Research On Surface Modification For Highly Efficient And Stable Perovskite Solar Cells

Organic lead halide perovskite has been an ideal optical absorption material for the photovoltaic devices due to tremendous advantages such as large photon absorption coefficient,high defect tolerance,high carrier mobility and long diffusion length.Over the past decade,perovskite solar cells（PSCs）have achieved unprecedented development and their power conversion efficiency（PCE）has increased rapidly,which has attracted the continuous attention research interest.However,the PCE of PSCs still lags behind the calculated theoretical limit.At the same time,the weak stability of perovskite is also one of the main factors that hinder the commercialization for PSCs.For PSCs devices,a large number of defects on the upper and lower surfaces of perovskite and the mismatched interfacial properties between the optical absorption layer and the charge transport layer will lead to non-radiative recombination loss,which still restrict the development of efficiency and stability.Therefore,in this paper,the planar structural PSCs were taken as the research object,and the surface/interface optimization between the charge transport layer and perovskite optical absorption layer was achieved through post-processing or pre-processing methods,so as to improve the efficiency and stability of the devices.The main research contents are as follows:（1）A fluorinated and non-centrosymmetric 4,4-difluorocyclohexylammonium（DFCHA⁺）cation was employed as an organic ligand for post-treatment of the surfaces of MA-based perovskite（MAPb I₃）films.The presence of an ultrathin 2D Ruddlesden-Popper（RP）phase perovskite was confirmed on the surface,and the3D/2D perovskite heterojunction was successfully constructed as the absorption layer.The 2D RP phase perovskite effectively passivated the surface defects of 3D perovskite.In addition,nonradiative recombination was suppressed,and the interface bands were aligned.The n-i-p PSCs based on this 3D/2D perovskite heterojunction was constructed by a one-step method,achieving an ultra-high efficiency of 21.93%.Furthermore,owing to the hydrophobicity of the DFCHA⁺cation,the unencapsulated device was able to maintain an initial efficiency of 82.3%after storage for 500 h at a relative humidity of～45%.（2）To passivate the defects enriched on the lower surface of the MAPb I₃ films,the bis（triphenylphosphine）cobalt dichloride（BTPPCC）was selected to treat the pre-buried interface of the absorption layer.Benefitting from the effective introduction of the surface modifier,the interface contact between MAPb I₃ and the hole transport layer（HTL）was optimized and the crystallization ability of the perovskite was significantly enhanced.At the same time,the BTPPCC effectively passivated the defects on the buried interface of the perovskite absorption layer,and the non-radiative recombination was suppressed.As a result,the optimal p-i-n device achieved a PCE over 20%.Futhermore,the long-term stability of the devices was remarkably improved.