Matching Items (4)
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- All Subjects: Silicon Solar Cells
- Creators: Tracy, Clarence
Description
Silicon solar cells with heterojunction carrier collectors based on a-Si/c-Si heterojunction (SHJ) have a potential to overcome the limitations of the conventional diffused junction solar cells and become the next industry standard manufacturing technology of solar cells. A brand feature of SHJ technology is ultrapassivated surfaces with already demonstrated 750 mV open circuit voltages (Voc) and 24.7% efficiency on large area solar cell. Despite very good results achieved in research and development, large volume manufacturing of high efficiency SHJ cells remains a fundamental challenge. The main objectives of this work were to develop a SHJ solar cell fabrication flow using industry compatible tools and processes in a pilot production environment, study the interactions between the used fabrication steps, identify the minimum set of optimization parameters and characterization techniques needed to achieve 20% baseline efficiency, and analyze the losses of power in fabricated SHJ cells by numerical and analytical modeling. This manuscript presents a detailed description of a SHJ solar cell fabrication flow developed at ASU Solar Power Laboratory (SPL) which allows large area solar cells with >750 mV Voc. SHJ cells on 135 um thick 153 cm2 area wafers with 19.5% efficiency were fabricated. Passivation quality of (i)a-Si:H film, bulk conductivity of doped a-Si films, bulk conductivity of ITO, transmission of ITO and the thickness of all films were identified as the minimum set of optimization parameters necessary to set up a baseline high efficiency SHJ fabrication flow. The preparation of randomly textured wafers to minimize the concentration of surface impurities and to avoid epitaxial growth of a-Si films was found to be a key challenge in achieving a repeatable and uniform passivation. This work resolved this issue by using a multi-step cleaning process based on sequential oxidation in nitric/acetic acids, Piranha and RCA-b solutions. The developed process allowed state of the art surface passivation with perfect repeatability and negligible reflectance losses. Two additional studies demonstrated 750 mV local Voc on 50 micron thick SHJ solar cell and < 1 cm/s effective surface recombination velocity on n-type wafers passivated by a-Si/SiO2/SiNx stack.
ContributorsHerasimenka, Stanislau Yur'yevich (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Bowden, Stuart G (Thesis advisor) / Tracy, Clarence (Committee member) / Vasileska, Dragica (Committee member) / Holman, Zachary (Committee member) / Sinton, Ron (Committee member) / Arizona State University (Publisher)
Created2013
Description
Silicon (Si) solar cells are the dominant technology used in the Photovoltaics industry. Field-effect passivation by means of electrostatic charges stored in an overlying insulator on a silicon solar cell has been proven to be a significantly efficient way to reduce effective surface recombination velocity and increase minority carrier lifetime. Silicon nitride (SiNx) films have been extensively used as passivation layers. The capability to store charges makes SiNx a promising material for excellent feild effect passivation. In this work, symmetrical Si/SiO2/SiNx stacks are developed to study the effect of charges in SiNx films. SiO2 films work as barrier layers. Corona charging technique showed the ability to inject charges into the SiNx films in a short time. Minority carrier lifetimes of the Czochralski (CZ) Si wafers increased significantly after either positive or negative charging. A fast and contactless method to characterize the charged overlying insulators on Si wafer through lifetime measurements is proposed and studied in this work, to overcome the drawbacks of capacitance-voltage (CV) measurements such as time consuming, induction of contanmination and hysteresis effect, etc. Analytical simulations showed behaviors of inverse lifetime (Auger corrected) vs. minority carrier density curves depend on insulator charge densities (Nf). From the curve behavior, the Si surface condition and region of Nf can be estimated. When the silicon surface is at high strong inversion or high accumulation, insulator charge density (Nf) or surface recombination velocity parameters (Sn0 and Sp0) can be determined from the slope of inverse lifetime curves, if the other variable is known. If Sn0 and Sp0 are unknown, Nf values of different samples can be compared as long as all have similar Sn0 and Sp0 values. Using the saturation current density (J0) and intercept fit extracted from the lifetime measurement, the bulk lifetime can be calculated. Therefore, this method is feasible and promising for charged insulator characterization.
