Cite this:Phys.Chem.Chem.Phys.,2015,17,19745
Inverted planar NH 2CH Q NH 2PbI 3perovskite solar cells with 13.56%e?ciency via low temperature processing ?
Da-Xing Yuan,a Adam Gorka,b Mei-Feng Xu,a Zhao-Kui Wang*a and Liang-Sheng Liao*a
In this work,NH 2CH Q NH 2PbI 3(FAPbI 3)was employed for light harvesting in inverted planer perovskite solar cells for the first time.Except for the silver cathode,all layers were solution-processed under or
below 1401C.The e?ect of the annealing process on device performance was investigated.The FAPbI 3solar cells based on a slowed-down annealing shows superior performance compared to the CH 3NH 3PbI 3(MAPbI 3)-based devices,especially for the short circuit current density.A power conversion e?ciency of 13.56%was obtained with high short circuit current density of 21.48mA cm à2.This work paves the way for low-temperature fabrication of e?cient inverted planer structure FAPbI 3perovskite solar cells.
Introduction
Since organo-metal halide perovskite was first employed for light harvesting in solar cells in 2009,1much e?ort has been made to improve device performance,including introducing solid state hole conductors,2modifying the fabrication process 3–10and the annealing process,11–16structure engineering,17–19interface engineering 20–30and so on.So far,the highest performance has overcome 20%.31However,most work was based on the CH 3NH 3PbI 3(or CH 3NH 3PbI 3àx Cl x )system,in which its energy bandgap (B 1.55eV)is beyond the ideal bandgap of photo-voltaic materials (1.1–1.4eV).What’s more,the CH 3NH 3PbI 3system was reported to have a very low phase transition tem-perature,which will strongly influence the device stability.32
Recently,a brand new perovskite system,known as NH 2CH Q NH 2PbI 3(FAPbI 3)perovskite,has drawn much attention,since it has superior properties compared with the CH 3NH 3PbI 3(MAPbI 3)system,such as extended absorption range,33–35higher phase transition temperature and better photostability.36
By introducing a small amount of hydroiodic acid (HI)to the FAPbI 3precursor solution,Snaith et al.obtained a compact and uniform one-step spin-coating processed FAPbI 3perovskite layer,and a high performance of 14.9%in power conversion efficiency (PCE)was realized.Park and co-workers 36achieved a current density–voltage (J –V )hysteresis-free and photostable FAPbI 3perovskite solar cell based on a mesoporous structure by using a sequential deposition technique.Zhao et al.reported that FAPbI 3perovskite films exhibit a very pure crystalline phase with a strong (110)preferred orientation when a new precursor compound of HPbI 3was introduced.37However,all related works were based on a conventional structure,in which condensed or mesoporous TiO 2acts as the bottom electron extraction layer and allows for a pretty high temperature (400–5001C).Such a high temperature is not suitable for low cost and flexibility.
In this work,FAPbI 3was employed as the light harvester in inverted planer perovskite solar cells for the first time.Except for the silver cathode,all layers were solution-processed at or below 1401C.With a slowed-down annealing process,the FAPbI 3perovskite layer showed high crystallinity,large grain size and full surface coverage.A power conversion e?ciency of 13.56%was obtained with a high short circuit current density of 21.48mA cm à2.
Experimental section
Device fabrication
The NH 2CH Q NH 2PbI 3(FAPbI 3)based perovskite solar cell with the device structure of ITO/PEDOT:PSS/FAPbI 3/PCBM/BCP/Ag (Fig.1)was fabricated according to the following steps.
