)technology has the potential to provide a virtually unlimited supply of usable energy that is sustainable and environmentally benign in operation.4–10However,economical implementation of PV technology on a global scale requires

critical advances in both materials and devices to decrease the cost and increase the power conversion efficiency.11–14Nano-chemistry and nanomaterials open up new opportunities to achieve higher solar energy conversion efficiencies at lower fabrication costs,as they allow the use of inexpensive materials and inexpensive processing technologies to harvest sunlight by efficiently capturing photon energy over a broad spectral range,and then quickly separating and collecting photo-generated charge carriers.15–22Spectral tuning in semiconductor quantum-confined nanomaterials,23sensitizing dyes,14and polymers,18,20allows the band-gap of a single-junction device to be optimally matched over a broad range of the solar spectrum to efficiently produce photon-generated charge carriers.It also allows fabri-cation of tandem or multi-junction solar cells which sequen-tially harvest the Sun’s constituent spectral components in tandem,24,25accomplishing power-conversion efficiencies potentially up to 68%through a significant reduction of losses associated with intra-band relaxations.3Nanochemistry can be utilized to tailor numerous nanointerfaces in these solution-processed nanostructured PV cells,which provides great oppor-tunity to enable efficient photo-induced charge separation and produce significantly improved charge transport and collec-tion.Multi-exciton generation,26singlet exciton fission,27plasmonic-induced light trapping,16,17as well as photon up-conversion and down-conversion,28–30realized in nano-structures,provide a range of novel approaches to harvest underutilized ultraviolet and currently unutilized infrared (IR)photons with high efficiencies,breaking the Shockley–Queisser limit set for a single junction solar device.31In addition,

a

School of Chemical Engineering and Technology,Harbin Institute of Technology,Harbin,Heilongjiang 150001,People’s Republic of China.E-mail:yangchh@https://www.doczj.com/doc/299020017.html, b

Department of Chemistry and the Institute for Lasers,Photonics,and

Biophotonics,University at Bu?alo,State University of New York,Bu?alo,New York 14260,United States.E-mail:pnprasad@bu?https://www.doczj.com/doc/299020017.html, c

Department of Chemistry,Korea University,Seoul 136-701,Korea

?Present address:Center for Supramolecular Optoelectronic Materials,Department of Materials Science and Engineering,Seoul National University,Seoul 151-744,Korea.

Received 7th February 2013DOI:10.1039/c3cs60054h

https://www.doczj.com/doc/299020017.html,/csr

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nanostructured light-absorbing and electrode materials can be applied to the substrate at low temperature by solution-processed techniques such as successive ionic layer-by-layer adsorption and reaction(SILAR),spray coating,and printing.32–34 This significantly reduces manufacturing and energy costs of solar-cell devices.In this review,we survey recent advances in nanochemistry and nanomaterials employed to intro-duce new concepts to improve the solar energy conversion efficiency,and reduce the device fabrication cost in PV technologies.A2.Fundamentals of photovoltaics

Photovoltaics or solar cells are devices that convert the energy from the sun into electric power.The power conversion e?-ciency Z of these devices can be given by,35

Z?

V oc J sc FF

P inc

(1)

where P inc is the incident solar power on the device,V oc is the open-circuit voltage,J sc is the short-circuit current density,

and Guanying Chen

Guanying Chen received his BS

and PhD degrees in optics in

2004and2009,respectively,

from Harbin Institute of

Technology,P.R.China.He

then worked as a postdoctoral

fellow(2010–2011)with Profes-

sor Paras N.Prasad at the Insti-

tute for Lasers,Photonics,and

Biophotonics,University at Buf-

falo,State University of New

York,and Professor Hans?gren

at the Department of Theoretical

Chemistry&Biology,Royal Insti-

tute of Technology.He is a research assistant professor at the

Institute for Lasers,Photonics,and Biophotonics,University at

Buffalo,State University of New York.He holds a joint affiliation

with Harbin Institute of Technology.His interests include photon

up-conversion and down-conversion nanomaterials,nanostruc-

tured solar cells,plasmonics,and nanoparticles-based diagnostics

and

therapy.

Jangwon Seo

Jangwon Seo received his BS

degree in Fiber and Polymer

Science in1998and MS degree

in2000from Seoul National

University,Korea.He obtained

his PhD degree in Materials

Science and Engineering from

Seoul National University under

the supervision of Professor Soo

Young Park in2006and worked

as a postdoctoral fellow by Aug.

2007.He then moved to the

Institute for Lasers,Photonics,

and Biophotonics,University at

Bu?alo,State University of New York where he worked with

Professor Paras N.Prasad as a postdoctoral fellow(2007–2011)

and a research assistant professor(2011–2012).He is currently a

research professor at the Center for Supramolecular Optoelectronic

Materials in Seoul National University.His research interests

include p-conjugated organic materials-polymers for optoelectronic

applications and organic–inorganic hybrid nanosystems for nano-

structured solar

cells.

Chunhui Yang

Chunhui Yang received her BS

degree in chemistry from Dalian

University of Technology in1991,

and obtained MS and PhD

degrees in chemistry from

Harbin Institute of Technology,

in1996and2001,respectively.

She then joined Harbin Institute

of Technology as an assistant

professor,and was promoted to

associate professor and full

professor in2001and2004,

respectively.She has been a

visiting professor at Stanford

University in2003and Oxford University in2004,respectively.

She has published more than80peer-reviewed scientific papers,

and co-edited two books.Her interests include solar cells,

lanthanide-doped nanomaterials,nonlinear infrared single

crystals,and poly-silicon

technology.

Paras N.Prasad

Paras N.Prasad is a SUNY Distin-

guished Professor of Chemistry,

Physics,Electrical Engineering

and Medicine;the Samuel P.

Capen Chair of Chemistry;the

Executive Director of the Insti-

tute for Lasers,Photonics,and

Biophotonics,University at

Bu?alo;the visiting Distin-

guished Professor at Korea Univer-

sity.He was named among the

top50science and technology

leaders in the world by Scientific

American in2005.He has

published nearly700scientific and technical papers;four

monographs;eight edited books.He has received many scientific

awards and honors(Morley Medal;Schoellkopf Medal;

Guggenheim Fellowship;Fellow of the APS,OSA,and SPIE,etc.).

His interests include solar cells,metamaterials,nanophotonics,

biophotonics,and nanomedicine.

Review Article Chem Soc Rev P

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FF is the fill factor.The short-circuit current density J sc is the current density derived at zero bias voltage,while the open circuit voltage V oc is the maximum voltage that a solar cell can generate.The physical processes in a solar device encompass:(1)Sun light irradiation.The solar spectrum irradiated on Earth’s surface is displayed in Fig.1a with an optical path length of air mass (AM)1.5in Earth’s atmosphere.The value of AM can be approximated by 1/cos j ,where j is the angle of the location of the sun observed on Earth’s surface with respect to the direction perpendicular to the Earth’s surface,i.e.,the zenith angle (see the inset of Fig.1a).It is worth to note that the standard solar spectrum used for e?ciency evaluation of a solar cell is AM 1.5(global),referring to an angle of about 48degrees.6The solar spectrum covers a wide range of wave-lengths,from ultraviolet to IR,resembling the spectrum of a blackbody radiation at a temperature of about 5700K.6Owing to influences from atmospheric absorption and the position of the sun,the ultraviolet light is mostly filtered out by ozone,and the dips in the IR range are mainly caused by absorptions of water and carbon dioxide molecules in the atmosphere.

(2)Light absorption.The photoactive layer in the solar device absorbs sun light,raising an electron from the ground state to a higher energy state and then generates an energy-bearing electron–hole pair,called an exciton.5–13An appropriate band-gap between the ground state and the higher energy state is required in order to eliminate cascaded nonradiative relaxa-tions.A variety of materials and processes can potentially satisfy the requirements for solar energy conversion,but in practice nearly all PV energy conversion uses semiconductor materials

(inorganic or organic).To make full use of the solar spectrum in Fig.1a to maximize J sc ,light-absorbing substances with a high extinction coefficient,low band-gap,and an appropriate Fermi level,are preferred.

(3)Electron–hole separation.An electron–hole pair is then separated at an energetically favorable charge-separating junc-tion into longer-lived charge carriers through a diffusion-driven process or by a built-in internal field.For example,c-silicon (crystalline)solar devices utilize a built-in electric field to drift the electron into the n-type silicon layer and the hole into the p-type silicon layer.10A proper alignment of electronic energy band structures or Fermi levels at the charge-separation junc-tion is of particular importance in dissociating the electron–hole pair to lead them into opposite directions.In addition,an efficient charge-separation also requires low electron–hole recombination rates (e.g.,radiative recombination,Shockley–Read–Hall recombination),which are generally determined by interfaces of light-absorbing materials.

(4)Transport of photo-generated charge carriers to electro-des.To ensure an e?cient collection of charge carriers,carrier-transporting layers are required to have high mobility as well as long di?usion lengths for electrons and holes.3An obvious but general principle is that electrons or holes move as majorities in carrier-transporting layers that have high mobility.The collecting junctions between metallic electrodes and charge-transporting materials also need optimization to allow a fluent flow of electrons or holes.

(5)Energy dissipation in the circuit.The collected electrons then dissipate their energy in the external circuit,returning to the solar cell,and annihilating with holes.

The open-circuit voltage is important for the performance of a solar cell device as it strongly a?ects the power conversion e?ciency (see eqn (1)).It is determined by Fermi levels or work functions involved in a solar device.A judicious alignment of these Fermi levels or work functions is,therefore,of particular importance to maximize the open-circuit voltage,which is also important for charge separation.In addition,the fill factor FF is related to series resistance,R s (the sum of film resistance,electrode resistance and the contact resistance between the film and the electrode),and shunt resistance,R sh .35The series resistance and shunt resistance are associated with carrier mobility and carrier recombination loss,respectively;high carrier mobility and low carrier recombination loss lead to a device with good FF .A3.

Current challenges in photovoltaics

Photovoltaics have significantly evolved through three genera-tions.36(i)The first generation of crystalline p–n junction silicon solar cells .Although relatively high in e?ciency,they su?er from high manufacturing and installation costs.37–40(ii)The second generation of thin-film solar cells ,e.g.,amorphous sili-con,41,42poly-crystalline silicon,43,44cadmium telluride,45,46and copper indium gallium selenide.47,48These solar cells are significantly less expensive than single-crystalline PV cells due to reduced materials and processing costs,and increased manufacturing throughput.The efficiencies of these cells

are

Fig.1(a)Solar spectrum irradiated on the Earth’s surface with a global air mass of 1.5,compared with the spectrum of blackbody emission at a temperature of 5700K.The red line is marked for the edge absorption of single crystalline silicon.The inset depicts the optical path length of sun light through Earth’s atmosphere,defined by air mass,at an angle j with respect to the zenith benchmark.(b)Cross-section of a general solar cell,illustrating the photon-generation of electron–hole pairs as well as processes of their dissociation,collection,and operation.(c)I –V curve of a typical solar cell,illustrating the open-circuit voltage of V oc ,the short-circuit current density of J sc and the fill factor of FF .V mp and J mp are the corresponding voltage and current of a solar cell when the device is operating at a maximum power out.

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generally lower than that of the first-generation PV cells.(iii)The third generation of solution-processed solar technologies that aim to lower costs,while maintaining high solar energy con-version efficiencies.They provide the advantage that large area and flexible structures with roll-to-roll coating/printing can be accomplished.These technologies comprise of dye-sensitized solar cells (DSSCs),49–51organic photovoltaics,52,53solution-processed bulk inorganic photovoltaics,54colloidal quantum-dot solar cells,23etc.Currently,solar cells based on crystalline,polycrystalline and amorphous silicon represent more than 90%of the world production.37However,for both the first and second generation PV cells,the band-gap energy of the semiconductors places a fundamental upper limit on solar energy conversion efficiency.This is known as Shockley–Queisser limit which restricts the efficiency to a maximum of about 31%for unconcentrated sun light irradiation,using a semi-conductor material with an optimized band-gap of around 1.35eV.31The Shockley–Queisser limit for silicon solar cells (B 1.1eV bandgap)is approximately 30%.

