Optoelectronic Properties of Impurity-Modified CuGaS2 Solar Cell Absorber Films

79 Pages·2011·22.25 MB·English

SCHRIFTENREIHE DES HZB · EXAMENSARBEITEN

Optoelectronic Properties
of Impurity-Modified
CuGaS Solar Cell Absorber
2
Films

Sascha Klemz
Diplomarbeit

Institut für Technologie E-I3
September 2011
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doi: http://dx.doi.org/10.5442/e0005

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Optoelectronic Properties of
Impurity-Modified CuGaS Solar Cell
2
Absorber Films
Diploma Thesis
Sascha Klemz
submitted to the
University of Applied Sciences of Brandenburg
Faculty of Engineering Sciences
Physical Engineering
prepared at the
Helmholtz-Zentrum Berlin für Materialien und Energie
Supervisor: Prof. Dr. sc. nat. K. P. Möllmann
Supervisor: Dr. Björn Marsen
.
Eidesstattliche Erklärung
Hiermit versichere ich, die vorliegende Arbeit selbstständig und unter ausschließlicher
Verwendung der angegebenen Quellen und Hilfsmittel erstellt zu haben.
Die Arbeit wurde bisher in gleicher oder ähnlicher Form keiner anderen Prüfungsbehörde
vorgelegt und auch nicht veröffentlicht.
Berlin, den 15. September 2011 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
.
Contents
1 Introduction 9
2 Theoretical background 11
2.1 Structure of chalcopyrite (CuFeS ) and gallite (CuGaS ) . . . . . . . . . . 11
2 2
2.2 Optical properties of solids . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1 Allowed direct transitions in semiconductors . . . . . . . . . . . . . 13
2.2.2 Absorption and dispersion described by Lorentzian oscillators . . . 16
2.2.3 Crystal field theory and d-d-transitions . . . . . . . . . . . . . . . . 18
2.2.4 Optical propagation model of light interacting with a stack of layers 20
2.3 Device structure of chalcopyrite thin film solar cells . . . . . . . . . . . . . 25
3 Experimental and Modeling Methods 27
3.1 Preparation of CuGaS and Cu-Ga-Fe-S films . . . . . . . . . . . . . . . . 27
2
3.2 VIS-NIR Spectrophotometer . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.3 Modeling of reflectance and transmittance curves . . . . . . . . . . . . . . 29
3.4 Optoelectronic characterization methods . . . . . . . . . . . . . . . . . . . 31
3.4.1 Photoconductivity of co-planar contacted solar cell absorber layers . 31
3.4.2 Current-Voltage measurements . . . . . . . . . . . . . . . . . . . . 33
3.4.3 Quantum efficiency of solar cells . . . . . . . . . . . . . . . . . . . . 34
4 Optical Properties of CuGa Fe S Films 37
1−x x 2
4.1 Basic characterization of iron modified CuGaS absorber layers and solar
2
cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Optical analysis of CuGa Fe S thin films . . . . . . . . . . . . . . . . . 39
1−x x 2
4.2.1 Experimental results from reflectance and transmittance measure-
ments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.2.2 Simulated reflectance and transmittance curves . . . . . . . . . . . 44
4.3 Optoelectronic effects of the iron incorporation . . . . . . . . . . . . . . . . 50
4.3.1 PhotoconductivitymeasurementsofCuGa Fe S absorberlay-
0.997 0.003 2
ers on glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
5 Solar cells with CuGa Fe S absorbers 53
1−x x 2
5.1 J-V characteristics of CuGa Fe S solar cell devices . . . . . . . . . . . . 53
1−x x 2
5.2 Quantum efficiency characteristics of CuGa Fe S solar cell devices . . . 54
1−x x 2
5.3 Intensity-dependent photocurrent measurements . . . . . . . . . . . . . . . 57
6 Summary and Conclusions 61
7 Appendix 63
Bibliography 69
Abstract
Transition-metal doped chalcopyrite thin films have been recommended as a suitable ab-
sorberlayerforintermediatebandsolarcells. ThinfilmsofCuGa Fe S hadbeengrown
1−x x 2
on glass and on glass/Mo in a coevaporation process at a substrate temperature of 400°C.
The amount of incorporated iron has been varied from x = 0.003 to x = 0.76 as measured
by energy dispersive x-ray spectroscopy. X-ray diffraction patterns have been analyzed to
investigate present phases and the current thin film absorber layers have been determined
to be a single-phase solid solution material. Transmittance and reflectance curves have
been recorded to study optical transitions and distinct absorption peaks around 1.3 and
1.9 eV have been evaluated which increase in prominence with increasing iron content.
An optical model including absorption process and the propagation path of the light has
beendevelopedtoquantifysub-bandgaptransitionsintermsoftransitionratesandoscilla-
tor strength respectively. Using a co-planar device and photoconductivity measurements,
higher sub-bandgap photocurrent for Fe-containing samples have been observed compared
to the CuGaS sample. Solar cell devices have been fabricated from CuGa Fe S ab-
2 1−x x 2
sorber layers using the standard chalcopyrite glass/Mo/absorber/CdS/ZnO device struc-
ture. The solar cell performance has been evaluated by current-voltage, spectral- and
intensity-dependent quantum efficiency. The device performance in general decreases
with increasing iron content which is attributed to iron defects induced interface recombi-
nation. However, forthesamplewiththelowestamountofiron, anincreasedsub-bandgap
photoresponse is observed, dropping off around 1.9 eV. This is interpreted as carrier col-
lection facilitated by Fe related states. The defect level around 1.9 eV have been further
investigated due to an intensity-dependent photocurrent generation. The photocurrent
of the sample with the lowest iron content increases linear with the incident photon flux
whichhasbeenvariedoverfiveordersofmagnitude. Thequantumefficiencyofthesample
have been shown no dependence of the incident light intensity.
.