The morphologies in bulk heterojunction (BHJ) thin films are directly related to the interaction strength between the conjugated polymers and fullerenes, degree of crystallinity, choice of processing conditions and substrate surface energy and roughness. These parameters provide huge degrees of versatility to the behavior of the system and consequently device efficiency. However, this flexibility produces significant challenges to the rational exploration of these materials without predictive models or well-established data and guidelines to determine the expected behaviour. We study model bulk heterojunction (BHJ) films based on mixtures of poly(3-alkyl thiophene)s (P3BT, P3HT and P3OT) and three different fullerenes (C60, PCBM and bis-PCBM), as well as more novel synthesized nanoparticle BHJ systems. Using these systems, we have measured the effect of typical thermal oven annealing processes as well as our recently developed dynamic thermal zone annealing on the BHJ film morphology-performance correlation. We have used neutron reflection, SANS and GIWAXS measurements to determine film morphology normal to the film surfaces in many real device configurations. The novelty of the approach over previous studies is that the BHJ layer is sandwiched between a PEDOT/PSS and Al layers. In all systems NR analysis shows that the composition is not homogeneous through the film thickness and demonstrate enrichment of the fullerenes at the Al interface and polythiophene at the PEDOT/PSS interface. The degree of segregation varies depending on the miscibility of the polymer and fullerene. The effects of processing-morphology-device performance have been evaluated directly on these systems.

Neutron and X-ray scattering based correlation of novel nanomaterials, processing, phase morphology and device efficiency in OPVs

247th National Meeting (2014)

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