Cu(In, Ga)Se2 (CIGS) thin films were deposited on Molybdenum (Mo) coated soda lime glass (SLG/Mo) substrates, using physical vapor deposition (PVD) 3-stage process. The Mo thin films were sputtered on SLG substrates using DC planar magnetron sputtering at a working gas (Ar) pressure that varies from 0.8 mT to 12 mT with a sputtering power density of 1.2 W/cm2. The sputtering pressure of Mo thin films was varied in order to induce variations in the sputtered films' morphology and porosity; as well as to subsequently induce variations in the Na out-diffusion from SLG substrate. The surface chemistry of CIGS thin films was investigated by X-Ray Photoelectron Spectroscopy (XPS). The XPS surface surveys (top 30A) and depth profiling survey (top 100A) for the elements, their chemical states, and their relative concentration were analyzed for CIGS thin films. The XPS surface analysis and composition of CIGS thin films were correlated to the bulk composition and Na out-diffusion in the CIGS films from SLG substrates.

Since their discovery in 1993,1,2 carbon single-wall nanotubes (SWNTs) have been of great interest because of their expected novel electronic,3 mechanical,4 and gas adsorption properties.5 Difficulties arise in measuring these various properties because the SWNTs currently available for analyses have very small bundle sizes (1020 nm) and are typically in random orientations. A vital step for the industrial application of SWNTs is to align the individual tubes into bundles of a size that allows facile measurement of the various physical and chemical properties. This paper is the first to illustrate a set of procedures for the preparation and isolation of aligned SWNT “superbundles”. The synthesis of raw SWNT soot is similar to the pulsed laser vaporization technique described by Guo et al.6 A Nd:YAG laser (1064 nm) was employed to synthesize the carbon nanotubes from 1.2 atom % metaldoped (50:50 Co/Ni) pressed graphite targets.7 The targets were placed in a quartz tube that was heated to a temperature of 1200 °C in a clam-shell furnace. With the laser operating at a frequency of 10 Hz, the laser power and beam size were adjusted to provide ∼20 J/(pulse cm2) at a ∼450-ns pulse width. An argon flow of 100 sccm at 500 Torr was maintained through the reaction vessel for the duration of the synthesis. The raw soot was purified by refluxing in 3 M nitric acid for 16 h, filtering and washing with deionized water on a polytetraflouroethylene (PTFE) filter, and then heating the obtained paper in air for 30 min at 550 °C. This procedure results in tubes of greater than 98 wt % purity when target material is not sputtered and trapped in vaporized soot.7 A representative TEM image of the resultant pure tubes is shown in Figure 1a. The random orientation and small size of the long bundles are apparent. The SWNTs exist as a random tangle because of the conditions under which they are synthesized and purified. The tubes are formed within the high-temperature plasma generated by the laser striking the graphite target, and their formation is rapidly quenched as the tubes diffuse out of the plasma plume. The resulting SWNTs exist in small bundles and are accompanied by other graphitic and amorphous carbon fractions as well as metal nanoparticles. The purification process succeeds in removing the non-nanotube carbon fractions and the metal, but the bundles are still randomly oriented with diameters of only ∼5-20 nm. We have discovered that through the use of ultrasound and polar solvents. it is possible to unwind the intertwined SWNT bundles. Subsequent removal of the ultrasonic perturbation results in realignment and collapse of the SWNTs into the much larger superbundle configuration. A 1.0-mg sample of purified SWNTs was placed in a cylinder containing 10 mL of deionized water or other polar solvents or solvent mixtures. A Heat SystemsUltrasonics Inc. model w-220F Cell Disrupter sonic horn was submersed into the solution and the power was slowly increased to 90 W/cm2. The ultrasonic agitation was continued for a maximum of 120 min. Normally, * Author for correspondence. † On sabbatical from Chemistry Department, Rochester Institute of Technology, 85 Lomb Memorial Drive, Rochester, NY 14623-5604. (1) Bethune, D. S.; Kiang, C.-H.; Vries, M. S. d.; Gorman, G.; Savoy, R.; Vasquez, J.; Beyers, R. Nature 1993, 363, 605. (2) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (3) Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Phys. Rev. B 1992, 46, 1804. (4) Yakobson, B. I.; Brabec, C. J.; Bernhole, J. Phys. Rev. Lett. 1996, 384, 2511-2514. (5) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (6) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49. (7) Dillon, A. C.; Gennett, T.; Jones, K. M.; Alleman, J. L.; Parilla, P. A.; Heben, M. J. Adv. Mater. 1999, 11, 1354. Figure 1. TEM images of (a) purified single-wall nanotubes with small diameter bundles, (b) SWNT superbundles extracted from a 50:50 water/methanol solvent mixture, and (c) a SWNT superbundle at higher magnification. 599 Chem. Mater. 2000, 12, 599-601

