Plenarvorträge - DPG-Tagungen

Dünne Schichten Dienstag
DS 22.48 Di 14:30 Poster B
Temperature-dependent surface photovoltage of polycrystalline
Cu(In,Ga)Se2 from photoemission studies using synchrotron radiation
— Thomas Schulmeyer, Ralf Hunger, and •Andreas
Klein — Technische Universität Darmstadt, Petersenstrasse 23, D-
64287 Darmstadt
A deep insight of surfaces and interfaces is essential for improving
the knowledge of the complex interface behaviour of Cu-chalcopyrites.
In this contribution, temperature-dependent photovoltage measurements
with synchrotron radiation induced photoelectron emission of polycrystalline
Cu(In,Ga)Se2 surfaces and Cu(In,Ga)Se2/CdS interfaces are presented.
Clean Cu(In,Ga)Se2 surfaces are prepared with the Se de-capping
procedure. Additionally, defined oxidized Cu(In,Ga)Se2 and de-capped
CuGaSe2 absorbers were investigated. All chalcopyrite surfaces show a
Cu-poor surface composition.
DS 22.49 Di 14:30 Poster B
Aluminum-induced crystallization of amorphous silicon: Al/a-
Si-interface influence on seed layer formation — •Jens Schneider,
Juliane Klein, Martin Muske, Stefan Gall, and Walther
Fuhs — Hahn-Meitner-Institut Berlin, Kekuléstr. 5, D-12489 Berlin,
Germany
Crystallization of seed layers on inexpensive foreign substrates and
their subsequent epitaxial thickening is a promising approach for the formation
of thin-film crystalline silicon cells. Such seed layers can be formed
by Al-induced crystallization of an amorphous silicon (a-Si) layer. The kinetics
of Al-induced crystallization of a-Si are studied by variation of the
thickness of an Al-oxide layer between the initial Al and a-Si films. The
results show that the thickness of this Al-oxide layer strongly determines
the nucleation time and the number of Si-grains (nucleation) and has only
little influence on the growth velocity of the grains. The diameter of the
grains increases in good approximation linearly with time and is strongly
influenced by the annealing temperature. The general behavior suggests
that the process kinetics of the aluminum-induced layer exchange are determined
by the silicon diffusion across the interface which can be altered
by the thickness of the oxide layer.
DS 22.50 Di 14:30 Poster B
Development system for advanced CdTe Thin Film Solar Cells
— •Jochen Fritsche, Daniel Kraft, Bettina Späth, Andreas
Thißen, Andreas Klein, and Wolfram Jaegermann — Darmstadt
University of Technology, Institute of Materials Science, Surface
Science Division, Germany
CdTe thin film solar cells promise a high potential to reach good conversion
efficiencies with a low cost production process. A large scale production
of CdTe modules (120 cm × 60 cm) is realized by ANTEC Solar
Energy GmbH with average conversion efficiencies of 6,7 %. In the production
a layer sequence is used, which is based on empirical development.
However, the highest obtained efficiency with CdTe is 16,5 %. We have
analyzed electronic and chemical properties of surfaces und interfaces
in the CdTe solar cell. Major obstacles to obtain higher efficiencies and
lower production costs within this system are related to interfacial issues
as barrier heights for charge transport, interface recombination and film
nucleation. A better understanding of these properties will help to prepare
improved front and back contacts as well as to reduce the CdS and
CdTe layer thickness. In order to explore possible advanced cells with
a better control and knowledge about interface properties, we have set
up DAISY-Sol (DArmstadt Integrated SYstem for Solar cell research),
which combines a full vacuum production of CdTe solar cells with an
in-situ analysis to investigate and modify the electrical and chemical
properties of all surfaces and interface.
DS 22.51 Di 14:30 Poster B
CuInS 2 Solar Cells Deposited by Reactive Magnetron Sputtering
— •Thomas Unold, Jan Hinze, and Klaus Ellmer — Hahn-
Meitner Institut, Glienicker Str. 100, D-14109 Berlin, Germany
We have deposited CuInS 2 films in a reactive magnetron sputtering
system, specially designed for the deposition of sulfides by sputtering
in Ar-H2S mixtures. The substrate temperature was varied from
350 to 500C and the deposition rate was about 50 nm/min. The magnetron
plasma was excited by a DC power supply. The contacts (Mo,
ZnO/ZnO:Al) and the buffer layer (CdS) were prepared by the standard
processes of the Hahn-Meitner-Institut. Up to now solar cell efficiencies
of up to 8.1 have been obtained. The electronic and optical properties
of the solar cell devices were analyzed by dark and illuminated current-
voltage characteristics, quantum efficiency and photoluminescence measurements.
