1-s2.0-S0021979715303143-main

Improved charge transfer and photoelectrochemical performance of CuI/Sb 2S 3/TiO 2heterostructure nanotubearraysFeng Yang a ,⇑,Jinfang Xi a ,Li-Yong Gan a ,Yushu Wang b ,Shuangwei Lu a ,Wenli Ma a ,Fanggong Cai a ,c ,Yong Zhang a ,Cuihua Cheng d ,Yong Zhao a ,d ,⇑aKey Laboratory of Advanced Technology of Materials (Ministry of Education of China),Superconductivity and New Energy R&D Center,Mail Stop 165#,Southwest Jiaotong University,Chengdu,Sichuan 610031,China bSchool of Materials Science and Engineering,Georgia Institute of Technology,Atlanta,GA 30318,USA cNational Engineering Laboratory for Superconducting Materials,Western Superconducting Technologies Co.Ltd,Xi’an,China dSchool of Materials Science and Engineering,University of New South Wales,Sydney,2052NSW,Australiaa r t i c l e i n f o Article history:Received 22August 2015Revised 2November 2015Accepted 4November 2015Available online 4November 2015Keywords:Sb 2S 3/TiO 2heterostructure NTAs CuIa b s t r a c tCharge transfer is important for the performance of a photoelectrochemical cell.Understanding photo-generated charge accumulation and separation is mandatory for the design and optimisation of photo-electrochemical cells.Unique stacked and embedded heterostructure of Sb 2S 3/TiO 2nanotube arrays (NTAs)was fabricated through anodic oxidation with thehydrothermal method.Surface photovoltage spectroscopy,phase spectra and photoluminescence measurements were performed to explore the mechanism by which theinorganic hole transport material CuI affects the charge transfer and photoelec-trochemical properties of Sb 2S 3/TiO 2heterostructure NTAs.The interfacial separation and transport of photoinduced charge carriers were also examined by applying current–voltage characteristics (J –V ),/10.1016/j.jcis.2015.11.0040021-9797/Ó2015Elsevier Inc.All rights reserved.⇑Corresponding authors at:Key Laboratory of Advanced Technology of Materials (Ministry of Education of China),Superconductivity and New Energy R&D Center,Mail Stop 165#,Southwest Jiaotong University,Chengdu,Sichuan 610031,China (Y.Zhao).E-mail addresses:yf@ (F.Yang),yzhao@ (Y.Zhao).Hole conductorCharge transfer Photoelectrochemical performance incident-photon-to-current conversion efficiency(IPCE)and Mott–Schottky techniques.Results show that CuI acts not only as a hole-conducting and electron-blocking material but also as a light-absorbing material in the ultraviolet range.Efficient charge transfer processes exist in CuI/Sb2S3/TiO2 heterostructure NTAs.The photoelectrochemical performance of CuI/Sb2S3/TiO2heterostructure NTAs is dramatically improved.Under AM1.5G illumination at100mW/cm2,the short-circuit current density and open-circuit voltage are3.51mA/cm2and0.87V,respectively.The photoelectric conversion effi-ciency of CuI/Sb2S3/TiO2heterostructure NTAs(0.95%)is36%higher than that of Sb2S3/TiO2heterostruc-ture NTAs(0.66%).Ó2015Elsevier Inc.All rights reserved.1.IntroductionPhotoelectrochemical cell is a promising technology for thefuture of renewable energy.The development of efficient and eco-nomical solar cells has recently become an urgent subject.Inor-ganic semiconductors and quantum dot-sensitised solar cellshave been investigated as potential alternative devices to conven-tional dye-sensitised solar cells because of their numerous uniquecharacteristics,such as large extinction coefficient,large intrinsicdipole moment and size dependent band gap,multiple excitongeneration,high potential for hot electron injection,good stabilityand solution processability[1–5].Previous studies have exploredvarious semiconductors,such