Characterization of a-Si:H films via analysis of multiple photocurrent spectra

The spectral response of photoconductive materials applicable to solar cells and photosensors, is commonly characterized using different experimental approaches, such as photocurrent spectra normalized to incident light intensity, photoconductivity spectra measured at constant photon flux, the constant photocurrent method (CPM) [3–5], and the dependence of the photocurrent on the photon flux.In this work, a procedure is proposed, which allows the complementary results of the above mentioned approaches to be gained by analysing several photocurrent spectra, obtained by routine measurements at different illumination levels (Multiple Spectra). This procedure is used for investigation of two a-Si:H samples. The spectra, corresponding to a constant photon flux and a constant photocurrent, as well as the dependencies of the photocurrent on the photon flux at various wavelengths I−Φ spectral map) are derived from the Multiple Spectra and are subsequently used to obtain the following characteristics of the investigated samples: the subbandgap optical absorption spectra; the bulk density of “defect” states; the Urbach tail width; and the power index of the I−Φ)_λ dependencies for the bulk and for the interface regions.     

Deposition of highly photoconductive wide band gap a-SiOx:H thin films at a high temperature without H2-dilution

Electrical and optical characterisation of hydrogenated amorphous silicon–oxygen alloy thin films (a-SiO_x:H, x<2) grown in a single chamber radio frequency plasma enhanced chemical vapour deposition (PECVD) system at a high substrate temperature of 300 °C is presented. The samples were investigated by Fourier transform infrared spectroscopy (FTIR), optical transmission, the constant photocurrent method (CPM), conductivity and steady-state photoconductivity measurements. With increasing oxygen concentration, the Tauc gap increases from 1.69 to 2.73 eV. The sample with an oxygen concentration of 26.2 at% and a reasonably high bandgap of 2.18 eV shows photoconductivity comparable to that of pure a-Si:H films. The Urbach parameter (E₀) increases almost linearly with oxygen concentration whereas the dangling bond defect density is found to be saturating at a value of about 7.1×10¹⁶ cm⁻³. One of the highly alloyed samples with exhibited a detectable photosensitivity.     

Variance analysis of estimates and measurements of terrestrial heat flow

A components-of-variance-analysis is applied to compare heat flow estimates with heat flow measurements. Three types of heat flow data are analysed; they are: “between depth interval within well” variances, “between well site-within region” variances and “between region” variances. “Between depth interval-within well” heat flow variances are typically greater for estimates than for measurements, implying a greater unsystematic uncertainty with the interval heat flow estimates as compared to the interval measurements. “Between well site-within region” variances of heat flow are about the same, or in some cases less, for groups of estimated data as compared with groups of measured data. Therefore unsystematic uncertainties for good heat flow estimates at sites within a region should compare favourably with similar uncertainties for heat flow measurements at sites within a region. Both the “F” test and the “between region” heat flow variances suggest that estimates and measurements of heat flow can reasonably differentiate between regions of different heat flow.Some of the estimated and measured data groups are from the same region. The very limited number of heat flow estimates and measurements available indicate that the mean of estimates and the mean of deep measurements in the northern Colorado Plateau are in close agreement. While this suggests the possibility that a number of carefully calculated heat flow estimates may reasonably define the mean heat flow for a region, more data in other regions will be needed to support the concept.     

Quality control and characterization of Cu(In,Ga)Se2-based thin-film solar cells by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/ Cu(In,Ga)Se₂ (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect “hard failures” following CIGS deposition, and both “hard” and “soft” failures following CdS deposition and following ZnO deposition. In addition, a semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS/CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO/CIGS structures is found.     

Precursor route poly(thienylene vinylene) for organic solar cells: Photophysics and photovoltaic performance

Photophysical studies and photovoltaic devices on a low bandgap, high-charge carrier mobility poly(thienylene vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the “dithiocarbamate route”, are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM), a soluble fullerene derivative), photoinduced absorption characteristic for charged excitations together with photoluminescence quenching are observed indicating photoinduced electron transfer. The “bulk heterojunction” photovoltaic devices using PTV and PCBM composites show short-circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset about 1.65 eV (750 nm), which corresponds to the absorption spectrum of the polymer.     

Effect of light-induced metastable defects on photocarrier lifetime

Creation and annealing of light-induced defects and their effect on photocarrier lifetime have been studied at 120 and 300 K using constant photocurrent method (CPM) and steady-state photoconductivity measurements. A hysteresis-like relation is observed between photoconductivity and light-induced defect density. This relation depends on both degradation temperature and light intensity used for the degradation. A broad, resembling a two-component distribution of defect annealing activation energies together with distribution of recombination coefficients account for the observed changes at 120 K. On the other hand, these distributions are narrower and sharply peaked at about 1 eV for the 300 K measurements. Results indicate that defects which are created at the earlier stages of the illumination have smaller annealing activation energies and higher recombination coefficient (capture cross-section) and these are better recombination centers than the defects with higher annealing activation energies.     

