Understanding How Polymer Properties Control OPV Device Performance: Regioregularity, Swelling, and Morphology Optimization Using Random Poly(3-butylthiophene-co-3-octylthiophene) Polymers

The performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaics is highly sensitive to the morphology of the polymer within the active layer. To tune this morphology, we constructed both blend-cast and sequentially processed BHJ devices from the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PCBM), in combination with a series of random poly(3-butylthiophene-co-3-octylthiophene)s with different fractions of each monomer, with the goal of controllably varying the average polymer side-chain length. What we found, however, was that the most important parameter for predicting device performance across this series of polymers was the regioregularity of the particular synthetic batch of polymer used, not the average side-chain length. Moreover, we found that regioregularity affected device performance in different ways depending on the processing route: lower regioregularity led to improved performance for sequentially processed devices, but was detrimental to the performance of blend-cast devices. We argue that the reason for this anticorrelation is that regioregularity is the single most important determinant of the relative crystalline of the polymer. The relative crystalline fraction, in turn, determines the ability of the polymer to swell in the presence of solvents. Polymer swelling is key to BHJ formation via sequential processing, but can lead to overly mixed systems using traditional blend-casting methods. As a result, we find that the best performing polymer for sequentially processed devices is the worst performer for blend-cast devices and vice versa, highlighting the importance of using both processing methods when exploring new materials for use in BHJ photovoltaics.