Search

A growing global population coupled with rising industrialization has led to an ever increasing energy demand, which in turn forced us to turn to fossils without sufficient consideration of the consequences. The most serious consequence, as is evident today, is the increase in CO2 levels responsible for adverse climate change, which compelled the United Nations to develop the Sustainable Energy Goal 13 (SDG-13), calling for immediate action to combat climate change and its effects. On the other hand, the increase in population has also resulted in a tremendous increase in waste generation, necessitating the need for safe and sustainable waste treatment. Over recent years, research has been concentrating on advanced thermal plasma technology to valorize diverse wastes into useful energy. The generated syngas can be utilized for energetic (such as heat and electricity) and chemical (such as methanol and dimethyl ether) end applications. The current investigation analyzes the viability of the thermal plasma system (a water-stabilized DC plasma torch) for the pyrolysis, partial pyrolysis, and gasification of refuse-derived fuel (RDF) coupled with the implementation of the CCUS concept using waste CO2 as a gasi-fying agent (which is planned to be separated from flue gas coming from waste combustion) and converting it into CO in the syngas through plasma valorization of RDF. A comparative evaluation of the attributes, impurities, and yield of syngas and the generation of solids (ash and char) is carried out along with the analysis of per-formance parameters for four different cases (with variable inputs of CO2, namely 173, 62, 26 and 0 L/m with gasification ratio ranging from 1.04 to 0.4). The syngas constituents are compared to the equilibrium compo-sitions deduced by developing an equilibrium model in Aspen Plus (V 11.0) thermodynamic environment and are found to be close to the theoretical compositions. The highest gas yield is found for Case-I with 3.44 m3/kg due to the maximum carbon conversion, while the minimum gas yield is assessed as 1.60 m3/kg for Case-IV with high char production. Furthermore, H2 production was found to be almost the same for all cases (1.06-1.12 m3/kg of fuel). Furthermore, energetic efficiencies for all the four cases (63 %, 60 %, 62 % and 58 %) clearly reveal an encouraging energy balance. The theoretical maximum values of contaminants such as NH3, H2S, HCl, HF and HBr are calculated and a design of a syngas cleaning system is proposed. The promising results of the study can be considered a small but significant step towards SDG-13.

Teoretické modely naznačují, že při depozici vrstev pomocí reaktivního HiPIMS lze dosáhnout vyšší depoziční rychlosti v porovnání s pulzním magnetronovým naprašováním s vyššími opakovacími frekvencemi (Mf-PDCMS, mid-frequency pulsed dc magnetron sputtering) díky nižšímu stupni otrávení terče. Pro potvrzení těchto teoretických výsledků byly provedeny série experimentů v laboratorním depozičním systému s různými terčovými materiály (Al, Cr, Ti, Zr, Hf, Ta, Nb). Dva terče stejného materiálu byly rozprašovány jak pomocí duálního Mf-PDCMS, tak pomocí duálního HiPIMS výboje při konstantním výkonu. Potvrzuje se, že vyšší depoziční rychlosti v oxidovém módu lze dosáhnout u HiPIMS depozice Ta2O5 a Nb2O5 vrstev. Experimentální výsledky jsou podpořeny Monte-Carlo simulacemi v programech SRIM a SDTrimSP. Theoretical models indicate that reactive HiPIMS can deliver higher deposition rates compared to the mid-frequency pulsed dc magnetron sputtering (Mf-PDCMS) due to a lower degree of target poisoning. To confirm these theoretical results, series of experiments were performed in laboratory deposition system with different target materials (Al, Cr, Ti, Zr, Hf, Ta, Nb). Two targets of the same material were employed both in dual Mf-PDCMS and in dual HiPIMS conditions at a fixed power. It is confirmed that higher values of the deposition rate for Ta2O5 and Nb2O5 films can be achieved by HiPIMS when operated in the poisoned regime. The experimental results are supported by Monte-Carlo simulations performed in SRIM and SDTrimSP simulation software.

