Crystalline Silicon Thin-film Solar Cells Research Articles

High-throughput zone-melting recrystallization for crystalline silicon thin-film solar cells

Crystalline silicon thin-film solar cells which shall exceed a conversion efficiency of 15% require a coarse-grained silicon layer of at least 5 μm thickness as an absorber layer, with minority carrier diffusion length and grain diameter at least twice the layer thickness. At Fraunhofer ISE, we use zone-melting recrystallization to recrystallize a silicon layer grown by chemical vapor deposition on high-temperature stable substrates. Our main focus is the development of tools and processes for achieving silicon layers of sufficient electrical quality at high throughput and low cost. A processor for in-line zone melting of 400-mm-wide samples was developed, which has gas curtain load locks and thus offers a theoretical maximum throughput of approx. 10 m 2/h at a melting velocity of 500 mm/min. Zone-melting experiments with varying melting velocity showed that high crystal quality can be maintained even for velocities up to 350 mm/min.

Progress in epitaxial deposition on low-cost substrates for thin-film crystalline silicon solar cells at IMEC

In recent years, research on epitaxial growth for photovoltaics became more important due to the increasing interest in thin-film silicon solar cells. Two significant challenges need to be resolved before this technique can become a competitive industrial alternative to the current dominating technology of bulk silicon solar cells: (i) the availability of a high-throughput and cost-effective epitaxial CVD reactor and (ii) efficiencies approaching those of bulk Si solar cells. In this paper, two CVD systems are studied: an AP-CVD commercial reactor, as a reference system, and an experimental LP-CVD system for optimization of a low-cost semi-industrial process. For low growth rates, an LP-CVD process is realized with a defect density around 5×10 3 defects/cm 2, comparable with the layers grown in the commercial reactor with a growth rate of 3.9 μm/min. First solar cells, grown in the LP-CVD reactor show an efficiency of 8.2% on mono-crystalline samples. Cells on various low-cost substrates, grown in the reference reactor, show efficiencies between 12% and 13% with IMEC's industrial screen-printing process. The short-circuit current of epitaxial cells is limited to 28 mA/cm 2 (typically 5 mA/cm 2 less than for bulk Si cells). Therefore, the thin epitaxial cell concept requires optimal light trapping, increasing the optical path length. Experiments show that a porous silicon (PS) intermediate layer as an internal reflector can fulfill this role adequately, giving an internal reflectance up to 80%. However, the effectiveness of this reflector depends on its influence on the quality of the epi-layer. Measurements show a lower-quality epi-layer for samples with a PS intermediate layer, but indicate that further optimization of the pre-deposition bake could lead to a compromise between current gain by internal reflectance and losses caused by the increased defect density.

Back-Surface-Field Effects at the Heterojunctions between Boron-Doped p-Type Hydrogenated Amorphous Silicon and Crystalline Silicon in Thin-Film Crystalline Silicon Solar Cells

We have found that the back-surface recombination velocity, Sb, of minority carriers in crystalline silicon (c-Si) thin film solar cells can be reduced to less than 103 cm/s when a boron (B)-doped p-type hydrogenated amorphous silicon (a-Si:H) layer is deposited on the back surface of a p-type c-Si substrate at 200°C, while the value of Sb is 106 cm/s when a B-doped p-type layer is epitaxially grown on the c-Si substrate. We have clarified, from internal photoemission (IPE) and attenuated-total-reflection Fourier transform infrared (ATR-FTIR) spectroscopy measurements, that the band lineup of the heterojunction between B-doped p-type a-Si:H and c-Si and the hydrogen passivation of defects in B-doped a-Si:H are possible reasons for the observed low value of Sb.

