High Mobility Indium Oxide Electron Transport Layer for an Efficient Charge Extraction and Optimized Nanomorphology in Organic Photovoltaics

The electron transport layer (ETL) plays an important role in determining the device efficiency of organic solar cells (OSCs). A rational design of an ETL for OSCs targets high charge extraction and induction of an optimized active layer morphology. In this Letter, a high mobility In₂O₃ synthesized via a solution-processed combustion reaction is successfully used as a universal ETL in an organic photovoltaic device. With the modification of a thin layer of polyethylenimine ethoxylated (PEIE), a device based on crystalline In₂O₃ outperforms its counterpart, ZnO, in both PBDTTT-EFT-based fullerene and nonfullerene systems. As ZnO is replaced by In₂O₃, the average efficiency increases from 9.5% to 10.5% for PBDTTT-EFT-PC₇₁BM fullerene-based organic solar cells and also increases from 10.8% to 11.5% for PBDTTT-EFT-IEICO-4F nonfullerene-based organic solar cells, respectively. Morphological studies have unraveled the fact that the crystalline In₂O₃ ETL with highly aligned nanocrystallites has induced the crystallization of polymer into a preferential molecular packing that favors the charge transport across an active layer. From the photophysical study, it is found that charge extraction in the crystalline In₂O₃ device is significantly faster than in the ZnO device due to the higher mobility of In₂O₃ and optimized nanomorphology of the active layer.