Approaches for third-generation photovoltaics aim to circum-vent the Shockley–Queisser limit for single-band-gap devices that are fabricated at low cost.Impressive progress on the solar-power conversion efficiency is being made in these solution-processed solar technologies,approaching or having exceeded an efficiency of 10%.3However,the technical challenge now is to continue this progress by tackling the inability to produce photovoltaics over the entire solar spectrum as well as to overcome the fundamental power-loss mechanisms (Fig.2).The key points to address are:(1)Sun light in the near infrared range contains almost half of the energy of sun irradiation,yet this portion of sun light is currently unutilized.For example,single crystalline silicon is transparent for sun light photons with a wavelength longer than 1100nm,as shown in Fig.1a.

(2)Thermalization losses of photon energies exceeding the band-gap.28This becomes serious for sun light at ultraviolet and short visible wavelengths.This power loss is caused by processes of cascaded nonradiative relaxations,i.e.,for elec-trons relaxing to the edge of the conduction band or the lowest unoccupied molecular orbital (LUMO),and holes to the edge of the valence band or the highest occupied molecular orbital (HOMO).In addition,these relaxations generating heat neces-sitate the management of heat load.

(3)A trade-o?between electron extraction and optical absorption,owing to weak absorption in the light-absorbing substance.Typical di?usion lengths for the performance-limiting photocarrier –minority charge carriers –are within the 5–500nm range in solution-processed semiconductors,3but the thickness of the light absorbing layer,required to achieve complete absorption of solar irradiation of interest,is often considerably more than this.

(4)Charge separation,transportation,and collection being adversely a?ected by the defects and interfaces in solution-processed solar cells.These interfaces include large surface area junctions between the photoelectron donors and accep-tors,the intra-layer grain boundaries within the absorber,and the interfaces between the photoactive layer and the top and bottom contacts.

Lowering of the fabrication cost of solar cells is another important challenge for PV technologies.For example,since manufacturing solar cells using conventional semiconductors such as single-crystal silicon is expensive and energy-consuming,the cost per kilowatt hour delivered is too high to compete with widely used energy technologies.5Third-generation solution-processed solar technologies aim to address this challenge using cost-e?ective fabrication techniques such as,SILAR,spin-casting,spray coating,and printing (see Section C).A4.Nanochemistry and nanomaterials meeting the challenges

Nanochemistry and nanomaterials provide numerous ways to address the challenges detailed in Section A3.Fig.2describes some of the strategies to meet these challenges using nano-chemistry and nanomaterials produced thereby.

To harvest IR light,the absorption of semiconductor quan-tum dots (QDs)can be tailored to be in the IR range by judicious manipulation of their compositions,sizes or shapes.55–60Another mechanism of utilizing IR photons involves photon up-conversion by (i)triplet–triplet annihilation in organic dyes 61–63or (ii)sequential multiphoton absorption in lantha-nide-doped dielectric inorganic materials.64–70These up-conversion processes transform IR light into visible light that can be absorbed by the photoactive material to produce charge carriers.

To minimize thermallization losses and to produce a more e?cient utilization of ultraviolet photons,the processes of multiple exciton generation (MEG)in QDs and singlet exciton fission in organic dyes can be useful.26,27Rather than dissipat-ing the excess energy of a highly energetic exciton as heat,these processes generate multiple electron–hole pairs upon the absorption of a single photon at short wavelength.Spectral down-converters of lanthanide-doped dielectric nanoparticles provide two alternative ways,quantum cutting and Stokes-shifting,to reduce thermallization losses.28–30

Tandem cells that contain several p–n junctions can be utilized to reduce both transmission losses and thermalization losses at the same time.24,25Each junction can be tuned to a different wavelength of light,significantly reducing losses associated with intra-band relaxations,and thereby increasing the

efficiency.

Fig.2Current challenges in solar cells as well as corresponding strategies to address them.Challenges are marked within the ellipses,while strategies are marked within the boxes.

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Traditional single-junction cells have a maximum theoretical efficiency of 31%,while a theoretical ‘‘infinite-junction’’cell would improve this to 68%under unconcentrated one sun irradiation.3

A trade-o?between charge extraction and optical absorption arises due to the weak absorption of solar devices;its allevia-tion requires the employment of a long optical path length,an increased e?ective absorption cross section of the absorber,or an enlarged interface area of the active junction.E?cient light trapping can be accomplished using metallic nanoparticles produced by nanochemistry to create plasmonically enhanced multiple scattering that induces multiple absorptions.16,17In addition,plasmonic nanoparticles can be used as sub-wavelength antennas to concentrate sun light by coupling the plasmonic near-field to light-absorbing substances,thereby increasing the e?ective absorption cross-section.Also nanochemistry can be used for interface engineering to provide a three-dimensional junction with a high-contact area that might be more than a thousand times the area of the solar cell,as in the case of dye-sensitized solar cells.3

Appropriate nanochemistry to tailor the numerous nano-interfaces in solution-processed nanostructured PV materials provides ways to manipulate the dynamics of charge carriers,allowing e?cient separation and collection of energy-bearing photocarriers,before they lose their excess energy through recombination.In addition,mobility boosting nanoinclusions can be used to enhance the carrier mobility and consequently increase the charge collection e?ciency.71,72In summary,nanochemistry and nanomaterials play vital roles in construct-ing highly e?cient,large area,and,at the same time,low cost solar cells.

B.Nanomaterials and interface engineering

B1.

Quantum-confined nanomaterials

E?cient solar devices rely on matching of their light absorption spectrum to the AM1.5G spectrum irradiated on the earth surface.AM1.5G is defined by a broadband solar spectrum,spanning from B 280nm in the ultraviolet to B 2500nm in the mid-infrared region (Fig.1a).Semiconductor quantum-confined nanomaterials,e.g.,QDs,and quantum rods (QRs),hold promise as light-absorbing materials to harvest the whole AM 1.5G spectrum,owing to their band-gap tuning features.55–60These QDs or QRs can be prepared using a hot colloidal nanochemistry described in Section C1,and thus are suitable for solution processing and casting.Specifically,these quantum-confined nanomaterials provide three main advantages for a solar cell device:

1.The energy gap of semiconductor QDs,which determines the portion of solar spectrum the material can absorb,can be tailored by the choice of the semiconductor material used,by the change of the size,and by the use of an alloyed composition.Importantly,by reducing the dimension of a semiconductor below its Bohr exciton radius,electrons and holes in QDs become quantum confined,producing molecular like discrete states,where the band gap (the gap between the highest discrete level in the valence band and the lowest discrete level in the

conduction band)increases as the size of the nanoparticle decreases.73,74In addition,according to Vegard’s law,the band-gap of an alloyed semiconductor is approximately equal to the compositionally-weighted average of the band-gaps of the constituent semiconductors.75,76Hence,the absorption of quantum-confined materials can be engineered,providing facile ways to build multi-junction tandem or ‘‘rainbow’’solar cells using QDs with several sizes or having varied alloyed composi-tions.A range of band gaps,in which each type of QDs is optimized to absorb a particular wavelength of the solar spec-trum,are important for constructing panchromatic solar cells with impressive PV performance.We utilize IV–VI semiconduc-tor lead sulfide (PbS)QDs,having the exciton Bohr radius of 18nm,to illustrate the quantum confinement for band-gap engineering of materials for tandem solar cells (Fig.3a).77,78By controlling the size of PbS QDs using nanochemistry,the absorption onset can be tuned from 3000to 600nm;a ternary junction solar cell is,thus,able to be prepared within a single material system.79The first (top)layer is composed of 2.6nm sized PbS QDs with absorption onset at 680nm;the second layer is composed of 3.6nm sized PbS QDs with absorption onset at 1070nm;the bottom layer is composed of 7.2nm sized PbS QDs with absorption onset at 1750nm.35For a single junction solar cell device,we would like to emphasize that low-band-gap materials absorb more light and produce a large current at the cost of a low open circuit voltage.In contrast,large-band-gap materials yield a high open circuit voltage,but low short circuit current due to their limited absorption.As a consequence,a balance between the voltage and the current necessitates an optimal choice of band-gap in the range of 1.1–1.4eV in order to yield the best power conversion efficiency.In this case,B 3.5nm sized PbS QDs are desired for a single junction device,which corresponds to an optimal band-gap of about 1.1eV.35

2.In addition to tuning the band-gap,the quantum con-finement e?ect also provides a way to adjust the absolute positions of quantized energy levels,allowing one to manip-ulate the dynamics of charge separation.80–84As illustrated in Fig.3b,modifying the size of II–VI semiconductor cadmium selenide (CdSe)QDs provides control over not only the

band

Fig.3(a)The concept of using IV–VI semiconductor PbS QDs with different sizes to build a triple-junction tandem solar cell,each absorbing in a different spectral range.(Reprinted with permission from ref.35.Copyright 2011,Wiley-VCH Verlag GmbH &Co.KGaA).(b)Varying and aligning band gaps of II–VI semiconductor CdSe QDs due to the quantum confinement,which enables the manipulation of charge separation at the junction interface.(Reprinted with permission from ref.80.Copyright 2008,American Chemical Society).

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gap,but also the position of energy levels with respect to that of the acceptor.80The smaller the size of CdSe QDs,the larger the difference between the conduction band of CdSe and that of Titanium oxide [TiO 2is generally utilized to accept forward injected electrons in QDs-sensitized solar cells (QDSSCs)or in dye-sensitized solar cells (DSSCs)].

3.A direct band gap semiconductor o?ers a stronger absorp-tion coe?cient of B 104cm à1than the indirect band gap bulk silicon,85,86which is of crucial importance in engineering films of suitable thicknesses to enable e?cient photovoltaics.Also,such a strong absorption imparts reduced material consump-tion and enables realization of e?cient devices based on materials having modest carrier transport lengths.Optical penetration depths for PbS or PbSe are estimated to be B 20nm for 400nm wavelength photons and B 500nm for 1700nm wavelength photons.35An implementation of complete absorption of most incident solar photons,therefore,requires only B 1m m thickness of the direct band-gap PbS or PbSe QD film,which is much thinner than that for indirect band-gap devices such as Si (typically 100m m is required).B2.

Plasmonic nanostructures

A limitation in all thin-film solar-cell technologies is the weak absorption,particularly at near-band-gap wavelengths,if a single optical pass is used.For example,the indirect band-gap semiconductor Si has a low absorption coe?cient between 600–1100nm,whereas sun emission in this range is quite intense (consult Fig.1a).A thick absorbance layer,therefore,has to be employed to efficiently harvest sun power (e.g.,

B 200–300m m for silicon,87in comparison with B 500nm for PbS QDs),23but high-efficiency solar cells at the same time must have minority carrier diffusion lengths several times the mate-rial thickness for all photocarriers to be collected.This issue requires ways of effectively trapping sun light inside the device to increase the absorbance.Plasmonic nanostructures provide unique technologies in this endeavor utilizing the collective oscillation of free dense charge carriers at the interface between a plasmonic substance (metals or doped semiconductors)and a dielectric.16,17,88–98An appropriate structuring of these surface plasmons in the device can concentrate and fold the sun light in a thin semiconductor layer,thereby increasing the device absorption by creating an increase in the optical path derived from strong plasmonically enhanced scattering.Surface plas-mon polaritons propagating at the plasmonic substance/semi-conductor interface are also promising for increasing the device absorption.64

