Density-Functional Theory (DFT) and Time-Dependent DFT Study of the Chemical and Physical Origins of Key Photoproperties of End-Group Derivatives of the Nonfullerene Bulk Heterojunction Organic Solar Cell Acceptor Molecule IDIC

TL;DR

This study uses DFT and TD-DFT to analyze how chemical modifications in nonfullerene acceptors derived from IDIC-4Cl influence their electronic properties and photovoltaic performance, proposing a new predictive Scharber plot model.

Contribution

It introduces a novel approach combining DFT calculations with a new Scharber plot to predict the efficiency of modified NFAs in organic solar cells.

Findings

The candidate molecule with the least good acceptor A is predicted to outperform IDIC-4Cl.

Modified NFAs show changes in frontier orbital energies and absorption spectra due to chemical modifications.

A new Scharber plot model better predicts PCE based on acceptor properties.

Abstract

As emphasized in a recent review article [Chem. Rev. 122, 14180 (2022)], 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:PC61BM solar cells then went through different stages to shift lately interest towards nonfullerene acceptors (NFAs) as a replacement of PC61BM acceptor (ACC) molecule. Here, we use density-functional theory (DFT) and time-dependent (TD) DFT to investigate four novel NFAs of A-D-A (acceptor-donor-acceptor) form derived from the recently synthesized IDIC-4Cl [Dyes and Pigments 166, 196 (2019)]. Our level of theory is carefully evaluted 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 (FMO) 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.