Figure 3.1: (a) Illustrating the absorption of a photon in a semiconductor with bandgap E G . The photon with energy E ph
= hν excites an electron from E i to E f . At E i a hole is created. (b) If E ph > E G , a part of the energy is thermalized.
In a real semiconductor, the valence and conduction bands are not flat, but vary
depending on the so-called k-vector that describes the momentum of an electron in the
semiconductor. This means that the energy of an electron is dependent on its momentum
because of the periodic structure of the semiconductor crystal. If the maximum of the
valence band and the minimum of the conduction band occur at the same k-vector, an
electron can be excited from the valence to the conduction band without a change in the
momentum. Such a semiconductor is called a direct bandgap material. If the electron
cannot be excited without changing its momentum, we refer to it as an indirect bandgap
material. The electron can only change its momentum by momentum exchange with the
crystal, i.e. by receiving momentum from or giving momentum to vibrations of the crystal
lattice. The absorption coefficient in a direct bandgap material is much higher than in an
indirect bandgap material, thus the absorbing semiconductor, often just called the
absorber, can be much thinner [24].
If an electron is excited from E i to E f , a void is created at E i . This void behaves like a
particle with a positive elementary charge and is called a hole. The absorption of a photon
therefore leads to the creation of an electron-hole pair, as illustrated in Figure 3.2 . The
radiative energy of the photon is converted to the chemical energy of the electron-hole
pair. The maximal conversion efficiency from radiative energy to chemical energy is
limited by thermodynamics. This thermodynamic limit lies between 67% for nonconcentrated sunlight and 86% for fully concentrated sunlight [25].
The basic physics required for describing semiconductors is presented in Chapter 6.
2. Subsequent separation of the photo-generated charge carriers in the junction
Usually, the electron-hole pair will recombine, i.e. the electron will fall back to the initial
energy level E i , as illustrated in Fig. 3.2 . The energy will then be released either as
photon (radiative recombination) or transferred to other electrons or holes or lattice
vibrations (non-radiative recombination). If one wants to use the energy stored in the
electron-hole pair for performing work in an external circuit, semipermeable membranes
= hν excites an electron from E i to E f . At E i a hole is created. (b) If E ph > E G , a part of the energy is thermalized.
In a real semiconductor, the valence and conduction bands are not flat, but vary
depending on the so-called k-vector that describes the momentum of an electron in the
semiconductor. This means that the energy of an electron is dependent on its momentum
because of the periodic structure of the semiconductor crystal. If the maximum of the
valence band and the minimum of the conduction band occur at the same k-vector, an
electron can be excited from the valence to the conduction band without a change in the
momentum. Such a semiconductor is called a direct bandgap material. If the electron
cannot be excited without changing its momentum, we refer to it as an indirect bandgap
material. The electron can only change its momentum by momentum exchange with the
crystal, i.e. by receiving momentum from or giving momentum to vibrations of the crystal
lattice. The absorption coefficient in a direct bandgap material is much higher than in an
indirect bandgap material, thus the absorbing semiconductor, often just called the
absorber, can be much thinner [24].
If an electron is excited from E i to E f , a void is created at E i . This void behaves like a
particle with a positive elementary charge and is called a hole. The absorption of a photon
therefore leads to the creation of an electron-hole pair, as illustrated in Figure 3.2 . The
radiative energy of the photon is converted to the chemical energy of the electron-hole
pair. The maximal conversion efficiency from radiative energy to chemical energy is
limited by thermodynamics. This thermodynamic limit lies between 67% for nonconcentrated sunlight and 86% for fully concentrated sunlight [25].
The basic physics required for describing semiconductors is presented in Chapter 6.
2. Subsequent separation of the photo-generated charge carriers in the junction
Usually, the electron-hole pair will recombine, i.e. the electron will fall back to the initial
energy level E i , as illustrated in Fig. 3.2 . The energy will then be released either as
photon (radiative recombination) or transferred to other electrons or holes or lattice
vibrations (non-radiative recombination). If one wants to use the energy stored in the
electron-hole pair for performing work in an external circuit, semipermeable membranes