The influence of working pressure and inter-electrode gap distance on the quality of microcrystalline silicon prepared by PECVD at 40 MHz in three different KAI™ reactors designs with inter-electrode gaps of 28 mm, 16 mm and 7 mm has been investigated. The microcrystalline devices were implemented and studied as bottom cells in MICROMORPH™ tandem modules or processed with an optical filter to simulate the absorption of a top cell on a module size of 1.43 m 2 . Increasing the working pressure from 250 Pa to 2000 Pa resulted in an improvement of the efficiency of 1% absolute due to improvement in the open circuit voltage V oc and the fill factor FF . The influence of the deposition rate on the quality of the intrinsic absorber layers has been investigated and optimized. An unconventional crystallinity profile throughout the intrinsic absorber layer has been developed to further improve either the current density J sc or V oc and FF . By controlling and tuning the Raman crystallinity a very high open circuit voltage V oc of >1.42 V in a tandem cell design in full size modules (1.43 m 2 ) could be realized.

Increased electrical power generated from a thin-film silicon (TF-Si) photovoltaic device can lead to a reduced cost of electricity production that will support the mass adoption of this technology as a renewable energy source. Extracting the highest conversion efficiency from ‘champion’ large area TF-Si modules has been the focus of development at TEL Solar AG, Trubbach. The layer deposition process adjustments and further module technology improvements that led to a significant increase in the absolute stabilized module conversion efficiency of large area (1.43 m 2 ) tandem MICROMORPH™ modules centered first on obtaining high quality amorphous TF-Si deposited materials for the top cell. This was integrated with microcrystalline TF-Si material for the bottom cell that was deposited under conditions close to the transition point between the amorphous and microcrystalline growth regimes. In an optimized solar cell design the TF-Si materials were then combined with effective light management technologies and an improved module layout. The end result of a world record large area (1.43 m 2 ) stabilized module conversion efficiency of 12.34% was certified by the European Solar Test Installation (ESTI). The main technology contributions in the device design for this breakthrough result that generated more than 13.2% stabilized efficiency from each equivalent 1 cm 2 of the active area of the full module are described.

Friction and wear resistance are important factors in almost all branches of industry, as they affect lifetime, power consumption and other properties of all mechanical tools and machines. Diamond-like carbon (DLC) and derived metal-carbon composite (WC:H) layers are known to substantially improve both of these factors and frequently find their use in tribological applications. One of the desired properties of the tribological layers is a smooth surface. However, microscopic analysis of the WC:H layers shows a frequent occurrence of a certain type of structural defects that have major influence on the layer roughness. The defects are conically shaped objects penetrating the layer all the way down to the substrate, being covered with a spherical cap on the top of the layer. The lateral dimensions of the defects vary from several hundreds of nanometers to a few micrometers. To study these defects we prepared several layers at various conditions and analysed them by different methods, such as SEM, FIB, EDX, AFM and Raman spectroscopy. We will discuss the structure of the defects and influence of the preparation conditions on their growth to find the way to control their occurence in the layer.

Mass-adoption of thin-film silicon (TF-Si) photovoltaic modules as a renewable energy source can be viable if the cost of electricity production from the module is competitive with conventional energy solutions. Increased module performance (electrical power generated) is an approach to reduce this cost. Progress towards higher conversion efficiencies for ‘champion’ large area modules paves the way for mass-production module performance to follow. At TEL Solar AG, Trubbach, Switzerland, significant progress in the absolute stabilized module conversion efficiency has been achieved through optimized solar cell design that integrates high-quality amorphous and microcrystalline TF-Si-deposited materials with efficient light management and transparent conductive oxide layers in a tandem MICROMORPH ™ module. This letter reports a world record large area (1.43m 2 ) stabilized module conversion efficiency of 12.34% certified by the European Solar Test Installation; an increase of more than 1.4% absolute compared with the previous record for a TF-Si triple junction large area module. This breakthrough result generates more than 13.2% stabilized efficiency from each equivalent 1cm 2 of the active area of the full module. Copyright © 2015 John Wiley & Sons, Ltd.

Columnar growth was observed in the amorphous part of mixed phase layers deposited at very low substrate temperatures. The width of the columns and the layer thickness at which they are first distinguishable in a cross-sectional transmission electron microscope (X-TEM) image, about 120 nm, is similar for the substrate temperature range of 40-100 °C, but the columns are less well developed when either the substrate temperature is increased or the dilution ratio is lowered. This growth behaviour and the incubation layer are attributed to hydrogen-induced surface diffusion of growth precursors resulting in an amorphous-amorphous roughness transition.

