Theoretical Study of Core Excitations of Fullerene-Based Polymer Solar Cell Acceptors

Abstract

First-principles simulations for the C K-edge X-ray photoelectron (XPS) and near-edge X-ray fine-structure (NEXAFS) spectra of representative polymer solar cell acceptor (PSCA) molecules have been carried out with special focuses on the [6,6]-phenyl-C n +1 -butyric acid methyl ester (PC n BM, n = 60, 70, 84). In the XPS spectra, evident red shift of core binding energies are observed in PC n BM as compared to the corresponding fullerene C n, which is due to weak electronic charge transfer from the side chain to the fullerene backbone and consequently increased electron screening. Special emphasis is paid to a spectral peak at ca . 284.7 eV, which is caused by resonances of phenyl-ring carbons in the side chain and is important in characterizing the electronic structures of different PSCA molecules. Additionally, we propose a modified version of the building block (MBB) approach for quick estimation of the NEXAFS spectra, which is useful when computational resources are limited or massive systems are under study. This method is based on our component analysis on the side chain and fullerene backbone parts and adds only a further empirical fine-tuning after the conventional BB approach. Our calculated results are in good agreement with available experiments.