In-depth photoluminescence spectra of pure CIGS thin films.

Photoluminescence spectra of pure CIGS thin films with different Ga in-depth gradients are systematically investigated. Pure Na-free films are prepared with an innovative hybrid deposition technique, whose optical luminescence emission is analyzed as a function of the depth and is correlated to the radiative intrinsic defects of the material. Finally, the highlighted features are correlated with the performances of test solar cells prepared with the same growths.

Care System Versus Transmitted Light Wavefront Pattern of Contact Lenses.

OBJECTIVES: This article compares the optical performance of soft contact lenses (CLs) treated with multipurpose or hydrogen peroxide care systems. METHODS: The investigated care systems were (1) 3% hydrogen peroxide solution Oxysept (Abbot Medical Optics, Abbott Park, IL) and (2) multipurpose solution Regard (Vita Research, Ariccia, Italy). Three types of silicone hydrogel CLs were studied (comfilcon A, lotrafilcon B, and balafilcon A), unworn and exposed for 30 times to the solutions, which were replaced every 8 hr. The optical performance of the CLs was evaluated through the on-eye transmitted light wavefront patterns by considering new CLs as references. The surface morphology of the CLs was investigated by scanning electron microscopy. RESULTS: Statistically significant modifications in the range 0.1 to 0.3 µm of Zernicke coefficients and modifications of the root mean square of the wavefront aberration function were found for CLs treated with multipurpose solution, in agreement with the observed modifications of the surface morphology. Statistically significant changes were also found after exposure to the hydrogen peroxide solution, but the variation of the Zernicke coefficients was found lower than 0.1 µm, thus being negligible in CL optical performances. CONCLUSIONS: In addition to disinfection ability and ocular surface reactions, CL care systems are different in solution-related CL optical performance. Multipurpose solutions may affect the CL surface morphology with significant modifications of the transmitted light wavefront pattern.

Titania as Buffer Layer for Cd-Free Kesterite Solar Cells.

Pure sulfide kesterite (Cu2ZnSnS4) is one of the most promising emerging photovoltaic technologies thanks to its excellent absorption coefficient, cost-effectiveness, and environmental sustainability. However, record efficiencies are not exceeding 11% due to several issues, such as absorber defects or a nonoptimal band alignment with the toxic but conventionally used CdS buffer layer. To get rid of it, several efforts have been made in the past few years. Among recent theoretical works, TiO2 has been suggested as a suitable buffer layer due to its optical and electrical properties, giving extremely promising results in device simulation. However, there are few experimental examples combining TiO2 with kesterite, and they generally show very modest performances. In this Letter, we report on the preliminary and promising results of our experimental procedure for the production of Cd-free kesterite photovoltaic devices featuring ALD-TiO2 as a buffer layer, leading to efficiencies comparable with our CZTS/CdS reference devices.

Band-Gap Tuning Induced by Germanium Introduction in Solution-Processed Kesterite Thin Films.

In the last few decades, the attention of scientific community has been driven toward the research on renewable energies. In particular, the photovoltaic (PV) thin-film technology has been widely explored to provide suitable candidates as top cells for tandem architectures, with the purpose of enhancing current PV efficiencies. One of the most studied absorbers, made of earth-abundant elements, is kesterite Cu2ZnSnS4 (CZTS), showing a high absorption coefficient and a band gap around 1.4-1.5 eV. In particular, thanks to the ease of band-gap tuning by partial/total substitution of one or more of its elements, the high-band-gap kesterite derivatives have drawn a lot of attention aiming to find the perfect partner as a top absorber to couple with silicon in tandem solar cells (especially in a four-terminal architecture). In this work, we report the effects of the substitution of tin with different amounts of germanium in CZTS-based solar cells produced with an extremely simple sol-gel process, demonstrating how it is possible to fine-tune the band gap of the absorber and change its chemical-physical properties in this way. The precursor solution was directly drop-cast onto the substrate and spread with the aid of a film applicator, followed by a few minutes of gelation and annealing in an inert atmosphere. The desired crystalline phase was obtained without the aid of external sulfur sources as the precursor solution contained thiourea as well as metal acetates responsible for the in situ coordination and thus the correct networking of the metal centers. The addition of KCl in dopant amounts to the precursor solution allowed the formation of well-grown compact grains and enhanced the material quality. The materials obtained with the optimized procedure were characterized in depth through different techniques, and they showed very good properties in terms of purity, compactness, and grain size. Moreover, solar-cell prototypes were produced and measured, exhibiting poor charge extraction due to heavy back-contact sulfurization as studied in depth and experimentally demonstrated through Kelvin probe force microscopy.

Defect-assisted photocatalytic activity of glass-embedded gallium oxide nanocrystals.

The use of glassceramics in photocatalysis is an attractive option for the realization of smart optical fibers and self-cleaning windows. Here we present the photocatalytic activity of germanosilicate glasses embedding Ga2O3 nanocrystals prepared by batch melting and glass heat treatment. The powdered material is used for UV-assisted degradation of rhodamine in water. The kinetics show changes after repeated experiments. In the first cycle, the apparent rate is governed by a second-order reaction with a Gaussian-like shape, whereas the second cycle follows a first-order reaction. The modification appears to be correlated with perturbations in the defect population. Photoluminescence has been used to monitor the evolution of such defects. Kinetic data on photoreactions and defect formation have been modelled in a combined frame in which the defect concentration determines the photocatalytic activity. The results prove the photocatalytic ability of the studied glassceramics. Moreover, the general validity of the kinetic model can be of interest for other systems in which the photocatalytic response depends on photoreactive species concentration.