QE and Suns-Voc study on the epitaxial CSiTF solar cells

In order to clarify the major factors having confined the efficiencies of as-prepared crystalline silicon thin film (CSiTF) solar cells on the SSP (silicon sheets from powder) ribbons, QE (quantum efficiency) and Suns-V-oc study were performed on the epitaxial CSiTF solar cells fabricated on the SSP ribbons, the SSP ribbons after surface being zone melting recrystallized (ZMR) and single crystalline silicon (sc-Si) substrates. The results show that the epi-layers deposited on the SSP ribbons have rough surfaces, which not only increases the diffusion reflectance on the surfaces but also makes the anti-reflection coatings become structure-loosened, both of which would deteriorate the light trapping effect; in addition, the epi-layers deposited on the SSP ribbons possess poor crystallographic quality, so the heavy grain boundary (GB) recombination limits the diffusion length of the minority carriers in the epi-layers, which makes the as-prepared CSiTF solar cells suffer the worse spectra response at long-wavelength range. Nearly all the dark characteristic parameters of the CSiTF solar cells are far away from the ideal values. The performances of the CSiTF solar cells are especially affected by too high I-02 (the dark saturation current of space charge region) values and too low R-sh (parallel resistance) values. The higher 102 values are mainly caused by the heavy GB recombination resulting from the poor crystallographic qualities of the silicon active layers in the space charge regions, while the lower R-sh values are attributed to the electrical leakage at the un-passivated PN junction or solar cell edges after the solar cells are cut by the laser scriber.

Study on epitaxial silicon thin film solar cells on low cost silicon ribbon substrates

Heavily B-doped silicon ribbons were prepared by modified silicon sheets from powder (SSP) technique using 9 N purity electronic-grade silicon powder. Utilizing the SSP ribbons as substrates, crystalline silicon thin film (CSiTF) solar cells have been fabricated by direct epitaxy method. As-prepared CSiTF solar cells were characterized by illuminated and dark I– V characteristic measurement, Suns- V oc measurement, quantum efficiency (QE) and reflectance measurement. The results show that the CSiTF solar cells have poor performance with typical efficiency η only about 4.25%, and nearly all the characteristic parameters are far away from the ideal values. By detailed analysis and discussion on the measured results, it was found that grain boundary (GB) recombination in the active layers and electrical leakage at the edge of the PN junctions play a major role in deteriorating the performance of the CSiTF solar cells, which lead to higher dark saturation current I s and lower parallel resistance R sh, respectively. By improving the preparation process, such as optimization of the epitaxy process, use of remote plasma hydrogen passivation (RPHP), optimization of the metallization and use of double layer anti-reflection (DLAR) coatings, etc., all the characteristic parameters of the CSiTF solar cells have been greatly improved, and the best efficiency value increased to 8.02%.

Crystalline silicon thin-film solar cells—recent results at Fraunhofer ISE

Crystalline silicon thin-film solar cells combine the advantages of the stability and high-efficiency potential of crystalline silicon solar cell technology with the low material utilization of the thin-film solar cell technology. At Fraunhofer ISE the wafer equivalent concept is currently pursued. Within this concept, the active silicon layers are deposited on high-temperature stable substrates. The resulting substrate/layer sandwich can be processed into a solar cell using the same techniques that are used in conventional crystalline silicon wafer solar cell processing, hence the name wafer equivalent. In the present paper we report on how we realized wafer equivalents and explain in detail our development work on processors for both large-area silicon deposition and for zone melting recrystallization. An overview is given on the solar cell results achieved in this area.

Preparation and characterization of Si sheets by renewed SSP technique

Silicon sheets from powder (SSP) ribbons have been prepared by modified SSP technique using electronic-grade (9N purity) silicon powder. The surface morphology, crystallographic quality, composition and electric properties of the SSP ribbons were investigated by surface profiler, X-ray diffraction (XRD), scanning electron microscopy (SEM), metallurgical microscope, Auger electron spectroscopy (AES) and four-point probe apparatus, respectively. The results show that the SSP ribbon made from electronic-grade silicon powder is a suitable candidate for the substrates of crystalline silicon thin film (CSiTF) solar cells, which could meet the primary requirements of CSiTF solar cell process on the substrates, including surface smoothness, crystallographic quality, purity and electric conductivity, etc.

Characterisation of direct epitaxial silicon thin film solar cells on a low-cost substrate

Direct epitaxial crystalline silicon thin film (CSiTF) solar cells on low-cost silicon sheets from powder (SSP) ribbons have been prepared using rapid thermal chemical vapour deposition (CVD) growth. The characterisation of CSiTF solar cells was investigated by electron and spectrally resolved light beam induced current (EBIC and SR-LBIC, respectively). All EBIC measurements were performed on both the front-side surface as well as on the cross-section of CSiTF solar cells. The electrical recombination was detected by EBIC and compared with their morphologies. The results of EBIC scan show that recombination centres are situated at grain boundaries (GBs); higher the density of grain, higher the recombination activities (higher contrast). Recombination of different intensity (strong and weak) takes place at vertical GBs. Compared with the high recombination at GBs, the contrast in intragrain is low. The dark contrast of the GBs and intragrain defects is clearly reduced near the surface due to the passivation by hydrogen, which indicates that the minority carrier diffusion length decreases gradually with the depth perpendicular to the surface. The diffusion length was determined by SR-LBIC. The results show that the diffusion length distribution is quite inhomogeneous over the whole cell area. A maximum L eff of about 25 μm and mean values around 15 μm are calculated for the best solar cell.