Plasmonic structures can o?er at least three ways of redu-cing the physical thickness of the PV absorber layers,while keeping their optical thickness constant.16First,sun light can be bounced into the light-absorbing thin film using plasmonic nanoparticles as subwavelength scattering elements,increasing the e?ective optical light pathway in the thin film (Fig.4a).99It has been shown that when the plasmonic particles are placed close to the interface between two dielectrics,light will be preferentially scattered into the dielectric with a larger permit-tivity.100Moreover,light scattered at an angle beyond the critical

angle for reflection (total internal reflection)will remain trapped in the cell.101The employment of a reflecting metal back contact can reflect sun light back to the plasmonic scatters,which enable multiple passes of scattered sun light through the light-absorbing film,providing an additional way to increase the e?ective path length.Considerable experimental evidences of increased photocurrent spectral response in thin-film solar cells by plasmonic scatterings have been reported for single-crystalline Si,102amorphous Si,103Si-on-insulator,104an InP/InGaAsP quan-tum well 105and GaAs solar cells.106The shape and size of plasmonic nanoparticles a?ect the incoupling e?ciency;smaller and anisotropic nanoparticles can couple a larger fraction of the incident light into the underlying semiconductor because of enhanced near-field coupling.107–109In addition,light-trapping effects are most pronounced at the peak of the plasmon reso-nance spectrum.107Plasmonic nanostructures with a peak near the band-gap are required for the use of scattering geometry to enhance device absorption.Tunability of the plasmonic absorp-tion peak is needed because of the varying band-gaps of light-absorbing materials in different types of solar cells.This tunability can be realized by varying the dielectric environment for a plasmonic structure or by selecting appropriate composi-tion or geometry of the plasmonic nanostructure as in Fig.4d.Second,sun light can be concentrated using plasmonic nanoparticles as subwavelength antennas in which the

plasmonic

Fig.4(a–c)Plasmonic light-trapping geometries for thin-film photovoltaics;(a)light trapping by scattering from plasmonic nanoparticles placed at the surface of the solar cell.(b)Light trapping by excitation of localized surface plasmons in plasmonic nanoparticles embedded in the semiconductor.(c)Light trapping by the excitation of surface plasmon polaritons at the plasmonic nanoparticles/semiconductor interface.(d)Normalized extinction of Ag and Ag 0.25Au 0.75alloyed nanoparticles dispersed in toluene,Au nanoparticles dispersed in toluene,Au nanorods (NRs)dispersed in water,Cu 2–x S nanoparticles dispersed in hexane,Cu 2–x Se nanoparticles dispersed in hexane,and indium tin oxide (ITO,tin 10mol%)nanoparticles dispersed in hexane.Tuning of the plasmonic absorbance peak between 400–550nm can be realized by varying the composi-tion in alloyed Ag/Au nanoparticles,while tuning between 550–900nm can be enabled by elongating the length (thus changing the aspect ratio)of gold NRs.In addition,the plasmonic absorption peak located in the wavelength range of 1000–2100nm can be achieved by using Cu 2–x S,Cu 2–x Se and alloyed Cu 2–x S y Se 2–y nanoparticles,as well as ITO nanoparticles with varied dopant concentration of tin.(These plasmonic nanoparticles were produced and characterized at our Institute for Lasers,Photonics,and Biophotonics.).

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near-field is coupled to the semiconductor,thereby,increasing its e?ective absorption cross-section (Fig.4b).Small sized (5–20nm diameter)particles work particularly well for this geometry,as a low fraction of the absorbed light is emitted as radiation in them.16These antennas are particularly useful in materials where the carrier diffusion lengths are small,and photocarriers must thus be generated close to the collection junction area.Doping small plasmonic metal nanoparticles in the active layer has resulted in enhanced efficiencies in bulk hetero-junction organic solar cells,110,111tandem polymer solar cells,112dye-sensitized solar cells,113,114as well as in Si-based solar cells.115–117

Third,a corrugated metallic film on the back surface of a thin PV absorber layer can couple sunlight into surface plas-mon polariton modes supported at the metal/semiconductor interface as well as into guided modes in the semiconductor slab,whereupon,the light is converted to photocarriers in the semiconductor (Fig.4c).118–121In this geometry,the incident solar flux is effectively turned by 90degrees,and light is absorbed along the lateral direction of the solar cell,which has dimensions that are orders of magnitude larger than the optical absorption length.A calculation of the absorption of an organic semiconductor (PF10TBT :[C60]PCBM),in contact with Ag or Al,indicates that about B 90%of surface plasmon polarition energy can be absorbed by the organic semiconductor over the entire spectral range below 650nm.122Experiments on amorphous Si thin-film solar cells deposited on a textured metal back reflector showed a 26%enhancement in short-circuit current,with the primary photocurrent enhancement in the near-infrared.123

Utilizing these three light trapping and concentration tech-niques,a considerable shrinkage (possibly 10-to 100-fold)of the PV layer thickness can be realized,while keeping the optical absorption (and thus e?ciency)constant.16The use of surface plasmon peaks at varying wavelengths can be provided by recent advances in the field of plasmonic materials with tuning capability covering a broad range of 400–2000nm in nano-structures of metals,124alloyed metals,124metal NRs,125self-doped semiconductors,95and transparent conductors (Fig.4d).B3.

Inorganic semiconductor and polymer nanocomposites

Inorganic semiconductor/polymer nanocomposites provide unique advantages in e?cient hybrid polymer–inorganic solar cells.The hybrid polymer–inorganic nanocrystal solar cells consist of a hole-conducting conjugated polymer containing inorganic semiconducting quantum materials,like CdSe,PbS and PbSe QDs or QRs.They effectively combine the advantages of the two classes of materials,including:(a)low fabrication costs due to simple solution-based processing;(b)light weight,and flexible substrate manufacturing (roll-to-roll production)of the conju-gated polymer matrix;and (c)high electron mobility,size-dependent optical band-gap,and carrier multiplication due to semiconducting nanoparticles.126–129The performance of these hybrid devices depends on energy band matching,blend film morphology,composition ratio,and semiconductor nano-particle shape,particularly,the electron-injection efficiency

from the conjugated polymer to the inorganic semiconductor nanoparticles.Examples of hybrid solar devices are described in detail in Section D2.An efficient electron injection process is of crucial importance for high power conversion efficiency,which,however,requires that the inorganic semiconductor and the conjugated light-absorbing polymer are chemically placed in intimate contact.Instead of physically blending,which does not generally produce efficient charge transfers,direct tethering of conjugated polymers to the surface of semiconductor nanoparticles can yield a well-defined interface that markedly facilitates photoinduced charge transfer between these two semiconductor components,leading to enhanced power conversion efficiency.There are at least three ways to fabricate inorganic and polymeric nanocomposites (Fig.5).(1)Ligand exchange of insulating ligand-capped inorganic nanoparticles with functionalized conjugated polymers.In this strategy,a strong coordinating group (e.g.,phosphoric acid,amine)functionalized polymer is first synthesized,and then directly anchored onto the inorganic nanoparticles by substi-tuting the original or intermediate ligands on the nanoparticle surface.130–132For example,a poly(3-hexylthiophene)(P3HT)with the amino end-functionality was employed to exchange the pyridine ligand capped on the surface of the CdSe NRs.The original ligands,trioctylphosphine oxide (TOPO)and tetra-decylphosphonic acid (TDPA)capped on the surface of CdSe NRs,had been replaced by refluxing the NRs in pyridine.An improved dispersion of CdSe NRs in the P3HT matrix in the P3HT–CdSe NRs hybrid solar cells was manifested,result-ing in an improved power conversion efficiency of 1.4%as compared to that of 0.5%from the device fabricated using non-functionalized P3HT.132Although ligand exchange provides derivatization of semiconductor nanoparticles with functional ligands,it suffers from incomplete surface coverage.133,134

(2)Directly tethering functionalized conjugated polymers to bifunctional ligand capped inorganic nanoparticles.This strat-egy relies on the employment of a bifunctional ligand that not only acts as the coordinating ligand (using the steric e?ect and the coordination capability)to synthesize high quality semi-conductor nanoparticles with controlled size and shape,but also is capable of providing another functional group to couple with appropriately functionalized conjugated polymers.133,135One-dimensional NRs are more preferable over QDs in hybrid solar cells,126,136because they naturally provide a longer

axial

Fig.5Advantages of inorganic–polymer nanocomposites displayed together with three synthetic methods for their preparation.

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path for electrical transport.A bifunctional ligand of bromo-benzylphosphonic acid (BBPA)has been employed for grafting vinyl-terminated P3HT onto CdSe QRs.137BBPA has two func-tional groups,a phosphonic group which allows the prepara-tion of the elongated CdSe QDs,and the aryl bromide group which allows Heck coupling with the vinyl group in the func-tionalized P3HT.137An enhanced charge transfer at the P3HT/CdSe QR interface was substantiated by the observation of quenching and a much shorter lifetime of the emission of P3HT.Although direct tethering of functionalized conjugated polymers to bifunctional ligand capped semiconductor nano-particles provides high grafting density at the interface,the need for a rational design of bifunctional ligands and polymers as well the necessary coupling reactions can be challenging.(3)Direct growth of semiconductor inorganic nanoparticles within a conjugated polymer,allowing an intimate contact of the polymer and the semiconductor nanoparticles through a simple process.138–140A good example is the work on the direct growth of PbS QDs within poly(2-methoxy-5-(20-ethyl-hexyloxy)-p -phenylenevinylene)(MEH-PPV).1382–6nm PbS QDs can be synthesized by injection of a sulfur precursor solution and a lead acetate mixture solution into the MEH-PPV at a high temperature.Since MEH-PPV possesses no charged functional groups,the growth is more likely to be directed by the steric effects of the long chains of MEH-PPV.138Although the result-ing PbS QDs have a wider distribution of size and shape than those synthesized by a hot colloidal method (see Section C1),this synthetic route for inorganic–polymer nanocomposites is quite simple,as it eliminates the needs for not only an initial surfactant to control the nanoparticle growth,but also a func-tional ligand that subsequently couples to conjugated poly-mers.Utilizing direct growth of semiconductor nanoparticles in a conjugated polymer,nanocomposites of P3HT/CdSe QDs,140polyalkyloxythiophene/ZnO QDs,141P3HT/CdS QRs have been synthesized,142offering versatile alternatives for hybrid solar cells.

B4.Interface engineering to manipulate charge carrier dynamics

E?cient solar cell operations rely on charge separation,trans-port,and collection.Appropriate engineering of the chemistry and resultant electronics of various interfaces to manipulate charge carrier dynamics are of particular importance.These interfaces comprise the interface of charge-separation,the interface of nanomaterials,as well as the interface at the metallic electrode.Here we summarize some general principles to engineer these interfaces.

A forward transfer in dye sensitized solar cells (DSSCs)is determined by the energetic position of the LUMO level of the sensitizer relative to the conduction band of the acceptor,and the strength of electronic coupling between the initial and the final states associated with the transfer.6,143,144In DSSCs,the light-absorbing dye molecules are anchored to the oxide nano-particle electrode through coordinative bonding of the linker to the metal ions exposed at the surface of the metal oxide.143It has been shown that adding a –CH 2–CH 2–spacer between the

dye and the metal oxide increases the electron injection time from 13to 57fs,while adding an unsaturated group of –CH Q CH–does not alter the electron injection time.145Molecular engineer-ing of a linker with delocalized pi electrons is important to ensure efficient electron injection efficiencies.

