New bithiazole-based sensitizers for efficient and stable dye-sensitized solar cells

A series of new push-pull or- ganic dyes (BT-I-VI), incorporating electron-withdrawing bithiazole with a thiophene, furan, benzene, or cyano moiety, as p spacer have been synthe- sized, characterized, and used as the sensitizers for dye-sensitized solar cells (DSSCs). In comparison with the model compound T1, these dyes con- taining a thiophene moiety between tri- phenylamine and bithiazole display en- hanced spectral responses in the red portion of the solar spectrum. Electro- chemical measurement data indicate that the HOMO and LUMO energy levels can be tuned by introducing dif- ferent p spacers between the bithiazole moiety and cyanoacrylic acid acceptor. The incorporation of bithiazole substi- tuted with two hexyl groups is highly beneficial to prevent close p-p aggre- gation, thus favorably suppressing charge recombination and intermolecu- lar interaction. The overall conversion efficiencies of DSSCs based on bithia- zole dyes are in the range of 3.58 to 7.51 %, in which BT-I-based DSSCs showed the best photovoltaic perform- ance: a maximum monochromatic inci- dent photon-to-current conversion effi- ciency (IPCE) of 81.1 %, a short-circuit photocurrent density (Jsc )o f 15.69 mA cm 2 , an open-circuit photo- voltage (Voc) of 778 mV, and a fill factor (ff) of 0.61, which correspond to an overall conversion efficiency of 7.51 % under standard global AM 1.5 solar light conditions. Most important- ly, long-term stability of the BT-I-III- based DSSCs with ionic-liquid electro- lytes under 1000 h of light soaking was demonstrated and BT-II with a furan moiety exhibited better photovoltaic performance of up to 5.75 % power conversion efficiency.