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Presentation preference: Poster
Themes: 2) Thin-film inorganic photovoltaics
Keywords: CIGS, solution processing, thin film
Solution processing of Cu(In,Ga)(S,Se)2 absorber layers for thin film solar cells
S.Ulicna, P.Arnou, C.S.Cooper, L.D.Wright, A.V. Malkov, J. M. Walls and J. W. Bowers
Centre for Renewable Energy Systems Technology (CREST), Wolfson School of Mechanical, Manufacturing and
Electrical Engineering, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK.
Corresponding author: [email protected]
Atmospheric solution processing of absorber materials in thin film solar cells is an attractive, simple and lowcost alternative compared to conventional deposition in vacuum-based systems which require high capital
investment. Typically, solutions are prepared which are then sprayed onto a supporting substrate to form the
desired film. The composition of the film is easily controlled depending of the precursors used in the solution,
allowing for fine control of the final material properties, whilst the addition of beneficial dopants makes this
method extremely versatile. The method is also suitable for large area roll to roll processing adding to its
economic value, as well as rapid combinatorial analysis, allowing for rapid materials development at an R&D
level.
A promising material for thin film photovoltaic absorbers is the chalcopyrite semiconductor CuInGaSe2
(CIGS). CIGS solar cells deposited by vacuum-based co-evaporation processes have reached record power
conversion efficiency of 21.7%, whilst 15.2% efficiency has been accomplished when deposited by spin coating
from a molecular solution of chalcogenide precursors dissolved in hydrazine. Whilst the method has extreme
promise, the use of hydrazine makes this process difficult at the industrial scale because of the toxic and
explosive nature of the solvent used.
With this in mind, the purpose of this work is to develop a hydrazine-free, environmentally friendly method of
CIGS absorber fabrication using metal chalcogenide precursor solutions. Metal chalcogenides are used as a
starting material compared to metal oxides or metal salts since they are free of impurities (O,Cl) which can
deteriorate device performance. In order to dissolve metal chalcogenide precursors, safer mixture of solvents is
used, compared to hydrazine. At Loughborough, copper and indium sulphides and selenides have been
successfully dissolved in 1,2-ethanedithiol/1,2-ethylenediamine in a 1:10 volumetric ratio and remained in pure
solution for several days. Elemental gallium and selenium have also been dissolved in the same solvents and
added to the solution. The solution is then sprayed in air onto molybdenum coated glass substrates at various
temperatures. In order to obtain desired absorber thickness, several layers are sprayed with a drying step in
between to allow excess solvent to evaporate. Once the precursor layers are deposited, an annealing treatment in
selenium atmosphere is performed inside a tube furnace. The devices are completed with deposition of CdS by
chemical bath deposition technique followed by intrinsic ZnO and Al doped ZnO (AZO) by rf magnetron
sputtering. The devices microstructure is studied with scanning electron microscopy (SEM) and its composition
is characterized with X-ray diffraction (XRD). The device performance is obtained with JV measurements under
1000 W/m2 illumination.
Another important factor influencing CIGS device performance considered in this work is the MoSe2 layer at
Mo/CIGS interface. MoSe2 contributes to the formation of quasi-ohmic back contact rather than Schottky type
barrier, but too thick a layer is detrimental to device performance due to its high resistivity and delamination
from the substrate. Molybdenum layer physical properties, influence of sodium diffusion from glass substrate,
selenisation process parameters and additional precursor solution doping are studied in this work in order to
achieve optimum MoSe2 layer thickness together with good grain growth of the CIGS absorber.