ContributorsYang, Qun (Author) / Bowden, Stuart (Thesis advisor) / Honsberg, Christiana (Committee member) / Tracy, Clarence (Committee member) / Arizona State University (Publisher)
Created2014
Description
A major obstacle to sustainable solar technologies is end-of-life solar modules. In this thesis, a recycling process is proposed for crystalline-Si solar modules. It is a three-step process to break down Si modules and recover various materials. Over 95% of a module by weight can be recovered with this process. Two new technologies are demonstrated to enable the proposed recycling process. One is sequential electrowinning which allows multiple metals to be recovered one by one from Si modules, Ag, Pb, Sn and Cu. The other is sheet resistance monitoring by the 4-point probe which maximizes the amount of solar-grade Si recovered from Si modules with high throughput. The purity of the recovered metals is above 99% and the recovery rate can achieve between 70~80%. The recovered Si meets the specifications for solar-grade Si and at least 91% of Si from c-Si solar cells can be recovered. The recovered Si and metals are new feedstocks to the solar industry and generate over $12/module in revenue. This revenue enables a profitable recycling business for Si modules without any government support. The chemicals for recycling are carefully selected to minimize their environmental impact and also the cost. A network for collecting end-of-life solar modules is proposed based on the current distribution network for solar modules to contain the collection cost. As a result, the proposed recycling process for c-Si modules is technically, environmentally and financially sustainable.
ContributorsHuang, Wenxi (Author) / Tao, Meng (Thesis advisor) / Alford, Terry (Committee member) / Sinha, Parikhit (Committee member) / Arizona State University (Publisher)
Created2018
Description
In order to ensure higher penetration of photovoltaics in the energy market and have an immediate impact in addressing the challenges of energy crisis and climate change, this thesis research focusses on improving the efficiency of the diffused junction silicon solar cells of an already existing line with established processes. Thus, the baseline processes are first made stable and demonstrated as a pilot line at the Solar Power Lab at ASU, to be used as a backbone on which further improvements could be made. Of the several factors that affect the solar cell efficiency, improvement of short circuit current by reduction of the shading losses is chosen to achieve the improvement.
The shading losses are reduced by lowering the finger width of the solar cell .This reduction of the front metal coverage causes an increase in the series resistance, thereby adversely affecting the fill factor and hence efficiency. To overcome this problem, double printing method is explored to be used for front grid metallization. Before its implementation, it is important to accurately understand the effect of reducing the finger width on the series resistance. Hence, series resistance models are modified from the existing generic model and developed to capture the effects of screen-printing. To have minimum power loss in the solar cell, finger spacing is optimized for the front grid design with each of the finger widths chosen, which are narrower than the baseline finger width. A commercial software package called Griddler is used to predict the results of the model developed to capture effects of screen-printing.
The process for double printing with accurate alignment for finger width down to 50um is developed. After designing the screens for optimized front grid, solar cells are fabricated using both single printing and double printing methods and an improvement of efficiency from 17.2% to 17.8%, with peak efficiency of 18% is demonstrated.
The shading losses are reduced by lowering the finger width of the solar cell .This reduction of the front metal coverage causes an increase in the series resistance, thereby adversely affecting the fill factor and hence efficiency. To overcome this problem, double printing method is explored to be used for front grid metallization. Before its implementation, it is important to accurately understand the effect of reducing the finger width on the series resistance. Hence, series resistance models are modified from the existing generic model and developed to capture the effects of screen-printing. To have minimum power loss in the solar cell, finger spacing is optimized for the front grid design with each of the finger widths chosen, which are narrower than the baseline finger width. A commercial software package called Griddler is used to predict the results of the model developed to capture effects of screen-printing.
The process for double printing with accurate alignment for finger width down to 50um is developed. After designing the screens for optimized front grid, solar cells are fabricated using both single printing and double printing methods and an improvement of efficiency from 17.2% to 17.8%, with peak efficiency of 18% is demonstrated.
ContributorsSrinivasa, Apoorva (Author) / Bowden, Stuart (Thesis advisor) / Tracy, Clarence (Committee member) / Dauksher, Bill (Committee member) / Arizona State University (Publisher)
Created2015