Regulating Back Electron Transfer through Donor and π-Spacer Alterations in Benzothieno[3,2-b]indole-based Dye-sensitized Solar Cells

Three metal-free organic D-pi-A dyes with benzothieno[3,2-b]indole as electron donor, cyanoacrylic acid as both electron acceptor and anchoring group with benzene (BID-1), thiophene (BID-2) and furan (BID-3) as pi-spacers were designed and synthesized for application in dye-sensitized solar cells (DSSCs). A planar and electron-rich heterocycle such as benzothieno[3,2-b]indole offers better backbone rigidity and improves charge transport properties in comparison to indolo[3,2-b]indole donor, previously reported from our group. Additionally, we synthesized a benzothieno[3,2-b]indole donor grafted with longer alkyl chains which efficiently prevented the approach of oxidized species in the electrolyte coming closer to semiconductor thereby arresting recombination. A power conversion efficiency of 4.11 % was achieved for dye-sensitized solar cells based on the furan pi-spacer benzothieno[3,2-b]indole dyeBID-3in comparison to the corresponding indolo[3,2-b]indole dye (IID-3) having an efficiency of 1.71 %. Detailed interfacial electrical measurements along with theoretical calculations disclosed the mechanism of back electron transfer and improvement in photovoltaic performance with respect to variation in both donor and pi-spacer.