Firstly,
a
Jiangsu Key Laboratory for Carbon-Based Functional Materials &Devices,Institute of Functional Nano &Soft Materials (FUNSOM),Soochow University,Suzhou,Jiangsu 215123,China.E-mail:zkwang@https://www.doczj.com/doc/305935074.html,,lsliao@https://www.doczj.com/doc/305935074.html,;Tel:+8651265880927b
Department of Physics &Astronomy,University of Waterloo,
200University Avenue West,Waterloo,N2L 3G1,Ontario,Canada
?Electronic supplementary information (ESI)available:EQE spectra of the one-step annealed and slowed-down annealed FAPbI 3perovskite solar cells,when PCBM acts as an electron transporting layer.J –V curve of a representative modified FAPbI 3device tested under forward and reverse bias,respectively.J –V curve of a representative slowed-down annealed FAPbI 3based solar cell tested every 10min in air under continuous light illumination.Photovoltaic parameters of some repre-sentative modified FAPbI 3perovskite solar cells.See DOI:10.1039/c5cp02705e
Received 11th May 2015,Accepted 10th June 2015DOI:10.1039/c5cp02705e
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pre-cleaned ITO-coated glass substrates were treated by ultraviolet-ozone for 15min.The poly(3,4-ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS,Clevious AI 4083)layer was deposited by spin-coating at 4000rpm for 40s and annealed at 1401C for 15min in air.Then the substrates were transferred into a nitrogen filled glovebox.The FAPbI 3precursor solution (40wt%),prepared by dissolving NH 2CH Q NH 2I (Xi’an Polymer Light Technology Corp.)and PbI 2(Alfa Aesar)in N ,N -dimethyl-formamide (DMF)solvent with a molar ratio of 1:1,was spin coated on the PEDOT:PSS layer following the fast deposition crystallization procedure as previously reported.6Then the FAPbI 3precursor solution was annealed to form black FAPbI 3perovskite.After cooling down to room temperature,PC 60BM (20mg ml à1in chlorobenzene)was spin-coated on the perovskite layer at 2000rpm,followed by drop casting an interfacial layer solution of BCP (0.5mg ml à1in anhydrous ethanol)at 4000rpm without further annealing.Finally,devices were transferred into the thermal evaporation system (OMV-FS300)for silver cathode evaporation.The active area of the devices (7.25mm 2)was defined through a shadow mask.The MAPbI 3-based solar cell with the same device structure was also fabricated for comparison.Measurements and characterization
Current density–voltage (J –V )characteristics of perovskite solar cells were measured in air using a programmable Keithley 2400source meter under AM 1.5G solar irradiation at 100mW cm à2(Newport,Class AAA solar simulator,94023A-U).The light intensity
was calibrated by a certified Oriel Reference Cell (91150V)and verified with an NREL calibrated Hamamatsu S1787-04diode.The external quantum efficiency (EQE)was measured by a certified IPCE instrument (Zolix Instruments,Inc.,Solar Cell Scan 100).The scanning electron microscope (SEM)images were obtained from a field emission scanning electron microscope (FEI Quanta 200).The ultraviolet-visible spectroscopy (UV-vis)spectra were achieved on a Perkin Elmer model Lambda 750instrument.X-ray diffrac-tion (XRD)patterns were collected on an analytical (Empyrean)apparatus.The steady-state photoluminescence spectra and time-resolved photoluminescence were measured by utilizing Horiba Jobin-Yvon LabRAM HR800and a single photon counting spectro-meter,which was combined with the Fluorolog-3spectrofluoro-meter (Horiba-FM-2015),respectively.A 625nm laser source was used in the time resolved PL measurement.
Results and discussion
Fig.2(a)shows the X-ray di?raction patterns of ITO/PEDOT:PSS,MAPbI 3and FAPbI 3perovskites on an ITO/PEDOT:PSS surface.Highly oriented crystallinity was observed in the FAPbI 3perov-skite.Its main di?raction peaks shift toward lower degrees compared with the MAPbI 3system,due to the replacement of the smaller organic CH 3NH 3+(MA)cation with the larger NH 2CH Q NH 2+(FA)cation.The zoomed in X-ray di?raction patterns between 12and 16degrees (Fig.2(b))shows a much more obvious change.In the FAPbI 3system,peaks labeled with a #are assigned to
the
Fig.1(a)Device structure of the FAPbI 3-based perovskite solar cell.(b)Crystal structure of the FAPbI 3
perovskite.
Fig.2(a)X-ray di?raction patterns of ITO/PEDOT:PSS,MAPbI 3and FAPbI 3perovskites on the ITO/PEDOT:PSS surface.(b)The zoomed in X-ray di?raction patterns between 12and 16degrees for the modified MAPbI 3and FAPbI 3perovskites,respectively.
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ITO/PEDOT:PSS substrate,and other peaks are assigned to the labelled reflections from a tetragonal perovskite lattice with cell parameters a =b =8.99?and c =11.0?.These results are in good agreement with the previously reported black phase of FAPbI 3.38,39In addition,no PbI 2peaks can be observed.It indicates that FAPbI 3perovskite also has a pure crystalline phase.
