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Multicrystalline silicon wafers used for solar cells exhibit defect clusters-localized crystal defects in and near grains of some specific orientations. Defect clusters are also dominant sites for impurity precipitation, and they remain ungettered and unpassivated through the solar cell processing. This paper describes characteristics of defect clusters, and shows, through theory and experiment, that defect clusters typically lower cell efficiency by 3 to 4 absolute percentage points. To recover this efficiency loss, it is necessary to getter precipitated impurities.

The Very High Efficiency Solar Cell (VHESC) program is developing integrated optical system–PV modules for portable applications that operate at greater than 50% efficiency. We are integrating the optical design with the solar cell design, and have entered previously unoccupied design space. Our approach is driven by proven quantitative models for the solar cell design, the optical design, and the integration of these designs. Optical systems efficiency with an optical efficiency of 93% and solar cell device results under ideal dichroic splitting optics summing to 42·7 ± 2·5% are described. Copyright © 2008 John Wiley & Sons, Ltd.

The temperature dependence of an additional 1H NMR signal that appears in a-Si:H only after light soaking is very different from the NMR signal due to bonded hydrogen and trapped molecular hydrogen. This `doublet' signal is attributed to a pair of hydrogen atoms approximately 2.3 A apart. From the temperature dependence we show that the lineshape of this site is consistent with a pair of hydrogen atoms. We also examine the spin-lattice relaxation from 6.5 to 20 K. At low temperatures, the spin-lattice relaxation time of these paired hydrogen sites has a strong dependence on temperature.

Amorphous silicon based solar cells were deposited by dc plasma decomposition of silane and hydrogen. As the deposition rate increases, the processing time decreases and becomes comparable to the time required to heat the glass superstrate to the deposition temperature. This allows de-coupling of the temperature of the growing film that determines the hydrogen out-diffusion processes from that of the gas, which influences the formation of multiple silicon containing molecules that increase the light induced degradation of the material. Consequently, reducing the initial temperature of the superstrate is shown to maximize the initial performance while maintaining the stability obtained at higher deposition chamber temperatures.

Since the fabrication of the first amorphous silicon solar cell in 1974 and the introduction of the first commercial products in 1980, shipments of amorphous silicon solar cells have grown to about 40 MW in 2001. The majority of these products have been fabricated on tin oxide coated glass in a monolithic series-connected configuration. In the current paper, the author describes the manufacturing process used to fabricate amorphous silicon monolithic series connected PV modules and also reviews the performance, reliability and stability of the modules. Issues such as manufacturing costs and environmental impacts are also discussed. Further research, development and engineering are required to assure that a-Si photovoltaics will play a major role in future world energy production.

Electrochemical corrosion effects can occur in thin-film photovoltaic (PV) modules that are fabricated on tin-oxide-coated glass when operating at high voltages and at elevated temperatures in a humid climate. The current study shows that this corrosion is associated with a delamination of the tin oxide layer from the glass, which is caused by sodium accumulation near the interface between the tin oxide and the glass and by the ingression of moisture into the PV module from the edges. This corrosion in thin-film PV modules can be significantly reduced by altering the growth conditions of the tin oxide or by using zinc oxide as a transparent conductive oxide electrode. Copyright © 2003 John Wiley & Sons, Ltd.

The growth rate of a-Si increases with dc plasma current with a single slope at high flow rates but with a reduced slope above a certain power for lower flow rates when using a showerhead type gas delivery system. This is in contrast to saturation of the growth rate under gas depletion conditions previously reported. The change to a smaller slope for lower flow rates at higher powers can be understood in terms of a transition to reactive radical dominated growth. We show that a-Si single junction cell performance deteriorates under gas depletion conditions.

DC discharge voltage monitoring is a convenient non-intrusive tool for in-line monitoring of plasma-deposited amorphous silicon deposition processes. We discuss the observed variations of the discharge voltage due to (i) changing hydrogen dilution of silane, (ii) etch back of silicon by a hydrogen plasma, (iii) admixing of germane and methane and correlate the results with in situ ion-mass spectroscopy. We suggest that negative ion formation by electron attachment to silane and/or higher-silane species causes the changes of discharge voltage. Time dependences following discharge initiation are discussed in terms of the relative concentrations of silane versus higher-silane species.

