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University Center of Excellence for Photovoltaics
University Center of Excellence for Photovoltaics
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ItemGreater than 16% Efficient Screen Printed Solar Cells on 115-170 μm Thick Cast Multicrystalline Silicon(Georgia Institute of Technology, 2006-05) Upadhyaya, A. D. ; Sheoran, Manav ; Ristow, Alan ; Rohatgi, Ajeet ; Narayanan, S. ; Roncin, SteveIn this paper we report on the impact of mc-Si wafer thickness on efficiency. We have obtained 16.8%, 16.4%, 16.2% and 15.7% efficient screen printed 4 cm(2) solar cells on 280 μm, 170 μm, 140 μm and 115 μm thick cast mc-Si respectively. Analysis of these cells showed that the efficiency of the 115 μm thick cell is limited by a BSRV of 750 cm/s, FSRV of 120,000 cm/s and a BSR of 67%. A module manufacturing cost model for a 25 MW plant was used to demonstrate that 15.7% efficient cells on 115 μm thick wafers are more cost effective than 16.8% cells on 280 μm wafers. The module manufacturing cost reduced from $1.82/W to $1.63/W when the wafer thickness was reduced from 280 μm (efficiency 16.8%) to 115 μm (efficiency 15.7%). A roadmap is developed for 115 μm thick wafers to demonstrate how cell efficiency can be increased to greater than 18% resulting in a module cost of less than $1.40/W.
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ItemHigh Efficiency Screen-Printed Solar Cells on Textured Mono-Crystalline Silicon(Georgia Institute of Technology, 2005-10) Rohatgi, Ajeet ; Ebong, Abasifreke ; Hilali, Mohamed M. ; Meemongkolkiat, Vichai ; Rounsaville, Brian ; Ristow, AlanIn this paper we report on high efficiency screen-printed 4 cm(2) solar cells fabricated on randomly textured float zone, magnetic Czochralski (MCz) and Ga-doped Cz silicon. A simple process involving POCl(3) emitters, low frequency PECVD silicon nitride deposition, Al back contact print, Ag front grid print followed by co-firing of the contacts produced efficiencies of 19.0% on textured float zone, 18.2% on MCz and 17.7% on Ga-doped Cz. A combination of high sheet resistance emitter (~100 Ω-/sq.) and the surface texturing resulted in short circuit current density of 37.3 mA/cm(2) for 0.6 Ω-cm float zone cell, 38.2 mA/cm(2) for 4.8 Ω-cm MCz cell and 37.4 mA/cm(2) for 1.5 Ω-cm Ga-doped Cz cell. Open circuit voltages were consistent with the base resistivity of the three materials. However, FF was highest for float zone (0.784) followed by MCz (0.759) and Ga-doped Cz (0.754). Model calculations performed using PC1D showed that, once the lifetime exceeds 200 μs for this cell design, the efficiency no longer has a strong dependence on the bulk lifetime. Instead, the performance is limited by the cell design including contacts, base resistivity, doping profiles, and front and back surface recombination velocities. Detailed analysis is performed to explain the high performance of these screen-printed cells and guidelines are provided for ≥20% efficient screen-printed cells.
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ItemModeling the Effects of Uncertainty and Reliability on the Cost of Energy from PV Systems(Georgia Institute of Technology, 2005-06) Ristow, Alan ; Begović, Miroslav ; Rohatgi, AjeetThe cost of energy produced by a photovoltaic system is dependent upon the amount of energy produced by the system and the amortized cost of the system's components. Existing simulation tools either use crude estimators of system reliability or ignore the effects of system downtime on energy production altogether. Furthermore, the costs associated with system components are often not known precisely during system planning. However, it is difficult to reflect this uncertainty in energy cost calculations using conventional deterministic techniques. This work attempts to address these deficiencies by applying a stochastic model of system reliability to the prediction of energy production over a system's life. Similarly, it uses a stochastic model that encompasses the uncertainties associated with system component prices to estimate the uncertainty in the total installed system cost. Finally, using these two results, it computes the uncertainty in the cost of energy produced by the system. Preliminary testing of this approach, using failure data obtained from an actual system, produces cost estimates of $0.300–0.400/kWh, with a mean of $0.349/kWh, consistent with contemporary residential system cost analyses. The link between engineering and economics suggests that the proposed method may be useful as an optimization tool if an appropriate database can be developed from which to draw realistic input distributions.
