Indole fused heterocycles as sensitizers in dye-sensitized solar cells: an overview

The past three decades have witnessed extensive research in developing a range of non-metallic organic dyes for dye sensitized solar cells (DSSCs). Dyes occupy a prominent position among components in DSSCs, and organic dyes have emerged as the most promising candidate for DSSCs due to their performance, ease of synthesis, stability, tunability, low cost and eco-friendly characteristics. In addition to this, so far, the best and highest performing DSSCs reported in the literature use metal-free organic dyes. Organic dyes also provide flexibility to be used along with alternate new generation cobalt and copper electrolytes. Among various organic dyes, heterocycles, mainly N- and S-containing, have found immense applications as sensitizers. Indole fused heterocycles were used by different research groups in their dye designs, mainly as a donor and π-spacer. The planarity of these electron-rich fused indole systems is advantageous as it helps to initiate a more prominent ICT transition in dye molecules. In addition, the possibility for selective functionalization of N-atoms with long or branched alkyl chains prevents the aggregation of the sensitizer, increasing the solubility and is effective in custom design dyes which are in turn capable of preventing back electron transfer (recombination). Fused indole moieties utilized in the design of sensitizers are stable and offer easy synthesis. In the present review, we examine different indole fused heterocycles as building blocks for sensitizers used in DSSCs.