Controlling adsorption of two dyes on TiO2 surface to improve the efficiency of see-through dye-sensitized solar cells

Transparent dye-sensitized solar cells (DSSCs) were fabricated utilizing judiciously selected blue-absorbing (D-131) and red-absorbing (SQ-140) two sensitizing dyes. These dyes exhibit complementary light absorption along with the least optical absorption in the 500 nm-600 nm wavelength region. This is helpful to impart not only high average visible transparency (AVT) but also high photoconversion efficiency (PCE). D-131 exhibits not only an approximately 1.5 times higher adsorption speed but also an approximately 4 times stronger binding on the TiO2. Stepwise dye adsorption on TiO2 was conducted to suppress unwanted and non-favourable inter-dye interactions. It has been demonstrated that when the SQ-140 adsorbed substrate was subjected to further dye adsorption by the D-131, there was a replacement of the SQ-140 by the D-131 reaching the optimal presence of both of the dyes in 20–30 min giving the best PCE of 4.5 %. However, in the opposite sequence of dye adsorption, there was no replacement of D-131 by SQ-140, rather SQ-140 was adsorbed in the available surface on the mesoporous TiO2 adsorbed with D-131. In this 2nd case, the optimal presence of both dyes is attained in 40 min of adsorption by SQ-140 giving the best PCE of 5.45 %. Transparent DSSCs fabricated using stepwise co-sensitized photoanodes demonstrated an AVT of 47.5 % and 26.5 % with thinner (single-layer) and thicker (double-layer) mesoporous TiO2, respectively.