Tailoring Thick Film Crystallization and Facet Orientation via Potassium Acetate for Efficient Bifacial Perovskite Solar Cells

Bifacial perovskite solar cells (PSCs) require micron-thick perovskite layers to compensate for optical loss from transparent electrodes, but high defect densities limit efficiency. Here, we introduce a potassium acetate (KAc) post-treatment on microthick PbI2 films to form an amorphous KAc-PbI2 intermediate, which facilitates ammonium salt incorporation and promotes perovskite crystallization. During the second step, released Ac– and K+ ions selectively adsorb onto (111) and (001) facets, respectively, directing vertical crystal growth with (001) orientation. This strategy reduces defect density and improves film quality in thick perovskite layers. As a result, the bifacial PSC achieves a power conversion efficiency (PCE) of 21.33%, bifaciality exceeding 80%, and a record power generation density (PGD) of 24.72 mW cm–2 under 0.2 albedo, representing the highest reported PGD for n-i-p bifacial PSCs. After 500 h of continuous illumination at the maximum power point, the KAc-treated bifacial device retained its initial PCE.