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NiO is a promising alternative to p-GaN as a hole injection layer for normally-off lateral transistors or low on-resistance vertical heterojunction rectifiers. The valence band offsets of sputtered NiO on c-plane, vertical geometry homoepitaxial GaN structures were measured by x-ray photoelectron spectroscopy as a function of annealing temperatures to 600 °C. This allowed determination of the band alignment from the measured bandgap of NiO. This alignment was type II, staggered gap for both as-deposited and annealed samples. For as-deposited heterojunction, ΔE V = 2.89 eV and ΔE C = −2.39 eV, while for all the annealed samples, ΔE V values were in the range of 3.2–3.4 eV and ΔE C values were in the range of −(2.87–3.05) eV. The bandgap of NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for much of the absolute change in ΔE V − ΔE C . At least some of the spread in reported band offsets for the NiO/GaN system may arise from differences in their thermal history.
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Efficient Heterojunction Thin Film CdSe Solar Cells Deposited Using Thermal Evaporation
CdSe is potentially an important material for making tandem junction solar cells with Si and CIGS. Thermodynamic calculations reveal the potential Shockley-Queisser efficiency of such a tandem cell to be in the 45% range. CdSe has the optimum bandgap (1.72eV) for a tandem cell with Si. In this paper, we show that this material system is indeed capable of achieving good electronic properties and reasonable devices can be made in the material. We report on fabricating CdSe materials and heterojunction CdSe solar cells in both superstrate and substrate configurations on FTO/glass and metal substrates. CdSe layer was deposited using thermal evaporation and then was post-treated with CdCl2 to enhance the grainsize and passivate grain boundaries. The device was an ideal heterojunction structure consisting of glass/FTO/n+CdS/ n-CdSe/p organic layer/NiO/ITO. The n+ CdS layer acted to prevent hole recombination at the n+/n interface, and the p organic layer (such as PEDOT:PSS or P3HT) acted to prevent electron recombination at the p+/n interface. The NiO layer was deposited on top of the organic layer to prevent decomposition of the organic layer during ITO deposition. World-record open-circuit voltages exceeding 800 mV and currents of ~15 mA/cm2 were obtained in devices. Detailed material measurements such as SEM revealed large grain sizes approaching 8 micrometer in some of the films after grain enhancement. Optical measurements and QE measurements show the bandgap to be 1.72 eV. XPS measurements showed the CdSe film to be n type. Space-charge limited current was used to measure electron mobilities which were in the range of 1-2 cm2/V-s. Capacitance spectroscopy showed the doping densities to be in the range of a few x 1015/cm3. For substrate devices, the quantum efficiency obtained was in the 90% range.
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- Award ID(s):
- 1664945
- PAR ID:
- 10151217
- Date Published:
- Journal Name:
- Conference record of the IEEE Photovoltaic Specialists Conference
- Volume:
- 46
- ISSN:
- 0160-8371
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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