Figure 117: Phase diagram for photoexcited electrons and holes in unstressed siliconIn fig. 118 we see the energy dependence of the wave vectork. Remember that the solution has
similarity with the tight binding model beside the sign.
Frenkel excitons occur in organic solar cells, organic light emitting diodes and in photosynthesis. They
can transport energy but no charge.
14.2 Optical measurements
14.2.1 Introduction
There are various types of optical measurements. It is possible to shine light on a sample and analyse
the reflected light or the electrons that come out. It is also possible to shine an electron beam on the
sample and analyse the light that comes out.
When light comes in, it appears as an oscillating electric field for the sample. This electric field changes
the polarization of the material, which is made of many little dipoles. The dipoles start to oscillate
with the incoming frequency and emit dipole radiation, which is perpendicular to the oscillation axis.
With the detected scattered radiation it is possible to determine what the dipole is, which generates
this radiation. Then it is possible to calculate the electric field and the polarization. With these two
quantities, we can calculate the susceptibility and the dielectric constant.
14.2.2 Ellipsometry
The incoming light gets polarized and the polarization of the outgoing light is measured. This is
done for many different polarizations of the incoming light, so that one can get an impression of the
dependence of the polarizations of the two light beams. The measured signal gives a lot of information
about the thickness and the dielectric constant of the involved layers. For this technique to work, it