With respect to QDs-based solar cell devices,solution-processed QDs are generally surrounded by organic ligands that employ long-chains to ensure their solution processability,but adversely act as an insulating matrix material.35Such an insulating layer drastically reduces electronic interaction at the interface of QDs.146Transport in the QD film occurs mainly through tunneling or hopping.An e?cient way to address the interface problem of QDs is to replace these long-chain ligands with shorter but more conductive ligands,promoting carrier transport while lowering the surface defect density to reduce recombination losses.147–153Various approaches of nanochem-istry to replace these long-chain ligands will be discussed in detail in Section C5.Ligands of short alkylthiols,154,155aromatic thiols,156alkylamines,157mercaptocarboxylic acids (MPA),147,158and atomic halide anions (such as Cl à,Br àand I àwith ligand size scaling down to 0.1nm),151have all shown promise in achieving effective surface passivation,while enhan-cing carrier mobility.Generally,the shorter the ligand is,the better the solar efficiency is,if employing the same PV device configuration (see Section D1).A recent investigation has shown that a hybrid passivation combining the MPA-treatment and the halide anions-treatment is found to have a more pronounced effect on passivation of surface trap sites,while retaining close inter-particle contacts and thus the power conversion efficiency (PCE).153

The use of appropriately energy matched structures of semiconductors and work functions of metals to produce a favorable charge-separating interface is very important for e?cient charge separation.3Early QDs solar cell performance relied on a metal-semiconductor Schottky junction (see Fig.14b),whereby the contact of a light metal (Al)of a lower work function with the film of a p-type colloidal QDs solid established a charge-separating electric field for charge carrier separation.159However,the performance of this Schottky junc-tion is largely influenced by the interface between the QD film and the Al metal;tailoring this interface by the deposition of a thin LiF layer (B 0.8nm)in between,not only reduces the recombination and resistance,but also improves the device stability against air by retarding the penetration of oxygen and moisture below the Al electrode.160Despite the advances,this metal-semiconductor junction or interface still remains ine?-cient for charge-separating,as it is placed far from the QDs layer where light absorption starts (see Fig.14b);thus,electrons are required to travel a long distance before reaching this charge-separation interface.147A shift of the charge-separating interface to the proximity of the initially absorbing QDs layer (p-type)of light absorption can be implemented by insertion of an n-type TiO 2layer in between the transparent electrode and the QDs film,generating a new type of p–n junction solar cell configuration,termed ‘‘depleted heterojunction’’solar devices (see Fig.14d).147Separating charges on the light-absorbing side

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of the device lessened the reliance on the long-distance trans-port of electrons within the p-type quantum-dot film that is required in a Schottky junction solar cell.In addition,the n-type TiO2,p-type colloidal quantum-dot heterojunction removed the limits on the open-circuit voltage that arose as a result of Fermi-level pinning in Schottky devices.The barrier-to-hole extraction presented by the large-band-gap TiO2was used to prevent back recombination.For photovoltaics using semi-conductors,it seems that the use of a p–n junction is still the best way to configure the interface for efficient operations,as is the case for inorganic solution-processed solar cells utilizing CIGS and CZTS.54

The absolute position of the lowest conduction band of TiO2 in‘‘depleted heterojunction’’solar devices sets a limit on the energy level,and thus,the size of QDs.A diameter below B4.3nm for PbS QDs is required by energy level matching to ensure an e?cient electron transfer from the QDs to TiO2.161To alleviate this limit,recent approaches are to engineer this interface by lowering the conduction band of TiO2via doping of Zr or Sb;these approaches proved to be beneficial for charge separation at the TiO2–PbS heterojunction.162Furthermore,the opposite side of the PbS film in this TiO2–PbS heterojunction device is in contact with a metal,intrinsically forming a Schottky junction that prevents the main electron flow at the interface.Thus,several efforts dedicated to incorporating a hole extraction layer between the PbS film and the metal electrode,eliminated this undesirable Schottky junction and significantly improved the current density together with enhanced V oc.162

B5.Novel approaches to harvest ultraviolet and infrared photons

B5.1.Multiple exciton generation and singlet exciton fission.Multiple exciton generation(MEG)in QDs involves the absorption of one photon possessing at least twice the band-gap energy,leading to the generation of a highly energetic exciton that breaks up to produce two or more excitons (Fig.6a).26,163–171Separation of electrons and holes in each exciton thus produces carrier multiplication,i.e.,more than one electron and one hole per absorbed photon(Fig.6b).This process can enable effective harvesting of high-energy photons in the violet and ultraviolet parts of the solar spectrum where part of the photon energy is lost as heat.163–171It has been indicated that the application of MEG in single junction solar cells has the potential to yield efficiency as high as44%,well exceeding the Shockley–Queisser limit of31%.165,168Although MEG was first observed from a bulk semiconductor in the 1950s,172its discovery in semiconductor QDs was in PbSe nanocrystals by Schaller and Klimov in2004.169Since then, MEG has been reported in a range of QD systems,such as PbS,77,173,174PbSe,77,169PbTe,175CdSe,176InAs,177,178 InP,179and Si,180,181and recently in single-walled carbon nano-tubes.182,183The efficiency of MEG is defined as the number of electron–hole pairs produced per absorbed photon.184,185How-ever,the measurement of the MEG quantum efficiency in colloidal QDs and its use on solar energy conversion remain controversial,as most works evaluate the MEG quantum effi-ciency utilizing an indirect approach based on ultrafast tran-sient absorption techniques.169,175–183A direct proof of carrier multiplication resulting from MEG was reported by Prasad et al. in2008,when irradiating PbSe QDs with photon energies greater than2times that of the QDs band-gap(Fig.6b).186A photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2single crystals was subsequently utilized to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon.187A direct evidence of an increase of solar cells efficiency due to MEG in QDs was given by Semonin et al.in2011,whereby they showed about4%of total photocurrent in PbSe-based solar cells arises from the MEG effect.188This is verified by a peak external quantum efficiency of114%and a peak internal quantum efficiency of130%obtained in their work.Never-theless,until now,the impact of MEG on the increase of the solar power conversion efficiency of QD solar cells is still quite limited,requiring an effective minimization of the com-peting intra-band relaxation processes.Appropriate control

of

Fig.6(a)Schematic of multiple exciton generation.(Reprinted with permission from ref.163.Copyright2011,American Chemical Society).(b)Extraction of excitons generated by MEG in a PbSe nanocrystal device.(Reprinted with permission from ref.186.Copyright2008,American Institute of Physics).

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nanoscale excitation dynamics by suitable nanochemistry is needed to be established.In addition,MEG was generally demonstrated utilizing deep ultraviolet photons excitation.The realization of MEG under near ultraviolet and visible light excitation can greatly expedite its use in solar cells,as carriers multiplication can be implemented for photons over a large fraction of the solar spectrum.

In analogy to MEG in QDs,singlet exciton fission is a promising counterpart in organic dyes.Specifically,singlet exciton fission is a process in which a singlet exciton splits into two triplet excitons,thus providing a pathway to enhance energy e?ciency in photovoltaics,as in the case of dye-sensitized solar cells.27,189The enhancement is possible if each of the generated triplet states independently provides an injected elec-tron.190,191A direct experimental observation of the multiexciton state resulting from singlet fission was reported in 2011using the model system of pentacene/fullerene bilayers and femto-second nonlinear spectroscopy.192The main challenge now is to synthesize photostable dyes with low rates of vibrational deacti-vation and triplet–triplet annihilation.Molecular engineering with appropriate organic moieties is needed to provide an appropriate electronic structure with the energies of the first excited singlet state and second triplet state equal to or greater than twice the energy of the first excited triplet state.

B5.2Photon down-conversion.Photon down-conversion comprises two types of energy conversion processes that are useful for solar cells (see Fig.7).193One is based on photon quantum cutting,194,195the other one is based on Stokes-shifted emission.196

Quantum cutting is a direct conversion of the energy of one absorbed photon into two (or more)emitted low-energy photons.This process is known to have quantum e?ciency more than 100%.195The ladder-like spacing of energy levels in lanthanide ions provides opportunities for a quantum cutting mechanism due to pronounced energy transfer between two lanthanide ions of the same or di?erent types.Quantum cutting in the visible spectral region with a quantum e?ciency of 190%was reported in LiGdF 4:Eu 3+phosphors by Wegh et al.in 1999,197but it has limited influence on PV e?ciency.Recently,the potential of using NIR quantum-cutting phosphors in c-Si solar cells has been explored in an e?ort to increase the power conversion e?ciency by alleviating thermalization losses (see Fig.7).28–30Theoretical calculations implemented by Trupke et al.indicate that a layer of quantum-cutting phosphors coated on the c-silicon solar cell can increase the efficiency up to 38.6%,

well surpassing the Schockley–Queisser limit.198NIR quantum-cutting has been realized in various Ln 3+/Yb 3+(Ln =Ce 3+,199–201Eu 2+,202Tb 3+,203,204Tm 3+,205,206Pr 3+,207,208Er 3+,209,210Nd 3+,211Ho 3+,212,213and Dy 3+)214co-doped materials,emitting NIR emis-sions at B 980nm which is above the band-gap of B 1.15eV of silicon.The efficiency of quantum cutting is reported to be theoretically in the range of 150–190%.Among them,broad-band excited quantum cutting in Ce 3+/Yb 3+and Eu 2+/Yb 3+co-doped systems are more promising for solar cell applications,as they can harvest photons over a broad spectrum of B 300–500nm,converting them into two NIR photons at B 980nm.199–202In addition,Ce 3+and Eu 2+utilize much stronger 4f-5d transi-tions,rather than the discrete and weak 4f–4f transitions in lanthanides to harvest photons.Quantum cutting utilizing an energy transfer between ZnO QDs and Yb 3+ions in a glass matrix,215Si nanocrystals and Er 3+ions,21,216as well as between Bi 3+ions and Yb 3+ions in YVO 4nanophosphors 217or Y 2O 3films,218is also being explored in order to impart strong and broad-band light harvesting capability.

Stokes-shifted luminescence can be utilized in solar devices where the photoactive component has a poor spectral photo-response to short-wavelength light.A Stokes-shifting material absorbs the short-wavelength light,typically in the 300–500nm range,and emits at a longer wavelength where the external quantum efficiency of the PV device is high.196While lumines-cence Stokes-shifting can increase the solar cell efficiency,it cannot overcome the Shockley–Queisser efficiency limit,as thermalization losses remain unchanged.28In practice,a planar Stokes-shifting layer is placed directly onto the front surface of a solar cell to improve the device performance by transforming short wavelength sun light photons to longer wavelength photons at a quantum yield near unity (see Fig.7).219,220An ideal Stokes-shifting material should possess the following char-acteristics:29(i)Strong and broadband absorption in the region where the spectral response of the solar cell is low,while having high transmission in the other spectral range;(ii)high extinc-tion coefficient and near unity quantum yield of emission;(iii)sharp or narrowband emission in the spectral region where the external quantum efficiency of the solar device is high;(iv)a large Stokes shift to minimize self-absorption energy losses due to any spectral overlap between the absorption and the emission bands;(v)long-term photo-stability.Until now,lanthanide-doped inorganic phosphors,221–224colloidal QDs,225–227organo-lanthanide complexes,228organic dyes 229,230have been widely investigated as Stokes-shifting materials,manifesting enhanced power conversion efficiencies in solar cell devices.

B5.3Photon up-conversion.The term of up-conversion describes anti-Stokes optical processes that convert two (or more)low-energy pump photons,even from an incoherent continuous-wave light source,to generate a higher-energy photon by sequen-tial absorption or ion-to-ion energy transfer.65This phenomenon was first discovered by Auzel in the 1960s.66As was already discussed above,only the absorption of photons with energy higher than the band-gap can generate electron–hole pairs con-tributing to the electric current.Indeed,the transmission of sub-band-gap photons is one of the major energy loss

mechanisms

Fig.7Spectral conversion design for solar cell applications involving stokes-shifting (SS),quantum-cutting (QC),and up-conversion (UC).

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in conventional solar cells(see Fig.2).In the case of c-Si solar cells,the transmission loss amounts to about20%of the incident solar energy,which is not substantially reducible by conventional approaches.28To this end,the use of up-conversion materials may provide a solution to the transmission loss by converting two sub-band-gap photons into one above-band-gap photon(see Fig.7).231–234Indeed,Trupke et al.in2002showed that the theoretical efficiency limit of a single-junction solar cell,modified with an up-converter,can reach63.2%for con-centrated sunlight and47.6%for non-concentrated sunlight, respectively.235

Without altering the already existing merits of a solar cell, the photon up-conversion approach utilizes the anti-Stokes up-conversion technique to convert long-wavelength light to visible photons that can be e?ciently absorbed by most solar cells(see Fig.7).236The up-conversion technique is realized by(1)rare-earth-doped nanocrystals.64–70The up-conversion process pro-duces a highly energetic excited state in the rare-earth ion by sequential absorption of two or more photons with lower energies,as shown in Fig.8a on an example of the Er3+ion absorbing photons with wavelength of B1500nm.237(2)Triplet–triplet annihilation(TTA)up-conversion in a pair of sensitizer-annihilator dyes.61–63In general,the singlet excited state of a sensitizer will be populated(S0-S1)with an excitation photon (Fig.8b).Along with an efficient intersystem crossing process (ISC,S1-T1)due to the heavy atom effect of the transition metal atom,the triplet excited state of the sensitizer,an organo-metallic,will be populated.Since the lifetime of the triplet excited state is much longer than that of the singlet excited state,energy can be transferred from the triplet sensitizer to the triplet acceptor(TTET process).Two nearby triplet acceptor molecules excited in their triplet states interact to combine their excitation energy which results in one acceptor molecule being excited to the higher singlet state,while the other one returns to the ground singlet state.This process is called triplet–triplet annihilation(TTA).The radiative decay from the generated singlet excited state of the acceptor produces an up-converted fluores-cence,for which the energy is higher than the excitation light.The excitation power density required for TTA up-conversion is quite low;a few mW cmà2(which is in the useful range for solar cells).62Furthermore,the excitation wavelength and the emission wave-length of TTA up-conversion can be readily tuned,simply by independent selection of the triplet sensitizer and the triplet acceptor(annihilator/emitter pair).In general,the unabsorbed long-wavelength transmission light in solar devices can be effi-ciently up-converted into visible photons through either rare-earth up-conversion nanocrystals or TTA annihilator/emitter pairs.The resulting visible emission is then to be absorbed by the solar cell devices,e.g.,by the dye in the DSSC cell configuration(Fig.8c)that is known to convert visible light into electricity with high efficiency.

