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Atom Probe Tomography Studies on the Cu(In,Ga)Se2 Grain Boundaries

Published: April 22nd, 2013



1Department of Microstructure Physics and Alloy Design, Max-Planck-Institut für Eisenforschung GmbH, 2Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg ( ZSW )

In this work, we describe the use of the atom-probe tomography technique for studying the grain boundaries of the absorber layer in a CIGS solar cell. A novel approach to prepare the atom probe tips containing the desired grain boundary with a known structure is also presented here.

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm).

Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, site-specific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In,Ga)Se2 thin-films to be studied by atom probe tomography.

Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se2 thin-film solar cells.

Thin-film solar cells based on the chalcopyrite-structured compound semiconductor Cu(In,Ga)Se2 (CIGS) as the absorber material have been under development for more than two decades because of their high efficiency, radiation hardness, long-term stable performance, and low production costs 1-3. These solar cells can be fabricated with only little material consumption due to the favorable optical properties of the CIGS absorber layer, namely, a direct bandgap and a high absorption coefficient 1,2. Absorber films of only a few micrometers in thickness are sufficient to generate a high photocurrent. Since the diffusion paths of photogenera....

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1. CIGS Layer Deposition

  1. Sputter-deposit 500 nm of molybdenum (back contact layer) onto a 3 mm thick soda lime glass substrate (SLG).
  2. Co-evaporate 2 μm of CIGS in an inline multistage CIGS process 24. The obtained CIGS layer deposited on Mo back contact is shown in Figure 1.
  3. Measure the integral composition of CIGS layer by X-ray fluorescence spectrometry (XRF). The obtained CIGS composition is shown in Table 1.

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Figure 3 shows a side view (x-z slice) elemental map of the random high-angle GB (HAGB) 28.5°-<511>cub selected in Figure 2 by site-specific preparation method. Co-segregation of Na, K, and O at a CIGS HAGB is directly mapped using APT. These impurities most likely diffused out of the SLG substrate into the absorber layer during the deposition of the CIGS layer at ~ 600 °C.

Figure 4a shows the Cu, In, Ga, and Se concen.......

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In the current work, we have presented APT results on a random HAGB in CIGS, a compound semiconductor material used for photovoltaic application. Furthermore, we have also shown that APT in conjunction with complementary techniques, such as EBSD and TEM, is a powerful tool to elucidate the structure-composition properties relationship for the CIGS solar cells. Unfortunately, the correlation between APT and EDX/EELS in TEM was not possible because firstly, EDX/EELS has not sufficient resolution to detect low Na and O conc.......

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This work is founded by the German Research Foundation (DFG) (Contract CH 943/2-1). The authors would like to thank Wolfgang Dittus, and Stefan Paetel from Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg for preparing the CIGS absorber layer for this work.


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