In this work we present a correlation between the structural and electro-optical properties of grain boundaries in CdTe thin films deposited by vapor transport technique. We were able to identify different types of grain boundaries using electron backscatter diffraction (EBSD), and investigated their recombination properties by cathodoluminescence (CL). The objective is to investigate the existence of “good” and “bad” boundaries in CdTe thin films, which will provide guidance for the growth of better films in the future. The crystallographic orientation, grain size, and relative fraction of different boundaries were determined by EBSD. For the comparison study, the grain boundaries were colored according to their character, and compared to the CL spectra. By applying focused ion beam (FIB) marks, we were able to analyze CL and EBSD maps taken at exactly the same areas. We present a correlation between the types of boundaries with recombination.

A deposition process has been developed which allows the growth of large grain (20+ μm) polysilicon films on SiO2 substrates at a growth temperature of 650° C. A thin layer of liquid Si-metal solution is formed on the substrate surface as the growth medium. This layer is kept saturated by Si flux from a DC magnetron sputter gun. XRD analysis of the deposited films show a strong (111) preferred orientation, with increasing integrated peak intensities with increasing depositon temperature and solution layer thickness. Films deposited using an In-Si solution are p-type, with carrier concentrations in the mid 1016 cm−3 range. Conductivities of ∼.2 (Ω cm)−1 were measured, with activation energies for both carrier generation and conductivity of about 135meV. The hole mobility was found to be ∼ 30 cm2 V−1s−1. A wetting layer is used which may have a detrimental effect on the minority carrier lifetime.

Crystal silicon is an excellent photovoltaic (PV) semiconductor: silicon is abundant, environmentally benign, capable of high solar conversion efficiencies, and profits from an unparalleled scientific knowledge base. However, the energy-intensive, inefficient and expensive processes that turn sand into a crystal silicon (c-Si) wafer account for more than half of today s Si PV module costs. Because the peak flux of solar energy on Earth is only about 1 kW/m2, large areas must be covered with inexpensive PV to provide for TW-scale electrical power needs. To circumvent the costly wafer fabrication step, it would be ideal to grow 2-20 micron thick PV-quality silicon absorber layers directly from silane gas onto inexpensive substrates at temperatures below 800 C.

It is shown that GaP layers grown upon Si at a single temperature of 900ºC can have a crystalline quality superior to that exhibited by previous two—step and one—step growth methods. The layers are characterized by a planar network of misfit dislocations confined to the interface plane an a reduced density of threading dislocations (low 106 cm-2; previously >108). Very few threading defects were observed in areas devoid of amorphous oxide contamination, as shown by HREM examination of cross—sectional samples. A low growth rate during nucleation enhances crystalline perfection, since it decreases the tendency toward three—dimensional islanding.

The use of nanoparticle precursors for electronic materials including sulfides, selenides, oxides and the elements has potentially wide ranging implications for improving device properties and substantially reducing the deposition costs. To realize this goal the complex interfacial chemistry of these small particles must be controlled. In this paper we present a number of cases demonstrating the complexity of this chemistry. These include CuInSe2 where the kinetics of phase formation dominate the sintering process; CdTe where sintering proceeds with and without the sintering enhancement of CdCl2, but produces materials different electronically than bulk materials; and the use of compound and elemental nanoparticles (Ag, Al, Hg-Cu-Te and Sb-Te) for contacts to elemental and compound semiconductors (Si and CdTe).

We report on the first known growth of high-quality epitaxial Si via the hot wire chemical vapor deposition (HWCVD) method. This method yields epitaxial Si at the comparatively low temperatures of 195° to 450°C, and relatively high growth rates of 3 to 20 Å/sec. Layers up to 4500-Å thick have been grown. These epitaxial layers have been characterized by transmission electron microscopy (TEM), indicating large regions of nearly perfect atomic registration. Electron channeling patterns (ECPs) generated on a scanning electron microscope (SEM) have been used to characterize, as well as optimize the growth process. Electron beam induced current (EBIC) characterization has also been performed, indicating defect densities as low as 8×104/cm2. Secondary ion beam mass spectrometry (SIMS) data shows that these layers have reasonable impurity levels within the constraints of our current deposition system. Both n and p-type layers were grown, and p/n diodes have been fabricated.

In this contribution, the authors report on near-field scanning optical microscopy measurements of the luminescence emitted from localized junction breakdown in epitaxial silicon solar cells. Our measurements suggest that the observed local reduction in breakdown voltage results from avalanche multiplication assisted by the reinforcing combination of (i) the local enhancement of the electrostatic field at the apex of inverted pyramid pits and (ii) the participation of defect states in the avalanche breakdown. Transmission electron microscopy reveals the microstructure of the defect responsible for the local junction breakdown.

