K-edge EXAFS and XANES studies of Cu in CdS/CdTe solar cells

Xiangxin Liu, Akhlesh Gupta and Alvin D. Compaan ( University of Toledo)

Nadia Leyarovska (Illinois Institute of Technology)

Jeff Terry (Illinois Institute of Technology and University of Notre Dame)

Abstract

Motivation

Significance of Copper in CdTe based solar cell

• Good performance of Cu back-contact cell

• Fast diffuser in CdTe

• Critically change CdS/CdTe junction property

Equipment introduction

Simple introduction of EXAFS and XANES

Some recent results

• XANES study on the chemical location of Cu in CdTe

• Cu K fluorescence peak study for Cu density in CdTe film and solar cell

• X-ray beam damage study

Preliminary conclusions

Copper has been identified as a very important dopant element in CdTe thin-film solar cells.  It is a deep acceptor in CdTe and is commonly used to obtain a heavily doped, low resistance back contact to polycrystalline CdTe.  Cu also helps to increase the open circuit voltage of the cell. 

However, Cu is also a fast diffuser in CdTe, especially along grain boundaries, and can accumulate at the CdS/CdTe junction.  It is suspected of leading to cell performance degradation in some cases. 

A substantial, thermally restorable “aging” behavior was also found in Cu-doped CdTe.  

The present study is designed to help identify the lattice location and amount of Cu in CdTe. Cu K-edge x-ray absorption (XAS) and Cu K

fluorescence measurements were conducted on thin films of CdTe and on thin-film solar cells based on CdTe.  Experiments were performed at the MR-CAT beamline at the Advanced Photon Source.  XAS spectra and full fluorescence spectra with 9.5 KeV beam energy were collected in fluorescence geometry with a 13 element Ge detector.

Motivation

The I-V characteristics of a Cu/Au back-contacted CdS/CdTe cell:

Voc = 0.826 (V)Jsc = 19.3(mA/cm2)FF= 68.66Efficiency = 11.4%

I-V of a ZnTe:N back-contacted CdS/CdTe cell:

Voc = 0.705 (V)Jsc = 19.0 (mA/cm2)FF= 67.7

Efficiency = 9.07%

Figure 1. I-V curve of Cu-contacted cell

Why is Cu used for contacts to CdTe?Currently, copper and ZnTe are the two kinds of back contact used on CdS/CdTe cells at The University of Toledo. Cells with back contacts based on Cu reach 13% whereas ZnTe:N based contacts have not exceeded 10%.

Figure 2. I-V curve of ZnTe:N-contacted cell

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Photoluminescence (PL) study of Cu-diffused CdTe crystal

Figure 3. The PL studies of single-crystal CdTe help to establish that the Cu diffuses rapidly even through single-crystal material. The spectra were taken from a) pure CdTe, b) CdTe crystal diffused 45 min at 200 C after evaporating a 200 nm Cu layer on one side, and c) the same crystal but studied from the back side opposite the Cu layer.

752nm Laser

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CdTe crystal

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CdTe x-tal referenceCdTe x-tal Cu sideCdTe x-tal no Cu side

PL

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Photon Energy (eV)

Cu accumulates at CdS/CdTe junction and introduces lead to great changes of the PL from the junction.

Figure 4. A) Change in junction PL for a CdS/CdTe structure after diffusion with Cu. b) junction PL from CdS/CdTe:Cu. Cu was introduced in case (b) by co-sputtering.

Experimental Hutch B

MRCAT Beamline, Sector 10ID-B, Advanced Photon Source

Feedback

Harmonic Rejection MirrorIncident IntensityMonitor, 20% N2 80% He

Defining slits

Sample

13-element Ge Detector

Transmitted IntensityMonitor, 100% N2

Ken Reference Monitor, Air Scatter, PIN Diode

Cu foil

Storage ring

Insertion device(ID)

Cryogenic Si-111 double-crystal monochromator

Hutch A

Monochromic X-ray beam

X-ray beam

Simple Introduction to EXAFS and XANES

The extended x-ray absorption fine structure (EXAFS) is the fine structure in the x-ray absorption coefficient starting somewhat past an absorption edge ~50eV and extending typically 1000eV further for the K-edge of copper metal.

