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Tandem modules combining a III–V top cell with a Si bottom cell offer the potential to increase the solar energy conversion efficiency of one-sun photovoltaic modules beyond 25%, while fully utilizing the global investment that has been made in Si photovoltaics manufacturing. At present, the cost of III–V cells is far too high for this approach to be competitive for one-sun terrestrial power applications. We investigated the system-level economic benefits of both GaAs/Si and InGaP/Si tandem modules in favorable future scenarios where the cost of III–V cells is substantially reduced, perhaps to less than the cost of Si cells. We found, somewhat unexpectedly, that these tandems can reduce installed system cost only when the area-related balance-of-system cost is high, such as for area-constrained residential rooftop systems in the USA. When area-related balance-of-system cost is lower, such as for utility-scale systems, the tandem module offers no benefit. This is because a system using tandem modules is more expensive than one using single-junction Si modules when III–V cells are expensive, and a system using tandem modules is more expensive than one using single-junction III–V modules when III–V cells are inexpensive. Copyright © 2016 John Wiley & Sons, Ltd.

As the world’s demand for energy grows, the search for cost competitive and earth abundant thin film photovoltaic absorbers is becoming increasingly important. A promising approach to tackle this challenge is through thin film photovoltaics made of elements that are abundant in the Earth’s crust. In this work, we focus on Cu2SnS3, a promising earth abundant absorber material. Recent publications have presented 3% and 6% device efficiencies using Cu2SnS3-based absorber materials and alloys, respectively. However, little is understood about the fundamental defect and doping physics of this material, which is needed for further improvements in device performance. Here, we identify the origins of the changes in doping in sputtered cubic Cu2SnS3 thin films using combinatorial experiments and first-principles theory. Experimentally, we find that the cubic Cu2SnS3 has a large phase width and that the electrical conductivity increases with increasing Cu and S content in the films, which cannot be fully explained ...

Thermochromic VO{sub 2} was directly synthesized using hydrothermal techniques. The effects of formation conditions on the structure and morphology of the final product were studied through X-ray diffraction (XRD), and scanning electron microscopy (SEM). Unique hollow sphere morphology was observed for the synthesized VO{sub 2} powders. Ex-situ XRD studies after heat treatment confirmed the thermal stability of the VO{sub 2} structure. Thermochromic properties, as a consequence of the reversible structural transformation between monoclinic VO{sub 2} and tetragonal phases, were observed by Fourier transform infrared spectroscopy (FTIR). - Graphical abstract: Thermochromic VO{sub 2} crystals with hollow spherical and asterisk shape were directly synthesized using hydrothermal techniques. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) studies confirmed the thermal stability and the reversible thermochromic properties of the VO{sub 2} structure. - Highlights: • One-step synthesis of thermochromic VO{sub 2} monoclinic phase, and VO{sub 2} (A and B phases) using hydrothermal technique. VO{sub 2} (A), VO{sub 2} (B). • Identification of the relationship between synthesis conditions and the morphology/structure of the final products. • Formation of VO{sub 2} monoclinic phase with an interesting hollow sphere shape. • Demonstration of superior thermal stability of the VO{sub 2} monoclinic phase. • Characterizing the thermochromicmore » properties of VO{sub 2} monoclinic phase.« less

Single-walled carbon nanotube (SWNT) radical anions will react with tetrahydrofuran and generate ethylene, enolates, and a partially hydrogenated nanotube backbone. The experimental evidence suggests that there are sp3 C-H binding interactions. The total gravimetric content of hydrogen on a sample averages from 3.5% to 3.9% w/w, about four times the total amount observed for nanotubes hydrogenated via traditional Birch reduction reactions. Furthermore, the hydrogen desorbs at temperatures up to 400 °C less than those observed for the hydrogenated SWNTs formed after the Birch reduction. Finally, the first room temperature electron spin resonance spectrum of a nanotube radical ion is also reported.

Atomic layer deposition (ALD) of Al2O3 is applied on a polypropylene separator for lithium-ion batteries. A thin Al2O3 layer (

Utilizing the concepts of a critical crystallite size and local film inhomogeneity, it is shown that nucleation in thermally annealed hydrogenated amorphous silicon occurs in the more well ordered spatial regions in the network, which are defined by the initial inhomogeneous H distributions in the as-grown films. Although the film H evolves very early during annealing, the local film order is largely retained in the still amorphous films even after the vast majority of the H is evolved, and the more well ordered regions which are the nucleation center sites for crystallization are those spatial regions which do not initially contain clustered H, as probed by H NMR spectroscopy. The sizes of these better ordered regions relative to a critical crystallite size determine the film incubation times (the time before the onset of crystallization). Changes in film short range order upon H evolution, and the presence of microvoid type structures in the as grown films play no role in the crystallization process. While the creation of dangling bonds upon H evolution may play a role in the actual phase transformation itself, the film defect densities measured just prior to the onset of crystallization exhibit no trends which can be correlated with the film incubation times.