A 15-nm lithium fluoride (LiF) thin film evaporated on glass substrate is shown to enhance the nucleation of microcrystalline Si grown by plasma enhanced chemical vapour deposition at the amorphous/microcrystalline boundary conditions. The effect is more pronounced at low substrate temperatures, nucleation density being 10 times higher at ∼ 80 °C. The effect is ascribed to the ionic chemical nature of LiF, the low work function material used in organic electronic devices, and we propose its use for micro patterning crystalline Si regions in otherwise amorphous Si film.

The authors report on the correlation of the oxygen content for three high growth-rate series of thin Si films crossing the boundary between amorphous and microcrystalline growth together with the evolution of the prefactor and the activation energy of the dark d.c. conductivity. The different roles of oxygen, such as doping, alloying or defect passivation, are discussed in the framework of the model of transport based on the formation of large grain boundaries with an increased band gap due to hydrogen and/or oxygen alloying.

Silicon thin films were grown by plasma enhanced chemical vapor deposition at high-pressure (700 Pa), high-power (4– W/cm2) depletion regime using multi-hole cathode. Series of samples were deposited by varying hydrogen/silane ratio or plasma power to study evolution of film structure and transport properties near a-Si:H/μc-Si:H transition. We suggest a simple “μc-Si:H layer quality factor” based on the ratio of subgap optical absorption α (1.4 eV)/α (1 eV) measured by constant photocurrent method. This ratio correlates well with the values of ambipolar diffusion lengths measured by surface photovoltage method perpendicularly to the substrate, i.e., in the direction of the collection of the photogenerated carriers in solar cells.

Four series of intrinsic thin Si films were prepared by plasma enhanced chemical vapor deposition at standard and high growth rate conditions. We suggest a simple ‘μc-Si:H layer quality factor’ based on the ratio of subgap optical absorption coefficient values: α (1.4 eV)/ α (1 eV). This ratio minimizes the light scattering effects for rough films and can serve as a reliable detection of the amorphous/microcrystalline structure transition and also as a figure of merit for the microcrystalline layer. The quality factor is evaluated for series of our samples with well known structure and also compared with samples from other laboratories with different deposition and measurement techniques.

We have proposed annealing in H 2 O vapor as a new effective and low-cost method for passivating polycrystalline silicon grain boundaries and for improving the performance of poly-Si based devices. The effect of H 2 O vapor treatment was experimentally found to differ from analogous anneal in nitrogen and hydrogen. Mechanism of the H 2 O vapor treatment was studied by Kelvin force microscopy, used to measure the potential change at individual grain boundaries and point defects. The potential change was dependent on the grain boundary character and it correlated with the crystalline disorder and internal stress observed by microscopic Raman spectroscopy. Effect of H 2 O vapor passivation was experimentally connected with the potential change at the grain boundaries before and after the treatment.

Preparation of thin film silicon at high growth rate is an important target for its application in solar cells. The properties of hydrogenated microcrystalline silicon, prepared with the help of PECVD multi-hole cathode in a high pressure and depletion regime in a wide range of thicknesses are described in detail. We illustrate the surprising result that we can prepare high growth rate microcrystalline silicon from 0.4 up to 30 μm thickness without great peel-off problems. The room temperature dark DC conductivity, as well as the crystallinity, increased up to 5 μm film thickness and then started to decrease again. These results are explained by the initial temperature profiling and a thickness-induced increase of the lateral inhomogeneity.

Lithographically prepared micrometer-sized permalloy ellipses were used to control the growth of nanocrystalline Si in otherwise amorphous Si film prepared by plasma enhanced chemical vapor deposition. Atomic force microscopy and micro-Raman spectroscopy were employed to study the surface structures before and after the deposition of the Si film. The possible applications of the controlled growth of nanocrystalline Si micro-patterns are discussed as well as the mechanisms leading to the growth of these patterns.

Surface morphology and roughness of amorphous spin-coated As–S–Se chalcogenide thin films were determined using atomic force microscopy. Prepared films were coated from butylamine solutions with thicknesses d ∼ 100 nm and then annealed in a vacuum furnace at 45 °C and 90 °C for 1 h for their stabilization. The root mean square surface roughness analysis of surfaces of as-deposited spin-coated As–S–Se films indicated a very smooth film surface (with R q values 0.42–0.45 ± 0.2 nm depending on composition). The nanoscale images of as-deposited films confirmed that surface of the films is created by domains with dimensions 20–40 nm, which corresponds to diameters of clusters found in solutions. The domain character of film surfaces gradually disappeared with increasing annealing temperature while the solvent was removed from the films. Middle-infrared transmission spectra recorded a decrease of intensities of vibration bands connected to N–H (at 3367 and 3292 cm −1 ) and C–H (at 2965, 2935 and 2880 cm −1 ) stretching vibrations. Temperature regions of solvent evaporation T = 60–90 °C and glass transformation temperatures T g = 135–150 °C of spin-coated As–S–Se thin films were determined using a modulated differential scanning calorimetry.

