We present the synthesis of regioregular polythiophenes with alkylthiophene side chains P3TC16 prepared by Ni-catalyzed polymerization from the branched, thiophene-based monomer 5-bromo-5 ''-hexadecyl-[2,2';3',2 '']-terthiophene. The optical properties in solution and thin films of the polymer were investigated in situ as a function of temperature and compared to the low regioregularity analogue FeP3TC6 synthesized by Fe(III) mediated oxidative polymerization of 5 ''-hexyl-[2,2';3',2 '']terthiophene. It was found that due to the regioregular structure, P3TC16 tends to strong aggregation in solution, which is ascribed to pi-pi interactions. The bandgap in thin films of 1.88 eV is slightly smaller than the bandgap of the reference polymer poly(3-hexylthiophene) (P3HT, 1.91 eV). Interestingly, it was found that the HOMO and LUMO levels of P3TC16 are shifted to significantly lower values as compared to P3HT. First results regarding the application of P3TC16 in FETs are shown and mobilities of up to 3.1 X 10(-2) cm(2)/(V s) were achieved. Open circuit voltages of up to 710 mV in combination with PC[60]BM in organic solar cells were found, which is about 30% higher than for P3HT, which can be attributed to the low HOMO energy level.