Dependence of the Structure and Electronic Properties of D–A–D Based Molecules on the D/A Ratio and the Strength of the Acceptor Moiety

Abstract

A series of donor–acceptor–donor (D–A–D) scheme based organic molecules was studied to examine the dependence of molecular structure and electronic properties on the D/A ratio and the strength of the acceptor moiety, using first-principles density-functional-theory based calculations. Thiophenes were taken as the donor moiety and a series of benzo-X-diazoles and benzobis-X-diazoles (X = O, S, Se, and Te) were considered to account the strength of the acceptor moieties. The role of different exchange–correlation functionals was also investigated to search for the functional that best describes the properties of such D–A–D based molecules. Our systematic calculations reveal that both the D/A ratio and the strength of the acceptor moiety largely affect the energy gap between energies of the highest occupied molecular orbital (H) and the lowest unoccupied molecular orbital (L). In thiophene–benzo-X-diazole molecules, the H–L gap varies from 7% to 25%, whereas in thiophene–benzobis-X-diazoles, it can be tuned from 40% to 80%, by changing the D/A ratio from 0.5 to 4.0. In the latter case, higher steric hindrance (>50°) between A–A units disrupts the conjugation length with the increase in acceptor units. This leads to a monotonic decrease of the H–L gap with the increase in the D/A ratio, and a larger variation as compared to the case for thiophene–benzo-X-diazoles. On accounting for the effect of strength of the acceptor moiety, we observed that the H–L gap of the bis molecule was roughly 1 eV smaller than its respective non-bis configuration. A decrease in the H–L gap was also found on moving from S to Se to Te. Quantitatively, the H–L gap of the investigated molecules was found within a wide range of 0.2–2.4 eV, which not only is smaller than the H–L gap of isolated thiophene or the benzo-(bis)­X-diazole molecules but also lies in the desired range for the applications in optoelectronic devices, including solar cells. Thus, our study affirms that by choosing a suitable acceptor moiety and the D/A ratio, the structural and electronic properties of D–A–D based materials can be widely tuned. Through this work we emphasize the need to understand the tuning of molecular properties by examining the structure–property correlation, which is essential for rational design of high performing novel organic materials