Previous work on CdS–CuInSe2 (CIS) solar cells, which reported distortions of their current-voltage (J–V) curves under red illumination, is expanded in this work to include CdS–CuIn1−xGaxSe2 cells with variable Ga and CIS cells with variable CdS thickness. Different amounts of J–V distortion were observed in these cells under red light. The details are in good agreement with predictions of a photodiode model, in which a secondary barrier caused by the positive conduction-band discontinuity (spike) at the buffer–absorber interface is responsible for the current limitation. The illumination of the cell with high-energy photons lowers the barrier due to buffer photoconductivity, and thus removes the J–V distortion.

Spectroscopic ellipsometry measurements of CuInSe2 (CIS) and CuIn1−xGaxSe2 (CIGS) over a range of Cu compositions reveal that there are important differences in electronic and optical properties between α-phase CIS/CIGS and Cu-poor CIS/CIGS. We find a reduction in the imaginary part of the dielectric function ϵ2 in the spectral region, 1–3eV. This reduction can be explained in terms of the Cu-3d density of states. An increase in band gap is found for Cu-poor CIS and CIGS due to the reduction in repulsive interaction between Cu-3d and Se-4p states. We also characterize the dielectric functions of polycrystalline thin-film α-phase CuIn1−xGaxSe2 (x=0.18 and 0.36) to determine their optical properties and compare them with similar compositions of bulk polycrystalline CuIn1−xGaxSe2. The experimental results have important implications for understanding the functioning of polycrystalline optoelectronic devices.

Room temperature spectroscopic ellipsometry measurement of Cu0.86In1.09Se2.05 reveals that shallow defect states are found in the sub-band-gap region. The energies of these defect levels are in quantitative agreement with those calculated and measured by photoluminescence, electrical conductivity, optical absorption, and deep level transient spectroscopy at low temperatures. The results can be explained in terms of the defect physics of this material and suggest that the defect levels are due to Cu deficiency. This work opens up the possibility of measuring defect levels of off-stoichiometric or heavily doped semiconductors by spectroscopic ellipsometry at room temperature.

Photoelectron spectroscopy was used to determine the compositional and electronic changes occurring in Cu(In,Ga)Se2 thin films as a result of immersion in aqueous ammonia solution. We find that NH4OH-treated CIGS surfaces are preferentially etched of indium and gallium, resulting in the formation of a thin layer of a degenerate Cu-Se compound that we tentatively identify as Cu2Se. The work function of ammonia-treated samples is found to increase by 0.6 eV relative to as-grown CIGS thin films. The uniformity of chemical bath effects (etching & deposition) was found to be improved by the addition to the bath of a non-ionic surfactant. Initial device results show that the new surfactant-based chemical bath deposition (CBD) method may lead to better and thinner CdS buffer layers.

Optical bowing coefficients are used to describe the band gap variation of a composite semiconductor alloy. It is known to be related to the electronic structure and the lattice deformation in the semiconductor alloys. Spectroscopic ellipsometry study shows that the optical bowing coefficient of slightly Cu-poor polycrystalline Cu0.9In1−xGaxSe2 is larger than that of stoichiometric polycrystalline CuIn1−xGaxSe2 and band gaps are larger when Cu becomes poor. This can be explained by an increase in valence band offset due to reduced p-d coupling and an increase of perturbation potential ΔV due to lattice deformation.

