Open-circuit Photovoltage Decay Research Articles

Effects of dye adsorption on the electron transport properties in ZnO-nanowire dye-sensitized solar cells

Mercurochrome and N3 dyes are employed to be the sensitizers in the ZnO-nanowire (NW) dye-sensitized solar cells (DSSCs). A lower fill factor is obtained in the N3-sensitized cell which results in comparable efficiencies in both ZnO-NW DSSCs although the N3 molecules possess a wider absorptive range for light harvesting. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements are employed to investigate the electron transport properties in both ZnO-NW DSSCs. The results indicate that more abundant electron interfacial recombination occurs in the N3-sensitized ZnO-NW DSSC due to the higher surface trap density in the ZnO-NW photoanode after N3 dye adsorption.

Analysis of Photovoltage Decay Transients in Dye-Sensitized Solar Cells

It is shown that application of the so-called quasi-static approximation greatly simplifies the theoretical treatment of the open circuit photovoltage decay of dye-sensitized nanostructured solar cells (DSCs), since it removes the need to treat the kinetics of trapping and detrapping explicitly and leads to a straightforward analytical solution in the case of an exponential trap distribution. To identify the conditions under which the quasi-static approach is valid, transients calculated using the quasi-static approximation are compared with the results of numerical calculations that treat trapping and detrapping of electrons explicitly. The application of the quasi-static approach to derive the rate constant for the back-reaction of electrons from experimental photovoltage decay data is illustrated for an optimized DSC.

Determination of the Density and Energetic Distribution of Electron Traps in Dye-Sensitized Nanocrystalline Solar Cells

Electron transport and recombination in dye-sensitized nanocrystalline solar cells (DSCs) are strongly influenced by the presence of trapping states in the titanium dioxide particles, and collection of photoinjected electrons at the contact can require times ranging from milliseconds to seconds, depending on the illumination intensity. A direct method of determining the density and energetic distribution of the trapping states responsible for slowing electron transport has been developed. It involves extraction of trapped electrons by switching the cell from an open circuit to a short circuit after a period of illumination. An advantage of this charge extraction method is that it is less sensitive than other methods to shunting of the DSC by electron transfer at the conducting glass substrate. Results derived from charge extraction measurements on DSCs (with and without compact TiO(2) blocking layers) are compared with those obtained by analysis of the open circuit photovoltage decay.

How Does Back-Reaction at the Conducting Glass Substrate Influence the Dynamic Photovoltage Response of Nanocrystalline Dye-Sensitized Solar Cells?

In dye-sensitized nanocrystalline solar cells (DSC), the transfer of electrons from the conducting glass substrate to triiodide ions in solution is an important loss mechanism that can be suppressed by using thin compact blocking layers of TiO(2). Whereas back-reaction at the substrate is relatively unimportant under short circuit conditions, it must be taken into account at the maximum power point or at open circuit. The influence of the back-reaction on open circuit photovoltage decay measurements and on intensity modulated photovoltage (IMVS) measurements has been studied by model simulations and by experimental measurements. The simulations demonstrate that reliable information about DSC properties such as trapping distributions can only be derived from transient or periodic photovoltage responses if the back-reaction is suppressed by the use of suitable blocking layers.

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.

Characterization of Titanium Dioxide Blocking Layers in Dye-Sensitized Nanocrystalline Solar Cells

The properties of thin blocking layers of titanium dioxide used to improve the performance of dye-sensitized nanocrystalline solar cells have been studied. TiO2 blocking layers prepared on fluorine-doped tin oxide-coated glass by spray pyrolysis have been characterized by electrochemical impedance spectroscopy, spectroscopic ellipsometry, and voltammetry. The impedance data reveal the presence of a distribution of surface states at the titanium dioxide−electrolyte interface that is similar to the one seen in the case of nanocrystalline TiO2 films. The influence of the blocking layers on the back transfer of electrons to tri-iodide ions in electrolyte-based dye-sensitized nanocrystalline cells has been studied by open circuit photovoltage decay. The results show that the ability of the blocking layer to prevent the back reaction of electrons with tri-iodide ions in the electrolyte is excellent under short circuit conditions, but is limited under open circuit conditions by electron accumulation at the surfa...