The film structure and H bonding of high deposition rate a-SiGe:H i-layers, deposited by HWCVD and containing ~ 40 at.% Ge, have been investigated using deposition conditions which replicate those used in n–i–p solar cell devices. Increasing the germane source gas depletion in HWCVD causes not only a decrease in solar cell efficiency from 8.64% to less than 7.0%, but also an increase in both the i-layer H preferential attachment ratio (PA) and the film microstructure fraction (R⁎). Measurements of the XRD medium range order over a wide range of germane depletion indicate that this order is already optimum for the HWCVD i-layers, suggesting that energetic bombardment of a-SiGe:H films may not always be necessary to achieve well ordered films. Preliminary structural comparisons are also made between HWCVD and PECVD device layers.

Transparent conducting oxides (TCOs) are increasingly critical components in photovoltaic cells, low-e windows, flat panel displays, electrochromic devices, and flexible electronics. The conventional TCOs, such as Sn-doped In{sub 2}O{sub 3}, are crystalline single phase materials. Here, we report on In-Zn-O (IZO), a compositionally tunable amorphous TCO with some significantly improved properties. Compositionally graded thin film samples were deposited by co-sputtering from separate In{sub 2}O{sub 3} and ZnO targets onto glass substrates at 100 C. For the metals composition range of 55-84 cation% indium, the as-deposited IZO thin films are amorphous, smooth (R{sub RMS} 90%). Furthermore, the amorphous IZO thin films demonstrate remarkable functional and structural stability with respect to heating up to 600 C in either air or argon. Hence, though not completely understood at present, these amorphous materials constitute a new class of fundamentally interesting and technologically important high performance transparent conductors.

Fourier transform infrared (FTIR) spectroscopic imaging was used to study the initial diffusion of different solvents in cellulose acetate butyrate (CAB) films containing different amounts of acetyl and butyryl substituents. Different solvents and solvent/non-solvent mixtures were also studied. The FTIR imaging system allowed acquisition of sequential images of the CAB films as solvent penetration proceeded without disturbing the system. The interface between the non-swollen polymer and the initial swelling front could be identified using multivariate data analysis tools. For a series of ketone solvents the initial diffusion coefficients and diffusion rates could be quantified and were found to be related to the polar and hydrogen interaction parameters in the Hansen solubility parameters of the solvents. For the solvent/non-solvent system the initial diffusion rate decreased less than linearly with the weight-percent of non-solvent present in the solution, which probably was due to the swelling characteristic of the non-solvent. For a given solvent, increasing the butyryl content of the CAB increased the initial diffusion rate. Increasing the butyryl content from 17 wt.% butyryl to 37 wt.% butyryl produced a considerably larger increase in initial diffusion rate compared to an increase in butyryl content from 37 wt.% to 50 wt.% butyryl.

We optimize InAs y P 1− y buffer layers and compositional grades for lattice-mismatched heteroepitaxy of Ga x In 1− x As/InAs y P 1− y double-heterostructures on InP. The strains of the active and buffer layers depend on the bulk misfit difference between these layers. The misfit difference is adjusted to eliminate strain in the active layer, thus avoiding misfit dislocations and surface topography that would otherwise form to relieve strain. The optimized structure uses an “overshoot” with respect to the conventional design in the misfit and As composition of the InAs y P 1− y buffer. Nearly optimized heterostructures typically show excellent structural quality and extended minority-carrier lifetimes.

We study the optical, electrical and structural properties of a-SiGe:H thin films deposited by hot-wire chemical vapor deposition (HWCVD). The best films are deposited with a Ta filament heated to 1750 °C and the substrate temperature held at 200 °C. For the same precursor gas ratio of GeH 4 to SiH 4 , a lower filament temperature produces films with higher Ge incorporation than with a higher filament temperature. For Tauc band gaps between 1.22 and 1.30 eV, the photoconductivity and photosensitivity are significantly higher than for plasma-enhanced CVD a-SiGe:H films exhibiting the same band gap. The filament lifetime is much longer for a Ta filament than for a W filament at these operating temperatures.

