Towards amorphous solution-processed small-molecule photovoltaic cells by design
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Organic photovoltaic cells incorporating a homogeneously blended amorphous active layer were fabricated using a novel glass-forming squarylium cyanine derivative (SQ-glass) as donor, and both a previously reported glass-forming diketopyrrolopyrrole (DPP-glass) and the benchmark PC61BM as acceptors. Whereas devices with DPP-glass always yielded an amorphous active layer, devices with PCBM could be either fully amorphous, or phase-segregated with crystalline PCBM, depending on processing conditions. Amorphous devices yielded efficiencies ranging up to 0.41-0.44 %, while the efficiency of devices with crystalline PCBM reached up to 0.56 %, which corresponds to only a 18 % increase relative to a fully amorphous active layer. Although phase-segregated active layers are known to show a higher efficiency, this difference is smaller than anticipated, and is mitigated by the absence in amorphous films of various defects common in their polycrystalline counterparts.