Twist and Degrade—Impact of Molecular Structure on the Photostability of Nonfullerene Acceptors and Their Photovoltaic Blends

Abstract

Nonfullerene acceptors (NFAs) dominate organic photovoltaic (OPV) research due to their promising efficiencies and stabilities. However, there is very little investigation into the molecular processes of degradation, which is critical to guiding design of novel NFAs for long‐lived, commercially viable OPVs. Here, the important role of molecular structure and conformation in NFA photostability in air is investigated by comparing structurally similar but conformationally different promising NFAs: planar O‐IDTBR and nonplanar O‐IDFBR. A three‐phase degradation process is identified: i) initial photoinduced conformational change (i.e., torsion about the core–benzothiadiazole dihedral), induced by noncovalent interactions with environmental molecules, ii) followed by photo‐oxidation and fragmentation, leading to chromophore bleaching and degradation product formation, and iii) finally complete chromophore bleaching. Initial conformational change is a critical prerequisite for further degradation, providing fundamental understanding of the relative stability of IDTBR and IDFBR, where the already twisted IDFBR is more prone to degradation. When blended with the donor polymer poly(3‐hexylthiophene), both NFAs exhibit improved photostability while the photostability of the polymer itself is significantly reduced by the more miscible twisted NFA. The findings elucidate the important role of NFA molecular structure in photostability of OPV systems, and provide vital insights into molecular design rules for intrinsically photostable NFAs.

A highly sensitive molecular structural probe applied to in situ biosensing of metabolites using PEDOT:PSS

“…From the absorption spectra, we determine the laser excitation wavelength required for resonance Raman spectroscopy measurements (Kim, Ho, Murphy, Baynes, & Friend, ; Limbu et al, ; Luke et al, ; Soltzberg et al, ). At 633 nm, the neutral band can be resonantly excited and this would provide more sensitive structural characterization to the changes in the dedoped PEDOT.…”

“…From the absorption spectra, we determine the laser excitation wavelength required for resonance Raman spectroscopy measurements (Kim, Ho, Murphy, Baynes, & Friend, 2002;Limbu et al, 2019;Luke et al, 2019;Soltzberg et al, 2006). At 633 nm, the neutral band TAN ET AL.…”

Progress in Stability of Organic Solar Cells

“…It is worth to mention that the photo‐degradation of fullerene acceptors can also cause the formation of persistent free radicals, which can accumulate to decrease the device performance from another pathway. [ 52 ] In addition to the photo‐oxidation of active layer, carrier transport layers also play a non‐negligible role in the photo‐degradation process. Carrier transport layers can take part in the photo‐oxidation process of the active layer in some cases.…”

Exceptionally low charge trapping enables highly efficient organic bulk heterojunction solar cells

“…We have previously shown how conformational changes can lead to a different energetic landscape in blends and molecular photostability in NFAs. 65 Here we implemented similar in situ accelerated photo-irradiation studies in N 2 to test the effect of conformational rigidity of the NFAs on photostability (see Fig. S12, ESI †).…”