Inverted Nonfullerene Polymer Solar Cells with Photoannealed Cs2CO3 Films as Electron Extraction Layers

This paper successfully demonstrates that solution-processable cesium carbonate (Cs2CO3) and photoannealing treatments can be used to replace thermal annealing treatments. The power conversion efficiency (PCE) is quite low in the initial stage, while the PCE dramatically increases following continuous light illumination and reaches a saturated state after 15 min of light illumination. Raman spectra and X-ray photoelectron spectroscopy (XPS) are then used to show that the Cs2CO3 molecule decomposes and transforms into a mixture of cesium oxide (CsOX), which is the major constituent of cesium oxide (Cs2O), and small amounts of cesium dioxide (Cs2O2) and cesium oxide (CsO2). In addition, following the spin coating of a fresh Cs2CO3 solution onto the indium tin oxide surface and 15 min of photoannealing, polymer solar cells show improved photovoltaic performance compared with the use of thermally annealed Cs2CO3 and ZnO films as the electron extraction layer.