We report the growth and characterization of record‐efficiency ZnO/CdS/CuInGaSe 2 thin‐film solar cells. Conversion efficiencies exceeding 19% have been achieved for the first time, and this result indicates that the 20% goal is within reach. Details of the experimental procedures are provided, and material and device characterization data are presented. Published in 2003 by John Wiley & Sons, Ltd.

Solar cells have been fabricated with partial electrolyte treatments of CuInGaSe2 (CIGS) thin film absorbers in lieu of a CdS layer. Treatment of the absorbers in a Cd or Zn containing solution is shown to produce conditions under which efficient solar cells can be fabricated. A similar effect is also observed in CuInGaSSe2 (CIGSS) graded band gap absorbers. These observations can be explained by the ability of Cd and Zn to produce n-type doping or inversion in the surface region. We also provide a brief review of similar work done elsewhere and identify directions for future investigations.

Cu(In,Ga)Se 2 (CIGS) solar cells have been designed for operation under mildly concentrated sunlight. The absorber was deposited via a three‐stage evaporation process that has consistently yielded high‐performance one‐sun devices. The device structure reported here was modified by reducing the thickness of the CdS window/buffer layer to enhance the short‐circuit current at the expense of the open‐circuit voltage. Operation of the devices under optical enhancement leads to significant increases in the voltage and fill factor. At 14 suns, the open‐circuit voltage for this device was 736 mV, the fill factor was 80.5%, and the efficiency was 21.5%. This result represents the first report of a polycrystalline thin‐film solar cell with an efficiency in excess of 20%. Published in 2002 by John Wiley & Sons, Ltd.

Molybdenum thin films were deposited on soda lime glass (SLG) substrates using direct-current planar magnetron sputtering with a sputtering power density of 1.2 W/cm(2). The working gas (Ar) pressure was varied from 0.6 to 16 mtorr to induce changes in the Mo films' morphology and microstructure. Thin films of Cu(In,Ga)Se-2 (CIGS) were deposited on the Mo-coated glass using the 3-stage co-evaporation process. The morphology of both the Mo-coated SLG and the CIGS thin films,grown on it was examined using high-resolution scanning electron microscopy. Na was depth profiled in the Mo and CIGS films by secondary ion mass spectrometry. The device performance was evaluated under standard conditions of 1000 W/m(2) and 25degreesC. Optimum device performance is found for an intermediate Mo sputtering pressure.

A chemical-bath treatment that does not form a CdS layer has been used on CIGS absorbers made at the National Renewable Energy Laboratory (NREL). The resultant cells have moderate to high efficiency, with improved current collection at shorter wavelengths. Room temperature quantum efficiency (QE) and capacitance-voltage (CV) results indicate that the different surface treatments yield electro-optical differences in the bulk of the absorber. Room temperature current density-voltage (JV) and AMPS modeling results are used to compare and contrast the results of the surface treatments, primarily from the NREL devices.

We study the influence of Cd partial baths on the photovoltaic properties of CuInGaSe2 (CIGS) and CuInGaSSe2 (CIGSS) thin film absorbers. We find that efficient solar cells can be fabricated by this treatment, and we compare their properties with those containing CdS window layers grown by chemical bath deposition. The results suggest that Cd plays a dominant role in establishing efficient photovoltaic junctions in CuInSe2 alloys. Micron scale photoluminescence scans show non-uniformity along the length probed. Cd treatment quenches one of the luminescence transitions, which indicates a change in shallow acceptor level density. We present a model that helps to explain the evolution of photovoltaic action.

II–VI and I–III–VI solar cells are promising for future thin‐film photovoltaics. In this paper, the roles of electron‐beam‐induced current (EBIC) and cathodoluminescence in evaluating the influence of interfaces on those solar cells are reviewed. CdTe and Cu(In,Ga)Se2 (CIGS) are the absorbers of the cells investigated. For CdTe/CdS solar cells, a detailed study has been conducted of the effects of grain boundaries and the Te/CdTe or ZnTe:Cu/CdTe interfaces for back‐contacting. For CIGS solar cells, we have investigated different buffer layer schemes, showing that these interfaces are critical in the definition of the mechanisms for carrier collection. Copyright © 2002 John Wiley & Sons, Ltd.

