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

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 recombi-nation, impeding the further improvement of efficiency. Herein, the synergies of potassium trifluoroacetate on the treatment of delta phase CsPbI2Br film, in which it not only passivated surface defects but also simultaneously benefited to the secondary growth of crystal during delta -> alpha 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 degrees C in N2 atmosphere for 800 h.