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Efficient Crystalline Si Solar Cell with Amorphous/Crystalline Silicon Heterojunction as Back Contact: Preprint

Description: We study an amorphous/crystalline silicon heterojunction (Si HJ) as a back contact in industrial standard p-type five-inch pseudo-square wafer to replace Al back surface field (BSF) contact. The best efficiency in this study is over 17% with open-circuit (Voc) of 0.623 V, which is very similar to the control cell with Al BSF. We found that Voc has not been improved with the heterojunction structure in the back. The typical minority carrier lifetime of these wafers is on the order of 10 us. We also found that the doping levels of p-layer affect the FF due to conductivity and band gap shifting, and an optimized layer is identified. We conclude that an amorphous/crystalline silicon heterojunction can be a very promising structure to replace Al BSF back contact.
Date: June 1, 2012
Creator: Nemeth, B.; Wang, Q. & Shan, W.
Partner: UNT Libraries Government Documents Department

Synthesis and optical properties of II-O-VI highly mismatched alloys

Description: We have synthesized ternary and quaternary diluted II-VI oxides using the combination of O ion implantation and pulsed laser melting. CdO{sub x}Te{sub 1-x} thin films with x up to 0.015, and the energy gap reduced by 150 meV were formed by O{sup +}-implantation in CdTe followed by pulsed laser melting. Quaternary Cd{sub 0.6}Mn{sub 0.4}O{sub x}Te{sub 1-x} and Zn{sub 0.88}Mn{sub 0.12}O{sub x}Te{sub 1-x} with mole fraction of incorporated O as high as 0.03 were also formed. The enhanced O incorporation in Mn-containing alloys is believed to be due to the formation of relatively strong Mn-O bonds. Optical transitions associated with the lower (E{sub -}) and upper (E{sub +}) conduction subbands resulting from the anticrossing interaction between the localized O states and the extended conduction states of the host are clearly observed in these quaternary diluted II-VI oxides. These alloys fulfill the criteria for a multiband semiconductor that has been proposed as a material for making high efficiency, single-junction solar cells.
Date: January 20, 2004
Creator: Yu, K.M.; Walukiewicz, W.; Shan, W.; Wu, J.; Beeman, J.W.; Scarpulla, M.A. et al.
Partner: UNT Libraries Government Documents Department

Raman spectroscopy and time-resolved photoluminescence of BN and BxCyNz nanotubes

Description: We report Raman and time-resolved photoluminescence spectroscopic studies of multiwalled BN and B{sub x}C{sub y}N{sub z} nanotubes. The Raman spectroscopy shows that the as-grown B{sub x}C{sub y}N{sub z} charge recombination, respectively. Comparison of the photoluminescence of BN nanotubes to that decay process is characterized by two time constants that are attributed to intra- and inter-BN sheet nanotubes as predicted by theory. nanotubes are radially phase separated into BN shells and carbon shells. The photoluminescence of hexagonal BN is consistent with the existence of a spatially indirect band gap in multi-walled BN.
Date: January 21, 2004
Creator: Wu, J.; Han, Wei-Qiang; Walukiewicz, W.; Ager III, J.W.; Shan, W.; Haller,E.E. et al.
Partner: UNT Libraries Government Documents Department

Pressure-dependent photoluminescence study of ZnO nanowires

Description: The pressure dependence of the photoluminescence (PL) transition associated with the fundamental band gap of ZnO nanowires has been studied at pressures up to 15 GPa. ZnO nanowires are found to have a higher structural phase transition pressure around 12 GPa as compared to 9.0 GPa for bulk ZnO. The pressure-induced energy shift of the near band-edge luminescence emission yields a linear pressure coefficient of 29.6 meV/GPa with a small sublinear term of -0.43 meV/GPa{sup 2}. An effective hydrostatic deformation potential -3.97 eV for the direct band gap of the ZnO nanowires is derived from the result.
Date: September 13, 2004
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Zhang, Y.; Mao, S.S. et al.
Partner: UNT Libraries Government Documents Department

Band anticrossing in dilute nitrides

Description: Alloying III-V compounds with small amounts of nitrogen leads to dramatic reduction of the fundamental band-gap energy in the resulting dilute nitride alloys. The effect originates from an anti-crossing interaction between the extended conduction-band states and localized N states. The interaction splits the conduction band into two nonparabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy. The changes in the conduction band structure result in significant increase in electron effective mass and decrease in the electron mobility, and lead to a large enhance of the maximum doping level in GaInNAs doped with group VI donors. In addition, a striking asymmetry in the electrical activation of group IV and group VI donors can be attributed to mutual passivation process through formation of the nearest neighbor group-IV donor nitrogen pairs.
Date: December 23, 2003
Creator: Shan, W.; Yu, K.M.; Walukiewicz, W.; Wu, J.; Ager III, J.W. & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Effect of Native Defects on Optical Properties of InxGa1-xNAlloys

