Photovoltaic Power Generation - Renewable Energy: Technology, Economics and Environment - page 233

kinds of doping must not be applied simultaneously, since the effects of acceptors
and donors cancel each other (also refer to /6-3/, /6-4/).
Semiconductors are further distinguished into "direct" and "indirect" semicon-
ductors. While for direct semiconductors only energy is required to transfer
charge carriers from the valence band to the conduction band, for indirect semi-
conductors also a momentum needs to be transferred to the charge carrier. This is
mainly due to the band structure and has a tremendous impact on the appropriate-
ness of semiconductor materials for solar cells. While for a direct semiconductor
absorption of an incident photon is sufficient to lift the charge carrier up to the
conduction band (i.e. exclusive energy transfer by the photon), for indirect semi-
conductors an appropriate momentum additionally needs to be transferred. This
process requires three particles: the charge carrier (1 st particle) simultaneously
receives sufficient energy quantities from the photon (2 nd particle) and the re-
quired momentum from a phonon (3 rd particle); quanta of a crystal momentum are
referred to as phonons. Only if all three particles meet simultaneously (i.e. three
particle process), charge carriers are lifted into the conduction band. In compari-
son to direct semiconductors (two body process) these conditions are much rarer.
This is why in case of indirect semiconductors the photon inside the semiconduc-
tor material has to travel a much longer distance until it is absorbed.
Crystalline silicon is such an indirect semiconductor, and silicon cells must
thus be relatively thick and/or contain an appropriate light-trapping scheme to
generate a prolonged optical path length. Amorphous silicon, CdTe or CIS (see
Chapter 6.2.1), are in contrast direct semiconductors. Solar cells made of these
materials can thus have a thickness clearly below 10 µm, while the thickness of
crystalline silicon solar cells typically stretches from 200 to 300 µm. Thinner crys-
talline silicon cells are under development, but must be provided with the dis-
cussed optical properties, resulting in increased manufacturing expenditure.
6.1.4 Photo effect
The term "photo effect" refers to the energy transfer from photons (i.e. quantum
of electromagnetic radiation) to electrons contained inside material. Photon en-
ergy is thereby converted into potential and kinetic energy of electrons. The elec-
tron absorbs the entire quantum energy of the photon defined as the product of
Planck's quantum and the photon frequency. External and internal photo effect is
distinguished.
External photo effect. If electromagnetic radiation hits the surface of a solid
body within the ultraviolet range, electrons can absorb energy from the photon.
Then they are able to surmount the required work function to escape from the
solid body, provided that there is sufficient photon energy. This process is re-
ferred to as the external photo effect.