Very High Frequency Glow Discharge Research Articles

Material and solar cell research in high efficiency micromorph tandem solar cell

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. This paper describes the use, within p–i–n- and n–i–p-type solar cells, of hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (_c-Si:H) thin films (layers), both deposited at low temperatures (200_C) by plasma-assisted chemical vapour deposition (PECVD), from a mixture of silane and hydrogen. Optical and electrical properties of the i-layers are described. Finally, present performances and future perspectives for a high efficiency ‘micromorph’ (mc-Si:Hya-Si:H) tandem solar cells are discussed.

A Wide Band Gap Boron-doped Microcrystalline Silicon Film Obtained with VHF Glow Discharge Method

AbstractA wide bandgap microcrystalline silicon film for the window layer of microcrystalline silicon thin film solar cells was obtained with very high frequency (VHF) glow discharge technology. The material was deposited on corning 7059 substrate at about 170 When H2/SiH4was more than 100, Raman spectra showed that this material was highly crystallized, and no peak correlation with amorphous silicon was observed. This material showed strong n type before any intentional doping. We considered that the unintentional doping of oxygen and unpurified gases. The doping performance of this material was investigated by introducing B2H6into the reacting gas. As increasing the rate of B2H6/SiH4from zero to 0.5%, the conductivity changed from 10-1S.cm-1(n type) to 10-8S.cm-1dramatically and than backed to 10-1S.cm-1(p type), which indicated that this material had excellent doping ability. Raman spectra also showed that the microstructure of these materials did not change obviously in this doping range. We gained the p-uc-Si:H film with thickness less than 30nm, and the conductivity was more than 10-2S.cm-1, and crystalline volume fraction no less than 40%, the Egoptcould be wider than 2.10eV. Using this p window layer in microcrystalline silicon solar cells with no ZnO rear reflection, the conversion efficiency was 8.30% (Voc=0.531V, Jsc=24.66mA/cm2, FF=63.41% ).

Amorphous solar cells, the micromorph concept and the role of VHF-GD deposition technique

During the last two decades, the Institute of Microtechnology (IMT) has contributed in two important fields to future thin-film silicon solar cell processing and design: (1) In 1987, IMT introduced the so-called “very high frequency glow discharge (VHF-GD)” technique, a method that leads to a considerable enhancement in the deposition rate of amorphous and microcrystalline silicon layers. As a direct consequence of reduced plasma impedances at higher plasma excitation frequencies, silane dissociation is enhanced and the maximum energy of ions bombarding the growing surface is reduced. Due to softer ion bombardment on the growing surface, the VHF process also favours the formation of microcrystalline silicon. Based on these beneficial properties of VHF plasmas, for the growth of thin silicon films, plasma excitation frequencies f exc in the range 30–300 MHz, i.e. clearly higher than the standard 13.56 MHz, are indeed scheduled to play an important role in future production equipment. (2) In 1994, IMT pioneered a novel thin-film solar cell, the microcrystalline silicon solar cell. This new type of thin-film absorber material––a form of crystalline silicon––opens up the way for a new concept, the so-called “ micromorph” tandem solar cell concept. This term stands for the combination of a microcrystalline silicon bottom cell and an amorphous silicon top cell. Thanks to the lower band gap and to the stability of microcrystalline silicon solar cells, a better use of the full solar spectrum is possible, leading, thereby, to higher efficiencies than those obtained with solar cells based solely on amorphous silicon. Both the VHF-GD deposition technique and the “micromorph” tandem solar cell concept are considered to be essential for future thin-film PV modules, as they bear the potential for combining high-efficiency devices with low-cost manufacturing processes.

Microstructure and surface roughness of microcrystalline silicon prepared by very high frequency-glow discharge using hydrogen dilution

The microstructure of a series of silicon films deposited by very high frequency glow discharge (VHF-GD) with silane concentration in hydrogen varying from 100% down to 1.25% has been observed with transmission electron microscopy (TEM). The surface topography of the layers has been analysed by atomic force microscopy (AFM). At silane concentration below 8.6%, a phase transition between amorphous hydrogenated silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) is observed by TEM. After this transition, the further decrease of silane concentration leads to complex changes of the crystalline microstructure of the layers. AFM observations of the surface reveal that the film rms roughness increases with the decrease of the silane concentration. The surface morphology is not related simply to the microstructure of crystalline grains as observed by TEM.