The morphology of the perovskite layer is pretty important for the device performance.Two annealing methods were conducted to improve the morphology of the FAPbI 3layer,the one-step annealing process (1401C/20min)and the slowed-down anneal-ing process (1001C/20min,1201C/20min and 1401C/20min).The SEM images of the one-step annealed and slowed-down annealed FAPbI 3perovskites on the ITO/PEDOT:PSS surface were shown in Fig.3(a)and (b).Compact films with full surface coverage were obtained for both methods.Noticeably,the slowed-down annealed film shows a much larger grain size,which will obviously result in fewer grain boundaries.
We fabricated both one-step annealed and slowed-down annealed FAPbI 3perovskite solar cells employing PCBM as the electron transporting layer.The related current density–voltage (J –V )curves and photovoltaic parameters of the perovskite solar cells measured under AM 1.5G solar illumination at 100mW cm à2are shown in Fig.4(a)and Table 1.The EQE spectra were also tested,as shown in Fig.S1(ESI ?).Obviously,higher performance
and EQE value were achieved for the slowed-down annealed device.The enhancement was ascribed to less carrier recombi-nation and better charge extraction,probably as a result of there being fewer grain boundaries.
To further explain the related mechanisms,we measured the steady-state photoluminescence (PL)spectra of the one-step annealed and slowed-down annealed FAPbI 3perovskites on the ITO/PEDOT:PSS surface,respectively.As shown in Fig.4(b),the slowed-down annealed FAPbI 3perovskite showed enhanced PL intensity compared to the one-step annealed FAPbI 3perovskite.It implies that the non-radiative decay is significantly suppressed through our slowed-down annealing process.The
time-resolved
Fig.3SEM images of the (a)one-step annealed and (b)slowed-down annealed FAPbI 3perovskite films on the ITO/PEDOT:PSS surface,
respectively.
Fig.4(a)J –V curve of the one-step and slowed-down annealed FAPbI 3perovskite solar cells employing PCBM and ICBA as electron transporting layers,respectively.(b)The steady-state photoluminescence spectra and (c)time-resolved photoluminescence of one-step annealed and slowed-down annealed FAPbI 3perovskites on the ITO/PEDOT:PSS surface,respectively.
Table 1Photovoltaic parameters of the inverted planar structure FAPbI 3based perovskite solar cells with di?erent configurations a
Configurations J sc
(mA cm à2)V oc (V)Fill factor PCE (%)FAPbI 3(one-step annealing)/PCBM 14.730.860.698.71FAPbI 3(slowed-down annealing)/PCBM 19.440.890.7312.56FAPbI 3(slowed-down annealing)/ICBA
17.50
0.83
0.66
9.65
a
All the photovoltaic parameters are the average of a batch of twelve devices.
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PL (TRPL)was also measured,as shown in Fig.4(c).The slowed-down annealed FAPbI 3perovskite gave a lifetime of approximately 59ns,whereas the slowed-down annealing process increased the lifetime to 112ns.The PL and TRPL measurements showed that the slowed-down annealing process significantly suppressed the non-radiative recombination channels and increased the PL lifetime,which will obviously reduce the number of crystal defects,resulting in fewer carrier recombinations and better charge extraction.
We also fabricated devices employing indene-C 60bisadduct (ICBA)as an electron transporting layer (ETL).The fabrication conditions of ICBA are the same as with PCBM.The related current density–voltage (J –V )curves and photovoltaic para-meters are shown in Fig.4(a)and Table 1.We find that poor performance was achieved when ICBA acted as the ETL,prob-ably as a result of there being higher trap densities of states (tDOS)in these devices.40This result indicates that electron transporting materials can obviously influence the performance of the FAPbI 3based perovskite solar cell.We can investigate superior electron and hole transporting materials to further improve device performances.
The modified FAPbI 3-based device with the modified MAPbI 3-based device were further fabricated for comparison.The J –V curves of the FAPbI 3and MAPbI 3based solar cells measured under AM 1.5G solar illumination at 100mW cm à2are shown in Fig.5(a).Their related photovoltaic parameters are summarized in Table 2.Both devices showed comparable open circuit voltage (V oc )and fill factor (FF).The reference device (MAPbI 3)shows an average PCE of 10.59%,while the FAPbI 3-based device shows a higher performance,with an average PCE of 12.56%.An increased PCE was ascribed to the
obvious enhancement of the short circuit current density (from 16.66mA cm à2to 19.89mA cm à2on average).
