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Thomas Unold - One of the best experts on this subject based on the ideXlab platform.
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high Surface recombination velocity limits quasi fermi level splitting in kesterite Absorbers
Scientific Reports, 2018Co-Authors: Alex Redinger, Thomas UnoldAbstract:Kelvin Probe Force Microscopy, Photoluminescence imaging and numerical simulations are used to study the Surfaces of Cu2ZnSnSe4 Absorber layers. In particular, the effect of NH4OH and annealing under ambient conditions is investigated. We observe drastic changes in the measured quasi Fermi-level splitting (QFLs) after chemical cleaning of the Absorber Surface with NH4OH, which is traced back to a removal of the Surface inversion. Air annealing recovers Surface inversion, which reduces the recombination current at the Surface. Annealing above 200 °C leads to a permanent change in the work function which cannot be modified by NH4OH etching anymore. This modification makes the QFLs insensitive to Surface cleaning and explains why air annealing in Cu2ZnSnSe4 is important. From numerical simulations we find that a large Surface recombination velocity needs to be present in order to describe the experimental observations.
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time resolved photoluminescence on cu in ga se 2 Absorbers distinguishing degradation and trap states
Applied Physics Letters, 2017Co-Authors: Alex Redinger, Dieter Greiner, Christian A. Kaufmann, Charles J Hages, Sergiu Levcenko, Thomas UnoldAbstract:Recent reports have suggested that the long decay times in time resolved photoluminescence (TRPL), often measured in Cu(In, Ga)Se2 Absorbers, may be a result of detrapping from sub-bandgap defects. In this work, we show via temperature dependent measurements, that long lifetimes >50 ns can be observed that reflect the true minority carrier lifetime not related to deep trapping. Temperature dependent time resolved photoluminescence and steady state photoluminescence imaging measurements are used to analyze the effect of annealing in air and in a nitrogen atmosphere between 300 K and 350 K. We show that heating the Cu(In, Ga)Se2 Absorber in air can irreversibly decrease the TRPL decay time, likely due to a deterioration of the Absorber Surface. Annealing in an oxygen-free environment yields a temperature dependence of the TRPL decay times in accordance with Schockley Read Hall recombination kinetics and weakly varying capture cross sections according to T0.6.
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investigation of the potassium fluoride post deposition treatment on the cigse cds interface using hard x ray photoemission spectroscopy a comparative study
Physical Chemistry Chemical Physics, 2016Co-Authors: Bünyamin Ümsür, Wolfram Calvet, Alexander Steigert, Iver Lauermann, Mihaela Gorgoi, Karsten Prietzel, Dieter Greiner, Christian A. Kaufmann, Thomas UnoldAbstract:The impact of the potassium fluoride post deposition treatment on CIGSe chalcopyrite Absorbers is investigated by means of depth resolved hard X-ray photoemission spectroscopy of the near Surface region. Two similar, slightly Cu-poor CIGSe Absorbers were used with one being treated by potassium fluoride prior to the chemical bath deposition of an ultrathin CdS layer. The thickness of the CdS layer was chosen to be in the range of about 10 nm in order to allow the investigation of the CIGSe/CdS interface by the application of hard X-rays, increasing the information depth up to 30 nm. Besides strong intermixing on both samples, an increased Cu depletion of the KF treated Absorber was observed in combination with an increased accumulation of Cd and S. In addition, a general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the Absorber Surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon is attributed to the impact of additional cadmium which acts as donor and releases further electrons into the conduction band of the Absorber. Finally, the electrons accumulate at the CdS Surface after having passed the interface region. This additional Surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe Absorber compared to the non-treated Absorber.
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improved performance of ge alloyed cztgesse thin film solar cells through control of elemental losses
Progress in Photovoltaics, 2015Co-Authors: Charles J Hages, Thomas Unold, R G Wilks, Sergej Levcenco, Caleb K Miskin, Jan H Alsmeier, Daniel Abouras, Marcus Bar, Rakesh AgrawalAbstract:Nanocrystal-based Cu2Zn(SnyGe1-y)(SxSe4-x) (CZTGeSSe) thin-film solar cell Absorbers with tunable band gap have been prepared. Maximum solar-conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu2ZnSn(SxSe4-x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the Absorber film during the high-temperature selenization treatment, although some Ge loss from the Absorber Surface is still observed following this step. Despite limitations imposed by elemental losses present at the Absorber Surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage-dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.
