14.2% Efficiency Dye-Sensitized Solar Cells by Co-sensitizing Novel Thieno[3,2-b]indole-Based Organic Dyes with a Promising Porphyrin Sensitizer

A new series of 4-hexyl-4H-thieno[3,2-b]indole (HxTI) based organic chromophores is developed by structural engineering of the electron donor (D) group in the D–HxTI–benzothiadiazole-phenyl-acceptor platform with different fluorenyl moieties, such as unsubstituted fluorenyl (SGT-146) and hexyloxy (SGT-147), decyloxy (SGT-148) and hexyloxy-phenyl substituted (SGT-149) fluorenyl moieties. In comparison to a reference dye SGT-137 with a biphenyl-based donor, the effects of the donating ability and bulkiness of the fluorenyl based donor in this D–π–A-structured platform on molecular properties and photovoltaic performance are investigated to establish the structure–property relationship. The photovoltaic performance of dye-sensitized solar cells (DSSCs) is improved according to the bulkiness of the donor groups. As a result, the DSSCs based on SGT-149 show high power conversion efficiencies (PCEs) of 11.7% and 10.0% with a [Co(bpy)₃]^2+/3+ (bpy = 2,2′-bipyridine) and an I⁻/I₃⁻ redox electrolyte, respectively. Notably, the co-sensitization of SGT-149 with a SGT-021 porphyrin dye by utilizing a simple “cocktail” method, exhibit state-of-the-art PCEs of 14.2% and 11.6% with a [Co(bpy)₃]^2+/3+ and an I⁻/I₃⁻ redox electrolyte, respectively.