Passivation Effect of Plasma Chemical Vapor Deposited SiNx on Single-Crystalline Silicon Thin-Film Solar Cells

Reduction of surface recombination velocity is one of the most important issues for high-efficiency single crystalline silicon thin-film solar cells. Surface passivation effects of silicon nitride (SiNx) films deposited by direct plasma chemical vapor deposition were analyzed focusing on the polarity of fixed charges and solar cell performance. For the solar cell, epitaxial single crystalline silicon layers with a typical thickness of 10 µm were grown by atmospheric pressure chemical vapor deposition. The solar cell passivated by SiNx film with positive fixed charges showed improved internal quantum efficiency in the wavelength range of 320–500 nm, which was attributed to the effect of an induced front surface field of an n++/n+ junction as holes. A very small surface recombination velocity of 4.6 ×103 cm/s and high cell performances (η=11%, Jsc=26.3 mA/cm2, Voc=609 mV) were realized.

Aluminium BSF in silicon solar cells

The purpose of this work is to develop a back surface field (BSF) for industrial crystalline silicon solar cells and thin-film solar cells applications. Screen-printed and sputtered BSFs have been realised on structures which already have a n +p back junction due to the diffusion of the phosphorus in both faces of the wafer during solar cell emitter elaboration. Rapid thermal annealing temperatures from 700°C to 1000°C have been used. Thickness of the BSF has been measured by SIMS and confronted to the theoretical expected value and simulations. Electrical and optical measurements have been done in order to characterise the BSF. For 250 μm thick industrial solar cells, 6% relative increase in photocurrent has been reached.

Crystalline Silicon Thin-Film Solar Cells on Silicon Nitride Ceramic Substrates

Photovoltaics - the generation of electricity from sunlight - is a technically challenging but environmentally benign technology to generate electricity with a large econnomical potential. However, the major hurdle for its widespread usage is its present high cost. Various thin-film solar cell technologies are investigated to bring down the total cost to an economic value. One of them, the crystalline silicon thin-film (CSiTF) solar cell combines the advantages of conventional wa fer-based silicon solar cells such as high efficiency and non-toxicity with the benefits of thin-film technologies such as serial interconnection and large area deposition. This paper reports for the first time the preparation of CSiTF solar cells on specially developed Si 3 N 4 ceramic substrates. Three different types of Si 3 N 4 recomme wafers were single-sided coated with 10 μm of microcrystalline silicon, which was recrystallized by a zone melting step and subse quently thickened to approx. 30 μm. Optical analysis of the layer surface and cross sections was done to determine the crystallographic properties of the silicon layers, as well as mass spectroscopy to measure the concentration of transition metal impurities. A one-side conducted solar cell process was applied on non-conducting Si 3 N 4 substrates. The best 1 cm 2 cells achieved an efficiency of 8.0% with an excellent fill factor of 74% and an open circuit volonge of 554 mV. The solar cell characterization was complemented by measurements of dark current-voltage characteristics spectrally resolved light beam induced current mapping, and external quantum efficiency.

Crystalline silicon thin-film (CSiTF) solar cells on SSP and on ceramic substrates

CSiTF solar cells are being intensively investigated due to their cost reduction potential compared to silicon wafer solar cells. At the Fraunhofer ISE several high-temperature approaches are under investigation. Our main interest is focussed on low-cost silicon ribbons and on ceramics as substrate material for such solar cells because they match most requirements best. The silicon substrates are being prepared by a ribbon technique, namely the silicon sheets from powder (SSP) technique using silicon powder of different purity as starting material. Onto these substrates silicon layer systems for CSiTF solar cells are deposited by APCVD from SiHCl3 with high deposition rates and recrystallized by a melting step in order to achieve large crystallites in the active layer. The same layer systems have been studied on different kinds of ceramic substrates like SiSiC, SiN, graphite and mullite. To prevent diffusion of impurities from the substrates into the active silicon layer, different types of barrier layers have been used. On SSP substrates made of EG-Si powder conversion efficiencies up to 11.2% on those of UMG-Si 8.6% could be achieved. On ceramic substrates, efficiencies were up to 11% on graphite, up to 9.3% on SiSiC, up to 8.0% on SiN, and up to 8.2% on mullite.

