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TiO2 DLAR coatings for planar silicon solar cells
RESEARCH TRENDS
Research Trends CELLO: Advanced LBIC measurement technique for solar cell local characterization A solar cell is a large-area device, and thus its global I/V characteristic and efficiency strongly depend on local properties. The existence of local defects, such as a locally reduced diffusion length, and strong local shunt or high local series resistances, may adversely influence the solar cell global properties. Experimental techniques suitable to map the spatial distribution of such local parameters can provide valuable information, and thus help to improve the technology for the production of efficient and reproducible solar cells. Workers from the Faculty of Engineering, Christian-Albrechts-University of Kiel in Germany have described a new light beaminduced current (LBIC) measurement technique, called ‘CELLO’. To the best of their knowledge, this is the first tool to allow the determination of all local parameters on largearea silicon solar cells, especially the local series and shunt resistances, and thus permit the identification of all material- and processinduced efficiency-relevant defects. In principle, the data obtained could also be used to simulate the behavior of the complete solar cell for any set of technological parameters. CELLO has been tested on mono- and multi-crystalline silicon solar cells. A solar cell is illuminated at near 1.5AM light intensity, and is additionally subjected to intensity modulated scanning local illumination by a focused IR laser. The linear response (current or potential) of the solar cell is measured for various fixed global conditions (different preset voltage or current values) during scanning. A large number of independent data with high spatial resolution are obtained. Applying an advanced fitting procedure to these data yields a set of local parameters for each point on the solar cell. This gives information on the spatial distribution of the photocurrent, the series and shunt resistances, the lateral diffusion of minority carriers, the quality of the back surface field, and even allows the calculation of local I/V curves. The theoretical and experimental approach to this technique is discussed, and the applicability of this new solar cell characterization tool is demonstrated. These first results demonstrate that CELLO is a universal method for detecting and characterizing local defects in all solar cells, since it is not restricted to silicon or crystalline materials. Inclusion of the CELLO results into a
May 2003
detailed simulation program for solar cells should provide a powerful tool for improving their efficiency, since this would allow one to systematically optimize the technology for particular materials and processes. J. Carstensen, G. Popkirov, J. Bahr and H. Föll: Solar Energy Materials & Solar Cells 76(4) 599–611 (1 April 2003).
Potential PV materials-based InN thin films: Fabrication, structural and optical properties The fabrication details, as well as basic structural and optical properties, of low temperature plasma enhanced reactively sputtered indium nitride thin films are reported by researchers from the Institute for Problems of Materials Science at the National Academy of Sciences of the Ukraine in Kiev. SEM and AFM studies of surface morphology, including a microstructural cross-section, were performed. Optical absorption and reflectance spectra of InN textured films at room temperature in the visible and near-IR regions were taken, to reproduce accurately the dielectric function as well as determine the optical effective mass of electrons (0.11) and the direct bandgap (2.03 eV). Some TO (445, 480 and 490 cm–1) and also LO (570 cm–1) phonon features of InN polycrystalline films in the near-IR and Raman spectra are presented and discussed. Notably, both recent results and results from a few years ago from identical samples just after preparation, are in good agreement. This demonstrates an extraordinary long-term stability of this compound, with respect to its optical and electrical characteristics. The attractive possibilities based on model calculations of InN/Si tandem film systems for potential applications in photovoltaic devices, including high efficiency thin film solar cells, are emphasized and discussed. V.Y. Malakhov: Solar Energy Materials & Solar Cells 76(4) 637–646 (1 April 2003).
