This article is an executive summary of a report from the Centers for Disease Control and Prevention Ventilator-Associated Pneumonia Surveillance Definition Working Group, entitled “Developing a new, national approach to surveillance for ventilator-associated events” and published in Critical Care Medicine. The full report provides a comprehensive description of the Working Group process and outcome.

In September 2011, the Centers for Disease Control and Prevention (CDC) convened a Ventilator-Associated Pneumonia (VAP) Surveillance Definition Working Group to organize a formal process for leaders and experts of key stakeholder organizations to discuss the challenges of VAP surveillance definitions and to propose new approaches to VAP surveillance in adult patients (Table 1).

Zinc oxide is increasingly being studied as an eventual replacement for indium tin oxide as a transparent conducting oxide (TCO) for thin film solar cells. In order to better understand the growth process of ZnO, as well as provide for accurate control on the National Renewable Energy Laboratory's TCO deposition system, undoped RF-magnetron sputtered ZnO films on silicon were analyzed in situ using real-time spectroscopic ellipsometry (RTSE). A large wavelength range (245-994 nm) was measured in order to derive information about optical properties both below and above the bandgap. Atomic force microscopy (AFM) measurements were also taken to confirm ellipsometry surface roughness results and provide additional insight into the surface morphology.

In light of observed poor fit of typical two-layer optical models to in situ ellipsometry data, we propose a model film structure consisting of two layers: a bulk ZnO layer of graded density that increases in thickness as the growth proceeds and a growth zone at the top of the film of approximately constant thickness with a slightly larger bandgap, lower density, and decreased excitonic absorption. Unfortunately, this model is insufficient to explain the evolution of the film during the early growth period, corresponding to film thicknesses less than 100 nm. Nevertheless, the presence of a growth zone for films above a certain thickness should provide some insight into the growth process of RF-magnetron sputtered ZnO.

Our research team has used hot wire chemical vapor deposition (HWCVD) to fabricate silicon heterojunction (SHJ) solar cells on p-type FZ silicon substrates with efficiencies as high as 18.2%. The best cells are deposited on anisotropically-textured (100) silicon substrates where an etching process creates pyramidal facets with (111) crystal faces. Texturing increases Jsc through enhanced light trapping, yet our highest Voc devices are deposited on un-textured (100) substrates. One of the key factors in maximizing the efficiency of our SHJ devices is the process of optimization of the material properties of the 3 - 5 nm thick hydrogenated amorphous silicon (a-Si:H) layers used to create the junction and back contact in these cells. Such optimization is technically challenging because of the difficulty in measuring the properties of extremely thin layers. This difficulty is compounded by the fact that the properties of such amorphous layers are substrate- and thickness-dependent. In this study, we have utilized spectroscopic ellipsometry (SE) and photoconductivity decay to conclude that a-Si:H films grown on (111) substrates are substantially similar to films grown on (100) substrates. In addition, analysis of the substrate temperature dependence of surface roughness evolution reveals a substrate-independent mechanism of surface smoothening with an activation energy of 0.28 eV. Analysis of the substrate temperature dependence of surface passivation reveals a passivation mechanism with an activation energy of 0.63 eV.

