Studies of Charge Transport Processes in Dye-sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Although efficiencies exceeding 10% in full sunlight have been presented, major improvements of the system are however limited. Electron transport is one of the processes in the cell and is of major importance for the overall performance. It is further a complex process because the transport medium is a mesoporous film and the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, present models describing electron transport include simplifications, which limit the practical use, in terms of improving the DSC, because the included model parameters usually have an effective nature. This thesis focuses in particular on the influence of the mesoporous film on electron transport and also on the influence of electron-ion interactions. In order to model diffusion, which is assumed to be the transport process for electrons in the DSC, Brownian motion simulations were performed and spatial restrictions, representing the influence of the mesoporous film, were introduced by using representative models for the structure. The simulations revealed that the diffusion coefficient is approximately half the value for electrons and ions in mesoporous systems. To study the influence of ions, a simulation model was constructed in where electric fields were calculated with respect to the net charge densities, resulting from the different charge carrier distributions. The simulations showed that electron transport is highly dependent on the nature of the ions, supporting an ambipolar diffusion transport model. Experimentally, it was found that the transport process is dependent on the wavelength of the incident light; we found that the extracted current was composed of two components for green light illumination, one fast and one slow. The slow component showed similar trends as the normal current. Also we found that the transport coefficient scaled linearly with film thickness for a fixed current, which questions diffusion as transport process. Other experiments, investigating various effects in the DSC, such as the effect of different cations in the electrolyte, are also presented.QC 2010070

Brownian dynamics simulations of electrons and ions in mesoporous films

A model to study charge transport processes in mesoporous films of dye-sensitized solar cells is presented. By simulating electron and ion transport by Brownian dynamics in these films, a direct relation between grain connectivity and effective diffusion coeffs. was obtained. By comparing the macroscopic properties of a simple cubic and a diamond-structured unit cell, the latter better resembles the properties of the mesoporous oxide films in comparison with exptl. results. The model was used to optimize the size of the contact area between the interconnected particles in the mesoporous film with respect to the photocurrent

Studies of coupled charge transport in dye-sensitized solar cells using a numerical simulation tool

In this paper, we present a simulation platform designed to study coupled charge transport in dye-sensitized solar cell (DSC) devices. The platform, SLICE, is used to study the influence of ions in the electrolyte on electron transport in the nanoporous medium. The simulations indicate that both cationic and anionic properties should be considered when modeling DSCs and similar systems. Addnl., it was found that the effective permittivity coeff., ε, has no influence on the electron transport when the ionic concn. is sufficiently high due to the strong coupling between the resp. charged species

The influence of cations on charge accumulation in dye-sensitized solar cells

The relation between the VOC, light intensity, φ, and accumulated charge, Q, was studied for dye-sensitized solar cells (DSCs) contg. different counterions for the iodide/triiodide redox couple. At higher light intensities, VOC scaled in the order Cs+ > K+ > Na+ > Li+, which was caused in part by shifts in the conduction band edge. The relation between VOC and Q was fitted to an exponential trap model. Inclusion of a capacitive term, C, improved the fit significantly. The detd. values of C are large, up to 75 μF/cm2, and dependent on the cation. The largest fraction of C can be ascribed to the TiO2 bulk or the TiO2/dye/electrolyte interface. The interpretation of the trap distribution broadening parameter, β, is dependent on the fitting model. Using the model including the linear CVOC term, β was independent of cation and could be viewed as a TiO2 material parameter, while in the model excluding CVOC, β was dependent of cation. Voltage decay expts. were performed to study the cationic influence on recombination. Electron lifetimes were calcd. from the voltage decay curves and the DSC contg. Li+ yielded the shortest lifetime followed by the DSCs contg. Na+, K+ and Cs+. Voltage decay curves include the effect of the TiO2 conduction band shifts in the comparison of electron lifetimes with different cations. It is accordingly suggested that electron lifetimes be calcd. from charge decay curves. From such a comparison, the DSC contg. Li+ yielded the shortest lifetime whereas the DSCs contg. Na+, K+ or Cs+ showed approx. identical lifetimes

Recombination and transport processes in dye-sensitized solar cells investigated under working conditions [Erratum to document cited in CA145:214248]

