We report a general light processing strategy for organic solar cells (OSC) that exploits the propensity of the fullerene derivative PC60BM to photo-oligomerize, which is capable of both stabilizing the polymer:PC60BM active layer morphology and enhancing the device stability under thermal annealing. The observations hold for blends of PC60BM with an array of benchmark donor polymer systems, including P3HT, DPP-TT-T, PTB7, and PCDTBT. The morphology and kinetics of the thermally induced PC60BM crystallization within the blend films are investigated as a function of substrate and temperature. PC60BM nucleation rates on SiOx substrates exhibit a pronounced peak profile with temperature, whose maximum is polymer and blend-composition dependent. Modest illumination (<10 mW/cm(2)) significantly suppresses nucleation, which is quantified as function of dose, but does not affect crystalline shape or growth, in the micrometer range. On PEDOT:PSS substrates, thermally induced PC60BM aggregation is observed on smaller (≈ 100 nm) length scales, depending upon donor polymer, and also suppressed by light exposure. The concurrent thermal dissociation process of PC60BM oligomers in blend films is also investigated and the activation energy of the fullerene-fullerene bond is estimated to be 0.96 ± 0.04 eV. Following light processing, the thermal stability, and thus lifetime, of PCDTBT:PC60BM devices increases for annealing times up to 150 h. In contrast, PCDTBT:PC70BM OSCs are found to be largely light insensitive. The results are rationalized in terms of the suppression of PC60BM micro- and nanoscopic crystallization processes upon thermal annealing caused by photoinduced PC60BM oligomerization.

A key challenge to the commercialization of organic bulk heterojunction solar cells is the achievement of morphological stability, particularly under thermal stress conditions. Here we show that a low-level light exposure processing step during fabrication of blend polymer:PC 60 BM solar cells can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerization of PC 60 BM that effectively hinders their diffusion and crystallization in the blend. We thus suggest that light processing may be a promising, general and cost-effective strategy to optimize fullerene-based solar cell performance. The low level of light exposure required suggests not only that this may be an easily implementable strategy to enhance performance, but also that light-induced PC 60 BM oligomerization may have inadvertently influenced previous studies of organic solar cell device behaviour.

Existing state of the art polymeric membranes are either integrally skinned asymmetric (ISA) or thin fi lm composite (TFC) membranes. [ 2 ] ISA membranes are produced by the phase inversion technique which leads to a dense separation layer a few hundred nanometres thick being formed on a highly porous support structure several microns in thickness. Of the ISA membranes, crosslinked polyimide (PI) membranes are probably the most widely used for OSN due to their ease of fabrication, high mechanical strength, and high stability even in harsh solvents such as tetrahydrofuran or dimethylformamide. [3][4][5] However, physical aging and compaction remains a challenge for ISA membranes, resulting in permeance reduction over time in operation, often by more than 50% in a few days. [ 6 ] Studies show that the intrinsic solvent permeance of ISA membranes formed from polyimide is negligible; solvent cast or annealed fi lms of polyimide, in which the polymer chain conformation relaxes to pack closer to equilibirum, typically have low or no fl ux. [ 4 ] The permeance difference between polyimide ISA membranes and dense polyimide fi lms arises because ISA membranes formed by phase inversion rapidly "freeze" the polyimide microporous structure, which can be far away from equilibrium; importantly, the resulting microporosity is due to the way the membrane is made. Under service conditions, or when the membranes are heated and then cooled gradually ("annealed"), these membranes age (lose permeance) as the polymer chains relax towards equilibrium packing.TFC membranes are typically fabricated by depositing or forming a thin separation layer on top of a porous ultrafi ltration (UF) support several microns thick. [ 7,8 ] These membranes are attracting widespread interest for OSN because the separation layer structure can be better controlled and, therefore, the separation performance improved. [ 6 ] Crucially, when the top layer is formed by coating of rubbery polymers [ 7 ] or by interfacial polymerisation, [ 8 ] both resulting in crosslinked matrices, there is less evidence of physical aging or compaction. However, fl uxes of ISA and most TFC membranes are still relatively low and large membrane areas are necessary for industrial applications. Recently, the preparation of ultrathin (35 -50 nm) free-standing Organic solvent nanofi ltration (OSN) membranes with ultrathin separation layers down to 35 nm in thickness fabricated from a polymer of intrinsic microporosity (PIM-1) are presented. These membranes exhibit exceptionally fast permeation of n-heptane with a rejection for hexaphenylbenzene of about 90%. A 35 nm thick PIM-1 membrane possesses a Young's modulus of 222 MPa, and shows excellent stability under hydraulic pressures of up to 15 bar in OSN. A maximum permeance for n-heptane of 18 Lm −2 h −1 bar −1 is achieved with a 140 nm thick membrane, which is about two orders of magnitude higher than Starmem240 (a commercial polyimide-based OSN membrane). Unexpectedly, decreasing the fi lm thickness below 140 nm results in an anomal...

This review highlights the opportunities and challenges in stability of organic solar cells arising from the emergence of non-fullerene acceptors.

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