Electronic transport in hydrogenated microcrystalline silicon: similarities with amorphous silicon

Undoped hydrogenated microcrystalline silicon (μc-Si:H) layers were grown by the very high frequency glow discharge (VHF-GD) technique under various deposition conditions. The electronic transport properties under illumination were investigated by means of steady-state photoconductivity and steady-state photocarrier grating methods. Similarly to hydrogenated amorphous silicon (a-Si:H), power law dependencies as a function of the generation rate are observed for the photoconductivity, for the ambipolar diffusion length, and for the parameter b (indicating the Fermi level). For μc-Si:H, as for a-Si:H, nearly constant product of (mobility × recombination time) of majority and minority carriers is observed as a function of the parameter b. Based on these similarities, we assume that the electronic transport model developed for a-Si:H remains valid for μc-Si:H.

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchâtel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency.

Structural properties and electronic transport in intrinsic microcrystalline silicon deposited by the VHF-GD technique

A series of microcrystalline samples was deposited by the very high frequency glow discharge (VHF-GD) technique, with various input powers while keeping all the other parameters of deposition constant. The goal was to correlate transport and structural properties and avoid as much as possible the problem of a variation of the Fermi level between the samples. The observed decrease of the photoconductivity and of the product mobility-lifetime of hole (as measured by time of flight, TOF) with the increase of the power was surprisingly not connected to the structural properties, which remain approximately unchanged, but with a surface contribution to the transport properties.

Low Defect Density Microcrystalline-Si Deposited by the Hot Wire Technique

ABSTRACTThe optical and electronic properties of a series of microcrystalline silicon (μ-Si) films, deposited by the hot wire (HW) technique, are reported. Preliminary results suggest, using moderate H2 /SiH4 dilution ratios and substrate temperatures (320°C), high filament temperatures, and no H gas purifier, that the subgap absorption for these films, measured using the constant photocurrent (CPM) method, can be as low as that obtained for films deposited by the very high frequency glow discharge (VHF-GD) technique. The film dark conductivities of the HW samples, ranging as low as 2.0 × 10−8 (ohm cm)−1, lend further credance to these low defect values. At the same time, the optical absorption in the region > 1.6 eV is higher than that previously observed for the VHF-GD deposited samples. The present results, discussed in the context of the film microcrystalline fraction, suggest that there is no unique, good quality, low defect density μ-Si material, and that different deposition techniques can be used to successfully deposit device quality gc-Si. We also present optical and structural data for films deposited at lower substrate temperatures and higher deposition rates, and suggest combinations of deposition parameters to be used that may further improve the electronic properties of these films.

Potential of VHF-plasmas for low-cost production of a-Si: H solar cells

Compared to the use of the standard glow discharge technique the production of amorphous silicon solar cells by the very high frequency glow discharge (VHF-GD) bears yet additional cost reduction potentials: Using VHF-GD at excitation frequencies higher than 13.56 MHz, a more efficient dissociation of silane gas is obtained; thus, higher deposition rates are achieved; this reduces considerably the deposition time for intrinsic amorphous and microcrystalline silicon layers. Furthermore, by itself and even more so, in combination with argon dilution, VHF-GD technique improves silane utilisation and leads, thus, to further cost reduction. Finally, by combining the VHF-GD technique and the “micromorph” concept “real” tandem cells (i.e. a superposition of two cells with distinctly different band gaps) can be deposited at low temperatures without the use of expensive germane gas.

Fast Deposition of μc-Si:H by Restrictive Dilution and Enhanced HF-Power

Note: IMT-NE Number: 258 Reference PV-LAB-ARTICLE-1997-009 Record created on 2009-02-10, modified on 2017-05-10

A-Si:H/a-Si:H stacked cell from VHF-deposition in a single chamber reactor with 9% stabilized efficiency

