PCBM derivatives and copolymer blends for organic photovoltaic devices: Computational modeling and analysis

Organic Photo Voltaic (OPV) devices have attracted much attention due to their profitable electronic and mechanical properties. However, the synthesis of high efficiency mixtures remains a challenge. Several calculations have been performed in order to increase the understanding of how the DFTB technique performs in modeling OPV components: Systems based on mixtures of a low band gap polymer, -in this research- poly(4,4-dihexyldithieno(3,2b:2’,3’-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl), (PSBTBT-6), and both [6,6]-phenyl C61 - butyric acid methyl ester, (PCBM), and its derivate [6,6]-phenyl-C61 -butyric acid (2-N,N-dimethylaniline-5-nitro)benzyl ester, (PCBPP). In order to successfully equilibrate and get relevant configurations of the systems, molecular dynamic (MD) simulations are used. DFTB quantum mechanical theory was embraced to perform the MD and the subsequent solid state calculations. From the former, it was possible to select a broad range of snapshots to statistically investigate some electronic properties (i.e., Density Of States (DOS), charge differences, molecular orbitals). The presented results suggest that SCC-DFTB is a computationally feasible and reliable method for modeling structure evolution and the properties of such a systems, comparable in accuracy with medium size basis set DFT calculations at the computational cost of standard semi-empirical methods. In conclusion, the presented work-flow will serve to generate with high accuracy initial structures for subsequent calculations. These will evaluate further how likely the substitution constituents would lead to an increase of charge transfer from the copolymer to the fullerene derivative.