Vapor Healed Perovskite Films With Low Trap Density

In recent decade,the power conversion efficiency?PCE?of perovskite solar cells?PSCs?has rocketed from 3.81%to 25.2%.As far away from the thermodynamic equilibrium,the perovskite films deposited from solution process are usually polycrystalline and defective,which result in serious non-radiative recombination loss and resultant PCE inferior to the Shockley-Queisser limit?about 33%?.In addition,for the large-area,high-thickness PSCs in commercialization,the traps will increase exponentially due to the film thickness increase.Therefore,effective defect passivation is of great importance for the deposition of low-trap perovskite films.In this thesis,we concentrate on reducing defect density of perovskite films to achieve high-quality and low-trap perovskite films.As a usual anion passivant,chloride is often used in the preparation of low-trap perovskite films.On the one hand,the crystallization of perovskite will be retarded by the volatility of MACl during annealing.Perovskite films with reduced grain boundaries and the increased grain size are achieved,which is conducive to the carriers transport in the perovskite layer.On the other hand,chloride incorporation can passivate the defect-induced deep-level traps at the interface and grain boundaries,thus prevent the formation of defects and non-radiative recombination.As a Lewis base,methylamine?MA?can effectively passivate the surface defects of perovskite.It can also induce perovskite recrystallization to form a high-crystallinity,dense high-quality perovskite film,thereby improving carriers transport,reducing charge recombination and enhancing device stability.In this thesis,a series of research will be carried out focusing on the preparation of chloride-assisted low-trap perovskite films and the effects of MA vapor healing on the redistribution of excess lead iodide?PbI₂?.On the one hand,chloride incorporation slowed the crystallization and passivated defects during the annealing process,the open-circuit voltage and fill factor of PSCs were significantly improved.Improved crystallization and grain sizes,reduced grain boundaries and enhanced carrier transport ability have been achieved with diffierent chlorides incorporation.Compared with the negative impact of the residual MACl on the perovskite film,the residual PbCl₂ can further passivate defects and suppress carrier recombination,resulting in further enhancement of the perovskite film quality and the device photoelectric performance.On the other hand,we precisely introduced excess PbI₂ into the grain boundary by MA vapor healing induced perovskite recrystallization,resulting in significantly increased short-circuit current density and negligible hysteresis in the corresponding PSCs.Compared with different methods of excess PbI₂ introduction,we found that MA vapor could cause the redistribution of PbI₂ during the perovskite recrystallization.The crystallinity and grain size were notably increased and the defect states density was significantly reduced,resulting in the effective charge extraction and extended carrier lifetime.In addition,the strict tolerance of PbI₂ excess on high quality perovskite films has also alleviated by MA vapor healing.