Surface segregation at the aluminum interface of poly(3-hexylthiophene)/fullerene solar cells

The effects of thermal annealing before and after Al deposition on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C(61) butyric acid methyl ester (PCBM) blend solar cells were investigated by current density-voltage measurements and x-ray photoelectron spectroscopy (XPS). Compared to the preannealed device, the postannealed device exhibited enhanced open-circuit voltage (V(OC)), which is ascribed to the decrease in the reverse saturation current density J(0). The XPS measurements demonstrated that P3HT is dominant at the Al interface in the preannealed device while PCBM is instead dominant in the postannealed device. This surface-segregated PCBM formed in the postannealed device can serve as a hole-blocking layer at the Al interface to reduce J(0), and therefore improve V(OC).