Toward the rational design of organic solar photovoltaics: A DFT study of substitutent effects on P3MT*

Organic polymers containing conjugated thiophene rings are among the candidates for the electron-donor materials in organic solar cells. Since material synthesis, device fabrication, and definitive characterization of those devices is tedious and expensive, it is desirable to apply computational methods to a systematic variation of the material chemistry to predict which materials will have the best properties and, potentially, which will yield the highest photon conversion efficiency. Reported here is a model study in which DFT calculations of poly-3-methyl thiophene (P3MT) with a range of 12 different substituents. For each candidate polymer, DFT calculations at the B3LYP-D2/6-31G(d,p) level were extrapolated to the long-chain limit. The following properties relevant to application in a photovoltaic device were estimated from the calculations: (i) the bandgap, (ii) the LUMO energy, and (iii) the steric hindrance to coplanarity of adjacent rings. While (ii) was found to be well correlated with the electron-donating property of the various substituents, (i) and (iii) were much less so indicating a design space in which these three critical properties could, to some extent, be varied independently.