Simple sensitizers of low band gap based on 4-nitro-α-cyanostilbene prepared from a one-step reaction for efficient dye-sensitized solar cells

Two simple dyes D1 and D2 based on 4-nitro-α-cyanostilbene were conveniently synthesized from a one-step reaction. These dyes were prepared from the condensation of 4-carboxybenzaldehyde or 4-hydroxybenzaldehyde with 4-nitrobenzyl cyanide to afford 4-nitro-4′-carboxy-α-cyanostilbene (D1) and 4-nitro-4′-hydroxy-α-cyanostilbene (D2) respectively. Photophysical and electrochemical properties of the dyes were investigated by UV–vis spectroscopy and cyclic voltammetry. Their absorption spectra were broad with long-wavelength absorption maximum at 617–655 nm and optical band gap of 1.63 eV. These two dyes were used as sensitizers in quasi solid state dye-sensitized solar cells (DSSCs). Photovoltaic devices with D1 showed a maximum monochromatic incident photon to current efficiency (IPCE) of 70% and overall power conversion efficiency (PCE) of 4.8%, under illumination intensity 100 mW/cm2. The photovoltaic performance of the cell with D2 was lower because of less dye loading on the TiO2 surface, since it has OH anchoring group, and lower electron lifetime. Additionally, we have investigated the photovoltaic response of the DSSCs with nitrogen doped TiO2 photoanode and found that the PCE has been enhanced. The enhancement in PCE has been attributed to the increase in open circuit voltage and fill factor.