Name: x-ray beam induced current measurements for multi-modal x-ray microscopy of solar cells
Uploaded: 2019-08-20T15:00:00
Duration: 10 s
Description: Watch this Scientific Journal Video about X-ray Beam Induced Current Measurements for Multi-Modal X-ray Microscopy of Solar Cells at JoVE.com

We greatly acknowledge J. Garrevoet, M. Seyrich, A. Schropp, D. Br&#252;ckner, J. Hagemann, K. Spiers, and T. Boese from the Deutsches Elektronen-Synchrotron (DESY) and A. Kolditz, J. Siebels, J. Fl&#252;gge, C. Strelow, T. Kipp, and A. Mews from the University of Hamburg for supporting measurements at beamline P06 at PETRA III, DESY; M. Holt, Z. Cai, M. Cherukara, and V. Rose from the Argonne National Laboratory (ANL) for supporting measurements at beamline 26-ID-C at the Advanced Photon Source (APS) at ANL; D. Salomon and R. Tucoulou from the European Synchrotron Radiation Facility (ESRF) for supporting measurements at beamline ID16B at ESRF; R. Farshchi, D. Poplavkyy, and J. Bailey from MiaSol&#233; Hi-Tech Corp., and E. Avancini, Y. Romanyuk, S. B&#252;cheler, and A. Tiwari from the Swiss Federal Laboratories for Materials Science and Technology (EMPA) for providing solar cells. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. We acknowledge the European Synchrotron Radiation Facility (Grenoble, France) for provision of synchrotron radiation facilities. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

The authors have nothing to disclose.

In a world where the demand for electrical energy is constantly rising, a clean and sustainable energy source is increasingly necessary. One possibility to tackle these demands are photovoltaic (PV) systems1,2,3. For a directed and efficient way of developing next-generation solar cells, it is necessary to understand how the composition and structure of the solar cells affect their performance4. Typical questions in solar cell development include: Which types of defects are most detrimental, and where are they located5,6? Are there inhomogeneities in the elemental distribution, and what is their impact7,8,9? How do the solar cells change upon module assembly and aging10,11?
As a solar cell is only as good as its weakest part, it is especially important to understand the effect of compositional and structural variation on the performance in polycrystalline solar cells that suffer inherently from inhomogeneities7,8. This is particularly true for thin film (TF) solar cells, which contain absorber layers with crystallite sizes in the micrometer range. Here, the effect of grain boundaries on performance is of highest interest, but their small size and the fact that they are buried in an entire layer stack pose unique characterization challenges. Furthermore, the complex chemistry of multi-component absorber layers with co-existing phases and internal gradients require sophisticated characterization methods12.
Synchrotron-based hard X-ray microscopes are able to meet the characterization challenges of TF solar cells: they provide X-ray spot sizes down to the nanometer scale13,14,15,16 and the penetration depth of hard X-rays allows to probe the different device layers17, including buried absorber layers. With a wealth of different measurement techniques at a scanning X-ray microscope, it becomes possible to simultaneously study not just one, but many different aspects of solar cells within multi-modal measurements and to correlate the observed characteristics. For example, X-ray beam induced current (XBIC) measurements have successfully been combined with X-ray fluorescence (XRF)7,18,19, X-ray excited optical luminescence (XEOL)20,21, and X-ray diffraction (XRD)22 to correlate the electrical performance with composition, optical performance, and structure, respectively23.
During XBIC measurements of solar cells or other devices under test (DUT)24,25, the incident X-ray photons set off particle showers consisting of electrons and photons, resulting in a multitude of excited electron-hole pairs per incident X-ray photon in the semiconducting absorber material. Finally, the electron-hole pairs thermalize to the band edges of the solar cell absorber. Therefore, these X-ray excited charge carriers can be treated like charge carriers that are generated by the absorption of photons with energies just above the bandgap during normal solar cell operation, and the resulting current or voltage can be measured as X-ray beam induced current23,26,27 or voltage (XBIV)28,29 similar to more common measurements like electron-beam induced current (EBIC) or laser-beam induced current (LBIC). Consequently, the XBIC/XBIV signal not only depends on the thickness of the absorber layer, but also on the electric performance of the DUT, both at the microscopic and macroscopic level, including the local bandgap, Fermi-level splitting, and recombination. Thus, we are able to map local variations of the charge-carrier collection efficiency that is defined as the probability that an externally excited electron-hole pair in the absorber layer is collected at the electrical contacts of the DUT.
Note that only electron-hole pairs that are generated in the absorber layer of the DUT contribute to the XBIC/XBIV signal. Charge carriers generated in other layers such as the metallic contacts or substrate will immediately recombine, as they have no possibility of being separated by the junction. Therefore, other layers only affect XBIC/XBIV measurements via secondary effects such as parasitic X-ray absorption or the emission of secondary photons and electrons that may be re-absorbed in the absorber layer. In contrast, all layers potentially contribute to the XRF signal.
Given that the XBIC and XBIV signals can be small (often, variations in the sub-picoampere and nanovolt range are of interest), the signals are easily buried in noise. Therefore, we suggested to utilize lock-in amplification to extract the XBIC and XBIV signals30. For this purpose, the incoming X-ray beam is modulated by an optical chopper as indicated in Figure&#160;1. This modulation carries over to the signal produced by the DUT. Before the signal is fed into the lock-in amplifier (LIA), a pre-amplifier (PA) is typically used to match the raw signal intensity with the range of the analog-to-digital converter at the input of the digital LIA. The LIA mixes the modulated measurement signal with the reference signal. By employing a low-pass filter, only frequencies close to the reference signal are passed through and amplified31. This allows for an effective extraction of the XBIC or XBIV signal from a noisy background.
In the protocol, we introduce the prerequisites and motions necessary for taking successful XBIC measurements including the raw signal (direct current, DC) and the modulated signal (alternating current, AC). Beyond describing technical details, we discuss an XBIC setup in the context of multi-modal measurements at beamline P06 at PETRA III13. Please note that, compared to most laboratory experiments, the environment of hutches at hard X-ray nanoprobes requires particular planning and consideration. Specifically, multi-modal measurements with nanometer-scale resolution challenge the experimentalists with a variety of specific constraints. For example, electronic noise is often present with great amplitudes from piezo-driven motors and other equipment, such as the power supplies of detectors. Furthermore, a multitude of devices and detectors needs to be arranged at optimized geometry without interfering with each other nor inducing vibrations. Figure&#160;1 depicts a typical setup for XBIC measurements in combination with XRF and small/wide angle X-ray scattering (SAXS/WAXS) measurements.

