A. H. Mahan

Bio: A. H. Mahan is an academic researcher from National Renewable Energy Laboratory. The author has contributed to research in topics: Chemical vapor deposition & Amorphous silicon. The author has an hindex of 25, co-authored 105 publications receiving 2011 citations.

Structural, defect, and device behavior of hydrogenated amorphous Si near and above the onset of microcrystallinity

Abstract: High-hydrogen-diluted films of hydrogenated amorphous Si (a-Si:H) 0.5 μm in thickness and optimized for solar cell efficiency and stability, are found to be partially microcrystalline (μc) if deposited directly on stainless steel (SS) substrates but are fully amorphous if a thin n layer of a-Si:H or μc-Si:H is first deposited on the SS. In these latter cases, partial microcrystallinity develops as the films are grown thicker (1.5–2.5 μm) and this is accompanied by sharp drops in solar cell open circuit voltage. For the fully amorphous films, x-ray diffraction (XRD) shows improved medium-range order compared to undiluted films and this correlates with better light stability. Capacitance profiling shows a decrease in deep defect density as growth proceeds further from the substrate, consistent with the XRD evidence of improved order for thicker films.

Microvoids in amorphous Si1-xCx: H alloys studied by small-angle x-ray scattering

Abstract: The microstructure of hydrogenated amorphous silicon‐carbon alloys has been analyzed by small‐angle x‐ray scattering, infrared absorption, and density measurements. Decreasing density with C incorporation is due to microvoids about 0.6 nm in average radius, which are either approximately spherical in shape or randomly oriented nonspheres. The microvoid number density increases from about 5×1019/cm3 for a‐Si:H to about 4×1020/cm3 for a‐Si0.7 C0.3 :H. The CH3 species probably causes the enhanced microvoid formation in these alloys. A large fraction of the microvoid surfaces is not hydrogenated.

Structural changes in a − S i : H film crystallinity with high H dilution

Abstract: Using infrared absorption (ir) spectroscopy, H evolution, and x-ray diffraction (XRD), the structure of high-H-dilution, plasma-enhanced chemical vapor deposition $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ films ``on the edge of crystallinity'' is examined. From the ir Si-H wag mode peak frequency and the XRD results, we postulate the existence of very small Si crystallites contained within the as-grown amorphous matrix with the majority of the bonded H located on these crystallite surfaces. Upon annealing, a low-temperature H-evolution peak appears, and film crystallization is observed at temperatures as low as 500 \ifmmode^\circ\else\textdegree\fi{}C, which is far below that observed for $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ films grown without H dilution. While the crystallite sizes and volume fraction are too small to be detected by XRD in the as-grown films, these crystallites catalyze the crystallization of the remainder of the amorphous matrix upon annealing, enabling the evolution of H at low temperatures. The large spatial inhomogeneity in the H bonding thus produced throughout the film is suggested to be one of the reasons for the reduced Staebler-Wronski effect observed in solar cells utilizing these films.

Structural properties of hot wire a-Si:H films deposited at rates in excess of 100 Å/s

Abstract: The structure of a-Si:H, deposited at rates in excess of 100 A/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 A/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 A/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in th...

Thin-film solar cells: an overview

Abstract: Thin film solar cells (TFSC) are a promising approach for terrestrial and space photovoltaics and offer a wide variety of choices in terms of the device design and fabrication. A variety of substrates (flexible or rigid, metal or insulator) can be used for deposition of different layers (contact, buffer, absorber, reflector, etc.) using different techniques (PVD, CVD, ECD, plasma-based, hybrid, etc.). Such versatility allows tailoring and engineering of the layers in order to improve device performance. For large-area devices required for realistic applications, thin-film device fabrication becomes complex and requires proper control over the entire process sequence. Proper understanding of thin-film deposition processes can help in achieving high-efficiency devices over large areas, as has been demonstrated commercially for different cells. Research and development in new, exotic and simple materials and devices, and innovative, but simple manufacturing processes need to be pursued in a focussed manner. Which cell(s) and which technologies will ultimately succeed commercially continue to be anybody's guess, but it would surely be determined by the simplicity of manufacturability and the cost per reliable watt. Cheap and moderately efficient TFSC are expected to receive a due commercial place under the sun.

Materials interface engineering for solution-processed photovoltaics

Abstract: Advances in solar photovoltaics are urgently needed to increase the performance and reduce the cost of harvesting solar power. Solution-processed photovoltaics are cost-effective to manufacture and offer the potential for physical flexibility. Rapid progress in their development has increased their solar-power conversion efficiencies. The nanometre (electron) and micrometre (photon) scale interfaces between the crystalline domains that make up solution-processed solar cells are crucial for efficient charge transport. These interfaces include large surface area junctions between photoelectron donors and acceptors, the intralayer grain boundaries within the absorber, and the interfaces between photoactive layers and the top and bottom contacts. Controlling the collection and minimizing the trapping of charge carriers at these boundaries is crucial to efficiency.

Coaxial MnO2/Carbon Nanotube Array Electrodes for High-Performance Lithium Batteries

Abstract: Coaxial manganese oxide/carbon nanotube (CNT) arrays deposited inside porous alumina templates were used as cathodes in a lithium battery. Excellent cyclic stability and capacity of MnO2/CNT coaxial nanotube electrodes resulted from the hybrid nature of the electrodes with improved electronic conductivity and dual mechanism of lithium storage. The reversible capacity of the battery was increased by an order compared to template grown MnO2 nanotubes, making them suitable electrodes for advanced Li ion batteries.