High open-circuit voltage (Voc) silicon heterojunction (SHJ) solar cells are fabricated in double-heterojunction a-Si:H/c-Si/a-Si:H structures using low temperature (<225°C) hydrogenated amorphous silicon (a-Si:H) contacts deposited by hot-wire chemical vapor deposition (HWCVD). On p-type c-Si float-zone wafers, we used an amorphous n/i contact to the top surface and an i/p contact to the back surface to obtain a Voc of 667 mV in a 1 cm2 cell with an efficiency of 18.2%. This is the best reported p-type SHJ voltage. In our labs, it improves over the 652 mV cell obtained with a front amorphous n/i heterojunction emitter and a high-temperature alloyed Al back-surface-field contact. On n-type c-Si float-zone wafers, we used an a Si:H (p/i) front emitter and an a-Si:H (i/n) back contact to achieve a Voc of 691 mV on 1 cm2 cell. Though not as high as the 730 mV reported by Sanyo on n-wafers, this is the highest reported Voc for SHJ c-Si cells processed by the HWCVD technique. We found that effective c-Si surface cleaning and a double-heterojunction are keys to obtaining high Voc. Transmission electron microscopy reveals that high Voc cells require an abrupt interface from c-Si to a-Si:H. If the transition from the base wafer to the a-Si:H incorporates either microcrystalline or epitaxial Si at c Si interface, a low Voc will result. Lifetime measurement shows that the back-surface-recombination velocity (BSRV) can be reduced to ~15 cm/s through a-Si:H passivation. Amorphous silicon heterojunction layers on crystalline wafers thus combine low-surface recombination velocity with excellent carrier extraction.

We have achieved an independently-confirmed 17.8% conversion efficiency in a 1-cm2, p-type, float-zone silicon (FZ-Si) based heterojunction solar cell. Both the front emitter and back contact are hydrogenated amorphous silicon (a-Si:H) deposited by hot-wire chemical vapor deposition (HWCVD). This is the highest reported efficiency for a HWCVD silicon heterojunction (SHJ) solar cell. Two main improvements lead to our most recent increases in efficiency: 1) the use of textured Si wafers, and 2) the application of a-Si:H heterojunctions on both sides of the cell. Despite the use of textured c-Si to increase the short-circuit current, we were able to maintain the same 0.65 V open-circuit voltage as on flat c-Si. This is achieved by coating a-Si:H conformally on the c-Si surfaces, including covering the tips of the anisotropically-etched pyramids. A brief atomic H treatment before emitter deposition is not necessary on the textured wafers, though it was helpful in the flat wafers. It is essential to high efficiency SHJ solar cells that the emitter grows abruptly as amorphous silicon, instead of as microcrystalline or epitaxial Si. The contact on each side of the cell comprises a thin (< 5 nm) low substrate temperature (~100°C) intrinsic a-Si:H layer, followed by a doped layer. Our intrinsic layers are deposited at 0.3-1.2 nm/s. The doped emitter and back-contact layers were deposited at a higher temperature (>200°C) and grown from PH3/SiH4/H2 and B2H6/SiH4/H2 doping gas mixtures, respectively. This combination of low (intrinsic) and high (doped layer) growth temperatures was optimized by lifetime and surface recombination velocity measurements. Our rapid efficiency advance suggests that HWCVD may have advantages over plasma-enhanced (PE) CVD in fabrication of high-efficiency heterojunction c-Si cells; there is no need for process optimization to avoid plasma damage to the delicate, high-quality, Si wafers.

We report on the photovoltaic properties of organic solar cells based on pentacene and C60 thin films. A peak external quantum efficiency (EQE) of 69 % at a wavelength of λ = 668 nm is achieved upon optimization of the exciton blocking layer (EBL) thickness. Complex optical functions of pentacene films are measured as a function of wavelength by spectroscopic ellipsometry and used to analyze the EQE spectra. Detailed analysis of the EQE spectra indicate that the pentacene layers exhibit large excitonic diffusion lengths of ∼70 nm and that the performance improvement in EQE can be attributed to the influence of the thickness of the EBL layer on the carrier collection efficiency.

We report the results of our studies on the optical and electronic structure of a wide range of polycrystalline thin-film CuIn1-xGaxSe2 (CIGS) alloys. The composition range includes CIS and nearly stoichiometric (slightly Cu-poor) (24.3±0.3 at.% Cu) CIGS with x values located around the value that has the best efficiency (x ∼ 0.28). Relative to nearly stoichiometric CIS and CIGS, we find a reduction in the absorption strength in the spectral range 1-3 eV. This reduction can be explained in terms of the predominance of Cu 3d and Se 4p states at the valence band maximum (VBM). In addition, Cu-poor CIS and CIGS materials show an increase in bandgap because the p-d repulsive interaction in Cu-poor CIGS is less than that in nearly stoichiometric CIGS. High efficiency is discussed in terms of optical properties.

