ABSTRACT
Controlling polymer film solubility is of fundamental and practical interest and is typically achieved by synthetically modifying the polymer structure to insert reactive groups. Here, we demonstrate that the addition of fullerenes or its derivatives (C60 or phenyl-C61-butyric acid methyl ester, PCBM) to polymers, followed by ultraviolet (UV) illumination can change the film solubility. Contrary to most synthetic polymers, which dissolve in organic solvents but not in water, the fullerene-doped polymer films (such as polystyrene) can dissolve in water yet remain stable in organic solvents. This photoswitchable solubility effect is not observed in either film constituents individually and is derived from a synergy of photochemistries. First, polymer photooxidation generates macroradicals which cross-link with radical-scavenging PCBM, thereby contributing to the films' insolubility in organic solvents. Second, light exposure enhances polymer photooxidation in the presence of PCBM via the singlet oxygen pathway. This results in polymer backbone scission and formation of photooxidized products which can form hydrogen bonds with water, both contributing to water solubility. Nevertheless, the illuminated doped polymer thin films are mechanically robust, exhibiting significantly increased modulus and density compared to their pristine counterpart, such that they can remain intact even upon sonication in conventional organic solvents. We further demonstrate the application of this solubility-switching effect in dual tone photolithography, via a facile, economical, and environmentally benign solution-processing route made possible by the photoactive nature of polymer-PCBM thin films.
ABSTRACT
With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation.
ABSTRACT
Solution-processed organic small molecule solar cells (SMSCs) have achieved efficiency over 11%. However, very few studies have focused on their stability under illumination and the origin of the degradation during the so-called burn-in period. Here, we studied the burn-in period of a solution-processed SMSC using benzodithiophene terthiophene rhodamine:[6,6]-phenyl C71 butyric acid methyl ester (BTR:PC71BM) with increasing solvent vapour annealing time applied to the active layer, controlling the crystallisation of the BTR phase. We find that the burn-in behaviour is strongly correlated to the crystallinity of BTR. To look at the possible degradation mechanisms, we studied the fresh and photo-aged blend films with grazing incidence X-ray diffraction, UV-vis absorbance, Raman spectroscopy and photoluminescence (PL) spectroscopy. Although the crystallinity of BTR affects the performance drop during the burn-in period, the degradation is found not to originate from the crystallinity changes of the BTR phase, but correlates with changes in molecular conformation - rotation of the thiophene side chains, as resolved by Raman spectroscopy which could be correlated to slight photobleaching and changes in PL spectra.
ABSTRACT
The photochemistry and stability of fullerene films is found to be strongly dependent upon film nanomorphology. In particular, PC61BM 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 PC61BM triplet states and the appearance of a carbonyl FTIR absorption band indicative of fullerene oxidation, suggesting PC61BM photo-oxidation is primarily due to triplet-mediated singlet oxygen generation. PC61BM 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 PC61BM-based films and devices.
