Performance evaluation of dye-sensitized solar cells (DSSCs) based on metal-free thieno[3,2-b]indole dyes

A series of thieno[3,2-b]indole-based dyes (IS 1–10) was readily synthesized in three steps from 2-(thien-2-yl)thieno[3,2-b]indole as the key precursor, and further applied as photosensitizers for dye-sensitized solar cells (DSSCs). In general, the prepared dyes have a push–pull (donor-π-linker-acceptor) structure, included thieno[3,2-b]indole ring system, bearing different aliphatic substituents at the nitrogen atom, as an electron-donating part, single thiophene unit as π-linker, and 2-cyanoacrylic acid (IS 1–5), or 5-(methylene)barbituric acid (IS 6–10) as an acceptor-anchoring group. The DSSC devices based on IS 1–10 dyes were fabricated using commercially available TiO2-coated photoanodes, and their photovoltaic characteristics were investigated. The DSSCs based on IS 1–5 dyes exhibited values of power conversion efficiency (PCE) in the range of 2.25–3.02%, while the DSSCs based on IS 6–10 dyes showed significantly low values of PCE in the range of 0.20–0.32% under AM 1.5G illumination (100 mW cm−2). The highest PCE value of 3.02% (Jsc = 7.59 mA cm−2, Voc = 0.62 V, FF = 0.64) was achieved for DSSC based on dye IS 4, bearing 2-cyanoacrylic acid as an acceptor-anchoring group among all dyes IS 1–10, whereas IS 9, bearing 5-(methylene)barbituric acid displays best PCE of 0.32% (Jsc = 0.87 mA cm−2, Voc = 0.53 V and FF = 0.68) among dyes IS 6–10. Therefore, additional studies, including thermogravimetric analysis, UV–Vis, and FTIR-measurements on the TiO2 surface, cyclic voltammograms, and photoelectrochemical measurements, were carried out for these two dyes.