Impermeable Charge Transport Layers Enable Aqueous Processing on Top of Perovskite Solar Cells

Several applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top-illumination or see-through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead-halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work-function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (approximate to 10(18) cm(-3)) in the ETL is required to achieve well-behaved J/V characteristics free of s-shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p-i-n perovskite solar cells with an AgNW top electrode.