Polycrystalline zinc oxide (ZnO) and nitrogen-doped zinc oxide (ZnO:N) films, about 1 μm thick, were grown by metalorganic chemical vapor deposition on crystalline silicon substrates. Infrared absorption measurements reveal a complex growth chemistry resulting in the presence of carbon, oxygen, and nitrogen-related functional groups not seen in single-crystal material. Noteworthy changes in the absorbance spectra that occur with the incorporation of nitrogen include a group of strongly absorbing bands around 1800 cm −1 and a band at 3020 cm −1 attributable to a N–H bond. These atomic configurations, in addition to the observed O–H bands around 3400 cm −1 , provide insight into the difficulties of creating p-type ZnO through the incorporation of nitrogen.

Indium–zinc-oxide (IZO) compositional libraries were deposited with dc magnetron sputtering onto glass substrates at 100 °C and analysed with high throughput, combinatorial techniques. The composition range from 4 to 95 at% In for Zn was explored. A peak in conductivity with σ > 3000 (Ω cm)−1 was observed at an indium content of ~70%. The mobility exceeded 30 cm2 (V s)−1 and the carrier concentrations were greater than 8 × 1020 cm−3. Crystalline phases were observed for In concentrations less than 45% and greater than 80% with an intermediate amorphous region. The low indium content films have a zinc oxide type structure with a ZnO (002) spacing ranging from ~2.61 to 2.85 A for 4% In and 45% In, respectively. For indium contents between 82% and 95%, the In2O3 (222) spacing varied from 2.98 to 2.99 A. Regardless of the composition or the degree of crystallinity, all films showed high optical transparency with the transmission >80% across the visible spectrum.

We examine different types of order measured in hydrogenated amorphous silicon (a-Si:H) and their effect on the optical absorption (band gap). We first review previous experimental work determining order on a short-range scale as probed by Raman spectroscopy, and provide, using molecular dynamics simulations, a theoretical explanation for why the band gap increases when this type of ordering is improved. We then present results on a-Si:H films deposited by hot wire chemical vapor deposition (CVD) and plasma enhanced CVD where the short-range order, from Raman spectroscopy, does not change, but order on a larger or medium-range scale does. This order is determined by measuring the width of the first x-ray diffraction peak, and was varied by depositing films at different substrate temperatures and∕or different hydrogen dilutions. We find that the film band gap also increases when this type of ordering improves, and we provide a possible mechanism to explain these trends. We also suggest that much of the pre...

High-quality amorphous silicon nitrides were deposited by hot-wire chemical vapor deposition using SiH4, NH3, and H2 gases. These films show a high deposition rate of 5A∕s, a low processing temperature of 300°C, an excellent conformal coverage, a low etching rate of 7A∕min, an index of refraction of 2.1, an optical band gap of 4.0eV, and a high breakdown field of 3MV∕cm. The effects of hydrogen dilution, substrate temperature, chamber pressure, and filament temperature on silicon nitride film property were studied to optimize the process. We found that adding H2 to the processing significantly enhances the silicon nitride films’ properties. The N content in the film increased significantly based on the infrared measurement. Hydrogen dilution is believed to play a key role for the conformal silicon nitride film. Hydrogen dilution also improves the process in that the gas ratio of NH3∕SiH4 has been greatly reduced with the assistance of the H2 gas. With substrate temperatures varying from 23°to400°C, this s...

Low-bandgap, lattice-mismatched GaxIn1−xAs (GaInAs) grown using InAsyP1−y (InAsP) compositional-step grades on InP is a primary choice for lightabsorbing, active layers in high-efficiency thermophotovoltaic (TPV) devices. The GaInAs/InAsP double heterostructures (DHs) show exceptional minority carrier lifetimes of up to several microseconds. We have performed a characterization survey of 0.4–0.6-eV GaInAs/InAsP DHs using a variety of techniques, including transmission electron microscopy (TEM). Dislocations are rarely observed to thread into the GaInAs active layers from the InAsP buffer layers that terminate the graded regions. Nearly complete strain relaxation occurs in buried regions of the InAsP grades. The buffer-layer strain prior to deposition of the active layer is virtually independent of the net misfit. Foreknowledge of this buffer-layer strain is essential to correctly lattice match the buffer to the GaInAs active layer.

