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

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The project’s goals are, hence, to be achieved by introducing innovation and optimisation in four majors
areas:
� Layers with new or better properties (e.g. new materials with higher transparency)
� Improved processes (more stable, faster processes, yielding higher quality layers)
� Increased device and module efficiency
� Enhanced cells and modules reliability.
Work performed and results achieved
1. Materials
1.1 Development of advanced SiOx based intermediate reflectors [Bue08].
The material properties of doped silicon oxide intermediate reflectors deposited in situ with plasma
enhanced chemical vapor deposition systems has been investigated. The challenge in the development
of these layers is to lower the refractive index without compromising the electrically conductive
properties. Intermediate reflectors are used in micromorph tandem cells to increase the current density
in thin top amorphous cells. Low refractive indices are therefore desirable because the reflectivity of
the layer depends on the index step between silicon oxide and silicon. However, lower refractive indexes
are typically associated with increasingly more insulating layers which can block the current flow
between top and bottom cell. In order to better understand the trade-off between optical and electrical
requirements of the silicon oxide intermediate reflector, the structure of the layers was analyzed with
bright field TEM images. In the images of Figure 2, silicon nanocrystals embedded in an amorphous
silicon oxide matrix can be seen. For increasing H2 dilution the silicon nanocrystals are reduced in
number and in size. Accordingly, the conductivity of the layers decreases (not shown). This correlation
suggests that the conductive properties of doped silicon oxide intermediate reflectors are associated
with the presence of densely distributed and large silicon nanocrystals. These nanocrystals most likely
are the conductive channels through which the current flows.
Deposition parameters were varied in order to find a conductive layer with the lowest possible refractive
index. Fig.3 shows the refractive index of intermediate reflective layers as a function of CO2/SiH4
and H2/SiH4 gas ratios. For both series, the refractive index decreases with increasing CO2/SiH4 gas
ratio. As a result of this process optimization a conductive silicon oxide layer with refractive index n =
1.71 has been developed and used in micromorph tandem cells.
Fig.2: TEM top-view images of ~ 100 nm thick nc-SiOx layers deposited at different dilutions
Fig.3: Refractive index of silicon oxide layers with various preparation conditions
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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