Solar-assisted absorption heat pumps feasibility

An absorption system can be used for space cooling as well as for space heating. This dual purpose may be achieved by using the system as heat pump in wintertime. Absorption heat pump heating may be an interesting alternative, particularly for countries where there is a shortage of electric power.When an absorption unit is used as heat pump, its mode of operation is not modified: the internal temperatures of the cycle are only raised. Commercially available LiBr units were tested as heat pumps. COP and heating capacity were considered as a function of cold source temperature for different temperatures of the useful heat. The COP arrived at 1.7, which must be considered a high value for a thermally driven heat pump.Simulations were carried out in order to compare the performance of “conventional” solar, solar assisted heat pump and the combined series system under two different climate conditions. The series system showed performance 25–75 per cent better than “conventional” solar alone.     

Variation of creation and annealing character of light induced metastable defects in a-Si1−xCx:H alloys

The creation and annealing kinetics of light induced metastable defects were studied in a set of good quality a-Si_1−xC_x:H alloys (x0.11) using the constant photocurrent method (CPM) at room temperature. Light induced metastable defects created at room temperature started annealing at higher temperatures, when the alloys had high carbon content. The annealing activation energy distribution functions of the hydrogenated amorphous silicon–carbon alloys were calculated using the method proposed by Hata and Wagner (J. App. Phys. 72 (1992) 2857). The annealing activation energy distribution function is a narrow Gaussian peaked at about 1 eV for the unalloyed sample. For the alloys, the peak position shifts to higher energies and the half-width of the distribution decreases with increasing carbon content. The results obtained are discussed within the framework of the “weak-bond breaking” (Phys. Rev. B 32 (1985) 23), and the “hydrogen-collision” (Phys. Rev. 59(8) (1999) 5498) models.     

Light trapping and reflection control in solar cells using tilted crystallographic surface textures

Surface textures made of <111> crystallographically oriented facets are examined for their capacity to enhance absorption and reduce reflection in solar cells. Some that employ microstructures with their axes of symmetry tilted from the surface normal can almost eliminate broadband top surface reflection and enhance the pathlength of weakly absorbed radiation as well as any schemes previously suggested. For example, tilting the axes of upright pyramids by about 24° towards the same base corner is estimated to enhance absorption of the solar spectrum in a 280 μm cell by up to 4.5%. Practical issues concerning the fabrication of “tilted” structures and suitability for reducing back surface losses are also discussed.     

A comparative study of optical, electrical and structural properties of CuGaSe2 and CuGaTe2 thin films

The growth conditions, the composition and the structural, optical and electrical properties of thin films of CuGaSe₂ and CuGaTe₂ have been studied using “flash” and “slow” evaporation in vacuum. Single phase films, when analyzing the absorption coefficient, present several energy gaps. For CuGaSe₂, they are 1.59, 1.66, 2.03 and 2.11 eV, for CuGaTe₂ 1.23 and 1.89 eV. Both the CuGaSe₂ and CuGaTe₂ evaporated films are p-type; the resistivities, carrier densities and mobilities are appropriate for thin film solar cells.     

Application of scanning mid-IR-laser microscopy for characterization of semiconductor materials for photovoltaics

Scanning mid-IR-laser microscopy was previously demonstrated as an effective tool for the characterization of different semiconductor crystals. Now the technique has been successfully applied for the investigation of CZ Si_xGe_1−x – a promising material for photovoltaics – and multicrystalline silicon for solar cells. In addition, this technique was shown to be appropriate for the imaging of polishing-induced defects as well as huge defects such as “pin holes”. Besides, previously unexplained “anomalous” (cubic power) dependence of signal of the scanning mid-IR-laser microscope in the optical-beam-induced light scattering mode on the photoexcitation power obtained for mechanically polished samples has now been attributed to the excess carrier scattering on charged linear defects, likely dislocation lines. The conclusion is made in the article that scanning mid-IR-laser microscopy may serve as a very effective tool for defect investigations in materials for modern photovoltaics.     