We have developed, and are using, optical reflection and transmission mapping as a characterization tool for analyzing compositionally graded thin film combinatorial libraries. Measurements cover the spectral range of 200 nm to 25 μm. For the UV-Vis–NIR region, a multichannel fiber optically coupled CCD array-based spectrometer is used for simultaneous reflection and transmission mapping. For the IR, a Fourier transform infrared (FTIR) spectrometer is used for sequential reflection and transmission maps. Depending upon the type of library being analyzed, the measured spectra can, with appropriate modeling, be analyzed for the optical band gap, film thickness, index of refraction, plasma frequency, conductivity, carrier scattering time and color, in addition to simple reflectance and transmittance. We discuss these techniques using examples taken from our work on both transparent conducting oxides and metal nitrides for optical coatings.

Compositionally graded, combinatorial libraries of indium-doped zinc oxide (IZO) were grown by DC-magnetron sputtering on glass at 100°C. The In content ranged from 5 to 50 cation-%. The highest conductivity, σ=2 000 Ω -1 .cm -1 with μ = 25 cm 2 . V -1 . s -1 , was found at 50% in content with a visible transmittance greater than 80%. The (001) layer spacing increased from 2.6 to 2.85 A as the In content increased from 5 to 50%.

The structure of thin film SiN, deposited by the hot wire chemical vapor deposition (HWCVD) technique using SiH4 and NH3 gas mixtures, is examined as H2 dilution is added to the gas flow mixture. For NH3/SiH4 gas flow ratios greater than 1/2, all films are a-SiN:H for H2/SiH4 gas dilution ratios as high as 20/1. While H2 dilution does not change the basic film structure, it does increase the efficiency of NH3 dissociation in the gas phase, and causes a further reduction in the already small amount of N-H bonding in a-SiN:H films deposited by HWCVD. Differences in local N bonding sites are observed when the nitrogen source gas is changed from NH3 to N2. For NH3/SiH4 gas ratios less than 1/2 and with high H2 dilution, deposition of μc-SiN by HWCVD is demonstrated. X-ray diffraction measurements show that the structure of these films consists of silicon crystallites embedded in an a-SiN:H matrix. An upper limit for N incorporation with the preservation of microcrystallinity is found, beyond which the films again become amorphous. The existence of this limit is explained in terms of structural disorder in the a-SiN:H tissue brought about by N incorporation.

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased...

We have improved the electronic properties of narrow-bandgap (Tauc gap below 1.5 eV) amorphous-silicon germanium alloys (a-SiGe:H) grown by hot-wire chemical vapor deposition (HWCVD) by lowering the substrate temperature and deposition rate. Prior to this work, we were unable to grow a-SiGe:H alloys with bandgaps below 1.5 eV that had photo-to-dark conductivity ratios comparable with our plasma-enhanced CVD (PECVD) grown materials [B.P. Nelson et al., Mater. Res. Soc. Symp. 507 (1998) 447]. Decreasing the filament diameter from our standard configuration of 0.5 mm to 0.38 or 0.25 mm provides first big improvements in the photoresponse of these alloys. Lowering the substrate temperature from our previous optimal temperatures ( T sub starting at 435 °C) to at 250 °C provides additional photo-to-dark conductivity ratio increasing by two orders of magnitude for growth conditions containing 20–30% GeH 4 in the gas phase (relative to the total GeH 4 +SiH 4 flow).

We measured the infrared reflectance of thin films of degenerate n-type InxGa1−xAs and n-InAsyP1−y as a function of doping for compositions that correspond to x=0.53, 0.66, and 0.78 (band gaps of 0.74, 0.60, and 0.50 eV, respectively) and y=0.00, 0.31, 0.52, and 0.71 (band gaps of 1.34, 1.00, 0.75, and 0.58 eV, respectively). We then used the Drude theory and Hall measurements to determine the effective electron mass for these samples, and checked the results using Raman spectroscopy. The effective electron mass for these compositions increases abruptly as a function of free-electron density and converges at 5×1019 electrons/cm3. Consequently, it is difficult to attain plasma edges at wavelengths shorter than 5 μm using these materials, and the plasma edge is nearly independent of composition at large electron density levels. Results from similar studies on InP, InAs, and GaAs have been compiled and compared with our data. It is clear that the Kane band model offers an accurate description of the conducti...

