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.

amaury.delamarre@etu.upmc.fr

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

1

2

3

4

5

6

7

8

9

10

x10

20

12020

40

10040

60

80600

80

100

120

140

160

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

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

X (µm)

Y (µ

m)

0,000

0,2000

0,4000

0,6000

0,8000

1,000

Integrated photoluminescence intensity, λexc = 532 nm

1

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

500

1000

1500

2000

2500

Cou

nts

Wavelength (nm)

Histogram of photoluminescence peaks positions

0 5 10 15 20 25 30 350

5

10

15

20

25

30

35

X (µm)

Y (µ

m)

White lamp image of probed area

0 5 10 15 20 25 300

5

10

15

20

25

30

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

0,0

800 850 900

0,2

0,4

0,6

0,8

1,0

8006004001,00E+008

2,00E+008

3,00E+008

4,00E+008

5,00E+008

6,00E+008

Inte

nsity

(a.u

.)

Integrated luminescence as a fonction of contact distance

X (µm)

00

100

3,700E+04

5,400E+04

7,100E+04

8,800E+04

1,050E+05

1,200E+05

100

200

200

300

300

400

400

500

500

600

700

800

Spatially resolved EL GaAs

X (µm)

Y (µ

m)

Inte

nsity

(a.u

.)