Preparation Of Flexible Perovskite Solar Cells With High Efficiency And Stability Based On Multi-functional Additive

Perovskite solar cells have attracted great attention in recent years due to their high efficiency,low cost and easy solution processing.During the past 10 years,the power conversion efficiency of single junction perovskite solar cells has soared from 3.8%to the highest 25.7%as certified currently.Owing to the small Young’s moduli,organic-inorganic hybrid lead halide perovskite materials provide great potential for the development of light-weight,efficient and bending-resistant flexible photovoltaic（PV）technology.Nowadays,the leading efficiency of flexible perovskite solar cells（F-PSCs）has reached 21%all around the world,manifesting application prospects in various important scenarios such as internet of things,wearable bioelectronics,building-integrated PVs and other fields.However,the average performance of F-PSCs is still inferior to their rigid-substrate counterparts.Such underperformance of F-PSCs originates from the discrepancy of thermal expansivity between substrates and functional layers,which leads to interfacial lattice mismatch and residual stress,and has greatly hindered the further advancement of F-PSC performance.Therefore,resolving the abovementioned scientific problems and overcoming the performance bottleneck of F-PSCs represents a challenge and hot topic within the current research field.This thesis studies the in-depth physical mechanism of cross-layer modification with embedded additive at buried interface in a systematic manner.（1）The HCOONH₄additive improves the physical and chemical properties of the Sn O₂ETL by reducing the oxygen vacancy defects,so as to optimize the interfacial energy levels of devices,and to effectively mitigate the problem of interfacial charge transport.（2）The migration of non-halide HCOO^-ions across layers by thermal diffusion is revealed by time-of-flight secondary ion mass spectrometry,which indicates a significantly reduced defect densities in the bulk and across the grain boundary of perovskite films.（3）The dynamic interfacial optimization is realized upon the migration of non-halide anions,which effectively release the residual stress and micro-strain within the perovskite films.In addition,the adhesion between the Sn O₂ ETLs and perovskite layers is also enhanced by the HCOONH₄ functionalization,which provides a foundation for the improved bending resistance of F-PSC devices.By following the international development trends,this thesis aims at the stress and defects at device functional layers and their interfaces under the influence of thermal stress.By focusing on buried interface between perovskite and neighboring electron transport layer（ETL）,this work develops a bottom-up,cross-layer modification strategy by adopting a volatile additive with low melting point-ammonium formate（HCOONH₄）as a pre-buried modifier within Sn O₂ ETL,achieving breakthrough of PV performance of F-PSCs.Such novel strategy achieves record PCE of 22.37%as measured in laboratory and a certified PCE of 21.9%by third-party test agency,representing the highest performance of F-PSCs reported to date.In addition,these F-PSCs exhibit excellent environmental and mechanical stabilities,where the devices can retain about 90%of the initial PCE after 4000 bending cycles at a radius of 7 mm.