A D–A−π–A dye ( PTZ-5 ) has been synthesized by introducing a benzothiadiazole (BTD) unit as an auxiliary acceptor in a phenothiazine-based D−π–A dye( PTZ-3 ) to broaden its spectral response range and improve the device performance. Photophysical properties indicate that the inclusion of BTD in the PTZ-5 effectively red-shifted the absorption spectra by reducing the E gap . However, the device measurements show that the open-circuit voltage ( V oc ) of PTZ-5 cell (640 mV) is obviously lower than that of the PTZ-3 cell (710 mV). This results in a poor photoelectric conversion efficiency (PCE) (4.43%) compared to that of PTZ-3 cell (5.53%). Through further comparative analysis, we found that the introduction of BTD increases the dihedral angle between the D and A unit, which can reduce the efficiency of intramolecular charge transfer (ICT), lead to a less q CT and lower molar extinction coefficient of PTZ-5 . In addition, the ESI test found that the lifetime of the electrons in the PTZ-5 cell is shorter. These are the main factors for the above unexpected result of PCE. Our studies bring new insights into the development of phenothiazine-based highly efficient dye-sensitized solar cells (DSSCs).

The molecular engineering of phenylamine- and phenothiazine-based porphyrin dyes is an advisable strategy for high-efficiency dye-sensitized solar cells (DSSC). Herein, we incorporated two and three phenothiazine units into the triphenylamine donor (D) unit of porphyrin dye (T-1) to replace the benzene units, resulting in two novel porphyrin dyes, T-3 and T-4. UV–vis absorption studies revealed that the molar extinction coefficients (ε) on TiO2 films of T-3 and T-4 were significantly higher than T-1, mainly owing to the increased loaded amount. Hence, the T-3- and T-4-devices exhibit higher IPCE and Jsc values. Furthermore, transient PL measurements and electrochemical impedance spectroscopy (EIS) results demonstrate that the electron injection efficiency (ηinj) and electron lifetime (τ) of the T-3-device were the highest. Thus, the T-3-device achieved the highest power conversion efficiency (PCE) of 8.02% (N719, 8.45%) with V oc = 670 mV, J sc = 16.84 mA cm–2, and FF = 70.02%. Meanwhile, the theoretical calculation studies suggest the smaller dihedral angle between the D unit and porphyrin macrocycle of T-3, leading to a favorable intramolecular charge transfer (ICT) process (t = 1.831, q ct = 0.922) compared to that of T-4. These results demonstrate that introducing a different number of phenothiazine units to replace the benzene units of triphenylamine in suitable positions can serve as an effective strategy for developing efficient DSSC.

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