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We present optoelectronic properties of liquid phase crystallised silicon (LPC Si) thin-film solar cells on abrade-textured glass. On the textured glass-Si interface, LPC Si film exhibits a large broadband absorption enhancement, compared to a planar reference. However, LPC Si solar cells on the textured glass demonstrate a significant electrical degradation. External quantum efficiency (EQE) for the cell on the textured glass is lower throughout the 300–1100 nm wavelength region, compared to the planar reference. Open-circuit voltage (Voc) and short-circuit current density (Jsc) for the cells on the textured glass also show lower performance. This is believed to be due to higher effective area of poorly-passivated glass-Si interface, lower bulk carrier diffusion length in the absorber material and shunting through cracks and pinholes in the LPC films on the textured glass.

Liquid phase crystallized poly-Si thin-film solar cells on glass with a-Si heterojunction emitters were irradiated with 68 MeV protons with different fluences up to 10 13 protons/cm 2 . The degradation of devices with n-type and p-type absorber has been compared. A significantly stronger decrease of the solar cell performance parameters for devices with n-type absorber has been observed as compared to the p-type absorber case. This result corresponds to the different decrease of the minority carrier diffusion lengths with regard to the different poly-Si absorber materials.

This work reports a laser firing technique applied to completed silicon heterojunction interdigitated back contact solar cells in order to lower contact resistance. Previously, the implementation of a-Si:H(i) at the electron contact of polycrystalline silicon solar cells on glass substrates led to an increase in series resistance. The cell architecture with the current record efficiency of 14.2% (with illumination through glass) utilizes only an a-Si:H(n+) layer and 2–2.9 mA cm−2 of short circuit current density is lost due to electrical shading under the electron contact [1,2]. The goal of implementing an a-Si:H(i) layer and laser firing at this contact is to achieve low contact resistance at fired spots while preserving a-Si:H(i) passivation in unfired regions. After the laser firing, VOC was retained, while up to 14% absolute increase in FF was obtained with a mere 0.2 mA cm−2 loss in JSC. In the best performing cell, a 72.1% FF was achieved with a 0.7 mA cm−2 loss in JSC. Two laser sources were used to first ablate a part of the silver contact metal, and then to laser fire through the Si(n)/a-Si:H(i/n+)/ITO/Ag contact. The optimal laser fluence was found to be 1.1–0.5 J cm−2 (355 nm, picosecond pulse duration) and 4.4–5.2 J cm−2 (532 nm, nanosecond pulse duration), respectively. The upper limit on specific contact resistance in the laser fired spots was calculated to be 38 ± 20 mΩ cm 2 as a conservative estimate.

Liquid-phase crystallization (LPC) has proven to be a suitable method to grow large-grained silicon films on commercially well-available glass substrates. Zone-melting crystallization with high-energy-density line sources such as lasers or electron beams enabled polycrystalline grain growth with wafer equivalent morphology. However, the electronic quality is strongly affected by the material used as the interlayer between the glass and the silicon absorber. Open-circuit voltages above 630 mV, and efficiencies up to 11.8% were demonstrated using n-type absorbers on a sputtered interlayer comprising a triple stack of SiO2/SiN $_{x}$ /SiO2. In this study, we present our results to further improve the device performance by investigating the influence of the interlayer on the open-circuit voltage of the devices and characterize the properties of the absorber and interface using bias light-dependent quantum efficiency data and transmission electron microscopy (TEM) images. Finally, we investigate the applicability of aluminum oxide (Al2 O3) for passivation of p-type LPC absorbers.

Liquid phase crystallization of silicon is a promising technology to grow crystalline silicon thin films on glass. It has already been demonstrated that open circuit voltages of up to 656 mV and efficiencies of up to 11.8% can be achieved by this technique. Nevertheless further improvements are required to become competitive with wafer based silicon solar cells. A possibility to improve the quality is to enlarge the grain size and to control the crystallographic orientation of the resulting layers. While a preferential {100} surface orientation can be triggered utilizing suitable crystallization parameters, the in-plane orientation still remains random. Also the grain size stays within the same range. By introducing a local monocrystalline seed at the beginning of the crystallization process, we are able to control both the surface and the in-plane orientation of the silicon films. In addition the grain size is significantly increased in scanning direction and only limited by the substrate size of 5 cm. This high morphological quality is accompanied by an improved electrical quality confirmed by photoluminescence imaging and Hall measurements. This is a big step towards the final goal to directly grow monocrystalline silicon thin films on glass substrates.

