Programm Photovoltaik Ausgabe 2009 ... - Bundesamt für Energie BFE

2. Processes
2.1. Microcrystalline process at high deposition rate and control tools [Fel08, Fel08 b, Bug08]
Introducing a reduction of the KAI-M inter-electrode gap distance allowed us to explore higher pressure
regime. A basic description of the various plasma regimes and the relation to the reactor configuration
are given in [Fel08, Fel08 b, Bug08]. As a result of these hardware and process optimization
steps, a microcrystalline single junction solar cell (0.25 cm2) with an efficiency of 7.1% (Voc=503 mV,
FF=70.0%, Jsc=20.2 mA/cm2, Fig.7) was obtained at a deposition rate of 1 nm/s. This deposition rate
is significantly higher than the usual 0.4-0.55 nm/s which was achieved in previous projects.
Further optimization is under way to increase the efficiency; in order to better understand the variation
in efficiency observed for different deposition rates (usually a decrease of efficiency), the quality of the
�c-Si:H material in the single-junction cells was checked with Fourier Transform Photocurrent Spectroscopy
(FTPS). Absorption spectra are presented in Fig.7b for both high and low silane depletion
regimes. Residual defects are observed, for instance, in the i-layer of the cell deposited at high deposition
rates in high depletion regimes.
In order to better understand the plasma processes, in particular the depletion and the onset of powder
formation, several tools have been developed in 2008. These will be implemented for various
measurements in 2009.
a) b)
Fig.7: a) Initial I-V characteristics of �c-Si:H single-junction solar cell deposited under high silane
concentration regime at 1 nm/s. The cell thickness is 1.2 �m
b) Defect densities in two different regimes measured by FTPS
2.2 UV-NIL replication process for nano-textured substrates
During the last reporting period, a new replication system has been developed and installed to fabricate
laboratory scale nano-textured substrates directly at the IMT (see Fig.8). Both substrates and
superstrate configuration are possible.
Fig.8: Photos of the membrane REPLICATION SYSTEM
Fig.9: Example of a SEM images of a LPCVD master (left) and the replica (right).
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New processes and device structures for the fabrication of high efficiency thin film silicon photovoltaic modules, C. Ballif, University of Neuchâtel
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