Hydrogenated amorphous silicon films with significantly improved stability

A new regime of plasma-enhanced chemical-vapor deposition (PECVD), referred to as “uninterrupted growth/annealing” method, has been proposed for preparation of high-quality hydrogenated amorphous silicon (a-Si:H) films. By using this regime, the deposition process no longer needs to be interrupted, as done in the chemical annealing or layer by layer deposition, while the growing surface is continuously subjected to an enhanced annealing treatment with atomic hydrogen created in the hydrogen-diluted reactant gas mixture at a relatively high plasma power. The intensity of the hydrogen plasma treatment is controlled at such a level that the deposition conditions of the resultant films approach the threshold for microcrystal formation. In addition, a low level of B-compensation is used to adjust the position of the Fermi level close to the midgap. Under these conditions, we find that the stability and optoelectronic properties of a-Si:H films have been significantly improved.     

Optoelectronic properties of doped layers for a-Si solar cells prepared using helium dilution

This paper is the first part of a work about the preparation and characterisation of doped layers for hydrogenated-amorphous-silicon (a-Si:H) thin film solar cells. An approach for RF-glow discharge deposition of a-Si consisting of dilution of silane (SiH₄) in helium and application of high RF-power densities, has been tested. In this first part the optimisation of n-type layers has been accomplished. The influence of preparation conditions on the optical and electrical properties of the films has systematically been studied. It has been found that the use of high RF-power densities and high dilution levels of SiH4 in He favour the doping efficiency and film quality when the substrate temperature is 300°C. As a result of these investigations, n-type layers with thicknesses between 250 and 360Å, an optical gap about 1.95 eV, a dark-conductivity of 0.1 (Ωcm)⁻¹ and an extended-state conductivity activation energy of 0.1 eV have been prepared. Such properties make them suitable for their use as n-type layers for a-Si:H thin-film solar cells.     

Process development of amorphous silicon/crystalline silicon solar cells

We have already investigated some crucial limiting process steps of the amorphous silicon (a-Si)/crystalline silicon (c-Si) solar cell technology and some specific characterization tools of the ultrathin amorphous material used in devices. In this work, we focus our attention particularlyon the technology of the ITO front contact fabrication, that also is used as an antireflective coating. It is pointed out that this layer acts as a barrier layer against the diffusion of metal during the annealing treatments of the front contact grid. The criteria of the selection of the metal to be used to obtain good performance of the grid and the deposition methods best suited to the purpose are shown. We were able to fabricate low temperature heterojunction solar cells based p-type Czochralski silicon, and a conversion efficiency of 14.7% on 3.8 cm² area was obtained without back surface field and texturization.     

High quality amorphous silicon materials and cells grown with hydrogen dilution

Hydrogen dilution of the active gas during deposition has been found to be a very effective way to improve the quality of amorphous silicon-based materials and solar cells. With increasing hydrogen dilution, the material is characterized by an improved order, and at a certain threshold dilution, the amorphous to microcrystalline transition takes place. The best material is grown just below the threshold and is heterogeneous consisting of tiny crystallites embedded in an amorphous matrix of improved order. In this paper, we discuss the effects of hydrogen dilution on the material and cell properties of amorphous silicon-based alloys and provide an explanation for their improved stability against light-induced degradation. We also discuss some special properties of the on-the-edge materials that are not seen in the conventional amorphous or microcrystalline alloys.     

Theory of amorphous silicon solar cell (b): a five layer analytical model

A theoretical analysis of recombination kinetics and space charge distribution in amorphous silicon is carried out with a view to bring out the underlying physics. A uniform excitation with a flat quasi-Fermi level and a constant np product has been used as a probe to estimate the relative importance of various parameters. Recombination rates have been calculated for various ratios of capture rates for Coulomb attractive and neutral traps. In practice a large ratio of capture rates exists and for this case two peaks of recombination maxima are found to lie in the space charge regions corresponding to transitions at the energy level E₁ (for D⁺–D⁰ transition) at the p–i edge and for E₂ energy level (corresponding to D⁰–D⁻ transition) at the i–n interface. A two independent level model therefore holds to a good approximation. The dangling bond density is found to determine both the space charge distribution and the recombination rate. Based on space charge density distribution i-layer can be divided in the five parts. The two recombination rate peaks are found to exist at the p–i and i–n space charge transitions respectively. This enables us to develop a simple model for the i-layer of the p–i–n diode.     

