X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films

Thin film solar cells (TFSC) are a promising approach for terrestrial and space photovoltaics and offer a wide variety of choices in terms of the device design and fabrication. A variety of substrates (flexible or rigid, metal or insulator) can be used for deposition of different layers (contact, buffer, absorber, reflector, etc.) using different techniques (PVD, CVD, ECD, plasma-based, hybrid, etc.). Such versatility allows tailoring and engineering of the layers in order to improve device performance. For large-area devices required for realistic applications, thin-film device fabrication becomes complex and requires proper control over the entire process sequence. Proper understanding of thin-film deposition processes can help in achieving high-efficiency devices over large areas, as has been demonstrated commercially for different cells. Research and development in new, exotic and simple materials and devices, and innovative, but simple manufacturing processes need to be pursued in a focussed manner. Which cell(s) and which technologies will ultimately succeed commercially continue to be anybody's guess, but it would surely be determined by the simplicity of manufacturability and the cost per reliable watt. Cheap and moderately efficient TFSC are expected to receive a due commercial place under the sun.

Thin-film solar cells: an overview

Molecular Semiconductors in Organic Photovoltaic Cells

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

We demonstrate the improvement of an indium tin oxide anode contact to an organic light emitting device via oxygen plasma treatment. Enhanced hole-injection efficiency improves dramatically the performance of single-layer doped-polymer devices: the drive voltage drops from >20 to <10 V, the external electroluminescence quantum efficiency (backside emission only) increases by a factor of 4 (from 0.28% to 1%), a much higher drive current can be applied to achieve a much higher brightness (maximum brightness ∼10,000 cd/m2 at 1000 mA/cm2), and the forward-to-reverse bias rectification ratio increases by orders of magnitude (from 102 to 106–107). The lifetime of the device is also enhanced by two orders of magnitude.

Surface modification of indium tin oxide by plasma treatment: An effective method to improve the efficiency, brightness, and reliability of organic light emitting devices

The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine‐doped tin oxide (FTO), and indium‐doped zinc oxide (IZO) films has been studied. X‐ray photoelectron spectroscopy analysis shows that ITO and FTO surfaces get reduced to yield elemental indium and tin, respectively. Annealing of the plasma treated films in air leads to re‐oxidation of the reduced surface and the electro‐optical properties are recovered. In contrast, IZO films are not reduced by plasma treatment and show no changes in the electrical and optical properties. The surface of plasma treated IZO films shows a higher binding energy O(1s) peak probably due to OH or OH...O species which appear to form a protective layer against plasma degradation.

Effect of hydrogen plasma treatment on transparent conducting oxides

2 Abstract: Soil enzyme activities are very sensitive to both natural and anthropogenic disturbances and show a quick response to the induced changes. Soil dehydrogenase enzymes are one of the main components of soil enzymatic activities participating in and assuring the correct sequence of all the biochemical routes in soil biogeochemical cycles. Dehydrogenase activity is measured by two methods using the TTC and INT substrate; however, various authors reported poor results when TTC is used as substrate. Different biotic and abiotic factors such as incubation time and temperature, pre-incubation, soil aeration and moisture content have significant effect on dehydrogenase activity in soil. Highest dehydrogenase activity is reported from forest soil in autumn seasons while the disturbed soil from coal mines soils containing lowest dehydrogenase activities along the soil erosion gradient of experimental slopes. Least value of enzymes activity is reported from polluted sites than restored and undisturbed sites. Dehydrogenase enzyme is often used as a measure of any disruption caused by pesticides, trace elements or management practices to the soil, as well as a direct measure of soil microbial activity.

Soil Dehydrogenase Enzyme Activity in Natural and Mine Soil - A Review

An optical fiber, partially stripped of its cladding is shown to sense refractive index of a liquid in which the uncladded sensing region is immersed, to a high degree of precision and over a wide range of refractive index. The slope of sensor response is found to be non linear, can have either sign and can change sign at around refractive index of the fiber. The sensitivity of the sensor to refractive index change is dependent on cladding thickness and is a maximum at an intermediate thickness value. It is insensitive to the presence of absorption at the wavelength at which refractive index is being measured and to the chemical nature of the solute. Experiments designed to show that cladding modes are responsible for sensing are described.

Fiber optic sensing of liquid refractive index

High sensitivity, high‐energy resolution, and the capability to obtain data in situ make ellipsometry a useful tool for addressing a number of problems in thin‐film deposition processes. Dielectric functions e=e1−i e2 of glow‐discharge‐produced microcrystalline silicon films (μc‐Si) are determined by using spectroscopic ellipsometry (SE). e2 spectra present a shoulder near 4.2 eV which corresponds to the E2 optical transition observed in crystalline silicon. This peak is not observed in amorphous silicon spectra. c‐Si is described as a mixture of amorphous phase, microcrystallites, and voids. The choice of the microcrystalline reference phase is discussed in particular by comparing the microcrystallite volume fraction determined from SE and x‐ray measurements. Strong variations in the nature of the material are observed during growth: the density deficiency and the surface roughness both increase as functions of film thickness up to 0.4–0.5 μm.

In situ spectroscopic ellipsometry study of the growth of microcrystalline silicon

First-order Raman spectra from nanocrystalline semiconductors reflect the influence of crystallite sizes on the Raman shifts and line shapes. A Gaussian distribution in crystallite sizes is explicitly included to calculate the Raman spectra of porous silicon. Several porous-silicon samples were prepared using electrochemical anodization, and Raman as well as photoluminescence measurements were carried out on the same spots using a micro-Raman probe. The size distribution obtained from fitting the Raman data using our procedure is able to predict the photoluminescence accurately in the quantum-confinement models.

Satyendra Kumar

Papers

Effect of hydrogen plasma treatment on transparent conducting oxides

Soil Dehydrogenase Enzyme Activity in Natural and Mine Soil - A Review

Fiber optic sensing of liquid refractive index

In situ spectroscopic ellipsometry study of the growth of microcrystalline silicon

Influence of crystallite size distribution on the micro-Raman analysis of porous Si