In order to reach high penetration levels of distributed PV electricity generation in the distrib... more In order to reach high penetration levels of distributed PV electricity generation in the distribution grids, without overvoltage and other grid related problems, different concepts for peak shavings will have to be introduced. One possible way to alter the PV electricity generation profile, and at the same time increase the selfconsumption, is by installing PV panels in alternative orientations to avoid a large peak production at noon. In this study the qualitative and quantitative effects of PV installations in alternative orientations is investigated. Three different theoretical neighborhoods with different building orientations are studied at three different PV penetration levels. The results show that with a low PV system price the highest PV potential is found in neighborhoods where the building are placed so that the main roof orientations are east-west rather than north-south. The neighborhood where the buildings are east-west oriented reaches the highest solar fraction and the highest total electricity production at a given upper limit of the maximum peak power injected to the grid. However, in order for the neighborhood with east-west oriented houses to be truly favorable, also from an economical perspective, the PV system prices need to be lower than today and/or the electricity market price needs to be lower during mid day peak generation hours.
The study of solar irradiance is of vital importance in order to quantify the fluctuation that th... more The study of solar irradiance is of vital importance in order to quantify the fluctuation that the transmission system needs to handle in case of large scale solar power integration. In this paper, early results from a low-cost solar irradiance logger network are presented. The main advantages of the solar logger are its low cost (~$50) and mobility. Our results confirm previous models of the station pair correlation of clear sky index step changes, with the modification that the isotropic correlation appears to decrease exponentially as a function of Δt^(-1/2) rather than Δt^(-1) as has been proposed earlier.
Electronic power devices made of silicon carbide promisesuperior performance over today's sil... more Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices.High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated.A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.
The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sampl... more The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sample during the growth process and especially during the conversion from copper-rich to copper-poor material in a two-step process. Starting from a very simple model of a directly heated substrate the model was refined until it qualitatively and quantitatively explained the features observed in the real experiment. The results can be used to determine more accurate criteria for the endpoint detection in a real-world evaporation system.
A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and ca... more A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and calibrated using measurement data from an actual mini-module with an aperture area of 80 cm 2 . Simulations using this model accurately reproduce the measurement results on mini-modules with different cell widths to within ±3% of the parameters V oc , I sc , fill-factor and efficiency. The optical and electrical properties of the transparent front-contact were obtained from quantum efficiency measurements on a series of solar cells with ZnO:Al layers of different thicknesses and incorporated into the model. Finally, computer simulations were used to optimize CIGS thin-film modules for the application in low-concentrating systems. Aperture area efficiencies exceeding 14% at up to 10x concentration are predicted by the simulations for CIGS thin-film modules with cell widths of w = 2 mm. These simulation results are supported by preliminary measurements.
Abstract. Device simulations of B implanted 4H-SiC p nn diodes were performed including transient... more Abstract. Device simulations of B implanted 4H-SiC p nn diodes were performed including transient trapping of carriers in the deep B level. Most traps be com filled at forward current densities above 3000 A/cm 2 thus enabling conductivity modulation by a carrier plasma and a negative resistance in agreement with pulsed forward bias measure ments. Reverse recovery measurements were performed to verify a significant conductivity modulation at 4000 A/cm 2
Epitaxially grown 4H-SiC structures with several heavily Al doped layers were used. The samples w... more Epitaxially grown 4H-SiC structures with several heavily Al doped layers were used. The samples were annealed in Ar atmosphere in a RF-heated furnace between 1500 and 2900 degreesC for 0.5 to 3h. S ...
High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to gro... more High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to grow thick low-doped epitaxial layers of 4H silicon carbide. These layers were used as drift layers in a combined process to manufacture both bipolar and unipolar high-voltage diodes. The resulting diodes were characterized electrically in order to gain knowledge about the electric quality of the HTCVD epitaxial layers to assess the high-voltage properties of this material.
ABSTRACT Two sets of pin-diodes with epitaxially grown anode regions and implanted anode regions,... more ABSTRACT Two sets of pin-diodes with epitaxially grown anode regions and implanted anode regions, respectively, were processed on the same low-doped n-type epitaxial layer. The designed breakdown voltage for the epitaxial layer was 5 kV with punch-through at about 2 kV. The almost ideal forward voltage drop of less than 3.5 V at current densities of 100 A cm(-2) of the epitaxial diodes indicates high-level carrier injection into the low-doped epitaxial layer, which is also supported by the results of reverse recovery measurements. At current densities above 10 A cm(-2) the forward voltage drop of the implanted pin-diodes is significantly higher than that of the epitaxial diodes.
... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-... more ... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-SiC wafer (Cree, n ... The trapezoidal features seen in the SEM images (Figs. ... Further investigation of the stacking fault-like defects is necessary in order to understand their presence in the ...
Transient measurements and device simulations were performed to investigate self-heating and swit... more Transient measurements and device simulations were performed to investigate self-heating and switching with 4H-SiC BJTs. A current gain decrease was found during self-heating presumably due to reduced electron mobility with increasing temperature. Surface recombination increased the simulated maximum temperature but the current gain decrease during self-heating was similar as for bulk recombination. A fast switching of 0.5 A and 200 V was shown with a voltage rise-time of about 70 ns and fall-time of 50 ns. Turn-off measurements show a noticeably delay time before fall-off of the emitter current, indicating a significant amount of stored carriers in the base.
In order to reach high penetration levels of distributed PV electricity generation in the distrib... more In order to reach high penetration levels of distributed PV electricity generation in the distribution grids, without overvoltage and other grid related problems, different concepts for peak shavings will have to be introduced. One possible way to alter the PV electricity generation profile, and at the same time increase the selfconsumption, is by installing PV panels in alternative orientations to avoid a large peak production at noon. In this study the qualitative and quantitative effects of PV installations in alternative orientations is investigated. Three different theoretical neighborhoods with different building orientations are studied at three different PV penetration levels. The results show that with a low PV system price the highest PV potential is found in neighborhoods where the building are placed so that the main roof orientations are east-west rather than north-south. The neighborhood where the buildings are east-west oriented reaches the highest solar fraction and the highest total electricity production at a given upper limit of the maximum peak power injected to the grid. However, in order for the neighborhood with east-west oriented houses to be truly favorable, also from an economical perspective, the PV system prices need to be lower than today and/or the electricity market price needs to be lower during mid day peak generation hours.
The study of solar irradiance is of vital importance in order to quantify the fluctuation that th... more The study of solar irradiance is of vital importance in order to quantify the fluctuation that the transmission system needs to handle in case of large scale solar power integration. In this paper, early results from a low-cost solar irradiance logger network are presented. The main advantages of the solar logger are its low cost (~$50) and mobility. Our results confirm previous models of the station pair correlation of clear sky index step changes, with the modification that the isotropic correlation appears to decrease exponentially as a function of Δt^(-1/2) rather than Δt^(-1) as has been proposed earlier.
Electronic power devices made of silicon carbide promisesuperior performance over today's sil... more Electronic power devices made of silicon carbide promisesuperior performance over today's silicon devices due toinherent material properties. As a result of the material'swide band gap of 3.2eV, high thermal conductivity, itsmechanical and chemical stability and a high critical electricfield, 4H-silicon carbide devices have the potential to be usedat elevated temperatures and in harsh environments. Shortercarrier lifetimes and a reduction in the necessary width of thelow-doped drift zone in silicon carbide devices compared totheir silicon counterparts result in faster switching speedsand lower switching losses and thus in much more efficientpower devices.High-voltage 4H-silicon carbide diodes have been fabricatedin a newly developed processing sequence, using standardsilicon process equipment. Epitaxial layers grown by chemicalvapor deposition (CVD) on commercial 4H-silicon carbidesubstrates were used as starting material for both mesa-etchedepitaxial and implanted p+n-n+ planar diodes, Schottky diodesand merged pn-Schottky (MPS) diodes, together with additionaltest structures. The device metallization was optimized to givea low contact resistivity on implanted and epitaxial layers anda sufficiently high Schottky barrier with a singlemetallization scheme. Different high-field termination designshave been tested and breakdown voltages of up to 4 kV onimplanted, field-ring terminated diodes were achieved,corresponding to 80% of the critical electric field. A 5kVepitaxial diode design with a forward voltage drop of 3.5V at acurrent density of 100Acm-2 equipped with an implanted junctiontermination extension (JTE) was also fabricated.A new measurement setup was designed and built with thecapability of measuring current-voltage and capacitance-voltagecharacteristics of semiconductor devices at reverse biases upto 10kV. Together with these electrical measurements, theresults of other characterization techniques were used toidentify performance limiting defects in the fabricated siliconcarbide diodes. Increased forward voltage drop of bipolardevices during on-state operation was studied and it was shownthat the stacking faults causing forward degradation arevisible in scanning electron microscopy. With the help ofsynchrotron white-beam X-ray diffraction topographs (SWBXT),electron beam induced current (EBIC) and electroluminescencemeasurements of silicon carbide diodes, the role of screwdislocations as a dominant source of device failure in the formof localized microplasma breakdown was identified. Screwdislocations with and without open core have been found tocause a 20-80% reduction in the critical electric field of4H-silicon carbide diodes, both for low-voltage (150V) andhigh-voltage (~5kV) designs. While micropipes have almost beeneliminated from commercial silicon carbide material,closed-core screw dislocations are still abundant withdensities in the order of 10000cm-2 in state-of-the-art siliconcarbide epitaxial layers.