In the present paper we present results on a-Si:H/a-Si:H stacked cells deposited in a single-chamber reactor by the very high frequency-glow discharge (VHF-GD) deposition technique at 70 MHz. Hydrogen dilution of the i-layer yields more stable amorphous p-i-n solar cells, similar to what is observed for RF deposition. High dilution ratios of the i-layer are found to enhance contaminations. This is, for the single-chamber reactor, due to the etching effect of the highly reactive H 2-plasma. Additionally, oxygen incorporation into the i-layer is favored by the high hydrogen dilution. Different means to suppress the contaminations are employed and discussed. Regarding the stacked cell design, we show by experiment and simulation that it is important to carefully adjust the current mismatch between the component cells such as to obtain a slight top-cell-limited behavior after degradation. We present an a-Si:H/a-Si:H stacked cell with an initial efficiency of 9.8% showing only 8% relative degradation which results in a stabilized efficiency of 9%. The deposition rate of the employed H 2-diluted i-layer material is 4 Å/s. It is therefore demonstrated that it is possible to make highly efficient stacked cells showing good stability also in a single-chamber system and employing the VHF technique to obtain higher rates.

Effect of ion bombardment in very-high frequency glow discharge on growth and properties of SiHx films

Ion and electron flow characteristics have been studied in different regimes both in rf (13.56 MHz) and VHF (58 MHz) glow discharges in silane. The measurements were conducted in a growth reactor and the data obtained was used for SiHx films deposition. Electronic properties of the films prepared under a variety of ion bombardment in rf, VHF discharges have been investigated. Correlation of these properties with ion parameters in rf, VHF discharges is discussed.

On the Way Towards High Efficiency Thin Film Silicon Solar Cells by the “Micromorph” Concept

AbstractRecently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

Passivation properties of amorphous and microcrystalline silicon layers deposited by VHF-GD for crystalline silicon solar cells

A comparative study of crystalline silicon (c-Si) surface passivation techniques is presented. The passivation is essential in increasing the total solar cell efficiency by enhancing the open-circuit voltage ( V oc ). Apart from the thermally grown oxides or nitrides widely used sofar, recent publications report on excellent passivating properties of amorphous silicon (a-Si:H)- and microcrystalline silicon (μc-Si:H) layers, deposited on n-type crystalline silicon [1–3], forming thereby heterojunctions. We present first results of a new structure called ⪡BAP (Both Sides Amorphous Passivated) cell⪢, where the wafer was sandwiched between two intrinsic amorphous silicon layers. These two layers, as well as the μc-Si:H emitter, and the layer forming the BSF (Back Surface Field) were all deposited by the VHF-GD (Very High Frequency Glow-Discharge) process which was developed in our laboratory [4]. BAP cells showed V oc -values as high as 635 mV, proving thus the excellent passivating properties of a-Si:H and the validity of this cell concept which makes a fully in-line processing possible (wafer in-cell out).

“Thick” a-Si:H P-i-n diodes, produced by the VHF-GD process: spectral response for visible/infrared light and current transients

AbstractWith the possibility to deposit thick amorphous silicon layers, applications like particle- or X-ray detectors can be envisaged. The present paper describes new results obtained with a series of thick p-i-n diodes produced by VHF-GD (Very High Frequency Glow-Discharge). The goal is to obtain the maximum output signal for given detection conditions (reverse voltage, generation rate,…). The problem of the collection of the electrons and holes is discussed in this paper, in relation with the distribution of the electric field. As is well known, trapped charges screen the inner part of the intrinsic layer of the detector. Due to this effect, even under strong reverse bias-voltage, a complete depletion of the whole intrinsic layer can in general not be obtained for thick diodes; however, even if the diode is not completely depleted, the signal increases with increasing diode thickness until a maximum is reached for the signal, and then it decreases again. Finally, different rise-times of the transient current (current after a jump in bias voltage is applied) have been Measured. Differing between one diode and the other, the rise-time of the transient current was found to vary between 0.1 seconds and 1 hour; a clear dependence on the deposition conditions is observed. Long rise-times are major difficulty to be removed, because most of the applications like imaging require short response time of the detector.