Electrical properties of Cl incorporated hydrogenated amorphous silicon

The electrical properties of chlorine incorporated hydrogenated amorphous silicon films were studied. The samples were deposited with dichlosilane/silane mixtures. The conductivity of these chlorine-incorporated hydrogenated amorphous silicon films decreases with increasing dichlosilane to silane ratio. The Fermi level shifts toward the valence band with increasing chlorine content, resulting in a corresponding conductivity activation energy and room temperature conductivity decrease. However, the defect density and Urbach energy are noot significantly changed in films containing chlorine.     

Control of plasma chemistry for preparing highly stabilized amorphous silicon at high growth rate

Recent progress in the growth process of hydrogenated amorphous silicon (a-Si:H) from a silane glow-discharge plasma is reviewed, being focused on the improvement of photo-induced degradation properties in a-Si:H deposited at high growth rate. Contribution of higher-silane related reactive species (HSRS) during film growth is suggested as a key event to increase the degree of photo-induced degradation in the resulting a-Si:H through an incorporation of excess Si–H₂ bonds in the network as far as the substrate temperature is kept constant. The contribution ratio of HSRS to film growth is derived using a couple of gas-phase reaction-rate equations, which shows strong dependence on electron temperature, hydrogen-dilution ratio, gas-flow rate, and gas temperature in the plasma. Various methods are adopted to reduce the contribution ratio of HSRS during film growth and the validity of theoretical approach is confirmed experimentally. Finally, stabilized conversion efficiency of 8.2% is demonstrated in the a-Si:H-based single junction solar cell with high deposition rate of 2.0 nm/s.     

Realization of high efficiency micromorph tandem silicon solar cells on glass and plastic substrates: Issues and potential

High conversion efficiency for (amorphous/microcrystalline) "micromorph" tandem solar cells requires both a dedicated light management, to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. Efficient light trapping is achieved here by use of textured front and back contacts as well as by implementing an intermediate reflecting layer (IRL) between the individual cells of the tandem. This paper discusses the latest developments of IRLs at IMT Neuchâtel: SiO_x based for micromorphs on glass and ZnO based IRLs for micromorphs on flexible substrates were successfully incorporated in micromorph tandem cells leading to high, matched, current above 13.8 mA/cm² for p-i-n tandems. In n-i-p configuration, asymmetric intermediate reflectors were employed to achieve currents of up to 12.5 mA/cm². On glass substrates, initial and stabilized efficiencies exceeding 13% and 11%, respectively, were thus obtained on 1 cm² cells, while on plastic foils with imprinted gratings, 11.2% initial and 9.8% stable efficiency could be reached. Recent progress on the development of effective front and back contacts will be described as well.     

Production technology for amorphous silicon-based flexible solar cells

We have developed a-Si-based solar cells with plastic film substrate and achieved a stabilized efficiency of 9% in a 40 cm×80 cm cell. The structure and fabrication process of flexible solar cells are presented. Then we discuss the merits and demerits of our process from the viewpoint of mass production, and clarify that the SCAF cell has a good adaptability to mass production.     

Effect of hydrogen on stability of amorphous silicon thin films

This paper presents results of the investigation of hydrogen influence on the stability of low pressure chemical vapour deposition a-Si films. We measured boron- or phosphorus-doped films post-hydrogenated by ion implantation with different hydrogen doses. The dark conductivity after fast quenching and slow cooling and the isothermal relaxation were measured at different annealing temperatures. It was found that higher hydrogen concentration causes greater metastable changes but shorter relaxation time of defects.     

