Dye-sensitized solar cells (DSCs) using titanium dioxide (TiO 2 ) electrodes with different haze were investigated. It was found that the incident photon to current efficiency (IPCE) of DSCs increases with increase in the haze of the TiO 2 electrodes, especially in the near infrared wavelength region. Conversion efficiency of 11.1%, measured by a public test center, was achieved using high haze TiO 2 electrodes. This indicates that raising the haze of TiO 2 electrodes is an effective technique for improvement of conversion efficiency.
Internal resistance in a dye-sensitized solar cell (DSC) was investigated using electrochemical impedance spectroscopy measurements. Four resistance elements were observed in the impedance spectra, and their dependencies on the applied bias voltage were characterized. It is found that the resistance element related to charge transport at the TiO2/dye/electrolyte interface displays behavior like that of a diode, and the series resistance elements largely correspond to the sum of the other resistance elements. An equivalent circuit for DSCs is proposed based on these results.
With the aim of increasing conversion efficiency, the series-internal resistance of dye-sensitized solar cells (DSCs) was investigated with electrochemical impedance spectroscopy measurement based on an equivalent circuit of DSCs. It was found that series-internal resistance correlates positively with the sheet resistance of the transparent conducting oxide and the thickness of the electrolyte layer and negatively with the roughness factor of the platinum counter electrode. A cell sensitized with a black dye with series-internal resistance of 1.8Ωcm2 was fabricated and showed conversion efficiency of 10.2% when measured with a metal mask under an air mass of 1.5 sunlight.
Using transient absorption spectroscopy, we investigated the effects of 4-tert-butylpyridine (tBP) and Li ions on photoinduced electron injection efficiency in nanocrystalline TiO 2 films sensitized by black dye. The efficiency increased with the addition of Li ions but decreased with the addition of tBP molecules. These results indicate that a conduction band shift was induced by the solvation effect of the additives. Using recombination kinetic measurements, we examined that such a solvation effect was induced by the intercalation of Li ions into TiO 2 particles and by the adsorption of tBP on the TiO 2 surface. When both additives were used simultaneously, the efficiency was suppressed more than when only tBP was added. This difference in efficiency suggests that the local concentration of tBP near the TiO 2 surface increased with the addition of Li ions. This cooperative effect may have been caused by the formation of a complex between tBP and Li in solution.
IntroductionSince highly efficient dye-sensitized solar cells (DSSCs) were first reported, 1 much research has been carried out to improve their performance. Solar cells consisting of N719 dye, in which two protons of N3 dye [cis-di(thiocyanato)-bis(2,2′-bipyridiyl-4,4′-dicarboxylate)ruthenium(II); Ru(dcbpy) 2 (NCS) 2 ] are replaced by tetrabutylammonium (TBA) cations, adsorbed on nanocrystalline TiO 2 films (N719/TiO 2 ) show high solar-energyto-electricity-conversion efficiency (η > 11%). 2 Black dye (BD; trithiocyanato(4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine)ruthenium(II); Ru(tcterpy)(NCS) 3 ) is also a promising sensitizer dye for DSSCs because its lowest unoccupied molecular orbital (LUMO) is located just above the conduction band of TiO 2 , and because its absorption edge is in the near-infrared range. [3][4][5] With further optimizations, such as the reduction of internal resistance and the preparation of a high-haze electrode, 6-8 η values greater than 11% can be achieved with BD-sensitized nanocrystalline TiO 2 (BD/TiO 2 ) DSSCs. 9 However, to further improve these DSSCs, more detailed knowledge of the mechanisms of DSSC processes, especially with regard to the electron injection mechanism, is required.To understand the primary processes that occur in DSSCs, the photophysical processes of dye-sensitized TiO 2 electrodes have been studied extensively by means of various experimental techniques. Electron injection, which is the most important primary process, has been studied mainly by means of transient absorption (TA) spectroscopy. 10-12 Electron injection dynamics can be studied by means of femtosecond TA measurements. For dye-sensitized films based on N3 and N719 complex dyes, nonexponential ultrafast electron injection has been observed in the 100-fs to 100-ps time range. 13-18 Furthermore, microsecond TA measurements are used to study the recombination
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