Surface-modification-induced synergies of crystal growth and defect passivation toward CsPbI2Br solar cells with efficiency exceeding 17%

Abstract

CsPbI₂Br material is considered ideal for both single and multi-junction solar cells due to its outstanding stability against heat and illumination and suitable band gap. Unfortunately, CsPbI₂Br perovskite solar cells (PSCs) still bear the severe energy loss owing to the inferior perovskite crystal quality and serious non-radiative recombination, impeding the further improvement of efficiency. Herein, the synergies of potassium trifluoroacetate on the treatment of δ phase CsPbI₂Br film, in which it not only passivated surface defects but also simultaneously benefited to the secondary growth of crystal during δ → α phase transition, were proposed. It was found that the optimized CsPbI₂Br film exhibited the enlarged crystal grains, preferable orientation and higher crystallinity. Meanwhile, the carboxyl groups within potassium trifluoroacetate could effectively bind to the under-coordinated Pb²⁺ ions on perovskite surface, thus improving the carrier interface dynamics. As a result, the assembled CsPbI₂Br PSCs reached a champion power conversion efficiency of 17.1 % (steady-state efficiency of 16.11 %) and an open-circuit voltage of 1.382 V, which are among the highest values for CsPbI₂Br PSCs based on dopant-free poly(3-hexylthiophene) (P3HT) as hole transporting layer. More importantly, the synergetic strategy enabled the outstanding thermal stability by retaining 81 % of their initial efficiency after annealing at 85 °C in N₂ atmosphere for 800 h.