Rare-earth up-conversion nanocrystals work e?ciently in the wavelength range of800–2000nm,while the triplet–triplet up-conversion materials are limited to wavelengths r750nm. Examples for rare-earth-doped up-conversion are:(i)Nd3+/Yb3+/ Er3+for up-conversion from B800nm to B550nm and B650nm;238,239(ii)Yb3+/Er3+(or Ho3+)for up-conversion from 980nm to B550nm and B650nm;240–246(iii)Yb3+/Tm3+for up-conversion from980nm to B800nm and B480nm;247–251 (iv)Ho3+for up-conversion from B1180nm to B550nm and B650nm;252,253(v)Er3+for up-conversion from B1500nm to B550nm and B650nm.237,254–257The Ln3+ions have narrow and low intensity(compared to the organic chromophores) absorption lines at discrete wavelengths,but a number of wavelengths in the range of800–1550nm can be used.258,259 Combinations of the doped ions in a fluoride nanocrystal with minimized cross-relaxations between different activators would help to increase the NIR light absorptivity and the useful up-conversion efficiency.In addition,dye-sensitized up-conversion in lanthanides circumvents the drawback of the low and narrow absorption of lanthanide ions,260opening up the possibility of using an antenna effect provided by appropriate dyes,to imple-ment broad-band up-conversion,similar to TTA up-conversion. The use of up-conversion in NaYF4:Er3+powders significantly improved the response of a silicon solar cell at1532nm,257while the use of LaF3:Yb3+/Er3+,261YF3:Yb3+/Er3+,262and NaYF4:Yb3+/ Er3+,263up-conversion nanoparticles made impressive improve-ment in the efficiency of dye-sensitized solar cells.In addition,an increase in the conversion efficiency of about(1.0?0.2)%at 720nm has also been reported in a hydrogenated

amorphous

Fig.8(a)Scheme of energy levels in an Er3+ion and up-conversion of energy corresponding to the wavelength of1490nm;the inset is the corresponding up-conversion photographic image of colloidal LiYF4:Er3+nanoparticles dissolved in chloroform.(Reproduced with permission from ref.237.Copyright2011, American Chemical Society).(b)The scheme of triplet–triplet annihilation up-conversion;(c)up-conversion of solar energy from different wavelengths of the NIR spectral range(B650–850nm,B800,B980,B1180and1500nm)using up-conversion nanoparticles.All absorbed energy is up-converted to the visible range where the solar cell absorbs.

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silicon (a-Si:H)thin-film solar cell due to the use of photochemi-cal TTA up-converting pairs (the absorption is in the range of 600–750nm,while the emission is in the range of 550–600nm)placed at the rear of the device.264However,the TTA up-conversion is limited only to shorter NIR wavelengths r 750nm;265synthesis of stable organic compounds with the required hierarchy of the triplet and singlet levels in the energy range beyond 0.9–1m m is very problematic.Moreover,organic materials are not so photo-/thermo-stable,and are very susceptible to oxygen which quenches the triplets.266,267Progress on stable TTA up-conversion in new nanostructure designs is being made to circumvent this pro-blem.266–269The simultaneous use of up-conversion in lantha-nide-doped inorganic nanoparticles and TTA in organic solutions or nanoformulations is a promising direction,as full use of sun light in the IR range can be made (see Fig.8c).

C.Nanochemistry

C1.

Solution-based colloidal chemistry

As discussed in Section B,semiconductor nanocrystals (e.g.CdSe,PbS,PbSe.),plasmonic metals or alloyed metal nano-particles,plasmonic semiconductors or transparent conductor nanoparticles with high free charge carrier concentrations,up-conversion/down-conversion nanoparticles,and inorganic/polymer nanocomposites are increasingly being used for high e?ciency solar cells due to their unique optical and electronic properties.Herein,we describe a general solution-based colloidal nanochemistry approach for the preparation of these nano-particles of varying type,size and shape,and with a well-defined composition,allowing a precise control over their optical or electronic attributes.270,271The hot colloidal synthesis approach,widely used for the synthesis of inorganic nanoparticles,gener-ally employs three components:(i)a high boiling point solvent,(ii)ligands (also called surfactants or capping groups,such as TOPO,oleic acid,and oleylamine),and (iii)organometallic pre-cursors.272–274A high boiling point solvent is required to achieve a temperature high enough for crystallization of the inorganic nanoparticles.The organometallic precursors provide the inor-ganic elements (e.g.Cd and Se for CdSe QDs)from which the particles are formed.The ligands or surfactants adsorb to the surface of the nanoparticles,limiting their growth.Typically,a solution containing surfactants is heated to 150to 3501C for synthesis of nanoparticles.65,274A typical apparatus for preparing inorganic nanoparticles is illustrated in Fig.9.

There are two ways for loading precursors into the three-neck round bottom flask,(i)the ‘‘heating-up method’’;248(ii)the ‘‘hot-injection method’’.273In the ‘‘heating-up’’process,the temperature of the reaction solution containing pre-loaded precursors is elevated from room temperature to a specific high temperature to produce nanoparticles.The ‘‘hot-injection’’process employs the injection of pertinent precursors into a hot solution containing the high-boiling solvent and the surfactants.Semiconductor QDs are generally synthesized by a hot-injection process.272,273Injection of organometallic precursors into the hot solvent leads to a rapid burst of particle nucleation,followed by a period of particle growth without further nucleation.

This separation of nucleation and growth is essential for produ-cing uniform nanoparticles.The size of QDs,and thereby their emission color,can be controlled by the temperature,the nature of the solvent and surfactant,and the aging time.Anisotropic growth of QRs can be accomplished by introducing surfactants,such as phosphonic acids,that preferentially bind to a particular crystal facet of the growing nanoparticles.56,275The nanocrystal then grows preferentially along a particular direction in the crystal lattice,producing an anisotropic (rod or wire)structure.In analogy to QDs synthesis,high quality monodispersed nano-particles of plasmonic metal like gold and silver have been successfully prepared by hot colloidal synthesis.124Monodipersed plasmonic semiconductor nanoparticles and lanthanide-doped up-conversion NaYF 4:Yb 3+/Er 3+and NaYF 4:Yb 3+/Tm 3+nano-particles have also been accomplished using the hot-injection method.124,244

The ‘‘heating-up’’approach plays a vital role for the synth-esis of alloyed plasmonic nanoparticles (e.g.,Ag/Au),124,276transparent conductor nanoparticles (ITO nanoparticles),96and lanthanide-doped up-conversion/down-conversion nano-particles.277–279The key for the ‘‘heating-up’’approach is to utilize the Ostwald-ripening process in which larger particles with smaller surface to volume ratios are favored over the energetically less stable smaller particles,resulting in the growth of larger particles at the expense of smaller ones.277It allows the synthesis of alloyed metal nanoparticles (Ag/Au).In contrast,when the hot injection method is used,Ag and Au prefer to nucleate by themselves (forming Ag and Au nano-particles,rather than the Ag/Au alloyed nanoparticles).124

The general hot-colloidal approach has also been applied to produce magnetic nanoparticles (Fe 3O 4,FePt),280,281core/shell nanoparticles,282and other nanocrystalline materials.283,284The nanocrystals prepared by the hot colloidal synthesis are dispersible in organic solvents such as toluene or chloroform because of the presence of nonpolar (hydrophobic)ligands or surfactants bound to their surface.This provides an opportu-nity for solution-processing of these nanoparticles.However,the capping ligands generally have long chains and are insulat-ing materials,impeding the charge separation at the interface and charge transport within the quantum dots layers,

thus

Fig.9A typical laboratory-scale apparatus for preparing inorganic nanoparticles using colloidal synthesis.

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reducing the performance of solar cells involving QDs.148,150Further surface modification is required to shorten or remove the ligand to impart e?cient exciton-dissociation and electron hopping for improved power conversion e?ciency (see Section C5).151–153C2.

Chemical reaction within nano-confined geometries

A widely used method to produce nanoparticles is to conduct nanoparticle synthesis in a nanoreactor such as a microemul-sion which is produced in a thermodynamically stable,optically isotropic dispersion of two immiscible liquids (e.g.water and oil),with at least one type of surfactant.285,286Surfactants are amphiphilic molecules having a polar (hydrophilic)head group and a long non-polar (hydrophobic or lipophilic)tail.These systems form dispersions of nanosize droplets,called micro-emulsions,of oil-in-water (o/w)if water is the majority bulk phase.The microemulsion droplets act to trap nonpolar reac-tants within them,and act as micelle nanoreactors.If water is dispersed in a large volume of oil,the surfactants assemble around them to form a reverse (water in oil,w/o)microemul-sion,in which the polar groups are directed into the aqueous nanodroplets,to form an aqueous (or polar)reverse micelle nanoreactor in which polar molecules can react.285–287The surfactants present in the microemulsion serve not only to stabilize the nanoreactors for chemical syntheses,but also act as steric stabilizers to inhibit the aggregation of reacting species during the reaction period.288The size of the micelle,and subsequently the volume of the aqueous pool contained within the micelle,are governed by the water to surfactant ratio,also termed W 0,where W 0=[H 2O]/[surfactant].285One can easily tune the size and the shape of the droplet to tune the resulting nanoparticle size and shape.Continuous exchange of the micellar contents through dynamic collisions enables the reaction to proceed.Since the reaction is confined within the nanocavity of the micelle,the growth of a nanoparticle beyond the dimensions of the cavity is inhibited.288The micro-emulsion method has been used for the preparation of nano-particles from a diverse variety of materials including metals (Pt,Pd,Au,Ag,Cu,Ni),289bimetallic nanoparticles (Ag/Au,Au/Pt),290–292semiconductor nanoparticles (ZnS,PbS,and CdS),293as well as for other types of nanoparticles (e.g.,silica,269polymer-based TTA up-conversion nanoparticles 266,267).C3.

Successive ionic layer-by-layer adsorption and reaction

Successive ionic layer adsorption and reaction (SILAR)is a novel technique to produce a thin film of the desired materials on a support by in situ chemical reaction involving a range of steps.33Instead of physically depositing particles or atoms on a surface,SILAR utilizes electrostatic force to attract charged ions in the solution to the ions of opposite charge located on the surface layer,thus implementing in situ chemical reaction at the interface for generation of the desired materials.A layer-by-layer deposition can be employed to allow the generation of nanoparticles and their deposition in the form of a solid film.During each step of SILAR,sub-monolayers of desired cations and anions with defined ratios and compositions are alternately

and selectively adsorbed on the surface,yielding the deposition of ultrathin layer of the desired product.294–296The basic SILAR principle is described in Fig.10.The simple equipment,large area deposition,low operating temperatures,and atmospheric pressure requirements of the SILAR process,make it a compara-tively low cost technology for in situ film deposition.