Thin Si films prepared by plasma enhanced chemical vapor deposition at low temperature and containing microcrystalline grains in amorphous tissue were studied by two complementary microscopy techniques. The conductive atomic force microscopy was performed in standard ambient conditions, whereas the presence of the surface oxide was overcome by more sensitive (pA) current detection. The cross-sectional transmission electron microscopy images of the amorphous phase revealed the columnar structure, which was successfully correlated with the bumpy surface detected by the atomic force microscope.

The effects of covering InAs quantum dot (QD) layer by In x Ga 1− x As thin strain-reducing layer (SRL) on QD properties were studied by photoluminescence (PL) and atomic force microscopy (AFM). Samples were grown by low-pressure metalorganic vapor phase epitaxy. The In content in SRL was changed in the range 0–29%. The red shift of room temperature PL maxima from 1.28 to 1.46 μm with increasing In content was observed. The energy difference between PL transitions of the ground state and the first excited state (Δ E SP ) was almost unchanged (∼85 meV) up to 23% of In in SRL. AFM results show an increase of QD height and no change in QD diameter with increasing In content in SRL. This can be the cause of the red shift of emission with increasing concentration of In in SRL and the conservation of Δ E SP .

InAs/GaAs quantum dots (QDs) in vertically correlated structures were studied by photoluminescence (PL) and atomic force microscopy (AFM). Samples were grown by low-pressure metalorganic vapor phase epitaxy (LP MOVPE). InAs QDs on the GaAs surface as well as GaAs hillocks covering the underlying QDs are elongated in the [−1 1 0] direction. This elongation is caused by higher lateral growth rates of both materials in the [−1 1 0] direction on the [1 0 0]-oriented substrates. The elongation and curvature of GaAs hillocks is probably the cause of the change of elongation direction of QDs in the upper layers of vertically stacked structures. With greater number of layers, QDs start to be circular. This is potentially a useful tool for controlling QD properties in laser structures.

Microcrystalline silicon (μc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PECVD) at 37 °C has been studied by cross-sectional TEM and ambient conductive AFM. We have succeeded in the combined measurement of topography and local conductivity under standard ambient conditions, overcoming the surface native oxide by more sensitive (pA range) current detection. We observed the columnar structure of the amorphous phase in the TEM micrograph and related it to the surface corrugation of the same size detected by AFM.

We present a complex characterization of thin silicon films, the growth of which has been influenced by permanent magnet, placed under the substrate. The pattern of microcrystalline regions in otherwise amorphous film varied with the orientation of the magnetic field. Study by atomic force microscopy and by micro-Raman spectroscopy revealed that the microcrystalline regions resulted from increased nucleation density of crystalline grains at the locations where the magnetron effect could be expected. This phenomenon allowed us to study in detail the transition between amorphous and microcrystalline growth. Moreover, it can be used as a kind of ‘magnetic lithography’ for preparation of predefined microcrystalline patterns in otherwise amorphous silicon films.

Hydrogen passivation technique is essential for improving the properties of polycrystalline silicon thin films. Elastic Recoil Detection Analysis (ERDA) indicated depth profiles of hydrogen concentration in poly-Si after hydrogen passivation. We have observed that plasma hydrogenation with duration up to 30 min effectively passivated the defects and improved photoluminescence intensity. Over 60 min of hydrogenation, PL intensity started to decrease. Raman scattering spectroscopy and X-ray rocking curve indicated that hydrogen created new defects and/or disorder with an increase in the hydrogen passivation time. Over 60 min, hydrogen started to form Si–H 2 bonding and hydrogen molecules (H 2 ), which lead to degradation of PL intensity. These peak positions were largely influenced by the grain size. These formations must be formed in quasi-stable sites, which are located at close to grain boundaries. Thus hydrogenation treatment may lead to defect passivation and new defects and/or disorder creation.

