Cadmium telluride (CdTe) solar cells are a promising photovoltaic (PV) technology for producing power in space owing to their high-efficiency (> 22.1 %), potential for specific power, and cost-effective manufacturing processes. In contrast to traditional space PVs, the high-Z (atomic number) CdTe absorbers can be intrinsically robust under extreme space radiation, offering long-term stability. Despite these advantages, the performance assessment of CdTe solar cells under high-energy particle irradiation (e.g., photons, neutrons, charged particles) is limited in the literature, and their stability is not comprehensively studied. In this work, we present the PV response of n-CdS / p-CdTe PVs under accelerated neutron irradiation. We measure PV properties of the devices at different neutron/photon doses. The equivalent dose deposited in the CdTe samples is simulated with deterministic and Monte Carlo radiation transport methods.
Thin-film CdTe solar cells were synthesized on a fluorine-doped tin oxide (FTO) coated glass substrate (≈ 4 cm × 4 cm). CdS:O (≈ 100 nm) was reactively RF sputtered in an oxygen/argon ambient followed by a close-spaced sublimation deposition of CdTe (≈ 3.5 μm) in an oxygen/helium ambient. The sample was exposed to a 10 min vapor CdCl2 in oxygen/helium ambient at 430˚C. The samples were exposed to a wet CuCl2 solution prior to anneal 200ºC. A gold back-contact was formed on CdTe via thermal evaporation. The final sample contains 16 CdTe devices. For neutron irradiation, we cleaved the CdTe substrate into four samples and exposed two samples to ≈ 90 kW reactor power neutron radiation for 5.5 hours and 8.2 hours, respectively, in our TRIGA (Training, Research, Isotopes, General Atomics) reactor.
We observed a noticeable color change of the glass substrates to brown after the neutron/gamma reactor exposure. Presumably, the injected high-energy neutrons caused the breaking of chemical bonds and the displacement of atoms in the glass substrates, creating point defects and color centers. The I-V characteristics showed noticeable deterioration with over 8 hour radiations. Specifically, the saturation current of the control devices was ≈ 25 nA increasing to 1 μA and 10 μA for the 5.5-hour and 8.2-hour radiated samples, respectively. The turn-on voltage of the control devices (≈ 0.85 V) decreased with the irradiated sample (≈ 0.75 V for 5.5-hour and ≈ 0.5 V for 8.2-hour exposures), implying noticeable radiation damage occurred at the heterojunction. The higher values of the ideality factor for irradiated devices (n > 2.2) compared to that of the control devices (n ≈ 1.3) also support the deterioration of the p-n junction. We observed the notable decrease in shunt resistance (RSH) and the increase in series resistance (Rs) with the neutron dose. It is possible that Cu ions introduced during the CuCl2 treatment may migrate into CdTe grain boundaries (GBs). The presence of Cu ions at GBs can create additional leakage paths for photocarrier transport, deteriorating the overall PV performance.
We estimated the radiation dose of CdTe in comparison to Si (conventional PV) using a UUTR model (e.g., MCNP6 2D UTR Reactor simulations). In this model, we simulated Si and CdTe at the center point of the triangular fuel lattice and used an “unperturbed flux” tally in the water. Our simulations yielded a dose rate of 6916 Gy/s of neutrons and 16 Gy/s of photons for CdTe, and 1 Gy/s of neutrons and 21 Gy/s of photons for Si (doses +/- <1%). The large dose rate of neutrons in CdTe is mainly attributed to the large thermal neutron absorption cross-section of 113Cd. Based on this estimation, we calculate that the exposure of our CdTe PVs is equivalent to several million years in LEO (Low-Earth Orbit), or about 10,000 years for Si in LEO. Currently, we are working on a low-dose neutron/photon radiation on CdTe PVs and their light I-Vs and microstructural characterizations to gain better understanding on the degradation of CdTe PVs.
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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
- NSF-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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