as CdS[5],PbS[6],In2S3[7],SnS[8]and Sb2S3[1,2,9].Crystalline Sb2S3is a promising candidatebecause of its high absorption(1.8Â105cmÀ1)and proper opticalband gap(1.7eV).A significant energy conversion efficiency(6.3%)under1sun irradiation can be attained in Sb2S3-sensitised solarcells[9].However,Sb2S3-based solar cells suffer from low elec-tron–hole separation efficiency because sulphide radical speciesinduce the substantial recombination of photogenerated charges[2].Conducting polymers,such as P3HT[3],PEDOT:PSS[4],PCPDTBT–PCBM[9]and spiro-MeOTAD[10],and inorganic materi-als,such as CuSCN[2],graphene oxide[11]and CuI[12],have beenemployed as hole transport materials without the drawbackscaused by liquid electrolytes.In particular,inorganic copper-based p-type semiconductors have shown potential advantagesbecause of their wide band gap and high hole mobility.Further-more,these semiconductors are cheaper and easier to synthesisein solution processing than other organic polymers[12,13].CuIexists in different polymorphs(a,b and c),whereas c-phase CuI is a p-type semiconductor with a band gap of 3.1eV[13]. c-phase CuI shows versatile applications for hole conductors in semiconductor-sensitised solar cells,light-emitting diodes,field-effect transistors and transparent excitonic devices[12–14].Givenits high hole conductivity and the compatibility of the solutiondeposition method with the organo-lead halide perovskite absor-ber,CuI has been employed as a hole conductor in perovskite solarcells;the use of CuI has resulted in a high photovoltaic conversionefficiency of6.0%with excellent photocurrent stability[12].There-fore,the formation of CuI/Sb2S3heterostructure can hinder elec-tron–hole recombination and achieve high photoelectrochemicalperformance.Charge transfer is important for a photoelectrochemical cell.Understanding how and where a charge is generated and trans-ferred is required to optimise the performance of photoelectro-chemical cells.Impedance spectroscopy is widely used to analysethe mechanism underlying carrier accumulation and transfer.Thismethod has been used to study hole transport and the recombina-tion of nanostructured TiO2/Sb2S3/CuSCN solar cells[2].Importantparameters can be obtained from impedance spectroscopy;theseparameters include carrier conductivity,lifetime,chemical capaci-tance,recombination resistance,diffusion coefficient and diffusion length[2,10,15].Surface photovoltage(SPV)spectroscopy based on the lock-in amplifier technique is a well-established non-destructive tool for investigating charge transfer[5,8,16].The metal–insulator–semiconductor structure has been employed for SPV spectroscopy measurement[16,17].SPV represents the varia-tion in the photovoltage induced by periodic light excitation[18]. The obtained SPV spectra and phase spectra(PS)can provide useful information about the behaviour of photoinduced charges,type and band gap of semiconductor and surface band bending [5,8,16–18].SPV has been systematically employed to study the photoinduced charge behaviour in SnS/TiO2heterostructure NTAs [8],ZnO/Cu2O heterostructurefilms[16],MgFe2O4/a-Fe2O3 heterostructure hollow nanospheres[17]and Fe2O3/TiO2heteroge-neous photocatalysts under different wavelength irradiation[18]. SPV has also been used to observe the charge separation in poly-mer P3HT[19]and organo-lead perovskite solar cells[20].In this study,Sb2S3was successfully deposited on TiO2NTAs through hydrothermal pared with common chemi-cal bath deposition[21,22]and successive ionic layer adsorption and reaction[23],hydrothermal synthesis can crystallise Sb2S3in one step without