Description: The energy position of the optical absorption edge and the free carrier populations in In{sub x}Ga{sub 1-x}N ternary alloys can be controlled using high energy {sup 4}He{sup +} irradiation. The blue shift of the absorption edge after irradiation in In-rich material (x > 0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical absorption measurements show that the irradiation-introduced native defects are inside the bandgap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical absorption edge and the carrier populations in In{sub x}Ga{sub 1-x}N are in excellent agreement with the predictions of the amphoteric defect model.
Date: May 9, 2005
Creator: Li, S.X.; Haller, E.E.; Yu, K.M.; Walukiewicz, W.; Ager III,J.W.; Wu, J. et al.
Partner: UNT Libraries Government Documents Department

Highly Mismatched Alloys for Intermediate Band Solar Cells

Description: It has long been recognized that the introduction of a narrow band of states in a semiconductor band gap could be used to achieve improved power conversion efficiency in semiconductor-based solar cells. The intermediate band would serve as a ''stepping stone'' for photons of different energy to excite electrons from the valence to the conduction band. An important advantage of this design is that it requires formation of only a single p-n junction, which is a crucial simplification in comparison to multijunction solar cells. A detailed balance analysis predicts a limiting efficiency of more than 50% for an optimized, single intermediate band solar cell. This is higher than the efficiency of an optimized two junction solar cell. Using ion beam implantation and pulsed laser melting we have synthesized Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys with x<0.03. These highly mismatched alloys have a unique electronic structure with a narrow oxygen-derived intermediate band. The width and the location of the band is described by the Band Anticrossing model and can be varied by controlling the oxygen content. This provides a unique opportunity to optimize the absorption of solar photons for best solar cell performance. We have carried out systematic studies of the effects of the intermediate band on the optical and electrical properties of Zn{sub 1-y}Mn{sub y}O{sub x}Te{sub 1-x} alloys. We observe an extension of the photovoltaic response towards lower photon energies, which is a clear indication of optical transitions from the valence to the intermediate band.
Date: March 21, 2005
Creator: Walukiewicz, W.; Yu, K.M.; Wu, J.; Ager III, J.W.; Shan, W.; Scrapulla, M.A. et al.
Partner: UNT Libraries Government Documents Department

Nature of room-temperature photoluminescence in ZnO

Description: The temperature dependence of the photoluminescence (PL) transitions associated with various excitons and their phonon replicas in high-purity bulk ZnO has been studied at temperatures from 12 K to above room temperature (320 K). Several strong PL emission lines associated with LO phonon replicas of free and bound excitons are clearly observed. The room temperature PL spectrum is dominated by the phonon replicas of the free exciton transition with the maximum at the first LO phonon replica. The results explain the discrepancy between the transition energy of free exciton determined by reflection measurement and the peak position obtained by the PL measurement.
Date: November 11, 2004
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Yuan, H.B.; Xin, H.P. et al.
Partner: UNT Libraries Government Documents Department

Investigation of microstructure and V-defect formation inInxGa1-xN/GaN MQW grown using temperature-gradient MOCVD

Description: Temperature-gradient Metalorganic Chemical Vapor Deposition was used to deposit In{sub x}Ga{sub 1-x}N/GaN multiple quantum well structures with a concentration gradient of indium across the wafer. These multiple quantum well structures were deposited on low defect density (2 x 10{sup 8} cm{sup -2}) GaN template layers for investigation of microstructural properties and V-defect (pinhole) formation. Room temperature photoluminescence and photomodulated transmission were used for optical characterization which show a systematic decrease in emission energy for a decrease in growth temperature. Triple-axis X-ray diffraction, scanning electron microscopy and cross-section transmission electron microscopy were used to obtain microstructural properties of different regions across the wafer. Results show that there is a decrease in crystal quality and an increase in V-defect formation with increasing indium concentration. A direct correlation was found between V-defect density and growth temperature due to increased strain and indium segregation for increasing indium concentration.
Date: November 19, 2004
Creator: Johnson, M. C.; Liliental-Weber, Z.; Zakharov, D. N.; McCready, D. E.; Jorgenson, R. J.; Wu, J. et al.
Partner: UNT Libraries Government Documents Department

Nitrogen-Induced Modification of the Electronic Structure of Group III-N-V Alloys: Preprint