We report here on the development of a rear-side point-contact scheme for liquid-phase-crystallized silicon on glass solar cells, which uses laser for all structuring and contact-post-treatment steps. Our contact scheme is based on the procedure developed by CSG Solar and contains two main innovations. First, we demonstrate here that it is possible to form contact holes in the back-reflector resist layer by laser ablation instead of printing droplets of an etch solution with inkjet. The use of laser for forming holes in the resist holds promise for enhancing the precision and reliability of this process. Second, we show that for both p and n-type absorbers, laser firing at the absorber-point-contacts can be used to increase the doping concentration beneath the metal contact and thereby achieve low-resistance contacts. Using the All-by-Laser Point-contact Scheme (ALPS) we were able to reach a solar cell conversion efficiency of 11.5% for a planar n-type laser-crystallized silicon absorber with an amorphous/crystalline heterojunction structure.

In a constantly evolving laser industry, new and innovative applications of laser radiation are emerging in scope as well as in numbers. While pulsed lineshaped laser beams are already well established as mainstream manufacturing tools in the production of flat panel displays, continuous wave line beam processes are still in their infancy. Nevertheless, they have shown great potential in research amp; development laboratories as well as in pilot plants for various thermal material processing applications

Liquid phase crystallization of 10 μm thin silicon layers on glass substrates was performed with a line-shaped continuous wave laser beam. The process window was investigated in terms of the scanning velocity of the laser, the pre-heating of the specimens and the applied laser intensity. We have identified the entire process window, in which large-scale crystallization without deformation or destruction of the substrate and cracking of the silicon layer can be obtained. The grain orientations of the resulting Si layers were analyzed using both electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). The influence of the critical crystallization parameters on the grain orientation of the silicon film was examined. EBSD and XRD measurements show that a preferential {100} surface texture and {100} and {101} orientations in scanning direction of the laser can be achieved if appropriate crystallization parameters are used. This texture formation is accompanied with a substantial decrease of high angle grain boundaries.

Liquid phase crystallization of silicon is a promising technology platform to grow multi crystalline silicon thin films on foreign substrates. For solar cell application it has already been demonstrated that open circuit voltages of up to 661 mV [1] and efficiencies of up to 15.9% [2] can be achieved on a silicon layer of a few microns only. However, while the quality of the material has been continuously improved, the cost factor of the utilized substrate has been given little attention. The present work focuses on the technology transfer from technical glass substrates to low cost soda-lime glass substrates to become more attractive for commercial applications. We demonstrate that despite a large difference in the expansion coefficient between soda lime glass and silicon absorber layer, adhesive silicon thin films can be produced under certain conditions. On these first layers, we were able to fabricate solar cells, which in their cell and material properties were only ~30% below the values of their technical glass counterparts.

Polycrystalline silicon thin-film solar cells produced by continuous-wave diode-laser crystallization at the University of New South Wales were recently reported to have reached a conversion efficiency above 10%. One drawback of these cells, however, was that they exhibited efficiency degradation within several hours after the cell fabrication was completed. In this work we show that by applying laser firing to the rear point contacts of the solar cells, it is possible to stabilize and even to enhance the performance of these devices. Our investigation indicates that it is the poor quality of the contact between the aluminum and the silicon absorber that causes the cell degradation and offers an elegant and industrial-compatible process to improve the cell performance. This is the first time that the laser firing process, initially developed for alloying an aluminum layer through a dielectric layer on crystalline silicon wafer solar cells, is being applied to polycrystalline silicon thin-film solar cells.

The influence of a subsequent ns laser-firing process on aluminium rear-side point contacts of laser-crystallized silicon thin-film solar cells was investigated by means of conductive probe atomic force microscopy. A significant increase in conductivity was observed in the laser-fired contact area. The spatial uniformity of this enhanced conductivity as well as changes in the aspect ratio of the induced pit allowed us to derive a suitable parameter window for firing a 100nm thin aluminium layer typically used in such thin film Si solar cells devices.

