Electron Transport Layers Based on Oligo(ethylene glycol)-Incorporated Polymers Enabling Reproducible Fabrication of High-Performance Organic Solar Cells

Interlayers in organic solar cells (OSCs) play a crucial role in determining their charge extraction properties, device performance, and stability. In this work, two naphthalene diimide (NDI)-based n-type polymers (P(NDIDEG-T) and P(NDITEG-T)) incorporating oligo(ethylene glycol) (OEG) side chains of different lengths are designed and used as new electron transport layers (ETLs) in OSCs. The hydrophilic OEG side chains enable the processing of these n-type polymers using eco-friendly water/ethanol mixtures. In addition, the polar OEG groups effectively modify the work function of the Ag cathode, enabling the polymers to function as efficient ETLs. Consequently, when these OEG-based ETLs are applied to PM6:Y6-based OSCs, a maximum power conversion efficiency (PCE) of 15.43% is achieved, which is substantially higher than that of a reference OSC without an ETL (9.93%) and comparable to that of an OSC with a representative high-performance PFN-Br ETL (15.34%). We demonstrate that the P(NDIDEG-T)-based OSCs show both enhanced PCEs and better reproducibility than their P(NDITEG-T)-based counterparts, which are attributed to the lower surface tension and improved film uniformity of the P(NDIDEG-T) ETL. Also, the P(NDIDEG-T) ETL realizes OSCs with higher storage stability and more thickness-tolerant performance compared to the P(NDITEG-T) and PFN-Br ETLs. Our study provides useful guidelines for the design of ETLs suitable for the fabrication of high-performance, reproducible, and stable OSCs.