In this paper, we report on the synthesis and properties of a novel series of multi-triarylamine-substituted carbazole-based dendrimers G2-OTP(n)-G2 (n = 2−5) with an oligothiophene core. The length of the oligothiophene core varied from 2 to 5 thiophene units. It was found that both the absorption and fluorescence emission peaks red-shifted with increasing oligothiophene core length. As a result, the power conversion efficiencies of the as-fabricated solar cells using the composite of these dendrimers and PCBM as the photoactive layer increased with increasing oligothiophene core length, from 0.04% for G2-DTP-G2 to 0.13% for G2-PTP-G2 under AM 1.5 simulated solar illumination at an irradiation intensity of 100 mW/cm2. After a thermal annealing at 150 °C for 15 min under nitrogen, however, the devices based on the G2-DTP-G2:PCBM blend exhibited the most dramatic improvement and almost best performance, although other devices also showed some improvement. Since only G2-DTP-G2 is the a crystalline material ...