Toledo, OH, United States
Toledo, OH, United States

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Vasko A.C.,University of Toledo | Vijh A.,Xunlight Corporation | Karpov V.G.,University of Toledo
Solar Energy | Year: 2014

We present data exhibiting the spontaneous emergence of hot spots in forward biased thin film photovoltaics based on a-Si:H technology. These spots evolve over time, shrinking in their diameter and increasing in temperature up to approximately 300. °C above that of the surrounding area. Our numerical approach explores a system of many identical diodes in parallel connected through the resistive electrode and through thermal connectors, a model which couples electric and thermal processes. The modeling results show that hot spots emerge from a rather large area of nonuniform temperature, then collapse to local entities. Finally, we present a simplified analytical treatment establishing relations between the hot spot parameters. The technological importance of our findings is that they open a venue to improving the large area device performance and reliability by properly scaling the device thickness, substrate material, and thermal insulation. © 2014 Elsevier Ltd.


Karpov V.G.,University of Toledo | Vasko A.,University of Toledo | Vijh A.,Xunlight Corporation
Applied Physics Letters | Year: 2013

We show that thin film diode structures, such as photovoltaics and light emitting arrays, can undergo zero threshold localized thermal runaway leading to thermal and electrical nonuniformities spontaneously emerging in originally uniform systems. The linear stability analysis is developed for a system of thermally and electrically coupled two discrete diodes, and for a distributed system. These results are verified with numerical modeling that is not limited to small fluctuations. The discovered instability negatively affects the device performance and reliability. It follows that these problems can be mitigated by properly designing the device geometry and thermal insulation. © 2013 AIP Publishing LLC.


Fan Q.H.,University of Toledo | Chen C.,University of Toledo | Liao X.,University of Toledo | Xiang X.,University of Toledo | And 5 more authors.
Journal of Applied Physics | Year: 2010

This work demonstrates a method to optimize the indium tin oxide (ITO) thin films as front transparent electrode to maximize the efficiency of substrate type amorphous silicon (a-Si) based thin film solar cells. It shows that the total light intensity absorbed by the a-Si layer can be predicted by combining a multilayer optical simulation with the nonuniform solar spectrum and the spectroscopic response of the absorption coefficient of the a-Si film. Consequently, an optimized ITO film can be identified. The photovoltaic performances of experimentally obtained a-Si single junction solar cells confirm the simulation results, indicating an ITO film about 56 nm thick leads to the highest efficiency. Furthermore, it is shown that the ITO films should be deposited at relatively low temperature around 132 °C to avoid damage to the a-Si top p -layer and p-i-n junction. It is found that introducing a small fraction, ∼0.61% flow ratio, of O2 in the sputtering Ar gas reduces the sheet resistivity of the ITO film and improves its transmittance, leading to higher efficiency a-Si solar cells. © 2010 American Institute of Physics.


Fan Q.H.,University of Toledo | Liao X.,University of Toledo | Xiang X.,University of Toledo | Chen C.,University of Toledo | And 4 more authors.
Journal of Physics D: Applied Physics | Year: 2010

Amorphous silicon (a-Si) based thin film tandem junction solar cells are simulated based on a uniform field collection model. From the photovoltaic parameters of a single junction a-Si top cell and a few amorphous silicon-germanium (a-SiGe) bottom cells, the optimized a-Si/a-SiGe tandem cell can be predicted. The simulation results are in good agreement with the experiment. The highest efficiency a-Si/a-SiGe tandem cells are obtained with a combination of a-SiGe characteristics and a relatively large mismatch in the short circuit current between the top and bottom cells. A key reason for this behaviour is that the tandem cell may exhibit a larger fill factor than either one of the component cells under a certain current mismatch. © 2010 IOP Publishing Ltd.