SILAR in principle is able to deposit a broad class of materials as thin films on the treated substrate surface.The only prerequisite is that appropriate cationic and ionic ions can be available in aqueous or organic solvent (water,ethanol,etc.)that allow an e?cient di?usion of charged ions.297–299Since no coordinating ligands are involved in the deposition of a desired film,it is of great interest for in situ deposition of solar materials in the device,allowing efficient charge transfers or injections at the interface.33SILAR has been applied to prepare various inorganic-semiconductor-modified electrodes,particu-larly of metal sulfides (PbS and CdS)on mesoporous TiO 2.300,301It also has been successfully used to deposit a thin film of PbSe and CdSe(Te)on mesoporous TiO 2.A six layer-by-layer SILAR deposition produces a thin film of CdSe (Te)QDs with a broad size distribution,yielding a power conversion of 4.2%at 100W cm à2.33The SILAR process could now be considered as one of the best ways to allow deposition of well-defined composition modulated (doped,alloyed,or multilayered)QD layers onto mesoporous metal oxides using the solution process by alternating change of cationic and anionic precursors.A modified SILAR process can be combined with atomic layer deposition and ion exchange reaction (ALDIER)to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO 2substrates.302C4.

Tailoring the band gap of polymers

Semiconducting polymers play a vital role for either a polymer solar cell or a hybrid solar cell.Earlier polymer solar cells utilized a conjugated polymer (e.g.,P3HT,poly(3-hexyl thio-phene))as an electron donor and a soluble fullerene derivative (e.g.,PCBM,[6,6]-phenyl-C-61-butyric acid methyl ester full-erene)as an electron acceptor,303creating nanoscopic inter-facial phase separation with abundant interface areas for electric-field-induced charge carrier dissociations.304–306In a hybrid polymer solar cell,the n-type PCBM is replaced by inorganic nanocrystals like CdS and CdSe that have deep

LUMO

Fig.10Successive ionic layer adsorption and reaction (SILAR)on a substrate involving the deposition of three layers.

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levels of B 4.0eV.126,307The band-gap of the polymer donor can affect polymer solar cells and hybrid solar cells in three ways:303,307–311(i)the LUMO–HOMO energy difference of the polymer (electron donor)determine the photon harvesting capability of the device,thus,determining the J sc of the PV device;(ii)an abrupt change between the LUMO level of the electron donor and the LUMO level of the electron acceptor produces an electric field at the interface to dissociate charge carriers;(iii)the energy difference between the HOMO level of the electron donor and the LUMO level of the electron acceptor determines the V oc of the device.A HOMO offset of the electron donor relative to that of the electron acceptor is favorable for the hole separation.Therefore,a judicious selection of existing pi-conjugated polymers or engineering of conjugated moieties in a specific polymer to produce an electron donor with an appropriate band-gap and a high hole mobility,is important for high efficiency PV devices.

For example,earlier investigations of CdSe QDs-based hybrid solar cells used P3HT or MDMO-PPV as an electron donor;126,312however,the device e?ciency was generally below 3%,as neither MDMO-PPV (E g =2.2eV)nor P3HT (E g =1.9eV)can e?ectively harvest photons from the solar spectrum due to the large band-gap.Utilization of lower band-gap polymers than P3HT can have a broader absorption spectrum,and thus,a higher solar conversion e?ciency.313,314Indeed,a device e?ciency over 3%has been obtained using PCPDTBT (E g =1.45eV)containing a cyclopenta[2,1-b :3,4-b 0]dithiophene and 2,1,3-benzothiadiazole.315,316We would like to mention that,in the case of PbS-based hybrid solar cells,fine-tuning of energy levels of the donor polymer is important for exciton dissocia-tion at the interface due to the ambipolar characteristic of PbS nanocrystals.The employment of P3HT of a small HOMO o?set relative to that of PbS (PbSe)nanocrystals generally led to low PCEs,317–319while utilization of high-lying HOMO polymers (PDTPQx and PDTPBT)with a large HOMO offset,has signifi-cantly increased the PV device performance (see Fig.11b).The practical performance of a hybrid PV device is a direct result of a trade-off between V oc and J sc that are determined by the energy difference between the band gap of the QDs (size,affecting the photoinduced charge separation)and the poly-mers.320,321It is worth noting that a majority of semiconducting polymers have band gaps higher than 2eV (620nm),which limits the possible harvesting of solar photons to about 30%.A low band gap of 1.1eV (1100nm)is capable of absorbing 77%of the solar irradiation on earth.303Nanochemistry for mole-cular engineering of polymers to tune the band-gap to the IR range is of particular interest.Fig.11c exhibits band gap-tuned polymers (PCPBBT,PFTBBT,PCPBTD and PFTBTD)that have been chemically synthesized,displaying tunable absorp-tion from visible to IR in Fig.11d.322The PCPBTD (band gap B 1.6eV)and PCPBBT (band gap B 1.01eV)polymers were obtained by the Stille coupling reaction of the organotin reagent (1)with 4,7-dibromobenzothiadiazole [Reagent (3)],and 4,8-bis(5-bromo-2-thiophenyl)-2l4d2-[1,2-c :4,5-c0]bis[1,2,5]-thiadiazole)[Reagent (5)],respectively.The Suzuki coupling reactions have been implemented to prepare PFTBTD (band

gap B 2.05eV)and PFTBBT (band gap B 1.2eV),by reaction between the diboronic acid fluorene [reagent (2)]and 4,7-bis(5-bromo-2-thiophenyl)-benzo[c][1,2,5]-thiadiazole [Reagent (4)],or 4,8-bis(5-bromo-2-thiophenyl)-2l4d2-[1,2-c :4,5-c0]bis[1,2,5]-thiadiazole [Reagent (5)],respectively.By using a varying type of the repeated moiety and by increasing the number of the repeated moiety in a precisely defined way,the absorption maxima of the pi-conjugated polymer can be gradually tuned from the short to the long IR wavelength range.These tunable polymers can harvest a broad solar spectral range and,thus,are able to produce a high efficiency single-junction hybrid PV cell (when one low band-gap polymer is utilized),or a high effi-ciency multi-junction polymer solar cell when multiple of them are simultaneously utilized.323C5.

Interface engineering

As described in the section on solution-based colloidal chem-istry,the hot colloidal synthesis approach generally employs ligands (also called surfactants or capping groups)which are able to cap on the surface of the quantum dot to control the size and shape as well as to produce a stable https://www.doczj.com/doc/299020017.html,ually,as-synthesized nanocrystals are shielded with long alkyl-chain ligands like TOPO (comprising of three 8-carbon chains)and oleic acid (comprising a 18-carbon chain),which enable easy solution-processability for device fabrication,but they also act as an insulating layer (1–3nm long).146–153These long-chain ligands produce two adverse effects on the charge dynamics in the film of semiconductor nanocrystals.One is that the insulat-ing characteristic of these ligands produces a barrier for electrons and holes to separate;the other one is that the steric length of the ligand generates wide inter-particle distances between nanocrystals,creating another barrier for electron or hole hopping.To accomplish an efficient solar device,it is necessary to perform ligand exchange with short-length ligands that have a higher binding affinity for QDs,but a shorter length scale for an improved carrier transport.35Manipulation of the interface of nanoparticles is a promising approach to deal with various interfacial problems of nanomaterials-based solar cell devices.Though Section B3provides the specific interface treatments to produce closely contacted inorganic and polymer nanocomposites for hybrid solar cells,the following discus-sions on the interface of nanoparticles are intended for general nanomaterials-based solar cells.

The original long-chain ligand capped on the nanoparticle surface can be replaced with a desired ligand using either a pre-ligand exchange method or a post-ligand exchange method.A pre-ligand exchange method is generally applied to II–VI CdSe or CdS QDs,while a post-ligand exchange method is generally applied to IV–VI PbSe or PbS QDs.In a typical procedure for the pre-ligand exchange,an adequate pre-ligand of pyridine is com-monly used to exchange the long alkyl chain ligands capped on the as-synthesized nanocrystals under high temperature.324,325After casting pyridine-capped CdSe or CdS nanocrystals into a film,heat treatment can efficiently remove pyridine that is weakly bound to the nanoparticle interface.Furthermore,removal of the interfacial pyridine reduces inter-nanoparticle distances,

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thus enhancing the charge transport in the device.324Approaches for in situ removal of ligands capped on CdSe nanocrystals have also been demonstrated by using a hexanoic acid treatment or a cleavable functional ligand (see Fig.12).326,327Although pre-ligand exchange is a promising approach to remove the insulating ligands from nanoparticles for an efficient device operation,it cannot be applied to IV–VI semiconductors of PbS and PbSe,as some participates are formed after pyridine treatment.

A post-ligand exchange method,therefore,has been developed for PbS and PbSe nanocrystals,which involves soaking the long chain ligand capped nanocrystal film in a solution comprising an excess of short-alkyl chain ligands.A good example is P3HT/oleic acid-capped PbS nanocrystal film,which becomes more conduc-tive after soaking with a solution of acetic acid (CH 3COOH)that has a shorter chain length than oleic acid.319,328Similarly,a higher carrier mobility of nanocrystals was substantiated in a shorter ligand (hydrazine,NH 2NH 2–)-capped PbSe nanocrystal composite using a field-effect transistor configuration.148It is important to

note that the process of post-ligand exchange can be carried out in a layer-by-layer fashion,creating a thick nanoparticle layer whereby each nanoparticle is capped by the desired short ligand.For example,the first layer of oleic acid-capped PbS nanoparticles can be soaked in 1,2-ethanedithiol (EDT,creating an interparticle distance of B 2.1nm)solution in acetonitrile (AcCN),allowing the post ligand exchange process of oleic acid by EDT to occur.The exchanged oleic acid and any residual EDT can be removed by washing with pure AcCN and n -hexane.Multiple layers of EDT-capped PbS QDs can be obtained by repeating these procedures,producing the desired thickness of EDT-capped PbS film.146Such a successive layer-by-layer deposition has been performed by the spin-coating process and then the soaking process.The success of the layer-by-layer post ligand exchange process is revealed by XPS results where sulfur-containing oxide products,lead sulfite (PbSO 3)and lead sulfate (PbSO 4),are confirmed to be produced.146In comparison to the EDT-treated nanoparticle,the density of ener-getic distribution of charge traps in QDs can be more

effectively

Fig.11(a)Molecular structures of conjugated polymers of MDMO-PPV,P3HT,PCPDTBT,PSBTBT,PDTPQx,PDTPBT that have been generally used in polymer-based solar cells.(b)HOMO–LUMO energy levels of polymers in (a)and quantum dots (CdS,CdSe,PbS,and PbSe),as well the Fermi levels of some metals that are usually employed as electrodes.Note that the band-gaps of semiconductor QDs are dependent on the size of nanoparticles.(c)Schematic illustration of chemical synthesis of low band-gap polymers PCPBTD,PCPBBT,PFTBTD,and PFTBBT.(d)Absorption spectra,with tuned maxima from visible to IR range,of polymers PFTBTD,PCPBTD,PFTBBT,and PCPBBT dispersed in chlorobenzene.(Reprinted with permission from ref.322.Copyright 2011,Royal Society of Chemistry).

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reduced by utilizing a shorter 3-mercaptocarboxylic acid ligand (MPA,creating an interparticle distance of B 1.8nm)that com-prises functional groups of thiol and carboxylic acid.147The use of the MPA ligand can cause a decrease in the trap state depth and recombination rate,improving the current density in the solar device.147The shortest length halide atomic ligand was also applied to the treatment of PbS QDs,binding to surface defect sites that are inaccessible by typical alkyl thiol ligands due to their steric hindrance.151Atomic halide passivation (Cl à,Br àand I à,ligand size scaling down to 0.1nm)by post ligand exchange produces more pronounced passivation of the QDs surface,achiev-ing an impressive solar cell efficiency of 6%under AM 1.5G solar irradiation.151Recently,a hybrid passivation method of post-ligand exchange using a combination of metal halide treatment and MPA treatment has resulted in immense efficiency of 7.4%in PbS nanocrystal heterojunction solar cells (see Fig.25).153

Another approach for interface engineering of nanoparticles is to use a functional ligand containing tert -butyl N -(2-mercapto-ethyl)carbamate for post ligand exchange,which can be in situ thermally shortened (see Fig.12a).146Utilizing a post ligand exchange,the typical TOPO ligand capped on the surface of CdSe NRs can be easily replaced by the tert -butyl N -(2-mercaptoethyl)-carbamate ligand in the film of the hybrid P3HT/CdSe NRs device.As the tert -butoxycarbonyl (t -BOC group)in the carbamate ligand become unstable at a temperature range of 150–2501C,heat treatment in this range can break the t -BOC group into isobutene and carbon dioxide (see Fig.12a)and shorten this functional ligand,improving the solar conversion efficiency by B 60times.146An acid-assisted washing method has also been employed to remove the ligands from as-prepared CdSe QDs,resulting in improvement of charge transfer between the P3HT polymer and the nanocrystals (see Fig.12b).326

As illustrated in Fig.12b,the hexanoic acid can directly react with the hexadecylamine ligand that is capped on the CdSe QDs,forming an ionic organic salt.This organic salt can readily dissolve in the washing solvent and thus,can be separated easily from the QDs by subsequent centrifugation.This acid treatment approach removes the insulating long-chain ligand,allowing an improved charge transfer between the P3HT and QDs as well as an improved electron transport between QDs.326

To conclude,major breakthroughs in this area can be achieved by the development of innovative nanochemistry methods to tailor the surface properties of nanocrystals.Inter-face engineering to control chemical,physical and electronic interactions at the nanocrystal surface in the nanoscale range plays a vital role for improving electrical transport in the nanocrystal-based devices.The impact of interface engineering of nanoparticles on practical PV devices is covered in Section D.

