Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C60(CN)2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C60(CN)2) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C60. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C60(CN)2 thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV–vis absorption spectra of C60(CN)2 thin films spin coated on quartz substrates, the optical band gap (Eg) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage (Voc) measurements on P3HT/C60(CN)2 based organic solar cell device are compared to the commonly used P3HT/PCBM device.