Defect recognition and impurity detection techniques in crystalline silicon for solar cells

The efficiency of multicrystalline solar cells is limited by defects and impurities, which include grain boundaries, dislocations, and transition metals. The density of these defects often varies from grain to grain. “Bad grains” with low minority carrier diffusion length generate low open circuit voltage and shunt the “good grains” with high minority carrier diffusion length, thus reducing the overall cell efficiency. It was found that it is more likely to find transition metal clusters in “bad grains” than in “good grains”, and that gettering is not efficient in improving the areas of low diffusion length. The primary objective of materials research in photovoltaics is identification of these lifetime-limiting defects. In this article we summarize the current state of understanding of lifetime-limiting defects in solar cells, summarize the advantages and limitations of traditional analytical tools and discuss novel emerging techniques, including X-ray fluorescence microprobe, X-ray absorption spectromicroscopy, and X-ray beam-induced current     

Visualization of unstable water flow in a fuel cell gas diffusion layer

Modeling two-phase flow in proton exchange membrane (PEM) fuel cells is hampered by a lack of conceptual understanding of flow patterns in the gas diffusion layer (GDL). In this paper, pore-scale visualizations of water in different types of GDLs were used to improve current understanding of flow and transport phenomena in PEM fuel cells. Confocal microscopy was used to capture the real-time transport of water, and pressure micro-transducers were installed to measure water breakthrough pressures. Three types of fuel cell GDLs were examined: TO series (Toray Corp., Tokyo, Japan), SGL series (SGL Carbon Group, Wiesbaden, Germany), and MRC series (Mitsubishi Rayon Corp., Otake City, Japan). The visualizations and pressure measurements revealed that despite difference in “pore” structures in the three types of GDLs, water followed distinct flow paths spanning several pores with characteristics similar to the “column flow” phenomena observed previously in hydrophobic or coarse-grained hydrophilic soils. The results obtained from this study can aid in the construction of theories and models for optimizing water management in fuel cells.     

Three-state regulator for electrochromic windows

Electrochromic (EC) windows enable to change their optical transmittance by applying appropriate electrical signals to the window structure. In many applications, such as “smart” EC buildings’ windows is required, an automatic control of the window's transmittance, in order to assure a constant level of transmitted daylight for different outdoors’ illuminations. For this purpose we develop an analogue three-state electronic regulator which controls an “all sol–gel” EC device intensity of transmitted light on a constant pre-selected level. In this paper, first of all, the structure and the performances of an all sol–gel EC window is given. Then the electrical schematic of the regulator is represented and explained in detail. The performances of the regulator are demonstrated by electrical and optical measurements. At the end a demonstrational application—EC sun-glasses with an accompanying handy regulator—is presented.     

First results of the application of the dipole electrical sounding method in the geothermal area of travale - radicondoli (Tuscany)

A new geoelectric prospecting method has been tested in the Travale - Radicondoli geothermal area. This method is based on the dipolar technique that permits investigation at very great depths with much fewer problems than encountered when using the classical electric prospecting techniques.The following steps were taken in order to operate with relatively low power from a 2 kW generator:

1. (i) the ground was energized with a series of current square waves at a frequency of less than 0.05 Hz in order to avoid the effects of electromagnetic coupling and induced polarization;

2. (ii) the voltage was recorded digitally at the measuring dipole;

3. (iii) the voltage recordings were processed by the spectral analysis method of “maximum likelihood”.

The resulting apparent resistivity diagrams were transformed into Schlumberger diagrams and then interpreted quantitatively.The six soundings are too limited in number to represent a real prospecting but refer to different geological and structural situations typical of a geothermal area. Two electrosoundings were sited for this purpose so as to be directly calibrated by the wells in the local geothermal field. The quantitative analysis of the resistivity diagrams in particular revealed the low resistivity values of the carbonate formation forming the geothermal reservoir, where the hot fluid circulation is particularly strong (15 Ω.m).The dipolar method has proved capable of distinguishing, in the geological situation of Travale area, the various structural features of the geothermal field such as “cover”, “reservoir”. substratum, uplifted structures and tectonic depressions.     

Evaluation of porous silicon carbide monolithic honeycombs as volumetric receivers/collectors of concentrated solar radiation

Porous monolithic multi-channeled silicon carbide (SiC) honeycombs employed as open volumetric receivers of concentrated solar radiation, were evaluated with respect to their porous structure and thermomechanical properties before and after long-time operation. Proper “tuning” of porosity, pore size distribution and microstructure can provide SiC honeycombs with improved mechanical properties (higher bending and compressive strength) in the “as-manufactured” state. Exposure under solar irradiation was found to affect both their pore structure and their mechanical characteristics. During the first stages of exposure, a re-structuring of the porous structure takes place shifting the mean pore size to higher values and slightly decreasing the total porosity; this re-structuring ceases after some “characteristic” exposure time. After solar exposure the honeycombs become harder and exhibit significantly higher compressive strength. Extension of anticipated lifetime can be achieved by materials with enhanced mechanical properties like silicon-infiltrated (siliconized) SiC.     