BP Solar started manufacturing small-area, single-junction amorphous silicon solar cells for consumer applications in 1984, began producing 1 ft2 (∼0.09 m2) single-junction modules for terrestrial applications in 1986 and initiated the production of 8.6 ft2 (∼0.77 m2) tandem modules for both remote and building-integrated applications in 1997. Over the last few years, the technical and manufacturing personnel at the BP Solar TF1 plant in Toano, Virginia have made tremendous progress in ramping up the plant to where it is now producing amorphous silicon tandem modules with electrical yields in excess of 95% and at a run rate of more than 7 MWp per year. Copyright © 2002 John Wiley & Sons, Ltd.

Amorphous silicon (a-Si) photovoltaic (PV) modules are generally manufactured in a single-junction p-i-n configuration and in sites ranging from a few square centimeters to about 4000 cm 2 . These modules are being used in a number of both indoor and outdoor low wattage (less than 20 W p (peak watt)) applications, but have not found widespread use in most higher wattage power applications owing to relatively low stabilized conversion efficiencies (approximately 4%–5%). The recent improvements in the performance and stability of a-Si based multijunction modules indicates that these modules should soon start to appear in the higher wattage outdoor applications. When multijunction modules are manufactured in totally automated facilities, the manufacturing costs should fall below $1 per W p , and these modules should then start penetrating the grid-connected power generation markets.

We investigate the spin-lattice relaxation of the dipolar order in $^{1}\mathrm{H}$ NMR in hydrogenated amorphous silicon $(a\text{\ensuremath{-}}\mathrm{Si}:\mathrm{H})$. We find that the relaxation is dominated by the cross relaxation between the hydrogen nuclei and the paramagnetic states. The relaxation is inhomogeneous, and can be described as a stretched exponential function. We proposed a possible mechanism for this relaxation. This mechanism applies to a rather broad range of paramagnetic states, including the deep neutral defects (dangling bonds), the light-induced metastable defects, the defects created by doping, and the singly occupied, localized band-tail states populated by light at low temperatures. The cross relaxation is only sensitive to the bulk spin density, and the surface spins have a negligible effect on the relaxation.

The acute trauma response consists of multiple biological components reacting to various signals from local wounded tissue and systemic alterations. In severe injury, exaggerated and unchecked release of mediators result in detrimental systemic effects and may lead to multiple organ failure. In addition to host endogenous modulation of these mediators, timely therapeutic interventions may modify local inflammation, neuroendocrine signals, and systemic alterations. Recovery from trauma ultimately depends upon the coordination of the neuroendocrine, immune, and metabolic systems toward restoring homeostasis and survival. Future research in this area may elucidate new therapeutic options for the critically injured trauma patient.

Multicrystalline Si wafers used in commercial solar cell fabrication exhibit a tendency to form large “clusters” of defects, which remain laterally separated from each other. Defect clusters are also sites of impurity precipitation. Because precipitated impurities cannot be gettered by the conventional processes used in Si solar cell fabrication, defect clusters constitute low-performing regions in the cell. They shunt the device and constitute the primary efficiency limiting mechanism in current solar cells. We show that the efficiency loss caused by defect clusters can exceed 3–4 absolute points.

We have investigated the effects of using hydrogen-diluted mixtures of either silane and germane or disilane and germane for deposition of amorphous silicon germanium intrinsic layers in a-Si/a-SiGe tandem devices with all the other layers were unchanged. The 'optimized' DC plasma induced deposition conditions have been found using screening, followed by response surface design of experiments. Multiple deposition runs of initial performance and its light induced degradation have been compared.

We have developed an in-situ diagnostic system that utilizes optical emission spectroscopy (OES) in conjunction with an array of fiber optic probes to measure the spatial distribution of plasmas during the fabrication of amorphous silicon (a-Si) PV modules. The uniformity of the plasma emission was influenced by structural changes in the plasma electrode assembly that can occur with heating and by variations in the substrate-electrode spacing that can occur with warped and misaligned substrates or electrodes. This diagnostic system has been used to improve our deposition reactor design and to optimize the deposition conditions for the fabrication of high performance thin-film tandem modules.

We investigate the temperature dependence of an additional 1H NMR signal that appears in aSi:H at T = 7 K only after light soaking. This “doublet” signal is attributed to a pair of hydrogen atoms approximately 2.3 Å apart. We present the first lineshape of this doublet, and we examine the spin-lattice relaxation from 6.5 K to 20 K at a magnetic field of about 3.5 T. At low temperatures, the spin-lattice relaxation time of these paired hydrogen sites has a strong dependence on temperature, which is probably the result of some form of local motion.