Diagnostics of thin-film silicon solar cells and solar panels/modules with variable intensity measurements (VIM)

A simple and low-cost method for analyzing amorphous silicon solar cells and modules, which have low values of the fill factor (FF), is proposed. Low fill factors can occur mainly because of 3 reasons: (a) excessive recombination due to “bad” intrinsic layers; (b) shunts and (c) very high series resistance. The method described here allows one to discriminate between (a), (b) and (c). It consists of measuring the J-V curves at different light intensities, varying typically from 0.05 to 1 sun. It has been called the “variable intensity method (VIM)”. Here, one plots R_sc=∂V/∂J (at V=0) and R_oc=∂V/∂J (at J=0) as a function of J_sc. From the slope of the R_sc-J_sc curve, one derives the “collection voltage V_coll”; from the asymptotic value of R_sc for low values of J_sc (<0.1 mA/cm²) one obtains the “true” shunt resistance R_shunt; from the asymptotic value of R_oc for high values of J_sc (around 10 mA/cm²) one obtains the “true” series resistance R_series. This paper shows quantitatively how too low values of V_coll and of R_shunt as well as how too high a value of R_series lead to a low value of FF for both cells and panels/modules.     

Advances in amorphous silicon alloy cell and module technology

The low material cost and proven manufacturability of amorphous silicon (a-Si) alloy solar panels make them ideally suited for low-cost terrestrial application. A major challenge for the researchers has been how to improve the light-to-electricity conversion efeiciency. Extensive R & D efforts have resulted in a significant improvement in stable cell and module efficiencies with the achievement of 12.8% active-area cell efficiency and 10.4% module efficiency using a spectral-splitting triple-band gap, triple-cell approach. Further gains in efficiency are expected through an improved understanding of plasma chemistry and growth kinetics. In this paper, we shall discuss the progress in science and technology of a-Si alloy photovoltaics with special emphasis on the opportunities and the challenges that exist.     

Medium-range order in amorphous silicon measured by fluctuation electron microscopy

Despite occasional experimental hints, medium-range structural order in covalently bonded amorphous semiconductors had largely escaped detection until the advent of fluctuation electron microscopy (FEM) in 1996. Using FEM, we find that every sample of amorphous silicon and germanium we have investigated, regardless of deposition method or hydrogen content, is rich in medium-range order. The paracrystalline structural model, which consists of small, topologically ordered grains in an amorphous matrix, is consistent with the FEM data, and is rendered diffraction amorphous by strain effects. We present measurements on hydrogenated amorphous silicon deposited by different methods, some of which are reported to have greater stability against the Staebler–Wronski effect. The matrix material of these samples is relatively similar, but the order changes in different ways upon both light soaking and thermal annealing. Some materials are inhomogeneous, with either nanocrystalline inclusions or large area-to-area variation in the medium-range order. We discuss the implications of and future directions for understanding medium-range order.     

Amorphous silicon solar cells

The perfectioning of the deposition techniques of amorphous silicon over large areas, in particular film homogeneity and the reproducibility of the electro-optical characteristics, has allowed a more accurate study of the most intriguing bane of this material: the degradation under sun-light illumination. Optical band-gap and film thickness engineering have enabled device efficiency to stabilize with only a 10–15% loss in the as-deposited device efficiency. More sophisticated computer simulations of the device have also strongly contributed to achieve the highest stable efficiencies in the case of multijunction devices. Novel use of nanocrystalline thin films offers new possibilities of high efficiency and stability. Short term goals of great economical impact can be achieved by the amorphous silicon/crystalline silicon heterojunction. A review is made of the most innovative achievements in amorphous silicon solar cell design and material engineering.     