Backside contacted cells were fabricated on p- and n-type, liquid phase crystallized silicon on glass. The heterojunction emitter and transparent contacting layer were opened locally using an inkjet printer and the absorber was contacted with laser fired point contacts. An analytical resistance model was derived to determine the resistance losses for the point contact geometry. Using test structures and the model it was found that for p-type cells the main contribution to the cell resistance came from the absorber contact and the absorber. Both were strongly reduced by increasing the absorber doping. For n-type cells the main contributions originated from the emitter and emitter-TCO contact, which was improved by changing the emitter type, resulting in an 11.6 % solar cell.

Three different optical interaction techniques have been employed to characterise the electrical and material parameters of polycrystalline silicon (poly-Si) thin-film solar cells with an interdigitated mesa structure. First, Light Beam Induced Current (LBIC) in the infrared range was used to locally analyse the light collection properties. Second, electroluminescence in forward bias (EL) yielding information on band to band recombination was performed. Third, electroluminescence in reverse bias (ELR) was utilized to gain information on the intraband relaxation. The EL and ELR measurements were performed using cooled Si-CCD (Silicon-based Charge Coupled Device) and InGaAs (Indium Gallium Arsenide) detectors. The high resolution IR-LBIC measurement equipped with a 1,064 nm wavelength laser has been applied to investigate the grain boundary characteristics in the absorber layer. Additionally, the local electrical characteristics of the absorber layer (diffusion length, doping concentration and built-in potential) have been extracted by performing a bias-dependent IR-LBIC measurement based on a simple theoretical model with the assumption of relatively small diffusion length compared to the absorber layer thickness. The local/spatial distribution of the diffusion length in the absorber layer of the thin-film solar cell has been extracted. Furthermore, the temperature dependence of the photocurrent of thin-film solar cells in a temperature range of −25 to +70 °C has been locally investigated using IR-LBIC. Additionally, the temperature dependence of the reverse bias characteristics of the poly-Si thin-film solar cell is analysed and compared with that of monocrystalline Si solar cell. For the EL and ELR measurements a spectral analysis of the emitted light has been performed. From the EL results material properties like diffusion length and process induced defects have been deduced and insights on the quality of production processes like metallization and etching were gained. The complementary information from the ELR measurements provides access to additional types of defects resulting from generation centres, such as lattice disorder, crystal defects and charged coulomb centers.

Polycrystalline Si (poly-Si) thin-film solar cells on glass feature the potential to reach high single-junction efficiencies at low costs. However, the preparation is challenging because the process temperatures are limited by the glass to about 600 °C. There are several methods to prepare poly-Si films on glass. So far, the best poly-Si thin-film solar cells on glass have been prepared by solid phase crystallization (SPC) of amorphous Si (a-Si). In this paper, we give an overview on the formation of poly-Si films by both SPC and the aluminum-induced layer exchange (ALILE) process (which is based on aluminum-induced crystallization (AIC) of a-Si). Due to the fact that the utilization of ZnO:Al-coated glass is an attractive option for future poly-Si thin-film solar cells, we discuss the influence of an additional ZnO:Al layer on the formation of poly-Si films. Such an additional ZnO:Al layer leads to enhanced nucleation. Thus, the time necessary to form the poly-Si film (process time) and the resulting grain size of the poly-Si film are reduced.

The crystallization of high-rate electron beam evaporated amorphous silicon on ZnO:Al-coated glass and SiN-coated glass was investigated by applying ex-situ optical microscopy and isothermal annealing. The nucleation process in solar cell structures consisting of n+-Si(PECVD)/p-Si/p+-Si layers was analyzed and subsequently compared to the phase transition in p-Si and n+-Si(PECVD)/p-Si on SiN and ZnO:Al-coated glass. With a high steady-state nucleation rate and a low activation energy (EIss = 2.7 eV), the nucleation process of Si on ZnO:Al has shown to differ significantly from nucleation on SiN (EIss = 4.5 eV) in the investigated temperature regime of 560 °C to 600 °C. The experiments revealed that the n+-Si(PECVD) layer did not serve as a nucleation layer and did not influence the nucleation of p-Si on SiN. On ZnO:Al, however, the nucleation rate of n+-Si(PECVD)/p-Si was found to be smaller than the nucleation rate of a single p-Si layer. While the p+-Si promotes the nucleation on SiN, it did not affect the nucleation process of Si on ZnO:Al. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the experiment of this study, amorphous silicon (a-Si) films deposited by electronbeam evaporation method on (100) p+, (110) p+ and (111) p+ Si wafer substrates were crystallized at 600 °C by the solid phase epitaxy method. The times for generating the nuclei were different from one another depending on crystallized directions of the substrates, because the number of dangling bonds existing on the surface of the substrates and also the intervals of lattices are different from one another depending on the direction of crystallization. The crystallized Si film showed lots of defects. With increasing annealing time, the number of defects decreased. Also, it included more cracks. However, crystallization of the a-Si film on the (111) Si substrate was not epitaxial growth. The crystallized Si film was composed of crystal grains smaller than 1 μm.