The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sampl... more The purpose of this investigation was to model the thermodynamic behavior of a Cu(In,Ga)Se2 sample during the growth process and especially during the conversion from copper-rich to copper-poor material in a two-step process. Starting from a very simple model of a directly heated substrate the model was refined until it qualitatively and quantitatively explained the features observed in the real experiment. The results can be used to determine more accurate criteria for the endpoint detection in a real-world evaporation system.
A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and ca... more A computer model for Cu(In,Ga)Se 2 (CIGS) based, monolithic solar cell modules was derived and calibrated using measurement data from an actual mini-module with an aperture area of 80 cm 2 . Simulations using this model accurately reproduce the measurement results on mini-modules with different cell widths to within ±3% of the parameters V oc , I sc , fill-factor and efficiency. The optical and electrical properties of the transparent front-contact were obtained from quantum efficiency measurements on a series of solar cells with ZnO:Al layers of different thicknesses and incorporated into the model. Finally, computer simulations were used to optimize CIGS thin-film modules for the application in low-concentrating systems. Aperture area efficiencies exceeding 14% at up to 10x concentration are predicted by the simulations for CIGS thin-film modules with cell widths of w = 2 mm. These simulation results are supported by preliminary measurements.
Abstract. Device simulations of B implanted 4H-SiC p nn diodes were performed including transient... more Abstract. Device simulations of B implanted 4H-SiC p nn diodes were performed including transient trapping of carriers in the deep B level. Most traps be com filled at forward current densities above 3000 A/cm 2 thus enabling conductivity modulation by a carrier plasma and a negative resistance in agreement with pulsed forward bias measure ments. Reverse recovery measurements were performed to verify a significant conductivity modulation at 4000 A/cm 2
Epitaxially grown 4H-SiC structures with several heavily Al doped layers were used. The samples w... more Epitaxially grown 4H-SiC structures with several heavily Al doped layers were used. The samples were annealed in Ar atmosphere in a RF-heated furnace between 1500 and 2900 degreesC for 0.5 to 3h. S ...
High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to gro... more High-temperature chemical vapour deposition (HTCVD) in a vertical chimney reactor was used to grow thick low-doped epitaxial layers of 4H silicon carbide. These layers were used as drift layers in a combined process to manufacture both bipolar and unipolar high-voltage diodes. The resulting diodes were characterized electrically in order to gain knowledge about the electric quality of the HTCVD epitaxial layers to assess the high-voltage properties of this material.
ABSTRACT Two sets of pin-diodes with epitaxially grown anode regions and implanted anode regions,... more ABSTRACT Two sets of pin-diodes with epitaxially grown anode regions and implanted anode regions, respectively, were processed on the same low-doped n-type epitaxial layer. The designed breakdown voltage for the epitaxial layer was 5 kV with punch-through at about 2 kV. The almost ideal forward voltage drop of less than 3.5 V at current densities of 100 A cm(-2) of the epitaxial diodes indicates high-level carrier injection into the low-doped epitaxial layer, which is also supported by the results of reverse recovery measurements. At current densities above 10 A cm(-2) the forward voltage drop of the implanted pin-diodes is significantly higher than that of the epitaxial diodes.
... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-... more ... in detail earlier [7]. The epitaxial layer was grown in a chimney reactor on a commercial 4H-SiC wafer (Cree, n ... The trapezoidal features seen in the SEM images (Figs. ... Further investigation of the stacking fault-like defects is necessary in order to understand their presence in the ...
Transient measurements and device simulations were performed to investigate self-heating and swit... more Transient measurements and device simulations were performed to investigate self-heating and switching with 4H-SiC BJTs. A current gain decrease was found during self-heating presumably due to reduced electron mobility with increasing temperature. Surface recombination increased the simulated maximum temperature but the current gain decrease during self-heating was similar as for bulk recombination. A fast switching of 0.5 A and 200 V was shown with a voltage rise-time of about 70 ns and fall-time of 50 ns. Turn-off measurements show a noticeably delay time before fall-off of the emitter current, indicating a significant amount of stored carriers in the base.
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