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experimental indication for band gap widening of chalcopyrite solar cell Absorbers after potassium fluoride treatment
Applied Physics Letters, 2014Co-Authors: Paul Pistor, Wolfram Calvet, Alexander Steigert, Iver Lauermann, Mihaela Gorgoi, Dieter Greiner, Christian A. Kaufmann, Stephan Brunken, R Klenk, Thomas UnoldAbstract:The implementation of potassium fluoride treatments as a doping and Surface modification procedure in chalcopyrite Absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 Absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 Absorber Surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the Absorbers after the KF PDT which is limited to the very near Surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...
Iver Lauermann - One of the best experts on this subject based on the ideXlab platform.
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investigation of the potassium fluoride post deposition treatment on the cigse cds interface using hard x ray photoemission spectroscopy a comparative study
Physical Chemistry Chemical Physics, 2016Co-Authors: Bünyamin Ümsür, Wolfram Calvet, Alexander Steigert, Iver Lauermann, Mihaela Gorgoi, Karsten Prietzel, Dieter Greiner, Christian A. Kaufmann, Thomas UnoldAbstract:The impact of the potassium fluoride post deposition treatment on CIGSe chalcopyrite Absorbers is investigated by means of depth resolved hard X-ray photoemission spectroscopy of the near Surface region. Two similar, slightly Cu-poor CIGSe Absorbers were used with one being treated by potassium fluoride prior to the chemical bath deposition of an ultrathin CdS layer. The thickness of the CdS layer was chosen to be in the range of about 10 nm in order to allow the investigation of the CIGSe/CdS interface by the application of hard X-rays, increasing the information depth up to 30 nm. Besides strong intermixing on both samples, an increased Cu depletion of the KF treated Absorber was observed in combination with an increased accumulation of Cd and S. In addition, a general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the Absorber Surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon is attributed to the impact of additional cadmium which acts as donor and releases further electrons into the conduction band of the Absorber. Finally, the electrons accumulate at the CdS Surface after having passed the interface region. This additional Surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe Absorber compared to the non-treated Absorber.
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complex Surface chemistry of kesterites cu zn reordering after low temperature postdeposition annealing and its role in high performance devices
Chemistry of Materials, 2015Co-Authors: Markus Neuschitzer, Yudania Sanchez, Tetiana Olar, Thomas Thersleff, Simon Lopezmarino, Florian Oliva, Moises Espindolarodriguez, Marcel Placidi, Victor Izquierdoroca, Iver LauermannAbstract:A detailed study explaining the beneficial effects of low temperature postdeposition annealing combined with selective Surface etchings for Cu2ZnSnSe4 (CZTSe) based solar cells is presented. After performing a selective oxidizing Surface etching to remove ZnSe secondary phases typically formed during the synthesis processes an additional 200 °C annealing step is necessary to increase device performance from below 3% power conversion efficiency up to 8.3% for the best case. This significant increase in efficiency can be explained by changes in the Surface chemistry which results in strong improvement of the CdS/CZTSe heterojunction commonly used in this kind of Absorber/buffer/window heterojunction solar cells. XPS measurements reveal that the 200 °C annealing promotes a Cu depletion and Zn enrichment of the etched CZTSe Absorber Surface relative to the CZTSe bulk. Raman measurements confirm a change in Cu/Zn ordering and an increase in defect density. Furthermore, TEM microstructural investigations indica...
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experimental indication for band gap widening of chalcopyrite solar cell Absorbers after potassium fluoride treatment
Applied Physics Letters, 2014Co-Authors: Paul Pistor, Wolfram Calvet, Alexander Steigert, Iver Lauermann, Mihaela Gorgoi, Dieter Greiner, Christian A. Kaufmann, Stephan Brunken, R Klenk, Thomas UnoldAbstract:The implementation of potassium fluoride treatments as a doping and Surface modification procedure in chalcopyrite Absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 Absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 Absorber Surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the Absorbers after the KF PDT which is limited to the very near Surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...
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experimental indication for band gap widening of chalcopyrite solar cell Absorbers after potassium fluoride treatment
Applied Physics Letters, 2014Co-Authors: Paul Pistor, Wolfram Calvet, Alexander Steigert, Iver Lauermann, Mihaela Gorgoi, Stephan Brunken, R Klenk, D Greiner, C A Kaufmann, Thomas UnoldAbstract:The implementation of potassium fluoride treatments as a doping and Surface modification procedure in chalcopyrite Absorber preparation has recently gained much interest since it led to new record efficiencies for this kind of solar cells. In the present work, Cu(In,Ga)Se2 Absorbers have been evaporated on alkali containing Mo/soda-lime glass substrates. We report on compositional and electronic changes of the Cu(In,Ga)Se2 Absorber Surface as a result of a post deposition treatment with KF (KF PDT). In particular, by comparing standard X-ray photoelectron spectroscopy and synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), we are able to confirm a strong Cu depletion in the Absorbers after the KF PDT which is limited to the very near Surface region. As a result of the Cu depletion, we find a change of the valence band structure and a shift of the valence band onset by approximately 0.4 eV to lower binding energies which is tentatively explained by a band gap widening as expected for Cu defic...
