Determination of the Electron Diffusion Length in Dye-Sensitized Solar Cells by Substrate Contact Patterning

A new method to est. the electron diffusion length in dye-sensitized solar cells is presented. Dye-sensitized solar cells were fabricated on conducting glass substrates that were patterned by laser ablation of the fluorine-doped tin oxide coating to form parallel contact strips sepd. by uncontacted strips of the same width. The relative collection efficiency was measured as a function of the gap between the contact strips, which dets. the lateral distance traveled by electrons to reach the contacts. To avoid complications arising from nonlinear recombination kinetics, current measurements were performed using small amplitude perturbations of the electron d. close to open circuit and the max. power point to minimize electron d. gradients in the film. One- and two-dimensional solns. of the continuity equation for electron transport and back reaction predict that the relative collection efficiency should fall as spacing between the contact strips exceeds the electron diffusion length and electrons are lost by back electron transfer during transit to the contacts. Measurements of the relative collection efficiency were fitted to the predicted dependence of the collection efficiency on the spacing between the contact strips to obtain the value of the electron diffusion length. The diffusion length is found to increase with voltage both at open circuit and at the max. power point

In Situ Detection of Free and Trapped Electrons in Dye-Sensitized Solar Cells by Photo-Induced Microwave Reflectance Measurements

In order to study the behavior of photoinjected electrons in dye-sensitized solar cells (DSC), steady-state microwave reflectance measurements (33 GHz, Ka band) have been carried out on a working cell filled with electrolyte. The experimental arrangement allowed simultaneous measurement of the light-induced changes in microwave reflectance and open circuit voltage as a function of illumination intensity. In addition, frequency-resolved intensity-modulated microwave reflectance measurements were used to characterize the relaxation of the electron concentration at open circuit by interfacial transfer to tri-iodide ions in the electrolyte. The dependence of the free and trapped electron concentrations on open circuit voltage were derived, respectively, from conductivity data (obtained by impedance spectroscopy) and from light-induced near IR transmittance changes. These electron concentrations were used in the fitting of the microwave reflectivity response, with electron mobility as the main variable. Changes in the complex permittivity of the mesoporous films were calculated using Drude–Zener theory for free electrons and a simple harmonic oscillator model for trapped electrons. Comparison of the calculated microwave reflectance changes with the experimental data showed that the experimental response arises primarily from the perturbation of the real component of the complex permittivity by the high concentration of trapped electrons present in the DSC under illumination. The results suggest that caution is needed when interpreting the results of microwave reflectance measurements on materials with high concentrations of electron (or hole) traps, since an <i>a priori</i> assumption that the microwave response is solely determined by changes in conductivity (i.e., by free electrons) may be incorrect. The intensity-modulated microwave reflectance measurements showed that relaxation of the free and trapped electron concentrations occurs on a similar time scale, confirming that the free and trapped electron populations remain in quasi-equilibrium during the decay of the electron concentration

Carrier density and interfacial kinetics of mesoporous TiO2 in aqueous electrolyte determined by impedance spectroscopy

Water splitting at a semiconductor/solution interface with the only input of sunlight to generate hydrogen is one of the most attractive strategies to produce and store chemical energy. In the present study we have investigated carrier dynamics and interfacial kinetics of mesoporous TiO2 in an aqueous solution. The applicability of the transmission line model for mesoporous semiconductors has been validated to identify chemical capacitance, transport resistance and charge transfer resistance in this system by testing samples of different thicknesses in the dark and under illumination. We found that both transport resistance and chemical capacitance scale well with sample thickness, while charge transfer resistance scales with thickness when the FTO substrate is not exposed to the solution. Otherwise, there is a competition between charge transfer through TiO2 and through the FTO substrate. Under illumination, the electron density is dominated by photogenerated carriers at biases below the open circuit potential, whereas at higher bias, the applied potential determines the electron density. Evidence of charge transfer via surface states has been experimentally observed and corroborated with a physical model, which explicitly includes charge transfer through a monoenergetic trap for electron and holes. This study may lay the basis for understanding more complex processes at anodic potentials on the TiO2/solution interface where water splitting reactions take place

Nanostructured Ternary FeCrAl Oxide Photocathodes for Water Photoelectrolysis

A sol–gel method for the synthesis of semiconducting FeCrAl oxide photocathodes for solar-driven hydrogen production was developed and applied for the production of meso- and macroporous layers with the overall stoichiometry Fe_0.84Cr_1.0Al_0.16O₃. Using transmission electron microscopy and energy-dispersive X-ray spectroscopy, phase separation into Fe- and Cr-rich phases was observed for both morphologies. Compared to prior work and to the mesoporous layer, the macroporous FeCrAl oxide photocathode had a significantly enhanced photoelectrolysis performance, even at a very early onset potential of 1.1 V vs RHE. By optimizing the macroporous electrodes, the device reached current densities of up to 0.68 mA cm^–2 at 0.5 V vs RHE under AM 1.5 with an incident photon-to-current efficiency (IPCE) of 28% at 400 nm without the use of catalysts. Based on transient measurements, this performance increase could be attributed to an improved collection efficiency. At a potential of 0.75 V vs RHE, an electron transfer efficiency of 48.5% was determined

Electron Collection in Host–Guest Nanostructured Hematite Photoanodes for Water Splitting: The Influence of Scaffold Doping Density

Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host–guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host–guest counterparts, and a quantitative evaluation of the benefits of the host–guest architecture is still lacking. In this paper, we examine the impact of host–guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO₂ scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host–guest electrodes based on an undoped SnO₂ scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO₂, showing that the scaffold conductivity is crucial for the device performance