Charge Transfer Mechanism And Process Regulation In Perovskite Solar Cells

In recent years,perovskite solar cells have received a lot of attention due to their good photovoltaic performance and simple preparation process.In the past decade or so,researchers have invested a lot of efforts to carry out the design and development of cell devices,and great progress has been made.However,most of these studies have focused on macroscopic optoelectronic performance optimization of micro and nano materials and devices,while there is a paucity of research on microscopic photophysical mechanisms at the molecular or even atomic level.In this thesis,we study the microscopic mechanisms of charge dynamics of photovoltaic materials in perovskite solar cells by ultrafast laser pumping-probe spectroscopy and time-resolved photoluminescence.The study includes the following main parts.Four types of perovskite films were prepared by annealing in"air","H₂O+air","DMF+air"and"DMSO+air"environments.The crystallinity,morphology and optical properties of the four films were analyzed,and the relationship between grain size and carrier transfer was investigated by combining transient fluorescence and transient absorption techniques.We found that films with large grains and few grain boundaries have superior crystallographic and optical properties,and their fewer defect states and longer radiative recombination lifetimes（～73.45 ns）allow for efficient charge transfer within the perovskite.Moreover,the PCBM electron transport layer shows not only good charge transfer（extraction efficiency of about 97%）but also some passivation effect on the perovskite film on the high-quality film.The resulting perovskite carrier transfer model and electron extraction model were developed to explain the processes involved.It is known from the previous study that the large grain size of perovskite films can ensure the effective charge transfer inside the perovskite films.Therefore,by adding CH₃NH₃Cl（MACl）to the perovskite precursor solution to effectively control the growth of perovskite grains,perovskite thin films with an average grain size of～940nm were prepared,and combined with the analysis of morphology,optical properties and carrier transfer properties,it was finally also found that the radiation recombination lifetime（～1877 ns）of large and uniform perovskite thin films can reach.And the perovskite carrier transport model is effectively verified.In addition,in the prepared TiO₂/TiO₂ and Sn O₂/TiO₂ electron transport layers,it was found that the Sn O₂/TiO₂ electron transport layer was able to reduce the energy loss caused by the large energy level difference between the electron transport layer and the FTO conducting substrate,and accelerate the extraction efficiency of photogenerated electrons（～82.2%）,and also blocked the electron-hole recombination,thus improving the cell efficiency by～5%.The electron extraction model of electron transport layer/perovskite layer is effectively validated.