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D.
Roy’s Group Home
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Electroanalytical
Characterization of Materials
for Photovoltaic Applications Background We use electroanalytical techniques to characterize various materials for photovoltaic (PV) applications. Our experimental strategy is based on combining the techniques of potentiodynamic probing and impedance spectroscopy to simultaneously monitor the D.C. and A.C. electrical response characteristics of solar cells. This approach is capable of probing a broad range of cell-parameters that often are not readily accessible with conventional D.C. techniques. Electro-analytical measurements provide current vs. voltage characteristics of PV cells, and these data can be used to determine the fill-factor, D.C. resistance, open circuit voltage, short circuit current, maximum power point, as well as (with the incorporation of an adequate light source) energy conversion efficiencies. The D.C. cell resistance contains a series and a shunt resistance, which can be decoupled using A.C. impedance spectroscopy. This information is useful for studying the ohmic power loss components in a PV cell. Impedance spectroscopy can also measure the different capacitive components of solar cells (such as the depletion and diffusion capacitances for Si cells), and this latter information is relevant for designing transient loads such as charge regulators for PV systems. Results obtained in this way for single PV cells can be “scaled-up” through computer simulations to analyze electrical characteristics of solar modules and arrays.
PV research facilities in our laboratory Our PV characterization workstation contains a Newport Model 91159 solar simulator (150 W, equipped with necessary AM filters) for measuring PV efficiencies, and a PAR VersaSTAT potentiostat interfaced with a data acquisition computer to measure electrical parameters of PV cells. The simulator is coupled with a temperature-controlled cell-test bench, and set up in a light-tight, air-filtered faraday cage stationed on an optical table. The simulator meets class B specifications of the ASTM E927 standards for beam characteristics (collimation of < ±10°, uniformity of ±5 % and light ripple of <1 % rms), as well as other international standards for spectral match, spatial uniformity and temporal stability, and can be used to study PV cells of surface area up to 2X2 in.
A
PV cell being studied under illumination. The cell is placed on a
test-platform maintained at 25 deg C using a Peltier-module temperature
controller. The air filter shown next to the simulator is run 24 hours
to keep the simulator optics dust-free. The test-chamber is equipped
with additional air filters not shown in this picture. A halogen lamp, controlled by a LAMBDA Model LK 350-0 Voltage
Regulated Power Supply, is also used for measuring illumination dependent
impedance parameters of solar cells. The voltage regulated power supply
helps to eliminate low-level fluctuations of light intensities during
the recording of A.C. impedance data using low amplitude voltage perturbations.
This allows for independently controlling illumination and temperature
of test cells during impedance measurements, and provides temperature
dependent characteristics of resistive power losses.
Our present work involving PV materials focuses on two specific
systems, namely dye-sensitized solar cells (DSSCs), and Si solar cells
based on upgraded metallurgical grade (UMG) Si. DSSCs are relatively
easy to fabricate and environmentally compatible, but improving their
efficiency and increasing their long-term stability still faces challenges.
Our studies of DSSC materials center mostly on the development of
efficient counter electrodes and electrolytes. |
