Interfacial Optimization Of Electron Transport Layer/Perovskite Heterojunction For High Performance Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells(PSCs)have been widely studied due to their excellent photoelectric properties and low cost.The power conversion efficiency(PCE)of PSCs has rapidly increased from 3.8%in 2009 to 25.5%today,showing great application prospects in flexible energy devices,low-power electronic devices,and self-driving electronic devices.This dissertation focuses on the electron transport layer(ETL)/perovskite interface in PSCs with a normal structure,and addresses four main issues including the surface morphology,band matching and mobility,surface free energy and surface microstructure of ETL.The authors employed interface engineering methods,including interface passivation,interface doping,heterojunction,to improve the PCE and stability of PSCs,systematically studied its internal mechanism,and successfully reduced the preparation temperature of ETL to 150℃,laying the foundation for the development of flexible optoelectronic devices.The related research results are as follows:(1)Research on TiO2/perovskite interface modification.In this study,TiCl4,PC61BM and TiCl4-PC61BM combination were used to deal with TiO2 to improve the performance and stability of PSCs.TiCl4 can significantly reduce the surface roughness of TiO2,thereby effectively increasing the perovskite grain size and enhancing crystalline of the perovskite film.PC61BM treatment can adjust the energy band mismatch between the TiO2 and the perovskite layer,accelerate the extraction and separation of photogenerated carriers,while PC61BM can inhibit the hysteresis behaviors.Finally,the combination of TiCl4 and PC61BM treatment largely enhances the PSC device performance to a PCE of 16.4%,and the device can still maintain 70%of the initial PCE after stored in an air environment with a humidity of<30%for 216 hours,which is higher than the device stability with other treatment conditions.(2)NaCl-doping TiO2 improves the performance of planar perovskite solar cells.In this study,the author employed the water bath immersion method to treat the surface of the TiOx interface layer with NaCl solution,and the conductivity and mobility of NaCl-doping TiO2 are nearly doubled,which reduces the defect density,effectively inhibiting interfacial charge recombination.In the end,the NaCl-doped TiO2-based PSC finally achieves an energy conversion efficiency of 18.3%with 2.7%hysteresis factor.In addition,NaCl doping significantly improves the stability of the device.After 350 hours of storage in an air environment with a humidity of<30%,the NaCl doped device still maintains 79%of the initial PCE.(3)Research on high-efficiency and stable PSCs based on low-temperature ZnO.In this study,the author proposed a low-temperature TiOx treatment method to passivate the ZnO surface and stabilize the ZnO/perovskite interface.The ZnO thin films prepared by the solgel method and annealed at 150～250 ℃ all show good crystallinity.TiOx completely covered the ZnO layer and formed a composite layer,which causes the electrical conductivity of ZnO-TiOx is greater than single-layer TiOx.The perovskite film based on ZnO-TiOx is dense with vertically growing grains,and show good thermal stability at the ZnO-TiOx/perovskite interface.PSCs based on 200/250℃ ZnO-TiOx finally achieve PCE of 19.87%and 20.71%,respectively,which are the highest efficiency of PSCs based on low-temperature ZnO reported in the same period.(4)Research on low-temperature ZnO-based all-inorganic CsPbI2Br PSCs.In this study,The author successfully introduced ZnO thin film prepared at a low temperature of 150℃ into all-inorganic PSCs to replace SnO2 as ETL.Compared with SnO2,ZnO can better match the energy band with perovskite,while the smaller surface free energy of ZnO can effectively increase perovskite grains sizes,improve the quality of film and interfacial water stability.The unencapsulated ZnO-based CsPbI2Br films have better air stability and thermal stability comparing to SnO2.The unencapsulated ZnO-based devices maintain more than 80%of the initial PCE after 200 minutes of continuous light in the air and 250 minutes of continuous heating at 85℃,respectively.It can still maintain 80%of the initial PCE after 35 days of storage in an air humidity<20%environment,showing a remarkable stored stability.In the end,the PSC based on ZnO at a low temperature of 150℃ achieves a PCE of 14.78%,verifying the application of ZnO in all-inorganic PSCs,and this study also provides a theoretical basis for the development of ZnO-based all-inorganic perovskites in the field of flexible optoelectronic devices.(5)Dual side interface optimization for efficient and stable all-inorganic PSCs.In this study,the author introduced a ZnO modified layer between ITO/SnO2 to reduce the energy band mismatch of the ETL/CsPbI2Br interface,greatly increase the Voc of the device to 1.25 V,and significantly improve the quality of the film.In addition,GAI is used to post-treatment the perovskite film to passivate the deep-level defects on the surface of the perovskite and increase the FF and Jsc.Through first-principles calculations,it is found that the defects of Ii,VI and VPb on the surface of the perovskite are significantly reduced after GAI passivation,inducing strong charge transfer processes.It can still maintain 86%and 79%of the initial PCE after the unencapsulated ZnO/SnO2-GAI-based devices exposed to continuous light for 600 minutes in ambient air and heated at 85℃ in a glove box N2 environment for 5 days,respectively,showing an excellent photostability and thermal stability.Finally,CsPbI2Br allinorganic PSCs based on 150℃ ZnO/SnO2-GAI achieves a high PCE of 16.25%,and it can still maintain 92%of the initial PCE after 60 days storage in an environment with a humidity of<20%.