Whilst the stability of organic solar cells operating under ambient conditions is still a major issue, the negative impact of oxygen and water can be evaded to a certain extent by employing advanced materials, novel architectures, and proper encapsulation. However, in the case of some polymer:fullerene combinations, device performance losses even emerge from mere exposure to light, i.e., in the absence of any accelerating agents.
For example, encapsulated photovoltaic devices based on the polymer poly[4,4' bis(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-2,6-diyl-alt-[2,5-bis(3-tetrade cylthiophen 2-yl)thiazole[5,4-d]thiazole)-1,8-diyl] (PDTSTzTz) and the fullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM) show substantial performance losses upon illumination (“burn-in effect”). Our electrical, spectroscopic, and analytic measurements on this model system provide evidence that photo-dimerization of PC60BM (i.e., a [2+2] cycloaddition between two adjacent fullerene cages) is responsible for the observed degradation. Simulation of the electrical device parameters reveals that this dimerization process results in a significant reduction of the charge carrier mobility. BisPC60BM, the bis-substituted analog of PC60BM, is shown to be resistant towards light exposure, which in turn enables the manufacture of photostable PDTSTzTz:bisPC60BM solar cells.
In addition, detailed investigations on the photo-degradation behavior of PC60BM and other fullerenes in both neat films and blend films with different polymers have been performed under different conditions. Within a series of eleven different fullerenes, multi-substituted fullerenes as well as C70-based fullerenes were observed not to undergo dimerization, in contrast to all mono-substituted C60-based fullerenes. Dimerization is found to be suppressed in the presence of oxygen and is independent of the temperature. At temperatures above 100 °C, however, both the dimerization of PC60BM and the associated photovoltaic device performance loss turn out to become reversible. Moreover, the photo-transformation yield of PC60BM in thin films is independent of the energy of the absorbed photons and exhibits first-order dependence on the incident photon flux density. The extent of PCBM dimerization in blend films with polymers depends strongly on the type of polymer. The dimerization yield is found to mainly depend on the morphology of the bulk-heterojunction, that is to say: the bigger the fullerene domains the more dimerization occurs.
Finally, the formation of different photogenerated species and their kinetics in PC60BM films and blends with polymer was studied by ultrafast transient absorption spectroscopy. By this means, we established a mechanistic understanding of the photo-induced fullerene dimerization in the solid state and found it to be contradictory to the reaction mechanism commonly discussed in the literature. In films of PC60BM, fullerene triplet states being the initiator of the dimerization process are not directly formed upon photo-excitation and subsequent intersystem crossing, but are a product of the bimolecular recombination of fullerene anions and cations, which are already formed on a sub-picosecond scale.