Category Thin-Film Crystalline Silicon Solar Cells

What is the maximum path length enhancement?

A structured surface of the thin Si films enhances the optical path length since light is internally reflected frequently. The path length enhancement depends on the shape of the surface texture. In the framework of geometrical optics, which means that the size of the surface texture and the film thickness are large compared to the wavelength, the maxi­mum path length enhancement was previously derived for solar cells having surfaces of zero reflectance [16]. In this work we give a generalization of this theorem to the more realistic case of cells with non-vanishing surface reflectance. One consequence of an upper limit of the average path length is an upper limit for the optical absorption and thus for the photogeneration in the cell...

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Physical problems with thin-film crystalline Si cells

Aiming at a high power conversion efficiency from Si cells with a thickness of only a few microns raises questions on the physical limits of power conversion, on device fab­rication, and on device characterization. The subsequent sections will introduce the questions treated in this work.

What is the maximum photogeneration?

For very thin Si films the optical absorption severely limits the device current. Figure 1.2 shows the optical absorption A on a path length of / = 1, 10, and 1000 pm in crystal­line Si. For a path length of 1 to 10 pm the near-infrared fraction of the solar spectrum is hardly absorbed since silicon is an indirect semiconductor with a small absorption coef­ficient in this spectral region...

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Thin-film crystalline Si cells

The reduction of the fabrication costs increases the relative significance of the mate­rial costs in industrial solar cells. In today’s commercial modules the cost of the raw Si wafer is about half of the total cost. This fraction will further increase with the steadily growing global production volume of Si solar modules that brings about a shortage in Si supply. Hence there is a necessity to use thinner Si wafers in order to save Si material. Wafers thinner than 200 pm are, however, difficult to process without breakage.

Thin-film crystalline Si cells are an alternative to wafer cells. By definition, these cells have a thickness of less than 50 pm and are deposited onto a suitable substrate. The substrate enhances the mechanical strength and avoids the breakage of the thin films...

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Industrial crystalline Si solar cells

For cost reasons, commercial cells cannot use most of the above-mentioned high- efficiency features: double-layer antireflection coatings are replaced by single-layer antireflection coatings, and a surface texture is often omitted. Monocrystalline Si wafers are increasingly replaced by block-cast multicrystalline wafers. The 30% kerf loss when sawing the Si wafers from an ingot is avoided by directly pulling multicrystalline wafer ribbons [8]. Screen printing replaces the photolithographic definition of the fingers. Local diffusions are not applied. These simplifications of the fabrication sequence re­duce the fabrication costs of industrial cells by a factor that is larger than the accompa­nying reduction of the cell efficiency...

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Highest-efficiency crystalline Si solar cells

The current world record in power conversion efficiency with Si solar cells is 24.7% and was achieved with a solar cell that is shown schematically in Figure 1.1 [1]. The design features that are decisive for high photogeneration and low carrier recombination are listed in Table 1.1. A double-layer antireflection coating and the photolithographi­cally defined front surface texture with regular inverted pyramids minimize the reflec­tion loss at the front surface. The Si wafer is 400 pm thick to offer a long optical path length that permits close to complete absorption of all those photons that have an energy larger than the electronic bandgap of Si...

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Introduction

Semiconducting photovoltaic cells convert solar radiation to electric power. A photon that enters the cell will contribute to the electric current if it is absorbed by exciting an electron from the valence band into the conduction band and if this electron recombines neither in the volume nor at the surfaces of the semiconductor. High optical absorption and little carrier recombination are therefore two prerequisites for an efficient power conversion. The physical mechanisms and the solar cell design which maximizes the power output are well understood.

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Preface

Photovoltaics with thick crystalline Si wafers is a mature technology that is currently entering large-scale production. For a widespread solar electric power generation, however, a substantial reduction of the fabrication cost is required. For this purpose thin – film technologies are being developed with only micron-thick semiconductor layers for light absorption. While thin-film modules from amorphous Si, Cu(In, Ga)Se2, and CdTe are already being commercially produced on a small scale, the development of thin-film modules from crystalline Si is still in the laboratory phase. This phase is characterized by competition of many different approaches for depositing and fabricating the thin crystalline Si solar cells.

This book is adressed to the physicist and the engineer who are interested ...

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Thin-Film Crystalline Silicon Solar Cells

This book by Rolf Brendel closes a gap in the literature on photovoltaics, in particular on silicon solar cells. While there are several books on the general aspects of this topic available, they are limited mostly to the theory and practice of bulk silicon solar cells. The present book emphasizes thin silicon solar cells and treats the subject in a very comprehensive manner. Dr. Brendel is exceptionally qualified to write such a book because he has contributed personally in important ways to this field.

The crystalline silicon solar cell in its conventional form dominates today, with about 90% of the world market...

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