Parameters Influencing Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films

Abstract

Optical excitation of Ru II (2,2′-bipyridyl-4,4′dicarboxylate) 2 (NCS) 2 -sensitized nanocrystalline TiO 2 films results in injection of an electron into the semiconductor. This paper addresses the kinetics of charge recombination which follows this charge separation reaction. These charge recombination kinetics were found to be strongly dependent upon excitation intensity, electrolyte composition, and the application of an electrical bias to the TiO 2 film. For excitation intensities resulting in less than one excited dye molecule/TiO 2 particle, the recombination kinetics were independent of excitation intensity. Increasing the excitation intensity above this level resulted in a rapid acceleration in the charge recombination kinetics. Similarly, for positive electrical potentials applied to the TiO 2 electrode, the recombination kinetics were independent of applied potential. If the applied potential was more negative than a threshold potential V kin , a rapid acceleration of the charge recombination kinetics was again observed, for example from ∼1 ms at +0.1 V vs Ag/AgCl to ∼3 ps at -0.8 V (∼10 8 fold increase in the rate). Moreover, at a constant applied potential the charge recombination kinetics were found to be strongly dependent upon electrolyte composition (up to 10 6 -fold change in rate). This strong dependence upon the electrolyte composition was found to be associated with shifts in the threshold potential V kin . Spectroelectrochemical measurements were used to monitor the shift in the trap/conduction band density of states induced by the electrolyte composition. A direct correlation was observed between the threshold voltage V kin observed from kinetic measurements, and the threshold voltage for electron occupation of conduction band/trap states of the TiO 2 observed from spectroelectrochemical measurements. This direct correlation was observed for a wide range of electrolyte compositions including protic and aprotic solvents and the addition of Li + ions and 4-tert-butylpyridine. We conclude that the charge recombination kinetics in such dye-sensitized films are strongly dependent upon the electron occupation in trap/conduction band states of the TiO 2 film. This occupation may be modulated by variations in light intensity, applied electrical potential, and electrolyte composition. These results are discussed with relevance to the function of dye-sensitized photoelectrochemical devices.

Enhanced Electron Collection Efficiency in Dye-Sensitized Solar Cells Based on Nanostructured TiO2 Hollow Fibers

“…5,6 This slow electron transport process competes with recombination of carriers with the electrolyte at the surface. The limitation imposed on the electron transport by the mesoporous structure hinders the progress in achieving higher conversion efficiencies.…”

“…The setup and method used for such measurements have been previously described. 5 Detailed photovoltaic parameters, namely, the open-circuit voltage (V OC ), short-circuit current density (J SC ), fill factor (FF), and the photovoltaic power conversion efficiency (η) are gathered in Table 1. Comparing devices containing nanostructured fiber-based photoanodes of increasing thickness, the open-circuit voltage V OC of the DSCs was observed to be reduced as the thickness of the photoactive layer increases.…”

“…Earlier studies performed with nanoparticulate mesoporous TiO 2 layers concluded that ecb -S + recombination is controlled to a large extent by trapping and detrapping of electrons at the surface of the oxide and at grain boundaries between spherical particles. 5,6 In the fibrous morphology, the number of contact points between long fibers, and therefore the number of traps, is expected to be considerably lower than between particles in mesoscopic layers. While k r is expected to be comparable for both particle-and fiberbased cells, it is then likely that k b is increased for nanostructured fibers films.…”

Parameters Influencing the Efficiency of Electron Injection in Dye-Sensitized Solar Cells

“…[20][21][22] However the influence of such additives upon the efficiency of electron injection in complete DSSCs, and their correlation with device performance, have received only limited attention to date. 7,8,24,32,33 This gap arises partly from the difficulty of measuring injection in complete devices using femtosecond transient absorption instruments. As a consequence, DSSC device optimisation studies to date typically have not focused on electron injection dynamics as being a significant factor determining device performance.…”

Comparison of Interfacial Electron Transfer through Carboxylate and Phosphonate Anchoring Groups†

“…8,[11][12][13][14][15][16] The incident photon-to-current conversion efficiency was found to be near unity in RuN3-sensitized TiO 2 solar cells, 2,10 and the high efficiency has been attributed to an ultrafast electron injection and a much slower charge recombination that happens on the microsecond to millisecond time scales. [17][18][19] The injection kinetics has been shown to be biphasic, consisting of a primary <100 fs component and slower components on a few to tens of picosecond time scales. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] The ultrafast component has been attributed to injection from the unrelaxed singlet metal-to-ligand charge transfer ( 1 MLCT) state and the slower components to injection from the 3 MLCT states near the band edge.…”