Structural and optical studies on hot wire chemical vapour deposited hydrogenated silicon films at low substrate temperature

Thin films of hydrogenated silicon are deposited by hot wire chemical vapour deposition technique, as an alternative of plasma enhanced chemical vapour deposition technique. By varying the hydrogen and silane flow rate, we deposited the films ranging from pure amorphous to nanocrystallite-embedded amorphous in nature. In this paper we report extensively studied structural and optical properties of these films. It is observed that the rms bond angle deviation decreases with increase in hydrogen flow rate, which is an indication of improved order in the films. We discuss this under the light of breaking of weak Si-Si bonds and subsequent formation of strong Si-Si bonds and coverage of the growing surface by atomic hydrogen.     

Thickness determination of thin (20 nm) microcrystalline silicon layers

For the development of thin, doped microcrystalline silicon (μc-Si) layers, it is necessary to have an accurate tool to determine the thickness and material properties of layers around 20 nm. Here, we report on the interpretation of UV-VIS-NIR spectroscopy (reflection/transmission) measurements using the O’Leary, Johnson, Lim (OJL) model in which we add extra information to compensate for the loss of density information due to the lack of fringes. Moreover, using this method we extract information that can be correlated to the crystalline ratio of μc-Si:H thin layers. We correlate thicknesses and material properties obtained from the optical method to the results obtained from various other techniques: Raman spectroscopy, Rutherford back scattering (RBS) and cross-sectional transmission electron microscopy (X-TEM).By analyzing the data of thin μc-Si:H layers (20 nm) as well as of thicker layers (100 nm) and comparing the results to thicknesses measured with X-TEM, we conclude that as long as the density of thin layers is identical to the thicker layers, with the optical method a good approximation of thickness of microcrystalline silicon layers is possible at a layer thickness down to 20 nm.     

Crystalline silicon surface passivation by amorphous silicon carbide films

This article reviews the surface passivation of n- and p-type crystalline silicon by hydrogenated amorphous silicon carbide films, which provide surface recombination velocities in the range of 10 cm s⁻¹. Films are deposited by plasma-enhanced chemical vapor deposition from a silane/methane plasma. We determine the passivation quality measuring the injection level (Δn)-dependent lifetime (τ_eff(Δn)) by the quasi-steady-state photoconductance technique. We analyze the experimental τ_eff(Δn)-curves using a physical model based on an insulator/semiconductor structure and an automatic fitting routine to calculate physical parameters like the fundamental recombination velocities of electrons and holes and the fixed charge created in the film. In this way, we get a deeper insight into the effect of the deposition temperature, the gas flow ratio, the doping density of the substrate and the film thickness on surface passivation quality.     

Phase engineering of a-Si:H solar cells for optimized performance

Until recently, the advances in hydrogenated amorphous silicon (a-Si:H) solar cell performance and stability have been achieved materials prepared with hydrogen dilution following primarily empirical approaches. This paper discusses the recently obtained insights into the growth, microstructure and nature of these materials. Such protocrystalline Si:H materials are more ordered than the a-Si:H obtained without dilution and evolve with thickness from an amorphous phase into first a mixed amorphous–microcrystalline and subsequently into a single microcrystalline phase. The development of deposition phase diagrams, characterize their microstructural evolution during growth which can be used to guide the fabrication of solar cell structures in a controlled way. Examples are presented and discussed of their application in solar cell fabrication to obtain a fundamental understanding of the properties of the phase transitions as well as the systematic optimization of cell performance.     

Thermal recovery effect on light-induced degradation of amorphous silicon solar module under the sunlight

The thermal recovery effect from the light-induced degradation under the sunlight is experimentally investigated on the amorphous silicon photovoltaic module (a-Si PV module) for installing directly to the roof flames of wooden houses. To enhance the recovery effect, the heat-insulating material is attached to the back side of the module for increasing the module temperature under the sunlight: the heat-insulated module.The generated power from the heat-insulated module is compared with that from the normal module (without the heat-insulating material) for 2 yr, and it has been cleared that the generated power normalized at 25°C from the heat-insulated module is approximately 7.3% higher than that from the normal one with the average temperature increase of 4.2°C under the sunlight.