We find that although the short circuit current is comparable
to that of cells prepared by the two-step process (sulfurization of sputtered
precursor metals), the open-circuit voltage is about 100meV lower.
This is consistent with the quantum efficiency analysis where we find a
decreased collection efficiency at long wavelengths compared to cells from
the two-step process. From photoluminescence we conclude that the efficiency
of our cells is limited by the defect density in the absorber layer.
DS 22.52 Di 14:30 Poster B
Epitaxial growth and characterization of Cu0.5In0.5Se (Te)
films grown on GaAs by pulsed laser deposition. — •Liudmila
Roussak, Gerald Wagner, and Klaus Bente — Institute of Mineralogy,
Crystallography and Materials Science, Linnestr. 3-5 (TA), D-
04103 Leipzig
Alloys of Cu0.5In0.5Se(Te), as an attractive material for solar cell applications,
have been grown on (001) oriented GaAs substrates by pulsed
laser deposition (PLD). Epitaxial growth was found to occur for substrate
temperatures ranging between 380 and 600 ◦ C. The structural, compositional,
morphological and electrical properties of the Cu0.5In0.5Se(Te)
films depend strongly on the substrate temperature, laser energy density
and deposition rate. Applying optimized growth conditions it is possible
to obtain the films of required electrical and structural characteristic.
The detailed examination of late the absorber and buffer forming the heterostructure
as well as their interface are very important for the efficient
of thin film solar cells.
DS 22.53 Di 14:30 Poster B
Influence of the substrate preparation on the low-temperature
epitaxy of Si by electron-cyclotron resonance chemical vapor
deposition — •Patrick Clemens, Björn Rau, Ina Sieber, Stefan
Gall, and Walther Fuhs — Hahn-Meitner-Institut, Abt. Silizium
Photovoltaik, Kekuléstr. 5, D-12489 Berlin
We report on a study of substrate preparation for the low-temperature
Si homo-epitaxy by electron-cyclotron resonance chemical vapor deposition
(ECRCVD). The background of the research is the development of
a low-temperature epitaxial thickening process for Si thin-film solar cells
on large-grained polycrystalline Si seed layers on glass.
Different from conventional high-temperature Si deposition methods
the conditions of the substrate surface, its crystal orientation and its
contamination are very critical for the successful epitaxial growth of Si
at substrate temperatures below 600 ◦ C. Both, wet chemical surface pretreatments
and in-situ treatments are applied to obtain an ideal substrate
surface for Si epitaxy. Using a HF/alcohol instead of HF/water solution
the roughness of the surface of Si wafers decreases, the surface contaminations
are reduced, and the H-termination of the Si surface is improved.
During substrate heating in-situ H-treatment is used to increase the stability
of this surface and prevent reoxidation. This substrate preparation
leads to a successful epitaxial growth of Si on Si(100) wafers at low temperatures.
The influence of the preparation on the Si epitaxy is analyzed
by structural and electrical characterizations of the grown films.
DS 22.54 Di 14:30 Poster B
Majority-carrier properties of low-temperature epitaxial Si —
•S. Brehme, U. Knipper, B. Rau, and W. Fuhs — Hahn-Meitner-
Institut Kékulestr. 5 12489 Berlin
In this contribution we report the results of electrical investigations on
thin Si films grown epitaxial at temperatures below 600 ◦ C. Such films
are possible candidates for absorber layers in thin-film solar cells on glass.
Films were grown by Electron Cyclotron Resonance Chemical Vapor Deposition
(ECR-CVD) on Si substrates. n- and p-type wafers of varying
conductivity were used in the same run to render possible different types
of electrical measurements such as capacitance voltage (CV), Hall effect
(HE), and Deep Level Transient Spectroscopy (DLTS). The nominally
undoped thin films were found to have room temperature electron concentrations
in the range 10 16 cm −3 to 10 17 cm −3 depending on certain details
of the growth process. From CV measurements similar results were
obtained. Hall mobilities in the range 400 to 700 cm 2 /Vs were observed.
Considering the ratio R of experimentally observed mobilities to values
in a Si wafer of the same doping level, R values of typical 50 % are found
in thin films with doping levels Nd of about 1 × 10 16 cm −3 . For layers
with higher Nd R increases indicating different degrees of compensation.
DLTS measurements show the presence of several electron-emitting trap
levels with a dominant peak P1 at EC - 0.27 eV. Concentrations of P1