Thin Silicon Solar Cells Research Articles

Effect of controlled dopant distribution in thin silicon solar cell

Dopant incorporation leading to drift field in a thin silicon solar cell may be controlled and compensated suitably to produce positive or negative drift field. A positive drift field at the base increases short-circuit current density simultaneously decreasing open-circuit voltage, resulting in some net benefit in efficiency for unpassivated front and back surface. In case of well-passivated front and back, however, positive drift field may result in a marginal drop in efficiency. If the dopant incorporation is compensated to produce a small negative drift field, open-circuit voltage increases to such an extent that the overall effect is an increase in efficiency for moderately passivated front and back. Moderate passivation and small negative field seems to produce the best results. The effect may yield 10% gain in overall efficiency for thin cells. Methods of incorporating small negative drift field in LPE grown thin silicon cells are also suggested.

Thin film polycrystalline silicon solar cell on ceramics with a seeding layer formed via aluminium-induced crystallisation of amorphous silicon

Thin film polycrystalline silicon solar cells on foreign substrates are viewed as one of the most promising approaches to cost reduction in photovoltaics. To enhance the quality of the film, the use of ‘seeding layers’ prior to deposition of active material is being investigated. It has been shown that a phenomenon suitable to create such a seeding layer is the aluminium-induced crystallisation of amorphous silicon. Previous work mainly considered glass as the substrate of choice, thereby introducing limitations on the deposition temperature. Results concerning the application of such a technique to ceramic substrates (allowing the use of high-temperature CVD) are described. Also, the first reported results of a solar cell made in silicon deposited on these seeding layers are presented.

Effect of design parameters on the efficiency of the solar cells fabricated using SOI structure

Abstract Thin cells demand new grid design concepts in that all the contacts (to the emitter and base) be located on the front surface. Hence, the aim of the investigation is to determine the potential and the basic limitation of the design. With this concept, an interdigitated front grid structure was realized and cells were fabricated through a set of photolithography processes. Confirmed efficiencies of up to 11.5% were achieved on bonded silicon-on-insulator wafers with a cell thickness of 50 μm in the case of finger spacing more than 1000 μm and a base width of 35 μm. It was also shown from the results that to adapt the design rules for optimizing the base fraction and the shadowing fraction was noted as an important technique to realize high-efficiency thin silicon solar cells.

Influence of substrate texture on microstructure and photovoltaic performances of thin film polycrystalline silicon solar cells

Thin film polycrystalline silicon (poly-Si) solar cells have been fabricated by plasma enhanced chemical vapor deposition on differently textured ZnO/Ag/SnO 2/glass substrates. Using a textured substrate with the root mean square (RMS) roughness ( σ) of 38 nm, the conversion efficiency of 8.22% has been achieved due to improvement of long wavelength responses. Whereas using textured substrates with σ>38 nm , (2 2 0) preferential growth is deteriorated, yielding decreases in both short circuit current and open circuit voltage. The field-dependent carrier collection behaviors reveal that the carrier diffusion length in the poly-Si layer on textured substrates with σ>38 nm decreases due to the change in poly-Si microstructure.

Optical modeling of finely textured amorphous silicon solar cells

It has long been known that the use of finely textured transparent conducting oxide layers substantially improves the performance of thin film amorphous silicon (a-Si:H) solar cells. Major efforts to understand the nature of this effect and to fully capture its potential have been made by researchers using advanced modeling techniques. In this work, modeling the oblique angle optical performance and use of an effective medium approximation to simulate microrough interfaces suggests that effective interface grading makes a significant contribution to optical enhancement.

Analysis of Device Performance by Quasi Three-Dimensional Simulation for Thin Film Polycrystalline Silicon Solar Cells with Columnar Structure

In order to realize high efficiency thin film polycrystalline silicon (poly-Si) solar cells, a novel method of quasi three-dimensional simulation using a cylindrical coordinate system was proposed. Optimum design of cell configuration and analysis of device performance have been demonstrated. In this simulation we used realistic physical values for parameters, such as interface recombination velocity at grain boundaries and diffusion length of minority carriers. Interface recombination velocity at grain boundaries (which has a strong effect on cell performance) should be less than 103 cm/s to obtain high cell performance. Even at a relatively short diffusion length of 50 µm, high efficiency of more than 16–18% can be expected at a film thickness of 5–20 µm when grain size is 10 µm, which is appropriate for the utilization of solar grade Si of reasonable material quality.

CRYSTALLINE SILICON BASED PHOTOVOLTAICS: TECHNOLOGY AND MARKET TRENDS

An overview is given concerning industrial technologies, IMECS's advanced pilot line crystalline silicon solar cell technologies and medium term developments for industrial crystalline silicon terrestrial solar cell fabrication. Also IMEC's work on thin film crystalline silicon solar cells is shortly presented, all of this taking into account the existing market and technology trends.