A review of thin-film crystalline silicon for solar cell applications. Part 2: Foreign substrates

One of the most promising ways to reduce the cost of photovoltaics is thin-film crystalline silicon solar cells. This paper, together with part 1, reviews the current state of research in thin-film crystalline silicon solar cells. Deposition on silicon, novel techniques which use a high-quality, reusable silicon substrate and light trapping have been described in part 1 of this paper. This paper describes deposition on glass and ceramics and discusses cell designs for thin-film crystalline silicon solar cells.

A review of thin-film crystalline silicon for solar cell applications. Part 1: Native substrates

Approximately half the cost of a finished crystalline silicon solar module is due to the silicon itself. Combining this fact with a high-efficiency potential makes thin-film crystalline silicon solar cells a growing research area. This paper, written in two parts, aims to outline world-wide research on this topic. The subject has been divided into techniques which use native substrates and techniques which use foreign substrates. Light trapping, vapour- and liquid-phase deposition techniques, cell fabrication and some general considerations are also discussed with reference to thin-film cells.

Experimental testing of a random medium optical model of porous silicon for photovoltaic applications

AbstractWe have developed a model for light propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin‐film crystalline silicon solar cell realized in an ultrathin (1–3 μm) epitaxially grown Si layer on PS. Three layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150–1000 nm epitaxial silicon layers on silicon wafers of which 150–450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400–1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high porosity layer from about 60% to 20–30%. Copyright © 2001 John Wiley & Sons, Ltd.

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.

Optical confinement in thin-film crystalline silicon solar cells by adhesive bonding of ceramic substrate

Thin film (5-/spl mu/m-thick) silicon solar cells by adhesive bonding of a near-Lambertian Al/sub 2/O/sub 3/ ceramic substrate have been fabricated, and the electrical and optical performance of the cells have been investigated. This cell structure shows a good optical confinement effect. From quantum efficiency measurements, it is found that a short circuit current that is as high as that of water-based cells can be obtained by reducing the surface reflectance. Cell and module fabrication processes can be simplified using this technique.

High-temperature processing of crystalline silicon thin-film solar cells

The crystalline silicon thin-film solar cell combines, in principle, the advantages of crystalline silicon wafer-based solar cells and of thin-film solar cell technologies. Its efficiency potential is the highest of all thin-film cells. In the “high-temperature approach” thin silicon layers are deposited on substrates that withstand processing temperatures higher than 1000 °C. The basic features of the high-temperature crystalline silicon thin-film cell technology are described and some important results are discussed.

Potential of amorphous silicon for solar cells

This paper reviews recent developments in the field of amorphous-silicon-based thin-film solar cells and discusses potentials for further improvements. Creative efforts in materials research, device physics, and process engineering have led to highly efficient solar cells based on amorphous hydrogenated silicon. Sophisticated multijunction solar cell designs make use of its unique material properties and strongly suppress light induced degradation. Texture-etching of sputtered ZnO:Al films is presented as a novel technique to design optimized light trapping schemes for silicon thin-film solar cells in both p-i-n and n-i-p device structure. Necessary efforts will be discussed to close the efficiency gap between the highest stabilized efficiencies demonstrated on lab scale and efficiencies achieved in production. In case of a-Si:H/a-Si:H stacked cells prepared on glass substrates, significant reduction of process-related losses and the development of superior TCO substrates on large areas promise distinctly higher module efficiencies. A discussion of future perspectives comprises the potential of new deposition techniques and concepts combining the advantages of amorphous and crystalline silicon thin-film solar cells.

Crystalline Si thin-film solar cells: a review

The present review summarizes the results of research efforts in the field of crystalline silicon thin-film solar cells on foreign substrates. The large number of competing approaches can be broadly classified according to the grain size of the crystalline Si films and the doping of the crystalline absorber. Currently, solar cells based on microcrystalline Si films on glass with an intrinsic or moderately doped absorber film achieve efficiencies around 10%, whereas thin-film cells fabricated from large-grained polycrystalline Si on high-temperature-resistant substrates have efficiencies in the range of 15%. The paper discusses the limitations of various approaches and describes recent developments in the area of thin, monocrystalline Si films that may open the way towards 20% efficient thin-film Si solar cells.