Semitransparent organic solar cells based on low temperature processed PEIE as electron transport layer with enhanced charge transfer ability

Abstract

In inverted structure-based semitransparent organic solar cells (OSCs), the electron transport layer (ETL) plays a crucial role in the improvement of the transparent cathode efficiency in collecting and extracting negative charge carriers. Zinc oxide (ZnO) thin film prepared by zinc acetate dihydrate precursor with various benefits is generally used as ETL. However, high temperature, less charge transfer ability, and irregular film surface due to fiber-like domain formation limit the device performance. In this work, a new approach is presented by using low-temperature processed polyethylenimine ethoxylated (PEIE) as ETL in semitransparent OSCs fabricated in an ambient environment with a blend of low-bandgap donor polymer PTB7-Th, and fullerene acceptor, PC71BM, based active layer. For semitransparent OSCs, the thickness of the silver electrode has been varied from 55 nm to 25 nm to investigate its effect on the electrical and optical properties of the devices. The power conversion efficiencies (PCE) of 5.1% and 4.6% were achieved for semitransparent devices (25 nm thickness of silver electrode) for PEIE and ZnO ETLs, respectively. Similarly, PCE of 7% and 6.7% have been achieved for opaque devices (85 nm thickness of silver electrode) using PEIE and ZnO ETLs, respectively. PEIE based devices with 25 nm Ag demonstrate about 25-30% transparency. The impedance spectroscopy measurements indicate low interfacial contact resistance and fast charge transfer capability for PEIE interlayer-based devices compared to the ZnO based devices. The encapsulated semitransparent devices processed and stored in ambient condition with PEIE and ZnO ETLs were found to retain ∽80% performance for up to 45 days.