9 research outputs found
Determination of the electron diffusion length in dye-sensitized solar cells by substrate contact patterning
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 DrudeâZener 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 solâgel 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<sub>0.84</sub>Cr<sub>1.0</sub>Al<sub>0.16</sub>O<sub>3</sub>. 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<sup>â2</sup> 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 HostâGuest 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
hostâguest 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 hostâguest counterparts, and a quantitative
evaluation of the benefits of the hostâguest architecture is
still lacking. In this paper, we examine the impact of hostâguest
architectures by comparing nanostructured tin-doped hematite electrodes
with hematite nanoparticle layers coated onto two types of conducting
macroporous SnO<sub>2</sub> scaffolds. Analysis of the external quantum
efficiency spectra for substrate (SI) and electrolyte side (EI) illumination
reveals that the electron diffusion length in the hostâguest
electrodes based on an undoped SnO<sub>2</sub> 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
SnO<sub>2</sub>, showing that the scaffold conductivity is crucial
for the device performance