Improved electron extraction by a ZnO nanoparticle interlayer for solution-processed polymer solar cells

Charge injection/extraction properties dictate the device characteristics of polymer solar cells. Based on a model system comprising PTB7-Th:PC₇₁BM blends, we investigate solvent effects and interface properties of ZnO nanoparticles used as electron-transporting/hole blocking interlayers (ETL/HBL) to improve the solar cell performance and stability. We find that the polarity of processing solvents and thickness of ZnO nanoparticles ETL both play a crucial role in the device behavior. Under the optimal conditions, the power conversion efficiency (PCE) is increased from 8.4% to 9.2% for PTB7-Th:PC₇₁BM solar cells with a conventional device architecture, which is among the highest efficiencies for this blend system. The simultaneous increase in short-circuit current (J_sc) and fill factor (FF) originates from an effective hole blocking mechanism provided by the ZnO layers which increase the selectivity of charge extraction and reduce non-geminate charge recombination. The dense and air-stable ZnO ETLs minimize the diffusion of moisture and O₂ into the photoactive layer, leading to drastically improved device stability.