Molecular doping mechanism for efficient and stable formamidine-cesium perovskite solar cells

The high efficiency and stability of perovskite solar cells (PSCs) are the key factors to ensure their industrialization. Surface doping of perovskite films has been proved as an efficient strategy to achieve more efficient and stable PSCs. Plenty of work has been focused on passivating perovskite films by organic molecules and studying the corresponding passivation mechanism. However, very few studies have paid attention to the effect of surface dopants on the crystallization process during the formation of perovskite films. Herein, we employ a simple organic molecule phenylphosphonic acid (PPA) as a surface doping agent to simultaneously improve the efficiency and stability of FA0.85Cs0.15PbI3 PSCs and focus on the effect of PPA on the crystallization process of perovskite films. The PPA molecules can slow down the crystal nucleation and growth rate of perovskites and inhibit the formation of δ-FAPbI3 phase to obtain α-phase perovskite films with high quality, due to the multifunction of PPA as crystal growth controller and passivator of perovskite films. Finally, the PCE of FA0.85Cs0.15PbI3 PSC with an effective area of 0.255 cm2 increased from 18.95% to 22.02%, which is among one of the highest efficiencies of FA/CsPbI3 PSCs with such big active area. Meanwhile, air environmental stability and thermal stability of the devices were also improved significantly. In addition, the 5 × 5 cm-module device with an effective area of 19.32 cm2 obtained a high PCE of 19.12%. This surface doping strategy has obtained great benefits from multifunctional small molecules and laid a promising seed for obtaining stable and efficient FACs-based PSCs.