Fig.5(b)shows the EQE spectra of the MAPbI 3and FAPbI 3based devices.We observed that the MAPbI 3-based device generates photocurrent up to 800nm,while the FAPbI 3-based device generates photocurrent up to 840nm.The FAPbI 3-based device showed higher EQE spectra compared to the MAPbI 3-based device on the whole,which will obviously result in higher short circuit current density.
The UV-vis absorption properties of both modified MAPbI 3and FAPbI 3perovskites on the ITO/PEDOT:PSS surface are shown in Fig.5(c).The zoom of the absorption for both perovskites between 700and 850nm is shown in the inset Fig.5(c).We can clearly see that the absorption cut-o?edges for both MAPbI 3and FAPbI 3perovskites are consistent with their EQE photocurrent generation cut-o?edges,respec-tively.The absorption cut-o?edge of the MAPbI 3system was set at 800nm,while the FAPbI 3system was set at https://www.doczj.com/doc/305935074.html,pared with the MAPbI 3system,the FAPbI 3perovskite film shows stronger absorption and a broadened absorption range,which will obviously enhance the short circuit current density.
Due the superior properties of slowed-down annealed FAPbI 3perovskite,such as high crystallinity,large grain size,compact film with full surface coverage,stronger absorption and broadened absorption range,an excellent PCE of 13.56%was obtained,with a short circuit current density (J sc )of 21.48mA cm à2,open circuit voltage (V oc )of 0.89and fill factor (FF)of 0.71,under 100mW cm à2AM 1.5illumination (Fig.6(a)).From the PCE histogram of 40devices (Fig.6(b)),we can see that our slowed-down annealed FAPbI 3perovskite solar cell also showed very good reproducibility.Here,we noticed that our slowed-down annealed FAPbI 3perovskite solar cell shows obvious hysteresis properties as well (Fig.S2,ESI ?).The origin of the anomalous hysteresis in perovskite solar cells is not clearly understood at present.40–42The photostability of our modified FAPbI 3based solar cells was also evaluated as shown in Fig.S3(ESI ?).When the device was tested under continuous illumination,the performance did not show an obvious
decrease.
Fig.5(a)Current density–voltage (J –V )curves of the perovskite solar cells measured under AM 1.5G solar illumination at 100mW cm à2.(b)The external quantum efficiency (EQE)spectra of the MAPbI 3and FAPbI 3based devices.(c)UV-vis absorption of MAPbI 3and FAPbI 3perovskites on the ITO/PEDOT:PSS surface,respectively.Inset figure shows the related zoom of the absorption between 700and 850nm.
Table 2Photovoltaic parameters of the inverted planar structure MAPbI 3and FAPbI 3based perovskite solar cells measured under AM 1.5G solar illumination at 100mW cm à2
Solar cells
J sc
(mA cm à2
)J sc (mA cm à2)V oc (V)
Fill
factor (%)PCE (%)PCE (%)Average Highest Average Highest
MAPbI 316.6618.22
0.890.7210.5911.71FAPbI 319.8921.48
0.890.71
12.56
13.56
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Conclusion
In summary,we have demonstrated an inverted planar structure perovskite solar cell with a power conversion e?ciency of 13.56%by using the FAPbI 3as a light harvester for the first time.Except for the silver cathode,all layers were solution-processed under or below 1401C.We investigated the e?ect of the annealing process on device performance.When the slowed-down annealing process was conducted,the FAPbI 3perovskite layer showed high crystallinity,large grain size and full surface coverage.Our slowed-down annealed FAPbI 3-based device shows superior performance to the modified MAPbI 3-based solar cell,especially for the short circuit current density.This work paves the way for low-temperature fabrication of e?cient inverted planer structure FAPbI 3perovskite solar cells.
Acknowledgements
We acknowledge financial support from the Natural Science Foundation of China (No.61307036and 61177016)and from the Natural Science Foundation of Jiangsu Province (No.BK20130288).This project is also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology,and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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