Tailored PEDOT:PSS phase segregation for high-efficiency flexible all-perovskite tandem solar cells and mini-modules

Flexible all-perovskite tandem solar cells (TSCs) promise lightweight power for portable and aerospace uses, yet performance remains limited by interfacial losses in narrow-bandgap (NBG) sub-cells, particularly arising from poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Here, we uncover a previously unrecognized vertical phase segregation in PEDOT:PSS films, with an insulating PSS-rich surface layer atop a conductive PEDOT-rich base. This architecture induces interfacial electric dipoles that hinder efficient hole extraction. We find that the inclusion of Triton X-100 disrupts this segregation by modulating polymer interactions, thereby suppressing surface dipole formation. NBG perovskite cells incorporating this modified interface exhibit enhanced performance and reproducibility. This strategy enables flexible TSCs with a power conversion efficiency (PCE) of 25.4% and a proof-of-concept flexible tandem solar mini-module reaches 19.7%, with theoretical modeling projecting module efficiencies exceeding 24%. These results provide key insights into PEDOT:PSS interfacial phenomena and elucidate design principles for optimizing interfaces for next-generation flexible perovskite tandems.