Strain gradient-induced diffusion of hydrogen in palladium and nickel membranes

The “uphill” diffusion of hydrogen during permeation through flat sheets of palladium and nickel has been studied by an electrochemical permeation method at 303 K. For both annealed and “as cold rolled” Pd samples, uphill diffusion effects on hydrogen absorption and desorption have been observed over a range of initial hydrogen contents from about H/Pd = 0.01, i.e. near or slightly less than the _max composition, up to H/Pd = 0.25–0.3. The occurrence of a non-Fickian component of permeation flux has been associated with temporary formation of lattice volume differences across the ( + β)/β and ( + β)/ interfaces during absorptions and desorptions, respectively. Influences of the magnitudes of galvanostatic hydrogen fluxes and of the membrane thickness on the uphill effects were examined. Analogous uphill effects were observed in similar studies with nickel membranes also in both annealed and “as cold rolled” states, which were much larger than those observed for palladium.     

Comments on: “Gallium-doped ZnO thin films deposited by chemical spray”: [Sol. Energy Mater. Sol. Cells 87 (2005) 107–116]

In this communication, I will discuss some aspects concerning a paper recently published by H. Gomez et al. on the properties of gallium-doped ZnO thin films deposited by chemical spray methods [H. Gomez, A. Maldonado, M. de la L. Olvera, D.R. Acosta, Sol. Energy Mater. Sol. Cells 87 (2005) 107.]. I believe that there are some wrong considerations of basic principles, some incorrect experimental procedures and inconsistencies on the analysis and throughout the paper. The “Introduction” reflects some knowledge on the subject, but it seems that the authors of the aforementioned paper use outdated background sources in some cases, and inconsistently apply prior research in others.The “Experimental procedure” shows some serious lack of knowledge concerning thin films that are potentially applicable as transparent electrodes in photovoltaic devices. In the “Results and discussion”, I found some of the arguments and statements contradictory, mainly when the effects of their vacuum annealing were being discussed. Considerations on the “Structural properties” are necessary, as they are not appropriate for comparative purposes. Finally, a last point, but not less important than the others, is the “Conclusions” of the paper. Some suggestions are made below for the improvement of this, and several others, aspects of the paper.     

Effects of grain boundaries in polycrystalline silicon thin-film solar cells based on the two-dimensional model

The two-dimensional calculation for polycrystalline Si thin-film solar cells was performed. Two models, “stripe structure” and “columnar structure”, were applied for the solar cells composed of grains. For the stripe structure of 20 μm active layer, to keep the efficiency distribution within 5% for individual unit cells, the stripe width requires more than 500 μm for a minority-carrier lifetime of 1×10⁻⁵ s and recombination velocity at the grain boundary of 1×10⁴ cm/s. For the columnar structure of 10 μm active layer, to keep the efficiency independent of grain size, the recombination velocity should be kept less than 1×10³ cm/s. If imperfect passivation of a grain boundary is given, the way of decreasing carrier concentration to 10¹⁴ cm⁻³ in an active layer may realize insusceptible output. An appropriate device modeling is needed in the two-dimensional calculation for polycrystalline Si thin films with an electron diffusion length close to or more than grain size and with a poorly passivated grain boundary. The calculated efficiency using bad model will include an error of about 1% as overestimation.     

An analytical method for reflected flux density calculations

Conception, evaluation and real time control of solar “power tower” systems require the use of fast and accurate computer programs for calculating the flux density distributions on the receiver. Since the classical methods of “cone optics” and “hermite polynomial expansion” have some limitations of speed and accuracy, we have built an analytical model for calculating the convolution of the solar brightness distribution with the principal image of a heliostat (i.e. the fictive image for a “point sun”). We first characterize a principal image of a focusing heliostat by its shape and its geometrical concentration factor. Then this image is projected back onto the central plane (which passes through the center of the mirror), and considered as a flat reflecting surface. And the problem is reduced to density calculation for a flat heliostat. For each point of the receiver, the density of flux reflected by a heliostat is obtained by direct resolution of a convolution integral. The different formulations used to express the density function correspond to the various types of intersections between the image of the solar disk for the considered point and the principal image of the heliostat. Confrontation of this method with a program based on “cone optics” shows a good concordance of results and a strong decrease of computation time. We want to apply this method to the existing “THEMIS” solar plant built in France and to compare our results with real observations. Our density calculation programs will help conceiving fields of focusing heliostats for a new generation of power systems (gaz turbine systems).