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

Introduction and objectives
Clearly a change in paradigm is required for photovoltaics to become cost competitive and cover a
significant part of the enormous future energy demands. This project wants to provide solutions based
on thin film PV technology. Several world class Swiss research institutes are involved in state-of-the
art research and industrial development of PV technologies. This project has the objective to significantly
strengthen and synergise the major efforts undertaken in Switzerland in this field. It can be divided
into three parts, where the first two parts include two different technological roads: Part A addresses
the short-to-medium term needs for the advancement of thin film silicon technology. This implies
the understanding and characterisation of the plasma conditions used in plasma enhanced
chemical vapour deposition reactors. Advanced device structures and materials are elaborated on the
laboratory scale. In a further step, large scale implementation is investigated. Part B focuses on the
development of high performance multi-junction cells based on inorganic and organic hybrid photovoltaic
devices using low cost manufacturing technologies, which could become competitive in the
medium to long term range. High efficiency devices are targeted by combining dye sensitized solar
cells (DSC) with CuIn(Ga)Se (CIGS) and organic photovoltaic devices (OPV) in a multi-junction approach.
Optimized individual cells are stacked in series before monolithical integration is addressed.
Novel concepts for hybrid organic-inorganic multiple junction cells are elaborated with the aim to produce
intellectual property and to promote industrial activities in this field. Part C is defined to be an
exchange platform to allow for workshops and educational activities.
Work performed and results obtained
Part A
Three diagnostic tools have been installed at IMT and two are already operational: optical emission
spectroscopy and infrared absorption spectroscopy. In addition a third diagnostic tool is being set up
for the measurement of poly-silane and powder particles using laser scattering.
Optical emission spectroscopy (OES)
This technique allows monitoring the optical emission of plasmas typically from the visible to the near
infrared range. In the case of thin film silicon deposition that involves the injection and dissociation of
hydrogen and silane molecules, emission lines from H2 and the SiH radical are of particular interest.
The plasma parameters that can be extracted from OES time-resolved measurements are silane depletion
and fluctuations of the electron temperature. The first parameter is relevant because it gives a
measure of the utilization rate of silane during the dissociation process in the plasma. This parameter
is not only important with respect to the efficiency of gas utilization during deposition, but under certain
process conditions literature reports indicate it as an important factor that determines the properties of
the microcrystalline material as well. In addition, the same technique allows the measurement of fluctuations
of the electron temperature. The importance of monitoring this parameter is related to the fact
that fluctuations of the electron temperature are related to the formation of poly-silane and powder
particles in the plasma.
Infrared absorption spectroscopy
The determination of silane depletion using OES is an easy, though not a direct measurement and
relies on model assumptions; i.e. electron temperature has to remain constant from plasma ignition to
steady state. As a result, this technique works best when powder formation is negligible. In many industrially
relevant cases microcrystalline silicon is deposited under process conditions where polysilane
and powder formation is present. As a result, an experimental tool that is able to measure silane
depletion directly without any model assumptions is of great value. In our lab a new technique for the
measurement of silane depletion has been attempted and successfully demonstrated. It relies on high
resolution infrared absorption of roto-vibrational states of the silane molecule. The setup allows performing
the measurements inside the reactor where the plasma is dissociated or outside the deposition
chamber, in the pumping line. In Fig. 1 an image and a sketch of the setup are displayed.
ThinPV, F. Nüesch, Empa
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