X-ray absorption near-edge structure (XANES) in an absorption spectra covers the range between the threshold and the point at the extended x-ray absorption fine structure, EXAFS, begins.

Comparing the XANES spectra of our samples with reference is helpful to identify the valence of designated element.

Figure 5. Reference XANES absorption spectrum of Cu foil, CuO, Cu2Te and crystalline CdTe:Cu

X-ray photon Energy(eV)

Figure 5. Reference XANES absorption spectrum of Cu foil, CuO, Cu2Te and CdTe:Cu film

The figure shows some of the reference spectra that provide a basis for comparison to the various XAS spectra from CdTe crystals and thin films. (One example of single crystal CdTe diffused with Cu is provided).

X-ray photon Energy(eV)

XTAL CdTe:Cu

Cu foil

CuO

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rma

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Figure 6. K-edge x-ray absorption spectrum of 3m CdTe film with 200 Ǻ diffused Cu

Some recent resultsThe XANES here

shows the typical spectrum obtained from a 3 micron RF sputtered film of CdTe on fused silica.

Absorption is monitored by the Cu K fluorescence with the 13 element Ge detector array.

Figure 7. The intensity of Cu fluorescence is a convenient measure of the Cu content in the film. We have used this to determine even the residual Cu in nominally pure CdTe films—and found some surprises!

Samples: all are 3m CdTe film magnetron sputtered on quartz3b---- 200Å Cu, diffused 45 min. at 250 C, HCl 18% etch to remove excess Cu 1a-----100Å Cu, diffused 45 min. at 150 C, no etch (reference sample)1c----- as-deposited CdTe film

Conclusion: 1) 90 ppm Cu was found in as-deposited CdTe film, which manufacturer analysis showed 9 ppm .

2) half of evaporated Cu was removed by HCl etching in ct3b.

Cu K fluorescenc peak

Cd

Te

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Cu density in CdTe target, film and CdCl2 treated film

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Figure 8. Cu K fluorescence study show not only much higher Cu density (50ppm) in target than manufactory claims (1ppm), but additional Cu were introduced into CdTe through deposition and CdCl2 treatment.

Part Per Million

Plasmaterial target 50

Plasmaterial film 170

Plasmaterial film+CdCl2 220

Effect of high energy x-ray photon beam damage on CdTe film

Photoluminescence (PL) intensity is know to be very sensitive to the defects in CdTe crystals, films, and solar cell structures. Current spatial dependent PL study on the 8 hours irradiated spots generated during XAS data collection is shown here.The near-band-edge PL intensity decreases to ~1/10.

Even the spots burned by the same X-ray beam for only 10 seconds are visible.

Thus, in subsequent scans we have rastered the sample during XAS data acquisition and also obtained XAS data with the sample held at 200 C. Sample deterioration is greatly reduced.

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Figure 9. PL mapping on two spots produced by X-ray.

Rastering the sample

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Figure 10. XANES spectra of rastered and long exposed points.

Difference at pre-edge peak is obvious. Exposure to the X-rays reduced the intensity of the pre-edge feature.

The sample was then rastered to produce a virgin XANES point.. The XAFS did not appear to show much change over time.

Preliminary conclusions

• X-ray fluorescence detection with the MR-CAT 13-element Ge system permits XAS studies of very low levels of Cu in CdTe thin films and solar cells.

• Comparisons with reference spectra indicate that much Cu in an aged CdTe is located in a local environment similar to Cu2Te. Preliminary evidence suggests that the local environment in some samples becomes similar to Cu2Te. However, measurements on fresh samples do not show this behavior.

• XAS can be performed on completed solar cell structures and is a promising tool for defect chemistry studies needed for further improvements in efficiency and long-term stability of thin-film solar cells.

• Cu k fluorescence is a convenient, sensitive, and powerful tool for quantitative trace element analysis of Cu in CdTe.

• High intensity X-ray beam can produce more defects in CdTe and change Cu chemical bond in CdTe.

Acknowledgements

• Work performed at MRCAT is supported, in part by funding from the Department of Energy, Office of Energy Research under grant number DEFG0200ER45811.