Reducing trap densities of perovskite films by the addition of hypoxanthine for high-performance and stable perovskite solar cells

Perovskite solar cells (Pero-SCs) have developed rapidly in the past decade and have demonstrated a power conversion efficiency (PCE) comparable to that of silicon-based solar cells quite recently; hence, they have shown great potential in commercialization. Solution processing of perovskite films is one of the advantages of Pero-SCs; however, uncontrollable quick crystallization through post-thermal annealing also brings voids and defects in and between the crystal grains, resulting in recombination traps of photoinduced excitons. To solve this problem, hypoxanthine was employed as an additive in the perovskite precursor to assist the preparation of perovskite films. The quality of the perovskite films was greatly improved in terms of defect passivation, crystal grain enlargement and alignment. The film improvement should be attributed to two effects caused by hypoxanthine: 1) hypoxanthine can slow down the crystallization process and thus is responsible for the enlarged and aligned crystal grains; 2) due to the nonvolatile nature, hypoxanthine stays in the film and passivates the uncoordinated Pb(II) defects. Consequently, planar n-i-p solar cells based on (FAPbI3)1-x(MAPbBr3)x presented a champion PCE of 22.14% and maintained over 95% of the initial PCE after being stored in a glovebox for 1500 h.