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

Abstract

Type-II photogeneration in dye-sensitized solar cells (DSSCs) involves a ground-state dye-to-TiO₂ charge transfer (DTCT) process and has received increasing interest. However, the dyes are typically limited to catechol anchors, which exhibit a low power conversion efficiency (PCE). Herein, new dyes comprising carbazole (Cz), phenothiazine (PTZ) and cyanocrylic acid (CA) moieties were synthesized, which exhibited obvious DTCT, representing the first DTCT dyes without a catechol anchor. The peri-Cz and PTZ-grafted alkyl chain influence the frontier electron population and PTZ butterfly twisting, respectively, both of which affect the DTCT process. Among the six adsorbed dyes, the meta-Cz dye with long alkyl-chain exhibits the highest Cz-population in HOMO and the smallest PTZ-twisting, leading to its most pronouncing DTCT process. However, in the device, DTCT was stronger in short-chain dyes than in their long-chain analogues, which was likely due to the significantly reduced back DTCT caused by the larger PTZ twisting in the short-chain dyes. The meta-Cz and short-chain-based dye coupled with the I⁻/I₃⁻ redox showed a PCE of 8.76%, which was much higher than the best-reported PCE of 2.04% for dyes comprising both type-I and type-II photoinjections.