Copper Zinc Tin Sulfide-Based Thin-Film Solar Cells

Kentaro Ito

• Publisher: Wiley
• Year: 2014
• Print ISBN:
  9781118437872
• Edition: 1
• eBook ISBN:
  9781118437858
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Beginning with an overview and historical background of Copper Zinc Tin Sulphide (CZTS) technology, subsequent chapters cover properties of CZTS thin films, different preparation methods of CZTS thin films, a comparative study of CZTS and CIGS solar cell, computational approach, and future applications of CZTS thin film solar modules to both ground-mount and rooftop installation.

The semiconducting compound (CZTS) is made up earth-abundant, low-cost and non-toxic elements, which make it an ideal candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe solar cells which face material scarcity and toxicity issues. The device performance of CZTS-based thin film solar cells has been steadily improving over the past 20 years, and they have now reached near commercial efficiency levels (10%). These achievements prove that CZTS-based solar cells have the potential to be used for large-scale deployment of photovoltaics.

With contributions from leading researchers from academia and industry, many of these authors have contributed to the improvement of its efficiency, and have rich experience in preparing a variety of semiconducting thin films for solar cells.

Part I
Introduction

1
An Overview of CZTS-Based Thin-Film Solar Cells

Kentaro Ito

Department of Electrical and Electronic Engineering, Shinshu University, 1-17-4 Wakasato, Nagano 380-8553, Japan

1.1 Introduction

This book deals with the thin-film solar cell with optical absorber layer composed of the copper-zinc-tin-sulphide-based quaternary semiconductor represented by chemical formula Cu₂ZnSnS₄ or related compound semiconductors. Throughout this book, we abbreviate the quaternary compound as CZTS. The concept of CZTS thin-film solar cells is based on the following principles. The compound semiconductor meets two necessary conditions for efficient solar cells. One is the direct nature of the band gap and the other is its width within a certain optimal range for photovoltaic cells. Because the pre-factor of absorption coefficient for the CZTS thin film is large enough the layer of just micron thickness is able to absorb sunlight sufficiently, and the use of it as an absorber does not have any damaging effects on photocurrents. The probability of radiative recombination in the film is able to exceed that of non-radiative recombination if both absorption and emission of photons are caused by an allowed direct transition of carriers between valence and conduction bands without any intermediaries such as crystal defects and phonons. It is therefore possible for cell efficiency to approach the theoretical limit if Shockley–Read–Hall-type recombination centers, which play a role in bypassing the direct recombination, are diminished and at the same time a device structure to confine excited electrons in the CZTS base layer is implemented. The CZTS semiconductor is potential candidate material for terawatt (TW) -scale photovoltaic energy conversion: a fractional amount of the elemental constituents produced annually is sufficient to fabricate CZTS thin-film solar cells which can supply renewable energy on a scale comparable to the world’s electricity consumption. The multiplicity of the compound is advantageous in designing the semiconductor material for photovoltaic devices, because we can control its physical properties depending on a substitution of the cation or anion included in the fundamental tetrahedron for another cation or anion and we can also avoid the undesirable use of rare or toxic elements. The incomplete (9%) substitution of sulfur for selenium is a typical example, which has lead to the achievement of alloy thin-film solar cells with over 10% efficiency [1, 2].

The physics of the photovoltaic effect are described in Section 1.2, including: the spectral irradiance of solar radiation and the influence of the Earth’s atmosphere on it; the upper limit of conversion efficiency of a single-junction solar cell which is evaluated on the basis of a detailed balance model; an optimal range of energy band gaps for photovoltaic energy conversion; optical absorption in semiconductor thin films and the estimation of the thickness of the absorber layer required for an efficient thin-film solar cell; and important roles of semiconductor pn- (positive or negative) homo- and hetero-junctions in the photovoltaic effect. In Section 1.3 we describe the pursuit of an optimal semiconductor for photovoltaic applications which have a band gap within the optimal range. The history of the thin-film solar cell is first discussed, including studies on some mono-crystalline semiconductor materials and their photovoltaic applications and the development of a chalcopyrite-type thin-film solar cell for comparison. We then describe how the concept of CZTS technology originated. Finally, we describe our synthesis and characterization of the CZTS absorber and n-type buffer layers to conclude the chapter.

1.2 The Photovoltaic Effect

1.2.1 Solar Radiation

1.2.1.1 Extra-terrestrial Radiation

At the core of the sun, nuclear fusion of hydrogen releases massive heat. The sun is surrounded by a thin atmosphere which consists mostly of hydrogen atoms. This is the so-called photosphere that absorbs the heat and emits electromagnetic radiation into outer space with almost the same spectral radiation as that of a black body in thermal equilibrium at a high temperature T _S. According to Planck’s formula, the power emitted per unit projected area of the black body into a unit solid angle per unit frequency interval is given by the spectral irradiance L_ν (T _S), defined

(1.1)

where ν is the frequency of radiation, c is the light speed, h is the Plank constant, and k _B is the Boltzmann constant. The photon energy of electromagnetic oscillation at frequency ν is given by hν. The solid angle Ω_S of the sun (in steradians) which is seen from the Earth is calculated as:

(1.2)

where r is the radius of the sun (i.e. 6.96 × 10⁵ km) and R is the mean orbital radius of the Earth rotating around the sun (i.e. 1.496 × 10⁸ km).

The spectral photon irrad...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• List of Contributors
• Part I: Introduction
• Part II: The Physics and Chemistry of Quaternary Chalcogenide Semiconductors
• Part III: Synthesis of Thin Films and Their Application to Solar Cells
• Part IV: Device Physics of Thin-Film Solar Cells
• Subject Index
• End User License Agreement