annealing in Ar or N2atmosphere.The typical hole transport material CuI was introduced to decorate Sb2S3/ TiO2heterostructure NTAs.SPV spectroscopy,PS and photolumi-nescence(PL)measurements were applied to explore the effect of CuI on the photogenerated charge transfer behaviour in Sb2S3/ TiO2heterostructure NTAs and identify the potential applications of this material in photoelectrochemical cells.Our results show that efficient charge transfer exists in CuI/Sb2S3/TiO2heterostruc-ture NTAs and that improved photoelectrochemical performances are obtained.2.Experimental2.1.Preparation of TiO2nanotube arraysHighly ordered1D TiO2NTAs were prepared through anodic oxidation at a constant voltage of60V for5h at room temperature. Ti foil(2cmÂ3cmÂ0.25mm,99.6%purity)and platinum foil (3cmÂ4cm)were used as the working and counter electrodes, respectively[8].Ethylene glycols with0.25wt%NH4F(80mL)were used as the electrolyte.After anodic oxidation,TiO2NTAs were washed with deionized water and then dried in air.Thereafter, the TiO2NTAs were cleaned through sonication in deionized water, dried naturally and then annealed at450°C in air for3.5h at a heating and cooling speed of3°C/min.2.2.Deposition of Sb2S3Sb2S3was deposited on TiO2NTAs through the hydrothermal method.SbCl3and Na2SÁ9H2O were used as antimony and sulphur precursors,respectively.SbCl3absolute alcohol solution(0.8M) and aqueous Na2SÁ9H2O solution(1.6M)were magnetically stir-red.The above solutions were immediately mixed and then2 F.Yang et al./Journal of Colloid and Interface Science464(2016)1–9transferred to Teflon-lined stainless steelity,60%filling)wherein the as-prepared TiO2 placed and sealed.These autoclaves were8h and then cooled to ambient temperature. Sb2S3/TiO2heterostructure NTAs were washed sufficient absolute alcohol followed bySb2S3/TiO2heterostructure NTAs were dried for further analysis.2.3.Deposition of CuICuI was deposited on the Sb2S3/TiO2 through the dipping–coating method.In brief, solved in20mL of acetonitrile with20min at room temperature.The Sb2S3/TiO2 were preheated to100°C in advance,for several seconds and then evenlytions were repeated several times to heterostructure NTAs.2.4.Characterisation and measurementsThe phases of the samples were characterised through X-ray powder diffraction(XRD)with Cu K a radiation(k=1.54178Å) under40V and40A.The morphologies were analysed under a field-emission scanning electron microscope(FESEM,JSM-7001F, JEOL,Japan)with an energy-dispersive X-ray(EDX).SPV spec-troscopy was conducted using a self-assembling measurement sys-tem that comprises a monochromatic light source provided by a Xenon lamp,a sample cell,a computer and a lock-in amplifier (SR830-DSP)with a light chopper(SR540)that chopped the monochromatic light frequency to23Hz.The scanning range of the monochromatic light was800–300nm,and the intensity of each wavelength wasfixed by relying on the spectral energy distri-bution of Xenon lamp.To characterise photovoltaic performance, the photocurrent–voltage curves were recorded using an electro-chemical workstation(LK2006A,Tianjin)and were measured with a traditional three-electrode system comprising a saturated calo-mel reference electrode(SCE)and a Pt foil counter electrode.A mixture of two solvents(methanol and water in a volume ratio of7:3)containing0.5M Na2S,2M S and0.2M KCl was used.All potentials were measured with respect to SCE.The electrode was illuminated under Air Mass1.5Global(AM1.5G)with a Xenon lamp(CHF-XM-500W)to calibrate the irradiation intensity to 100mW/cm2.The effective area of the electrode for illumination was0.5cm2.The