Interface Optimization Strategy To Improve The Performance Of Low Bandgap Perovskite Solar Cells

Organic inorganic metal halide perovskite has advantages such as adjustable bandgap,low cost,high absorption coefficient,and solvable preparation,and has been widely used in the photovoltaic field in recent years.The power conversion efficiency of perovskite solar cell has improved rapidly,and the certification efficiency of single junction perovskite solar cell has reached 25.7%,reaching the efficiency bottleneck and limited improvement space.In the past two years,the efficiency improvement rate of single junction perovskite photovoltaic devices has slowed down,and the efficiency of perovskite tandem solar cells has exceeded the performance of single junction devices.Therefore,double junction and multi junction tandem devices based on perovskite have become the focus of development in the next stage.Compared to single junction devices,tandem solar cells have a wider range of light absorption and lower thermal relaxation energy loss,which is an effective way to break through the efficiency Shockley Queisser(SQ)limit of single junction devices.All-perovskite tandem solar cells typically contain a wide bandgap perovskite top cell and a low bandgap perovskite bottom cell,which are mechanically stacked or monolithic integrated to achieve a four or two tandem cell structure.At present,the low bandgap tin lead mixed perovskite used for bottom cell has problems such as rapid crystallization rate,high defect state density,and easy oxidation of divalent tin elements,resulting in the loss of photo generated carriers at the device interface,affecting the performance of low bandgap devices,and severely limiting the further improvement of the efficiency of all-perovskite tandem solar cells.Therefore,developing effective interface optimization strategies,reducing energy loss paths,and significantly improving the performance of single junction low bandgap solar cells are important means to achieve efficient and stable perovskite tandem devices.This article mainly focuses on the preparation of high-quality tin lead low bandgap perovskite thin films,suppressing the oxidation of divalent tin elements,while modulating the interface energy level structure,promoting efficient transport of photo generated carriers,and improving the conversion efficiency of single junction low bandgap solar cells.Further combining the wide gap perovskite absorbing layer and intermediate connecting layer,highperformance all-perovskite tandem solar cells are prepared,achieving an power conversion efficiency of over 26%,which is at the international advanced level.This article adopts strategies such as surface passivation of perovskite,interface energy level engineering,and optimization of new hole transport layers to improve the performance of low bandgap perovskite photovoltaic devices.Through multi characterization methods,the physical mechanism of performance improvement in low bandgap perovskite cells is deeply studied.The details are as follows:(1)Introducing maltol on the surface of perovskite can passivate shallow level defects on the surface of perovskite films,reduce surface carrier recombination,and significantly increase the current of low bandgap perovskite solar cells.Furthermore,in response to the issue of poor open circuit voltage in low bandgap perovskite devices,an additional PCBM is introduced at the cell back interface,and a dual electron transport layer structure is formed with thermal evaporation C60 to enhance the built-in electric field,promote efficient transport of photogenerated electrons,and thereby enhance the open circuit voltage of low bandgap perovskite solar cells.Finally,the synergistic effect of maltol and PCBM promotes a comprehensive improvement in device performance.The open circuit voltage and short circuit current density of the optimized device increased to 0.854 V and 30.28 mA cm-2,respectively,and the power conversion efficiency significantly increased from 18.86%to 20.62%.(2)Looking for a new interface(ITO/Perovskite)modification materials for low bandgap perovskite solar cell,taurine with excellent photoelectric performance and stability was selected as the new ultra-thin modification layer,which improved the problems of severe parasitic absorption and poor stability caused by interface corrosion in traditional PEDOT:PSS hole transport materials.The low bandgap perovskite solar cell based on the taurine charge layer has a short circuit current density of over 32 mA cm-2 and an open circuit voltage of 0.911 V.Its power conversion efficiency reaches 22.5%,which is superior to the low bandgap perovskite photovoltaic device based on PEDOT:PSS.Further design the structure of all-perovskite tandem solar cells,optimize the taurine spin coating process and preparation method,and successfully apply the taurine modification layer to tandem devices.Combining high-quality wide bandgap perovskite absorbing layer and intermediate connecting layer,a all-perovskite tandem solar cell with an open circuit voltage exceeding 2.1 V and a conversion efficiency exceeding 26%was prepared.With the excellent stability of taurine,tandem devices have good stability.The packaged tandem device operates continuously for 600 hours under 1 sun intensity,maintaining 90%of its original efficiency.