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 − /I 3 − 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 − /I 3 − redox electrolyte, respectively.

Zn(II)–porphyrin sensitizers, coded as SGT‐020 and SGT‐021, are designed and synthesized through donor structural engineering. The photovoltaic (PV) performances of SGT sensitizer‐based dye‐sensitized solar cells (DSSCs) are systematically evaluated in a thorough SM315 as a reference sensitizer. The effect of the donor ability and the donor bulkiness on photovoltaic performances is investigated for establishing the structure–performance relationship in the platform of porphyrin‐triple bond‐benzothiadiazole‐acceptor sensitizers. By introducing a more bulky fluorene unit to the amine group in the SM315, the power conversion efficiency (PCE) is enhanced with the increased short‐circuit current (Jsc) and open‐circuit voltage (Voc), due to the improved light‐harvesting ability and the efficient prevention of charge recombination, respectively. As a consequence, a maximum PCE of 12.11% is obtained for SGT‐021, whose PCE is much higher than the 11.70% PCE for SM315. To further improve their maximum efficiency, the first parallel tandem DSSCs employing cobalt electrolyte in the top and bottom cells are demonstrated and an extremely high efficiency of 14% is achieved, which is currently the highest reported value for tandem DSSCs. The series tandem DSSCs give a remarkably high Voc value of >1.83 V. From this DSSC tandem configuration, 7.4% applied bias photon‐to‐current efficiency is achieved for solar water splitting.

Thieno[3,2-b]indole-based organic sensitizers were synthesized to enhance the light absorption ability and exhibited superior performance for dye-sensitized solar cells.

Three new thieno[3,2‐b][1]benzothiophene (TBT)‐based donor–π–acceptor (D–π–A) sensitizers, coded as SGT‐121, SGT‐129, and SGT‐130, have been designed and synthesized for dye‐sensitized solar cells (DSSCs), for the first time. The TBT, prepared by fusing thiophene unit with the phenyl unit of triphenylamine donor, is utilized as the π‐bridge for all sensitizers with good planarity. They have been molecularly engineered to regulate the highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) energy levels and extend absorption range as well as to control the electron‐transfer process that can ensure efficient dye regeneration and prevent undesired electron recombination. The photovoltaic performance of SGT‐sensitizer‐based DSSCs employing Co(bpy)32+/3+ (bpy = 2,2′‐bipyridine) redox couple is systematically evaluated in a thorough comparison with Y123 as a reference sensitizer. Among them, SGT‐130 with benzothiadiazole‐phenyl (BTD‐P) unit as an auxiliary acceptor exhibits the highest power‐conversion efficiency (PCE) of 10.47% with Jsc = 16.77 mA cm−2, Voc = 851 mV, and FF = 73.34%, whose PCE is much higher than that of Y123 (9.5%). It is demonstrated that the molecular combination of each fragment in D–π–A organic sensitizers can be a pivotal factor for achieving the higher PCEs and an innovative strategy for strengthening the drawbacks of the π‐bridge.