Theoretical study of the influence of fluorination/chlorination on the electronic properties of donor:acceptor complexes

Abstract

In this study, the electronic properties of J50:N2200 (benzodithiophene-alt-benzotriazole: NDI-bithiophene) interface before and after fluorination/chlorination were investigated based on the first-principles density functional theory (DFT). The results reveal that the donor (D) and acceptor (A) molecules exhibit direct band gap whether to be fluorinated/chlorinated or not, and the six D:A pairs constructed all display indirect band gap. Next, for the fluorinated/chlorinated D molecule J50, the slope of total density of states (TDOS) curve edge at the highest occupied molecular orbital (HOMO) energy level enlarges, indicating high electron locality; the fluorination/chlorination of the A molecule N2200 reduces the slope of the TDOS at the HOMO level, and the electron delocalization strengthens. Then, the difference ΔE1 of the lowest unoccupied molecular orbital (LUMO) levels between D and A, the difference ΔE2 of HOMO levels between D and A, and the difference ΔE3 between the HOMO level of the D and the LUMO level of the A were calculated about the D:A complexes. The consequences present that by using fluorine/chlorine (F/Cl) substitution at J50, ΔE1 and ΔE2 both decrease, and ΔE3 increases; for N2200, both ΔE1 and ΔE2 increase, and ΔE3 decreases. Since the higher open circuit voltage (V OC) is directly proportional to ΔE3, again ΔE1 and ΔE2 afford the driving force for charge transport, these expose that the fluorination/chlorination of J50 is beneficial to obtain the higher V OC, meanwhile, the F/Cl replacement in N2200 facilitates the separation of excitons. In addition, by the Bader charge analysis, the F/Cl substitution at D in D:A blends will promote the intramolecular charge transfer and enhance the molecular polarity; moreover, the substitution at A will improve the intermolecular charge transfer and the dipole electric field may be enhanced. Finally, the details also depend on the type of element and the position of substitution.