Dye-sensitized solar cells (DSSCs) with the capability of photon harvesting from both front and rear sides, consisting of newly developed NIR dye (SQ-140), iodine-based redox electrolyte, and transparent counter electrode with high bifaciality factor (BFF) and good cumulated photoconversion efficiency (PCE), are successfully fabricated and characterized. The use of transparent TiO2-based photoanode leads to improved BFF as compared to its opaque TiO2 counterparts: from 74% to 86% under similar experimental conditions and device parameters. The high molar extinction coefficient of the NIR dye SQ-140, needing a thinner TiO2 layer, was found to be advantageous to maintaining the high BFF even in thicker TiO2 films without serious compromise in the PCE. The lower efficiency of the bifacial DSSC under rear illumination was attributed to the hampered optical penetration as well as absorption of light by the intense colored iodine-based redox electrolyte.
Herein, transparent dye‐sensitized solar cells (DSSCs) are fabricated using two sensitizing dyes aiming toward panchromatic photon harvesting following two different approaches of dye adsorption such as using dye cocktail and sequential dye adsorption. A combination of complementary yellow‐colored dye (D‐131) and green‐colored far‐red‐sensitive squaraine dye (SQ‐140) is judiciously selected not only for panchromatic light absorption but also for bypassing the absorption in the high eye‐sensitivity wavelength region (500–600 nm) region to improve the transparency. The DSSC fabricated using a dye cocktail of D‐131 and SQ‐140 in a 9:1 molar ratio exhibits a photoconversion efficiency (PCE) and average visible transparency (AVT) of 3.89% and 36.9%, respectively. In contrast, stepwise and controlled dye adsorption of SQ‐140 followed by D‐131 on mesoporous TiO2 leads to further improvement in the PCE from 3.89% to 4.50%, while AVT is reduced from 36.9% to 15.6%. This marked improvement in the PCE for the stepwise co‐sensitized transparent DSSCs is attributed to the suppression of unfavorable inter‐dye interaction leading to synergistic photon harvesting by both of the constituent sensitizing dyes.
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