Polymer Lewis Base for Improving the Charge Transfer in Tin-Lead Mixed Perovskite Solar Cells

The poor film stability of Sn-Pb mixed perovskite film and the mismatched interface energy levels pose significant challenges in enhancing the efficiency of tin-lead (Sn-Pb) mixed perovskite solar cells. In this study, polyvinylpyrrolidone (PVP) is introduced into the PVK perovskite precursor solution, effectively enhancing the overall stability of the film. This improvement is achieved through the formation of robust coordination bonds between the carbonyl (C=O) in the pyrrole ring and the undercoordinated SnII and PbII, thereby facilitating the passivation of defects. Furthermore, the introduction of PVP inhibits the oxidation of tin (Sn), thereby enhancing the n-type characteristics of the perovskite film. This adjustment in the energy level of the PVK perovskite film proves instrumental in reducing interface energy loss, subsequently improving interface charge transfer and mitigating device recombination. Consequently, perovskite solar cells incorporating PVP achieve an outstanding champion power conversion efficiency (PCE) of 21.31%.

Surface Defect Passivation and Energy Level Alignment Engineering with a Fluorine-Substituted Hole Transport Material for Efficient Perovskite Solar Cells

The surface and boundary defects present in the perovskite film are reported to be nonradiative recombination and degradation centers, restricting further improvement of the power conversion efficiency (PCE) and long-term stability of perovskite solar cells. To address this problem, herein, we introduce a fluorine-substituted small molecular material 2FBTA-1 as a bifunctional buffer layer to efficiently passivate the surface defects of perovskite and tune the energy level alignment between the perovskite/2,2',7,7'-tetrakis(N,N-di-(p-methoxyphenyl)amino)-9,9'-spirobifluorene (Spiro-OMeTAD) interface. X-ray photoelectron spectroscopy shows that with the insertion of 2FBTA-1 between perovskite and Spiro-OMeTAD, the metallic Pb⁰ defects and uncoordinated Pb²⁺ defects are well restricted. Consequently, the average PCE is distinctly improved from 18.4 ± 0.51 to 20.3 ± 0.40%. Moreover, the long-term stability of unencapsulated devices with 2FBTA-1 treatment under ambient conditions (relative humidity 40-60%) is effectively enhanced, retaining 87% of the initial efficiency after storage for 500 h.