Bifacial dye-sensitized solar cells (DSSCs) were fabricated utilizing dye cocktails of two dyes, Z-907 and SQ-140, which have complementary light absorption and photon harvesting in the visible and near-infrared wavelength regions, for panchromatic photon harvesting. The investigation of the rate of dye adsorption and the binding strengths of the dyes on mesoporous TiO2 corroborated the finding that the Z-907 dye showed a rate of dye adsorption that was about >15 times slower and a binding that was about 3 times stronger on mesoporous TiO2 as compared to SQ-140. Utilizing the dye cocktails Z-907 and SQ-140 from ethanol, the formation of the dye bilayer, which was significantly influenced by the ratio of dyes and adsorption time, was demonstrated. It was demonstrated that the dyes of Z-907 and SQ-140 prepared in 1:9 or 9:1 molar ratios favoured the dye bilayer formation by subtly controlling the adsorption time. In contrast, the 1:1 ratio counterpart was prone to form mixed dye adsorption; the best performance of the BF-DSSCs was shown when a dye cocktail of Z-907 and SQ-140 in a molar 9:1 ratio was used to prepare a photoanode for 1 h of dye adsorption. The BF-DSSCs thus exhibited PCEs of 4.23% and 3.48% upon the front and rear side light illuminations, a cumulated PCE of 7.71%, and a very good BBF of 83%.
Keywords: NIR dye; adsorption behavior; bifacial solar cells; dye cocktail; dye-bilayer; dye-sensitized solar cells; squaraine dye.