Incorporating formamidinium into 4-fluoro-phenethylammonium based quasi-2D perovskite films and their application in n-i-p perovskite solar cells

Recently, perovskite solar cells (PSCs) basing on quasi-two-dimensional (quasi-2D) perovskites with hydrophobic bulky organic cations have received a lot of interests owing to their superior moisture stability. While quasi-2D perovskites have achieved great success in planar p-i-n PSCs, their application in planar n-i-p devices and the corresponding efficiencies lag far behind. In our work, (FPEA)2(MA)4Pb5I16 perovskite films with high crystal-linity, compact and smooth surface were deposited on electron transport layer with NH4SCN as an additive. Formamidinium (FA+) were then incorporated into the FPEA based perovskite films to partially replace meth-ylammonium (MA+), and effects of FA+ on the film's vertical orientation, light absorption capacity, carrier lifetime, defect density and electron mobility were studied. It was found that when molar ratio of FA/MA = 0.5/ 9.5, the film had the highest vertical orientation, enhanced light absorption, less carrier recombination loss. The obtained (FPEA)2(FA0.05MA0.95)4Pb5I16 based n-i-p PSCs gave a maximum efficiency of 12.17% under standard illumination, retaining 83% of the initial efficiency after stored in air environment (25-30 degrees C) with 40 +/- 5% RH for 1080 h. The results demonstrate an efficient approach to prepare high-quality quasi-2D perovskite films in a n-i-p structure as well as improved device efficiency and stability.