John D. Webb

On the role of Na and modifications to Cu(In,Ga)Se/sub 2/ absorber materials using thin-MF (M=Na, K, Cs) precursor layers [solar cells]

The growth and characterization of Cu(In,Ga)Se/sub 2/ polycrystalline thin film solar cells under the presence of thin-MF (M=Na, K, Cs) precursor layers is presented. Some electrical, structural and electronic absorber properties due to the presence of such Group Ia impurities are quantified along with their influence in device performance. The authors present a growth model for the role of Na in Cu(In,Ga)Se/sub 2/ that attributes the enhancements in electrical conductivity and photovoltaic device performance to the extinction of a finite number of donor states (i.e., In/sub Cu/) at the bulk and grain-boundary regions.

Si–H bonding in low hydrogen content amorphous silicon films as probed by infrared spectroscopy and x-ray diffraction

A systematic series of hydrogenated amorphous silicon (a-Si:H films) has been deposited by the hot wire chemical vapor deposition (HWCVD) technique onto crystalline silicon substrates, and the H bonding has been examined by infrared spectroscopy. All deposition parameters were kept the same, except that the substrate temperature (TS) was varied to affect changes in the film H content. Although the peak position of the Si–H stretch mode changes minimally with increasing substrate temperature, the stretch mode shape changes, becoming more intense (compared to the height of the wag mode) and considerably narrower. We show, through annealing experiments, that this narrow stretch mode may be a universal feature of low H content films, and suggest interpretations for this finite (narrow) linewidth. By correlations with x-ray diffraction data, we also show that the narrowing of the stretch mode peak for low H content HWCVD films is an indication of improved lattice ordering, and suggest that this improved orderi...

Prospects for in situ junction formation in CuInSe2 based solar cells

Abstract In this paper we describe our research efforts directed towards the understanding of the CdS/CuInGaSe 2 junctions and, specifically, the interaction of the chemical bath with the CuInGaSe 2 . Information gained from these studies has been used to develop a set of criteria for forming junctions without the need for chemical bath deposition or CdS. Our approach differs from many others previously used “alternative buffer layer” methods which appear to be somewhat problematic in implementation as well as in the quality of the results. This “buffer-free” technology has resulted in a 13.5% efficiency cell.

Fourier transform infrared quantitative analysis of sugars and lignin in pretreated softwood solid residues

Hydrolysates were obtained from dilute sulfuric acid pretreatment of whole-tree softwood forest thinnings and softwood sawdust. Mid-infrared (IR) spectra were obtained on sample sets of wet washed hydrolysates, and 45°C vacuum-dried washed hydrolysates, using a Fourier transform infrared (FTIR) spectrophotometrer equipped with a diamond-composite attenuated total reflectance (ATR) cell. Partial least squares (PLS) analyiss of spectra from each sample set was performed. Regression analyses for sugar components and lignin were generated using results obtained from standard wet chemical and high-performance liquid chromatography methods. The correlation coefficients of the predicted and measured values were >0.9. The root mean square standard error of the estimate for each component in the residues was generally within 2 wt% of the measured value except where reported in the tables. The PLS regression analysis of the wet washed solids was similar to the PLS regression analysis on the 45°C vacuum-dried sample set. The FTIR-ATR technique allows mid-IR spectra to be obtained in a few minutes from wet washed or dried washed pretreated biomass solids. The prediction of the solids composition of an unknown washed pretreated solid is very rapid once the PLS method has been calibrated with known standard solid residues.

Fourier Transform Infrared Quantification of Sugars in Pretreated Biomass Liquors

The process of converting renewable lignocellulosic biomass to ethanol requires a number of steps, and pretreatment is one of the most important. Pretreatment usually in volves a hydrolysis of the easily hydrolyzed hemicellulosic component of biomass using some form of thermal/chemical/mechanical action that results in a product that can be further hydrolyzed by cellulase enzymes (the cellulosic portion). The sugars produced can then befermented to ethanol by fermentative microorganisms. If the pretreatment step is not severe enough, the resultant residue is not as easily hydrolyzed by the cellulase enzyme. More severe pretreatment conditions result in the production of degradation products that are toxic to the fermentative microorgan ism. In this article, wereport the quantitative analysis of glucose, mannose, xylose, and acetic acid using Fourier transform infrared (FTIR) spectroscopy on liquors from dilute-acid-pretreated softwood and hard wood slurries. Comparison of FTIR and high-performance liquid chromatography quantitative analyses of these liquorsare reported. Recent developments in infrared probe technology has enabled the rapid quantification of these sugars by FTIR spectroscopy in the batch reactor during optimization of the pretreatment conditions, or interfaced to the computer controlling a continuous reactor for on-line monitoring and control.