Outdoor testing of single crystal silicon solar cells Researchers in the Department of Physics, University Brunei Darussalam have reported on an evaluation and assessment of the performance of PV cells. PV measurements include current as a function of voltage, temperature, intensity, wind speed and radiation spectrum. Most noticeable of these parameters is the PV conversion efficiency (maximum electrical power Pmax produced by the cell divided by the incident photon power Pin), which is measured with respect to standard test conditions (STC). These
refer to the solar spectrum, solar radiation intensity, cell temperature and wind speed (2 mph). Tests under STC were carried out in a laboratory-controlled environment. With an increase in ambient temperature, there is a deficiency in the electrical energy that the solar cell can supply. This situation is especially important in hot climates. Outdoor exposure tests of solar cells were conducted in the Department of Physics. Preliminary results demonstrate that the efficiency of the single crystal silicon solar cell strongly depends on its operating temperature. It has been noted that at the operating temperature of 64°C, there was a decrease of 69% in the efficiency of the solar cell compared with that measured at STC. Investigation of the effect of variation in intensities of sunlight on the solar cell performance showed that the efficiency of the cell is reduced as sunlight intensity is reduced, but at a rate different from the reduction in intensity. Among the conclusions drawn by this project, cold temperatures were seen to produce more efficient photo conversion for monocrystalline solar cells. The efficiency for these decreases as the operating temperature of the cells increases: the decrease is approximately 0.06 in absolute value per °C increase. Also, for the same irradiance level, the output power, and therefore the efficiency, decreases with the increasing cell temperature. A.Q. Malik and S.J.B.H. Damit: Renewable Energy 28(9) 1433–1445 (July 2003).
TiO2 DLAR coatings for planar silicon solar cells At the Centre for Photovoltaic Engineering, University of New South Wales in Sydney, in conjunction with the Department of Electrical & Electronic Engineering at the University of Western Australia in Perth, researchers have demonstrated that a double-layer anti-reflection (DLAR) coating can be fabricated using only titanium dioxide. Two TiO2 thin films were deposited onto planar silicon wafers using a simple atmospheric pressure chemical vapor deposition system under different deposition conditions. Weighted average reflectances of 6.5% (measured) and 7.0% (calculated) were achieved for TiO2 DLAR coatings in air and under glass, respectively. An increase in the short-circuit current density of Jsc = 2.5 mA/cm2 can be expected for an optimized TiO2 DLAR coating when compared with a commercial TiO2 single-layer antireflection coating. B.S. Richards, S.F. Rowlands, C.B. Honsberg and J.E. Cotter: Progress in Photovoltaics: Research & Applications 11(1) 27–32 (January 2003).
Photovoltaics Bulletin
11
Research Trends CELLO: Advanced LBIC measurement technique for solar cell local characterization A solar cell is a large-area device, and thus its global I/V characteristic and efficiency strongly depend on local properties. The existence of local defects, such as a locally reduced diffusion length, and strong local shunt or high local series resistances, may adversely influence the solar cell global properties. Experimental techniques suitable to map the spatial distribution of such local parameters can provide valuable information, and thus help to improve the technology for the production of efficient and reproducible solar cells. Workers from the Faculty of Engineering, Christian-Albrechts-University of Kiel in Germany have described a new light beaminduced current (LBIC) measurement technique, called ‘CELLO’. To the best of their knowledge, this is the first tool to allow the determination of all local parameters on largearea silicon solar cells, especially the local series and shunt resistances, and thus permit the identification of all material- and processinduced efficiency-relevant defects. In principle, the data obtained could also be used to simulate the behavior of the complete solar cell for any set of technological parameters. CELLO has been tested on mono- and multi-crystalline silicon solar cells. A solar cell is illuminated at near 1.5AM light intensity, and is additionally subjected to intensity modulated scanning local illumination by a focused IR laser. The linear response (current or potential) of the solar cell is measured for various fixed global conditions (different preset voltage or current values) during scanning. A large number of independent data with high spatial resolution are obtained. Applying an advanced fitting procedure to these data yields a set of local parameters for each point on the solar cell. This gives information on the spatial distribution of the photocurrent, the series and shunt resistances, the lateral diffusion of minority carriers, the quality of the back surface field, and even allows the calculation of local I/V curves. The theoretical and experimental approach to this technique is discussed, and the applicability of this new solar cell characterization tool is demonstrated. These first results demonstrate that CELLO is a universal method for detecting and characterizing local defects in all solar cells, since it is not restricted to silicon or crystalline materials. Inclusion of the CELLO results into a
May 2003
detailed simulation program for solar cells should provide a powerful tool for improving their efficiency, since this would allow one to systematically optimize the technology for particular materials and processes. J. Carstensen, G. Popkirov, J. Bahr and H. Föll: Solar Energy Materials & Solar Cells 76(4) 599–611 (1 April 2003).