Thin base-layer single crystal silicon solar cells with ECR plasmaCVD grown emitters

Thin (~16 µm) base-layer monocrystalline silicon solarcells have been investigated with microcrystalline or epitaxial n-typeemitters grown at low temperatures ({<}550 °C) by electron cyclotronresonance (ECR) plasma-enhanced chemical vapour deposition (PECVD). Thep-type, 1-2 Ωcm, base layers were epitaxially deposited byconventional thermal CVD onto monocrystalline Si p+ substrates. Anefficiency of 13.72% was achieved in the best epitaxial emitter cell afterECR hydrogen passivation and the application of a SiNx anti-reflectioncoating deposited by ECR PECVD. Cells with microcrystalline Si emittersprocessed in a similar fashion gave a maximum efficiency of 10.73%. The cellperformance was analysed using the two-diode model and the solar cellmodelling programme PC-1D. The results are presented. It was necessary toinvoke a three-layer PC-1D model to obtain self-consistent fits to the lightand dark I-V characteristics and spectral response data.

Performance of 6 μm thick crystalline silicon solar cells on glass substrate

Thin crystalline silicon (c-Si) solar cells with a thickness of 6 μm on glass substrate are fabricated. In this study, the top thin layer of silicon-on-insulator (SOI) wafers is used to realize single c-Si films. It is found that the achieved efficiency of the cell is about 8% by only preparing surface passivation without the textured structure. This value is much higher than the case of bulk c-Si solar cells if the efficiency is converted with respect to the sample thickness. It is suggested that thinner c-Si solar cells are useful for saving materials by maintaining cell efficiency properly.

Large-grained polycrystalline Si films obtained by selective nucleation and solid phase epitaxy

We investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin film crystalline silicon solar cells. Since the use of glass substrates constrains process temperatures, our chosen approach to form large-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selective crystallization of an initially amorphous silicon film, at lithographically predetermined sites, enables grain sizes larger than those observed via random crystallization. Selective heterogeneous nucleation centers were created for both P-doped, B-doped and undoped, 100 nm thick amorphous silicon films, by masked implantation of In or Ni islands, followed by annealing at temperatures below 600°C. Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy (SPE), thus obtaining crystallized regions of several tens of square microns. The maximum achievable grain size depends on the product of the SPE rate and the incubation time for the spontaneous nucleation. We have studied the dependence of the SPE rate and the incubation time on the type of metal (In and Ni) inducing the nucleation and on the electronic dopant (e.g. P and B) concentration in the 10 19–10 21 cm −3 range.

Modeling the Optical Quantum Efficiency of Thin Film Amorphous Silicon Solar Cells

ABSTRACTThe optical quantum efficiency and spectral response of p-i-n thin film amorphous silicon (a-Si:H) solar cells have been modeled using software based on optical admittance analysis. The optical constants of a-Si:H and Indium Tin Oxide (ITO) thin film layers have been measured by Variable Angle Spectroscopic Ellipsometry (VASE) and used as inputs into the optical admittance analysis program in order to model cells constructed from these films.Amorphous silicon thin films and p-i-n assemblies have been deposited by glow discharge and reactive sputtering techniques. The optical constants of doped and intrinsic a-Si:H thin films were determined by VASE and the film thickness verified by Scanning Electron Microscopy studies. The optical constants of commercially available transparent conducting oxide (TCO) coated substrates have also been determined by VASE.The experimental transmission spectra of p-i-n assemblies are compared with transmission spectra that have been modeled using the measured optical constants. Results of modeling different a-Si:H solar cell structures using these materials are presented, including a study of the optimal TCO layer thickness for p-i-n a-Si:H solar cells. This work shows that optical admittance modeling gives a good prediction of the optical behavior of p-i-n assemblies, but that accurate measurements of the optical constants of the component films are required in order to model effectively the optical quantum efficiency and photocurrent.

Light trapping in Silicon-Film™ solar cells with rear pigmented dielectric reflectors

This paper presents the novel method of using pigmented dielectric reflectors to provide light trapping in thin-film silicon solar cells. This type of reflecting material offers many potential advantages over specular metallic reflectors, including low cost, compatibility with high temperatures common to solar cell processing, and high, broadband and diffuse reflectance. As this is the first time this concept is described, the basic theory of the optical behavior of pigmented materials is presented by connecting the basic material properties of pigment and medium to their light-trapping benefit in thin silicon solar cells. Several general principles leading to maximum light-trapping benefit are identified, and experimental evidence is presented corroborating these general principles. Light trapping is demonstrated in thin silicon solar cells with pigmented dielectric reflectors by measurement and analysis of external quantum efficiency curves. Copyright © 1999 John Wiley & Sons, Ltd.

