Introducing an Organic Hole Transporting Material as a Bilayer to Improve the Efficiency and Stability of Perovskite Solar Cells

Although perovskite solar cells (PSCs) have achieved high power conversion efficiency (PCE) by utilizing 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-Spirobifluorene (Spiro-OMeTAD) as hole transporting material (HTM), the reproducibility and stability of PSCs are still a pressing concern. Herein, we introduced a solvent processed organic-organic bilayer based on 2-(4-(7-(9,9-dimethylacridin-10(9H)-yl)-9,9-diethyl-9H-fluoren-2-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (AFpPPI) and Spiro-OMeTAD in layer to layer as HTM in PSCs. The devices configured with AFpPPI/Spiro-OMeTAD bilayer achieved a maximum PCE of 19.9% in mesoporous-TiO2 (mp-TiO2) structure with perovskite absorber of Cs0.05Rb0.05(FAPbI3)0.76 (MAPbBr3)0.14. The properties of the bilayer structure were analyzed with steady-state photoluminescence, ultra-violet photoelectron and impedance spectroscopy. The AFpPPI/Spiro-OMeTAD improved open-circuit voltage (Voc) by lowering the quasi-Fermi energy level for holes and reducing the charge recombination, resulting in high Voc (1.14 V in the champion cell) and high fill factor (FF) that lead to high PCE. The addition of AFpPPI layer improves the quality of Spiro-OMeTAD and provides pinhole-free film. Moreover, the stability is improved in controlled temperature and humid conditions. This work affords a new approach for commercial applications of PSCs with better stability.