• The product CuSe undergoes another decomposition at 377 − 379 °C.
377 − 379°C : 2γ −CuSe β − Cu2Se + Se (14.5)
• Above 523 °C Cu2Se melts in the presence of liquid Se and forms liquid L3 (53% at Se)
523°C : β −Cu2Se + L4 L3 (14.6)
14.3. Copper - Indium - Selenium
Figure 14.3.: Quasibinary section of the copper - indium - selenium system. The diagram was taken from the review [127]. The Greek letters abbreviate the following phases: α-CISe: CuInSe2 (chalcopyrite), β-CISe: CuIn3Se5, γ-CISe: CuIn5Se8, δ-CISe: CuInSe2 (sphalerite). Below 500 °C no phase transitions are observed and the diagram can be extrapolated.
The ternary phase diagram of the copper-indium-selenium system has three degrees of freedom. Instead of taking into account the complete three dimensional phase diagram, it is often sufficient to restrict to the section between Cu2Se and In2Se3 (figure 14.3). Gödecke et al. have confirmed in a
comprehensive study that this section is indeed a “quasibinary section” [128]. The term “quasibinary“ means that the section can be considered as a two-dimensional phase diagram with the binary compounds Cu2Se and In2Se3 acting as elements.
The chalcopyrite CuInSe2 phase is part of the quasibinary section. Due to copper vacancies its
composition can vary, i.e. Cu1−xIn1+xSe2+x. At equilibrium the copper deficiency x reaches values
up to 0.02. In the preparation of thin films the final layer is often a supercooled non-equilibrium state and Cu deficiencies up to x ≈ 0.16 are observed [128]. Also the addition of sodium impurities can increase the composition range of Cu1−xIn1+xSe2+x to the indium-rich side [102]. If the In-
content is further increased, an ordered vacancy compound is formed together with the chalcopyrite Cu1−xIn1+xSe2+x. This vacancy compound (phase β in figure 14.3) is usually assigned to CuIn3Se5 [127]. A stannite structure and a “P-chalcopyrite” structure (space group P¯42c) have been proposed for CuIn3Se5. The latter one has been supported by determining the bond distances with XRD
14. Phase diagrams
refinements [129]. CuIn3Se5 is usually called an “ordered vacancy compound” or “ordered defect
compound”. This refers to the ordered lattice sites which the copper vacancies can occupy. It does not mean that there exists a long range order of the copper vacancies [127]. Increasing the indium amount further leads to a CuIn5Se8 phase. While it is possible to form In-rich Cu1−xIn1+xSe2+x,
it is not possible to form a Cu-rich chalcopyrite containing more than 25% (at) Cu. In this case the excess Cu is consumed in a Cu2Se phase. If in the following a CuInSe2 absorber is labelled
“Cu-rich”, it refers to the growth conditions and not to the sample composition.
At high temperatures above 600 °C chalcopyrite CuInSe2 can transform into sphalerite CuInSe2.
15. Experimental
In this chapter the methods of annealing the stacked indium selenide copper selenide precursors and of their structural characterisation are described. Furthermore the synthesis of reference samples prepared by other methods is introduced. The characterisation of both annealed stacked precursors and reference samples allows direct comparison and more knowledge and insight to be gained. All electrodeposited precursors are annealed in a closed tube furnace with elemental selenium, whilst vacuum deposited samples are prepared in a physical vapour deposition machine. Finished absorber layers and absorber formation processes are characterised by SEM, EDX, XRD, AES, and EBSD.
15.1. Synthesis
15.1.1. Electrodeposited stacked binary selenide layers
The main analysis is performed on electrodeposited binary selenide precursors. These precursors have been prepared according to chapter 11. Stacks with different Cu/In ratio will be compared (chapter 16).
15.1.2. Indium selenide + Cu + Se precursors
The in-situ XRD experiments in figure 19.7 have been performed on a precursor where the electrode- posited copper selenide layer has been substituted by an elemental Cu and Se layer. Unfortunately the electrodeposition of a smooth and compact Cu layer on top of an electrodeposited indium sel- enide film was not successful in this thesis. The obtained films consisted of small particles and the lateral uniformity was insufficient. Therefore the elemental Cu layer was deposited by sputtering, which resulted in a smooth film. An elemental Se layer has been evaporated on top of the Cu layer. Both layers have been deposited by Guillaume Zoppi (Northumbria University). The Se layer was supposed to provide the necessary Se during annealing, because no elemental Se was added in the hotstage experiment. Additionally it protects the sputtered copper film from oxidation. The overall composition of the complete stack is slightly Cu-rich (Cu/In atomic ratio = 1.14). The thicknesses of the Cu and Se layer were chosen to match the Cu/Se ratio in the electrodeposited Cu-Se film. The sputtering was done at low temperatures in order to avoid a reaction between Cu and Se. A XRD diffractogram of the complete stack shows a dominating Cu peak. Also some minor reflections of Cu3Se2 are observed. The Se capping layer is amorphous.
15.1.3. Coelectrodeposited CuInSe2 sample
In chapter 18 the grain size and coherence length (section 18.1.1) of CuInSe2absorbers prepared from
electrodeposited binary selenide precursors are compared to an annealed coelectrodeposited CuInSe2
sample. The preparation of the annealed coelectrodeposited sample follows the procedure described by Dale et al. [21]. All elements (Cu, In, Se) are simultaneously electroplated at room temperature from a bath containing 2.6 mM CuCl2, 9.6 mM InCl3, 5.5 mM H2SeO3, 240 mM LiCl in a pH 3
buffered aqueous solution. The electrodeposition potentials are −0.476 V (vs. Ag | AgCl | 3 M KCl) for 20 min followed by −0.576 V for 50 min. The precursor has been annealed in the same set-up under the same conditions (30 min at 550 °C) as the electrodeposited binary selenide precursors described in section 15.2. A background gas (10% H2 / 90% N2) pressure of 10 mbar and excess
15. Experimental
same graphite box as the binary selenide precursors. The annealed absorber is typically single-phase Cu-poor (Cu/In atomic ratio ∼ 0.8) CuInSe2 chalcopyrite.
15.1.4. PVD deposited CuInSe2 absorbers
For comparison purpose samples prepared by physical vapour deposition (PVD) have been added. The sample labelled “PVD-Cu-poor” (table 16.1) has been prepared in a “3-stage-process” that is usually applied for chalcopyrite solar cells with the highest efficiency [130]. In the first stage In and Se are evaporated onto the substrate having a temperature around 350 °C. In the second stage the substrate temperature is increased to 540 °C while Cu and Se are evaporated. The substrate temperature is kept constant during the last step, when again In and Se are evaporated. The absorber composition is Cu-poor.
In the preparation of the absorber labelled “PVD-Cu-rich” constant fluxes of Cu, In, and Se have been evaporated ending up with an atomic Cu/In ratio of 1.55. The substrate temperature during the deposition has been 540 °C. The Cu excess is contained in copper selenide phases at the surface and can be removed by KCN etching [131, 132].