The suitability of ZnO:Al thin films for polycrystalline silicon (poly-Si) thin-film solar cell fabrication was investigated. The electrical and optical properties of 700 -nm-thick ZnO:Al films on glass were analyzed after typical annealing steps occurring during poly-Si film preparation. If the ZnO:Al layer is covered by a 30 nm thin silicon film, the initial sheet resistance of ZnO:Al drops from 4.2 to 2.2 Ω after 22 h annealing at 600 °C and only slightly increases for a 200 s heat treatment at 900 °C. A thin-film solar cell concept consisting of poly-Si films on ZnO:Al coated glass is introduced. First solar cell results will be presented using absorber layers either prepared by solid-phase crystallization (SPC) or by direct deposition at 600 °C.

Poly-Si thin-film solar cells on glass feature the potential to reach single-junction efficiencies of 15% or even higher at low costs. In this paper innovative approaches are discussed, which could lead to substantial efficiency improvements and significant cost reductions: (i) preparation of large-grained poly-Si films using the ‘seed layer concept’ targeting at high material quality, (ii) utilization of ZnO:Al-coated glass enabling simple contacting and light-trapping schemes, (iii) utilization of high-rate electron-beam evaporation for the absorber deposition offering a high potential for cost reduction.

Hydrogen passivation (HP) of polycrystalline silicon (poly-Si) thin film solar cells was performed in a parallel plate radio-frequency (rf) plasma setup. The influence of hydrogen pressure p and electrode gap d on breakdown voltage Vbrk is presented showing that the minimum in Vbrk shifts with higher pressures towards higher p · d values. Cell test structures provided by CSG Solar AG were used to examine the influence of p and d on the open circuit voltage VOC. The highest VOC's were achieved for p · d values that correspond to a minimum in Vbrk. HP strongly improved the VOC. After the hydrogen plasma treatment the VOC improved significantly by a factor of 2 and amounted to 450 mV. Optimized parameters were then applied to different poly-Si solar cells prepared by electron beam evaporation.

In this report, we discuss the influence of rapid thermal annealing (RTA) on the performance of polycrystalline Si (poly-Si) thin-film solar cells on glass where the poly-Si layers are differently prepared. The first part presents a comprehensive study of RTA treatments on poly-Si thin-films made by solid phase crystallization (SPC) (standard material of CSG Solar AG, Thalheim). By varying both plateau temperature (up to 1050 °C) and duration (up to 1000 s) of the annealing profile, we determined the parameters for a maximum open-circuit voltage (VOC). In addition, we applied our standard plasma hydrogenation treatment in order to passivate the remaining intra-grain defects and grain boundaries by atomic hydrogen resulting in a further increase of VOC. We found, that the preceding RTA treatment increases the effect of hydrogenation already at comparable low RTA temperatures. The effect on hydrogenation increases significantly with RTA temperature. In a second step we investigated the effect of the RTA and hydrogenation on large-grained poly-Si films based on the epitaxial thickening of poly-Si seed layers.

Thin-film polycrystalline-silicon (pc-Si) solar cells with a grain size in the order of 1–100 μm could substantially lower the price of photovoltaic energy if sufficiently high efficiencies are obtained on low-cost foreign substrates. A promising approach is the epitaxial thickening of seed layers made by aluminum-induced crystallization (AIC) of amorphous silicon. A clear difference in performance was found for AIC based solar cells grown by low- and high-temperature epitaxy. A large intragrain defect density was observed for this type of pc-Si solar cells that probably limits the cell performance. This paper studies intragrain defect formation in AIC based pc-Si layers by the investigation of state of the art absorber layers grown both by low- and high-temperature epitaxy on seed layers made on various substrates. The samples were characterized by defect etching in combination with scanning electron microscopy (SEM) and by cross-section transmission electron microscopy (TEM). The AIC seed layers themselves were found to be major sources of intragrain defects. Moreover, AIC seed layers prepared under different conditions can lead to different intragrain defect densities.