Techniques for measuring the composition of hydrogenated amorphous silicon–germanium alloys

Abstract We grow hydrogenated amorphous silicon–germanium alloys by the hot-wire chemical vapor deposition (HWCVD) technique at deposition rates between 0.5 and 1.4 nm per second. We prepared a set of these alloys to determine the concentrations of the alloying elements as measured by various techniques. This set consists of samples throughout the range of germanium alloying from 0% (a-Si:H) to 100% (a-Ge:H). We find that by making the appropriate calibrations and corrections, our compositional measurements agree between the various techniques. Nuclear reaction analysis (NRA), Fourier transform infrared spectroscopy (FTIR)and secondary ion mass spectrometry (SIMS) all yield similar hydrogen contents, within ±20% for each sample. Electron probe micro-analysis (EPMA) and SIMS yield silicon and germanium contents within ±7% of each other with results being confirmed by Rutherford backscattering (RBS). EPMA oxygen measurements are affected by oxidized surface layers, thus these data show larger O concentrations than those measured by SIMS.

Advances in the CIS research at NREL

This paper summarizes the research of the CIS Team at NREL in three major areas: absorber deposition; understanding the role of chemical bath deposited (CBD) CdS in CIS junctions; and in the development of devices without CdS. Low cost, scaleable processes chosen for absorber fabrication include sputtering, electrodeposition (ED), and close spaced sublimation (CSS). The interaction between the CBD and the CIS has been investigated and the results show that Cd might be instrumental in shaping the interface. We have also developed a process to fabricate a 13.5% efficiency ZnO/CulnGaSe/sub 2/ device without CdS or other buffer layers.

Recombination lifetimes in undoped, low-band gap InAsyP1−y/InxGa1−xAs double heterostructures grown on InP substrates

High-quality, thin-film, lattice-matched (LM) InAsyP1−y/InxGa1−xAs double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap InxGa1−xAs layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In0.53Ga0.47As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAsyP1−y/InxGa1−xAs DH system attractive for applications in high-performance, infrared-sensitive devices.

Effect of nitrogen doping on microdefects and minority charge carrier lifetime of high-purity, dislocation-free and multicrystalline silicon

We studied the effects of Si growth in atmospheres containing N2 on minority charge carrier lifetime τ using a high-purity, induction-heated, float-zone (FZ) crystal growth method. Ingots were grown with purge gases that ranged from pure argon (99.9995%) to pure N2 (99.999%). τ was measured as a function of position along the ingots using the ASTM F28–75 photoconductive decay (PCD) method. We found that Ga-doped, multi-crystalline silicon ingot growth in a partial or total nitrogen ambient has a negligible effect on minority charge carrier lifetime and no significant grain boundary passivation effect. Values of 40 μms < τ < 100 μs were typical regardless of ambient. For dislocation-free (DF) growth, the degradation of τ is minimal and τ values above 1000 μs are obtained if the amount of N2 in the purge gas is below the level at which nitride compounds form in the melt and disrupt DF growth.

Effect of heat treatments and window layer processing on the characteristics of CuInGaSe/sub 2/ thin film solar cells

Interaction between chemical bath deposited CdS and ZnO window layers are a focus of this paper. Low temperature anneals were used to follow the changes at the interface. Optical absorption spectra show that CdS and ZnO intermix upon annealing. Heat treatments applied to ZnO/CdS/CuInGaSe/sub 2/ thin film solar cells produced changes in the short and long wavelength responses. The latter is attributed to an increase in the energy gap of the absorber by diffusion of S, and it is confirmed by SIMS. The interdiffusion is shown to increase the short wavelength collection, and hence the current density of the devices. Photoluminescence data provides some indication of the quality of the interface.