Optimization And Interface Control Of CsPbI₂Br Perovskite Solar Cells

Perovskite,as a new generation of photovoltaic materials for solar cells,exhibits the great research potential due to its excellent semiconductor properties such as high absorption coefficient,low exciton binding energy and long carrier diffusion length.Among many types of perovskite solar cells(PSCs),the organic-inorganic hybrid perovskite solar cells have reached the power conversion efficiency of 25.5%,which greatly encourages researchers to further explore PSCs.However,due to the volatility and thermal instability of organic cations,the conventional organic-inorganic hybrid PSCs can not meet the commercial requirement.Therefore,in order to improve the environmental stability of perovskite materials,the substitution of volatile organic cations by inorganic Cs⁺not only could maintain good stability of all-inorganic perovskite materials at high temperature but also possesses the PCE exeeded 19%with the support of cutting-edge scientific research and technology.This is an indication that all-inorganic PSCs have the future commercial application value.In recent years,Cs Pb I₂Br PSCs became a promising all-inorganic PSCs due to its well-balanced ability between the phase stability and light absorption ability.In this work,based on carbon-based Cs Pb I₂Br PSCs,the structure mismatch,charge loss caused by interface defects and poor crystallization process are explored and discussed as following.Aiming at the exploration of the device structure,Cs Pb I₂Br PSCs with the structure of FTO/c-Ti O₂/m-Ti O₂/Cs Pb I₂Br/C are established.And then the faster electron transport are obtained by controlling the concentration of m-Ti O₂slurry and the optimal thickness of the mesoporous layer adjusted by the spin speed.Accordingly,the PCE of Cs Pb I₂Br PSCs are to be 10.4%.Based on the above experimental results,the more uniform and dense perovskite film are yielded by using the anti-solvent method during the preparation of the perovskite layer.Due to the interface defects between functional layers in Cs Pb I₂Br PSCs,a bi-functional organic small molecules(p-fluorobenzoic acid(P-FBA))is used in the interface layers betweenthe mesoporous/perovskite layer.P-FBA not only anchors the Ti O₂surface through the carboxyl group but also interacts with the uncoordinated Pb on the surface with F ions,reducing the interface defects,improving the compatibility of the interface and decreasing the charge accumulation between the electron transport and perovskite layers.Finally,the PCE of Cs Pb I₂Br PSCs improved by P-FBA increases from10.4%to 11.7%,and the fill factor increases to be 71.9%.The crystallization process of perovskite is crucial for the fabrication of Cs Pb I₂Br PSCs.The p-chlorobenzoic acid(CBA),used as a cross-linking agent,is introduced into the Cs Pb I₂Br precursor.The anchoring effect between the functional groups of CBA and the ligand of Cs Pb I₂Br is used to assist the crystal growth and stabilize the perovskite phase,improving the surface morphology of the films and the performance of the Cs Pb I₂Br PSCs.The experimental results found that the carboxyl group of CBA molecule and Pb²⁺could form an anchor Pb X₆framework,and chloride ions replace iodine or bromide ions,indicating that CBA provides a crystal interconnection bridge for the perovskite crystallization process and improving the stability of peroskite.The improved Cs Pb I₂Br films not only increase the PCE to be 13.25%but also makes the PSCs maintain for 600 h under low-humidity environment without almost degradation.