Thin film polycrystalline silicon (poly-Si) films have been fabricated by the aluminium-induced layer exchange (ALILE) process on ZnO:Al coated glass. The formation of the poly-Si films was observed during the annealing process using an optical microscope. Poly-Si films formed on ZnO:Al coated glass consist of high-quality poly-Si material, as evidenced by Raman measurement. The average grain size of the poly-Si films slightly increases with decreasing annealing temperature. The formation of poly-Si films on ZnO:Al coated glass led to a preferential (001) orientation at all annealing temperatures (425 °C ~ 525 °C).

In this work, we present structural and electrical properties of thin Si films which are homoepitaxially grown at low substrate temperatures (Ts = 450–700 °C) by high-rate electron beam evaporation. As substrates, monocrystalline Si wafers with (100) and (111) orientations and polycrystalline Si (poly-Si) seed layers on glass were used. Applying Secco etching, films grown on Si(111) wafers exhibit a decreasing etch pit density with increasing Ts. The best structural quality of the films was obtained on Si(100) wafers. Defect etching on epitaxially grown poly-Si absorbers reveal regions with different crystalline quality. Solar cells have been prepared on both wafers and seed layers. Applying Rapid Thermal Annealing (RTA) and Hydrogen plasma passivation an open circuit voltage of 570 mV for wafer based and 346 mV for seed layer based solar cells have been reached.

In this paper we report on homoepitaxial growth of thin Si films at substrate temperatures T s = 500–650 °C under non-ultra-high vacuum conditions by using electron beam evaporation. Si films were grown at high deposition rates on monocrystalline Si wafers with (100), (110) and (111) orientations. The ultra-violet visible reflectance spectra of the films show a dependence on T s and on the substrate orientation. To determine the structural quality of the films in more detail Secco etch experiments were carried out. No etch pits were found on the films grown on (100) oriented wafers. However, on films grown on (110) and (111) oriented wafers different types of etch pits could be detected. Films were also grown on polycrystalline silicon (poly-Si) seed layers prepared by an Aluminum-Induced Crystallisation (AIC) process on glass substrates. Electron Backscattering Diffraction (EBSD) shows that the film growth proceeds epitaxially on the grains of the seed layer. But a considerably higher density of extended defects is revealed by Secco etch experiments.

The aluminium-induced layer-exchange process allows to grow thin large-grained polycrystalline Si films on foreign substrates. A characteristic feature of these films is the preferential (100) orientation of Si grains, favourable for subsequent epitaxial thickening at low temperatures. In this work, a model based on the preferential nucleation is proposed, which elucidates a possible origin of the preferential (100) orientation and its sensitivity to the preparation and process conditions. The probability of Si nuclei to have respective orientation is attributed to the nucleation barrier, i.e. the critical value of the change of the Gibbs energy during nucleation. The preferential orientation is formed statistically by the nuclei having the lowest nucleation barriers.

This work reports on the low temperature ( ⩽ 550 ∘ C ) epitaxial growth of Si films on polycrystalline seed layers made by aluminium-induced crystallisation (AIC) on glass. The seed layers were chemically mechanically polished before use and electron–cyclotron resonance chemical vapour deposition (ECRCVD) was used to deposit the Si films. The quality of the deposited films depends strongly on the addition of Ar to the hydrogen rich silane growth environment. With Ar, epitaxial growth has been achieved to a thickness of ∼ 2.7 μ m . Electron backscatter diffraction (EBSD) analysis shows a similar grain structure to the starting seed layers with a preferred (1 0 0) orientation and grain sizes approaching ∼ 10 μ m for samples up to ∼ 1.7 μ m thick. Thicker films ( 2.7 μ m ) show a more mixed orientation with an increase in the density of smaller grains. Nevertheless, more than 95% of the film surface can be indexed suggesting large grained crystal growth has occurred on all grain orientations present in the seed layer. Similar behaviour is also exhibited by the films grown without Ar but these films are more defective and the deterioration in the EBSD orientation maps occurs at a significantly lower thickness. The role of Ar in the growth process is briefly discussed.