We have investigated the changes in the cathode potential in a dc discharge of silane and hydrogen used to deposit the intrinsic layer of p-i-n type solar cells at deposition rates from 1 to 10Å/s with the superstrate temperature at 200°C and 250°C. Under plasma conditions that lead to higher deposition rates (5-10Å/s), fluctuations of the cathode potential which are suggestive of the formation and de-trapping of particulates in/from the plasma, are observed at 200°C but disappear at 250°C. Improvement of the temperature uniformity over the plasma region from 1.7°C/cm to 0.7°C/cm removes the fluctuations of the cathode potential even at 200°C, indicating that the particulates are formed predominantly at the plasma boundary. Consequently, the stability of solar cells with i-layers deposited at ~10Å/s in the center of the plasma region at the same superstrate temperature improved by 26% suggesting that multiple silicon containing molecules diffuse from the edge to the center of the plasma region.

The current markets for a-Si solar cells are reviewed. The different manufacturing approaches being used in the United States are discussed. An update is also presented on recent progress in improving the properties of a-Si based alloys and on the development of relatively high-performance multijunction thin-film solar cells. >

Irreversible light-induced degradation has been observed in amorphous silicon p-i-n solar cells exposed to intense illumination at normal operating temperatures. In addition, subjecting the cells to a strong reverse bias while exposed to intense illumination caused increases in both the open-circuit voltage and the fill factor. Both effects appear to be associated with the motion of hydrogen in the vicinity of the p/i interface.

We report a hydrogen-related defect that establishes the direct role of hydrogen in stabilizing the silicon dangling bonds created in the Staebler-Wronski effect in hydrogenated amorphous silicon. A specific NMR signal due to paired hydrogen atoms occurs only after optical excitation, exists at an intensity that is consistent with the density of optically induced silicon dangling bonds, and anneals at temperatures that are consistent with the annealing of the optically induced silicon dangling bonds. At this defect the hydrogen atoms are $2.3\ifmmode\pm\else\textpm\fi{}0.2\AA{}$ apart.

We have investigated the cause for continued changes in the silane related species observed using mass spectroscopy during i-layer deposition process lasting several tens of minutes. Silane decomposition is found to influence the mass spectrometer filament and therefore, pressure changes were compared to ascertain differences in pumping speed using a rotary, mechanical pump. The cathode voltage was used as a measure of effective hydrogen to silane ratio in real time and deliberate tailoring of this ratio is seen to result in a discernible difference in the degradation of single junction amorphous silicon solar cells. Some obvious consequences of this subtle, pump-type dependent transient behavior are discussed.

We study the H NMR line shapes of a sample of a-Si:H under several conditions: 1) as grown, 2) light-soaked for 600 hours, and 3) light-soaked followed by annealing at different temperatures. At T = 7 K, the NMR line shape of the sample after light soaking exhibits an additional doublet compared to that of the sample as-grown. This doublet is an indication of a closely separated hydrogen pair. The distance between the two hydrogen atoms is estimated to be about (2.3 ± 0.2) Å. The concentration of these hydrogen sites is estimated to be between 1017 and 1018 cm-3 consistent with ESR measurements of the defect density after light soaking. This doublet disappears after the sample is annealed at 200°C for 4 hours.

BP Solar started commercial production of amorphous silicon tandem solar cells in Toano, Virginia in 1997. The scale-up process has involved overcoming technology challenges in several areas. It was necessary to develop high speed, multiple-beam laser scribing systems that could pattern photovoltaic modules over an area of 0.8 square meters with close dimensional tolerances. In addition, the deposition rate of amorphous silicon was doubled while production was being ramped up, and the deposition system was modified to meet capacity requirements. The utilization of the germane feedstock gas was increased by about 25% in order to reduce material costs. In addition, the dimensional tolerances of the deposition system geometry were determined and controlled in order to assure uniform deposition of the amorphous silicon alloys. Effects of contaminants (such as pump oils and residual dopants) on device performance were quantified. Sources of debris that could cause shunts and shorts in the devices were identified and minimized. Current research efforts are focused on further increases in the amorphous silicon deposition rate, improvements in device performance and the development of in-situ diagnostic tools to monitor and control the manufacturing process.