C6.Spray/printing

A key factor in developing third generation photovoltaics is the ability to utilize low cost production techniques,while main-taining a high device quality.Nanochemistry provides a very attractive approach of in situ generation of desired composition solution-processed thin films towards this goal,particularly for the ones involved in copper indium gallium selenide (CIGS)or copper zinc tin sulfide (C2ZTS4)solar cell devices.329,330Utilizing this approach,the reported performance of solution-processed C2ZT(SSe)4solar cells has already exceeded that of traditionally vacuum-processed counterparts.330

Unlike particle-based solution processing,this approach utilizes fully dissolved precursor complexes in solvents,in which each of the elemental constituents are mixed on a molecular scale with precisely defined ratios and concentra-tions.For example,cupric chloride dehydrate (CuCl 2á2H 2O),zinc chloride (ZnCl 2),stannic chloride (SnCl 4)and thiourea are mixed in stoichiometric ratios in deionized water for the formation of a copper zinc tin sulfide C2ZTS4absorber layer.331A controlled spray-based system can be applied to deposit these absorber precursors on a temperature-controlled hot plate (see Fig.13a);then in situ chemical reaction is implemented to prepare the film of C2ZTS4.Excellent film homogeneity can

be

Fig.12(a)Schematic illustration of the thermal deprotection of the t -BOC moiety in the tert -butyl N -(2-mercaptoethyl)carbamate ligand linked to the surface of the CdSe nanocrystals (reprinted with permission from ref.146.Copyright 2009,American Institute of Physics),and (b)acid treatment-assisted removal of the insulating hexadelyamine ligand liked to the QDs surface.(Reprinted with permission from ref.326.Copyright 2010,American Institute of

Physics).

Fig.13(a)A set-up for spray-based nanochemical fabrication of a thin film for a solar cell device.(b)Confocal images of a printed and patterned metallic gold structure with a 1m m period:1st quadrant shows transmission image,2nd quadrant shows reflectance image,3rd quadrant shows overlapped images,4th quadrant shows 3D intensity profile.(Reprinted with permission from ref.332.Copyright 2010,Wiley-VCH Verlag GmbH &Co.KGaA).

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realized by placing the sample at an appropriate distance from the spray head which is moved back and forth at a precisely defined speed.The film thickness can be easily and precisely controlled by the control of the spraying-time,the regulated gas rate,and the concentration of precursors.In addition,in situ nanochemical reactions can be implemented in a layer-by-layer manner,finally building up a low-cost solar cell device with impressive power conversion e?ciency of B 4.4%.331The high performance of the resulting low-cost devices provides an opportunity to extend the impact of spray-based nanochemical approaches for new generation solar cells.

Similar to chemical reactions in spray-based films,in situ nanochemical reactions can also be performed in patterned structures.A patterned-film comprising involved precursors can firstly be produced by a range of approaches,such as conventional lithography,screen-printing,inkjet printing,and pad printing.32Then,implementation of a nanochemical reaction using heat,light,or other parameters can enable the fabrication of desired and patterned materials.A two-photon lithography approach has been utilized to prepare sub-wavelength metallic structures in a polymer matrix through in situ photon-reduction of a gold precursor (HAuCl 4á3H 2O)during pattern writing (see Fig.13b).332Simultaneous writing and reduction of gold precur-sors provides a good example for light-induced printing of patterned metallic structures,playing an important role for the improvement of the solar cell e?ciency using plasmonic e?ects (see Section B2).

D.Nanomaterials-based solar cells

D1.

Introduction

This section describes various types of QDs-based solar cells that have been fabricated,exploring the advantages of the use of nanomaterials.333–335These solar cell structures are schema-tically represented in Fig.14.Examples for each type are presented in the following.

The first type described in Section D2is a hybrid bulk-heterojunction cell.Since the first report by Alivisatos and coworkers who used P3HT and CdSe NRs,126this type of cell has shown e?cient photovoltaic performance.336This device configuration has the advantage of a bulk-heterojunction of nanocomposite that yields bicontinuous percolation pathways in the nanoscale phase.In particular,it is possible to choose a variety of polymers with tuned energy levels and absorption,for compatibility and exciton dissociation in QDs.

The second type of QDs-based solar cell is a Schottky junction cell,described in Section D3.It has a simple device configuration where the QDs film has an ohmic contact with ITO,and on the other side of the QDs film,a low work function metal is deposited to create a built-in-potential at the Schottky junction.154,156Various sizes of PbS and PbSe QDs have been adopted in this device to harvest infrared light and produce high current density.However,the open circuit voltage is limited to an order of half of the band gap of the QDs.

The third type,described in Section D4,is QDs-sensitized solar cells which utilize photoinduced charge separation at the CdSe–TiO 2interface,similar to that in dye-sensitized solar cells which have been widely investigated.3,337They are easy to fabricate.

Another type of cells described in Section D5is depleted heterojunction cells which involve the heterojunction of TiO 2–PbS QDs.35,79Currently,the highest power conversion efficiency (of B 7%)is reported for this type of heterojunction cell among the various device architectures of QDs-based solar cells.153The advantage of this device architecture and the materials science of QDs are described in Section D5.35,79,338,339

These types of cells are described in some detail in the respective subsections below.In addition,subsection D6describes tandem and multijunction cells that couple many cells in tandem and utilize multiple heterojunctions.D2.

Hybrid bulk-heterojunction solar cells

A polymer solar cell is a bulk-heterojunction device composed of a p -conjugated polymer (e.g.P3HT,poly(3-hexylthiophene))and a soluble fullerene derivative (e.g.PCBM,[6,6]-phenyl-C-61-butyric acid methyl ester fullerene).308To date,the investigation of varying composition,morphology and annealing conditions,has led to power conversion e?ciency (PCE)up to 8–9%.308One promising direction to further increase the PCE is to replace the organic electron acceptor (PCBM)with solution-processed inorganic semi-conductor nanoparticles,producing a new type of hybrid bulk-heterojunction solar cell.The advantages of inorganic and polymer hybrid nanocomposites have been described earlier in Section B3.A broad variety of nanocrystals have been utilized in the hybrid polymer nanocrystal solar cells,including cadmium selenide (CdSe),cadmium sulfide (CdS),lead selenide (PbSe),lead sulfide (PbS),copper indium disulfide (CuInS 2),silicon (Si)spherical nanocrystals,CdSe NRs as well as CdSe hyper-branched nanocrystals.333–336In analogy to polymer solar cells,nanoscopic arrangement of the polymer and the inorganic QDs as well as their morphologies produced in the nanocomposite film,have pronounced effects on the PCE.This is because the polymer–inorganic interface is important for electron–hole charge separation,while a hopping electron transport is accom-plished through the percolation pathway of inorganic QDs.Since the as-prepared QDs are covered with insulating long alkyl chain ligands like TOPO or OA,an efficient charge transfer from a p -conjugated polymer to the QDs requires appropriate inter-face engineering (see Section C5).It is also found that the percolation pathway of QDs is sensitive to the solvent and the composition of the blend,as well as to the

morphological

Fig.14Various device configurations for QDs-based solar cells.(a)Hybrid polymer–QDs solar cell,(b)Schottky junction solar cell,(c)QDs sensitized solar cell,and (d)depleted heterojunction solar cell.

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structures of QDs (e.g.,the aspect ratio of nanorods,and the three dimensional (3D)structure of hyperbranched (or tetrapod)nanocrystals).333–336Optimizations of the interface and the percolation pathway of QDs are important for improving solar conversion efficiency.Here,we describe some examples of QDs based bulk hetero-junction hybrid cells that utilize the principles described in Sections B3and C5to implement the optimization of these devices.In addition,in the development of hybrid PV cells,much attention is dedicated to semiconducting nanocrystals such as CdSe,CdS,PbSe and PbS.Hence,in the following,we focused on CdSe (CdS)-based and PbSe (PbS)-based hybrid hetero-junction cells.

D2.1CdSe (CdS).Alivisatos et al.reported 1.7%of power conversion e?ciency by controlling the aspect ratio of CdSe NRs dispersed in P3HT polymer (see Fig.15).126In their work,a high aspect ratio of nanorod was found to be favorable for forming a vertically aligned orientation during solvent evapora-tion which improved charge transport.Also they developed hyperbranched CdSe nanocrystals which exhibited better per-formance (2.2%),comparable to that utilizing NRs.340This internal 3D structure of dendritic inorganic nanocrystals con-trolled nanoscale blending and morphology,leading to a preformed percolation network in the active layer.Several reports have described the removal of the surface ligand and morphology control by choosing a suitable solvent for spin-coating.324,325,341Cao et al.reported that pyridine-capped CdS exhibited improved compatibility with MEH-PPV,resulting in a stronger donor–acceptor interaction in homogeneous blends and thereby better charge generation.325On the other hand,in order to enhance miscibility of a polymer with nanocrystals and suppress nanocrystal segregation,a chemical modification of the end group of P3HT polymer was also demonstrated to control the morphology and yield a larger interfacial area for charge separation.In addition,other approaches such as direct coordination of thiophene-based dendrimers and oligo-mers to nanocrystals as well as layer-by-layer assembly of P3HT and nanocrystals through hydrogen bonding have been explored.132,342A power conversion efficiency of 2.6%has been obtained in hybrid photovoltaic cells,where the P3HT nano-fiber morphology with CdSe nanorod was optimized using a

high boiling point solvent,1,2,4-trichlorobenzene (TCB).343Similarly,improved efficiencies (2.4–2.8%)were also achieved in a blend film of CdSe tetrapods with poly(2,7-(9,9-dioctyl-fluorene)-alt -5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole))or poly(2-meth-oxy-5-(30,70-dimethyloctyloxy)-p -phenylenevinylene).344,345Another approach to enhance interfacial interaction between the nano-scopic inorganic nanocrystal and the polymer for charge gen-eration is the use of a thermally cleavable solubilizing ligand as described in Section C5.146Prasad and coworkers demon-strated a relative improvement in hybrid polymer composite PV cells of P3HT :CdSe nanocrystals (see Fig.12a).146Thermal deprotection processing of the tert -butoxycarbonyl (t -BOC)moiety in the carbamate ligand surrounding the surface of a CdSe nanocrystal significantly shortened the length of the ligand,and thus the distance between the nanocrystal and

the polymer matrix.Furthermore,Kru

¨ger and coworkers pre-sented an acid-assisted washing method to remove the ligand of as-prepared CdSe quantum dots,resulting in an improve-ment in charge transfer between the P3HT polymer and the nanocrystals (PCE =B 2%)(see Fig.12b).326In situ formation of CdS nanocrystals in P3HT polymer was achieved by Leventis et al.346This approach yielded a PCE of 0.7%.So far,there have been tremendous efforts to improve the device efficiency in a hybrid CdSe nanocrystal–P3HT polymer solar cell.Nevertheless,PCEs of all those devices could not exceed 3%.Very recently,the use of a low band gap polymer (poly[2,6-(4,4-bis(2-ethylhexyl)-4H -cyclopenta[2,1-b ;3,4-b 0]dithiophene)-alt -4,7-(2,1,3-benzothiadia-zole)],PCPDTBT)with CdSe tetrapods gave rise to a significant enhancement in PCE (B 3.2%)due to a broad band absorption in the range of 350–800nm.315Recently,Meerholz and cow-orkers have enhanced the device performance (up to 3.6%)using a blend of both CdSe QDs and QRs that yielded a well-interconnected pathway for electrons within the p-type PCPDTBT polymer matrix (see Fig.16).