We report on the effects of replacing both In2O3:Sn (ITO) and the hole transport layer (HTL) in organic photovoltaic (OPV) cells with single-walled carbon nanotube (SWNT) network transparent electrodes. We have produced an OPV device without an HTL exhibiting an NREL-certified efficiency of 2.65% and a short-circuit current density of 11.2 mA/cm(2). Our results demonstrate that SWNT networks can be used to replace both ITO and the HTL in efficient OPV devices and that the HTL serves distinctly different roles in ITO- and SWNT-based devices. (C) 2010 American Institute of Physics. [doi:10.1063/1.3453445]

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

The material-quality limiting factors of evaporated solid-phase crystallized (SPC) poly-Si thin films fabricated on planar glass for photovoltaic applications are investigated by a study combining scanning electron microscopy and transmission electron microscopy. The grains in the investigated thin films are found to be randomly oriented, with an average grain size of ~2.1 μm. In general, the grains are found to have a high defect density, although some grains are more defective than others. We also observe a high level of impurity incorporation, in particular, oxygen, into the film. The optical activity of the Si films is dominated by deep band tail states. We conclude that the high intragrain defect densities and the high impurity levels are two major limiting factors for obtaining high-quality evaporated SPC poly-Si thin films for photovoltaics.

A photovoltaic conversion efficiency of 40.8% at 326 suns concentration is demonstrated in a monolithically grown, triple-junction III–V solar cell structure in which each active junction is composed of an alloy with a different lattice constant chosen to maximize the theoretical efficiency. The semiconductor structure was grown by organometallic vapor phase epitaxy in an inverted configuration with a 1.83 eV Ga.51In.49P top junction lattice-matched to the GaAs substrate, a metamorphic 1.34 eV In.04Ga.96As middle junction, and a metamorphic 0.89 eV In.37Ga.63As bottom junction. The two metamorphic junctions contained approximately 1×105 cm−2 and 2–3×106 cm−2 threading dislocations, respectively.

Hot-wire chemical vapor deposition (HWCVD) has been employed as an economically scalable method for the deposition of crystalline tungsten oxide nano-rods and nano-particles. Under optimal synthesis conditions, only crystalline WO3 nano-structures with a smallest dimension of similar to 10-50 nm are observed with extensive transmission electron microscopy (TEM) analyses. The incorporation of these particles into porous films led to profound advancement in state-of-the-art electrochromic (EC) technologies. HWCVD has also been employed to produce crystalline molybdenum oxide nano-rods, particles and tubes at high density. TEM analyses show that the smallest dimension of these nano-structures is similar to 5-30 nm. XRD and Raman analyses reveal that the materials are highly crystalline and consist of Mo, MoO2 and MoO3 phases. It is also possible to fabricate large-area porous films containing these MoOx, nano-structures. Furthermore, these films have been tested as the negative electrode in lithium-ion batteries, and a surprisingly high, reversible capacity has been observed. (c) 2007 Elsevier B.V. All tights reserved.

We demonstrate high efficiency performance in two ultra-thin, Ge-free III–V semiconductor triple-junction solar cell device designs grown in an inverted configuration. Low-stress metamorphic junctions were engineered to achieve excellent photovoltaic performance with less than 3 × 10 6 cm −2 threading dislocations. The first design with band gaps of 1.83/1.40/1.00 eV, containing a single metamorphic junction, achieved 33.8% and 39.2% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 131 suns, respectively. The second design with band gaps of 1.83/1.34/0.89 eV, containing two metamorphic junctions achieved 33.2% and 40.1% efficiencies under the standard one-sun global spectrum and concentrated direct spectrum at 143 suns, respectively.

The origin of intragrain defects in polycrystalline silicon films grown by ion-assisted deposition (IAD) on aluminum-induced crystallization seed layers on glass is investigated. The microstructure of these polycrystalline Si films is bimodal, with near defect-free regions of [001] orientation along the growth direction and highly defective regions containing smaller grains of [111] orientation. In the defective regions, the dominant structural defects are twins in the seed layer and stacking faults in the IAD-grown epitaxial layer, both lying on {111} planes. The stacking faults originate at the seed layer surface due to surface imperfections, indicating that the quality of the seed layer surface plays an important role for the quality of the epitaxial Si film. We find a clear correlation between the structural crystal quality and defect-related radiative transitions at sub-bandgap wavelengths. Two dominant defect levels (similar to 0.20 eV and similar to 0.29 eV below the conduction band edge) are observed and identified as impurity-related. Published by Elsevier B.V.

