Efficient polymer bulk heterojunction solar cells with cesium acetate as the cathode interfacial layer

The enhanced performance of polymer solar cells based on regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend was achieved by using cesium acetate (CH3COOCs) as cathode buffer layer. Under 100 mW/cm2 white light illumination, the device with 0.8 nm thick CH3COOCs as cathode buffer layer exhibits power conversion efficiency (PCE) as high as (4.16 ± 0.02) %. Compared to the control devices without cathode buffer layer and with LiF as cathode buffer layer, the PCE is enhanced ∼100% and ∼31%, respectively. The introduction of the CH3COOCs buffer layer effectively improves the photo-generated charge collection. The Kelvin Probe measurement shows that the work function of the CH3COOCs is estimated to be −4.0 eV, which has an ideal energy band match with PCBM and a good property for electron collection. The static contact angle results indicated that the CH3COOCs with the hydrophobic CH3COO- group has an improved wettability between the buffer layer and the hydrophobic organic active layer surface, resulting in better interfacial contact and reduced contact resistance. The improved performance may be attributed to the dissociation of semi-conducting CH3COOCs upon deposition to liberate Cs with a low work function, which reduces the interface resistance of the active layer and the cathode and enhances the interior electric field that may result in efficient charge transportation. Therefore, the CH3COOCs interlayer could be a promising alternative to LiF to improve the efficiency of the electron collection of polymer bulk heterojunction solar cells.