Description: Incorporation of nitrogen in conventional III-V compound semiconductors to form III-N-V alloys leads to a splitting of the conduction band into two nonparabolic sub-bands. The splitting can be described in terms of an anticrossing interaction between a narrow band of localized nitrogen states and the extended conduction-band states of the semiconductor matrix. The downward shift of the lower sub-band edge is responsible for the N-induced reduction of the fundamental band-gap energy. The modification of the conduction-band structure profoundly affects the optical and electrical properties of the III-N-V alloys.
Date: April 1, 1999
Creator: Walukiewicz, W.; Shan, W.; Ager III, J. W.; Chamberlin, D. R.; Haller, E. E. (Lawrence Berkeley National Laboratory); Geisz, J. F. et al.
Partner: UNT Libraries Government Documents Department

Effect of pressure on the luminescence emissions in CuGaSe2

Description: We present the results of a pressure-dependent photoluminescence (PL) study on CuGaSe{sub 2} films grown on GaAs substrate by metalorganic vapor phase epitaxy. The low-temperature PL spectra of the CuGaSe{sub 2} samples measured at atmospheric pressure are dominated by one near band-edge exciton luminescence line and two strong and relatively broad emissions associated with donor acceptor pairs (DAP). All the observed luminescence emission lines shift toward higher energy with increasing pressure at almost the same rate. The nearly identical pressure coefficients of the two DAP emissions as compared to that of the exciton emission confirm the suggestion that the recombination processes associated with the DAPs involve one shallow donor and two different acceptor species with different binding energies and related to two different native defects.
Date: July 13, 2004
Creator: Shan, W.; Walukiewicz, W.; Wu, J.; Yu, K.M.; Ager III, J.W.; Siebentritt, S. et al.
Partner: UNT Libraries Government Documents Department

Valence band hybridization in N-rich GaN1-xAsx alloys

Description: We have used photo-modulated transmission and optical absorption spectroscopies to measure the composition dependence of interband optical transitions in N-rich GaN{sub 1-x}As{sub x} alloys with x up to 0.06. The direct bandgap gradually decreases as x increases. In the dilute x limit, the observed band gap approaches 2.8 eV; this limiting value is attributed to a transition between the As localized level, which has been previously observed in As-doped GaN at 0.6 eV above the valence band maximum in As-doped GaN, and the conduction band minimum. The structure of the valence band of GaN{sub 1-x}As{sub x} is explained by the hybridization of the localized As states with the extended valence band states of GaN matrix. The hybridization is directly confirmed by soft x-ray emission experiments. To describe the electronic structure of the GaN{sub 1-x}As{sub x} alloys in the entire composition range a linear interpolation is used to combine the effects of valence band hybridization in N-rich alloys with conduction band anticrossing in As-rich alloys.
Date: May 4, 2004
Creator: Wu, J.; Walukiewicz, W.; Yu, K.M.; Denlinger, J.D.; Shan, W.; Ager III, J.W. et al.
Partner: UNT Libraries Government Documents Department

Mutual passivation of group IV donors and isovalent nitrogen in diluted GaN{sub x}As{sub 1-x} alloys

Description: We demonstrate the mutual passivation of electrically active group IV donors and isovalent N atoms in the GaN{sub x}As{sub 1-x} alloy system. This phenomenon occurs through the formation of a donor-nitrogen bond in the nearest neighbor IV{sub Ga}-N{sub As} pairs. In Si doped GaInN{sub 0.017}As{sub 0.983} the electron concentration starts to decrease rapidly at an annealing temperature of 700 C from {approx} 3 x 10{sup 19}cm{sup -3} in the as-grown state to less than 10{sup 16}cm{sup -3} after an annealing at 900 C for 10 s. At the same time annealing of this sample at 950 C increases the gap by about 35 meV, corresponding to a reduction of the concentration of the active N atoms by an amount very close to the total Si concentration. We also show that the formation of Si{sub Ga}-N{sub As} pairs is controlled by the diffusion of Si via Ga vacancies to the nearest N{sub As} site. The general nature of this mutual passivation effect is confirmed by our study of Ge doped GaN{sub x}As{sub 1-x} layers formed by N and Ge co-implantation in GaAs followed by pulsed laser melting.
Date: July 23, 2003
Creator: Yu, K.M.; Wu, J.; Walukiewicz, W.; Shan, W.; Beeman, J.; Mars, D.E. et al.
Partner: UNT Libraries Government Documents Department

Effect of oxygen on the electronic band structure in ZnO{sub x}Se{sub 1-x} alloys

Description: The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnO{sub x}Se{sub 1-x} epitaxial films (0 {<=} x {<=} 1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.
Date: March 14, 2003
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Wu, J.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department