Both the origins of the high open circuit voltages (VOC) in amorphous silicon solar cells having p layers prepared with very high hydrogen dilution and the physical structure of these optimum p layers remain poorly understood topics, with several studies offering conflicting views. This work attempts to overcome the limitations of previous studies by combining insights available from electronic measurements, real time spectroscopic ellipsometry, atomic force microscopy, and both high-resolution transmission electron microscopy (TEM) and dark field TEM of cross sections of entire solar cells. It is found that solar cells fabricated with p layers having a low volume fraction of nanocrystals embedded in a protocrystalline Si:H matrix possess lower recombination at the i∕p interface than standard cells and deliver a higher VOC. The growth of the p layers follows a thickness evolution in which pure protocrystalline character is observed at the interface to the i layer. However, a low density of nanocrystallites nucleates with increasing thickness. The advantages offered by the protocrystalline character associated with the amorphous phase of the mixed-phase (amorphous+nanocrystalline) p layers prepared with excess H2 dilution account for the improved VOC of the optimum p layers. In this model, the appearance of a low volume fraction of nanocrystals near the top transparent conductor interface is proposed to be incidental to the high VOC.

The new organometallic C 60 compounds demonstrate sites with higher binding energies. Theory has been performed to confirm the most probable binding interaction for Fe(C 60 ). Good agreement with experiment has been achieved

Ar ion beam and electron beam-induced damages in Cu(In,Ga)Se, thin films are investigated by transmission electron microscopy and X-ray energy-dispersive spectroscopy. We find that a high-energy Ar ion beam can cause severe damage in Cu(In,Ga)Se-2 surface regions by preferentially depleting Se and In. The depletion can occur with an Ar ion beam at energy as low as 0.5 keV High-energy electron beams also cause damage in Cu(In,Ga)Se, thin films by preferentially depleting In and Ga. Our results imply that special care must be taken for measurements involving surface treatments using high-energy Ar ion beams or electron beams. (c) 2006 Elsevier B.V. All rights reserved.

The performance of state-of-the-art, series-connected, lattice-matched (LM), triple-junction (TJ), III-V tandem solar cells could be improved substantially (10-12%) by replacing the Ge bottom subcell with a subcell having a bandgap of approx.1 eV. For the last several years, research has been conducted by a number of organizations to develop approx.1-eV, LM GaInAsN to provide such a subcell, but, so far, the approach has proven unsuccessful. Thus, the need for a high-performance, monolithically integrable, 1-eV subcell for TJ tandems has remained. In this paper, we present a new TJ tandem cell design that addresses the above-mentioned problem. Our approach involves inverted epitaxial growth to allow the monolithic integration of a lattice-mismatched (LMM) approx.1- eV GaInAs/GaInP double-heterostructure (DH) bottom subcell with LM GaAs (middle) and GaInP (top) upper subcells. A transparent GaInP compositionally graded layer facilitates the integration of the LM and LMM components. Handle-mounted, ultra-thin device fabrication is a natural consequence of the inverted-structure approach, which results in a number of advantages, including robustness, potential low cost, improved thermal management, incorporation of back-surface reflectors, and possible reclamation/reuse of the parent crystalline substrate for further cost reduction. Our initial work has concerned GaInP/GaAs/GaInAs tandem cells grown on GaAs substrates. In this case, the 1- eV GaInAs experiences 2.2% compressive LMM with respect to the substrate. Specially designed GaInP graded layers are used to produce 1-eV subcells with performance parameters nearly equaling those of LM devices with the same bandgap (e.g., LM, 1-eV GaInAsP grown on InP). Previously, we reported preliminary ultra-thin tandem devices (0.237 cm2) with NREL-confirmed efficiencies of 31.3% (global spectrum, one sun) (1), 29.7% (AM0 spectrum, one sun) (2), and 37.9% (low-AOD direct spectrum, 10.1 suns) (3), all at 25 C. Here, we include recent results of testing similar devices under the concentrated AMO spectrum, and also present the first demonstration of a high-efficiency, ultra-thin GaInP/GaAs/GaInAs tandem cell processed on a flexible kapton handle.

We use the combination of high-resolution electron microscopy and density-functional theory to study the atomic structure and electronic effects of structural defects, such as lamellar twins, stacking faults, and double-positioning twin boundaries in polycrystalline photovoltaic materials such as Si, CdTe, and CuInSe2. We find that individual lamellar twins and stacking faults do not create deep levels in all these materials. However, areas with high density of these defects can form buried wurtzite layers that introduce a barrier to the majority carriers. Double-positioning twin boundaries, which contain dislocation cores, create deep levels in Si and CdTe. Surprisingly, however, they do not create deep levels in CuInSe2. These results may explain the fact that Si and CdTe solar cells usually require special passivation, whereas CuInSe2 solar cells do not. Our further study on the passivation effects indicates that grain boundaries in Si cannot be passivated completely by H alone. On the other hand, grain boundaries in CdTe can be passivated well by Cl and I.