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

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

Strategical Designing of Donor–Acceptor–Donor Based Organic Molecules for Tuning Their Linear Optical Properties

Low-energy linear absorption spectrum of a series of 48 donor–acceptor–donor (D–A–D) scheme based thiophone–benzo­(bis-)­X-diazole molecules with X = O, S, Se, or Te are calculated using time dependent density functional theory in order to propose strategical design of molecules that can efficiently absorb light in the infrared and visible region of the solar spectrum. Our study establishes that optical properties of the D–A–D based organic molecules significantly depend on the donor-to-acceptor (D/A) ratio and the strength of the acceptor moiety. Thus, by choice of a suitable D/A ratio and type of the acceptor moiety, the linear absorption spectrum can be largely shifted, in general, while the optical gap can be engineered over a wide energy range of ∼0.2–2.3 eV, in particular. It is also noticed that the increase in acceptor units (i.e., when D/A ≤ 1) leads to increase in steric hindrance in between them. This, in turn, disrupts the effective conjugation length and increases the optical gap. However, this effect is found to dominate strongly in the bis-configurations of the molecules as compared to the nonbis compositions. In order to reduce this effect for rational designing of effective D–A–D type chromophores with less steric hindrance, the role of π-conjugated ethylene (−CHCH−) linkage/spacer between the A–A units is explored further. Here, it is found that introduction of such linkage substantially decreases the steric hindrance and, thereby, the optical gap as well. Besides this, our study also highlights and explains the impact of the acceptor moiety in improving the absorption capabilities of these molecules in the low-energy region

Molecular-Shape-Induced Efficiency Enhancement in PC₆₁BM and PC₇₁BM Based Ternary Blend Organic Solar Cells

Evolution of a self-organized molecular packing at the microscopic scale in a bulk heterojunction active layer is critical for improving the performance of organic solar cells. We demonstrate that the molecular-shape-induced effects improve the morphology of the ternary thin films as PC₆₁BM molecules were added to the host binary blend, PTB7-Th:PC₇₁BM. Compared to the binary PTB7-Th:PC₇₁BM devices, the ternary devices with 20% PC₆₁BM content (relative to PC₇₁BM) exhibited an enhanced efficiency, from 6.4% to 8.5%. In particular, we find that the spherical PC₆₁BM molecules with better precipitation kinetics than ellipsoidal PC₇₁BM alter the morphology of ternary thin film and modulate interfaces to expedite charge transport and collection by reducing various recombination losses