Efficient Metal Sulfide Window Layers for Organic Photovoltaics

The n-type window layer is a critical component of thin-film and organic photovoltaics (OPVs) that serves to 1) eliminate Shottkey barriers that impede efficient charge collection, 2) block excitons from becoming quenched at the cathode, and 3) protect the active layers of the device during cathode deposition and environmental exposure that can prolong the lifetime. While bathocuproine (BCP) has been widely utilized, it suffers from low conductivity that limits the  optimal thickness and a low glass transition temperature which can result in poor stability. In this work we demonstrate the structural and electronic control of cadmium-free, thermally evaporated, metal sulfide layers in OPV devices. We show through x-ray diffraction, optical spectroscopy, and device characterization that metal sulfide window layers offer improved power conversion efficiency and yields over equivalent transparent and opaque OPVs with standard BCP layers. This demonstration adds to the short catalog of high performance cadmium-free options for n-type window layers that could also prove useful in a range of other thin film photovoltaics.