Cesium usage in solar cells!Cesium layer improves performance of solar cells by 30%. Solar cells based on evaporated Cs2O/GaSe9 and Cs2O/Se (as comparison) active layers have been investigated. We correlate their performance with devices previously reported without Cs2O. GaSe9 appears as a very promising solar absorber because of its attractive optical and electrical properties. We improved device performance by around 30% by inserting a Cs2O thin interlayer.