Research On The Stability Of Perovskite Solar Cells Based On Interface And Bulk Modifications

Photovoltaic performance of perovskite solar cells（PSCs）has been rapidly developing over the past decade,where the power conversion efficiency（PCE）of PSCs have reached 26%.However,stability issue of PSCs remains one of the critical factors that limits their large-scale deployment.The instability of PSCs mainly stems from two root causes.First,interface between perovskite and charge transport layer decomposes or degrades during device operation due to interfacial defects,lattice mismatch,residual stress,and energy level offsets,thus affecting the device stability.Second,being a core component of solar cell devices,perovskite layer is featured with a large number of grain boundaries in the bulk that results in the formation of rampant defects.Plus,perovskite materials are prone to decompose under elevated temperatures,water and illumination conditions owing to their active chemical properties,which ultimately leads to the deterioration of device stability.Based on such observations,the keys to improve the stability of PSCs is to optimize the interfaces of charge transport layers/perovskites and conduct bulk regulation of the perovskite layers.The thesis first targets the interfaces of PSCs.By doping methylene diamine dichloride（MDACl₂）as an additive in the Sn O₂ electron transport layer（ETL）,we solve the problem of device instability as caused by the excessive lead iodide（PbI₂）in perovskite layers.Through the modification effects of MDACl₂,one not only harvest enhanced light transmittance of the Sn O₂ ETL,but also achieves reduced content of PbI₂in the perovskite films,which contributes to the optimized nucleation and reduced interfacial defects of perovskite films.As a result of these modifications,lattice strain of the perovskite thin films is alleviated,improving the environmental and operational stabilities of PSCs on a high PCE（23.02%）basis.By systematically regulating the nucleation,growth and coalescence processes within the perovskite thin films,this thesis comes up with a low-temperature solvent-vapor annealing approach to assist the film formation process.Subsequently,through a series of orthogonal experiments adopting annealing temperature,solvent amount,and annealing time as the variables,we develop a method of fabricating perovskite thin films featured with large grains with maximum sizes exceeding 10μm.Because of the decreased densities of grain boundaries and PbI₂ residues,bulk defects are depleted,with the phase compositions,micro-strain,and interfacial adhesion being improved,thus enhancing the stability and photovoltaic performance of PSCs in comparison with the low-temperature-processed control devices without the solvent vapor effects.In summary,combining interfacial treatments and bulk regulation,this thesis achieves optimized growth of perovskite crystals that results in perovskite thin films with reduced PbI₂ residues micrometer-size grain sizes.Owing to the decreased defects and grain boundary densities,stability and photovoltaic performance of PSCs are systematically enhanced.This work establishes scientific modification approaches for large-scale perovskite devices in the future,and provided insights for further advancements of photovoltaic performance and stability.