Siva Sivananthan

High-Efficiency Polycrystalline CdS/CdTe Solar Cells on Buffered Commercial TCO-Coated Glass

Multiple polycrystalline CdS/CdTe solar cells with efficiencies greater than 15% were produced on buffered, commercially available Pilkington TEC Glass at EPIR Technologies, Inc. (EPIR, Bolingbrook, IL) and verified by the National Renewable Energy Laboratory (NREL). n-CdS and p-CdTe were grown by chemical bath deposition (CBD) and close space sublimation, respectively. Samples with sputter-deposited CdS were also investigated. Initial results indicate that this is a viable dry-process alternative to CBD for production-scale processing. Published results for polycrystalline CdS/CdTe solar cells with high efficiencies are typically based on cells using research-grade transparent conducting oxides (TCOs) requiring high-temperature processing inconducive to low-cost manufacturing. EPIR’s results for cells on commercial glass were obtained by implementing a high-resistivity SnO2 buffer layer and by optimizing the CdS window layer thickness. The high-resistivity buffer layer prevents the formation of CdTe-TCO junctions, thereby maintaining a high open-circuit voltage and fill factor, whereas using a thin CdS layer reduces absorption losses and improves the short-circuit current density. EPIR’s best device demonstrated an NREL-verified efficiency of 15.3%. The mean efficiency of hundreds of cells produced with a buffer layer between December 2010 and June 2011 is 14.4%. Quantum efficiency results are presented to demonstrate EPIR’s progress toward NREL’s best-published results.

Development of high performance radiation hardened antireflection coatings for LWIR and multicolor IR focal plane arrays

High Performance Radiation Hardened LWIR and Multicolor Focal Plane Arrays are critical for many space applications. Reliable focal plane arrays are needed for these applications that can operate in space environment without any degradation. In this paper, we will present various LWIR and Multicolor Focal Plane architectures currently being evaluated for LWIR and Multicolor applications that include focal plane materials such as HgCdTe, PbSnTe, QWIP and other Superlattice device structures. We also present AR Coating models and experimental results on several promising multi-layer AR coatings that includes CdTe, Si3N4 and diamond like Carbon, that have the necessary spectral response in the 2-25 microns and are hard materials with excellent bond strength. A combination of these materials offers the potential of developing anti-reflection coatings with high optical quality with controlled physical properties.

Nitrogen Plasma Doping of Single-Crystal ZnTe and CdZnTe on Si by MBE

We have investigated inxa0situp-type doping of ZnTe and CdZnTe on Si(211) by molecular beam epitaxy using a radiofrequency (RF)-nitrogen plasma source for application to multijunction II–VI-based solar cells. CdZnTe would be used as a wide-gap top cell in a monolithic multijunction device, and ZnTe or CdZnTe could be used for the p-side of tunnel junctions. Highly p-type material is required for producing the high-quality tunnel junctions crucial for maintaining current flow, and p-doping of order 1017xa0cm−3 is required for the generation of a large built-in potential in the absorber region of solar cells. Our uniformly doped films exhibited good Hall characteristics, especially considering the large lattice mismatch between Si and either ZnTe or CdZnTe. Crystal quality was examined by x-ray diffraction. Nitrogen incorporation was examined as a function of the source-gas dilution with argon. A sample with layers of CdZnTe doped using 1% to 100% nitrogen was grown on nominally undoped CdZnTe and analyzed using secondary-ion mass spectrometry. The nitrogen incorporation differed by only a factor of 10, despite the factor of 100 difference in the nitrogen concentration in the plasma, indicating a saturation effect.

Design and development of multicolor detector arrays

Multi-color infrared (IR) focal planes are required for high performance sensor applications. These sensors will require multi-color focal plane arrays (FPA) that will cover various wavelengths of interest in MWIR/LWIR and LWIR/VLWIR bands. There has been a significant progress in HgCdTe detector technology for multi-color MWIR/LWIR and LWIR/VLWIR focal plane arrays [1,2,3]. Two-color IR FPA eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in a simpler, reliable and affordable systems. The complexity of multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally material characteristics are only relatively controllable and depend on the process repeatability. In this context the ability of performing simulation experiments where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications. Complex multi-color detector pixels cannot be designed and optimized by using a conventional 1D models. Several additional physical phenomena need to be taken into account. In designing a conventional photovoltaic IR detector array, a trade off exists on the choice of the pixel pitch, the pixel area and its height. The main goal of the device optimization is to reduce the pixel cross talk while keeping a high filling factor and detection efficiency. If the pixel height is made comparable to the lateral pixel dimension the contribution of the lateral photocurrent and lateral generation-recombination current becomes relevant and a full 2D simulation needs to be performed. It also important to point out that the few attempts to perform 2D simulations have reached the conclusion that for advanced IR arrays a full 3D approach should be used. The most challenging aspect of the array design and simulation is the pixel cross-talk effects. Since this is caused by the interaction with the four nearest neighboring pixels, even a description based on a 2D simulation model in most cases is not adequate. It is consequently important to include results from 3D simulation models as a guide to build lower dimensionality models.