We have examined the common methods for determination of the crystallinity of mixed phase silicon thin films from the TO–LO phonon band in Raman spectra. Spectra are decomposed into contributions of amorphous and crystalline phase and empirical formulas are used to obtain crystallinity either from the integral intensities (peak areas) or from magnitudes (peak maxima). Crystallinity values obtained from Raman spectra excited by Ar+ laser green line (514.5 nm) for a special sample with a profile of structure from amorphous to fully microcrystalline were compared with surface crystallinity obtained independently from atomic force microscopy (AFM). Analysis of the Raman collection depth in material composed of grains with absorption depth 1000 nm in an amorphous matrix (absorption depth 100 nm), was used to explain reasons for systematic difference between surface and Raman crystallinities. Recommendations are given for obtaining consistent results.

Amorphous/microcrystalline transition was studied in the high growth-rate depositions of hydrogenated silicon films at a high pressure (700 Pa) in a depletion regime using a series of samples with the ratio of hydrogen to silane flows from 10 to 32. Results show the characteristic features of the amorphous/microcrystalline transition: abrupt change of dark conductivity and crystallinity accompanied by peaks of roughness and diffusion length, observed previously at standard growth rates.

A boundary between amorphous and microcrystalline growth of silicon thin films was explored for the study of grain growth by changing parameters of plasma enhanced deposition (substrate temperature, silane dilution or deposition duration and thus the film thickness). Resulting series of a-Si:H/μc-Si:H samples were characterized by values of dark conductivity and corresponding activation energy and prefactor, diffusion length, hydrogen content and morphology. The abrupt change of conductivity at the boundary is accompanied by a peak in surface roughness and diffusion length, which were connected to the formation of a percolating network of microcrystalline large grains. Results are discussed using the idea of barriers for electronic transport forming at the large grain boundaries. Comparison of transport properties with the hydrogen content pointed out the fundamental role of hydrogen present at the grain boundaries.

Polycrystalline silicon films for solar cells grown by atmospheric pressure chemical vapour deposition require hydrogenation to passivate defects at grain boundaries. Passivation by remote plasma hydrogenation of 12-μm-thick poly-Si films increased Hall effect carrier mobility from 3 to 20 cm2/Vs, photoluminescence intensity at 0.98 eV band more than 2 times and decreased contrast of local electronic conductivity between grains, observed by combined AFM. However, excessive hydrogenation led to surface damage and defect creation evidenced by widths of Raman LO–TO peak at 520 cm− 1 and X-ray rocking curve of (220) diffraction line. Depth profile by repeated etching of the surface and following the signature of Si–H2 and H2 bonding in Raman spectra showed that the damage extended up to 100 nm from the surface.

a so-called ‘mixed phase’. The performance of solar cell depends on this mixed phase structure. This is due to the amount of crystalline phases and the content of hydrogen and how it is bonded. They all influence the electronic transport properties. 10 Hydrogen is an essential element in both materials (polySi and µc-Si:H) for their transport properties. The content of hydrogen and the role of its presence are very important for improving photovoltaic properties. Elastic recoil detection analysis (ERDA), a technique that determines the content of hydrogen, was used in the present investigation.

Hydrogen passivation is essential for improving the properties of polycrystalline silicon thin films. We have observed that remote plasma hydrogenation with duration up to 30 min effectively passivated the defects and improved the Hall mobility, trap density and photoluminescence intensity. Over 60 min of hydrogenation caused the photoluminescence intensity to decrease. It seems that excessive hydrogenation not only passivated defects but also created new defects (Si–H2 bonds and hydrogen molecules) in the grains. Raman spectroscopy detected that hydrogen formed Si–H2 bonds in the poly-Si up to 100 nm from surface. Creation of these defects corresponded to a decrease of the photoluminescence intensity. These defects might be harmful to poly-Si-based devices.

A description of mixed phase hydrogenated microcrystalline silicon (μc-Si:H) geometry by a Voronoi network based on the growth in the grain neighborhoods is used as a basis for modelling electronic transport in μc-Si:H. The experimentally observed evolution of the grain shape during growth is used to discuss enhancement of the internal electric field at the grain tip. A possibility of light focussing by a spherical grain cap is suggested. Coincidence of the focussed photogeneration with the high electric field region could lead to solar cells with photogeneration of carriers spatially separated according to the photon energy.

