Name: atom probe tomography studies on the cu(in,ga)se2 grain boundaries
Uploaded: 2013-04-22T12:00:00
Description: Watch this Scientific Journal Video about Atom Probe Tomography Studies on the CuIn,GaSe2 Grain Boundaries at JoVE.com

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 f&uuml;r Sonnenenergie- und Wasserstoff-Forschung Baden-W&uuml;rttemberg for preparing the CIGS absorber layer for this work.

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

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 photogenerated charge carriers to the electrodes are relatively short, CIGS absorbers can be produced in polycrystalline form. The maximum efficiency of a Cu(In,Ga)Se2 (CIGS) solar cell achieved so far is 20.4% 4, which is the highest value among all thin-film solar cells.
To further establish the CIGS thin-film photovoltaic technology, both the reduction of production costs and the enhancement of solar cell efficiency are essential. The latter is strongly dependent on the microstructure and chemical composition of the CIGS absorber layer. Internal interfaces, in particular grain boundaries (GBs) within the absorber, play a pivotal role, as they can affect the transport of photogenerated charge carriers.
One of the main unresolved issues with respect to CIGS solar cells is the benign nature of CIGS GBs, i.e. polycrystalline CIGS absorber films yield outstanding cell efficiencies despite a high density of GBs and lattice defects.
Several authors studied GBs in solar-grade CIGS films with respect to their electrical properties 5,6, character and misorientation 7-9 as well as impurity segregation 10-13. However, no clear link between these properties could be established so far. In particular, there is a substantial lack of information regarding the local chemical composition and impurity content of the GBs.
In the past two decades, Atom Probe Tomography (APT) has emerged as one of the promising nano-analytical techniques 14-17. Until recently APT studies of solar cells have been largely restricted by difficulties in the sample preparation process and the limited capability of analyzing semiconductor materials using conventional pulsed-voltage atom probes. These restrictions have been largely overcome by the development of the 'lift-out method' based on focused ion beam (FIB) milling 18 and the introduction of pulsed laser APT 16. Several papers about APT characterization of CIGS solar cells have been published 19-23, which are strongly encouraging for further investigations.
This paper gives a guideline of how to study internal interfaces in CIGS thin-film solar cells by the atom probe tomography technique.

atom probe tomography studies on the cu(in,ga)se2 grain boundaries

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.

Figure 3 shows a side view (x-z slice) elemental map of the random high-angle GB (HAGB) 28.5&deg;-&lt;511&gt;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 &deg;C.
Figure 4a shows the Cu, In, Ga, and Se concen...

Watch this Scientific Journal Video about Atom Probe Tomography Studies on the CuIn,GaSe2 Grain Boundaries at JoVE.com

Atom Probe Tomography Studies on the CuIn,GaSe2 Grain Boundaries

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.

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

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the ...

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