In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

We present a surface passivation study of a B-diffused emitter and P-diffused back-surface field (BSF) of n-CZ Si substrates. The optimized passivation layers are subsequently incorporated into a 20%-efficient passivated emitter, rear totally-diffused (PERT) cell with V oc of 672 mV. On the P-diffused, concentrated KOH-planarized BSF side, we compare different passivating plasma enhanced chemical vapor deposition (PECVD) SiN x layer compositions. We demonstrate that a favorable combination of best passivation quality is achieved by a stack of thermal oxide grown at ∼700°C, followed by a bilayer SiN x , consisting of stoichiometric PECVD nitride and capped with Si-rich nitride, or H-dilution nitride. The stack results in surface passivation quality of J o ∼ 17 fA/cm2 for bilayer SiN x and 14 fA/cm2 for H-SiN x on lightly P-doped BSF, and is very resistive to HF-containing wet etches. Surface preparation, deposition parameters, and post-growth annealing collaborate to define the effectiveness of the passivation. Their optimization is critical for integration of SiN x :H into our high-efficiency solar cells. On the B-diffused textured emitter side, we use atomic layer deposition (ALD)-deposited Al 2 O 3 for surface passivation and low-temperature stoichiometric PECVD SiN x for the anti-reflection coating. We discuss deposition conditions and thermal treatments for both ALD Al 2 O 3 and PECVD nitride that result in the optimized passivation resulting in J o ∼ 52 fA/cm2 and that prevent the blistering of the film.

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...

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 degrees C vacuum-dried washed hydrolysates, using a Fourier transform infrared (FTIR) spectrophotometer equipped with a diamond-composite attenuated total reflectance (ATR) cell. Partial least squares (PLS) analysis 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 were0.9. 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 degrees 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.

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.

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 ordering might also exist for other types of low H content a-Si:H films as well. However, for the as-grown HWCVD films the narrowing of the stretch mode peak width at lower H contents does not completely compensate for the increase in peak height, and as a result the integrated intensity of the peak mode (relative to that of the wag mode) increases. We comment on the differences between as-grown, low H content a-Si:H HWCVD films and high H content films annealed to reduce the film H content to comparable levels, and discuss possible reasons for these intensity changes versus sample H content.

We have been successful in adapting Fourier transform (FT) Raman accessories and spectrophotometers for sensitive measurements of the photoluminescence (PL) spectra of photovoltaic materials and devices. In many cases, the sensitivity of the FT technique allows rapid room-temperature measurements of weak luminescence spectra that cannot be observed using dispersive PL spectrophotometers. We present here the results of a number of studies of material and device quality obtained using FT-luminescence spectroscopy, including insights into bandgap variations, defect and impurity effects, and relative recombination rates. We also describe our approach to extending the range of the FT-Raman spectrophotometer to cover the region from 11,500 to 3700 cm-1, enabling FT-luminescence measurements to be made from 1.42 to 0.46 eV, and our investigation of FT-PL microspectroscopy.

Cu(In,Ga)Se 2 (CIGS) solar cell junctions prepared by chemical-bath-deposited (CBD) ZnS(O,OH), Zn–Cd–S(O,OH), and CdS buffer layers are discussed in this paper. The device performances are compared by applying CBD ZnS(O,OH), CBD Zn–Cd–S(O,OH), and CBD CdS buffer layers on similar CIGS absorbers. The C N impurities in CBD ZnS(O,OH) are identified with Fourier transform infrared spectroscopy (FTIR) techniques. The impurities containing carbon-nitrogen bonds are most likely cyanamide (NCN 2− ) or thiocyanate (SCN − ), which resulted from the chemical reaction of thiourea and ammonia.

We have measured and compared surface roughness and the degree of damage for wafers cut by three different sawing techniques - slurry, Ni-based diamond wire, and resin-based diamond wire sawing. The local damage was determined by angle polishing followed by defect etching, TEM, SEM/EBSD imaging and Raman imaging. It showed that each of the cutting processes produces a thin layer of amorphous Si at the surface and dislocation loops that can go about 1 μm deep below the surface. A new approach was used to quantify the average damage over a large area. We determined the effective surface recombination (SRV) as a function of depth. Because the effective SRV is a function of the carrier loss close to the surface, it is well-suited to define damage distribution at and below the surface. Wafers with surface damage were step etched in (HF:HNO3:CH3COOH::1:1:5), and the effective lifetime was measured with a Sinton system after each etching step, with iodine-ethanol passivation. The SRV plots as a function of depth, representing depth distribution of the damage, were compared for large groups of wafers cut by each technique. Our results show that for optimized cutting, all three cutting methods produce damage depth of about 5μm (each surface). However, the degree of damage is higher for slurry cut wafers.

Highly conductive and transparent Mo-doped indium oxide (IMO) thin films were grown on glass and (100) yttria-stabilized zirconia (YSZ) single-crystal substrates by pulsed laser deposition. The electrical, optical, and structural properties were measured for films grown from 0, 1, 2, and 4 wt % Mo-doped targets. Films grown from the 2 wt % Mo-doped target had the best overall properties. In particular, for biaxially textured 2 wt % Mo IMO films grown on (100) YSZ, the conductivity was ∼3000 S cm−1 with a mobility greater than 95 cm2 V−1 s−1. In the visible, the optical transmittance normalized to the substrate was greater than 90%.

