Unveiling Popular PEDOT:PSS-Derived Composition Segregation Effect in Tin–Lead Mixed Perovskite Solar Cells and Elimination

As the most popular hole-transport material for promising tin-lead mixed perovskite (TLP) solar cells, poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) would cause the composition segregation of TLP, besides exacerbating degradation and parasitic absorption reported previously. However, the segregation phenomenon is now crucial but was rarely discussed previously, and the mechanism behind it was hardly investigated. In this work, we reveal unambiguously that PSS with a sulfonic acid group in PEDOT:PSS has the propensity to coordinate Sn prior to Pb and causes the uneven nucleation at the surface of PEDOT:PSS, further affecting the growth of the TLP film. This resulted in severe tin enrichment, and voids frequently occurred at the buried interfacial layer; herein, the Sn/Pb distribution was uneven through the TLP film. To address this concern, we designed 4-(10<i>H</i>-phenoxazin-10-yl)butyl)phosphonic acid (PXZPA) to exclude that negative effect. For the Cs_0.1FA_0.7MA_0.2Sn_0.3Pb_0.7I₃ TLP material with a bandgap of 1.30 eV, the defect density declined obviously from 1.33 × 10¹⁶ cm^-3 to 5.78 × 10¹⁵ cm^-3, and the champion device conversion efficiency surged remarkably from 17.27% to 22.43%.