Fan Q.H.,University of Toledo | Chen C.,University of Toledo | Liao X.,University of Toledo | Xiang X.,University of Toledo | And 8 more authors.
Solar Energy Materials and Solar Cells | Year: 2010

Amorphous silicon-germanium (a-SiGe) solar cells with graded Ge fraction along the film depth profile are deposited at elevated rate of 4 Å/s in a pressure range of 2-4 Torr. A properly graded GeH4 flow pattern characterized by a low starting ratio of GeH4:H2 is the key towards achieving highly stable a-Si/a-SiGe tandem cells of 12.94% initial and 11.22% stable efficiencies. A fully laminated module with 38×38 mm2 aperture area made from the tandem cell exhibits an initial efficiency of 10.81%, which shows that the high rate deposited a-Si/a-SiGe tandem cells are promising for practical module applications. © 2010 Elsevier B.V. All rights reserved.


Hou G.,University of Toledo | Fan Q.,University of Toledo | Liao X.,University of Toledo | Chen C.,University of Toledo | And 3 more authors.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films | Year: 2011

This paper presents our recent results on the high-rate deposition of high-efficiency and highly stable hydrogenated amorphous silicon (a-Si:H) solar cells with all layers deposited by 13.56 MHz radio frequency glow discharge. Using a linear disilane (Si 2H 6) grading process, high initial active-area efficiency of 11.42% has been obtained for the a-Si:H top cells with an effective i-layer deposition rate of 8 Å/s. It is also found that the light-soaking stability of the a-Si:H top cells is much improved by the Si 2H 6 grading process with the best a-Si:H top cell exhibiting only 11.2% light-induced degradation after 1000 h of light-soaking. Integrating the high-rate deposited a-Si:H top cell in an amorphous silicon/amorphous silicon germanium (a-Si:H/a-SiGe:H) tandem cell, an initial active-area efficiency of 12.57% is achieved. After light soaking for 1008 h, the stable efficiency is still as high as 11.02%, corresponding to only a 12.31 degradation. To the best of our knowledge, this is the best performance for a-Si:H based solar cells at such a high deposition rate by 13.56 MHz RF-PECVD. Possible mechanisms responsible for the superior stability of the a-Si:H solar cells deposited by the Si 2H 6 grading process are discussed. © 2011 American Vacuum Society.


Chen C.,University of Toledo | Liao X.,University of Toledo | Xiang X.,University of Toledo | Cao X.,Xunlight Corporation | And 5 more authors.
Conference Record of the IEEE Photovoltaic Specialists Conference | Year: 2011

This paper reports numerical modeling and experimental investigation for the impacts of an intrinsic a-Si buffer layer between the p-type nc-Si layer and the intrinsic a-SiGe layer with a narrow bandgap of 1.40-1.55eV on the performances of a-SiGe single junction solar cells. The effects of bandgap and thickness of the buffer layer were simulated by using Analysis of Microelectronic and Photonic Structures (AMPS) computer model developed at Penn State University. The results obtained by the simulation show that the intrinsic a-Si buffer layer can lead to an increase in the open circuit voltage (V oc), but cause a decrease of the fill factor (FF) and the conversion efficiency (Eff), depending on how large the band gap and thickness of the buffer layer are. Our experimental results are consistent with the simulation's results; i.e., a thick and wide-bandgap buffer layer between i and p layers can cause a serious deterioration in FF. An optimal a-SiGe single junction solar cell without the a-Si buffer layer has achieved an efficiency of 9.41% with V oc=0.576V, J sc=23.34 mA/cm 2, and FF=70.0%. © 2011 IEEE.


Patent
Xunlight Corporation | Date: 2010-11-09

A photovoltaic structure is provided. The photovoltaic structure includes a photovoltaic module having a top surface, a bottom surface, and a perimeter. The photovoltaic structure also includes a plurality of flexible tabs attached to the photovoltaic module bottom surface, wherein the plurality of flexible tabs extend beyond the perimeter of the photovoltaic module. A method for mounting a photovoltaic module is also provided.


A photovoltaic apparatus is provided. The photovoltaic apparatus includes a photovoltaic module. The photovoltaic apparatus also includes at least one support strip attached to a bottom surface of the photovoltaic module. Each support strip has a first edge portion, a center portion, and a second edge portion. The second edge portion includes a flange having a first end and a second end. A photovoltaic array is also provided. The photovoltaic array includes a first photovoltaic apparatus and a second photovoltaic apparatus. The photovoltaic array is attached to a support structure.


Trademark
Xunlight Corporation | Date: 2010-09-14

photovoltaic cells and panels.

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