B. N. Baron

Luminescence of Crystals, Molecules, and Solutions

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Molecular beam distributions from high rate sources

A mathematical model that describes the evaporated flux from high rate sources like those used in the deposition of semiconductor films were developed from mass balances with suitable constitutive equations describing the molecular beam distribution. The model was experimentally verified in laboratory scale equipment. Good agreement between the measured distributions and model results were obtained using a single fitting parameter. The fitting parameter is material specific and was correlated to the Knudsen number at a fixed location in the nozzle. A generalized procedure to evaluate this parameter and estimate the incident flux from a molecular beam source is outlined.

Economics of processing thin‐film solar cells

The commercial‐scale processing potential for photovoltaic devices based on polycrystalline and amorphous silicon thin films is evaluated by analyzing in detail the semiconductor deposition process for the following material layers: (CdZn)S/CuInSe2, CdS/CdTe, (CdZn)S/Cu2S, and amorphous silicon. The current status of both laboratory‐scale and unit‐operations‐scale experiments is tabulated and the goals required for reasonable commercial‐scale production detailed. The costs of manufacture are estimated and it is shown that all thin‐film materials which have achieved 10% efficiency in the laboratory can be manufactured at a cost well below one dollar per watt, provided the manufacturing facilities can be designed to produce between 100 000 and 1 000 000 square meters per year. The critical thin‐film research areas are identified for each of the materials listed above.

Properties of a-Si:H and a-SiGe:H Films Deposited by Photo-Assisted CVD

Chemical vapor deposition techniques, in particular plasma enhanced CVD, have been used to produce high quality a-Si:H materials. Continuing research is directed toward increased device performance, improved stability, and translation of scale to commercial production. A part of this effort is the evaluation of alternate CVD techniques which in addition to providing technical options for high efficiency and long term stability are likely to lead to improved understanding of the relationships between deposition processes and material properties. A relatively new technique for depositing a-Si:H is photo-CVD which utilizes ultraviolet light to initiate the decomposition of silane or disilane. The best results from both materials properties and device efficiency points of view have been achieved using mercury sensitized photo-CVD. Recently, a 10.5% efficient a-Si:H p-i-n photovoltaic cell, fabricated by photo-CVD, was reported [1]. A limitation in photo-CVD has been preventing deposition on the UV transparent window. In this paper we describe a new photo-CVD reactor with a moveable UV-transparent Teflon film and secondary gas flows to eliminate window fouling. The deposition and opto-electronic characterization of intrinsic a-Si:H and a-SiGe:H and p-type a-SiC:H are described. Finally, preliminary results of p-i-n solar cells are presented.

Density of midgap states and Urbach edge in chemically vapor deposited hydrogenated amorphous silicon films

Intrinsic or lightly boron‐doped amorphous silicon films were produced by chemical vapor deposition (CVD) from disilane at temperatures from 380 to 460 °C and at growth rates from 0.1 to 40 A/s. The density of states (DOS) near the middle of the mobility gap was determined by space‐charge‐limited conduction on n‐i‐n structures and by steady‐state capacitance‐temperature spectroscopy on p‐i‐n structures. Very close agreement was found between the two techniques for intrinsic layers deposited under similar conditions. The DOS distribution is rather flat from 0.60 to 0.75 eV from the conduction‐band edge and has a range of 1–6×1017 cm−3 eV−1 for intrinsic films, with the minimum occurring for depositions at 440 °C independent of growth rate or contaminants in the disilane. Diborane levels from 9 to 18 ppm in the gas phase reduces the DOS at 440 °C to 3–6×1016 cm−3 eV−1. The exponential absorption below the band edge in the range 1.4–1.6 eV was determined from primary photocurrent spectra of p‐i‐n structures....

