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 a priori 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.
The use of solar panels is becoming one of the options for some of the rural communities in Limpopo Province, South Africa, to get electrical energy for their radio and television sets as the national grid may not reach them in the near future. Hence, dissemination of knowledge of how to use the solar devices and their maintenance is crucial for these communities. This will be possible only if there is appropriate information available for the potential end-users, installers and extension workers. With this in mind, an attempt has been made to evaluate the performance of an amorphous and a crystalline solar panel at our experimental site. Outdoor tests were conducted to measure solar radiation, open-circuit voltage, short circuit current, current-voltage (I-V) curve, fill-factor and conversion efficiency and hence to compare the performance of the two types of panels. It was found that both types give a satisfactory performance for the climate of this region.
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