Studies On Drop-Coating Preparation And Photovoltaic Performances Of CsPbI_xBr_3-x Inorganic Perovskite Semiconductor Thin Films

Volatile and unstable organic cations（CH₃NH₃⁺（MA⁺）or CH₃（NH₂）₂⁺（FA⁺））make organic-inorganic hybrid perovskite solar cells（PSCs）vulnerable against high temperature and/or light illumination.Without instability issue caused by organic cations,all-inorganic perovskites（CsPb X₃,X=I,Br and Cl）become very promising light absorption materials for PSCs.I and Br mixed inorganic perovskites(CsPb I_2.25Br_0.75,CsPb I₂Br etc.)show better phase-stability than that of Br-poor inorganic perovskites(CsPb I₃,CsPb I_2.85Br_0.15,etc.)and higher performance than that of Br-rich inorganic perovskites（CsPb IBr₂ and CsPb Br₃）.Therefore,I and Br mixed inorganic perovskites were regarded as the most promising materials for future commercialization application.This thesis mainly focused on I and Br mixed inorganic perovskites.A low-cost,simple and large-scale-compatible drop-coating method was developed to prepare I and Br mixed inorganic perovskites films.Efficient and stable inorganic PSCs were fabricated through modified drop-coating method by additive technology and interface engineering.The mechanisms of the enhanced efficient and stability of films and PSCs were studied.Furthermore,the drop-coating inorganic perovskite films were applied into large-scale PSCs,hole transporting layer（HTL）-free carbon electrode PSCs and the tandem solar cell.The main results and novelty are as following:（1）A drop-coating method was developed to make CsPb I₂Br films.Isopropanol was added into the precursor solution to improve the wettability of the solution,resulting in improved film morphology,reduced trap states,and enhanced performance of PSCs.A power conversion efficiency（PCE）of 16.27%was achieved,which is higher than that of the PSC based on spin-coating method.Thanks to high quality films,all-drop-coating（all layers were prepared by drop-coating method expect Ag electrode）devices were fabricated and a PCE of 16.57%with remarkable fill factor of 84.9%was achieved.（2）An additive-assisted airflow drying（AAD）method based on drop-coating was developed to make mixed-halide inorganic perovskite films in ambient air.The nucleation and crystal growth during crystallization process are controlled by using airflow dying and lead acetate trihydrate additive,producing pinhole-free film with low defect density.Consequently,efficiency of CsPb I_2.25Br_0.75 PSCs is improved to 18.49%,which is the highest for wide-bandgap（>1.8 e V）inorganic PSCs at that time.An efficiency of 17.08%is achieved for PSCs made under moderate relative humidity（40-50%）.Moreover,PCEs of 17.39%and 11.54%are achieved for CsPb I₂Br and CsPb IBr₂ PSCs,respectively,which are among the best PCEs for CsPb I₂Br and CsPb IBr₂ PSCs.The long-term stability,thermal and light stability of AAD films and PSCs were also enhanced.（3）A polylactic acid（PLA）modification method was used to improve the performance of mixed-halide inorganic perovskites.The surface defects could be effectively passivated through strong interaction between C=O in PLA and undercoordinated Pb²⁺.A secondary grain growth is induced by PLA modification,resulting in larger grain size.PLA modification makes the surface region of perovskite changing from n-to p-type,favoring charge transport from perovskite to HTL.The PLA modified films enable PSCs with less nonradiative recombination and lower energy loss.Consequently,record PCEs of 19.12%and 18.05%were achieved for CsPb I_2.25Br_0.75and CsPb I₂Br PSCs respectively.In addition,the PSC with PLA show significantly improved air stability due to the hydrophobic PLA coating.（4）The inorganic perovskite film was prepared on larger substrate by the modified drop-coating method.The uniformity of the film was studied.It was found that drop-coating method could be used to prepare large-area inorganic perovskite films.The PSC with an active area of 1 cm² shows a PCE of 16.41%.A PCE of 14.70%was achieved for HTL-free PSC with carbon electrode.Without using glovebox,spin-coater,vacuum evaporation and expensive HTL and metal electrode,the carbon-electrode inorganic PSCs based on drop-coating method have better economic benefit and is meaningful for future commercialization.Moreover,the drop-coating inorganic perovskite film was applied into 4-terminal inorganic perovskite/organic tandem solar cell which deliver a PCE of22.34%.This dissertation contains 122 figures,19 tables,and 214 references.