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Recent advances in device design and process optimizations have enabled the production of CdTe devices on flexible substrates, but the necessary high‐temperature processing (>450 °C) to recrystallize grains limits the use of alternative lightweight substrates. Here, a new synthesis method is reported to create a freestanding CdS/CdTe film by combining high‐temperature depositions (CdS/CdTe on Si/SiO₂) and a simple lift‐off process in a water environment at room temperature. Analysis of the results indicate that the delamination is facilitated by the innate lattice mismatch as well as the presence of an unexpected Te‐rich layer (≈20 nm), which accumulates on the SiO₂surface. High‐resolution electron microscopy and spectroscopy measurements confirm that the CdS/CdTe film is physically liberated from the substrate without leaving any residue, while also preserving their initial structural and compositional properties.

 

Leading photovoltaic technologies such as multicrystalline Si, CdTe, Cu(In,Ga)Se₂, and lead halide perovskites are polycrystalline, yet achieve relatively high performance. At the moment polycrystalline photovoltaic technologies stand at a juncture where further advances in device performance and reliability necessitate additional characterization and modeling to include nanoscale property variations. Properties and implications of grain boundaries are previously studied, yet chemistry variations along individual grain boundaries and its implications are not yet fully explored. Here, the effects of bromine etching of CdTe absorber layers on the nanoscale chemistry are reported. Bromine etching is commonly used for improving CdTe back contacts, yet it removes both cadmium and chlorine along grain boundaries to depths closer to 1 µm. 2D device simulations reveal these composition modifications limit photovoltaic performance. Since grain boundaries and their intersections with surfaces and interfaces are universal to thin film photovoltaics, these findings call for similar studies in each of the photovoltaic technologies to further enable advances.

 

Cadmium telluride (CdTe) thin-film semiconductors exhibit many desirable properties for low-cost and high-efficiency photovoltaic (PV) technology, including inherent robustness of inorganic absorber, a direct bandgap that allows full absorption of the solar spectrum with thicknesses of only few microns, and inexpensive and high-throughput manufacturing processes. At the best efficiency of 22 %, the power conversion efficiency of CdTe PVs is still well below the maximum theoretical limit (approximately 30 %). It has been suggested that the inferior efficiency is mainly attributed to the inherent polycrystalline nature of CdTe absorber (e.g., grains, grain boundaries). Understanding local photocarrier dynamics is vital to overcoming roadblocks toward higher efficiency CdTe PVs. However, conventional cell-level PV measurements often limit the microstructural analysis. In this work, we present a local PV characterization technique using point back-contacts. The thin-film CdTe solar cells used in this work were prepared by CSS (close-spaced sublimation) on a stack of n-type window layer (e.g., CdS) / transparent conductive layer (TCO; e.g., SnO2) / glass substrate. 

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. 

In this work, we report the different effects of CdCl 2 treatment on CdTe films deposited by thermal evaporation onto CdS and MgZnO (MZO) buffer layers. The main finding, which is relevant for understanding recent advances in CdTe device efficiency, is that few-μm thick CdTe films deposited on MZO can be induced to completely recrystallize forming a film consisting of grains that span the film thickness and are up to 30 μm laterally. On CdS buffer layers, the changes in microstructure with Cl treatment are much less pronounced and the final microstructure is less ideal for thin film photovoltaics. We propose a thermodynamic framework for understanding the microstructural changes during CdCl 2 treatment which can assist in understanding the wide range of behaviors observed across the many CdTe thin film solar cell fabrication procedures.