We have studied silicon nitride thin films deposited by hot-wire chemical vapor deposition as a function of the substrate temperature and hydrogen dilution. We found that adding H2 to the process significantly enhances silicon nitride film deposition. High-quality films can be grown at low substrate temperatures (

The researchers extensively studied the effects of annealing or thermal history of cell process on the minority carrier lifetimes of FZ n-type c-Si wafers with various i-layer thicknesses from 5 to 60 nm, substrate temperatures from 100 to 350°C, doped layers both p- and n-types, and transparent conducting oxide (TCO). Hot-Wire Chemical Vapor Deposition (HW-CVD) was used to achieve high lifetime, high open circuit voltage (V oc ), and high efficiency in crystalline silicon (c-Si) heterojunction (HJ) solar cells. The minority carrier lifetime with i-layer passivation in as-grown state was found to peak at 200°C substrate temperature. Annealing c-Si with as-grown layers affects the lifetime significantly. The optimized annealing temperature is from 250–350°C. It was also found that the lifetime of c-Si wafers with a very thin i/p passivation decreases significantly when annealed at temperatures higher than 250°C. However, the lifetime of the i/p passivated c-Si wafers is not affected by the p-layer even when the i-layer is as thin as 10 nm. Fourier Transform Infrared Spectroscopy (FTIR) was used to understand the annealing effect. For the c-Si wafers with i/n passivation, the minority carrier lifetime is usually longer than 2 ms and slightly improved by annealing. Minority carrier lifetime greater than 1 ms in a double side HJ structure with i/n and i/p layers can be achieved by controlling thermal history of the cell process. HJ cells were fabricated with an efficiency >18% on n-type wafers without texturing, and an efficiency of 19.2% with texturing.

Background Recently developed iron cocatalyst enhancement of dilute acid pretreatment of biomass is a promising approach for enhancing sugar release from recalcitrant lignocellulosic biomass. However, very little is known about the underlying mechanisms of this enhancement. In the current study, our aim was to identify several essential factors that contribute to ferrous ion-enhanced efficiency during dilute acid pretreatment of biomass and to initiate the investigation of the mechanisms that result in this enhancement. Results During dilute acid and ferrous ion cocatalyst pretreatments, we observed concomitant increases in solubilized sugars in the hydrolysate and reducing sugars in the (insoluble) biomass residues. We also observed enhancements in sugar release during subsequent enzymatic saccharification of iron cocatalyst-pretreated biomass. Fourier transform Raman spectroscopy showed that major peaks representing the C-O-C and C-H bonds in cellulose are significantly attenuated by iron cocatalyst pretreatment. Imaging using Prussian blue staining indicated that Fe2+ ions associate with both cellulose/xylan and lignin in untreated as well as dilute acid/Fe2+ ion-pretreated corn stover samples. Analyses by scanning electron microscopy and transmission electron microscopy revealed structural details of biomass after dilute acid/Fe2+ ion pretreatment, in which delamination and fibrillation of the cell wall were observed. Conclusions By using this multimodal approach, we have revealed that (1) acid-ferrous ion-assisted pretreatment increases solubilization and enzymatic digestion of both cellulose and xylan to monomers and (2) this pretreatment likely targets multiple chemistries in plant cell wall polymer networks, including those represented by the C-O-C and C-H bonds in cellulose.

Deep oxygen related defects form in hydrogenated nanocrystalline silicon (nc-Si:H) as a consequence of thermal annealing, but their microscopic origins and formation mechanisms are not well understood. To gain insight to this behavior we intentionally drive-out hydrogen from nc-Si:H films by thermal annealing and monitor accompanying changes in the electronic and vibrational structure of the films with photoluminescence (PL) and Fourier transform infrared (FTIR) absorption spectroscopy. Hydrogen effusion (HE) data provide additional insight, because the annealing temperature range shown to induce a defect band, centered at ∼ 0.7 eV in PL studies, and that corresponding to the onset of thermally activated hydrogen desorption from grain boundaries, coincide. This coincidence suggests a probable link between the two processes. The activation energy obtained from correlated annealing-PL experiments, of ∼ 0.6 eV, for defect formation with thermal exposure, provides substantial insight regarding the mechanism.

