Study On Self-Assembled Interface Modification For High-Performance Perovskite Solar Cells

Due to the extraordinary photovoltaic properties of perovskite,perovskite solar cells（PSCs）have attracted much attention in the field of photovoltaics.After just a decade of development,their photovoltaic conversion efficiency（PCE）has increased from 3.8%to 25.7%.According to reports,the theoretical efficiency of perovskite solar cells can reach 40%,and the actual theoretical available efficiency can exceed 30%.Compared with the cells on the market,perovskite solar cells still have a huge room for improvement in efficiency.Moreover,with the perfection of perovskite solar cell packaging technology and the improvement of device operational stability,there will be huge commercial potential in the future.However,there are also some problems with perovskite solar cells,such as the sensitivity of perovskite to the environment,and a large number of defects distributed inside and on both sides of the perovskite layer.These problems can degrade the photovoltaic performance and stability of perovskite solar cells.Among them,good contact between interfaces and reasonable arrangement of energy bands are crucial for achieving high efficiency.The interface energy barrier can affect the extraction and transmission rate of charge carriers;the chemical properties of the interface are important for the formation and stability of perovskite thin films,which determine the morphology of the film and the stability of the device.Good interface properties are the key to achieving high-performance PSCs.Therefore,in order to improve charge extraction through better energy level alignment and reduced defect density,this paper introduces self-assembled monolayers（SAM）at the interface of Zn O/Sn O₂ ETL and perovskite layer,and systematically studied the effects of different self-assembled modifying molecules on the crystallinity of perovskite thin films,interface defects,and energy level alignment.As a result,the PCE and stability of the devices were significantly improved.The main research content is divided into the following two parts:（1）The self-assembled molecules 2-thiopheneacetic acid（2-TA）and 3-thiopheneacetic acid（3-TA）were deposited as interface modifiers between the Zn O electron transport layer and the perovskite layer（Cs-FA）.TA molecules with carboxyl groups can bond with hydroxyl groups on the surface of zinc oxide,slowing down the deprotonation reaction caused by contact between zinc oxide and perovskite,and changing the interface properties through chemical bonding.The interface dipole effect generated by TA molecules can reduce the energy level barrier.The sulfur on the thiophene ring with lone pair electrons can passivate the Pb²⁺defects on the surface of the perovskite layer,reducing the defect density.We finally obtained perovskite solar cells with larger grain size,smaller energy level difference,effective defect reduction,and significant increase in device efficiency.The thermal stability of the perovskite films and the long-term stability of the devices were significantly improved,and the PCE was significantly increased from 18.1%of standard devices to 20.6%.（2）The interface of Sn O₂/perovskite was modified by self-assembled molecules3,4,5-trimethoxyphenylacetic acid（PAA）and 3,4,5-trimethoxyphenylpropionic acid（PPA）as interfacial modifiers.One end of the PAA/PPA molecule contains a carboxyl acid group,which can adsorb on the surface of Sn O₂ by condensation reaction with the residual hydroxyl groups on the surface of Sn O₂.The other end of the molecule contains a methoxy group,which can form hydrogen bonds with the methylammonium cations in the perovskite layer.At the same time,PAA/PPA molecules can passivate the Pb²⁺vacancies in the perovskite and the oxygen vacancies in Sn O₂,which helps to improve the crystalline quality of the perovskite film.In addition,both PAA and PPA have strong dipole moments,which can improve the interface energy level alignment,promote charge extraction and reduce losses.The results showed that the introduction of PAA and PPA interlayers can effectively passivate defects,reduce non-radiative recombination,and promote charge extraction and transfer.Correspondingly,the device efficiency increased significantly from 20.0%to 22.2%,and exhibits good thermal stability and long-term stability.