Cationic and Anionic Vacancy Healing for Suppressed Halide Exchange and Phase Segregation in Perovskite Solar Cells

All-inorganic wide-bandgap (WBG) perovskite solar cells are best suited as the top cells for tandem devices. However, they suffer from photoinduced halide segregation (PIHS) and a quick anion exchange reaction (AER). Herein, polyvinylpyrrolidone (PVP) polymer-assisted in situ crystalization of CsPbBr3 and CsPbBr1.5I1.5 compounds is shown to suppress these effects by passivation of both positively charged (halide vacancy) and negatively charged (cation vacancy) defects. Modifying the perovskite precursor solution with this polymer improved the chemical stability and photostability in both nanocrystal solution and solution-processed films. Ultrafast transient absorption measurements elucidate the excited state dynamics of PVP-modified perovskite and probe an improved lifetime of charge carriers. As a proof of concept, PVP-modified CsPbI1.5Br1.5 retained nearly 98% Voc and 90% PCE under 1 sun AM 1.5G illumination for 12 h, while the control devices lost 25% of their Voc within 6 h. Using simple polymer additives to protect WBG perovskites is a huge step toward new device architectures involving all-perovskite heterojunctions.