The Effects Of Titanium And Halide On The Performance Of Planar Perovskite Solar Cells

Solar cells can directly convert solar energy into electricity,which helps to alleviate the current energy crisis.Among them,perovskite solar cells?PSCs?have attracted extensive attention due to their high power conversion efficiency?PCE?and low manufacturing cost.The device structure of PSCs is divided into mesoporous and planar structure,especially the planar structure,which has received tremendous attention due to its simple preparation process.Typically,the structure of the planar PSCs consists of a conductive glass FTO/electron transport layer?ETL?/perovskite/hole transport material/gold electrode.ETL plays a vital role in transporting electrons and blocking holes,which has a significant influence on the efficiency of PSCs,and the common used ETL is TiO₂.Therefore,improving the photoelectric properties of TiO₂ ETL,and studying the effects of ETL as well as ETL/perovskite interface on the performance of PSCs are of great significance for improving cell efficiency.Moreover,perovskite materials are the light absorbing materials of PSCs,and the composition of the perovskite precursor also has an important influence on the film formation and photoelectric properties of perovskite.In view of these key issues above,the specific work of this paper are as follows:Firstly,we synthesized the precursor solution of TiO₂ ETL by using tetrabutyl titanate?TBOT?,ethanol and triethanolamine.When the volume ratio of ethanol,TBOT and triethanolamine was 30:3:1,the ETL?TT-TiO₂?prepared by spin coating has the optimal photoelectric properties.In addition,in order to investigate the effects of titanium source on the photoelectric properties of ETL and device performance,we compared it with ETL?TDB-TiO₂?prepared by the commonly used titanium diisopropoxide bis?acetylacetonate??TDB?,both of which use the optimal volume ratio.The experimental results demonstrated that the crystal phase of TiO₂ prepared by two different titanium sources have not changed,but the surface state,morphology,electron extraction ability and interface recombination resistance have changed significantly.Compared with the widely used titanium source of TDB,the surface of the TiO₂ film synthesized by TBOT is smooth and uniform,the particle size of TiO₂films is increased,and the electron extraction ability is enhanced.Therefore,the PSCs based on TT-TiO₂ ETL have excellent ETL/perovskite interface quality,increased interface recombination impedance,and reduced interface recombination,resulting in enhanced PCE.Finally,the device based on TT-TiO₂ ETL exhibits a higher PCE of17.4%.However,the devices based on TDB-TiO₂ ETL only obtain PCE of 14.5%.Secondly,the quality of the perovskite film,which is closely related to the composition of the precursor,determines the efficiency of PSCs.We used MAPbI₃-based PSCs as a template to investigate the influence of precursor composition and halide on the crystallization behavior and photovoltaic properties of perovskite films by adding 5 mol%excess cationic halides such as MAI,CsI,and lead metal halides such as PbI₂,PbBr₂ and PbCl₂ into the precursor solution.This can help to find the process of transforming from precursors into the perovskites crystal.It is demonstrated that the cationic halides such as MAI and CsI can accelerate the nucleation of perovskite,and the excess MA and Cs ions will aggregate around the grain boundary and Pb-I skeleton to form a layered structure similar to two-dimensional perovskite.The effect of lead metal halides on the optoelectronic properties of perovskite films is mainly affected by halogen atoms.PbCl₂ can hinder the nucleation of perovskites by forming intermediates,which increases the perovskite grains,but also increases the grain surface roughness.5 mol%excess PbI₂and PbBr₂ can passivate defects and improve the quality of perovskite film.The efficiency of PSCs with adding 5 mol%excess PbI₂ and PbBr₂ is significantly improved,achieving efficiency of 16.5%and 19.3%,respectively,while the devices based on stoichiometric precursor only achieved efficiency of 14.5%.