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.  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 power losses.

Ongoing 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 PV cells.

Recent publications by our group in the area of photovoltaic systems:

J.E. Garland, D.J. Crain and D. Roy, "Utilization of electrochemical impedance spectroscopy for experimental characterization of the diode features of charge recombination in a dye sensitized solar cell", Electrochimica Acta 148 (2014) 62-72.

S.E. Rock, X. Shi, J.E. Garland and D. Roy, "Experimental Considerations for Temperature Controlled Measurements of Fast Charge Recombination Times in Dye Sensitized Solar Cells using Open Circuit Voltage Decay and Impedance Spectroscopy", Measurement 53 (2014) 71-82.

D.J. Crain, S.E. Rock, J.E. Garland and D. Roy, "Comparison of D.C. and A.C. Electro-analytical Methods for Measuring Diode Ideality Factors and Series Resistances of Silicon Solar Cells", Current Applied Physics 13 (2013) 2087-2097.

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 electrical parameters",
Anal. Methods 4 (2012) 106 - 117.
Included in the journal's themed issue on "Future Electroanalytical Developments."

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 and Temperature”,
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.

Recent presentations by our group in the area of photovoltaic systems:

Krishnan, S., Losenge Lebga,, J., Rock, S. E., Wu, L., McLaughlin, J., Roy, D., AIChE 2012 Annual Meeting, Pittsburgh, Pennsylvania, AIChE, Pittsburgh, Pennsylvania, "Thermophysical and Electrochemical Properties of Self-Assembling Amphiphilic Ionic Liquids", October 2012

Garland, J. E., Crain, D. J., Rock, S. E., Roy, D., CAMP Technical Meeting, Clarkson University, Potsdam, NY, "Electroanalytical Characterization of Resistive Power Losses in a Single Crystal Silicon Solar Cell", October 2012

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 10-11, 2011.

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.