High Deposition Rate Pecvd Processes For Next Generation Tft-Lcds

AbstractThe demand for lower cost per panel in TFT-LCD production is driving the PECVD market to deposition systems that combine high throughput and uptime with high yield. Today it is generally believed that a multichamber system that combines a number of single-panel deposition chambers is the best way to achieve these goals.For these PECVD systems to be economical, the deposition rate of a-Si:H, SiNx and SiO2 has to be in the 1200–1500 Å/min range. In 13.56 MHz parallel-plate glow discharge systems SiNx and SiO2 deposition rates exceeding 1500 Å/min are commonly achieved, whereas the deposition rate of a-Si:H is limited to 100–200 Å/min due to powder formation. Over the last 5 years significant progress has been made to increase the deposition rate of a-Si:H. Methods include the use of very-high-frequency glow discharge (VHF-GD) and pulsing of the rf discharge. However, substrate sizes never exceeded 100mm×100mm.We have developed a multichamber PECVD system for TFT-LCD production where VHFGD is used to obtain uniform high deposition rates for (doped) semiconductors and insulators, such as a-Si:H, n+ a-Si:H, SiNx and SiO2 over areas as large as 470mm×370mm. Even at deposition rates well above 1200 Å/min hydrogen in a-Si:H is exclusively bound as monohydride. The optoelectronic properties of the films are at least as good as those of their 13.56 MHz counterparts and thus good-quality TFTs can be obtained. At the same time the number of added particles is low allowing for high production yields.

Influence of higher deposition temperature on a-Si:H material properties, powder formation and light-induced degradation, using the VHF (70 MHz) glow discharge technique

a-Si:H films were prepared using pure silane in the temperature range from 200°C up to 400°C by the very high frequency glow discharge (VHF-GD) technique. During deposition a He/Ne laser beam directed across the discharge monitored powder formation. H-content and microstructure were determined by IR-spectroscopy. The impurity contents of oxygen (O), nitrogen (N) and carbon (C) were analysed by SIMS. A surface profiler was used to measure the internal stress and thickness of the films. The samples were further characterised by dark- and photo-conductivity and by PDS-measurements, both in annealed and light-soaked states. Finally, we will show how far changes in film properties affect light-induced degradation and the saturated defect density.

An Approach to High Quality a-Ge:H by VHF Deposition

Amorphous hydrogenated germanium a-Ge:H was deposited by very high frequency glow discharge (VHF-GD) at frequencies between 25 and 220MHz, low pressure (2.5Pa) and high deposition rates (≤4Á /s) on both electrodes of a parallel plate reactor. The films are comparable to material deposited on the powered electrode of a conventional RF-GD. Mobility-lifetime products ηµτ for photogenerated charge carriers around 10-7cm2/V and a Fermi level position at EC-EF≈400meV indicate good opto-electronic properties. The IR spectra show that the samples are free of oxydization and incorporate slightly more voids than the RF material. Measurements of ion energy Ei and flux Фi in the substrate plane show that “hard&” preparation conditions are obtained comparable to the deposition on the powered electrode of an RF-GD.

Microstructure, Optoelectronic Properties and Saturated Defect Density of A-SL:H Prepared in VHF-Glow Discharge Using AR and XE Dilution

ABSTRACTFilms were prepared under argon and xenon dilution with silane concentrations ranging from 3 to 100% using the very high frequency glow discharge (VHF-GD) technique. The H-content and the microstructure were determined by IR-spectroscopy. A surface profiler was used to measure the stress and thickness of the films. The surface roughness of the films was evaluated by the UV-light reflectance loss. The samples were further characterized by dark- and photoconductivity, by CPM and by PDS, both in annealed and light-soaked state.Down to silane concentrations of about 10–20% film properties change only little; however, for both dilution series the microstructure parameter shows a minimum and the internal stress a maximum near 20% silane concentration. At still higher rare gas dilution the film properties change drastically. Surprisingly the photoconductivity remains almost constant for all gas dilutions. It is shown how these changes in the film properties are linked with the light induced degradation.

Preparation and Characterization of Highly Conductive (100 S/cm) Phosphorus Doped νc-Si:H Films Deposited Using the VHF-GD Technique

AbstractWe report on the preparation and characterization of phosphorus doped gc-Si:H films produced by the very high frequency glow discharge (VHF-GD) at a plasma excitation frequency of 70 MHz. We present a systematic study of the deposition parameters i.e. hydrogen dilution of silane, VHF power density, gas phase doping ratio and deposition temperature and their influences on the electrical and structural properties of the material. In contrast to 13.56 MHz GD the VHF plasma conditions favour microcrystalline formation at low power densities; the resulting conductivities are significantly higher than those obtained at 13.56 MHz.