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

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Abstract

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 (Jsc) and open circuit voltage (Voc). 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 Jsc from 13.77 (WS-22) to 16.91 mA cm–2 (WS-23). Moreover, the improvement of the Voc in WS-23 is originated by a synergy contribution of the uplifting of ECB 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 Voc. The incorporated BTD unit can increase Voc 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.

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