Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Introduction / Project Goals
High efficiency Cu(In,Ga)Se2 solar cells can be produced from (In,Ga)2Se3 precursor films using the
three stage process [1]. In the second stage of this process, the (In,Ga)2Se3 films are exposed to a
flux of Cu and Se atoms at temperatures >540°C, leading to the formation of Cu(In,Ga)Se2 and Cu2Se.
The current record efficiency Cu(In,Ga)Se2 solar cells (19.5%) were produced using this method [2].
Work performed with In2Se3 has shown that Cu vapour can be diffused into In2Se3 layers at 200°C [3].
In this case a subsequent high temperature annealing stage in the presence of Se vapour was used to
produce CuInSe2 layers that gave solar cell efficiencies of 13.7%.
Other solar cell technologies have made use of low temperature (�100°C) copper ion exchange reactions
to incorporate copper into thin films. Most notable perhaps is the Clevite process used to make
CdS/Cu2S solar cells. In this process Cu + ions in an aqueous solution of CuCl exchange places with
Cd 2+ ions in a CdS layer to form a surface layer of Cu2S [4]. A similar process (from aqueous
Cu(NO3)2 solution) has been used to incorporate Cu into CdTe solar cells, leading to the formation of
CuTe and excess Cu. In this case, the layers were annealed at high temperature after immersion in
the copper solution to promote further diffusion.
The objective of this work is to develop a novel low-cost process for the production of thin film
Cu(In,Ga)Se2 solar cells.
The ion-exchange reaction will be used to incorporate copper ions into thin films of indium selenide
from aqueous solution. These layers will then be homogenised by annealing at high temperature in the
presence of selenium vapour, leading to the formation of chalcopyrite CuInSe2. Further, the process
idea will be protected by a patent.
Work performed and results obtained
700-800nm thick indium selenide films were deposited onto 5x5cm 2 molybdenum-coated soda lime
glasses substrates by co-evaporation. Deposition of the indium selenide films was performed at
450°C. Due to the style of sample holder, an uncoated border of Mo was left around the edge of each
substrate.
Graded copper-indium-selenium precursor films were prepared by suspending the indium selenide
layers in an aqueous solution of CuSO4 (0.2M) and acetic acid (0.2M) for 1 hour. The acetic acid was
required to prevent the formation of copper hydroxide precipitates. The solutions were stirred constantly
and heated from room temperature to boiling point whilst the indium selenide films were immersed.
After immersion in the copper solution the films were rinsed thoroughly in deionised water
and dried.
Films were selenized in a two-zone tube furnace under flowing N2 and selenium vapour. The two zones
allowed the temperature of the selenium source to be controlled separately from the temperature
of the substrate. Typically the substrate was ramped to 575°C whilst the Se source was maintained at
400°C. After selenization films were etched in aqueous potassium cyanide solution (10%ww) for
30 seconds. One such indium selenide film was quartered after deposition and each quarter was analysed
by XRD and XPS at different stages of processing. The “As-Deposited” quarter was left as
In2Se3, the “Cu-Treated” quarter was analysed after immersion in the Cu-solution, the “Selenized”
sample was analysed after selenization and the “Etched” sample was analysed after KCN etching.
Another indium selenide film was processed into solar cells after completion of the above steps by
deposition of a CdS buffer layer by chemical bath deposition and a ZnO and ZnO:Al bilayer window by
RF sputtering. IV measurements were performed under a tungsten-filament lamp calibrated against a
crystalline silicon reference cell.
During immersion in the copper solution the Mo exposed around the border of the samples was removed
from the substrate. This is due to oxidation of the Mo into soluble molybdenum oxide. When
viewed through the substrate glass, it was evident that areas of the Mo beneath the In2Se3 had been
removed as well. This is thought to be due to pin holes in the In2Se3 layers (themselves potentially
due to defects in the Mo film) allowing contact between the solution and the Mo. This degradation of
the back contact limited the thickness of the In2Se3 layers into which relevant quantities of copper
could be incorporated.
Thin Film CIGS Solar Cells with a Novel Low Cost Process, A. N. Tiwari, ETHZ
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