An Investigation of the Role of Macular Pigment in Attenuating Photostress through Comparison between Blue and Green Photostress Recovery Times.

PURPOSE: Photostress recovery time (PSRT) is the time required for the macula to return to its normal functioning after the bleaching of cone photopigments due to light exposure, usually white. This work investigates the role of macular pigment (MP) as an optical filter that attenuates photostress by analyses of PSRT at different wavelengths. METHODS: Thirty-nine subjects (19-28 years) were exposed to blue/green photostress varying in irradiance. During photostress, pupil constriction (Cp) was measured. Twenty-seven subjects (20-27 years) were exposed to white photostress. After 25 s of photostress, the time (PSRT) required to read correctly a 0.2 logMAR letter was measured. Correlation was studied between PSRT, CP, and irradiance. Statistical significance of differences between PSRTs was evaluated at Log(irradiance(quanta s-1 cm-2)) = 14 by Student's t statistics. RESULTS: Cp and PSRT were found linearly correlated to Log(irradiance) for blue, green, and white. At Log(irradiance(quanta s-1 cm-2)) = 14, blue and green mean PSRTs resulted different (p < 0.001) with 3.8 ± 0.8 s and 6.7 ± 1.7 s, respectively. After correcting irradiance for the optical absorption of MP, mean blue PSRT became 6.6 ± 0.8 s, at the logarithm of MP-corrected irradiance in quanta s-1 cm-2 equal to 14 (p = 0.571 compared to green PSRT). For white light, at the logarithm of MP-corrected irradiance in quanta s-1 cm-2 equal to 14, mean PSRT was 7.5 ± 2.2 s, not significantly different from blue and green PSRT (p > 0.05). CONCLUSIONS: MP plays the role of an optical filter attenuating photostress. PSRT was substantially proportional to the number of incident photons corrected for the MP optical absorption, regardless of their wavelength.

Droplet Controlled Growth Dynamics in Molecular Beam Epitaxy of Nitride Semiconductors.

The growth dynamics of Ga(In)N semiconductors by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) at low temperatures (T = 450 °C) is here investigated. The presence of droplets at the growth surface strongly affects the adatom incorporation dynamics, making the growth rate a decreasing function of the metal flux impinging on the surface. We explain this phenomenon via a model that considers droplet effects on the incorporation of metal adatoms into the crystal. A relevant role is played by the vapor-liquid-solid growth mode that takes place under the droplets due to nitrogen molecules directly impinging on the droplets. The role of droplets in the growth dynamics here observed and modeled in the case of Nitride semiconductors is general and it can be extended to describe the growth of the material class of binary compounds when droplets are present on the surface.

Gallium In-Depth Profile in Bromine- Etched Copper-Indium-Galium-(Di)selenide (CIGS) Thin Films Inspected Using Raman Spectroscopy.

In the thin film solar cells domain, copper indium galium (di)selenide (CIGS) is a material with well-established photovoltaic purpose. Here the presence of a suitable [Ga]/([Ga]+[In]) (GGI) in-depth profile has proved to play a key role in the performance of cells. The implementation of a routine method based on reliable but easily available experimental techniques is mandatory to obtain information on the GGI profile of any CIGS layer, in order to achieve high efficiency chalcogenide layers. In this vein, we here propose and systematically test a simple method for the GGI profile determination based on repeated bromine etching of CIGS thin films followed by Raman analysis of the A1 peak position. The reliability of the proposed approach is verified using a methodical comparison with energy-dispersive X-ray spectroscopy (EDS) analysis and secondary ion mass spectroscopy (SIMS) profiles, showing a good agreement with the GGI in-depth profiles determined using Raman analysis on bromine etched samples.

Conifer needles as passive biomonitors of the spatial and temporal distribution of DDT from a point source.

Needles of two conifer species, Picea abies and Pinus nigra, were used as passive samplers for monitoring air contamination by sampling at increasing distances from a suspected point source of DDT. Needle concentrations declined with increasing distance downwind of the point source allowing to identify spatial and temporal trends of accumulation. This suggested that conifer needles are effective biomonitors of contamination levels in areas characterized by the presence of semi-volatile substances. Differences in uptake were apparent between the species. Needle morphology and structure were studied with scanning electron microscope (SEM) as were dimensional parameters (surface area, volume). The results suggest that the concentrations depend on a mechanism involving the inner structure of the needles, specifically the number and accessibility of resin channels rather than their surface area. Pine needles have more channels with greater accessibility than spruce. The results suggest that spruce is more suitable for short term measurement while pine for determining long term cumulative exposure.

Thiocyanate-free cyclometalated ruthenium sensitizers for solar cells based on heteroaromatic-substituted 2-arylpyridines.

The first examples of thiocyanate-free thiophene-substituted Ru(II) cyclometalated complexes, based on thiophene-derived 2-(2,4-difluorophenyl)pyridine ligands, are presented and investigated as photosensitizers in DSCs. Upon thiophene substitution the complexes presented enhanced optical properties compared to the reference dye with no thiophene substitution. DSCs based on the dithienyl-derived dye showed power conversion efficiencies up to 5.7%, more than twice that containing the complex without the thiophene substitution.