Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

Abstract

The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PCBM blend films, dispersed with polystyrene, are found to be more susceptible to photobleaching in air than the more aggregated neat films. This enhanced photobleaching correlated with increased oxygen quenching of PCBM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PCBM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PCBM films were observed to undergo photo-oxidation in air for even modest (≤40 min) irradiation times, degrading electron mobility substantially, indicative of electron trap formation. This conclusion is supported by observation of red shifts in photo- and electro-luminescence with photo-oxidation, shown to be in agreement with time-dependent density functional theory calculations of defect generation. These results provide important implications on the environmental stability of PCBM-based films and devices.

From fullerene acceptors to non-fullerene acceptors: prospects and challenges in the stability of organic solar cells

“…1(a)). 44 For example, at a lower loading of PC 61 BM, a lower degree of PC 61 BM aggregation in conjunction with a higher yield of triplet states was observed, resulting in more severe photo-oxidation of PC 61 BM. This is consistent with tripletmediated photo-oxidation, analogous to that observed in polymer:fullerene blends via the donor polymer triplets.…”

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

“…Typically, the small molecules used have been fullerene derivatives such as [6,6]-phenyl-C-61-butyric acid methyl ester (PC 60 BM), [6,6]-phenyl-C-71-butyric acid methyl ester (PC 70 BM), and 1′,1″,4′,4″-tetrahydro-di [1,4]methanonaphthaleno [5,6]fullerene-C 60 (ICBA), achieving efficiencies of 11% [1,2] and lifetimes exceeding several years. Stability issues are also common, with light-induced effects causing strong burn-in degradation, [5,6] poor thermal stability, [7][8][9] and photo-oxidation, [10,11] reducing the viability of using fullerene-based acceptors in commercial modules. An inability to tune the chemical structure inhibits morphological or energetic optimization, to the effect that progress in the field has mostly been driven by the design Nonfullerene acceptors (NFAs) dominate organic photovoltaic (OPV) research due to their promising efficiencies and stabilities.…”

Impact of intentional photo-oxidation of a donor polymer and PC70BM on solar cell performance

“…On the other hand, the photooxidation of the PC 60 BM itself was found to be accelerated by its dispersion in a TQ1 matrix, which Hansson et al 27 assigned to the enhanced absorption range. Interestingly, Speller et al 28 showed a similar acceleration of the PC 60 BM photodegradation when PC 60 BM is dispersed in a transparent non-conjugated polymer (polystyrene) matrix. They also found that the epoxide defects that are formed in the early stages of photo-oxidation, prior to carbonyl formation and cage opening, hamper the electron transport.…”