Study On The Stability And Photoelectric Properties Of Formamidine Perovskite Single Crystal

Organic-inorganic hybrid perovskite materials,which have been increasingly promising in photovoltaic technology in recent years and are regularly employed in the fabrication of solar cells,photodetectors,and other devices,are a highly photoactive material.Most of the research at this stage is based on polycrystalline thin films.However,the imprecise stoichiometric ratio,high defect density,poor stability and low crystallinity of polycrystalline films make it difficult to further improve the performance of solar cells.Due to the absence of grain boundaries,single crystal perovskite exhibit higher photovoltaic performance.However,the presence of single crystal surface defects would cause carrier complexation,which leads to poor crystal stability.And the thickness of single crystal is difficult to control below the carrier diffusion length,which leads to poor performance of single crystal devices.For polycrystalline perovskite based devices,the traditional perovskite layer prepared by precursor salt has imprecise stoichiometry ratio,high defect density,poor stability and low crystallinity,which leads to low performance of polycrystalline perovskite devices.The FAPbI₃ perovskite is susceptible to phase transition by water erosion and also faces the problem of interfacial complexation,which causes the degradation of PCE of perovskite solar cells.To address the above issues,this thesis revolves around surface passivation of single crystals,optimization of the device perovskite layer and interfacial modifications,which are discussed in the following work:（1）Due to the presence of grain boundaries,polycrystalline thin films usually have charge transport problems.Single crystal perovskite have superior charge transport properties than polycrystalline perovskite.The single crystal’s surface defects cause its stability to deteriorate,resulting in carrier complexation.To overcome these limitations,perovskite single crystal was synthesized and applied owing to the more superior bulk charge transport properties compared with the polycrystalline counterpart.Nevertheless,due to the defective nature of the crystal surface,the surfaces of single crystals keep as the bottleneck that not only induce stability issues but also cause interfacial charge carrier recombination.In this work,we synthesized the triple cation perovskite single crystals,FA_0.8MA_0.15Cs_0.05PbI₃（denoted as FAMACs）and reported a facile surface modification strategy for single crystals by polymer monolayer coupling,which showed high effectiveness for improving stability and reducing surface trap state density for single crystals.The polymer-wrapped single crystals exhibited robust stability and a well-passivated surface,which showed an order of magnitude lower trap density compared with those of unmodified single crystals and achieved a high responsivity of 5.069A/W when used in photodetector applications.（2）The narrow band gap of FAPbI₃ has made it the most sought-after photoelectric functional material in perovskite solar cells^[92],garnering considerable attention in recent years.The traditional perovskite layer,created by formamidine iodide（FAI）and lead iodid（PbI₂）,has a low stoichiometric ratio,high defect density,low stability,and crystallinity,making it hard to further enhance the performance of perovskite solar cells.In contrast,the FAPbI₃ single crystal perovskite film has a high crystallinity,stability,accurate stoichiometric ratio,and low defect density.The large grain size and few grain boundaries of single crystal perovskite film lead to infewer defects in the grain boundaries,thus enhancing both the short-circuit current density(J_SC)and open-circuit voltage(V_OC)of perovskite solar cells and significantly increasing the photoelectric conversion efficiency^[92].This work offers an effective approach for creating perovskite solar cells with high stability,crystallinity,and low defect density^[92].（3）The instability and ease of decomposition of FAPbI₃ perovskite,caused by moisture,has caused the interface composite problem,drastically reducing the photoelectric transformation efficiency（PCE）of perovskite solar cells.This interface is the weakest point to link to the outside world.Water and oxygen in the air are easy to corrode the perovskite through the interface.Therefore,it is necessary to use interface engineering to optimize device performance.In this chapter,based on FAPbI₃ single crystal perovskite,the structure of ITO/TiO₂/FAPbI₃/（PMMA）/Spiro-OMe TAD/Au perovskite solar cell was prepared.Polymethyl methacrylate（PMMA）as the interface modification layer,isolates perovskite from the electron transport layer,prevents the electron transport layer from contacting with it directly,fills the holes on the surface of perovskite,and passivates the surface defects.Due to the hydrophobicity of PMMA,when it is used as the interface modification layer,It also isolates the perovskite from moisture and oxygen in the air.The PCE of solar cells with PMMA modified layer increased from 17.64% to 19.06%.