Effect of pn coupling on open-circuit photovoltage decay in a silicon pn junction solar cell

A theory of open-circuit photovoltage decay in a pn junction solar cell is presented which accounts for the pn coupling, i.e. the coupling between the diffused layer and the base of the solar cell. The results have been analysed in some detail which show how the voltage decay is influenced by the various parameters of the diffused layer. It is found that the decay is sensitive to pn coupling in the initial stages, particularly in those solar cells in which a heavy doping of the diffused layer may have caused sufficient band-gap narrowing. A measurement of the voltage decay in the initial stages can therefore yield the parameters of the diffused layer in a solar cell.

Measurements of the open-circuit photovoltage decay in a silicon solar cell

Experimental results are presented for the open-circuit photovoltage decay in a silicon p-n junction solar cell for monochromatic light of five different wavelengths and for composite light. Analysis of the results in terms of the existing theories gives a reasonable estimate of the minority carrier lifetime and other material parameters of the solar cell.

Photovoltage decay in p-n junction solar cells including the effects of recombinations in the emitter

The expressions for the open circuit photovoltage decay in a p-n junction solar cell are derived, including the effects of recombinations in the emitter. It is shown that for a cell with base thickness wB≫LB, the base diffusion length, the voltage decay rate for small values of time depends on the emitter dark saturation current JE0; the larger the value of JE0 , the faster is the initial rate of voltage decay. For large values of time, the rate of voltage decay is solely determined by the minority carrier lifetime in the base τB and is independent of JE0 . However, for a cell with wB≲LB, the voltage decay is linear from the very beginning and the decay rate is of the form (kT/e)[(1/τB) +(1/t1)]. The time constant t1 is independent of τB . It, however, depends on the base thickness, the effective back surface recombination velocity, and the emitter dark saturation current JE0 . The value of JE0 depends on emitter thickness, front surface recombination velocity, drift field in the emitter, band-gap narrowing, and Auger recombinations in the emitter.

Theory of open-circuit photovoltage decay in a finite base solar cell with drift field

A theory of open-circuit photovoltage decay has been developed in a pn junction solar cell by using the Sturm-Liouville R-transform technique. The theory accounts for finite thickness of the base, drift field in the base and a finite surface recombination velocity at the back surface. The theory is applied to analyse some experimental results on photovoltage decay in a solar cell induced by monochromatic light. A good fit is obtained between the theoretical and the experimental results which gives an estimate of excess carrier lifetime, diffusion length and drift field in the base as well as the base thickness. It is shown that the excess carrier lifetime, as determined by the slope of the voltage decay curve, will be an effective lifetime which depends upon the sign and the magnitude of the drift field as well as the thickness of the base.

The meteorology of Philadelphia

Water is a promising source of clean hydrogen. Besides water electrolysis, the direct photoelectrochemical dissociation of water that combines light absorption and electrochemical water splitting into a single device is actively investigated by the scientific community. We report results on the p-type rhodium-doped strontium titanate (Rh:SrTiO3) surface-modified by adsorption of a 3d-transition metal complex which is known to be a good catalyst of the hydrogen evolution reaction (HER) from water. The tris-dioximate cobalt hexachloroclathrochelate with an encapsulated cobalt(II) ion was selected as a surface HER co-catalyst for this study. Samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The kinetics of the photoelectrochemical water dissociation was studied using the open circuit photovoltage decay (OCPD) and photoelectrochemical impedance spectroscopy (PEIS) methods. Under open circuit conditions, the cobalt cage complex did not substantially affect the HER kinetics compared to the bare doped Rh:SrTiO3 semiconductor. But when a reverse bias was applied, a significant difference caused by presence of the clathrochelate was observed. The rate constants of the charge transfer and the recombination processes were both affected, leading to the conclusion that the macrobicyclic cobalt-encapsulated species not only increased the rate of charge transfer, but also played a role as recombination centers for photoexcited minority carriers. The space charge capacitance was determined under the inversion conditions. Only under strong reverse bias it was found that, whereas bulk Rh:SrTiO3 has a p-type conductivity, the surface of both bare Rh:SrTiO3 and the cobalt clathrochelate-covered Rh:SrTiO3 show the properties of a n-type semiconductor.