Synthesis and properties of multi-triarylamine-substituted carbazole-based dendrimers with an oligothiophene core for potential applications in organic solar cells and light-emitting diodes

In this paper, we report on the synthesis and properties of a novel series of multi-triarylamine-substituted carbazole-based dendrimers G(2)-OTP(n)-G(2) (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 G(2)-DTP-G(2) to 0.13% for G(2)-PTP-G(2) under AM 1.5 simulated solar illumination at an irradiation intensity of 100 mW/cm(2). After a thermal annealing at 150 degrees C for 15 min under nitrogen, however, the devices based on the G(2)-DTP-G(2):PCBM blend exhibited the most dramatic improvement and almost best performance, although other devices also showed some improvement. Since only G(2)-DTP-G(2) is the a crystalline material among these four dendrimers, the dramatic improvement of the associated devices after annealing is attributed to the formation of some nanocrystalline structures in the active layer, as indicated by DSC analysis and AFM studies. In addition, we have demonstrated that both G(2)-DTP-G(2) and G(2)-PTP-G(2) can be used as hole-transporting emitters in organic light-emitting diodes (OLEDs) with 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as a hole-blocking layer and LiF/Al as the cathode. Because of the low hole injection barrier, the turn-on voltages of the OLED devices were as low as 4 V. G(2)-DTP-G(2) exhibited blue electroluminescence with a maximum luminance of 1660 cd/m(2) at 15.5 V, while G(2)-PTP-G(2) emitted yellow light with a highest luminance of 1000 cd/m(2) at 19 V.