The aluminium induced layer exchange process is used for growing crystalline Si films on amorphous substrates like glass, which are further epitaxially thickened and can be used for photovoltaic applications. In this work we investigated Al 2 O 3 membranes separating the Al and Si layer during the layer exchange by means of high-resolution transmission electron microscopy and studied the influence of the membranes' structure on crystallization of Si. We observed a phase transformation of the membrane from the amorphous state to γ-Al 2 O 3 at exchange temperatures at above 450 °C. At higher temperatures this transformations induces cracks into the thin membrane. We further discuss the influence of this phase transformation on the preferential orientation of the growing Si grains.

Liquid phase crystallization (LPC) has proven to be a suitable method to grow large-grained silicon films on commercially well available glass substrates. Zone-melting crystallization with high energy density line sources such as lasers or electron beams enabled polycrystalline grain growth with wafer equivalent morphology. A lot of effort was put into interlayer optimization by different groups. Open circuit voltages above 630 mV and efficiencies up to 11.8% were demonstrated using n-type absorbers on a sputtered interlayer between glass substrate and silicon absorber comprising a triple stack of SiO2/SiNx/SiO2. In this work we report on our results to further improve device performance by investigating the influence of the interlayer on the open circuit voltage of the devices and demonstrate first results achieved on an interdigitated back contacted silicon heterojunction solar cell to simplify device fabrication.

Large-grained polycrystalline silicon (poly-Si) films were prepared on glass using the 'seed layer concept' which is based on the epitaxial thickening of large-grained seed layers. The aluminium-induced layer exchange (ALILE) process was used to form p + -type seed layers on glass. The seed layers feature a preferential (100) orientation of the grain surface. Epitaxial growth of the p-type absorber layers was carried out at temperatures § 600°C by either electron cyclotron resonance chemical vapour deposition (ECRCVD) or electron-beam evaporation. Secco etching was used to analyse extended defects in the films epitaxially grown on ideal substrates (Si wafers). Opposed to ECRCVD, films grown with electron-beam evaporation show no etch pits when deposited on Si(100) wafers. Post-deposition treatments (defect annealing and defect passivation) were used to improve the quality of the absorber layers. Thin-film solar cells were prepared on both Si(100) wafers (ideal seed layer) and seed layer covered glass substrates by the deposition of an n + -type a-Si:H emitter and a TCO layer. The best solar cell results were

In this work, Si grain nucleation during the aluminium-induced layer-exchange (ALILE) process is studied theoretically. A combined kinetic and thermodynamic model is derived to describe the nucleation and initial growth of Si grains. In the framework of the model, the dependence of the radial distribution of grains and the silicon concentration in the Al are calculated as a function of annealing time. The suppression of Si grain nucleation and the transition to pure grain growth at a certain stage of the ALILE process is demonstrated in qualitative agreement with experimental results.

The formation of polycrystalline silicon (poly-Si) films by Al-induced crystallization (ALILE process) was studied in situ by optical microscopy. The characteristic feature of this process is that nucleation is strongly suppressed after an initial nucleation period. The use of cooling periods in the course of annealing leads to enhanced nucleation and reveals that the formation of Si depleted regions around the growing grains prevents further nucleation. The growth mechanism is discussed starting from the phase diagram of the Al/Si system. It turns out that the critical parameter is the actual concentration of Si in Al and the value of supersaturation.

We present an investigation of line defects in epitaxially grown silicon layers using Secco defect etching and transmission electron microscopy (TEM). 1 μ m thick layers were deposited onto Si (1 0 0) wafers at a substrate temperature of 560 ∘ C using electron cyclotron resonance chemical vapour deposition (ECRCVD). Defect etching reveals a variety of etch pits related to extended defects. A detailed analysis of the orientations and shapes of etch pits related to line defects is carried out. Using this information it is then possible to assign different types of etch pits to line defects observed by TEM. The investigations show, that one type of defect are extended dislocations parallel to 〈 1 1 2 〉 , while the direction of two other types are 〈 1 1 0 〉 as well as 〈 3 1 4 〉 , a direction uncommon for line defects in silicon.

