A solution-processed n-doped fullerene cathode interfacial layer for efficient and stable large-area perovskite solar cells

A novel solution-processed cetyltrimethylammonium bromide (CTAB)-doped [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) film prepared by an extremely facile method is demonstrated as an effective cathode interfacial layer for perovskite solar cells (PSCs). Our results indicate that efficient doping takes place via anion-induced electron transfer between the bromide anions (Br^-) on CTAB and PC₆₁BM in the solid state, leading to a dramatic increase in electrical conductivity by more than five orders of magnitude. In addition, the CTAB-doped PC₆₁BM layer is capable of turning a more air-stable, high work-function (WF) Ag layer into an efficient low WF electrode as a result of the formation of favorable interfacial dipoles between Ag and the active layer. These characteristics enable the CTAB-doped PC₆₁BM layer to function as both an electron transport layer and a cathode buffer layer (CBL) in PSCs, thus simplifying the manufacturing process. This doped layer also exerts multi-positive effects for use in PSCs, including efficient interfacial charge transfer ability, superior charge selectivity, good film coverage on the perovskite layer, relatively weak thickness-dependent performance properties, general applicability to different perovskite materials, and good ambient stability. With this n-doped PC₆₁BM layer, the device delivers a high power conversion efficiency (PCE) up to 17.11%, which is superior to those of the devices with undoped PC₆₁BM layers (2.15%) and state-of-the-art CBL ZnO nanoparticles (10.45%). The application of the CTAB-doped PC₆₁BM layer in large-area solar cells (active area = 1.2 cm²) is also demonstrated, and a remarkable PCE of 15.42% is achieved, which represents one of the highest PCE values for PSCs with a similar active area. More significantly, the resulting devices possess good ambient stability without the need for rigorous encapsulation.