(1) Institute of Solar Energy Research (ISFH), Sokelantstr. 5, D-30165
Hannover, Germany
(2) Tallinn Technical University, Chair of Semiconducting Materials
Technology, Ehitajate tee 5, EE-0026 Tallinn, Estonia
(3) present address: AQR; Wendelinusstr. 85, D-52425 Juelich, Germany
Solid solutions of cadmium chalcogenides are extremely interesting materials
for solar energy conversion since their band gap varies with composition. Thereby a
tailoring of the absorption edge becomes possible which allows to optimize the
material for different applications. However, when the composition of the
semiconductor is changed, also the flat band potential, the conductivity and the
surface chemistry of the material changes.
Moreover, in our careful investigation of the photoelectrochemical
consequences of these effects we also developed a new strategy to understand the
influence of charge trapping in the space charge region on the impedance behavior
of these electrodes. This is of general interest also for impedance measurements
with solid state devices. Strong interactions with the electrolyte lead to a pronounced
shift of the flat band potential of cadmium chalcogenides in polysulfide as well as in
ferro/ferri cyanide solutions compared to alkaline solutions without additional redox
couples by several tens of a volt.
Upon illumination the flat band potential of CdSe is not shifted up to
photocurrent densities of 40 mA/cm2, whereas that of CdTe is shifted in positive
direction already by low light intensities. This can be explained by the different
positions of the valence band edges of the two materials leading to a pronounced
decreased of the overpotential of holes being transferred from the valence band to
the redox couple from 0.9 V for CdSe to less than 0.3 V for CdTe. The obviously
high necessary overvoltages may even lead to a blocking of the charge transfer
reaction at CdTe for stronger illumination conditions. This is the reason for the
instability of the CdTe/polysulfide-system.
Adding cyanide to the ferro/ferri cyanide redox couple leads to an additional
shift of the flat band potential by about 0.2 V in negative direction. Whereas CdSe
remains still stable CdTe is unstable already without additional cyanide. An
additional shift of the band edges by cyanide even leads to a stable corrosion
current for low current densities without the formation of a blocking layer. Here a
diffusion controlled six hole corrosion process can be determined experimentally.
Therefore the cyanide supported ferri/ferro cyanide solution is well suitable for
photoetching processes of CdTe.
Ternary CdSexTe1-x electrodes exhibit a
minimum in
their band gaps with a selenium content of 40% with an ideal band gap of 1.36 eV.
However, their corrosion behavior becomes rather complex due to their chemical
behavior being determined by Te as well as Se. Therefore only redox couples
strongly interacting with the surface of these material have yet led to a stable
photoelectrochemical behavior. Further investigations using very fast redox couples
and/or organic electrolytes need to be performed.
Acknowledgement
Financial support by the Volkswagenstiftung under contract No. I/71902 is
gratefully acknowledged.