We have previously reported the efficiency enhancement of gel electrolyte-based SnO2 Dye-sensitized Solar Cells (DSCs) through the use of an ultra-thin (∼5 nm) coverage of the SnO2 nanopaticles with CaCO3 in order to reduce the surface recombination. Also, we have reported the effect of the salt used in the gel electrolyte on the efficiency of these devices. Another vital component of the DSCs is the dye used for light absorption. In our previous studies, the indoline dye D358 was primarily used because of its relatively strong anchorage on to the nanoparticles.
However, the absorption spectra of the other dyes in the indoline dye family such as D149 and D131 indicate that they have complementary absorption in the visible region suggesting that, a mixture of these dyes could produce better efficiencies. We have observed a significant improvement in efficiency of these devices when a double-dye sensitization is employed. We have attempted to interpret the observed results using the molecular geometry and other properties of the dyes.
We used a 1:1 double-dye mixture having combinations of D358, D149 and D131 in DSC devices based on CaCO3 coated SnO2. Among the different combinations, D149/D131 mixture gave the best efficiencies. This device produced efficiencies of 5.10% and 4.80% for a liquid and a gel electrolyte respectively under 1.5 AM (100 mW cm−2) simulated sunlight. The general trend of the observed results indicates that there is an efficiency enhancement of about 30% for the D149/D131 mixture compared to their respective individual performances.
- A 1:1 ratio of double-dye mixture having combinations of D358, D149 and D131 in DSC devices based on CaCO3 coated SnO2 was used. The effects of different salts present in the electrolyte on the efficiency of these devices were also examined. Among the different combinations, D149/D131 mixture gave the best efficiencies. This device produced efficiencies of 5.10%, 4.80%, and 4.31% respectively for a liquid electrolyte and two different gel electrolytes containing LiI and Pr4N+I salts, under 1.5 AM (100 mW cm−2) simulated sunlight.
- The general trend of the observed results indicates that there is an efficiency enhancement of about 30% for the D149/D131 mixture compared to their respective individual performances. It can be inferred that the linear nature of D131 makes it possible for D131 to adsorb on the surface areas not covered by the bulky D149 dye molecules when a mixture of these two dyes is used. The amounts of D131, D149 and the mixture of D131/D149 dyes adsorbed on to the samples were calculated to be 2.80 × 10−5 mol L−1 cm−2, 2.25 × 10−5 mol L−1 cm−2 and 5.65 × 10−5 mol L−1 cm−2.
- The D358/D131 mixture does not significantly improve the device efficiency compared to D149/D131. Co-sensitization may require somewhat linear molecules for better dye attachment which also helps reduce dye aggregation. D149 dye molecules show somewhat better linear attachment compared to the D358 dye molecules as it has one –COOH anchoring group. Second possibility is the formation of a new molecular complex in the mixture due to the interaction between carboxyl and hydroxyl groups of D131 and D358 dye molecules.
Wanninayake, W.M.N.M.B., Premaratne, K. and Rajapakse, R.M.G., 2017. Photovoltaic performance of SnO2/CaCO3-based dye-sensitized solar cells co-sensitized using metal-free organic dyes. Journal of Photochemistry and Photobiology A: Chemistry, 346, pp.541-547.
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