A Resonance Raman spectroscopic study on charge transfer enhancement in photosensitizers

The charge transfer (CT) properties of photosensitizers largely determine the photovoltaic performances of dye sensitized solar cells (DSSCs). Thus, understanding the CT properties of photosensitizers is key to further improving the performances of DSSCs. We herein investigated the underlying relationship between the molecular structures and CT properties of the photosensitizers using resonance Raman (RR) spectroscopy and density functional theory (DFT) calculations. RR spectroscopy combined with DFT calculations showed that the presence of a triple bond (T-D1, T-D2, and T-D3) enhanced the degree of CT from the donor to the acceptor. In addition, the presence of electron donating groups (EDGs) on the donor (T-D2 and T-D3) further increased the CT properties of the donor. Moreover, DFT analysis based on the harmonic oscillator model of aromaticity revealed that the presence of a triple bond and an EDG increased the quinoidal character of the photosensitizer in the excited state. Finally, it was found that the degree of CT properties exhibited by each photosensitizer was in good agreement with the order of the DSSC performances.