Illumination intensity dependence of the photovoltage in nanostructured TiO2 dye-sensitized solar cells

The open-circuit voltage (V(oc)) dependence on the illumination intensity (phi0) under steady-state conditions in both bare and coated (blocked) nanostructured TiO2 dye-sensitized solar cells (DSSCs) is analyzed. This analysis is based on a recently reported model [Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 8774] which describes the rate of interfacial electron transfer from the conduction band of TiO2 to acceptor electrolyte levels (recombination). The model involves two possible mechanisms: (1) direct, isoenergetic electron injection from the conduction band and (2) a two-step process involving inelastic electron trapping by band-gap surface states and subsequent isoenergetic transfer of trapped electrons to electrolyte levels. By considering the variation of V(oc) over a wide range of illumination intensities (10(10) < phi0 < 10(16) cm(-2) s(-1)), three major regions with different values of dV(oc)/d phi0 can be distinguished and interpreted. At the lower illumination intensities, recombination mainly involves localized band-gap, deep traps at about 0.6 eV below the conduction band edge; at intermediate photon fluxes, recombination is apparently controlled by a tail of shallow traps, while, for high enough phi0 values, conduction band states control the recombination process. The high phi0 region is characterized by a slope of dV(oc)/d log phi0 congruent with 60 mV, which indicates a recombination of first order in the free electron concentration. The study, which was extended to different solar cells, shows that the energy of the deep traps seems to be an intrinsic property of the nanostructured TiO2 material, while their concentration and also the density ([symbol: see text]t approximately 10(18)-10(19) cm(-3)) and distribution of shallow traps, which strongly affects the shape of the V(oc) vs phi0 curves, change from sample to sample and are quite sensitive to the electrode preparation. The influence of the back-reaction of electrons from the fluorine-doped tin oxide (FTO) conducting glass substrate with electrolyte tri-iodide ions on the V(oc) vs phi0 dependence characteristic of the DSSC is analyzed. It is concluded that this back-reaction route can be neglected, even at low light intensities, when its rate (exchange current density, j0), which can vary over 4 orders of magnitude depending on the type of FTO used, is low enough (j0 < or = 10(-8)A cm(-2)). The comparison of V(oc) vs phi0 measurements corresponding to different DSSCs with and without blocking of the FTO-electrolyte contact supports this conclusion.     

Extending the current collector into the nanoporous matrix of dye sensitized electrodes

A new type of high surface area TiO(2) electrode for DSSCs is proposed. The new electrode consists of a transparent conductive nanoporous matrix that is coated with a thin layer of TiO(2). This design ensures a distance of several nanometers between the TiO(2)-electrolyte interface and the current collector throughout the nanoporous electrode, in contrast to several micrometers associated with the standard electrode. In addition the new electrode contains inherent screening capability due to the high doping level of the conducting core matrix. Theoretically, this electrode should overcome the collection and image field problems associated with solid-state DSSCs. Using a flat analogue of the new electrode we show that unless the TiO(2) coating is thicker than 6 nm, the electrode performance is very low due to fast recombination. Two mechanisms for the thickness effect on the recombination rate, that are proposed, provide new insight to the DSSC operation.     

UV-Vis-NIR spectroelectrochemical and in situ conductance studies of unusual stability of n- and p-doped poly(dimethyldioctylquaterthiophene-alt-oxadiazole) under high cathodic and anodic polarizations

Combined CV studies and UV-Vis-NIR spectroelectrochemical investigations revealed an unusual stability of the p- and n-doped PMOThOD in the wide potential window of 4 V. The n-doping process occurs in this polymer down to -2.7 V (vs. Ag/Ag+) in a non-destructive way with the characteristic development of the omega3 transition as a function of the doping level. In situ electronic transport studies revealed a high conductivity of the n-doped polymer which implies high mobility of the negatively charged carriers in the freshly doped PMOThOD film electrodes. An increase in the cathodic polarization, long-term cycling of the film electrodes, especially of higher thickness, results in a growing contribution of the negatively charged carriers trapping to the redox properties of the PMOThOD. The trapping of the charged carriers reduces gradually the electronic conductance of the PMOThOD film, but its effect on the redox-capacity of the film (in a typical scan rates range up to 50 mV s(-1)) is only minor.     

Sub-micrometer polarimetry of chiral surfaces using near-field scanning optical microscopy

In this work we have studied the optical activity of chiral crystal surfaces with polarized near-field scanning optical microscopy (NSOM); our studies clearly demonstrated that polarized NSOM can be utilized to determine chirality at crystal surfaces.     

Determination of rate constants for charge transfer and the distribution of semiconductor and electrolyte electronic energy levels in dye-sensitized solar cells by open-circuit photovoltage decay method

A combination of electron lifetime measurement in nanoparticles as a function of the Fermi level position at high resolution in the potential scale with a new model to describe this dependence provides a powerful tool to study the microscopic processes and parameters governing recombination in dye-sensitized solar cells. This model predicts a behavior divided in three domains for the electron lifetime dependence on open-circuit voltage that is in excellent agreement with the experimental results: a constant lifetime at high photovoltage, related to free electrons; an exponential increase due to internal trapping and detrapping and an inverted parabolla at low photovoltage that corresponds to the density of levels of acceptor electrolyte species, including the Marcus inverted region.     

The trap-limited diffusivity of electrons in nanoporous semiconductor networks permeated with a conductive phase

The transport of photogenerated electrons in nanocrystalline semiconductor networks permeated with a conducting phase is studied, with a particular emphasis on dye-sensitized nanoporous TiO₂ solar cells. We extend the classical approach to the trap-limited mobility according to specific features of the nanoporous configuration: electron transport by diffusion, the capacitive behavior of the nanoporous film and the possible bandshifts due to the charging of surface states. We show that the trap-limited diffusivity, as measured by small-signal techniques, is proportional to the ratio of the conduction-band capacitance and the trap capacitance. These capacitances are defined in terms of a pseudopotential related to the chemical energy of the free electrons, in order to account for possible band unpinning. Several specific distributions of bandgap states are investigated. The dependence of the trap capacitance on the number of free electrons takes the general form C_trap=An^1-a, where 0a1 depends on the distribution of the traps. The trap-limited diffusivity depends on the number of free electrons as D_n=Bn^a, and D_n also shows a power-law dependence with the light intensity. We describe the correlation of the electron conductivity with the photovoltage in the solar cell and the photon irradiation intensity. PACS 81.07.Bc; 73.30.+y     

Properties of the electronic density of states in TiO<Subscript>2</Subscript> nanoparticles surrounded with aqueous electrolyte

Results on the density of sates of nanostructured TiO₂ as a function of particle size and temperature are reported. In TiO₂ nanoparticles with a mean diameter 10 nm, the density of states (DOS) is strongly temperature-dependent, indicating a rearrangement of the bandgap states in which the exponential energy parameter (width of the distribution) increases from 0.080 to 50 °C. For nanoparticles with mean diameters of 20 and 30 nm the DOS is much closer to an exponential distribution, and is much less sensitive to temperature variations. It is suggested that nanometer confinement has a significant influence on the density of electronic states for 10-nm particles, while band tailing is similar to that occurring in bulk semiconductors for the larger particles.

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Unpredicted electron injection in CdS/CdSe quantum dot sensitized ZrO2 solar cells

A quantum dot sensitized solar cell based on a porous ZrO(2) film, sensitized with CdSe quantum dots using CdS as an intermediate layer is presented. We observe electron injection from photo-excited quantum dots into the ZrO(2), which is unexpected due to the much higher conduction band edge (closer to the vacuum level) of bulk ZrO(2) compared to TiO(2).