With rapid and brilliant progress in performance over recent years, perovskite solar cells have drawn increasing attention for portable power source applications.

By employing the neutral plane concept, we demonstrated ultra-flexible perovskite solar cells that can withstand 100 cycles of crumpling.

Black orthorhombic (B-γ) CsSnI3 with low toxicity and excellent optoelectronic properties is a promising candidate for perovskite solar cell (PSC). However, the performance of the B-γ CsSnI3-based PSCs is much lower than their lead-based or organotin-based counterparts due to the heavy self-doping of Sn2+ to form Sn4+ under ambient-air conditions. Here, this undesirable oxidation in CsSnI3 is restricted by engineering the localized electron density with phthalimide (PTM) additive. The lone electron pairs of NH and two CO units of PTM are designed to form trigeminal coordination bonding with Sn2+, resulting in reduced defect density and relatively grain-ordered perovskite film. The champion efficiencies of 10.1% and 9.6% are obtained for the modified rigid and flexible B-γ CsSnI3-based PSCs, respectively. These encapsulated devices maintain 94.3%, 83.4%, and 81.3% of their initial efficiencies under inert (60 days), ambient (45 days), and 1 Sun continuous illumination at ∼70 °C (2000 min) conditions, respectively.