Showing papers by "Miro Zeman published in 2009"

A scattering model for surface-textured thin films

Abstract: We present a mathematical model that relates the surface morphology of randomly surface-textured thin films with the intensity distribution of scattered light. The model is based on the first order Born approximation [see e.g., M. Born and E. Wolf, Principles of Optics, 7th ed. (Cambridge University Press, Cambridge, England, 1999) ] and on Fraunhofer scattering. Scattering data of four transparent conductive oxide films with different surface textures were used to validate the model and good agreement between the experimental and calculated intensity distribution was obtained.

Determination of the mobility gap of intrinsic μc-Si:H in p-i-n solar cells

Abstract: Microcrystalline silicon (?c-Si:H) is a promising material for application in multijunction thin-film solar cells. A detailed analysis of the optoelectronic properties is impeded by its complex microstructural properties. In this work we will focus on determining the mobility gap of ?c-Si:H material. Commonly a value of 1.1?eV is found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48–1.59?eV, depending on crystalline volume fraction. Indeed, for the accurate modeling of ?c-Si:H solar cells, it is paramount that key parameters such as the mobility gap are accurately determined. A method is presented to determine the mobility gap of the intrinsic layer in a p-i-n device from the voltage-dependent dark current activation energy. We thus determined a value of 1.19?eV for the mobility gap of the intrinsic layer of an ?c-Si:H p-i-n device. We analyze the obtained results in detail through numerical simulations of the ?c-Si:H p-i-n device. The applicability of the method for other than the investigated devices is discussed with the aid of numerical simulations.

Modulated photonic-crystal structures as broadband back reflectors in thin-film solar cells

Abstract: A concept of a modulated one-dimensional photonic-crystal (PC) structure is introduced as a back reflector for thin-film solar cells. The structure comprises two PC parts, each consisting of layers of different thicknesses. Using layers of amorphous silicon and amorphous silicon nitride a reflectance close to 100% is achieved over a broad wavelength region (700–1300 nm). Based on this concept, a back reflector was designed for thin-film microcrystalline silicon solar cells, using n-doped amorphous silicon and ZnO:Al. Simulations show that the short-circuit current of the cell with a modulated PC back reflector closely resembles that of a cell with an ideal reflector.

First-principles study of hydrogenated amorphous silicon

Abstract: We use a molecular-dynamics simulation within density-functional theory to prepare realistic structures of hydrogenated amorphous silicon. The procedure consists of heating a crystalline structure of Si64H8 to 2370 K, creating a liquid and subsequently cooling it down to room temperature. The effect of the cooling rate is examined. We prepared a total of five structures which compare well to experimental data obtained by neutron-scattering experiments. Two structures do not contain any structural nor electronic defects. The other samples contain a small number of defects which are identified as dangling and floating bonds. Calculations on a bigger sample (Si216H27) show similar properties (radial distribution functions, band gap, and tail states) compared to the Si64H8 sample. Finally the vibrational density of states is calculated and compared to inelastic neutron-scattering measurements.

Structural properties of amorphous silicon prepared from hydrogen-diluted silane

Abstract: A systematic structural analysis was carried out on amorphous silicon films prepared from hydrogen-diluted silane using plasma-enhanced chemical vapor deposition. Hydrogen dilution of silane during the growth of a-Si:H absorber layers is used to suppress light-induced degradation of a-Si:H solar cells. Transmission electron microscopy (TEM) shows that for higher hydrogen dilution ratios the growth of films becomes strongly inhomogeneous and the transition from amorphous to microcrystalline phase occurs at a smaller thickness. A detailed X-ray diffraction (XRD) analysis of the amorphous films reveals that the strongest peak in the XRD patterns is located around 27.5° and corresponds to the signal from the ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum of this peak narrows from 5.1° to 4.8° as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The presence of silicon ...

Photonic Crystal Back Reflector in Thin-film Silicon Solar Cells

Abstract: One-dimensional photonic crystals having desired broad region of high reflectance ( R ) were fabricated by alternating the deposition of amorphous silicon and amorphous silicon nitride layers. The effect of the deposition temperature and angle of incidence on the optical properties of photonic crystals deposited on glass substrate was determined and an excellent matching was found with the simulated results. The broad region of high R of photonic crystals deposited on flat and textured ZnO:Al substrates decreases when compared to the R of photonic crystals de-posited on glass. The performance of amorphous silicon solar cells with 1-D photonic crystals integrated as the back reflector was evaluated. The external quantum efficiency measurement demonstrated that the solar cells with the photonic crystals back reflector had an enhanced re-sponse in the long wavelength region (above 550 nm) compared to the cells with the Ag reflector.

Application of Photonic Crystals as Back Reflectors in Thin-Film Silicon Solar Cells

