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
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
research facilities in our laboratory
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.
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.
The potentiostat (seen
near the top right of the Figure in the upper panel) is used for
I-V and impedance measurements.
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
PV Projects in our Laboratory
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.
Improving the performance of UMG-Si based solar cells depends to
a large degree on developing a fundamental understanding of the
roles of various structural defects and impurities of UMG-Si in
the PV behavior of these materials. Understanding their electrical
characteristics (resistive power losses, carrier lifetime, diffusion
length, etc.) in full PV cells also is critical for improving power
generation efficiencies of UMG-Si based cells. Reaching these targets
requires quantitative characterization of PV cells based on UMG-Si
(unblended as well as blended with PV grade Si). Our effort in this
area focuses on the development of a general framework for detailed
characterization of Si based PV cells that can aid the design of
UMG-Si processing/blending for PV applications. To a large extent,
the essential foundation for this work can be set up using commercially
available Si solar cells, since the same set of D.C. and A.C. parameters
that determine the performance of these cells also are necessary
to evaluate UMG-Si solar cells. The general electroanalytical procedures
standardized in this way will also be useful for studying organic/hybrid
publications by our group in the area of photovoltaic systems:
D. J. Crain, J. E. Garland, S. E. Rock and D. Roy "Quantitative
characterization of silicon solar cells in the electro-analytical
approach: Combined measurements of temperature and voltage dependent
Anal. Methods 4 (2012) 106
Included in the journal's themed issue on "Future
J.E. Garland, D.J. Crain and D. Roy, "Impedance spectroscopy
coupled with voltammetry for quantitative evaluation of temperature
and voltage dependent parameters of a silicon solar cell",
Solar Energy 85 (2011) 2912-2923.
L. V. N. R. Ganapatibhotla, L. Wu, J. P Zheng, X. Jia, D.
Roy, J. B. McLaughlin and S. Krishnan, "Ionic liquids with fluorinated
block-oligomer tails: Influence of self-assembly on transport properties",
Journal of Matererials Chemistry, 21 (2011) 19275-19285.
J.E. Garland, D.J. Crain, J.P. Zheng, C.M. Sulyma and D.
Roy, “Electro-analytical Characterization of Photovoltaic Cells
by Combining Voltammetry and Impedance Spectroscopy: Voltage Dependent
Parameters of a Silicon Solar Cell under Controlled Illumination
Energy and Environmental Science, 4 (2011) 485-498.
L. V. N. R. Ganapatibhotla, J.P. Zheng, D. Roy and S. Krishnan,
“PEGylated Imidazolium Ionic Liquid Electrolytes: Thermophysical
and Electrochemical Properties”,
Chemistry of Materials, 22 (2010) 6347–6360.
presentations by our group in the area of photovoltaic systems:
J. B. McLaughlin, S. Krishnan, L. Wu, L. V. N. R. Ganapatibhotla,
X. Jia, D. Roy and J.P. Zheng, "Self-Consistent Field Modeling of
Microstructure Formation in Fluorinated "Block" Ionic Liquids for
Photovoltaic Cells", Talk
presented at the 2011 AIChE Annual Meeting, October 16-21,
2011, Minneapolis, MN.
J. E. Garland, D. J. Crain, S. E. Rock and D. Roy, "Electroanalytical
Characterization of Resistive Power Losses in a Single Crystal Silicon
Solar Cell", Poster presented at the CAMP Fall Meeting, October
J. E. Garland, D. J. Crain, S. E. Rock, D. Roy, “Electroanalytical
Characterization of Resistive Power Losses in a Single Crystal Silicon
Solar Cell”, Poster presented at the Annual Technical Meeting
of CAMP; May 19, 2011.
L. Wu, S. Krishnan, X. Shi, D. Roy, “Electrochemical Properties
of Novel PEGylated Electrolyte Blend for Dye Sensitized Solar Cells
and Lithium Ion Batteries”, Poster presented at the Annual Technical
Meeting of CAMP; May 19, 2011.
L. Ganapatibhotla, J. P. Zheng, D. Roy and S. Krishnan, "Solid
Organic Electrolytes and Ionic Liquids, with Poly(ethylene glycol)
and Semifluorinated Alkyl Side Chains, for Photovoltaic and Energy
Storage Applications", Talk
Presented at the 2010 AIChE Annual Meeting, Salt Lake
City, Utah, November 7-12, 2010.
D. Roy, “Electro-Analytical Characterization of Solar Cells”,
Talk presented at the Center for Advanced Materials Processing Fall
2010 Symposium, Potsdam, NY; October 14, 2010.
J. E. Garland, D.J. Crain, and D. Roy, “Dye Sensitized Solar
Cell Parameters Studied using Voltametry, Impedance Spectroscopy,
and Open Circuit Voltage Decay”, Poster presented at the Center
for Advanced Materials Processing Fall 2010 Symposium, Potsdam,
NY; October 14, 2010.
L. Wu, L. Ganapatibhotla, J. P. Zheng, D. Roy and S. Krishnan,
“Solid State Organic Electrolytes for Dye-Sensitized Solar Cells”,
Poster presented at the Center for Advanced Materials Processing
Fall 2010 Symposium, Potsdam, NY; October 14, 2010.
L. Ganapatibhotla, J. Zheng, D. Roy and S. Krishnan, PEGylated
and fluorinated ionic salts for dye-sensitized solar cells”, Poster
presented at the Northeast Regional Meeting (NERM 2010),
Potsdam, NY June 2 -5, 2010
J. E. Garland, D. J. Crain and D. Roy, “Temperature Dependencies
of Dye Sensitized Solar Cell Parameters Studied using Voltametry,
Photo-electrochemical Impedance Spectroscopy, and Open Circuit Voltage
Decay Techniques”, Poster presented at CAMP Technical Meeting,
Canandaigua, NY, May 19-21, 2010.
L. Ganapatibhotla, J.P. Zheng, D. Roy and S. Krishnan, “PEGylated
and Fluorinated Ionic Salts for Dye-Sensitized Solar Cells and Supercapacitors”,
Poster presented at CAMP Technical Meeting, Canandaigua,
NY, May 19-21, 2010: Received first prize among 38 contributed posters.
S.V. Babu and D. Roy, “Silicon Material Characterization
for Solar Application”, Presented at the Semicon China 2010
Conference (Session V, Device Engineering and Application), Shanghai,
China, during March 16-18, 2010.
D. Roy, “Materials Processing and Characterization for Advanced
Lithium Ion Batteries and Photovoltaic Cells”, Talk presented at
General Electric Global Research Center, Schenectady, NY;
July 10, 2009.
J. E. Garland and D. Roy, “Measurement of the Electrical
Parameters of a Monocrystalline Silicon Solar Cell by Combining
Potentiodynamic and Impedance Techniques”, Poster presented at CAMP
Technical Meeting, Canandaigua, NY, May 13- 15, 2009.
J. E. Garland, D. J. Crain and D. Roy, “Examination of Resistive
Power Losses in a Single Crystal Silicon Solar Cell”, Poster presented
at CAMP Technical Meeting, Canandaigua, NY, May 13-15, 2009.