Lieven Stalmans

Porous silicon in crystalline silicon solar cells: A review and the effect on the internal quantum efficiency

Crystalline silicon (c-Si) is the dominant semiconductor material in use for terrestrial photovoltaic cells and a clear tendency towards thinner, active cell structures and simplified processing schemes is observable within contemporary c-Si photovoltaic research. The potential applications of porous silicon and related benefits are reviewed. Specific attention is given to the different porous silicon formation processes, the use of this porous material as anti-reflection coating in simplified processing schemes and for simple selective emitter processes and its light trapping and surface passivating capabilities, which are required for advantageous use in thin active cell structures. Our analysis of internal quantum efficiency data obtained on both conventional and thin-film c-Si solar cells has been performed with the aim of describing the light diffusing behaviour of porous Si as well as investigating the surface passivating capabilities. An effective entrance angle of 60° is derived, which corresponds to totally diffuse isotropic light, and the importance of a correction for absorption losses in the porous layer is illustrated. Furthermore, photoconductivity decay measurements of freshly etched porous Si on float-zone p-type Si indicate a strong bias-light dependency and a fast degradation of the surface recombination velocity.

Rapid thermal oxidation of porous silicon for surface passivation

Abstract Rapid thermal oxidation with dry oxygen has been carried out on porous silicon (PS) films formed by electrochemical etching. The purpose of the paper was to investigate the surface passivation capability of the oxidized PS layers and to understand the oxidation mechanism. Rutherford back scattering (RBS) and X-ray photoemission spectroscopy (XPS) analyses confirmed the formation of a stoichiometric quasi-silicon dioxide. Besides, elastic recoil diffusion analysis (ERDA) demonstrated that a high concentration of hydrogen is still present in the PS film even after oxidation. RTO resulted in a good surface passivation effect at high temperature (>1000°C) as seen by internal quantum efficiency analysis. However, lifetime in bulk silicon is affected by the RTO process.

Comparative Analysis of Chemically and Electrochemically Formed Porous Si Antireflection Coating for Solar Cells

From the viewpoint of the porous Si (PS) application as an antireflection coating (ARC) in crystalline Si solar cells, a comparative study has been made on the morphological, kinetic, and optical properties of both chemically and electrochemically etched PS. These results are correlated and interpreted in terms of a simplified empirical model for the simultaneous PS formation and dissolution of Si. The fact that PS mainly carries potential as an ARC stresses the importance of the reflectance study. Chemically etched (stain-etched) PS has a pronounced porosity gradient in depth which is the result of a simultaneous etching at the PS/Si interface and Si dissolution throughout the existing porous layer. This implies that a part of the solar cell emitter is removed during stain etching. The predominant etching at the PS/Si interface in the case of electrochemically etched PS only converts part of the solar cell emitter into a porous layer and results in a nearly constant porosity in the thickness range of interest (up to 200 nm) for solar cell applications. Multicrystalline Si (mc-Si) behaves strongly similar to monocrystalline Si when applying an electrochemical porous etching, while stain etching is intrinsically more sensitive to the Si surface structure. The typical reflectivity behavior of both types of PS is interpreted in terms of the morphological differences. The integrated reflectance of both electrochemically formed and stain-etched porous layers is comparable to the value obtained for a silicon nitride coating on textured mc-Si, which stresses the potential of a PS ARC for crystalline Si solar cells.

Transmission electron microscopy investigation of the crystallographic quality of silicon films grown epitaxially on porous silicon

Epitaxial growth of thin crystalline layers on porous silicon can provide opportunities for silicon-on-insulator applications and silicon-based solar cells provided that the epitaxial layer has a sufficient crystallographic quality. Transmission electron microscopy (TEM) is used in this work to study the microstructural properties of porous silicon (PS) and of epitaxial Si layers grown on top of the PS. A more dense silicon layer exists in the upper part of the as-prepared porous silicon. The quality of the epitaxial layers is found to depend strongly on the morphology of the initial porous Si layers, and on the deposition temperature of the epitaxial silicon growth. The porous structure is completely destroyed after thermal CVD deposition of Si at too high temperature, resulting in a highly defective epitaxial layer. A high-quality epi-Si layer is obtained when depositing on a low porosity layer at 725°C. A stacked porous layer with a low porosity in the upper-part and a high porosity in the bulk can be formed by changing the conditions during the formation of the layer. On such a dual porous layer, an epitaxial silicon layer with a low defect density can be grown.

Efficient luminescence from porous silicon

Abstract Photoluminescence measurements are carried out on porous silicon layers. We show the enhancement and stabilization of the luminescence when depositing a silicon nitride layer on top of porous layers.We also demonstrate that direct- and remote-plasma nitridation are good ways to reduce the ageing effect of porous silicon layers due to a passivation of dangling bonds.

Low-thermal-budget treatments of porous silicon surface layers on crystalline Si solar cells: A way to go for improved surface passivation?