Changes in the growth of thin silicon films at very low substrate temperatures (35 °C

Silicon thin films grown near the boundary between the amorphous/microcrystalline growth offer superior properties for industrial applications. Series of silicon samples, in which crossing of this transition region was achieved by changing a single technological parameter (dilution of silane in hydrogen, deposition temperature, sample thickness) were used to test our model of transport, connecting the macroscopically observed transport properties and the crystallinity, hydrogen content, grain size and grain boundaries. Microscopic study by AFM led to the formulation of the geometrical model of growth of mixed phase Si. The demand for research of microcrystalline or polycrystalline silicon prepared at low substrate temperatures is stimulated by the use of cheap plastic substrates. In addition to a direct deposition an alternative technology, such as metal-induced crystallization supported by the electric field is discussed. Possible future application of thin silicon films, for example in a “nanolithography”, is also shown. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A series of investigations were performed to study the influence of the substrate temperature on the structure and properties of silicon thin films. Substrate temperature was varied in the wide range of 35–200 °C. It has been shown that the films grown below 60 °C exhibit an unusual structural behavior. A sharp TO phonon peak at 520 cm −1 was detected in Raman spectra, which is associated with the crystalline structure. In contrast to these results, the same samples do not show any crystallite-related peak by X-ray diffraction and their optoelectronic properties (dark conductivity, activation energy and subgap absorption spectra) show amorphous features. A similar discrepancy was observed for a hydrogen dilution ratio ( r H =([SiH 4 ]+[H 2 ])/[SiH 4 ]) series of samples deposited at 60 °C. Hydrogen dilution ratio was varied from 25 to 170. It seems that at low substrate temperature a parameter window exists where the silicon thin films can be grown with the properties combining both crystalline and amorphous behavior.

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.

A way in which thin films of hydrogenated amorphous silicon (a-Si: H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si: H, the crystallization temperature is reduced by more than 350°C. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si: H films (hydrogen content between 20 and 45at.% H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 105Vcm−1. We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of...

We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C ) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm ) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.

Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) (L∥) and by surface photovoltage (SPV) (L⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.

Hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared by plasma enhanced chemical vapour deposition (PECVD) and transferred without breaking vacuum into an ultra high vacuum ( 10 −10 mbar ) atomic force microscope (AFM). The AFM is used to characterize surface morphology and electronic properties with high lateral resolution. This combined AFM/current measurement leads to a modification of the local electronic properties. This modification is detected as a significant decrease in local current compared to that in newly scanned regions. Kelvin probe microscopy shows that the contact potential difference is reduced by 0.25 eV in the area with decreased conductivity. This area with decreased conductivity remains on the surface for at least several hours. When the cantilever is held at a particular point on the surface, the current exhibits a long-term decay ( ≈10 min ). It is suggested that may be due to a local change of a gap state distribution.

We have used plasma enhanced chemical vapor deposition (PECVD) to deposit silicon thin films (∼0.2–1 μm) with different crystallinity fractions on Nanosensors PtIr5 coated atomic force microscopy (AFM) cantilevers (450 × 50 × 2 μm 3 ). Microscopic measurements of Raman scattering were used to map both internal stress and extrinsic stress induced in the films by bending the cantilevers using a nanomanipulator (Kleindiek Nanotechnik MM3A). Thanks to the excellent elasticity of the cantilevers, the films could be bent to curvature radii down to 300 μm. We observed the stress induced shift of the TO–LO phonon Raman band of more than 3 cm −1 for fully microcrystalline film corresponding to the stress ∼0.8 GPa. The shift of the similar film with amorphous structure was ∼2.5 cm −1 .

Thin intrinsic silicon films containing microcrystalline grains embedded in amorphous tissue were studied by two complementary microscopy techniques. The conductive atomic force microscopy was performed in standard ambient conditions with very sensitive (pA) current detection. The cross-sectional transmission electron microscopy images of the amorphous phase revealed the columnar structure, which was attributed to the bumpy structures on the surface.

Thin hetero-junction composite films of polymer (electron donor) and fullerene (electron acceptor) are prepared on indium-tin-oxide coated glass by spin-coating from solution in dichlorobenzene. Optimized atomic force microscopy (AFM) parameters allowed us to scan these soft composite films in contact mode and to measure their local conductivity with high lateral resolution by current-sensing AFM. The morphology and local conductivity data are correlated with Kelvin force microscopy and micro-Raman mapping and discussed with view to their photovoltaic properties. Regions with both compounds present are compared to areas where the components segregated, acting as shunts of the junction. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Hydrogenated amorphous silicon (a-Si:H) layers are prepared by plasma-enhanced chemical vapor deposition on metallized glass substrates. Ambient atomic force microscopy (AFM) is employed for both modification and characterization of a-Si:H films. Voltage pulses of up to 35 V are applied as a cantilever scans the amorphous silicon surface in contact mode AFM. Subsequent detection by Kelvin probe microscopy reveals a persistent negative charge stored in the a-Si:H layers. The stored charge is always negative independent of voltage polarity and results in an upward shift of the Fermi level by as much as 0.1 eV. Only at higher negative voltages (