Influence of CdTe Deposition Temperature and Window Thickness on CdTe Grain Size and Lifetime After CdCl 2 Recrystallization

Grain structure influences both transport and recombination in CdTe solar cells. Larger grains generally are obtained with higher deposition temperatures, but commercially it is important to avoid softening soda-lime glass. Furthermore, depositing at lower temperatures can enable different substrates and reduced cost in the future. We examine how initial deposition temperatures and morphology influence grain size and lifetime after CdCl2 recrystallization. Techniques are developed to estimate grain distribution quickly with low-cost optical microscopy, which compares well with electron backscatter diffraction data providing corroborative assessments of exposed CdTe grain structures. Average grain size increases as a function of CdCl2 temperature. For lower temperature close-spaced sublimation CdTe depositions, there can be more stress and grain segregation during recrystallization. However, the resulting lifetimes and grain sizes are similar to high-temperature CdTe depositions. The grain structures and lifetimes are largely independent of the presence and/or interdiffusion of Se at the interface, before and after the CdCl2 treatment.

Effects of stoichiometry in undoped CdTe heteroepilayers on Si

Crystalline CdTe layers have been grown heteroepitaxially onto crystalline Si substrates to establish material parameters needed for advanced photovoltaic (PV) device development and related simulation. These studies suggest that additional availability of the intrinsic anion (i.e., Te) during molecular beam epitaxy deposition can improve structural and optoelectronic quality of the epilayer and the interface between Si substrate and the epilayer. This is seen most notably for thin CdTe epitaxial films (<; ~10 μm). Although these observations are foundationally important, they are also relevant to envisioned high-performance multijunction II-VI alloy PV devices-where thin layers will be required to achieve production costs aligned with market constraints.

Arsenic doped heteroepitaxial CdTe by MBE for applications in thin-film photovoltaics

Open circuit voltages in CdTe based solar photovoltaics can be improved through increasing the acceptor carrier concentration in the absorber. Arsenic doped heteroepitaxial CdTe layers deposited by MBE are investigated as a means to understand the viability of arsenic as an alternative dopant source without the complication of polycrystalline grain boundaries or high temperature deposition processes. Crystal quality, thickness, and minority carrier lifetimes are correlated with arsenic incorporation and p-type carrier concentrations for both doped and undoped films. Films with carrier concentrations greater than 1015 cm-3 have been produced using both an arsenic cracker source and a Cd3As2 effusion source though incorporation differs drastically between these two. As previous work has found, arsenic incorporation is shown to degrade crystal quality. Despite the lower crystal quality, minority carrier lifetimes greater than 1 ns have been achieved in samples with high carrier concentrations when the Cd3As2 source is used suggesting the benefit of cadmium overpressure. While the feasibility of arsenic doping during high temperature CdTe deposition processes is still not known, arsenic is shown to be a viable dopant source for continued investigation of heteroepitaxial model systems.

A semiconductor saturable absorber for mid-infrared wavelengths

Saturable absorption of nanosecond and picosecond duration pulses in a thin HgCdTe film is reported at 4.6 μm. The material shows promise as a mode locker for mid-infrared ultrafast lasers.

An analysis of gamma radiation effects on ZnS- and CdTe-passivated HgCdTe photodiodes

At present, infrared photodetectors are being increasingly used in space systems, where they are exposed to the space radiation environment. Consequently, the radiation-hardness-related problem in HgCdTe photodetectors has become a critical issue. In this study, the gamma radiation effects on ZnS- and CdTe-passivated mid-wavelength infrared (MWIR) HgCdTe photodiodes were investigated. Although ZnS has an excellent insulating property, its radiation-tolerant property was revealed very poor in comparison with CdTe. After 1 Mrad of gamma irradiation, the resistance-area product at zero bias (R0A) value of the ZnS-passivated photodiode was drastically reduced by roughly 5 orders from ~107 Ω cm2 to 102 Ω cm2, whereas the CdTe-passivated photodiode showed no degradation in R0A values.