Porous silicon (PS) has several potential interests for crystalline Si solar cells. Besides the use as an anti-reflection coating, the porous layer also acts as a light-diffusor. However, major drawbacks are the light absorption within the porous layer and both insufficient as well as unstable surface passivating capabilities. This work deals with a comparative analysis of different PS treatments with the aim of maintaining the light diffusing property while both the absorption losses are reduced and surface passivation is improved and stabilized. In order to obtain a surface layer with a controlled and stable structure and composition, rapid thermal oxidation (RTO), plasma-nitridation and anodic oxidation have been selected as potentially interesting pathways with a low thermal budget in common. The effects of these different treatments are studied simultaneously on the level of the porous material as well as on solar cell structures (IQE-analysis). A solar cell process is applied which provides an identical emitter for all conditions allowing an analysis of the blue response and an assessment of the most suited porous Si treatment. An improvement of the blue response is observed for RTO treatments at high temperatures, which is due to the creation of an intermediate oxide at the PS/Si interface. No passivation effects are observed in the case of nitridation or anodic oxidation. The modified porous material preserves its light diffusing properties and suffers less from light absorption. The conclusions are drawn up as a strength-weakness analysis for each of the investigated treatments. This balance is not in favour of applying any of the PS modification techniques since in all three cases important drawbacks are the presence of an additional process step as well as the fact that the refractive index decreases which is unfavourable from the viewpoint of ARC-properties.

Effects of Low-Thermal-Budget Treatments on the Porous Si Material Properties

Our interest in porous silicon is due to its potential benefits in crystalline Si solar cells. Besides the use as an anti-reflection coating, the porous layer also acts as a light-diffusor. However major drawbacks are the significant light absorption within the porous layer and both insufficient as well as unstable surface passivating capabilities. The unstable nature of the porous Si is also reflected in the presence of suboxides after storage in ambient. In this work we focus on rapid-thermal-oxidation (RTO) and plasma-nitridation as low-thermal-budget chemical modification techniques in order to obtain a surface layer with a controlled and stable structure and composition. RTO of porous Si converts the material into SiO2 in conjunction with a drastically decreased porosity. Both a remote- and a direct-plasma nitridation of porous Si are able to incorporate nitrogen uniformly throughout the porous layer while preserving the porous character.

Morphological properties of porous-Si layers for n+-emitter applications

Abstract Porous silicon layers prepared on n + emitters are investigated by a combination of spectroscopic ellipsometry, transmission electron microscopy and sheet resistance measurements. The porous silicon is formed by an anodic surface treatment of uniformly doped emitters prepared by chemical vapour deposition and of diffused emitters. The results show a good correspondence between the thicknesses obtained with the different techniques. The porosity of the layers increases for increasing doping level and saturates at a doping above ∼5×10 19 /cm 3 . The formation rate shows an inverse doping dependence. The top surface layer of the porous silicon is in all cases more dense than the bulk, which is related to an initially faster formation rate.

Pore Propagation Directions in P+ Porous Silicon

A comparative study is presented on the pore propagation directions of porous silicon layers (PSL) formed on p+-type substrates of different orientations. PSLs were formed on plain (0 0 1) and (1 1 1) silicon wafers as well as on structured (0 0 1) wafers containing facets of various orientations. During anodization, regular pores follow the 〈0 0 1〉 direction on the (0 0 1) planes. While on the (1 1 1) planes fewer regular pores develop and seemingly propagate closely to the 〈1 1 1〉 direction. These results indicate that the pores propagate perpendicular to the surface i.e. along the field lines when the surface orientation is either (0 0 1) or (1 1 1).When the silicon surface provided (1 1 0) orientation (Chuang, Collins, and Smith, 1989), or its position is in between the (0 0 1) and (1 1 1) planes then the pores do not propagate perpendicular to the surface but along the 〈0 0 1〉 direction.All the phenomena exhibited might be explained by presuming that during formation, the pores propagate along the 〈1 0 0〉 directions, and that those 〈1 0 0〉 directions are preferred which are closely to the field lines. In PSLs formed on (0 0 1) surfaces the field lines and the 〈0 0 1〉 crystallographic direction are coincident. However, in the (1 1 1) oriented wafer where three equally probable 〈1 0 0〉 directions exist around the field lines, more irregular structure of PSLs will develop.

Advanced concepts and options for thin-film crystalline Si solar cells

The purpose of this paper is twofold. Firstly, it presents a concise overview of the benefits to be expected from the development of a thin-film crystalline Si solar cell technology on a low-cost substrate as well as the technical and physical challenges encountered. It becomes clear that a large number of options are open in terms of substrate, deposition technology and solar cell technology. Secondly, focus is placed on the approach chosen by the thin-film crystalline Si solar cell team of IMEC and the results and progress of understanding achieved by the group over the last five years. Basically, their technical strategy is based on an evolutionary scheme for the cell design. More specifically, a transition is made from a classical two-side contacted cell design to a one-side contacted cell type which paves the way for a monolithic module design. For the technical realisation of the thin crystalline Si layers, chemical vapour deposition is expected to be the preferred technique, at least in a short and mid-term perspective.