Dual-passivation strategy on CsPbI2Br perovskite solar cells for reduced voltage deficit and enhanced stability

The notorious defects in the grain boundaries (GBs) and surfaces trigger serious nonradiative recombination and interfacial charge losses, concomitantly exacerbating the deterioration of photovoltaic performance and phase stability of CsPbI2Br perovskite solar cells. In this work, a comprehensive dual-passivation strategy enabled by formamidine salts (FADP) is developed to annihilate defects both inside the GBs and over the surface. It is shown that formamidinothiourea (FATU) additive could effectively modulate crystallization and afford to passivate both shallow-/deep-level defects at GBs, thereby endowing CsPbI2Br films with reduced lattice micro-strains and enlarged grains over 2 mu m. Meanwhile, formamidinium bromine (FABr) post-treatment can efficiently heal the defective surface and realize cationic exchange with below CsPbI2Br films, forming a gradient band alignment at the CsPbI2Br/HTL interface. Profiting from ameliorated crystallization, inhibited GBs/surface charge recombi-nation and facilitated hole transport ability, the novel FADP strategy substantially lifts the Voc from 1.22 V to 1.34 V, yielding a champion PCE of 16.74% with greatly reduced hysteresis. Coincidently, the unencapsulated FADP devices maintained 86.2% of initial PCE after aging at 25% RH for 40 days and 83.8% after 480 h aging under continuous illumination, which is pertinent to the inhibited defective region as well as Br-rich surface.

Automated summary

This study developed a comprehensive dual-passivation strategy to repair defects in the grain boundaries and surfaces of CsPbI2Br perovskite solar cells. The improved CsPbI2Br films showed reduced lattice strains and enlarged grains after the dual-passivation. The strategy significantly improved the photovoltaic performance by suppressing charge recombination and enhancing hole transport ability.