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cu accumulation at the interface between sputter zn mg o and cu in ga s se 2 a key to understanding the need for buffer layers
Thin Solid Films, 2007Co-Authors: Iver Lauermann, R Klenk, Ch Loreck, A Grimm, Harry Monig, Ch M Luxsteiner, Ch H Fischer, S Visbeck, T P NiesenAbstract:Abstract Zn0.85Mg0.15O buffer layers can replace both i-ZnO and CdS in n-ZnO/i-ZnO/CdS/Cu(InxGa1 − x)(SySe1 − y)2/Mo/glass (CIGSSe) solar cells without significant loss of efficiency. We found that the efficiency of Zn0.85Mg0.15O buffered solar cells decreased with increasing sample temperature when we sputter-deposited Zn0.85Mg0.15O directly onto the CIGSSe Absorber Surface, e.g. from 9.5% without deliberate heating to 6.5% at 240 °C. To find an explanation for this behavior we sputter-coated bare, KCN-etched CIGSSe Absorbers with about 420 nm of Zn0.85Mg0.15O at different sample temperatures and subsequently removed the Zn0.85Mg0.15O layers by wet-chemical etching with dilute acetic acid. The exposed CIGSSe Surfaces were examined by Surface-sensitive X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). We found a strong increase in the [Cu]/([In] + [Cu]) ratio compared to a bare, Acetic-acid-etched CIGSSe reference. The Surface of samples that had been sputter-coated at 150 °C changed from being initially Cu-poor to Cu-rich. The chemical shift of Cu Auger peaks from the same Surface confirmed this finding. The increased Cu/In ratio and the chemical shift were reversed after KCN etch. These findings are discussed in the context of the model of a Cu-depleted, wide-band gap Surface region in CIGSSe solar cells as a prerequisite for high efficiency.
Marc A. Rosen - One of the best experts on this subject based on the ideXlab platform.
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Performance evaluation of a double pass PV/T solar air heater with and without fins
Applied Thermal Engineering, 2011Co-Authors: Rakesh Kumar, Marc A. RosenAbstract:Abstract A photovoltaic/thermal (PV/T) solar air heater with a double pass configuration and vertical fins in the lower channel is investigated. The fins are arranged perpendicular to the direction of air flow to enhance the heat transfer rate and efficiency. Air enters the upper channel of the air heater and subsequently flows to the lower channel in the opposite direction. A comprehensive steady state analysis is performed, including energy balances for the upper glass cover, the superstrate of the photovoltaic module, the Absorber Surface, the back plate and the air in upper and lower columns. The effects of design, climatic and operating parameters are evaluated on outlet air temperature, cell temperature, thermal (energy) efficiency, electrical efficiency and total equivalent thermal efficiency. Thermal performance characteristic curves are also developed for the PV/T collector. The effects of the presence of fins in the lower air channel, the depth of ducts of the air channels, flow rate, inlet air temperature and packing factor are evaluated on the thermal and electrical efficiencies. The extended fin area reduces the cell temperature considerably, from 82 °C to 66 °C. Impact assessments on thermal and electrical outputs of the packing factor are reported, recognizing that higher packing factors are beneficial as they lead to the production of more electrical output per unit collector area and help in controlling the cell temperature.
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thermal performance of integrated collector storage solar water heater with corrugated Absorber Surface
Applied Thermal Engineering, 2010Co-Authors: Rakesh Kumar, Marc A. RosenAbstract:An investigation is reported of the thermal performance of an integrated solar water heater with a corrugated Absorber Surface. The thermal performance of the rectangular collector/storage solar water heater depends significantly on the heat transfer rate between the Absorber Surface and the water, and on the amount of solar radiation incident on the Absorber Surface. In this investigation, the Surface of the Absorber is considered to be corrugated, with small indentation depths, instead of plane. The modified Surface has a higher characteristic length for convective heat transfer from the Absorber to the water, in addition to having more Surface area exposed to solar radiation. The corrugated Surface based solar water heater is determined to have a higher operating temperature for longer time than the plane Surface. It means during the operation of water heater, more solar energy is converted into useful heat. However, this modification has reduces the efficiency of the system marginally.
R G Wilks - One of the best experts on this subject based on the ideXlab platform.
