One-Step Preparation of ZnO Electron Transport Layers Functionalized with Benzoic Acid Derivatives

ZnO is a common electron transport layer in organic solar cells that typically requires treatment with an additional layer to passivate surface traps to optimize the device performance. Here, we present a “one-step” approach to modifying ZnO electron transport layers (ETLs) used in organic solar cells. This approach involves adding benzoic acid (BZA) derivatives directly to the ZnO precursor solution, which are then present at the surface of the resulting ZnO film. We demonstrate this approach for three different BZA derivatives, namely, benzoic acid, 4-chlorobenzoic acid, and 4-hydrazinobenzoic acid. For all molecules, improved device performance and stability is demonstrated in solar cells using an active layer blend of PTQ10 (donor) and ITIC-Br (nonfullerene acceptor) compared to such cells prepared using untreated ZnO. Furthermore, similar or improved device performance and stability is demonstrated compared to conventional PEIE treatment of ZnO. The presence of the BZA derivatives at the surface after processing is established using X-ray photoelectron spectroscopy and near-edge X-ray absorption fine-structure spectroscopy. From atomic force microscopy analysis and X-ray diffraction studies, the addition of BZA derivatives appears to restrict ZnO grain growth; however, this does not negatively impact the device performance. ZnO layers treated with BZA derivatives also exhibit a higher water contact angle and lower work function compared to untreated ZnO. Improved device performance is attributed to reduced surface recombination at the ETL/active layer interface, while improved device stability is attributed to the increased hydrophobicity of the ETL. This approach enables simplification of device manufacture while still allowing for optimization of the surface properties of metal oxide ETLs.