Electrical conductivity transients in chemical bath deposited ZnO thin films as a func-
tion of UV illumination wavelength
P. Fuchs
1*
, J. Steinhauser
1
, D. Hariskos
2
, W. Wischmann
2
, Y.E. Romanyuk
1
, A.N. Tiwari
1
1
Laboratory for Thin Films and Photovoltaics, Empa - Swiss Federal Laboratories for Mate-
rials Science and Technology, 8600 Duebendorf, Switzerland
2
ZSW, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Würtenberg
Industriestrasse 6, DE-70565 Stuttgart
*
Contact:
Tel: +41 58 765 6124
Transparent conductive zinc oxide thin films are deposited by chemical bath deposition
(CBD) followed by an exposure to UV-visible light to increase their electrical conductivity.
Selected spectral bandwidths (10nm ± 2nm) are filtered from a 1kW Hg(Xe) lamp in the
range 280-550 nm to investigate conductivity transients as a function of the illumination
wavelength. The most significant conductivity increase is found for illumination wavelengths
just above the bandgap (~365nm), as predicted by the ionized oxygen species adsorption
model. Furthermore the transients depend on the photon flux, sample temperature, atmos-
phere and film thickness. Surface and bulk chemical modifications upon UV-visible light ex-
posure and degradation in humid atmosphere are examined by Fourier transform infrared
(FTIR) and X-ray photoelectron spectroscopy (XPS) measurements. UV illumination decom-
poses citrate impurities in CBD ZnO, but the residual carbon compounds are not effectively
removed from the bulk part of the layer. The exposure to humid atmosphere decreases the
electrical conductivity of CBD ZnO, which is caused by the chemisorption of environmental
oxygen species and the formation of zinc carbonate. Finally, CBD ZnO thin films with resis-
tivities down to 3*10
-3
Ωcm were implemented as a front contact on CIGS/CdS thin film so-
lar cells, reaching an active area efficiency of η = 16.6 % which is somewhat lower than the
value of η = 17.5% for the reference CIGS/CdS/sputtered AZO cell.
O 44
-106-
