Tailoring the benzotriazole (BTZ) auxiliary acceptor in a D-A′-π-A type sensitizer for high performance dye-sensitized solar cells (DSSCs)

Molecular engineering toward decreasing the band gap and thus extending the absorption spectrum of benzotriazole-based dye is a challenge. Herein, the benzotriazole structure is finely tuned by fusing another electron deficient ring to the benzo unit leading to a series of benzotriazole-based electron-withdrawing heterocyclic rings including benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole. These together with benzotriazole are incorporated into the π-bridge, respectively, as an auxiliary acceptor between the triphenylamine donor and the cyanoacrylic acid anchoring group for the construction of donor-auxiliary acceptor-π-acceptor type sensitizers (LC-2, LC-3, LC-4 and LC-5). The benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole units show greatly increased electron-withdrawing capability and the corresponding sensitizers (LC-3, LC-4 and LC-5) exhibit broadened spectral response range compared to the reference benzotriazole-based dye LC-2 due to the decreased energy gap. Thus, the LC-3 and LC-4 cells exhibit significantly enhanced photocurrent density (Jsc) values due to the greater light-harvesting abilities compared to LC-2. However, the electrochemical impedance analysis suggests the electron lifetime and charge recombination resistance are decreased due to the introduction of benzobitriazole, benzotriazolequinoxaline and benzotriazolethiadiazole, resulting in a slight loss of open-circuit voltage (Voc). The findings provide a guidance for the future molecular design of highly efficient organic sensitizers for use in dye-sensitized solar cells.