Light induced changes to 1 sun degraded steady state (DSS) have been investigated on hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells and corresponding films fabricated with and without hydrogen dilution of silane. Striking similarities are found for the degradation kinetics, between the electron mobility lifetime (μτ) products and the corresponding fill factors (FF). These correlations that exist for both intrinsic materials at temperatures between 25 and 100°C, are present for the DSS as well as in the kinetics, which exihibit distinctly different dependence on temperature. No such correlations are present between μτ, FF and densities of D0 defects, measured with subgap absorption α(E) at 1.2eV, and electron spin resonance (ESR). The creation of non-D0 defects is also clearly indicated by the temperature dependence of the kinetics and the changes in the shape of α(E) with the results suggesting the presence of more than one mechanism for the creation of light induced defects associated with the Staebler-Wronski effect (SWE).

We have recently observed that the initial performance of amorphous silicon (a-Si) solar cells can be improved by up to several % by annealing the cells at successively lower temperatures for successively longer times. For some devices, we also observed an improvement in performance after light soaking that was statistically significant. We have also observed an improvement in the stabilized performance of cells that were subjected to reverse bias annealing after light soaking. This effect was clearly evident in single-junction p-i-n cells with amorphous silicon-carbon (a-SiC) i-layers where the density of metastable centers is very high (> 1018cm-3). When we light-soaked a-SiC cells and then subjected them to low-temperature reverse bias annealing, they exhibited clear evidence of improved short-wavelength response after a second light soaking. These annealing effects are explained in terms of hydrogen motion within the a-Si network and within microvoids.

Submodule efforts have resulted in improved module design concepts as is demonstrated by: (1) a world record 13% conversion efficiency for a 40.4 cm/sup 2/ laminated 20 segment submodule, and (2) the ability to produce submodules of high FF (>70%) without other substantial losses relative to reference devices. The quality of our deposition systems for larger areas is tested and calibrated by the use of 3/spl times/3 matrices of 3"/spl times/3" substrates covering an area of over 500 cm/sup 2/. A high quality, high throughput (down to 10 min. deposition times) CIGS absorber process is established and exhibits a high degree of controllability as well as tolerance to compositional fluctuations. Results of laser interconnect patterning are also shown.

Some of the significant steps in technological development for large-scale commercialization of amorphous silicon (a-Si:H) based multijunction photovoltaic modules are presented. These developments are establishing a high quality baseline process for manufacturing large-area ( ˜ 8 ft2) a-Si:H/a-SiGe:H tandem junction modules with improved stabilized conversion efficiency, throughput, yield, and reduced materials usage.

The impact of front contact window optimization on module design for different absorber bandgaps is examined, wider bandgaps being simultaneously less demanding of the ZnO IR transparency as well as its sheet conductivity. Based on small area device measurements, examples are given demonstrating the need and deposition method of the high resistive ZnO buffer layer as a function of the CdS buffer layer thickness. Questions concerning the validity of using doped ZnO characteristics of films deposited on glass are addressed. It is shown that the doped ZnO in devices can be potentially quite different from that on glass, some of this difference is tracked to the CdS. This should be accounted for when used in module prediction models.

Solarex, a Business Unit of Amoco/Enron Solar, is constructing a plant in James City County, VA that will have the capacity to produce 10 MW/yr of multijunction amorphous silicon PV modules. The plant is scheduled to start commercial production of 8.6 ft/sup 2/ tandem modules in early 1997. The tandem structure utilizes a front p-i-n junction of amorphous silicon and a rear p-i-n junction containing amorphous silicon-germanium alloys. The manufacturing process utilizes plasma-enhanced CVD of the amorphous silicon alloys and computerized laser scribing combined with machine vision to fabricate monolithic thin-film PV modules. These modules are protected with a glass-EVA-glass encapsulation and pass all accelerated environmental tests. The stabilized conversion efficiencies of the tandem modules are projected to be about 8% based on results obtained with 4 ft/sup 2/ prototype modules produced on a pilot line. The principle product will be a monolithic module (/spl sim/26"/spl times/48") that is expected to be widely used in grid-tied applications such as PV roofing and solar farms.

