The approach of using alternating electron rich donor (D) and electron poor acceptor (A) moieties in the repeat unit of light absorbing and hole carrying conjugated DA polymers, which yield active photovoltaic materials when blended with electron carrying fullerenes, has lead to a set of materials with AM1.5 power conversion efficiencies (PCEs) in excess of 10%.  Often, minor changes in polymer structure, including main chain elemental composition, solubilizing side chain structure, end group identity, molecular weight, and dispersity, along with small modifications in the composition and processing of the active material (such as the use of processing additives, processing temperature, annealing, and method for film deposition), have major impacts on ultimate OPV performance.  In light of this, we report on the syntheses of a series of DA polymers where we have worked to minimize structural and handling modifications in order to separate the impact of a specific variable on optical, transport and morphological properties, which ultimately control device performance.  For example, changing one atom from carbon, to silicon, to germanium in the repeat unit of a fused bithiophene donor, subsequently incorporated into DA polymers with thienopyrrolodione (TPD), and blended with PC<sub>71</sub>BM, shows distinct differences in light absorption, intermolecular orientation, triplet energy levels and charge recombination efficiencies across the family, leading to PCEs that change by 35% (C-5.7%, S-7.0%, G-7.7%).  As a testament to the complexity of utilizing these materials in OPV devices, we find that all aspects of molecular and macromolecular structure, controlled morphologies, interfacial effects and material processing must be understood and controlled for success.<br/>

Organic polymers for photovoltaics: Small structural differences yield large performance changes

253rd National Meeting (2017)

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