incident-photon-to-current conversion efficiency (IPCE)was measured using a light source(300W Xenon lamp, 66902;Newport)aligned with a monochromator(Cornerstone 260;Newport)and a multimeter(Keithley2635A).Mott–Schottky plots were investigated in a polyiodide electrolyte(0.5M(KCl +KI)+0.01M I2)at1kHz.All measurements were conducted at room temperature.3.Results and discussion3.1.CharacterisationThe phases and structures of pure TiO2NTA,Sb2S3/TiO2and CuI/ Sb2S3/TiO2heterostructure NTAs were determined through XRD (Fig.1).Fig.1(A)shows an experimental XRD profile of pure TiO2 NTAfilm before Sb2S3deposition.Results show that TiO2NTAs are well crystallized and that the main characteristic peaks match well with the anatase phase(JCPDSfile:89-4921).The other peaks that cannot be assigned to TiO2are resulted from the Ti substrate (JCPDSfile:44-1294)[8].Compared with the XRD patterns of pure TiO2NTAs,those of Sb2S3/TiO2show obvious Sb2S3peaks.All peaks can be indexed to a well-crystallized orthorhombic phase of Sb2S3 (JCPDSfile:42-1393).For CuI/Sb2S3/TiO2heterostructure NTAs,the main characteristic peaks of CuI(111),(220)and(222)are assigned to c-phase CuI(JCPDSfile:06-0246).Fig.2shows the FESEM images of pure TiO2NTAs,Sb2S3/TiO2 and CuI/Sb2S3/TiO2heterostructure NTAs.An average inner diame-ter of111nm,a wall thickness of32nm and a nanotube length of approximately10l m can be observed in pure TiO2NTAs.The insert of Fig.2(B)displays that the outer surface of TiO2NTAs is smooth.Sb2S3nanoparticles are either stacked on the top of TiO2 NTAs or homogeneously embedded in the gaps among TiO2NTAs with a size of15–22nm in Sb2S3/TiO2heterostructure NTAs. Well-stacked and embedded structures in Sb2S3/TiO2heterostruc-ture NTAs can provide large contact areas,which increase carrier channels and accelerate charge transmission.Furthermore,the well-ordered pore and nanotube structure of TiO2NTAs are well maintained in the hydrothermal process.The highly oriented TiO2NTAs exhibit excellent electron percolation pathways for the vertical transfer of photogenerated electrons across the length of the nanotubes[24].Comparison of Fig.2(E),(F)and EDX spectra (Fig.2(F),inset)reveals that CuI nanoparticles are uniformly scat-tered on the Sb2S3/TiO2heterostructure NTAs.In Fig.2(F)a top view of CuI/Sb2S3/TiO2NTAs is shown and a CuI deposit is clearly observed on the top.Quantitative analysis of EDX spectra gives that the quality percentage of CuI is1.06%with respect to that of CuI/Sb2S3/TiO2heterostructure NTAs.3.2.Optical,surface photovoltage and PL propertiesUV–visible absorption spectra were obtained from pure TiO2NTAs,Sb2S3/TiO2and CuI/Sb2S3/TiO2heterostructure NTAs (Fig.3(A)).TiO2NTAs have a featured absorption band at 300–380nm because of its large band gap pared with pure TiO2NTAs,both Sb2S3/TiO2and CuI/Sb2S3/TiO2heterostructure NTAs exhibit obvious redshifts and high absorption in the visible region because Sb2S3has a high absorption coefficient and a narrow band gap of1.7eV.The absorbance of CuI/Sb2S3/TiO2heterostruc-ture NTAs is slightly higher than that of Sb2S3/TiO2heterostructure NTAs at300–800nm,especially at300–400nm.Fig.3(B)schematically shows the photovoltaic cell configura-tion in the SPV spectroscopy measurement.The construction of the photovoltaic cell is a sandwich-like structure of conductive glass(ITO)/sample/Ti foil.That is,a piece of transparent conductive1.XRD patterns of(A)pure TiO2NTAs,(B)Sb2S3/TiO2and(C)CuI/Sb2S3/TiO heterostructure NTAs.F.ITO glass was used as a top electrode,which was connected to the pole of the light incident side.The Ti substrate of the sample acted