In this work, the initial stage of the aluminium-induced layer-exchange (ALILE) process is studied theoretically. The diffusion of silicon atoms from an amorphous Si layer into an Al layer through a permeable membrane prior to Si grain nucleation within the Al layer is considered. A kinetic model is derived to describe this process. In the framework of this model, the evolution of the Si concentration within the Al layer prior to Si grain nucleation and the nucleation time are investigated for different annealing temperatures and preparation conditions of the membrane. From the comparison of results of calculations with experimental data, the pre-exponent factors and activation energies of diffusion coefficients of Si in membranes are determined.

Si films grown epitaxially by low-temperature electron–cyclotron resonance chemical-vapor deposition (ECRCVD) were investigated by analyzing extended defects in these films. Extended defects were made visible by Secco etching and scanning (SEM) and transmission (TEM) electron microscopy were used to analyze them in detail. Films grown on Si(1 0 0) wafers show different kinds of etch pits with characteristic shapes, sizes and crystallographic alignments. Their correlation to crystallographic defects is discussed. The typical total etch pit density for boron-doped samples grown at 580 °C on Si wafers is in the range of 10 8 cm −2 . Films grown on poly-Si seed layers on glass are strongly affected by Secco etch due to growth regions of different structural qualities and a high density of extended defects. Solar cell test structures were prepared from both types of samples and their results discussed in view to the density of extended defects.

In the aluminum-induced layer exchange process polycrystalline silicon thin films can be formed on foreign substrates at low temperatures. These films exhibit a preferential (1 0 0) crystal orientation, that depends on the annealing temperature. Further, the preferential orientation is sensitive to the nature of the Al oxide layer between the Si and the Al. A model based on preferential nucleation is presented which elucidates a possible origin of the preferential (1 0 0) orientation and its sensitivity to temperature and the aluminum oxide interlayer.

Annealing of amorphous silicon/aluminum layer stacks below the eutectic temperature leads to a layer exchange and concurrent crystallization of silicon. The resulting polycrystalline silicon layers are of great interest for large area thin film devices. This paper reports about the influence of annealing temperature profiles on the layer exchange process. It is shown that temperature profiles have pronounced influence on the nucleation process. The obtained results are helpful for a better understanding of the physics of the layer exchange process and for an improvement of the layer exchange process by allowing for a reduction of the process time with a simultaneous increase in grain size. It is shown in particular that the characteristic suppression of nucleation is caused by Si-depletion around the growing Si-grains.

This short review article gives an overview on the current status of polycrystalline silicon thin-film solar cells on glass and discusses the future perspectives of this technology.

An alternative approach is described for the fabrication of a polycrystalline silicon thin-film solar cell on inexpensive substrates. In a first step amorphous silicon is recrystallized in an aluminum-induced crystallization process forming a large-grained polycrystalline silicon layer on glass or metal substrates. In a second step this layer is used as a template for epitaxial growth of the absorber layer (2–3 μm thick) at T

The epitaxial thickening of polycrystalline Si films on glass substrates is of great interest for the realization of crystalline Si thin film solar cells and other large-area thin film devices. In this paper we report on the epitaxial growth of Si at temperatures below 600 ∘ C on polycrystalline seed layers using electron–cyclotron resonance chemical vapor deposition. The Si seed layers were prepared by aluminum-induced crystallization. The quality of the ECRCVD-grown films strongly depends on the orientation of the underlying seed layer grains. Due to a mainly favorable orientation of the seed layers more than 73% of the substrate area were epitaxially thickened. It turned out that a (1 0 0) preferential orientation is favorable for epitaxial thickening. This, however, is not the only requirement for successful low-temperature epitaxial growth of Si.

The kinetics of Al-induced crystallization of amorphous silicon (a-Si) are studied by variation of the thickness of an Al-oxide layer between the initial Al and a-Si films. The results show that the thickness of this Al-oxide layer strongly determines the nucleation time and the number of Si-grains (nucleation) and has only little influence on the growth velocity of the grains. The diameter of the grains increases in good approximation linearly with time and is strongly influenced by the annealing temperature. The activation energies for the growth velocity vg and the nucleation time tn were determined to be 1.8 and 1.9 eV, respectively. The general behavior suggests that the process kinetics of the aluminum-induced layer exchange are determined by the silicon diffusion across the interface which can be altered by the thickness of the oxide layer.