Photoluminescence (PL) spectra obtained with correlated set of experiments investigating grain boundary characteristics and diffusion processes in nanocrystalline silicon alloys (nc-Si:H), provide insight regarding formation and passivation of electronic defects in these regions. Based upon current results and previous works we believe thermally driven processes induce a PL band centered at 0.7 eV upon thermal annealing, and most likely involve diffusion of hydrogen and oxygen near interfaces. A nc-Si:H sample set with varied crystal volume fraction, Xc, was subject to thermal annealing treatments at different temperatures – each exceeding the deposition temperature. Fourier-transform photoluminescence (FTPL) and Fourier-transform infrared absorption spectroscopy (FTIR), were employed to correlate the relative 0.7 eV defect band emergence with compositional changes indicative of Si–Hx and Si–O species, for each sample, at each temperature, respectively. Hydrogen effusion data provide additional perspective.We find the Xc to strongly affect susceptibility of nc-Si:H to oxygen related effects. The higher the Xc, the more readily oxygen penetrates the nc-Si:H network. We attribute this relationship to elevated diffusivity of oxygen in highly crystalline nc-Si:H materials, owing to their abundance of gain boundaries and interfaces, which serve as pathways for impurity migration. These findings corroborate the expectation that oxygen impurities and diffusion processes contribute to development of microstructural features giving rise to radiative recombination through deep defects in nc-Si:H.

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.

The alloy GaInAsN has great potential as a lower-band-gap material lattice matched to GaAs, but there is little understanding of what causes its poor optoelectronic properties and why these improve with annealing. This study provides information about the structural changes that occur when GaInAsN is annealed. The Fourier transform infrared spectra exhibit two primary features: a triplet at ∼470 cm−1 (Ga–N stretch) and two or three bands at ∼3100 cm−1 (N–H stretch). The change in the Ga–N stretch absorption can be explained if the nitrogen environment is converted from NGa4 to NInGa3 after annealing. The N–H stretch is also changed after annealing, implying a second, and unrelated, structural change.

Concentrating photovoltaic (CPV) technology has recently gained interest based on their expected low levelized cost of electricity, high efficiency, and scalability. Many CPV systems use Fresnel lenses made of poly(methyl methacrylate) (PMMA) to obtain a high optical flux density. The optical and mechanical durability of such components, however, are not well established relative to the desired service life of 30 years. Specific reliability issues may include: reduced optical transmittance, discoloration, hazing, surface erosion, embrittlement, crack growth, physical aging, shape setting (warpage), and soiling. The initial results for contemporary lens- and material-specimens aged cumulatively to 6 months are presented. The study here uses an environmental chamber equipped with a xenon-arc lamp to age specimens at least 8x the nominal field rate. A broad range in the affected characteristics (including optical transmittance, yellowness index, mass loss, and contact angle) has been observed to date, depending on the formulation of PMMA used. The most affected specimens are further examined in terms of their visual appearance, surface roughness (examined via atomic force microscopy), and molecular structure (via Fourier transform infrared spectroscopy).

Concentrator solar cells are almost always characterized near room temperature (RT) conditions, but are operated at system-dependent elevated temperatures in the field. Cell designs that are optimized for RT can quickly lose their advantage at higher temperatures. Cell designers have traditionally considered thermal management to be the problem of system designers, but some initial consideration of these issues at the cell level can reduce the thermal resistances and heat-load that must be removed by system level active or passive cooling. We consider the location of heat generation within multijunction solar cells and its flow through thermal resistances caused by the substrates and mixed III–V alloy layers. We show advantages of inverted designs by rejection of much of the unused light below 1 eV, which makes up about 16% of the power flux. A 40% efficient IMM solar cell with a 1 eV bottom cell will therefore have about a 20% lower heat flux than a comparable 40% efficient Ge-based solar cell.

The infrared (IR) portion of the solar spectrum with photon energies below the lowest bandgap in III–V multijunction photovoltaic devices does not contribute to electrical power and is typically ignored during solar cell design. In a three junction inverted metamorphic multijunction (IMM) solar cell with a 1.0 eV bottom junction, this infrared light makes up 16% of the total power and can be extracted from the cell by reflective or transparent designs. We demonstrate several IMM designs that are about 39% efficient under concentration, allow this IR power to be used elsewhere, and reduce the heat load by about 20% relative to Ge‐based solar cells that are also 40% efficient.