Chemical Vapor Deposition of Hydrogenated Amorphous Silicon from Disilane

The authors describe hydrogenated amorphous silicon (a-Si:H) thin films deposited at growth rates of 1 to 30 A/s by chemical vapor deposition (CVD) from disilane source gas at 24 torr total pressure in a tubular reactor. The effects of substrate temperature and gas holding time (flow rate) on film growth rate and effluent gas composition were measured at temperatures ranging from 360{sup 0} to 485{sup 0}C and gas holding times from 3 to 62s. Effluent gases determined by gas chromatography included silane, disilane and other higher order silanes. A chemical reaction engineering model, based on a silylene (SiH/sub 2/) insertion gas phase reaction network and film growth from both SiH/sub 2/ and high molecular weight silicon species, Si/sub n/H/sub 2n/, was developed. The model predictions were in good agreement with experimentally determined growth rates and effluent gas compositions.

Hydrogen content and the goal of stable efficient amorphous-silicon-based solar cells

Abstract Solar cell and film analyses indicate that electron mobility in amorphous hydrogenated silicon-germanium decreases with increasing hydrogen C H and germanium C Ge contents. The hole mobility-lifetime product μτ is less dependent on germanium content than the electron μτ product. Thin (less than 1000 A) graded band gap alloy solar cells were prepared by photochemical vapor deposition with greater than 5% efficiency (at air mass 1.5) and 40% quantum efficiency at 800 nm. Unalloyed a-Si:H with C H values of 7% and 11% having similar annealed state dangling bond densities was prepared by photochemical vapor deposition. Under light exposure or high temperature current injection, high C H materials were markedly less stable.

Performance and analysis of amorphous silicon p-i-n solar cells made by chemical-vapor deposition from disilane

The photovoltaic performance of amorphous silicon p‐i‐n solar cells made by chemical‐vapor deposition (CVD) from disilane is reported and analyzed. Intrinsic layers were deposited at rates from 0.2 to 50 A/s at temperatures from 380 to 460 °C with and without boron doping. Device performance was insensitive to substantial differences in disilane purity. A cell efficiency of 4% was achieved. The primary limitation to higher efficiency was low fill factor ( 18 Ω cm2). Analysis of the series resistance indicated a contact‐related resistance of 4–12 Ω cm2 and a photoconductive resistance composed of intrinsic layer thickness‐independent (10 Ω cm2) and thickness‐dependent terms. Analysis of the voltage dependence of the current collection indicated a fill factor of 60% would be expected in the absence of series resistance. The maximum short‐circuit current of 12.5 mA/cm2 (normalized to 100 mW/cm2) resulted with a boron‐doped i layer deposited at 440 °C at 3.3 A/s. Modeling ...

Continuous Deposition Of Photovoltaic Grade CdS Sheet at the Unit Operations Scale

Uniform photovoltaic grade CdS sheet has been reproducibly deposited on a continuously moving flexible substrate in a reel to reel vacuum coater. Materials characterization by scanning electron microscopy, photoluminescence, and resistivity revealed that continuously deposited CdS is essentially equivalent to material that was deposited for making high efficiency Cu2S/CdS cells in the laboratory scale process. Cells made using continuously deposited CdS sheet had efficiencies as high as 7.85%.

Photovoltaics and electric utilities: An evaluation of utility attitudes and expectations

Abstract The U.S. Department of Energy has argued that the long-term market potential for photovoltaics (PVs) lies in central-station utility applications. Based on results of a nationwide survey of U.S. electric utilities, this study evaluates utility experience and information about PVs, utility perceptions of the viability of both central-station and dispersed applications, and utility expectations about the future prospects for PVs in the power systems market. While a number of utilities are actively involved with PV demonstration projects and remote applications, the attitudes and investment plans of most utilities are not supportive of central-station PV applications. Moreover, while some utilities anticipate growth in dispersed applications in their markets, most utilities are unprepared to deal with the challenges or business opportunities created by the introduction of dispersed systems. In general, limited knowledge and uncertainty characterize the general state of utility thinking about PVs as a power source.