as the bottom electrode grounding to the earth.The illuminated area is approximately 0.5cm 2.Fig.3(C)and (D)respectively dis-play the SPV spectra and PS of pure TiO 2NTAs,Sb 2S 3/TiO 2and CuI/Sb 2S 3/TiO 2heterostructure NTAs.The peaks at 475nm in the SPV spectra of CuI/Sb 2S 3/TiO 2and Sb 2S 3/TiO 2heterostructure NTAs are attributed to the inhomogeneous spectral distribution of the Xenon lamp [17].A weak SPV response is observed in pure TiO 2NTAs at 380–300nm;this response corresponds to a band gap of 3.2eV.Once light illuminates,TiO 2NTAs absorb UV light (6380nm)to generate electron–hole pairs.Under the effect of a built-in electric field and concentration gradient of excess carriers,excited electrons transfer to the bulk while holes localise on the surface [25];this finding is consistent with the PS result (À60°)shown in Fig.3(D).The SPV response region is broadened to 730nm in Sb 2S 3/TiO 2heterostructure NTAs,and the SPV intensity is remarkably stronger than that of pure TiO 2NTAs.This result can be ascribed to two aspects.On the one hand,Sb 2S 3has a high absorption coefficient and a proper optical band gap of 1.7eV,which can increase light absorption throughout the visible range.On the other hand,the heterostructure forms between the Sb 2S 3and TiO 2interface,which can enhance the separation efficiency of photogenerated electron–hole pairs in Sb 2S ing first-principle simulations,Patrick and Giustino investigated the Sb 2S 3/TiO 2interface at atomic scale and demonstrated an interface between Sb 2S 3and TiO 2,which is free of structural defects and recombination centres.A type-II heterojunction in a Sb 2S 3/TiO 2system has been reported [26].Thus,upon illumination,excited electrons in the conduction band of Sb 2S 3absorb visible light to have sufficient driving force to be injected into the conduction band of TiO 2because the conducting band minima of Sb 2S 3is 0.5eV higher than that of TiO 2.By contrast,the photogenerated holes accumulate on the surface of Sb 2S 3and are collected by the top ITO electrode;this finding is consistent with the PS value near 70°of Sb 2S 3/TiO 2heterostructure NTAs.Moreover,the almost con-stant PS value of the Sb 2S 3/TiO 2heterostructure NTAs implies that the charge transfer direction does not change throughout the whole SPV response region.However,despite the outstanding optical properties of Sb 2S 3,Sb 2S 3-sensitised solar cells yield a low power conversion efficiency because sulphide radical species in Sb 2S 3induce thesubstantial(B)cross-section images of pure TiO 2NTAs;(C)and (E)top view and (D)cross-section images of Sb 2S 3/TiO 2heterostructure 3/TiO 2heterostructure NTAs.recombination of photogenerated charges [4].In Sb 2S 3-sensitised solar cells,the photoinduced holes transferring from the light absorber to the p -type hole conductor plays an important role in charge separation.Sb 2S 3is an n -type semiconductor and conducts electrons [27].No direct channels for photoinduced holes from Sb 2S 3to the top ITO electrode exist in the Sb 2S 3/TiO 2heterostruc-ture NTAs;thus,the recombination of photoinduced charges in Sb 2S 3is unavoidable and yields a low separation efficiency.Upon introducing the typical hole conductor CuI in Sb 2S 3/TiO 2heterostructure NTAs,the SPV intensity becomes two to four times of the Sb 2S 3/TiO 2heterostructure NTAs.The PS value of CuI/Sb 2S 3/TiO 2heterostructure NTAs lies approximately between À15°and 0°at 300–730nm.This result indicates that holes accumulate on the CuI surface and are collected by the top ITO electrode.In addition,the band gap of CuI is 3.1eV,which corresponds to a light absorption of 6400nm (Fig.4(A)).The manner by which this light absorption affects the SPV intensity of CuI/Sb 2S 3/TiO 2heterostructure NTAs should be identified.However,Fig.4(B)dis-plays that the SPV intensity of pure CuI is very weak (i.e.