We use in-situ real-time spectroscopic ellipsometry to observe the breakdown of silicon epitaxy during growth by hot-wire chemical vapor deposition (HWCVD) on Si (100) substrates. Representative data is presented for the two types of epitaxy breakdown that we have observed: 1) an immediate transition to hydrogenated amorphous silicon (a-Si:H), and 2) a slower transition where a-Si:H cones nucleate and grow until they eclipse further epitaxial growth. Simple models, consistent with transmission-electron and atomic-force micrographs, describe the evolution of both types of breakdown showing that real-time spectroscopic ellipsometry is a useful tool for monitoring the growth of epitaxial silicon.

The highest efficiency CuIn1-xGaxSe2 (CIGS) solar cells use thin-film polycrystalline CIGS absorber layers. We have applied variable angle spectroscopic ellipsometry (VASE) to characterize the dielectric functions of polycrystalline thin films of CIGS with Ga: (In + Ga) ratios ranging from 0.18 to 1.0. The Cu: (In + Ga) ratios in these films are approximately 0.90, which is the ratio that yields the highest efficiency CIGS devices. Spectra were measured over the energy range 0.7 to 5.0 eV at room temperatures. Models used to analyze the ellipsometry data include the full multi-layer structure of the sample, which enables us to report the actual dielectric function rather than the pseudo-dielectric function. We present data on how the critical points change with composition, and compare and contrast our results with measurements of single-crystal and bulk polycrystalline samples reported in the literature. Auger electron spectroscopy, atomic force microscopy, and X-ray diffraction have been used to verify the homogeneity, surface roughness, and phase purity, respectively.

In-situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during film growth. We have used in-situ RTSE to characterize the film morphology and crystallinity of hot-wire CVD (HWCVD) Si:H films as a function of hydrogen dilution R=[H]/[H+SiH4], substrate temperature Ts, and film thickness db. Transitions from one mode of film growth to another are indicated by abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the bulk dielectric function. We have studied the growth parameter space consisting of R from 0 to 12, Ts from 150°C to 550°C, and db from 0 to 1 um. For each set of R and Ts values, the structural evolution of the film can be characterized by the shape of the surface roughness thickness ds versus bulk thickness db curve. In contrast to studies done by Collins et al on PECVD growth of Si:H films, our studies of HWCVD growth find no conditions where ds remains constant after coalescence of the initial nucleation centers. Most of the films grown within the range of parameters studied exhibit a secondary nucleation and coalescence signature. The transition between a-Si:H and uc-Si:H growth is near the R=3 to R=4 dividing line. Initial coalescence of purely uc-Si:H material doesn't occur until R>8. We have verified the RTsE crystallinity classification using ex-situ Raman scattering.

We use real-time spectroscopic ellipsometry (RTSE) for in-situ characterization of the optical properties and surface roughness (Rs) of hydrogenated amorphous silicon (a-Si:H) grown by hot-wire chemical vapor deposition (HWCVD) with varying deposition rates (5 to 120 Å/s). Early time evolution of the Rs during growth is remarkably similar for all deposition rates. During the first few Ås of growth, there is a sharp increase in Rs as the a-Si:H nucleates in separate islands. This is followed by a reduction of Rs as these areas coalesce into a bulk film, which occurs at an average thickness of 100 Å. After coalescence the Rs rises to a stable value that is dependent upon growth conditions with a general tendency for the Rs to increase with growth rate. However, neither the Rs nor the material electronic properties are unique for a given deposition rate. Films grown under high silane flow and low pressure have a better photoresponse and a lower Rs than films grown at the same deposition rate but with low silane flow and high pressure. We observe a stronger correlation of film properties with Rs than with deposition rate; namely a monotonic decrease in photo-response, and increase in optical gap, with increasing Rs.