We report the coupled optical and electrical properties of a-IZO films as a function of both the metals composition and the %O 2 in the Ar/O 2 sputter gas mix. In particular, a-IZO films with conductivity δ ≫ 2000 S/cm can be grown for a broad range of metals compositions (60 to 85 at.% In) as long as the corresponding optimal oxygen level is used in the deposition. The amount of oxygen required increases with increasing indium content. When too much oxygen is used, the a-IZO remains clear but the conductivity is reduced due to a decreased carrier concentration. Whereas, when too little oxygen is used, the conductivity is reduced due to decreased electron mobility and there is a concurrent increase in the optical absorption from 400 to 1000 nm which renders the oxygen deficient a-IZO samples gray.

Two types of commercial electrically conductive adhesive (CA) tapes, one isotropic and the other anisotropic, were investigated for their potential application as interleafing connects for flexible thin-film photovoltaic modules. The performance stability of their vacuum-laminated sample constructs between two 50-μm stainless steel (SS) foils and between the SS foil and bilayer ZnO/Mo-coated SS foil was evaluated upon damp-heat and dry-heat exposure. Preliminary results indicated that the isotropic CA tape was more stable than the anisotropic tape.

Amorphous mixed metal oxide TCOs are of increasing interest due to the excellent opto-electronic properties and smoothness (R RMS ≪ 0.5 nm) obtained for sputtered films deposited at less than 100 °C. In particular, for amorphous In-Zn-O (a-IZO) films grown from a ceramic target with 10 wt. % ZnO in In 2 O 3 , the current industry standard, conductivities σ ≥ 2500 S/cm are common. Here, we have investigated the combined materials phase space of oxygen stoichiometry and metals composition (In:Zn ratio) and made two key discoveries. First, that high conductivity a-IZO thin films can be made with substantially less indium provided that a corresponding change is also made in the oxygen content. And second, that for all compositions of a-IZO, the electron mobility (μ) and carrier concentration (N) fall on a single common curve when plotted as μ vs N.

We have employed a combinatorial high-throughput approach to explore transparent conducting oxide (TCO) materials deposited at TS ⩽ 100 °C. For In-Zn-O (IZO) thin films deposited by sputtering from ceramic metal oxide targets at 100 °C in argon, a broad maximum in the as-deposited conductivity is found for x ~ 0.55 to 0.85 in InxZn1-xOy with σmax ≈ 3000 Ω-1-cm-1 for x ≈ 0.8. This conductivity maximum correlates with the composition range found to be amorphous. These amorphous In-Zn-O (a-IZO) films are smooth, with an RMS roughness of ~ 0.3 nm. Post-deposition annealing experiments show that the a-IZO materials remain amorphous after 1 hour anneals at up to 600 °C in either air or argon.

We report the deposition of improved hydrogenated amorphous silicon germanium (a-SiGe:H) films by the hot wire CVD (HWCVD) technique using a tantalum filament operating at a low temperature. We gauge the material quality of the a-SiGe:H films by comparing infrared, small angle X-ray scattering (SAXS), photocapacitance and conductivity measurements to earlier results, and fabricate single junction n-i-p solar cell devices using these i-layers.

Transparent conducting oxides (TCOs) can serve a variety of important functions in thin film photovoltaics such as transparent electrical contacts, antireflection coatings and chemical barriers. Two areas of particular interest are TCOs that can be deposited at low temperatures and TCOs with high carrier mobilities. We have employed combinatorial high-throughput approaches to investigate both these areas. Conductivities of /spl sigma/ = 2500 /spl Omega//sup -1/-cm/sup -1/ have been obtained for In-Zn-O (IZO) films deposited at 100/spl deg/C and /spl sigma/ > 5000 /spl Omega//sup -1/-cm/sup -1/ for In-Ti-O (ITiO) and In-Mo-O (IMO) films deposited at 550/spl deg/C. The highest mobility obtained was 83 cm/sup 2//V-sec for ITiO deposited at 550/spl deg/C.

Using infrared (IR) spectroscopy and x-ray diffraction, the nature of the grain boundaries in twom c-Si films deposited by hot wire CVD, displaying similar crystalline volume fractionsbut very different behavior upon exposure to atmospheric contaminants, is analyzed. For the film exhibiting significant post deposition oxidation, the IR spectrum in the 2100 cm−1 Si-H stretch mode region contains two sharp and very narrow peaks, suggesting that the crystallites have been incorporated into them c-Si films with their hydrogenated surfaces relatively intact.By comparing these peak frequencies to those in the literature for Si-H bonding on c-Si surfaces, we identify certain crystallite orientations which, when comprising the c-Si grain boundaries, are particularly susceptible to oxidation. We further suggest that the distribution of H in this grain boundary/crystallite surface region is crucial for depositing c-Si films with good electronic properties and minimal post deposition oxidation.