<0.3l V).As shown in the insert of Fig.4(B),the disordered PS of pure CuI can further prove the negligibly weak SPV response.PL is the emission of light originating from the recombination of photogenerated electron–hole pairs.PL emission spectra are widely used to investigate the efficiency of charge carrier trapping,immigration and transfer [7].High PL intensity corresponds to low separation efficiency [14].To investigate the PL behaviour of pure CuI,CuI/TiO 2NTAs,CuI/Sb 2S 3and CuI/Sb 2S 3/TiO 2heterostructure NTAs,samples were irradiated by 370nm excitation light.The PL spectra were then recorded (Fig.4(C)).The PL intensity of pure CuI is the highest but decreases when CuI is coupled with TiO 2,Sb 2S 3and Sb 2S 3/TiO 2heterostructure NTAs.After UV light excita-tion,the hot electrons in CuI can be relaxed through two completive processes,namely,radiative and non-radiative relax-ation processes [14].Hence,in pure CuI,low SPV intensity results from the strongest PL intensity because of radiative relaxation.In situ-generated iodine can be absorbed by the precipitate,thereby creating a surface trapping centre $0.2eV above the valence band edge of CuI [28].The emissions at $420nm in pure CuI,CuI/TiO 2,CuI/Sb 2S 3and CuI/Sb 2S 3/TiO 2heterostructure NTAs are ascribed to the recombination of photo-excited free excitons and electrons (in conduction band)with trapped holes in the trapping sites.The decreased PL intensity when CuI is coupled with TiO 2,Sb 2S 3and Sb 2S 3/TiO 2heterostructure NTAs indicates a decreased recom-bination of excitons generated in CuI and that effective charge transfer occurs between CuI and TiO 2,Sb 2S 3and Sb 2S 3/TiO 2.Although CuI/TiO 2and CuI/Sb 2S 3/TiO 2heterostructure NTAs show different quenching efficiencies,the quenching efficiency of CuI/TiO 2NTAs is 70%because of the larger conductor band offset between CuI and TiO 2,whereas PL is almost quenched with an effi-ciency of 90%in CuI/Sb 2S 3/TiO 2heterostructure NTAs.In CuI/Sb 2S 3/TiO 2heterostructure NTAs,an interfacial electric field with a direc-tion from n -type Sb 2S 3to p -type CuI is formed [7,8],Sb 2S 3acts as a bridge between CuI and TiO 2,and electron transport material extracts electrons from CuI.This phenomenon enhances the sepa-ration of carriers generated in CuI and promotes transfer via a non-radiative process.On the basis of the above analysis,the high PL quenching degree of CuI indicates that strong charge transport exists in CuI/Sb 2S 3/TiO 2heterostructure NTAs.As shown in Fig.3(D),the PS value of CuI/Sb 2S 3/TiO 2heterostructure NTAs is approximately À15°at 300–400nm and changes to 0°at 400–730nm.This result indicates the new charge transfer behaviour of photogenerated carriers at 300–400nm.If only one charge transfer channel (between Sb 2S 3and TiO 2)exists in CuI/Sb 2S 3/TiO 2heterostructure NTAs,the PS value will beaUV–visible absorption spectra of pure CuI,CuI/TiO 2,CuI/Sb 2S 3and CuI/Sb 2S 3/TiO 2heterostructure NTAs;(B)schematic of SPV spectroscopy measurement;of pure TiO 2NTAs and Sb 2S 3/TiO 2and CuI/Sb 2S 3/TiO 2heterostructure NTAs.constant similar to that of Sb2S3/TiO2heterostructure NTAs[24]. The light energy of400nm is approximately3.1eV,which is lower than the band gap of TiO2(3.2eV),excluding that the change is induced by TiO2.However,the slight PS change is consistent with the band gap of CuI(3.1eV).Therefore,this