Density-functional theory (DFT) and time-dependent DFT study of the chemical and physical origins of key photoproperties of end-group derivatives of a nonfullerene acceptor molecule for bulk heterojunction organic solar cells

Abstract

As emphasized in a recent review article (Chem. Rev. 2022, 122, 14180), organic solar cell (OSC) photoconversion efficiency has been rapidly evolving with results increasingly comparable to those of traditional inorganic solar cells. Historically, OSC performance improvement focused first on the morphology of P3HT: ${\text{PC}}_{61}$ BM solar cells then went through different stages to shift lately interest towards nonfullerene acceptors (NFAs) as a replacement of ${\text{PC}}_{61}$ BM acceptor (ACC) molecule. Here, we use density-functional theory (DFT) and time-dependent DFT to investigate four novel NFAs of A-D-A (acceptor-donor-acceptor) form derived from the recently synthesized IDIC-4Cl (Dyes Pigm. 2019, 166, 196). Our level of theory is carefully evaluated for IDIC-4Cl and then applied to the four novel NFAs in order to understand how chemical modifications lead to physical changes in cyclic voltammetry (CV) frontier molecular orbital energies and absorption spectra in solution. Finally we design and apply a new type of Scharber plot for NFAs based upon some simple but we think reasonable assumptions. Unlike the original Scharber plots where a larger DON band gap favors a larger PCE, our modified Scharber plot reflects the fact that a smaller ACC band gap may favor PCE by filling in gaps in the DON acceptor spectrum. We predict that only the candidate molecule with the least good acceptor A, with the highest frontier molecular orbital energies, and one of the larger CV lowest unoccupied molecular orbital (LUMO) $-$ highest unoccupied molecular orbital (HOMO) gaps, will yield a PM6:ACC PCE exceeding that of the parent IDIC-4Cl ACC. This candidate also shows the largest oscillator strength for the primary ${}^1$ (HOMO, LUMO) charge- transfer transition and the largest degree of delocalization of charge transfer of any of the ACC molecules investigated here.

Keywords: density-functional theory; nonfullerene acceptors; organic solar cell; time-dependent density-functional theory; transition density matrix.