Regioregularity has been shown to be an important aspect for the properties of poly(3-hexylthiophene) both in the pristine material and in bulk heterojunction photovoltaics with the fullerene derivative PCBM. Here we present a straightforward method to adjust the effective regioregularity of poly(3-hexylthiophene) by introducing 3,4-dihexylthiophene into the polymer chain. By investigating bulk heterojunction morphology and photovoltaic performance upon annealing, we observe that polythiophene with 91% regioregularity maintains equivalent electronic properties but forms a more thermally stable interpenetrating network with PCBM than polythiophene with regioregularity >96%.