The status of a-Si solar cell technology is reviewed. This review includes a discussion of the types of solar cell structure that are being used in commercial products. An overview of the development efforts under way involving new materials, such as alloys and microcrystalline films, and their impact on device performance is given. The status of stability in a-Si solar cells and projections for costs for large-scale manufacturing facilities are reviewed. The development of markets for a-Si photovoltaics is also discussed. >

The electrical and chemical properties of palladium Schottky barrier cells were investigated for hydrogenated amorphous silicon (a-Si:H) films made in d.c. proximity and d.c. cathodic glow discharges in silane. Water vapor adversely affects the performance of palladium Schottky barrier cells made with either type of a-Si:H film. For palladium contacts on proximity films, exposure to humidity causes oxygen and hydrogen to accumulate in the vicinity of the Pd-(a-Si:H) interface. However, for palladium contacts on cathodic films the presence of water vapor promotes a chemical reaction, even at room temperature, to form a new material consisting of palladium, silicon, oxygen and hydrogen.

In the last few years, light-induced changes have been observed in many of the properties of hydrogenated amorphous silicon (a-Si:H). This article reviews the light-induced effects in properties such as conductivity, photoconductivity, photoluminescence, electron spin density, density of gap states, diffusion length, optical absorption and photovoltaic performance. Some of the experimental results indicate that more than one type of metastable center is operative in a-Si:H. We discuss several proposed models for these centers and also suggest some new models.

Conversion efficiencies as high as 11.95% have been obtained in small-area, single-junction amorphous silicon (a-Si) solar cells and as high as 10.51% in a-SiC/a-SiGe stacked-junction cells. These high performance cells use a combination of textured tin oxide as a front contact and In 2 O 3 :Sn/Ag as a back contact to ensure efficient light trapping. For large-area submodules (1000 cm 2 ), active-area efficiencies of 8.7% have been obtained in single-junction structures and 7.03% in stacked-junction structures.

The diffusion length and space charge width of undoped a-Si:H has been measured by means of the constant surface photovoltage technique for films grown at different substrate temperatures and in the presence of various gaseous impurities.

Cells taken from the spleens of tumor-cured (immunized), tumor-bearing or untreated control mice were employed in a test tube Leukocyte Adherence Inhibition (L.A.I.) assay. Immunized animals showed a strong L.A.I. reactivity to MOPC 104E tumor cell antigens but not to those from an unrelated lymphoma. The assay was quite sensitive to even limited tumor growth, and a positive L.A.I. index was observed at least 6 days prior to the appearance of a palpable tumor. The L.A.I. index decreased substantially, however, in animals bearing large tumors. When the L.A.I. assay was performed in conjunction with the Winn tumor neutralization assay to determine the kinetics of development of tumor immunity in spleens of tumor-bearing mice, relatively little agreement was observed between the two assays. In contrast, when spleen cells from groups of animals with varying levels of in vivo immunity were compared, L.A.I. indices showed a substantial correlation with the magnitude of in vivo immunity.

We have studied the stability of hydrogenated amorphous silicon (a-Si:H) using several new techniques such as diffusion length measurements, photovoltage profiling and IR absorption via multiple internal reflections. We find that prolonged illumination generally causes a decrease in the diffusion length and an increase in the space charge density of undoped a-Si:H films.

The barrier heights, φB, of Schottky barriers formed between Al, Ni, Cr, Pd, Au, Rh, and Pt metal films and undoped discharge-produced amorphous silicon (a-Si) have been measured. The dependence of φB on the metal work function, φM, has been characterized for values of φM between ≈4.0 and 5.5 eV. Experiments were performed to compare the effects of surface states on φB of both the undoped (n-type) amorphous and n-type single crystal Si. These effects of surface states are very similar, indicating similar densities and energy distributions of surface states. The densities of surface states on a-Si are found to be ≳1013cm−2eV−1 which result in the pinning of φB to values ≳0.7 eV even with φM as low as 4.0 eV.

Some market estimates indicate that a-Si:H based solar cells now account for the majority of sales. Although much of this market is consumer electronics, other conventional power markets may be expected to grow as the performance, reliability and cost of the devices improve. Prospects in each of these areas appear excellent. Although we believe efficiencies of 14% are realistic for single junction devices, multijunction cells will have better efficiencies and stabilities. The high performance in these devices will require improvements in the properties of compatible wide and low bandgap materials. Two of the most promising candidates are a-SiGe and a-SiC:H alloys. In this paper we examine recent developments in these materials and devices.

