Interfacial ZnO Modification Using a Carboxylic Acid Functionalized N-Annulated Perylene Diimide for Inverted Type Organic Photovoltaics

This study reports the modification of the zinc oxide cathodic interlayer with an N-annulated perylene diimide molecule functionalized with a carboxylic acid, namely, (HOOC5-triazole) PDIN-hex, referred to as IL-1, for the improvement of inverted-type organic photovoltaics. The new interlayer "IL-1" was synthesized using "click" chemistry and incorporated into the photovoltaic device structure using solution processing methods. The organic modified zinc oxide layer was characterized using optical absorption spectroscopy, contact angle measurements, and Kelvin probe force microscopy. IL-1 was retained on the zinc oxide surface and lowered the electrode work function from 4.5 to 4.1 eV, the low work function is crucial for any cathodic interlayer to match with the LUMO energy levels of acceptor materials for better charge extraction and electron collection properties. Bulk heterojunction organic photovoltaic devices utilizing this interlayer (compared to only zinc oxide) exhibited increases in power conversion efficiency ranging from 20% to 33% for those with active layer blends based on both fullerene and non-fullerene acceptors.