<table border="0" cellpadding="0" cellspacing="0" style="width:728px;" width="728">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">BNC cabling and connectors</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td style="width:172px;">From generall cable suppliers</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Chopper blade</td>
			<td style="width:134px;">Thorlabs</td>
			<td style="width:123px;">MC1F10HP</td>
			<td>Apart from technical compatibility of the chopper wheel with the chopper system, it should be checked that the chopper blade sufficiently blocks the X-ray beam.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Conductive silver paint</td>
			<td style="width:134px;">Conrad</td>
			<td style="width:123px;">530042</td>
			<td>Alternative products can be obtained from Pelco and others</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Copper wires</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td>From cable suppliers for contacting of the solar cell</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Current Preamplifier</td>
			<td style="width:134px;">Standford</td>
			<td style="width:123px;">SR570</td>
			<td>Alternatives include the Keithley 487 or 6487 Picoammeter.&#160;</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Device under test (DUT)</td>
			<td style="width:134px;"></td>
			<td></td>
			<td>Suitable device for XBIC measurements.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Holder with printed circuit board</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td>Custom design</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Kinematic sample mount</td>
			<td style="width:134px;">Thorlabs</td>
			<td style="width:123px;">KB25/M</td>
			<td>Optional, allows easy positioning and changing of sample. Alternatives include the M-BK-1A from Newport</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:300px;">Lock-in Amplifier</td>
			<td style="width:134px;">Zurich Instruments</td>
			<td style="width:123px;">UHFLI or MFLI</td>
			<td>Whereas the MFLI has current preamplifiers included, the UHFLI requires an external current preamplifier but offers more options. Therefore, the UHFLI was used for the presented experiment.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:300px;">Measurement control/data acquisition unit</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td>Available at different synchrotrons.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Optical Chopper</td>
			<td style="width:134px;">Thorlabs</td>
			<td>MC2000B(-EC)</td>
			<td>Alternatives include the choppers SR540 from Stanford Research Systems, or model 3502 from Newport.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">Polyimide tape</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td>Rolls with different widths and thicknesses are available</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:300px;">X-ray source</td>
			<td style="width:134px;"></td>
			<td style="width:123px;"></td>
			<td>Available at different synchrotrons</td>
		</tr>
	</tbody>
</table>

x-ray beam induced current measurements for multi-modal x-ray microscopy of solar cells

X-ray beam induced current (XBIC) measurements allow mapping of&#160;the nanoscale performance of electronic devices such as solar cells. Ideally, XBIC is employed simultaneously with other techniques within a multi-modal X-ray microscopy approach. An example is given herein combining XBIC with X-ray fluorescence to enable point-by-point correlations of the electrical performance with chemical composition. For the highest signal-to-noise ratio in XBIC measurements, lock-in amplification plays a crucial role. By this approach, the X-ray beam is modulated by an optical chopper upstream of the sample. The modulated X-ray beam induced electrical signal is amplified and demodulated to the chopper frequency using a lock-in amplifier. By optimizing low-pass filter settings, modulation frequency, and amplification amplitudes, noise can efficiently be suppressed for the extraction of a clear XBIC signal. A similar setup can be used to measure the X-ray beam induced voltage (XBIV). Beyond standard XBIC/XBIV measurements, XBIC can be measured with bias light or bias voltage applied such that outdoor working conditions of solar cells can be reproduced during in-situ and operando measurements. Ultimately, the multi-modal and multi-dimensional evaluation of electronic devices at the nanoscale enables new insights into the complex dependencies between composition, structure, and performance, which is an important step towards solving the materials&#39; paradigm.

Watch this Scientific Journal Video about X-ray Beam Induced Current Measurements for Multi-Modal X-ray Microscopy of Solar Cells at JoVE.com

X-ray Beam Induced Current Measurements for Multi-Modal X-ray Microscopy of Solar Cells

A setup for X-ray beam induced current measurements at synchrotron beamlines is described. It unveils the nanoscale performance of solar cells and extends the suite of techniques for multi-modal X-ray microscopy. From wiring to signal-to-noise optimization, it is shown how to perform state-of-the-art XBIC measurements at a hard X-ray microprobe.

X-ray beam induced current (XBIC) measurements allow mapping of&#160;the nanoscale performance of electronic devices such as solar cells. Ideally, XBIC is ...

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