A theoretical study on elementary building blocks for organic solar cells – Influence of a donor molecule on electronic spectrum of PCBM

Modifications in the electronic spectrum of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) caused by noncovalent interactions with smaller molecules, like benzene, thiophene, pyridine, methanimine, and porphyrin were studied by means of time-dependent density functional theory (TD-DFT) with the CAM-B3LYP functional. The calculated excited states for the complex with the first four molecules have mostly local character within the PCBM unit, only slightly perturbed by the second molecule, e.g. red shift of the spectrum with respect to this of a free PCBM is observed. The states with a predominantly intermolecular charge-transfer character are absent within the first 20 states of complexes with PCBM, with the exception of the complexes with porphyrin, where charge-transfer states and states delocalized on both molecules were observed. In addition to this, the lowest excited states of the recently synthetized 25Th-cardo molecule, its dimer and trimer, as well as its complex with PCBM were calculated and analyzed. This complex has been recently studied experimentally by Iwan et al. (2013) as a potential candidate for a donor–acceptor system in the solar cells. It has been found that in this case some higher excited states possess a partial charge-transfer character from 25Th-cardo to PCBM. The reliability of CAM-B3LYP for complexes of two molecules with aromatic rings or conjugated double bonds has been examined on a smaller example of pyrene interacting with benzene by comparing of the TD-DFT results with those of CIS, local CC2, and local ADC(2) methods.