Abstract: University of Ljubljana, Faculty of Electrical Engineering, Trzaska 25, SI-1000 Ljubljana, Slovenia ABSTRACT: Light trapping techniques are essential for increasing efficiency of thin-film silicon solar cells. Scattering at textured interfaces, introduced by surface-textured substrates, and highly reflective back reflectors have led to important improvements in solar cells performance. Beside metal, novel dielectric back reflectors are nowadays of great interest. One-dimensional photonic crystals in the role of distributed Bragg reflector can exhibit high reflectance in a broad wavelength range. Low temperature amorphous silicon/amorphous silicon nitride based one-dimensional photonic crystals eligible to act as dielectric back reflector in thin-film amorphous silicon solar cells have been developed. The conformal growth and optical properties of our photonic crystals on flat and on corrugated substrates (both randomly-textured and periodically-textured) are reported. Using one-dimensional photonic crystals as back reflectors we have obtained higher long-wavelength quantum efficiency of the solar cells with respect to the cells with metal back reflectors. Keywords: photon management, photonic crystal back reflector, surface texture, diffraction gratings, thin-film silicon solar cells 1 INTRODUCTION Thin-film solar cells present an important alternative to crystalline silicon photovoltaic technology. Due to the small thickness, the low deposition temperature and the use of advanced light management techniques, they offer attractive price/performance ratio, which results in one-fifth of the photovoltaic trade share [1]. Electrical and optical properties of the cells should be further improved, since there is still a lot of potential to increase their conversion efficiency in terms of band-gap engineering and light management [2]. By using surface-textured transparent conductive oxide (TCO) substrates [3, 4, 5], which enable efficient light scattering, and highly reflective metallic back contacts [6], light trapping in the absorber layers can be established. The excellent conductivity and the good reflectance of metal back reflectors are mediated by some disadvantage which constraints the cell performance. Textured silver reflectors suffer from parasitic absorption due to surface plasmon effects, which can reduce the reflectivity properties of the reflector and representing an important source of loss in the solar cell [7]. Furthermore, silver is an expensive material for manufacturing processes and it is sensible to moisture in the environment. In order to avoid these drawbacks, novel back reflectors based on dielectric materials such as white paints [8] and photonic crystal structures [9] are of interest. The implementation of a photonic crystal (PC) structure in the role of a Distributed Bragg reflector (DBR) as a back reflector in thin-film silicon solar cells has been already investigated [10, 11]. Recently we have successfully fabricated one-dimensional (1-D) photonic crystals deposited at a temperature consistent with the deposition of amorphous silicon layers [12]. Formed by alternating pair of amorphous silicon/amorphous silicon nitride, these photonic crystals can act like an ideal DBR in a broad long-wavelength range. The thickness, the optical constants of the individual layers, and the number of repeated layers of the desired photonic crystals can be designed by means of optical simulations [13]. In this paper we focus on the application of our low temperature a-Si:H/a-SiNx:H based 1-D PC deposited on periodic and random surface textures in order to combine the light scattering promoted by corrugated surfaces with the high reflective properties of the photonic crystal. To verify the conformal growth of our 1-D PC we have applied the spatial-frequency surface representation determined from atomic force microscopy (AFM) scans and to characterize the reflectivity properties spectral reflectance measurements were carried out. Amorphous silicon single-junction solar cells with PC-based back reflectors were fabricated. External quantum efficiencies of the cells are presented. We demonstrate the potential of 1-D photonic crystals as back reflectors for enhancing the absorption in the absorber layer with respect to metal contacts. We also show the performance of solar cells when photonic crystal is deposited on textured surfaces, both periodic and random. Using 1-D PC we have obtained higher long-wavelength quantum efficiency with respect to metal back reflectors. 2 EXPERIMENTAL The 1-D PC that was designed for the back reflector in a-Si:H solar cell consists of three pairs of alternating a-Si:H/a-SiN

Structural Properties of a-Si:H Films with Improved Stability against Light Induced Degradation

Abstract: X-ray diffraction (XRD) analysis of thin silicon films was carried out using both the symmetric Bragg-Brentano and asymmetric thin-film attachment geometries. The asymmetric configuration allows quantitative phase analysis of the films and reveals that amorphous silicon films deposited from silane diluted with hydrogen have the strongest peak in the XRD patterns located around 27.5 degrees. This peak corresponds to the signal from ordered domains of tetragonal silicon hydride and not from cubic silicon crystallites. The full width at half maximum (FWHM) of this peak narrows from 5.1 to 4.8 degrees as the ratio of hydrogen to silane flow (R) increases to 20 and does not change significantly for higher hydrogen dilutions. The amorphous silicon films fabricated at different hydrogen dilution were applied as absorber layers in single-junction solar cells. Degradation experiments confirm a substantial reduction of the degradation when the dilution ratio is increased from R=0 to R=20. The light induced degradation of solar cells with absorber layers prepared at R > 20 is not further reduced by increasing R.

Amorphous semiconductors studied by first-principles simulations: structure and electronic properties

Abstract: Atomistic models of amorphous solids enable us to study properties that are difficult to address with experimental methods. We present a study of two amorphous semiconductors with a great technological importance, namely a- Si:H and a-SiN:H. We use first-principles density functional theory (DFT), i.e. the interatomic forces are derived from basic quantum mechanics, as only that provides accurate interactions between the atoms for a wide range of chemical environments (e.g. brought about by composition changes). This type of precision is necessary for obtaining the correct short range order. Our amorphous samples are prepared by a cooling from liquid approach. As DFT calculations are very demanding, typically only short simulations can be carried out. Therefore most studies suffer from a substantial amount of defects, making them less useful for modeling purposes. We varied the cooling rate during the thermalization process and found it has a considerable impact on the quality of the resulting structure. A rate of 0.02 K/fs proves to be sufficient to prepare realistic samples with low defect concentrations. To our knowledge these are the first calculations that are entirely based on first-principles and at the same time are able to produce defect-free samples. Because of the high computational load also the size of the systems has to remain modest. The samples of a-Si:H and a-SiN:H contain 72 and 110 atoms, respectively. To examine the convergence with cells size, we utilize a large cell of a-Si:H with a total of 243 atoms. As we obtain essentially the same structure as with the smaller sample, we conclude that the use of smaller cells is justified. Although creating structures without any defects is important, on the other hand a small number of defects can give valuable information about the structure and electronic properties of defects in a-Si:H and a-SiN:H. In our samples we observe the presence of both the dangling bond (undercoordinated atom) and the floating bond (over-coordinated atom). We relate structural defects to electronic defect states within the band gap. In a-SiN:H the silicon-silicon bonds induce states at the valence and conduction band edges, thus decreasing the band gap energy. This finding is in agreement with measurements of the optical band gap, where increasing the nitrogen content increases the band gap.