Electronic structure of fullerene derivatives in organic photovoltaics

Abstract

The electronic structures of the fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), [6,6]-diphenyl C62 bis (butyric acid methyl ester) (bisPCBM), C70, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM), [6,6]-phenyl-C61-butyric acid butyl ester (PCBB), [6,6]-phenyl-C61-butyric acid octyl ester (PCBO), [6,6]-thienyl-C61-butyric acid methyl ester (TCBM), and indene-C60 bisadduct (ICBA), which are frequently used as n-type materials in organic photovoltaics, were studied by ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy. We also performed molecular orbital calculation based on density functional theory to understand the experimental results. The electronic structures near the energy gap of the compounds were found to be governed predominately by the fullerene backbone. The side chains also affected the electronic structures of the compounds. The ionization energy and electron affinity were strongly affected by the number of carbons and functional groups in the side chain.