Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

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.
An alternative route to incorporate copper, which allows large-area non-vacuum processing, is to use
copper ion exchange reactions at low temperatures (�200°C). 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.
During the 1st year of the current project, the low-temperature incorporation of Cu into In2Se3 layers by
ion-exchange from aqueous solutions was developed [2]. This process results in thin films with a
graded composition containing the crystalline phases �-Cu2-xSe and �-In2Se3. Annealing the films in
the presence of Se vapour formed chalcopyrite CuInSe2 and homogenised the depth profile of the
films. However, it was found out that in the case of aqueous solutions containing Cu ++ ions, the corrosion
of the Mo back contact took place, and attempts to use alternative contacts resulted in no incorporation
of Cu into the In2Se3 films.
The main motivation behind this project is to develop a novel low-cost process for the production of
thin film Cu(In,Ga)Se2 solar cells using the ion-exchange reaction in an appropriate solution. In order
to improve the copper incorporation and prevent the corrosion of the metal back contact, the following
specific points are addressed:
1. Investigate alternative solvents, e.g. ethylene glycole, which are suitable for the dissolution of
Cu+ salts and prevent the electrochemical corrosion of the metal contact.
2. Add gallium to initial In2Se3 precursors in order to increase the open circuit voltage of the final
CIGS photovoltaic devices.
3. Test various selenization regimes to improve the homogeneity of the CIGS absorber.
4. Protect the process idea by a patent.
Short project description
The Cu incorporation process was performed with (In,Ga)2Se3 (IGS) layers deposited onto 5x5cm 2
molybdenum-coated soda lime glass substrates. The Mo was deposited by DC sputtering and the
IGS layers were deposited by co-evaporation of the elements. The In and Ga were evaporated from
line sources and an excess of Se was provided from an effusion cell, substrate temperature was maintained
at 400°C. Throughout deposition the substrates moved back and forth over the line sources, 15
passes were typically required to deposit the �2 micron thick layers used in this work. Vacuum deposition
was used to provide a high degree of control over the IGS layers, however it would be desirable
in future work to use a non-vacuum technique to deposit these layers.
The incorporation of Cu into the indium gallium selenide layers was performed in ethylene glycol containing
0.6M CuCl and 1M NaCl. The relatively high concentration of NaCl is to stabilise the solution
and improve the solubility of the copper salt. The ethylene glycol bath was heated in a glycerol bath to
improve the temperature stability throughout the process, typically to better than ±2°C. Temperatures
from 140-190°C have been investigated but all layers reported here were processed at 160°C. At this
temperature it takes around 40 minutes to achieve 21 at.% Cu content in a 2 micron thick layer, as
measured by EDX after annealing.
The precursor layers were annealed in a two temperature-zone quartz tube furnace under flowing N2.
Se was supplied from a removable quartz crucible that was heated separately from the substrates to
allow control of the Se supply during annealing.
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Thin Film CIGS Solar Cells with a Novel Low Cost Process, A. N. Tiwari, ETH Zürich
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