Synthesis and Photovoltaic Properties of Conjugated Polymers Based on 1,2,4-Triazole Derivatives

Polymer solar cells (PSCs) have attracted much attention due to their unique features, such as low cost, light weight, solution processibility, fast roll-to-roll production, and applications in large area flexible panels. High performance photovoltaic materials are usually low bandgap polymers, which are constructed as donor-acceptor (D-A) alternating co-polymers, in order to better absorb solar energy. In the current work, three D-A conjugated polymers incorporating 1,2,4-triazole derivative as electron-withdrawing units and thiophene or benzo[1,2-b:4,5-b']dithiophene as electron-donating units have been synthesized. Their chemical structures of the corresponding intermediates and the polymers were confirmed with H-1 NMR, GC-MS or MALDI-TOF. All the polymers are readily dissolved in chloroform, THF, and toluene at room temperature, and the heat resistance and thermal stability of the three polymers are good enough for the application of PSCs. In chloroform solution, polymer PT-TZ shows only a absorption peak at 384 nm corresponding to the intramolecular charge transfer (ICT) interaction between thiophene unit and 1,2,4-triazole derivative. Whereas, polymers PB-TZ and PB-TTZT show three absorption peaks. The absorption peaks of PB-TZ and PB-TTZT in the UV region are attributed to the absorption of 1,2,4-triazole. Those in the visible region are ascribed to the pi-pi* transition derived from the polymer backbone and the ICT interaction respectively. Compared with PT-TZ and PB-TZ, the maximum absorption peak (lambda(max)) of PB-TTZT is obviously red-shifted because of extending thiophene units in the conjugated main chain which increases effective conjugation of the main chain and broadens the absorption band. The highest occupied molecular orbital (HOMO) energy levels of three polymers are lower than -5.2 eV and the lowest unoccupied molecular orbital (LUMO) energy levels of them are higher than -3.8 eV, so these polymers are promising candidates for the effective applications of PSCs. The obvious two phase separation can be seen in the photoactive layer of PT-TZ and PC61BM (1 : 2, w/w), so the monochromatic incident photon-to-electron conversion efficiency (IPCE) value of the PSC based on PT-TZ is very little. However, the photoactive layers of PB-TZ or PB-TTZT and PC61BM (1: 2, w/w) just show micro phase separation which is favourable to diffusion of exciton, so the IPCE values of the PSCs based on them are obviously higher than that of PT-TZ. The bulk-hetero junction photovoltaic cells of the polymers were prepared and investigated by blending the polymers and PC61BM. The employed device structure was ITO/PEDOT:PSS (40 nm)/polymer:PC61BM (1: 2, w/w)/LiF (0.7 nm)/A1 (100 nm). Under the illumination of AM1.5 G, 100 mW/cm(2), the BHJ devices based on PT-TZ, PB-TZ and PB-TTZT showed the power conversion efficiencies (PCEs) of 0.01%, 0.20% and 1.18% respectively. The short-circuit current density (J(sc)) and PCE value of the cell based on PB-TFZT is higher than that of PT-TZ and PB-TZ because of the red-shifted and broadened absorption band.