CHARACTERISATION OF SOLAR CELLS USING HYPERSPECTRAL IMAGER
OBJECTIVESWe develop a new characterization method based on a hyperspectral imager recording spectrally resolved images. We are able to cartography electroluminescence (EL) and photoluminescence (PL) spectra of solar cells with an absolute calibration. This will allow to study spatial variations of cell properties, like open circuit voltage and transport mechanisms [1,2]. The hyperspectral imager is compared to a classical confocal microscope.
PL measurement on electrodeposited CuInS2 with the hyperspectral imager allows to detect spatial inhomogeneities at sub-micron scale. We can monitor several features: PL maxima are related to bandgap, and fluctuation of quasi Fermi level splitting can be determined from spectra [1,2].
A confocal microscope coupled to a spectrometer provides similar data. The 532nm laser is focused onto the cell front contact, and the cartography of PL spectra is obtained by scanning the sample.
The acquisition time with the imager is much faster, 150*150µm² at 107W/m² would take hundreds of hours in confocal, but only 8min in hyperspectral. Moreover, surface excitation and detection allows to get rid of diffusion and roughness troubles for quantitative analysis.
CONCLUSIONS AND PERSPECTIVESHyperspectral imager produces spectrally resolved images of luminescence from multicristalline CIS solar cell, from which we can study its spatial inhomogeneities. On high efficiency GaAs solar cells, we got absolute measurements of EL and successfully investigated reciprocity relations. Our next step is to record quantitative maps of CIGS physical properties from PL and EL images, such as VOC , transport parameters…
HYPERSPECTRAL IMAGER AND ABSOLUTE CALIBRATION OF LUMINESCENCE
CIS PHOTOLUMINESCENCE MEASUREMENT
COMPARISON WITH A CONFOCAL SETUP
From the detailed balance principle, one can derive a reciprocity relation linking the External Quantum Efficiency of a solar cell (EQE) with the EL emitted at a voltage V [3]. An absolute measurement of EL on high efficiency GaAs solar cells* shows a good agreement with this relation. With this method we determine EQE precisely around band gap. However, to get such results we need to take into account series and sheet resistances. Otherwise voltage is overestimated and EQE underestimated.
*Cells provided by Fraunhofer ISE
ABSOLUTE MEASUREMENT OF GaAs ELECTROLUMINESCENCE AND EXPERIMENTS ON RECIPROCITY RELATIONS
REFERENCES ACKNOWLEDGEMENTS
A. Delamarre1*, L. Lombez1, J.F. Guillemoles1, M. Verhaegen2, B. Bourgoin2
1- Institute of R&D on Photovoltaic Energy (UMR 7174, EDF-CNRS-ChimieParisTech), 6 Quai Watier-BP 49, 78401 Chatou cedex, France2- Photon etc, 5795 avenue de Gaspé, #222, Montréal, Québec, H2S 2X3, Canada
[1] P. Würfel, J. Phys. C, 15, 3967 (1982)[2] L. Gütay, G.H. Bauer, Thin Solid Films, 515, 6212 (2007)[3] U. Rau, Pys. Rev. B, 76, 085303 (2007)[4] www.photonetc.com
Thanks to Marc Verhaegen and Brice Bourgoin from Photon etc. for our fruitful collaboration.
Our data are recorded using a Hyperspectral Imager. This original setup measures spectrally resolved images, therefore providing considerable advantages such as:
• Absolute calibration of intensity• Micrometer scale resolution• Excitation and detection on a surface(no information loss from lateral diffusion and roughness)
Based on volume Bragg gratings, the Hyperspectral Imager is developed in collaboration with Photon etc [4].
In luminescence imaging, absolute calibration is a main concern. This can be done here thanks to surface detection in two distinct steps:
• Absolute calibration at a determined point (spatially and spectrally) with a laser
• Relative calibration on the whole space and the whole spectrum, with a calibrated lamp coupled to an integrating sphere
photons/s
y (µ
m)
x (µm)
PL on GaAsat a contact, at 870nm
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y
x
λ
EL from GaAs solar cell at 1,05V
Wavelength (nm)
EL
(pho
tons
/s)
750-1,00E+012
0,00E+000
1,00E+012
2,00E+012
3,00E+012
4,00E+012
5,00E+012
6,00E+012
7,00E+012
850 950800 900
Hyperspectral Imager developed in collaboration with
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X (µm)
Y (µ
m)
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Integrated photoluminescence intensity, λexc = 532 nm
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2 3
750 800 850 900 9500,0
0,2
0,4
area 1 area 2 area 3
Inte
nsity
(a.u
.)
Wavelength (nm)
Spectra from different locations
820 840 860 880 9000
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Cou
nts
Wavelength (nm)
Histogram of photoluminescence peaks positions
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X (µm)
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White lamp image of probed area
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X (µm)
Y (µ
m)
1000
1,080E+04
2,060E+04
3,040E+04
4,020E+04
5,000E+04
Integrated spectra obtained by confocal microscopy on CIS sample
Confocal microscope
Confocal spot (NA dependant)
Focal depth (NA dependant)
Large area depending on system magnification
Depends on sample absorptivity
~0.5 µm
0.1 nm 2 nm
~0.6 µm
Probed area
Depth probed
Spectral resolution
Best spatial resolution
Hyperspectral imager
−= 1exp)(),(),( kTq(V-RsI)rEQEr eqssem λφλλφ ]
]
(
(
1,00 1,05 1,10 1,15
1E14
1E15
1E16
Pho
tons
/ s
EL measured Ideal slope
Voltage (V)
Intergrated EL as a fonction of voltage
Absolute measurement of EL on GaAs solar cell
Wavelength (nm)
EL
(pho
tons
/s)
750
1E9
1E10
1E11
1E12
1E13
850 950800 900
Measured EQESC and determined from EL spectra
EQ
ESC
Wavelength (nm)
EQESC measured
EQESC from EL measurement
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1,0
8006004001,00E+008
2,00E+008
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6,00E+008
Inte
nsity
(a.u
.)
Integrated luminescence as a fonction of contact distance
X (µm)
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3,700E+04
5,400E+04
7,100E+04
8,800E+04
1,050E+05
1,200E+05
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Spatially resolved EL GaAs
X (µm)
Y (µ
m)
Inte
nsity
(a.u
.)