The Interplay of Stability between Donor and Acceptor Materials in a Fullerene‐Free Bulk Heterojunction Solar Cell Blend

Abstract

With rapid advances in material synthesis and device performance, the long‐term stability of organic solar cells has become the main remaining challenge toward commercialization. An investigation of photodegradation in blend films of the donor polymer poly(3‐hexylthiophene) (P3HT) and the rhodanine‐flanked small molecule acceptor 5,5′‐[(9,9‐dioctyl‐9H‐fluorene‐2,7‐diyl)bis(2,1,3‐benzothiadiazole‐7,4‐diylmethylidyne)]bis[3‐ethyl‐2‐thioxo‐4‐thiazolidinone] (FBR) is presented in an ambient atmosphere. The photobleaching kinetics of the pure materials and their blends is correlated with the generation of radicals and triplet excitons using optical and magnetic resonance techniques. In addition, spin‐trapping methods are employed to identify reactive oxygen species (ROS). In films of P3HT, FBR, and the P3HT:FBR blend, superoxide is generated by electron transfer to molecular oxygen. However, it is found that the generation of singlet oxygen by energy transfer from the FBR triplet state is responsible for the poor stability of FBR and for the accelerated photodegradation at later times of the P3HT:FBR blend. In the early stage of degradation of the neat blend, it is protected from singlet oxygen by the fast donor–acceptor charge transfer, which competes with triplet exciton formation. These results provide initial input for a rational design of donor and acceptor materials through tuning the molecular singlet and triplet energy levels to prevent ROS‐related photodegradation.

n-Type organic semiconducting polymers: stability limitations, design considerations and applications

“…40 Instabilities can also arise in the presence of photogenerated singlet oxygen, which can cause photobleaching of OSC materials. 48 Consideration should therefore be given to both the material design to lower the LUMO below À4.0 eV and improve delocalisation of the LUMO across the entire polymer backbone (through improved planarity and an acceptor-acceptor configuration), in addition to the device architecture and processing conditions to maximise charge transport.…”

“…47 More specifically, water is reduced at potentials lower than À0.658 V (À3.7 eV), and oxygen can undergo reduction to hydrogen peroxide by electron transfer from the excited OSC negative polaron to a dioxygen molecule at +0.024 V (À4.4 eV). 47,48 Oxygen can also undergo a four electron reduction at +0.571 V (À4.9 eV), so without taking overpotentials into account, for an n-type polymer to be stable to water, the doped (reduced) polymer should be oxidised at a potential higher than À0.658 V, furthermore for it to be stable to both oxygen and water, it needs to be oxidised higher than +0.571 V (Fig. 2).…”

“…This 1 O 2 is then able to undergo a 1,4 Diels-Alder addition in thiophene containing polymers, which leads to photobleaching. 48 The simple solution to this is to encapsulate the device, removing all light sources, however this type of degradation must be predicted and accounted for in order to prevent it. OSC stability can also be improved by considering the close packed distances between polymer chains.…”

Emerging Chemistry in Enhancing the Chemical and Photochemical Stabilities of Fused‐Ring Electron Acceptors in Organic Solar Cells

“…[ 35b ] Recently, the study of photooxidation of P3HT:FBR system ( Figure ) demonstrated that fast donor–acceptor charge transfer leaves a negligible amount of triplet excited states for the generation of 1 O 2 , but the mobile electrons of FBR can energetically transfer to O 2 , inducing a higher production rate of O 2 •− than that in the neat P3HT film. [ 32 ] Note that the acceptor with a shallow LUMO (lowest unoccupied molecular orbital) level would further facilitate the formation of O 2 •− . [ 36 ] Furthermore, it was also shown that the degradation of P3HT will reduce the exciton dissociation efficiency, thereby increasing the triplet excitons in FBR, which would generate more 1 O 2 to photobleach both materials.…”

“…c) Jablonski diagrams for the P3HT:FBR blend with the 1 O 2 generation mechanism. Reproduced with permission [32]. Copyright 2020, Wiley-VCH.…”