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naf rbf treated cu in ga se2 thin film solar cell Absorbers distinct Surface modifications caused by two different types of rubidium chemistry
ACS Applied Materials & Interfaces, 2020Co-Authors: Jakob Bombsch, Enrico Avancini, Romain Carron, Evelyn Handick, Raul Garciadiez, Claudia Hartmann, Roberto Felix, Shigenori Ueda, R G WilksAbstract:The underlying beneficial mechanism of heavy alkali postdeposition treatment (PDT) of Cu(In,Ga)Se2 thin-film solar cell Absorbers that led to new record efficiencies in recent years is studied using photoelectron spectroscopy. Excitation energies between 40.8 eV and 6 keV were used to examine the near-Surface region of Cu(In,Ga)Se2 thin-film solar cell Absorbers that underwent NaF and combined NaF/RbF PDT. The already Cu-deficient Surface region after NaF PDT, which is modeled as a Cu:(In + Ga):Se = 1:5:8 phase, shows further depletion after NaF/RbF PDT and seems to incorporate some Rb. Additionally, we have found strong indications for the NaF/RbF PDT-induced formation of a Rb-In-Se-type compound with a 1:1:2 stoichiometry partially covering the Absorber Surface. The electronic Cu(In,Ga)Se2 structure is modified due to the RbF treatment, with a pronounced shift in the valence band maximum away from the Fermi level in the immediate vicinity of the Surface.
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improved performance of ge alloyed cztgesse thin film solar cells through control of elemental losses
Progress in Photovoltaics, 2015Co-Authors: Charles J Hages, Thomas Unold, R G Wilks, Sergej Levcenco, Caleb K Miskin, Jan H Alsmeier, Daniel Abouras, Marcus Bar, Rakesh AgrawalAbstract:Nanocrystal-based Cu2Zn(SnyGe1-y)(SxSe4-x) (CZTGeSSe) thin-film solar cell Absorbers with tunable band gap have been prepared. Maximum solar-conversion total area efficiencies of up to 9.4% are achieved with a Ge content of 30 at.%. Improved performance compared with similarly processed films of Cu2ZnSn(SxSe4-x) (CZTSSe, 8.4% efficiency) is achieved through controlling Ge loss from the bulk of the Absorber film during the high-temperature selenization treatment, although some Ge loss from the Absorber Surface is still observed following this step. Despite limitations imposed by elemental losses present at the Absorber Surface, we find that Ge alloying leads to enhanced performance due to increased minority charge carrier lifetimes as well as reduced voltage-dependent charge carrier collection. Copyright © 2013 John Wiley & Sons, Ltd.
Mohd Nazari Abu Bakar - One of the best experts on this subject based on the ideXlab platform.
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performance studies on a finned double pass photovoltaic thermal pv t solar collector
Desalination, 2007Co-Authors: Mohd Yusof Othman, Kamaruzzaman Sopian, Baharudin Yatim, Mohd Nazari Abu BakarAbstract:A hybrid photovoltaic–thermal (PV/T) solar air heater system which generates both electricity and heat energy simultaneously was studied. Mathematical model and test results are presented. This hybrid system consists of monocrystalline silicon cells pasted to an Absorber plate with fins attached at the other side of the Absorber Surface. Air as heat removing fluid is made to flow through an upper channel and then under the Absorber plate or lower channel of the collector. Only a small part of the absorbed solar radiation is converted to electricity, while the rest increases the temperature of the cells. Improvements to the total efficiency of the system can be achieved by the use of a double-pass collector system and fins. It gives more interesting hybrid photovoltaic-thermal system design for practical applications.
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double pass photovoltaic thermal solar air collector with compound parabolic concentrator and fins
Journal of Energy Engineering-asce, 2006Co-Authors: Mohd Yusof Othman, Kamaruzzaman Sopian, Baharudin Yatim, Mohd Nazari Abu BakarAbstract:A solar energy heat collector was combined with photovoltaic cells to form one single hybrid energy generating unit. This system produces both thermal and electrical energies simultaneously. An experimental setup of a double-pass photovoltaic-thermal solar air collector with a compound parabolic concentrator (CPC) and fins was designed and fabricated to study the performance over a range of operating conditions. The collector is designed in such a way that the Absorber Surface is partially covered by solar cells. A CPC is used to enhance more electrical and thermal energy output from the collector. However, solar cells perform poorly at high operating temperatures. Therefore, fins are attached at the back side of the Absorber Surface to improve this situation. A circulating air stream passes through an upper channel and then under the Absorber plate or lower channel of the collector carrying away the excess heat and thus maintaining the cells at the optimum operating conditions. The performance of the system has been measured. The thermal, electrical, and combined electrical thermal efficiencies of the collector are presented and discussed.