Potential PV materials-based InN thin films: Fabrication, structural and optical properties The fabrication details, as well as basic structural and optical properties, of low temperature plasma enhanced reactively sputtered indium nitride thin films are reported by researchers from the Institute for Problems of Materials Science at the National Academy of Sciences of the Ukraine in Kiev. SEM and AFM studies of surface morphology, including a microstructural cross-section, were performed. Optical absorption and reflectance spectra of InN textured films at room temperature in the visible and near-IR regions were taken, to reproduce accurately the dielectric function as well as determine the optical effective mass of electrons (0.11) and the direct bandgap (2.03 eV). Some TO (445, 480 and 490 cm–1) and also LO (570 cm–1) phonon features of InN polycrystalline films in the near-IR and Raman spectra are presented and discussed. Notably, both recent results and results from a few years ago from identical samples just after preparation, are in good agreement. This demonstrates an extraordinary long-term stability of this compound, with respect to its optical and electrical characteristics. The attractive possibilities based on model calculations of InN/Si tandem film systems for potential applications in photovoltaic devices, including high efficiency thin film solar cells, are emphasized and discussed. V.Y. Malakhov: Solar Energy Materials & Solar Cells 76(4) 637–646 (1 April 2003).
Outdoor testing of single crystal silicon solar cells Researchers in the Department of Physics, University Brunei Darussalam have reported on an evaluation and assessment of the performance of PV cells. PV measurements include current as a function of voltage, temperature, intensity, wind speed and radiation spectrum. Most noticeable of these parameters is the PV conversion efficiency (maximum electrical power Pmax produced by the cell divided by the incident photon power Pin), which is measured with respect to standard test conditions (STC). These
refer to the solar spectrum, solar radiation intensity, cell temperature and wind speed (2 mph). Tests under STC were carried out in a laboratory-controlled environment. With an increase in ambient temperature, there is a deficiency in the electrical energy that the solar cell can supply. This situation is especially important in hot climates. Outdoor exposure tests of solar cells were conducted in the Department of Physics. Preliminary results demonstrate that the efficiency of the single crystal silicon solar cell strongly depends on its operating temperature. It has been noted that at the operating temperature of 64°C, there was a decrease of 69% in the efficiency of the solar cell compared with that measured at STC. Investigation of the effect of variation in intensities of sunlight on the solar cell performance showed that the efficiency of the cell is reduced as sunlight intensity is reduced, but at a rate different from the reduction in intensity. Among the conclusions drawn by this project, cold temperatures were seen to produce more efficient photo conversion for monocrystalline solar cells. The efficiency for these decreases as the operating temperature of the cells increases: the decrease is approximately 0.06 in absolute value per °C increase. Also, for the same irradiance level, the output power, and therefore the efficiency, decreases with the increasing cell temperature. A.Q. Malik and S.J.B.H. Damit: Renewable Energy 28(9) 1433–1445 (July 2003).
TiO2 DLAR coatings for planar silicon solar cells At the Centre for Photovoltaic Engineering, University of New South Wales in Sydney, in conjunction with the Department of Electrical & Electronic Engineering at the University of Western Australia in Perth, researchers have demonstrated that a double-layer anti-reflection (DLAR) coating can be fabricated using only titanium dioxide. Two TiO2 thin films were deposited onto planar silicon wafers using a simple atmospheric pressure chemical vapor deposition system under different deposition conditions. Weighted average reflectances of 6.5% (measured) and 7.0% (calculated) were achieved for TiO2 DLAR coatings in air and under glass, respectively. An increase in the short-circuit current density of Jsc = 2.5 mA/cm2 can be expected for an optimized TiO2 DLAR coating when compared with a commercial TiO2 single-layer antireflection coating. B.S. Richards, S.F. Rowlands, C.B. Honsberg and J.E. Cotter: Progress in Photovoltaics: Research & Applications 11(1) 27–32 (January 2003).
Photovoltaics Bulletin
11