Lecture

Optics: Electronic Materials

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In course

MICRO-566: Large-area electronics: devices and materials

Introduction to the physical concepts involved in the description of optical and electronic transport properties of thin-film semiconductor materials found in many large-area applications (solar cells, displays, imagers, etc) and introduction to the physic ...

Description

This lecture covers the optical properties of large area electronic materials, focusing on band diagrams, band gaps, absorption coefficients, and disorder effects in materials like c-Si and a-Si. It discusses the formation of band gaps, absorption below the gap, sub-gap absorption, and temperature dependence of absorption. The lecture also explores the concepts of Urbach absorption, hopping conductivity, extended state conductivity, activation energy, drift mobility, and thermally activated drift mobility in materials like a-Si and a-Si:H.

Instructors (2)

Christophe Ballif

Christophe Ballif is director of the Phototovoltaics and Thin Film Electronics Laboratoryb) (PV-Lab at the institute of microengineering (IMT) in Neuchâtel (part of the EPFL since 2009). The lab focus is on the science and technology of high efficiency heterojunction crystalline cells,so-called passivating contacts for solar cells, multi-junction solar cells include novel generation Perovskite on innovative optical high speed detector and on various macroelectronics application. It also deals with energy management with a focus on integration of solar electricity into the energy system. The PV-Lab has strongly contributed to technology transfer and industrialization of novel devices and full technology with numerous companies. Christophe Ballif graduated as a physicist from the EPFL in 1994, where he also obtained in 1998 his Phd degree working on novel PV materials. He accomplished his postdoctoral research at NREL (Golden, US) on compound semiconductor solar cells (CIGS and CdTe). He worked then at the Fraunhofer ISE (Ge) on crystalline silicon photovoltaics (monocrystalline and multi-crystalline) until 2003 and then at the EMPA in Thun (CH) before becoming full professor at the University of Neuchâtel IMT in 2004, taking over the chair of Prof. A. Shah. Since 2013, C.Ballif is also the director of the new CSEM PV-Center, also located in Neuchâtel. The CSEM PV-Center is focussing more on industrialisation and technology transfer in the field of solar energy, including solar electricity management and storage. At the core of the CSEM PV-center activities lies several "pilot lines" for various kinds of solar cells manufacturing, with a focus coating technologies, wet chemistry processes for crystalline silicon, metalisation techniques for solar cells, and a platform for developing "ideal packaging solutions and polymers" for PV modules. In addition, joined facitilites between CSEM and EPFL of over 800 m2 are available for modules manufacturing, measuring and accelerated aging. CSEM PV-center has also full team dedicated to storage and energy systems and operates a joined center with BFH in Biel for research on electrochemical storage. He (co-) authored over 500 journal and technical papers, as well as several patents. He is an elected member of the SATW, member of the scientific council of the Swiss AEE, and member of the board of the EPFL Energy center. In 2016, he recieved the Becquerel prize for his contributions to the field of high efficiency photovoltaics.

Franz-Josef Haug

I studied Physics between 1990 and 1996 at the Universities of Ulm (Germany) and Waikato (New Zealand). While specializing mostly on Theoretical Physics and Quantum Theory during my studies, I turned towards Experimental Physics for a graduate work on gas-phase epitaxy of silicon. After that, I pursued a PhD program at ETH Zürich (Switzerland), studying Cu(In,Ga)Se2 solar cells. As PostDoc, I went to EMPA (Switzerland) to work on plasma processes for hard coatings of TiN and to the Jülich Research Centre in Germany to study light scattering ZnO front contacts for solar cells. In 2005 I joined the Institute of Microengineering (IMT) at the University of Neuchatel to lead a research group on thin film silicon solar cells within the PV-Lab. In 2009 the IMT was integrated into EPFL. My main research interests are optics, charge transport in semiconductors and renewable energy in general. My current work is devoted to passivating contacts for crystalline silicon solar cells, using thin silicon films as contact layers. In 2015 I was appointed Maitre d'Enseignement et Recherche (Senior Lecturer and Scientist) and member of the Faculty of STI. From 2013 to 2015 I was member of the EFPL Teachers Council (CCE) and from 2016 and 2018 I was elected into the Council of the Engineering School (CF-STI). In 2020 I was elected to become a member of CF-STI. My main objective for the legislature is to represent the role of EPFL's intermediate staff and to raise the awareness for concerns of the satellite campuses.

Official source

Ontological neighbourhood

Physics

Condensed matter physics: Solid-state physics

Related concepts (32)

Electrical resistivity and conductivity

Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current. Resistivity is commonly represented by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm-metre (Ω⋅m).

Semiconductor

A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. Its conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure. When two differently doped regions exist in the same crystal, a semiconductor junction is created.

Band bending

In solid-state physics, band bending refers to the process in which the electronic band structure in a material curves up or down near a junction or interface. It does not involve any physical (spatial) bending. When the electrochemical potential of the free charge carriers around an interface of a semiconductor is dissimilar, charge carriers are transferred between the two materials until an equilibrium state is reached whereby the potential difference vanishes.

Electronic band structure

In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called band gaps or forbidden bands). Band theory derives these bands and band gaps by examining the allowed quantum mechanical wave functions for an electron in a large, periodic lattice of atoms or molecules.

Band diagram

In solid-state physics of semiconductors, a band diagram is a diagram plotting various key electron energy levels (Fermi level and nearby energy band edges) as a function of some spatial dimension, which is often denoted x. These diagrams help to explain the operation of many kinds of semiconductor devices and to visualize how bands change with position (band bending). The bands may be coloured to distinguish level filling. A band diagram should not be confused with a band structure plot.

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