Effect Of Alkali Metal Sulfide On The Performance Of Hybrid Cation Perovskite Solar Cells

The certified efficiency of perovskite solar cells(PSCs)has increased from 3.8%to 25.2%,and it has become the most promising cell device in the next generation of photovoltaic technology.Due to its advantages of high photoelectric conversion performance,simple preparation method and low cost,more and more researchers are devoted to the research of perovskite solar cells.The quality of perovskite crystal and electron transport layer and the interface defect between the two layers are important factors restricting the further improvement of perovskite solar cells' performance.A small amount of Na2S can improve the conductivity of TiO2,slow the crystallization process of perovskite,and significantly reduce the defect density of the device,finally,a high-efficiency and high-stability device is obtained.Na2S was introduced between perovskite and TiO2 as inorganic passivation layer to study the distribution of Na and S in perovskite layer.Na2S,Na2S2O3 and Na2SO4 were introduced into TiO2 layers to explore the influence of sulfur valence state on the crystallization process and device performance of perovskite.The specific contents are as follows.(1)In this thesis,Na2S was introduced into the TiO2 precursor as a "dual-effect"dopant,while NaCl was introduced as a reference sample to distinguish the effects of Na and S.The results show that Na+could improve the conductivity of TiO2,and S2"could slow down the crystallization process of perovskite.The perovskite grown on the Na2S doped TiO2 substrate has large grain size and less grain boundary,and the defect state density of the device is significantly reduced.Finally,the photovoltaic conversion efficiency of solar cells based on the normal device structure is 21.25%,and the open circuit voltage is up to 1.221 V.The device can maintain 90%of the initial efficiency after 90 days' storage in the air.(2)Na2S was inserted into the interface of perovskite and TiO2 as the inorganic passivation layer,and the position distribution of Na and S was determined by using HAADF-STEM-EDS.The result of PL shows that Na2S layer can effectively reduce the interface defect state.The main reason is that Na and S are diffused into the bulk of perovskite,and Na+ can passivate the perovskite grain boundary,while S2-could bond with perovskite to enhance the chemical binding at the interface,thus improving device performance and photo stability.(3)It is first proposed that different valence states of S may have different effects on TiO2 and perovskite films.The valence states of S in Na2S,Na2S2O3 and Na2SO4 compounds were compared by XPS.The photoelectric performance of the devices prepared on the substrates of the three S-containing compounds is significantly different:the performance of the devices prepared based on Na2S is better than that of Na2S2O3,while the performance of the devices prepared based on Na2SO4 declined significantly.The valence state of S also plays a different role in the crystallization process of perovskite.Therefore,based on the perovskite composition and preparation process in this experiment,S2-shows excellent modification performance.