Alternative contact designs for thin epitaxial silicon solar cells

Two major opportunities for increasing the performance of crystalline silicon solar cells involve reducing their thickness and reducing the losses associated with their front metallic grid contacts. Front grid contacted thin epitaxial silicon solar cells based on the growth of crystalline silicon films on a substrate or superstrate have been reported for many years, as have wafer-based solar cells with alternative contact approaches. Integrating these two concepts into a single device presents an opportunity for simultaneously reducing two major loss mechanisms associated with crystalline silicon solar cells. The opportunities that exist and challenges that must be overcome in order to realize such a device are described in this paper. The design space is defined and explored by considering a wide range of possible approaches. A specific approach was chosen and used to design, grow, and fabricate a proof-of-concept thin epitaxial silicon solar cell with an embedded semiconductor grid as an alternative to a conventional front metallic grid. The work presented here has resulted in a thin epitaxial silicon solar cell with a 7·8% designated area conversion efficiency, well isolated contacts, negligible series resistive power loss, and less than 1% shading of the designated area. Copyright © 1999 John Wiley & Sons, Ltd.

Analytical study of carrier photogeneration and photocurrent in thin silicon solar cells with specular and diffuse back reflection

An analytical study of the effect of back reflection in thin silicon solar cells implementing a practical (nonideal) diffuse reflector at the back side is presented. Both the diffuse and specular reflection components are considered. The reflection properties of the back reflector are described by means of three parameters, which are interpreted in terms of a simple physical model. An exact expression for the generation profile resulting from specular and diffuse reflections is derived and its dependence on the reflector parameters is studied. A mechanism through which specular reflection can significantly enhance light trapping in the case of relatively weak diffuse reflection is proposed. The photocurrent boost doe to both types of reflections acting simultaneously is estimated.

Very thin film crystalline silicon solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 /spl mu/m on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p-type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si laser was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 /spl mu/m demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), the open-circuit voltage of 0.539 V and the short current density of 25.8 mA/cm/sup 2/ as independently confirmed by Japan Quality Assurance, which shows the no clear light-induced degradation. The optical and transport properties of poly Si cells are summarized.

Ni-Induced Selective Nucleation and Solid Phase Epitaxy of Large-Grained Poly-Si on Glass

AbstractWe investigated the formation of large-grain polycrystalline silicon films on glass substrates for application in low-cost thin film crystalline silicon solar cells. Since use of glass substrates constrains process temperatures, our approach to form large-grain polycrystalline silicon templates is selective nucleation and solid phase epitaxy (SNSPE). In this process, selective crystallization of an initially amorphous silicon film, at lithographically predetermined sites, enables grain sizes larger than those observed via random crystallization. Selective heterogeneous nucleation centers were created on undoped, 75 nm thick, amorphous silicon films, by masked implantation of Ni islands, followed by annealing at temperatures below 600 °. At this temperature, the Ni precipitates into NiSi2 particles that catalyze the transition from the amorphous to the crystalline Si phase. Seeded crystallization begins at the metal islands and continues via lateral solid phase epitaxy (SPE), thus obtaining crystallized regions of several tens of square microns in one hour. We have studied the dependence of the crystallization rate on the Ni-implanted dose in the seed, in the 5×1015/cm3 - 1016/cm3range. The large grained polycrystalline Si films were then used as a substrate for molecular beam epitaxy (MBE) depositions of 1 [.proportional]m thick Si layers. Transmission electron microscopy (TEM) analysis showed a strong correlation between the substrate morphology and the deposited layer. The layer presented a large grain morphology, with sizes of about 4 [.proportional]m.

Photoinduced Space Charge Effects in a-Si:H Solar Cells

AbstractWe have investigated charge collection in thin amorphous silicon solar cells under light bias illumination, both experimentally and by numerical simulation. In such charge collection experiments, space charge due to trapped bias-light generated carriers leads to an enhancement of a small signal probe beam charge collection. It is found that this enhancement of the small signal charge collection is strongly dependent on the diode thickness and the defect density in the samples. In particular for thin diodes (d &lt; 0.5 microns) the charge collection enhancement can be shown to increase with light-induced degradation of the devices. The effect of these material parameters as well as other experimental parameters, such as light bias and probe beam photon flux, will be demonstrated by means of numerical simulation.

98/03939 Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

98/03939 Optimum two-dimensional short circuit collection efficiency in thin multicrystalline silicon solar cells with optical confinement

Optical intensity of light in layers of silicon with rear diffuse reflectors

Light trapping is essential to the performance of thin-layer polycrystalline silicon solar cells. One relatively new method of light trapping is the use of textured or pigmented dielectric layers on the rear surface which diffuse and reflect light reaching the rear surface. This article presents a one-dimensional optical model of thin silicon solar cells with such rear diffuse dielectric reflectors. Experimental front reflectance measurements and ray tracing simulations of thin, planar silicon layers with rear diffuse dielectric rear reflectors are presented and compared to the model with good agreement. Finally, an expression for the upper limit of the optical enhancement factor is developed for this type of light trapping.

Micromachined thin solar cells with a novel light trapping scheme

Micromachined thin crystalline silicon solar cells have been fabricated with beam steering optical components on the surface. Microlens arrays focus light onto beam steering mirrors in V-grooves on the cell surface. One face of a V-groove is coated with aluminium to form the mirror so that the reflected light is coupled into the cell through the opposite face. The use of micromachining has allowed most of the cell surface to be covered by aluminium making it more effective in trapping light and giving good electrical contact.