Kelvin Probe Force Microscopy Measuring and Compensating Electro
In the nearly 20 years of Kelvin probe force microscopy an increasing interest in the technique and its applications has developed. This book gives a concise introduction into the method and describes various experimental techniques. Surface potential stu
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Optoelectronic Studies of Solar Cells S. Sadewasser
Abstract Solar cells are a most promising candidate to supply future energy needs in a sustainable and renewable way. Currently, solar cell devices based on semiconductor materials achieve the highest power conversion efficiencies. A typical solar cell consists of a semiconductor pn-junction where the semiconductor band gap is in the range between 1 and 2 eV, well adapted to absorb a large part of the solar spectrum. Issues as band alignment and spatial homogeneity of the materials are essential in providing the optimum achievable efficiencies. Kelvin probe force microscopy has been applied to a wide range of solar cell materials and devices, ranging from crystalline and amorphous silicon to polycrystalline CdTe and Cu(In,Ga)(S,Se)2 to organic semiconductors and molecules. On these systems, KPFM has been applied in different ways, from surface characterization to effects of illumination to cross-sectional studies. From many of these local work function measurements, understanding of functional principles and limiting factors has been gained. This chapter reviews the results that have been obtained by Kelvin probe force microscopy on solar cell devices and materials and describes how the gained understanding promotes the improvement of solar cell devices for renewable energy conversion.
8.1 Introduction to Solar Cells The world’s energy demand is continuously increasing and it is clear that fossil fuels will in the future not be able to supply sufficient energy. The only solution to this energy problem is the conversion of our energy systems to renewable energies. One
S. Sadewasser () International Iberian Nanotechnology Laboratory - INL, Avda. Mestre Jos´e Veiga s/n, 4715-330 Braga, Portugal e-mail: [email protected] S. Sadewasser and T. Glatzel (eds.), Kelvin Probe Force Microscopy, Springer Series in Surface Sciences 48, DOI 10.1007/978-3-642-22566-6 8, © Springer-Verlag Berlin Heidelberg 2012
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conduction band
EFn
ohmic contact
EFp
ohmic contact
valence band
Fig. 8.1 Band diagram of the pn-junction in a solar cell
of the prime suppliers of such renewable energy is the sun, and it is predicted that conversion of the radiation from the sun into usable energy will cover a main part of our future energy needs. One way is the direct conversion of sunlight into electricity by means of photovoltaic solar cells [1, 2]. The development of the currently most widespread solar cells started half a century ago. This crystalline silicon solar cell, however, has the disadvantage that it is based on an indirect band gap semiconductor, which results in the requirement to fabricate solar cells with a sufficient thickness of about 300 µm. Subsequent developments implement direct band gap materials, thereby providing the possibility to use less material and fabricate thinner devices. Such thin film solar cells consist of amorphous silicon, Cu(In,Ga)Se2, CdTe, III–V semiconductors or organic materials. At the core of a semiconductor solar
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