Chapter 22
Light-to-Electricity Conversion
Abstract The use of diodes for the conversion of electromagnetic radiation (light) into electrical
signals is discussed with many examples for devices such as photoconductors, pn-, pin-, MSM- and
avalanche photodiodes and charge-coupled devices. Energy conversion with solar cells is explained
and standard and advanced solar cells concepts are introduced.
22.1 Photocatalysis
The absorption of light in a semiconductor across the band gap creates free electrons and holes.
In particular, for small particle size in powders
1 these charge carriers can reach the surface of the
semiconductor. At the surface they can react with chemicals. The hole can form •OH radicals from OH
−
attached to the bead. The electron can form O 2 •
− . These radicals can subsequently attack and detoxify,
e.g., noxious organic pollutants in the solution surrounding the semiconductor. Such photocatalytic
activity has been found, e.g., for TiO 2 and ZnO powders. A review of photocatalysis, in particular
with TiO 2 particles and their surface modifications with metals and other semiconductors, is found
in [1816].
The efficiency of the photocatalytic activity depends on the efficiency of the charge separation
(Fig. 22.1). Any electron-hole pair that recombines within the bulk or the surface of the particle is lost
for the catalytic activity. Thus, surfaces must exhibit a small density of recombination centers. Surface
traps, however, can be beneficial for charge-carrier separation when they ‘store’ the charge-carrier
rather than letting it recombine. Small particles are expected to exhibit more efficient charge-carrier
separation than larger ones. Electrons at the surface can be donated and reduce an electron acceptor,
typically oxygen, A → A
− . A hole at the surface can oxidize a donor species, D → D
+ .
An example of increased photocatalytic activity are TiO 2 powders with deposited metal particles
(such as Pt) for H 2 evolution and metal-oxide particles (such as RuO 2 ) for O 2 evolution. Such a system
behaves as a short-circuited microscopic photoelectrochemical cell in which Pt is the cathode and
RuO 2 is the anode [1817]. Excitation with light energy above the band gap in the TiO 2 particle (3.2 eV)
injects electrons into the Pt particles and holes into the RuO 2 particles. Trapped electrons in Pt reduce
water to hydrogen and trapped holes in RuO 2 oxidize water to oxygen.
The photocatalytic activity is also tied to the geometrical shape of the semiconductor. Generally,
powders with nanosized grains have much higher activity than those with microsized particles [1818].
In Fig. 22.2 it is shown that nanosized objects with high surface-to-volume ratio are more effective
catalysts than rather compact surfaces.
1 ‘Small’ is here in relation to the diffusion length and does not need to be in the range where quantization effects (quantum
dots) are present.
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_22
589
Light-to-Electricity Conversion
Abstract The use of diodes for the conversion of electromagnetic radiation (light) into electrical
signals is discussed with many examples for devices such as photoconductors, pn-, pin-, MSM- and
avalanche photodiodes and charge-coupled devices. Energy conversion with solar cells is explained
and standard and advanced solar cells concepts are introduced.
22.1 Photocatalysis
The absorption of light in a semiconductor across the band gap creates free electrons and holes.
In particular, for small particle size in powders
1 these charge carriers can reach the surface of the
semiconductor. At the surface they can react with chemicals. The hole can form •OH radicals from OH
−
attached to the bead. The electron can form O 2 •
− . These radicals can subsequently attack and detoxify,
e.g., noxious organic pollutants in the solution surrounding the semiconductor. Such photocatalytic
activity has been found, e.g., for TiO 2 and ZnO powders. A review of photocatalysis, in particular
with TiO 2 particles and their surface modifications with metals and other semiconductors, is found
in [1816].
The efficiency of the photocatalytic activity depends on the efficiency of the charge separation
(Fig. 22.1). Any electron-hole pair that recombines within the bulk or the surface of the particle is lost
for the catalytic activity. Thus, surfaces must exhibit a small density of recombination centers. Surface
traps, however, can be beneficial for charge-carrier separation when they ‘store’ the charge-carrier
rather than letting it recombine. Small particles are expected to exhibit more efficient charge-carrier
separation than larger ones. Electrons at the surface can be donated and reduce an electron acceptor,
typically oxygen, A → A
− . A hole at the surface can oxidize a donor species, D → D
+ .
An example of increased photocatalytic activity are TiO 2 powders with deposited metal particles
(such as Pt) for H 2 evolution and metal-oxide particles (such as RuO 2 ) for O 2 evolution. Such a system
behaves as a short-circuited microscopic photoelectrochemical cell in which Pt is the cathode and
RuO 2 is the anode [1817]. Excitation with light energy above the band gap in the TiO 2 particle (3.2 eV)
injects electrons into the Pt particles and holes into the RuO 2 particles. Trapped electrons in Pt reduce
water to hydrogen and trapped holes in RuO 2 oxidize water to oxygen.
The photocatalytic activity is also tied to the geometrical shape of the semiconductor. Generally,
powders with nanosized grains have much higher activity than those with microsized particles [1818].
In Fig. 22.2 it is shown that nanosized objects with high surface-to-volume ratio are more effective
catalysts than rather compact surfaces.
1 ‘Small’ is here in relation to the diffusion length and does not need to be in the range where quantization effects (quantum
dots) are present.
© Springer Nature Switzerland AG 2021
M. Grundmann, The Physics of Semiconductors, Graduate Texts in Physics,
https://doi.org/10.1007/978-3-030-51569-0_22
589