This article reviews recent advances in the development of amorphous silicon solar cells. Both the glow-discharge deposition conditions and the solar-cell structures are discussed in some detail. The performance characteristics of present amorphous silicon cells are described, and the loss mechanisms that limit performance are considered. An effort has been made to point out those areas where further research is needed. Recently, amorphous silicon p-i-n cells with areas of 1.19 cm2 have been fabricated with conversion efficiencies as high as 6.1%.

Secondary ion mass spectrometry (SIMS) was used to characterize glow-discharge-deposited hydrogenated amorphous silicon (a-Si:H) as a function of depth. These depth profiles were used to determine hydrogen concentration as well as levels of impurities (carbon, oxygen and nitrogen) unintentionally grown into the films. Concentration profiles of dopants are also shown. Whenever possible, SIMS data are correlated with electrical measurements.

High performance amorphous silicon solar cells require high quality undoped hydrogenated amorphous silicon (diffusion lengths greater than or equal to 0.5 μm), a conductive p layer or n layer window ( σ p > 10 −6 ω −1 cm −1 , ϵ opt ⪸ 2.0 eV ), an effective light trapping geometry such as textured tin oxide, a reflective back contact (e.g. silver) and low contact resistance (less than 0.5 ω cm2). Requirements for high module performance require low interconnect resistance (e.g. less than 5 × 10−3ω cm2 for the AlSnO2 contact), large percentage of active area and good uniformity of material properties over large areas. New developments such as superlattice doped layers and improved tin oxide texturing have led to efficiencies as high as 10.9% in small cells (1 cm2). Processing improvements have led to efficiencies of 8.1% in 1 ft2 modules patterned entirely by laser scribing.

For the first time, structure has been found in the electron tunneling characteristics of a metal-oxide-semiconductor tunnel junction of an amorphous semiconductor. In the present study the amorphous semiconductor is phosporus-doped hydrogenated amorphous silicon. The results have been analyzed to determine the surface density of states over part of the forbidden gap. The basic features of the density-of-states distribution are in agreement with the results of field-effect measurements reported for this material.

The magnetic-field dependence of the penetration of radio-frequency fields has been measured in type-II superconductors. We discuss information obtained concerning the mixed state, the superconducting surface sheath, and superconducting cylindrical thin films. Some of the results, such as structure near ${H}_{c2}$, are interpreted in terms of vortex motion and effects of the sheath. Results pertaining to the sheath are compared with the calculations of Fink and Kessinger and of Maki. Other results in the mixed state are related to surface impedance calculations of Maki and of Caroli and Maki, based on the microscopic theory. Penetration depth measurements in the films agree better with a naive model than with Pincus's theory.

The efficiency of amorphous silicon (a-Si) solar cells may approach 18% by 1990. Large-area (about 1 ft 2 ) a-Si modules with efficiencies of about 5% – 6% may become available by the end of 1984. In the 1990s, a-Si photovoltaics may represent a billion (10 9 ) dollar business.

In Section II, Theoretical Modeling, theories for the capture of electrons by deep centers in hydrogenated amorphous silicon (a-Si:H) and for field-dependent quantum efficiency in a-Si:H are presented. In Section III, Deposition and Doping Studies, the optimization of phosphorus-doped a-Si:H carried out in four different discharge systems is described. Some details of the dc proximity and rf magnetron discharge systems are also provided. Preliminary mass spectroscopy studies of the rf magnetron discharge in both SiH/sub 4/ and SiF/sub 4/ are presented. In Section IV, Experimental Methods for Characterizing a-Si:H, recent work involving photoluminescence of fluorine-doped a-Si:H, photoconductivity spectra, the photoelectromagnetic effect, the photo-Hall effect and tunneling into a-Si:H is presented. Also, studies of the growth mechanism of Pt adsorbed on both crystalline Si and a-Si:H are described. Measurements of the surface photovoltage have been used to estimate the distribution of surface states of phosphorus-doped and undoped a-Si:H. Section V, Formation of Solar-Cell Structures, contains information on stacked or multiple-junction a-Si:H solar cells. In Section VI, Theoretical and Experimental Evaluation of Solar-Cell Parameters, an upper limit of approx. = 400 A is established for the hole diffusion length in undoped a-Si:H. A detailed description of carrier generation, recombination and transport inmore » a-Si:H solar cells is given. Finally, some characteristics of Pd-Schottky-barrier cells are described for different processing histories.« less