Inverted organic solar cells comprising a solution-processed cesium fluoride interlayer

We investigate the influence of solution-processed cesium fluoride (CsF) interlayers on the performance of inverted polymer solar cells comprising a blend of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61-butyric acid methyl ester. The thickness of the CsF layer is optimized in terms of current-voltage characteristics by a variation of the solid content in solution. Capacitance-voltage characteristics reveal a shift of the built-in voltage at the cathode interface by about 0.3 V as compared to devices without a CsF layer, giving rise to an increase in open-circuit voltage by the same value. The vertical distribution of Cs+ and F+ ions is studied by secondary ion mass spectroscopy, indicating a strong diffusion of the alkaline fluoride into the organic layer stack.