Research On Bottom Transport Layer Of Perovskite Solar Cells

Solar cells based on organic-inorganic lead halide perovskite have attracted tremendous attention in the photovoltaic communities due to its unique optical and electronic properties.The power conversion efficiency?PCE? of laboratory scale devices has been dramatically improved with an incredible rate.In fact,the rapid increase of the devices' photoelectric conversion efficiency is mainly due to the continuous improvement of perovskite films as well as optimization of each functional layer,especially the performance of the bottom transport layer?hole or electron transport layer?of the device.In this paper,we investigated two kinds?n-i-p and p-i-n? of planar heterojunction perovskite solar cells as the model to study the photoelectric properties of the bottom transport layer and the effects of the bottom transport layer on perovskite layer.The effects include the improvement of interfacial compatibility,good energy level matching,the decrease in carrier recombination and the increase in electron mobility,which is to improve photovoltaic performance of the perovskite solar cells.The research results are summarized as follows:Firstly,the inorganic lead salts?PbI₂,PbBr₂ and PbCl₂? and the organic methylammonium compounds?MAI,MABr and MACl? were used to prepare organolead trihalide perovskite absorbers,CH₃NH₃PbI_3-aBr_a,CH₃NH₃PbI_3-aCl_a and CH₃NH₃PbBr_3-aCl_a perovskite films by one-step anti-solvent spin-coating method.The variations of the crystal transformation,morphological framework,thermal decomposition and optical performance of the hybrid perovskites were investigated.The result provides visual evidence that the pure MAPbI₃ film was obtained with the purest phase,the optimal surface morphology,the best covering and the greatest grain size.Therefore,the photovoltaic performance of the MAPbI₃-based solar cell is the optimal,while the other devices exhibit the poor photovoltaic performance.Secondly,the moderately reduced graphene oxide?rGO? was prepared by a simple thermal reduction method,which not only maintained the hydrophilic property,but also had a certain conductivity.The rGO-PEDOT:PSS?rGO-PEDOT? hole transport layer?HTL? was prepared by incorporating rGO aqueous solution into the conventional PEDOT:PSS hole transport layer.It is found that the incorporation of rGO into PEDOT:PSS improves the energy matching and increases the electron extraction and carrier transfer.As a result,perovskite solar cells?PSCs? with rGO-PEDOT as HTL exhibit the improved power conversion efficiency than that of PSCs with PEDOT:PSS as HTL.The PSCs with rGO-PEDOT HTL exhibit an open-circuit voltage of 0.95V,a short-circuit current of 17.1mA·cm^-2,and a PCE of 10.6%,which was improved by 22%.Afterwards,in order to further improve the photovoltaic performance of perovskite solar cells,sulfated graphene oxide?sGO? as a HTL replace the conventionally used GO and PEDOT:PSS in PSCs,but pristine sGO as simple HTL cannot improve photovoltaic performance of PSCs with a maximum efficiency of 9.9%.The PSCs fabricated with sGO-PEDOT HTL show a dramatically enhanced PCE of 13.9% with a lower J-V hysteresis and superior reproducibility.This is because the work function of sGO-PEDOT HTL with sGO doping is more compatible with the energy band of the perovskites,and the carrier extraction efficiency and the transport ability are improved,thereby enhancing the photovoltaic performance of the PSCs.Subsequently,to carry out a contrastive study on the influence of the bottom transport layer on planar heterojunction PSCs,a composite electron transport layer?ETL? was designed.TiO₂/SnO₂ nanocomposite as an ETL in a formal planar heterojunction PSCs was prepared by partial incorporation of SnO₂ nanoparticles in a thin compact TiO₂layer.Compared to pristine TiO₂ ETL,TiO₂/SnO₂ ETL has high electrical conductivity and suitable band-level alignment at the ETL/perovskite interface.Using the optimized TiO₂/SnO₂ ETL,CH₃NH₃PbI₃-based PSCs exhibit an average PCE of 15.9±0.36% and the champion efficiency of 16.8% with less current density-voltage?J-V? hysteresis and high repeatability,which is significantly superior to that of TiO₂-based PSCs?the average PCE is 13.7±0.35%?.This work highlights that the successful use of TiO₂/SnO₂nanocomposite ETL can offer an efficient method for further improvement of the performance for planar PSCs.Finally,SrSnO₃ and Y-SrSnO₃ nanoparticles were prepared by a simple low temperature solution method,and then a series of SrSnO₃ and Y-SrSnO₃ nanoparticles can be prepared by different heat treatments,which can be an effective alternative ETL for PSCs by a low-temperature solution-deposited process.Compared with the SrSnO₃ETL-based on PSCs?the average PCE? is 14.9±0.91%,the PSCs with Y-SrSnO₃ ETL has a higher average PCE of 17.8±0.50%,and its maximum PCE is 19.0% with the significant reduction of J-V hysteresis.In addition,these devices also exhibit high long-term stability.More importantly,the introduction of yttrium dopant results in a significant improvement in the optoelectronic properties of Y-SrSnO₃,exhibiting higher electron conductivity and faster electron transfer,as well as better band alignment at ETL/perovskite interface compare to undoped SrSnO₃,which is also supported by theory calculation.In summary,this research is expected to make predictions on the potential new solar-cell materials and leads to a better understanding of perovskite solar cells.Moreover,this work will increase the focus on low-cost,high-efficiency,industrializable and eco-friendly perovskite solar cells.