new charge transfer behaviour is induced by CuI.Charge carriers are generated because of the band-to-band transition in CuI and separated by the CuI/ Sb2S3interface.Finally,photoinduced electrons of CuI transfer into TiO2through Sb2S3,and holes can be extracted and transported by CuI.The absorption coefficient of CuI(7Â106cmÀ1)[29]is an order of magnitude larger than that of Sb2S3(1.8Â105cmÀ1)at 400nm;thus,the absorption of CuI is stronger than that of Sb2S3 at6400nm,and Sb2S3plays the major role in light absorption at 400–730nm.Therefore,the absorption of CuI contributes to the enhanced SPV intensity of CuI/Sb2S3/TiO2heterostructure NTAs in the UV region.The PS spectra of CuI/Sb2S3/TiO2heterostructure NTAs can also prove that charge transfer exists between CuI, Sb2S3and TiO2NTAs.3.3.Photoelectrochemical propertiesFig.5(A)presents the J–V characteristics for pure TiO2NTAs, Sb2S3/TiO2and CuI/Sb2S3/TiO2heterostructure NTAs.The corre-sponding parameters are summarised in Table1.The measure-ments were conducted under AM 1.5G illumination(100mW/ cm2).The pure TiO2NTAs electrode exhibits a J sc of0.35mA/cm2 and a negligible overall g of0.07%.The J sc of the CuI/Sb2S3/TiO2 heterostructure NTAs electrode(3.51mA/cm2)is higher than that of the Sb2S3/TiO2heterostructure NTAs electrode(1.92mA/cm2). Accordingly,the g of the CuI/Sb2S3/TiO2heterostructure NTAs electrode(0.95%)is36%higher than that of the Sb2S3/TiO2heterostructure NTAs(0.66%).The J–V characteristics are closely related to the photon collection and separation efficiency of photo-generated carriers[8].The trends in the J sc of CuI/Sb2S3/TiO2 heterostructure NTAs are similar to those in their SPV intensity. The FF of the CuI/Sb2S3/TiO2heterostructure NTAs(0.33%)is slightly lower than that of Sb2S3/TiO2heterostructure NTAs (0.43%).J sc is proportional to IPCE,where IPCE is the product of light-harvesting efficiency(g lh),electron injection efficiency(g e-inj),hole injection efficiency(g h-inj)and charge collection efficiency(g cc) [10].g lh is dependent on the absorption of the sensitiser,and the absorbance of CuI/Sb2S3/TiO2heterostructure NTAs is slightly higher than that of Sb2S3/TiO2heterostructure NTAs.Thus,g lh is relevant to the change in J sc.Therefore,CuI/Sb2S3/TiO2heterostruc-ture NTAs is expected to hold better IPCE value due to the slightly stronger absorption than that of Sb2S3/TiO2heterostructure NTAs. As shown in Fig.5(B),IPCE results of CuI/Sb2S3/TiO2and Sb2S3/ TiO2heterostructure NTAs agreed well with their aforementioned J sc values.The slightly higher IPCE value of CuI/Sb2S3/TiO2 heterostructure NTAs at longer wavelengths(400–800nm)may be attributed to the light harvesting ability induced by light scat-tering effect of CuI nanoparticles.At shorter wavelengths(300–400nm),the enhancement of IPCE value of CuI/Sb2S3/TiO2 heterostructure NTAs highlighted by the blue1in Fig.5(B),is signif-icantly greater than that at longer wavelengths.The highest IPCE value(21.35%)of CuI/Sb2S3/TiO2heterostructure NTAs is found at 392nm.This phenomenon is caused by the fact that the generated charge carriers in CuI by light(6400nm)can be effectivelyhar-UV–visible absorption,(B)SPV spectra and PS(insert)for pure CuI;(C)PL spectra of pure CuI,CuI/TiO2,CuI/Sb2S3and CuI/Sb2S3/TiO2heterostructure1For interpretation of colour in Fig.5,the reader is referred to the web version ofthis article.vested by TiO 2NTAs through the Sb 2S 3intermediate.g e-inj is similar regardless of the CuI/Sb 2S 3/TiO 2and Sb 2S 3/TiO 2heterostructure NTAs because the interfacial property between the Sb 2S 3and the TiO 2surface is identical.By