Preparation of amorphous silicon/microcrystalline silicon superlattices allowed us a systematic study of transition from isotropic amorphous silicon to microcrystalline silicon with anisotropic (columnar) microstructure. The fact that just a few nm of amorphous interlayers are sufficient to interrupt columnar growth of crystallites is reflected in a clearly demonstrated isotropy of transport properties of all superlattice samples. Values of dark conductivity and diffusion length as well as grain size vary with changing crystallinity and so we can tailor the properties of the resulting material by adjusting thicknesses of amorphous and microcrystalline layers repeated to achieve a total desired thickness. Properly selected design of superlattice can lead to transport properties more suitable for solar cells than with pure microcrystalline silicon.

A series of silicon thin films was made by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at substrate temperatures below 100 °C at different hydrogen to silane dilution ratios. The electronic properties of these layers were studied as a function of the surface crystalline fraction as determined accurately from a combination of microscope images at different length scales (gathered by using different types of microscopes). The results show that the electrical conductivity increases monotonously as a function of crystalline surface coverage and no discontinuity is observed at the percolation threshold. An increase in conductivity of four orders of magnitude for layers with a high crystalline content is observed after annealing at temperatures up to 170 °C. Combined with the information that oxygen is incorporated at Si-H surface bond sites, this suggests that doping of the intergrain boundaries by oxygen might be dominantly responsible for the electronic properties of mixed phase silicon.

Silicon thin films were grown by PECVD at high-pressure (700 Pa), high-power (4-8 W/cm2) depletion regime using a multi-hole powered electrode. Series of samples were deposited varying hydrogen/silane ratio, plasma power and film thickness to study evolution of film structure and transport properties around a-Si:H/muc-Si:H transition. We suggest a simple figure-of-merit for the films based on alpha(1eV)/alpha(1.4eV) ratio from CPM, which correlates well with the values of ambipolar diffusion length measured by surface photovoltage method (SPV) perpendicular to the substrate, i.e., the direction of the photogenerated carriers collection in solar cells

Layers of densely packed luminescent Si nanocrystals embedded in fused silica act as wavelength-specific planar waveguides that filter the wide-band spontaneous emission. The waveguides’ light output consists of two spectrally narrow (∼10 nm), orthogonally polarized, and spatially directed bands. This effect is shown to result from leaky modes of the lossy waveguides. The results have general applicability to lossy, asymmetric waveguides and show the way to produce spectrally narrow emission without the use of optical cavities.

A combined cross-sectional transmission electron microscope (XTEM) and atomic force microscope (AFM) study of a hydrogen to silane dilution series of thin silicon films deposited by very high frequency plasma enhanced chemical vapor deposition at a substrate temperature that is low enough (39°C) to neglect the role of the surface diffusion in the growth process is reported. XTEM images of a mixed amorphous/microcrystalline layer reveal a structure of isolated conically shaped crystalline conglomerates (surface diameter ∼570±75nm) embedded in an amorphous phase of columns with diameters of ∼51±3nm. Detailed closeups of these crystallites, combined with AFM images of the hydrogen dilution dependent evolution of the surface, reveal similarities between the nucleation of amorphous and crystalline columnar structures at this low substrate temperature.

Silicon thin films were grown near the microcrystalline/amorphous boundary at substrate temperatures TS = 35–200°C and dilutions [H2]/[SiH4] = 25–167. The conductivity percolation threshold occurred at crystallinity ~60%, above the random composite threshold 33.3%, due to amorphous tissue coating crystalline grains and limiting the electronic transport. Higher content of hydrogen at lower TS facilitates formation of ordered silicon phase even close to room temperature (the sample grown at 35°C had 30 at% of hydrogen and crystallinity ~60%), providing a technological window for deposition of silicon thin films on cheap polymer substrates with electronic properties suitable for solar cells.

Charge transport in hydrogenated microcrystalline silicon (µc-Si:H) is determined by structure on several size scales: i) local atomic arrangement (