Significant Efficiency Improvement of the Black Dye-Sensitized Solar Cell through Protonation of TiO2 Films

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Abstract
This paper describes the influence of acid pretreatment of TiO2 mesoporous films prior to dye sensitization on the performance of dye-sensitized solar cells based on [(C4H9)4N]3[Ru(Htcterpy)(NCS)3] (tcterpy = 4,4‘,4‘ ‘-tricarboxy- 2,2‘,2‘ ‘-terpyridine), the so-called black dye. The HCl pretreatment caused an increase in overall efficiency by 8%, with a major contribution from photocurrent improvement. It is speculated, from the analysis of incident photon-to-electron conversion efficiency, UV−vis absorption spectra, redox properties of the dye and TiO2, and the impedance spectra of the dye-sensitized solar cells, that photocurrent enhancement is attributed to the increases in electron injection and/or charge collection efficiency besides the improvement of light harvesting efficiency upon HCl pretreatment. Open-circuit photovoltage (Voc) remained almost unchanged in the case of significant positive shift of flat band potential for TiO2 upon HCl pretreatment. The suppression of electron transfer from conduction band electrons to the I3- ions in the electrolyte upon HCl pretreatment, reflected by the increased resistance at the TiO2/dye/electrolyte interface and reduced dark current, resulted in a Voc gain, which compensated the Voc loss due to the positive shift of the flat band. Using the HCl pretreatment approach, 10.5% of overall efficiency with the black dye was obtained under illumination of simulated AM 1.5 solar light (100 mW cm-2) using an antireflection film on the cell surface.

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