Thin polycrystalline silicon (poly-Si) layers can be formed by aluminium-induced crystallisation of amorphous silicon (a-Si). During annealing the initial glass/Al/a-Si layer structure is transformed into a glass/poly-Si/Al+Si structure. For the overall process the structure of the Al layer is very important. In this paper the effect of oxygen present during deposition of the Al-layer on process time and grain size is discussed. It was found that low oxygen flows ( f O 2 =1 sccm) during Al depositions cause a significant reduction in process time. However, the grain size of the resulting poly-Si films is not affected. Further increase of f O 2 does not result in further acceleration of the process. Instead of that the layer exchange becomes more and more incomplete and the resulting poly-Si films are discontinuous. Because of the considerable reduction in process time for samples with Al-layers deposited at f O 2 =1 sccm the annealing temperature could be lowered from 500 to 460 °C in order to obtain larger grains. The process was still completed within 90 min.

We studied the homo-epitaxial growth of Si absorber layers at temperatures of 420–560 °C by electron-cyclotron resonance chemical vapour deposition. We deposited the Si films at rates of 15 nm min −1 on (1 0 0)-, (3 1 1)-, (1 1 1)- and multicrystalline Si wafers. Both substrate temperature and orientation have a pronounced influence on the structural quality. On Si(1 0 0) substrates films grew epitaxially above 480 °C and are of excellent crystallographic quality up to thicknesses of 2.5 μm. On Si(3 1 1) the epitaxy breaks down rapidly by the formation of polycrystalline regions. On Si(1 1 1) the films are always fine polycrystalline. Using multicrystalline substrates the quality of the films is related to the orientation of the grains ranging from excellent epitaxy to pure fine polycrystalline growth. The electrical properties of the intentionally undoped films grown on Si(1 0 0) are discussed in relation to the crystal quality.

Polycrystalline silicon (poly-Si) films on glass were prepared by aluminium-induced crystallisation of amorphous silicon (a-Si). During an annealing step at temperatures well below the eutectic temperature of the Al/Si system ( T eu =577 °C) a glass/Al/a-Si stack is transformed into a glass/poly-Si/Al+Si stack. In this aluminium-induced layer exchange process the oxidation of the Al layer during the preparation of the initial glass/Al/a-Si stack plays an important role. Here we show that the aluminium oxide layer, which was formed during the preparation of the initial stack, is still present after the annealing step. The Al 2 O 3 layer has corundum structure. The thickness of this layer is laterally inhomogeneous. The nucleation of the poly-Si layer starts at the Al/Al 2 O 3 interface.

We investigated the aluminum-induced crystallization of amorphous silicon (a-Si) during the aluminum-induced layer exchange (ALILE) process, in which a stack of glass/Al/a-Si is transformed into a glass/polycrystalline silicon (poly-Si)/Al(Si) structure by an annealing step well below the eutectic temperature of the Al/Si system. Our experiments resulted in continuous large-grained poly-Si films on glass substrates. The nucleation and the growth of the crystalline phase during the ALILE process was observed using an optical microscope. We found an activation energy of 1.8 eV for the nucleation process and we related this energy to a large barrier at the a-Si/Al interface.

Previous studies have indicated that recombinant cellular prion protein (PrP c ), as well as a synthetic peptide of PrP c , affects intracellular calcium homeostasis. To analyze whether calcium homeostasis in neurons is also affected by a loss of PrP c , we performed microfluorometric calcium measurements on cultured cerebellar granule cells derived from prion protein-deficient (Prnp 0/0 ) mice. The resting concentration of intracellular free calcium ([Ca 2+ ] i ) was found to be slightly, but significantly, reduced in Prnp 0/0 mouse granule cell neurites. Moreover, we observed a highly significant reduction in the [Ca 2+ ] i increase after high potassium depolarization. Pharmacological studies further revealed that the L-type specific blocker nifedipine, which reduces the depolarization-induced [Ca 2+ ] i increase by 66% in wild-type granule cell somas, has no effect on [Ca 2+ ] i in Prnp 0/0 mouse granule cells. Patch-clamp measurements, however, did not reveal a reduced calcium influx through voltage-gated calcium channels in Prnp 0/0 mice. These data clearly indicate that loss of PrP c alters the intracellular calcium homeostasis of cultured cerebellar granule cells. There is no evidence, though, that this change is due to a direct alteration of voltage-gated calcium channels.