One-step solution-processed low surface roughness silver nanowire composite transparent electrode for efficient flexible indium tin oxide-free polymer solar cells

We present a kind of all-solution-processed transparent electrode comprising of the mixture of silver nanowires (AgNWs) and ZnO nanoparticles modified with polyethylenimine ethoxylated (PEIE) solution by one-step doctor-blade-coating (DBC) processing. The AgNWs-ZnO-PEIE (AZP) composite films prepared with DBC exhibit highly transparent and conductive properties as well as low surface roughness. The optimized AZP film is highly transparent in the wavelength range of 350-1000 nm and the sheet resistance can be as low as 22-25 Omega/square with transmittance over 91% at 550 nm. A small content of PEIE can effectively prevent two kinds of nano components, AgNWs and ZnO nanoparticles, from agglomeration in solution and aggregation during solution processing, and therefore results in a very smooth and transparent AZP film without sacrificing its conductivity. Since the AgNWs are well embedded within the electron-transporting ZnO matrix, the prepared AZP film can serve as a dual function of both the transparent electrode and electron-transporting layer. The flexible PSCs based on the one-step prepared AZP electrode without an electron-transporting layer attained a power conversion efficiency (PCE) of 8.31%. A comparison study on the commercial indium tin oxide (ITO)/polyethylene terephthalate (PET) and AZP/PET-based flexible PSCs with the same device structure shows quite similar PCE (9.14% and 8.94%), but the polymer solar cells fabricated on the AZP/PET substrate show more excellent flexibility than the ITO/PET-based PSCs.