The built-in electrical potential on cross sections of Cu(In, Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. In the conditions of open and short circuits, no significant potential variation on the p-n junction was probed due to the surface Fermi-level pinning. With an external reverse-bias voltage applied to the device, we were able to probe the potential on the junction; the potential profiles demonstrate that the p-n junction is a buried homojunction, located 30-80 nm from the CIGS/CdS interface in the CIGS film. The potential measurement over the CdS and ZnO layers, which is consistent with the band diagram calculations, indicates that the CdS and ZnO layers are inactive for the collection of photoexcited carriers.

Cu(In,Ga)Se 2 (CIGS) thin films were deposited using the three-stage process. At the third stage, an amount of Indium was added to the CIGS that is greater than the standard used in processing high-efficient CIGS solar cells. The effects of Indium excess and substrate temperature were then investigated by electron-beam-induced-current (EBIC) and cathodoluminescence (CL). The addition of more indium compared to the standard noticeably affects the ZnO/CdS/CIGS heterojunction. On the other hand, the substrate temperature primarily affects the luminescence behavior of these films. It is suggested than In enrichment and Na incorporation play a main role in the electronic properties of the film. From these results, the efficiencies obtained for this set of CIGS cells are finally understood.

Graded-band-gap CuIn1−xGaxSe2 (CIGS) absorbers with Ga/Ga+In value in the 20%-30% range have a demonstrated efficiency of 18.8%. For CdS-containing devices, the shortcircuit current density (Jsc) has almost reached its expected maximum. However, the open-circuit voltage of CIGS solar cells is limited by the surface microstructure and chemistry. In this work, we examine the microstructural properties and chemistry of CIGS. We also attempted to correlate the above observations and device performance.

Thin-film polycrystalline photovoltaic devices based on Cu(In,Ga)Se2 have a demonstrated efficiency approaching 19%. The best performance was achieved when the Ga/In+Ga ratio was in the 25–30% range. The short-circuit current density exhibited for the device containing CdS was almost at its expected maximum. The open-circuit voltage was relatively low considering the optical bandgap (Eg) of the above absorber (∼1.15 eV); at best, it is 0.6×Eg. In this work, we examined the microstructural properties, e.g. defects due to misorientation, micro-twinning, stacking faults, and dislocations, for films prepared by our ‘ three-stage’ process, including the CIGS and Mo back-contact. We also attempted to make a correlation between the above observations and device performance.

Polycrystalline CdTe is a promising candidate for solar cells due to its nearly ideal band-gap, high absorption coefficient, and ease of film fabrication. Small-area CdTe/CdS cells with efficiencies of 16.0% have been demonstrated. The structure of a typical CdTe/CdS solar cell (Figure 1) consists of a glass superstrate, on which a thin layer of SnO2 is deposited (front contact), n-type CdS, p-type CdTe, and a back contact. Prior to applying the back contact to the CdTe, etching of the CdTe surface using a mixture of nitric and phosphoric (NP) acids is normally needed. It is known that the etching depletes a crystalline CdTe surface of Cd and creates a Te-rich layer. Two effects of the Te-rich layer has been proposed, namely, forming a Te-CdTe low-series-resistance contact and improving CdTe device stability by the gettering of Cu. Thus, the NP etching is an important process in the CdTe device fabrication. in this paper, we report on transmission electron microscopy (TEM) study of the microstructure of the surface of NP etched CdTe thin films.

Atomic force microscopy (AFM) can be used to image cross-sections of thin-film samples. So far, however, it has mainly been used to study cross-sections of epitaxial systems or integrated circuits on crystalline substrates. In this paper, we show that AFM is a powerful tool to image fractured cross-sections of polycrystalline thin films deposited on crystalline and non-crystalline substrates, yielding unique information on the three-dimensional properties of the cross-sections, with a spatial resolution in the nm range. Original images of three different heterostructure systems are presented: Si(wafer)/SnO2/CdS/CdTe, glass/Mo/Cu(In,Ga)Se2,/CdS/ZnO, and glass/SnO2/WO3. We discuss the results by comparing AFM and scanning electron microscopy (SEM) images, and explain, for the different materials, why the AFM provides useful additional information.