Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Introduction and first results
As the project started recently, we only present a short project overview and some initial results.
In October 2007, IMT and the company VHF-Technologies SA started a collaboration within a CTI
project that addresses two particular aspects for the improvement of VHF’s commercial product; these
aim at the development of textured back reflectors for current enhancement, and the introduction of a-
Si/a-Si tandem cells. The latter aspect allows using thinner absorbers with less degradation as well as
reducing the area losses of the series connection.
For the development of textured substrates it is intended to apply a variety of new structures on the
plastic substrates compatible with VHF’s industrial production. Those can range from periodic gratings
to random textures.
A particular emphasis is set on:
� finding out substrate structures that allow a high gain in current without loss in open circuit and
Fill Factor of the solar cells, for the specific devices used in the project,
� achieving reliable and reproducible ways to apply texture to the plastic foil substrates, in order to
be able to achieve high processing yields.
The development of a-Si/a-Si tandem cells is carried out in close collaboration with the developing
team of VHF-Technologies SA. Fig. 1 and Table 1 present first results of amorphous tandem cells
from IMT, in the n-i-p / n-i-p configuration. The initial efficiency of 8.8% on the flexible substrate with
specific texture is a promising result, and one of the best cell efficiencies on plastic substrate, based
exclusively on amorphous Si (no microcrystalline Si). Also, these cells will degrade significantly less
than the single junction counterpart, which have the same initial efficiency (cf report project OFEN
101191).
Fig 1: Current-voltage characteristics (initial, left) and corresponding external quantum efficiencies of
a-Si/a-Si tandem cells, newly developed on glass and flexible plastic substrates. All the devices are
grown in the n-i-p configuration.
Table 1: Best results for the solar cell parameters of a-Si/a-Si tandem cells on different substrates
(initial values) obtained in this project.
Substrate Voc jsc (top)
[mV] [mA/cm 2 jsc (btm)
] [mA/cm 2 FF �
] [%] [%]
Textured TCO /glass 1742 7.41 7.81 70.2 9.1
Lowly textured TCO/glass 1723 6.66 6.46 70.5 7.9
Textured plastic 1711 7.43 7.53 69.3 8.8
Flat plastic 1698 6.58 5.53 70.0 5.5
Flexible photovoltaics: next generation high efficiency and low cost thin film silicon modules, Haug/Ballif, IMT, Neuchâtel
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