The German Advisory Council on Global Change (WBGU) draws in its report “World in Transition – Towards Sustainable Energy Systems” [1] the conclusion that
“it is essential to turn energy systems towards sustainability worldwide – both in order to protect the natural life-support systems on which humanity depends, and to eradicate energy poverty in developing countries. Nothing less than a fundamental transformation of energy systems will be needed to return development trajectories to sustainable corridors.”
In one of the scenarios developed by the German Advisory Council on Global Change (see Fig. 1.1) a part and parcel is the
“substantial development and expansion of new renewable energy sources, notably solar.”
Fig. 1.1: Transforming the global energy mix: The exemplary path until 2050/2100 (from [1]).
The Federal Ministry for the Environment, Nature Conservation and Nuclear Safety investigated in its report “Renewable energies – Innovations for the future” [2] the technical potential for use of the available energy sources, namely the continental solar
irradiation, the kinetic energy from the wind, waves, and ocean currents, the biomass which grows again each year, the potential energy of water, the geothermal energy, and the thermal energy from the seas (see Fig. 1.2).
Fig. 1.2: Rear cubes: The natural availability of renewable energy is extraordinarily large. Front cubes: The technically available energy in the form of electricity, heat, and chemical energy carriers exceeds the present-day energy demand (grey cube, left) by a factor of six (altered from [2]).
Solar energy has a superior position amongst all renewable energies, firstly due to the nearly unlimited natural supply by the sun. Even the sustainable use with state-of-the-art technology exceeds the long-term global energy demand by far. Secondly, photovoltaic systems feature a good modularity, hence being very well suited for developing countries, too
Many different materials and technologies are available for producing solar cells. Fig.
1.3 gives an overview of the market share of the four major cell technologies used in the global photovoltaic market. All variants are based on silicon as starting material, which is available in nearly unlimited quantity. Silicon is nontoxic and is the driving force behind the microchip industry; hence a broad knowledge exists already.
The relatively expensive, but high-purity monocrystalline silicon is costly in its production, but yields the solar cells with the highest conversion efficiencies. On the
1.1 Motivation 3
other hand multi- or polycrystalline silicon is cheaper in its production, but resulting solar cell conversion efficiencies are lower due to the higher amount of impurities and crystallographic defects in the silicon material. With ribbon silicon, the wafers for solar cells are not cutted from large blocks but are produced on continuous ribbons.
Even cheaper in production are solar cells based on thin-film technology or amorphous silicon. It should be stressed that besides these materials and technologies mentioned above many more already exist and are being heavily developed.
Fig. 1.3: Development of the global photovoltaic market from 1980–2007. The top bar indicates the annually produced peak power. The vast majority of produced solar cells are based on mono- and multicrystalline silicon (from [3]).
The present work is settled in the field of crystalline silicon solar cells. The starting materials of such solar cells are typically 200 – 300 µm thick silicon wafers which are subsequently processed. These silicon wafers are in reality contaminated with impurities and crystallographic imperfections. These defect centers reduce the effective lifetime of excess carriers within the material, thus also limiting the conversion efficiency of the finished solar cells.
The characterization of these electrically active defect centers in silicon for solar cells is the challenge of the present work. The motivation for this undertaking is twofold.
On the one hand, the properties of foreign atoms and crystallographic imperfections are part of fundamental research on silicon material in order to understand the interactions and consequences of such defects. Since the 1950s researchers all over the world are investigating the defect parameters of different defect centers in silicon and other semiconductor materials. Besides this theoretical approach, on the other hand,
determination of the defect parameters has also a large technological relevance. Only the exact knowledge of the properties and characteristics of these electrically active defects allows for an effective suppression or avoidance of such defect centers in wafer manufacturing as well as in solar cell processing.
The reliable and significant detection of very small defect concentrations is the major challenge for defect characterization. As few as 1010 cm-3 foreign atoms (which means one impurity per 5 trillion silicon atoms) can lead to the reduction of solar cell conversion efficiency. One characterization technique, the deep-level transient spectroscopy, emerged in the 1970s. Its main principle is based on the thermal emission of charge carriers from the defect level into the majority carrier band. This technique is the working horse of defect characterization in semiconductor research and industry.
Different lifetime spectroscopic approaches for analyzing electrically active defects emerged in the 1990s. These lifetime spectroscopic techniques measure the actual recombination rate of excess carriers via defect levels. Due to this measured quantity, lifetime spectroscopy is sensitive to the recombination channels actually limiting the effective lifetime of the excess carriers and hence the conversion efficiency of solar cells.
In this work, both different characterization techniques are used so as to access the defects parameters of electrically active defects in silicon.