contrast,g h-inj is directly affected by the hole conductor of CuI.J sc is significantly increased when CuI is introduced;this increase is related to the improved hole injection and light absorption of CuI.To identify the main role of CuI,an optical filtered (k >400nm,100mW/cm 2)light was set to measure the J –V characteristic of CuI/Sb 2S 3/TiO 2heterostructure NTAs (Fig.5).Compared with the J sc of CuI/Sb 2S 3/TiO 2heterostructure NTAs,the Jgradually decreases under filtered light.Obviously,the g identical in CuI/Sb 2S 3/TiO 2heterostructure NTAs irrelevant to the change in J sc .By contrast,the g lh to the increased J sc .Thus,the non-zero decrease in J light further confirms the above analysis of CuI material in the UV range (6400nm),which is in the IPCE results.However,the J sc of CuI/Sb 2S 3/TiO 2NTAs under filtered light is still larger than heterostructure NTAs.This result suggests that CuI is to act as a hole conductor that collects and generated holes rather than absorbs UV light.Therefore,the improved overall power CuI/Sb 2S 3/TiO 2heterostructure NTAs is mainly hole conductor of CuI,which provides channels and transport.The slight deterioration of the fill attributed to the generation of the surface [28].Although the g of CuI/Sb 2S 3/TiO 2our work is lower than that of P3HT/Sb 2S 3/TiO 2ical cell (4.2%)in cobalt electrolyte [30],this g is Sb 2S 3-sensitised photoelectrochemical cells (0.08%)polysulphide electrolyte [31].To further understand the mechanism of the ical performance enhancement of CuI/Sb 2S 3/TiO 2NTAs,Mott–Schottky (MS)measurements were using the impedance technique.On the basis of rier between the electrolyte and semiconductor plots were obtained at a fixed frequency of 1the flat-band potential from the V -axis intercept and carrier den-sity.According to the Mott–Schottky equation [32,33],1C 2¼2N d e e 0e E ÀE FB ÀkT e ð1Þwhere C is the space charge capacitance in the semiconductor,N d is the electron carrier density,e is the elemental charge,e 0is the per-mittivity of a vacuum,e is the relative permittivity of the semicon-ductor,E is the applied potential,E FB is the flat band potential,T is the temperature,and k is the Boltzmann constant.Fig.6displays the Mott–Schottky plots of 1/C 2as a function of the applied poten-tial.Positive slopes (i.e.,lines)are observed,suggesting n -type behaviours,consistent with the PS results.In CuI/Sb 2S 3/TiO 2heterostructure NTAs,the quality percentage of CuI is 1.06%,which is too low to change the conductivity type.Furthermore,the plots are extrapolated to 1/C 2=0to estimate the values of E FB (Eq.(1)),giving À1.26,À035and À0.03V for TiO 2NTAs,Sb 2S 3/TiO 2heterostructure NTAs and CuI/Sb 2S 3/TiO 2heterostructure NTAs,respectively.Obviously,E FB in CuI/Sb 2S 3/TiO 2heterostructure NTAs is higher,demonstrating a decrease of band edge bending.The flat band shift has a major effect on the increase of the photocurrent for the CuI/Sb 2S 3/TiO 2heterostructure NTAs photoelectrode.This characteristics of pure TiO 2NTAs,Sb 2S 3/TiO 2and CuI/Sb 2S 3/TiO 2heterostructure NTAs under AM 1.5G illumination (100mW/cm /TiO 2heterostructure NTAs.Table 1Photovoltaic performances and parameters of pure TiO 2NTAs,Sb 2S 3/TiO 2and CuI/Sb 2S 3/TiO 2heterostructure NTAs.Photoelectrode J sc (mA/cm 2)V oc (V)FF g (%)Pure TiO 20.350.690.280.07Sb 2S 3/TiO 21.920.80.430.66CuI/Sb 2S 3/TiO 23.510.870.310.95J sc ,V oc ,FF and g are the short-circuit current density,open-circuit voltage,fill factorand power conversion efficiency,respectively.Mott–Schottky plots of pure TiO 2NTAs and Sb 2S 3/TiO 2and CuI/Sb 2S heterostructure NTAs.。