347

Fig.15(a)Schematic device structure consisting of P3HT and pyridine-capped CdSe nanorods.The molecular structure of P3HT and the energy band diagram between P3HT and CdSe.(b)EQEs of the resulting devices depending on the aspect ratio of the nanorod.(Reprinted with permission from ref.126.Copyright 2002,American Association for the Advancement of

Science).

Fig.16(a)The molecular structure of a low band gap polymer (PCPDTBT)and the pyridine-capped CdSe QDs and NRs.(b)The device performance (PCE and J sc )as a function of the QDs/NRs weight ratio.The QDs/NRs composite (B)forms the best percolation pathway for electron transport compared to QDs (A)and NRs (C)alone.(Reprinted with permission from ref.347.Copyright 2012,Wiley-VCH Verlag GmbH &Co.KGaA).

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Besides binary CdSe nanocrystals,several approaches utiliz-ing multinary compounds such as type II CdTe/CdSe nanocrystal heterostructures and tetrapod ternary CdSe x Te1–x nanocrystals have been investigated,but their performance is still low.348–350 D2.2PbS(PbSe).Much research e?ort has been focused on lead chalcogenide nanocrystals(PbS and PbSe)to harness energy from near-or mid-infrared wavelengths beyond700nm,where almost one-half of the total solar AM1.5power resides.35These Pb chalcogenide nanocrystals have tunable broad absorption bands throughout the IR region(from800nm to1600nm)due to their strong quantum-confined nature(see Section B1).In addition,NIR nanocrystals with low band gap can potentially provide e?cient use of the short wavelength(ultraviolet)range of the solar spectrum via the MEG e?ect discussed earlier in Section B5.1.However,an important consideration in hybrid polymer nanocrystal solar cells is the ligand passivation of nanocrystals.Contrary to CdSe,ligand exchange of PbS(PbSe) nanocrystals by pyridine could not assure solution-processing after the capped oleic acid ligand was removed from the nano-crystal surface(see Section C5).Instead,Sargent et al.prepared octylamine-capped PbS nanocrystals by post ligand exchange, and reported a200-fold increase in the short circuit current and a600-fold increase in the maximum power output,as compared to that of the oleic acid-capped PbS and poly[2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene](MEH-PPV)blend film.351 As a consequence,enhanced exciton dissociation at the improved interfacial contact and improved electron transport by a closer interparticle distance resulted in an enhancement of the photo-voltaic performance after ligand exchange.352Feng et al.utilized MDMO-PPV as the capping ligand for the synthesis of PbS nanocrystals and prepared a MDMO-PPV-capped PbS blend film, reporting more than one order higher power conversion e?-ciency than that of the pristine MDMO-PPV.353A facile ligand exchange by post-chemical treatment using acetic acid as a short-length ligand was demonstrated for a OA-capped PbS nanocrystal and P3HT blend film,yielding an improvement of solar cell performance.319By evaluating the photoluminescence decay,it was confirmed that the electronic interaction(charge generation) was more e?cient between the polymer and the PbS nanocrystals after the chemical treatment of the nanoparticle surface.Jiang et al.reported a sizable photovoltaic response from PbSe nanocrystals in P3HT polymer,with an open circuit voltage of 0.3–0.4V,and a short circuit current of B0.2mA cmà2.354In particular,this device showed a spectral response in the NIR range up to2m m,which was contributed by the embedded PbSe nanocrystals.A power conversion efficiency of0.14%was achieved with PbSe nanocrystals(80wt%)in P3HT(20wt%) under AM1.5G illumination by Cui et al.,who reported1.3%of current conversion efficiency(IPCE)derived at805nm.355They improved the device efficiency(by two times)by modifying the device configuration from integrated planar(PbSe)/bulk hetero-junction(PbSe-P3HT)structures.356Pal et al.introduced TiO2 NRs into the PbS-P3HT blend film to make a ternary bulk-heterojunction device,enhancing the photoinduced electron-transfer from PbS to TiO2(see Fig.17a).357However,the P3HT-based hybrid PbS(PbSe)nanocrystal-solar cells,discussed above,still exhibited very low PCE.It was mainly attributed to inefficient carrier generation by their poor type-I heterojunc-tions,because of a small energy difference between the valence band(5.0–5.2eV)of PbS and the highest occupied molecular orbitals(HOMO)(5.1eV)of P3HT(see Fig.11b).Recently,the problem of P3HT-PbS NC devices due to a poor type I-hetero-junction has been overcome by the use of multi-walled carbon nanotubes(NWCTs).358Izquierdo,Ma et al.prepared NIR-active PbS QDs-NWCTs nanostructures that are blended with P3HT (see Fig.17b).The resulting nanohybrid bulk-heterojunction cell (P3HT:PbS QDs-NWCTs)exhibited a high PCE of3.03%due to effective charge generation from PbS QDs,and better charge extraction toward each electrode through P3HT and MWCTs. With respect to the band alignment between the polymer and the PbS QDs,it is essential to use p-type polymers with a higher-lying HOMO level than that of P3HT(see the discussion on energy level matching in Section C4).Recently,Ginger,Jenekhe et al.prepared a new polymer,poly(2,3-didecyl-quinoxaline-5, 8-diyl-alt-N-octyldithieno[3,2-b:20,30-d]pyrrole)(PDTPQx),with a HOMO of4.61eV and confirmed a significant photoinduced electron transfer from PDTPQx(band gap,E g=2eV)to PbS QDs (B3nm diameter)in bulk heterojunction blends by photo-induced absorption(PIA)spectroscopy(see Fig.18a).320Using the ligand exchange procedure described earlier in Section

C5,

Fig.17(a)EQE spectra of the device(P3HT:PbS)with or without a TiO2layer.TiO2concentration was varied.(Reprinted with permission from ref.357.Copyright 2010,American Institute of Physics).(B)I–V curves of the P3HT blend devices with PCBM and with PbS QDs-MWCNT nanostructures(inset),respectively.(Reprinted with permission from ref.360.Copyright2010,Wiley-VCH Verlag GmbH&Co.KGaA).

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the oleic acid ligands of PbS QDs were replaced by butyl amine,prior to blending.This simple device (ITO/PEDOT :PSS/active layer/Al)of PDTPQx/PbS (90:10w/w)blend exhibited a power conversion efficiency of B 0.55%.Prasad and coworkers have introduced the dithienopyrrole-based low band gap polymer (band gap,E g =1.4eV)with a high-lying HOMO energy level to make bulk heterojunction blends with PbS QDs.321Direct post chemical treatment of 1,2-ethanedithiol (EDT)for ligand exchange was applied to the blend film and consecutive deposi-tion of a TiO 2layer resulted in an optimized hybrid bulk heterojunction device,giving rise to a broad spectral response and an impressive PV performance of 3.78%(see Fig.18b).Such a post-deposition ligand exchange by thiol chemicals has also been adopted to a CdSe QDs :P3HT hybrid solar cell by Chen and coworkers to achieve a performance of 3.09%due to surface passivation of the nanocrystals (see also Section C5).359Very recently,Tao and coworkers have observed direct hole extraction from PbS QDs to a low band gap polymer by quenching of PbS photoluminescence.360The bilayered planar hybrid device configuration (PbS/polymer)established a type II-heterojunction in combination with hole and electron extraction layers of MoO x and ZnO,to yield a PCE of 3.8%.

Among hybrid organic–inorganic nanocrystal solar cells,another design approach used is a planar heterojunction of bilayered device architecture which utilizes C60and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM).Silva et al.demonstrated an external quantum efficiency (EQE)of 1.2%at 1150nm in a butylamine-capped PbS/C60hybrid cell,about two orders higher than that for a oleic acid-capped PbS/C60device.361Additional solvent treatment of butylamine-capped PbS resulted in an improvement in PCE,but was still low (0.14%).Bawendi et al.fabricated a PbS QDs-PCBM bilayered device where PCBM acted as the electron-transporting and hole-blocking layer.362A two-step process consisting of air-annealing and thiol treatment caused a large decrease in charge recombination at the nanocrystal surface,yielding a FF as high as B 60%.Tao and coworkers improved the device fabrication by performing thiol treatment and introducing a thin hybrid blend layer of PbS QDs :PCBM (wt ratio 30:1)between

the p-type PbS QDs film and the n-type PCBM film,yielding a PCE of 3.7%.363

Besides cadmium and lead chalcogenide nanocrystals,copper indium disulfide (CuInS 2)and silicon spherical nanocrystals have also been employed in hybrid solar cells,with an advantage that they are non-toxic and made from earth abundant com-pounds.364,365In particular,a 35%Si NC (3–5nm):P3HT blend device exhibited a PCE of 1.15%.365

D3.Schottky junction solar cells

For this type of cell,a colloidal nanocrystal film is in ohmic contact with the indium tin oxide (ITO)substrate,while a low work function metal (Al,Ca,Mg etc.)is deposited on the other side of the nanocrystal film,forming a Schottky junction with a built-in-potential where electrons are extracted and holes are repelled.339Generally,chemical treatment of PbS and PbSe nanocrystals provides an excellent p-type semiconductor behav-ior to fabricate a typical Schottky junction with shallow metal contacts.The first PbS Schottky junction device with a PCE of 1%was demonstrated by replacing the long alkyl oleic acid ligand with a much shorter butylamine ligand to improve carrier transport.366In particular,1,2-ethanedithiol (EDT)or 1,4-benzenedithiol (BDT)treatment in a layer-by-layer deposi-tion of PbS QDs resulted in a shorter interparticle distance and significantly decreased surface defects.156,367–369The detailed procedure for post-ligand exchange has already been shown in Section C5.Moreover,the insertion of a thin LiF buffer layer between the nanocrystal film and the Al metal improved the Schottky device performance through enhancing the air-stability of the device.160The use of PbS nanocrystals synthesized by a slow-growth method gave rise to a remarkable improvement in V oc and FF due to a largely reduced number of traps on the particle surface.370As a result,an optimized PbS QDs-Schottky junction cell reported a PCE as high as 3–4%.370,371On the other hand,for PbSe QDs Schottky cells,despite their high short-circuit current densities (J sc ),a relatively low V oc was found,where V oc is determined by the band-gap (E g )of the QDs according to the equation,154

V oc ?0:49E g

q à0:253V (2)For example,a PbSe QDs Schottky cell with Au contacts,reported by Nozik and coworkers,exhibited a V oc of B 0.05V,due to the high work function of Au.Thus,contacts consisting of air-sensitive Ca or Mg metal,overcoated with Al were required to increase the observed built-in potential in the Schottky junction (V oc of 0.2–0.3V)(see Fig.19).Recently,the development of high quality small-sized PbSe QDs through low temperature synthesis resulted in a PCE of 2.8–4.57%(see Fig.20).372,373As an alter-native,Alivisatos and coworkers have developed PbS x Se 1–x ternary QDs to improve V oc and J sc .374Schottky cells incorporating these ternary QDs yielded a PCE of 3.3%under AM 1.5(global)illumination.However,in terms of the device geometry and the open-circuit voltage,as mentioned in D1,the Schottky junction devices exhibit several drawbacks as follows (see also Section B4on interface

engineering):

Fig.18(a)X-Channel (in-phase)photoinduced absorption spectra of the poly-mer :PbS QDs (B 3nm)nanocomposite film where PDTPQx (red circles),PDTPPPz (green diamonds),and PDTPBT (blue squares)are used.(Reprinted with permis-sion from ref.320.Copyright 2010,American Chemical Society).(b)Schematic diagram of the fabrication procedure and the device structure.The scanning electron microscopy (SEM)image shows the side view of the device and the surface morphology of the EDT-treated blend film of PDTPBT/PbS (10/90wt%).(Reprinted with permission from ref.321.Copyright 2011,Wiley-VCH Verlag GmbH &Co.KGaA).

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