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ItemNumerical Approach to Uncertainty and Sensitivity Analysis in Forecasting the Manufacturing Cost and Performance of PV Modules(Georgia Institute of Technology, 2004-06) Ristow, Alan ; Begović, Miroslav ; Rohatgi, AjeetForecasting of the manufacturing cost of PV modules is governed by a large number of uncertain factors. Cost estimates are frequently based upon imperfect information and, as a result, may not be perfectly accurate. Existing studies of these uncertainties focus on the sensitivity of the manufacturing cost to individual cost inputs, examining the effects of each input in isolation. Such methods of analysis neglect statistical correlations between inputs, provide no measure of the uncertainty in the projected manufacturing cost, and do not permit the assignment of probability distributions to the inputs in the case that one range of values is thought to be more likely than another. This work describes the development of a stochastic modeling framework that addresses these deficiencies. Furthermore, it demonstrates how sensitivity to particular inputs may be ranked in order to help determine the most effective path to cost reduction. The result is a method with great potential for exploring the link between engineering design, PV module cost, and the manufacturing process.
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ItemCost and Technology Roadmaps for Cost-Effective Silicon Photovoltaics(Georgia Institute of Technology, 2003-12) Rohatgi, Ajeet ; Ristow, Alan ; Yelundur, VijayThe cost of photovoltaics (PV) is expected to decrease by a factor of two to four within the next two decades, making PV an integral part of the solution to the problems of fossil fuel depletion and growing energy demand. This paper describes cost and technology roadmaps for achieving 17–18%-efficient crystalline Si solar cells at a competitive manufacturing cost of less than $1/W.
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ItemEstimation of PV System Reliability Parameters(Georgia Institute of Technology, 2001-10) Pregelj, Aleksandar ; Begović, Miroslav ; Rohatgi, Ajeet ; Ristow, AlanIn evaluating the payback time and energy price per kWh generated by a renewable energy system, the system is usually assumed to work without interruptions over its entire life. Renewable energy systems are fairly reliable, but like any complex system, they may fail, and the effects of failures should be analysed and accounted for. This paper presents a procedure for filtering-out failures associated with the "infant-mortality" failure mode, and enables the estimation of distribution parameters of the constant failure rate mode that governs the behavior of the system throughout its useful service life. The procedure is best suited for cases when a large number of data are available, such as when a large number of similar systems are monitored.
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ItemScreen-Printed Back Surface Reflector for Light Trapping in Crystalline Silicon Solar Cells(Georgia Institute of Technology, 2001-10) Ristow, Alan ; Hilali, Mohamed M. ; Ebong, Abasifreke ; Rohatgi, AjeetEvaporated metal back surface reflectors have been shown to yield high values of internal rear reflectance, and are particularly eff ective when combined with a thin dielectric layer between the silicon and the metal. However, evaporated metals are not compatible with low-cost solar cell fabrication processes and generally do not scatter light well, resulting in inefficient trapping of light. In this work, a ffordable screenprinted metal pastes have been employed to fabricate eff ective low-cost back surface reflectors. The best of these, fabricated from screen-printed silver paste on a thin silicon nitride dielectric layer, yield back surface reflectance values similar to those of evaporated metal reflectors. Furthermore, the screen-printed back surface reflectors in this study are shown to be highly diff use, thus enhancing light trapping in planar silicon solar cells. PC1D simulations suggest that a solar cell with a screen-printed metal/dielectric back surface reflector should outperform one with a high-quality aluminum back surface field.
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ItemGuidelines for More Accurate Determination and Interpretation of Effective Lifetime from Measured Quasi-Steady-State Photoconductance(Georgia Institute of Technology, 2001-08) Brody, Jed ; Rohatgi, Ajeet ; Ristow, Alan