Insight into Benzothiadiazole Acceptor in D–A−π–A Configuration on Photovoltaic Performances of Dye-Sensitized Solar Cells

The option of conjugated π-linkers is critical for molecular engineering toward the energy-level strategy of donor−π–acceptor (D−π–A) sensitizers. There is always a balance in the optimization of a π-linker. The π-conjugation should be enlarged to expand the light-harvesting capability of sensitizers for an increase in photocurrent; however, the oversized π-linker also would affect seriously the photovoltage and photostability. Two sensitizers, WS-22 and WS-23, are constructed without or with benzothiadiazole (BTD) in a molecular skeleton, aiming to gain insight into the effect of an auxiliary acceptor in D–A−π–A sensitizers on the photophysical and photovoltaic performances, especially focusing on the exploitation of the short circuit current density (J sc) and open circuit voltage (V oc). Compared with the typical D−π–A sensitizer WS-22, the incorporation of an auxiliary acceptor of BTD in WS-23 can improve the light-harvesting ability both in red-shifting the absorption peaks and the increment of absorption coefficient. The predominant increase by 15.6% in light-harvesting efficiency (LHE) of WS-23 results in a relatively higher J sc from 13.77 (WS-22) to 16.91 mA cm–2 (WS-23). Moreover, the improvement of the V oc in WS-23 is originated by a synergy contribution of the uplifting of E CB and inhibition of charge recombination. The stepped light-induced transient (SLIT) measurements indicate that the introduction of BTD can negatively shift the conduction band of the TiO2 film. For WS-23, the higher molecular dipole moment can bring forth a more effective charge separation between donor and acceptor units, also resulting in an increase in V oc. The incorporated BTD unit can increase V oc by 57 mV, arising from the CB edge shift of TiO2 (accounting for 40%, 23 mV) and the retarding charge recombination (accounting for 60%, 34 mV). As a consequence, WS-23 realizes an optimizing photovoltaic efficiency (η = 8.15%), with an improvement of 36.5% with respect to WS-22.