New phenothiazine-bridged dyes comprising both type-I and type-II photoinjections for DSSCs

Abstract

Type-II photogeneration in dye sensitized solar cell (DSSC) involves ground-state-dye to TiO₂ charge transfer (DTCT) process and has received increasing-interest. However, the dyes are typically limited to catechol-anchor, which exhibit low power conversion efficiency (PCE). Here, new dyes comprising carbazole (Cz), phenothiazine (PTZ) and cyanocrylic acid (CA) moieties exhibit obvious DTCT, which are the first DTCT dyes without catechol-anchor. The peri-Cz and the PTZ-grafted alkyl-chain influence frontier electron population and PTZ-butterfly twisting, respectively, both of which affect DTCT process. Owning to the most Cz-population in HOMO and the smallest PTZ-twisting, the DTCT in the meta-Cz and long alkyl-chain dye is the most pronounced. However, in the device, DTCT is stronger in short-chain dyes compared to their long-chain analogue, which is likely due to the significantly reduced back DTCT accounting from the larger PTZ twisting in the short-chain dyes. The meta-Cz and short-chain based dye in coupled with I-/I₃- shows a PCE of 8.76%, much higher than the best-reported one of 2.04% for dyes comprising both type-I and type-II photoinjections.