Buried surface engineering for efficient and stable back-contacted bifacial perovskite solar cells

Back-contacted perovskite solar cells (PSCs) have been demonstrated with merits of low material cost and weak ion migration, while the inferior buried surface restricts their performance and bifacial response. Herein, polyvinylidene fluoride (PVDF) with similar thermal expansion coefficient to perovskites and low tensile modulus is introduced at the substrate/crystal interface to release interface lattice strain and enhance crystallinity. Besides, PVDF can release free fluoride ions to interact with bare Pb2+ ions, reducing interface charge trap density and nonradiative recombination. As a result, an impressive efficiency of 13.37 % is obtained, setting a new efficiency benchmark for back-contacted PSCs. Moreover, the PVDF-modified devices retain 100 % of their initial efficiency after 1,200 h of maximum power point tracking at 60 °C. Finally, a high bifaciality factor of 0.96 is obtained, leading to obvious increase of power output under simulated circumstance with reflected light.