The effect of RC attenuation was not correctly taken into account. This led to a misinterpretation of the photocurrent response data that was in Figure 3a on page 17717 and further discussion. Under operating conditions, electrons are accumulated throughout the nanostructured TiO2, rendering the film conducting. The photocurrent response under such conditions is affected or dominated by the RC time of the solar cell, where the resistance R is the sum of the resistances of the conducting glass and that of the TiO2 film, and the capacitance C the sum of the capacity of the conducting glass/electrolyte interface and the chem. capacitance of the nanostructured TiO2. Figure 3b (3b has been reprinted) shows the transport time calcd. from the data of Figure 3a using appropriate correction for the RC time. At lower light intensities transport time decreased with voltage, whereas little change was obsd. at the highest light intensity. At voltages high than 0.65 V transport times were independent of light intensity. On page 17718, Table 2 is incorrect; supplemental Table 2 shows calcd. effective diffusion coeffs. and diffusion lengths using the cor. values of τtr

Recombination and Transport Processes in Dye-Sensitized Solar Cells Investigated under Working Conditions

The transport and recombination of electrons in dye-sensitized TiO2 solar cells were studied by anal. of the current and voltage response to a small square-wave light-intensity modulation. Solar cells were studied under working conditions by using potentiostatic and galvanostatic conditions. An increase in applied voltage, i.e., from 0 V toward open-circuit voltage, was found to lead to faster electron transport at low light intensities, while it slowed transport at higher light intensities. This observation seems to be conflicting with the multiple trapping model with diffusive transport. An effective diffusion length at the max. power point was calcd., and it was shown that it decreases with increasing light intensity

Interpretation of small-modulation photocurrent transients in dye-sensitized solar cells - A film thickness study

Electron transport in dye-sensitized solar cells with varying mesoporous TiO2 film thicknesses was investigated using exptl. and computational methods. More specifically, photocurrent transients resulting from small-amplitude square-wave modulation of the incident light were recorded for a series of solar cells, whereby the dependence of the wavelength and direction of the illumination was investigated. The responses were compared to simulations using different models for diffusional charge transport and analyzed in detail. The photocurrent transients are composed of 2 components: an initial fast response in case of illumination from the working electrode side, or an initial apparent delay of photocurrent decay for illumination from the counter electrode side, followed by a single exponential decay at longer times, with a time const. that is identified as the electron transport time. The initial response depends on the thickness and the absorption coeff. of the film. Transport times for different films were compared at equal short-circuit c.d., rather than at equal light intensity. Exptl., the transport time showed a power-law dependence on the film thickness with an exponent of about 1.5. Anal. using the quasi-static multiple trapping (MT) formulation demonstrates that this behavior originates from differences in quasi-Fermi level in the TiO2 films of different thickness when equal photocurrents are generated. The Fokker-Planck relation was used to derive expressions for the electron flux in porous TiO2 films with a position-dependent diffusion coeff

On the Influence of Anions in Binary Ionic Liquid Electrolytes for Monolithic Dye-Sensitized Solar Cells

Five ionic liqs. (ILs) Im+A-, where Im+ = 1-methyl-3-n-butyl-imidazolium, A- = I- (1), BF4- (2), SCN- (3), CF3CO2- (4), and CF3SO3- (5), were used in electrolytes for dye-sensitized monolithic solar cells. The properties of the electrolytes and various characteristics of the solar cell performance, such as electron transport and electron lifetime, were studied. The compn. of the binary electrolytes, i.e., the different anions, have a significant effect on the viscosity, but only a modest effect of the measured diffusion coeff. for triiodide. No significant effect of the electrolyte compn. on the electron transport time in the mesoporous TiO2 film was found, while there was a pronounced effect on the electron lifetime. Monolithic solar cells with thiocyanate, IL 3, showed overall light-to-electricity conversion efficiency up to 5.6% in 250 W m-2 simulated sunlight and have promising stability

Using a molten organic conducting material to infiltrate a nanoporous semiconductor film and its use in solid-state dye-sensitized solar cells

We describe a method to fill thin films of nanoporous TiO2 with solid organic hole-conducting materials and demonstrate the procedure specifically for use in the preparation of dye-sensitized solar cells. Cross-sections of the films were investigated by scanning electron microscopy and it was observed that a hot molten organic material fills pores that are 10 mu m below the surface of the film. We characterized the incident photon to current conversion efficiency properties of the solid TiO2/organic dye/organic hole-conductor heterojunctions and the spectra show that the dye is still active after the melting process. (C) 2008 Elsevier B.V. All rights reserved