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

Introduction
Cyanine dyes were developed at the beginning of the 20 th century, mainly as sensitizers for silver halide
emulsions in the photographic process. Above all, cyanines exhibit extraordinarly high extinction
coefficients and tunable absorption spectra. Most interesting is the possibility to achieve strong light
absorption in the near-infrared domain, which is presently thought to be one possibility to enhance
power conversion efficiency of organic solar cells.
Objectives
So far only few works have studied thin solid cyanine films as active layers in solar cells. Simple
bilayer heterojunction devices were fabricated, proving the concept of using cyanines as electron donors
or acceptors. Unfortunately the power conversion efficiency has been typically around 0.1 % or
lower, which is too modest for most applications. The cause of this poor performance has been unclear
so far. This work highlights the importance of charge carrier transport in cyanine dye based solar
cell [1].
Work performed and results obtained
When cyanine films were exposed to ambient atmosphere under white light irradiation, a steep rise of
the conductivity of the film could be observed (Fig. 1). Clearly light is required to induce the conductivity
increase. By separately investigating the effect of oxygen and water it could be demonstrated that
both are needed for the doping process. Photochemical reactions of cyanine dyes with oxygen involve
either energy transfer or electron transfer. While the former leads to reactive singlet oxygen, the latter
-
leads to the transient superoxide anion O2 species that further react with a neighboring cyanine molecule.
Contrary to the energy transfer mechanism, the electron transfer mechanism produces a cationic
cyanine species which can be regarded as positive charge carrier.
Conductivity (S/cm)
6.0x10 -4
5.0x10 -4
4.0x10 -4
3.0x10 -4
2.0x10 -4
1.0x10 -4
0.0
white light irradiation
dark
0 5 10 15 20 25 30
Exposure time to air (min)
Fig. 1: Four probe conductivity measurement of a thin cyanine film as a function of exposure time to
ambient atmosphere in the dark (black squares) and under white light irradiation (red squares).
Cyanine – fullerene C60 bilayer solar cells were fabricated on transparent conducting glass coated by a
conductive polymer interlayer (PEDOT:PSS). Photochemical doping of the cyanine film had a tremendous
impact on the device performance. Most importantly the power conversion efficiency of the device
increased from 0.14% to 1.2% which is among the best efficiencies for organic bilayer devices.
The short circuit current increase from 0.46 mA/cm 2 to 1.83 mA/cm 2 as well as the fill factor improvement
from 0.19 to 0.27 further emphasize the ameliorated charge transport due to the doping process.
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Doping of cyanine solar cells: enhancing charge transport, B. Fan, F. Nüesch, Empa
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