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

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

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29.09.2012 Views

Eidgenössisches Departement für Umwelt, Verkehr, Energie und Kommunikation UVEK Bundesamt für Energie BFE DOPING OF CYANINE SOLAR CELLS: ENHANCING CHARGE TRANSPORT Annual Report 2007 Author and Co-Authors Bin Fan 1 , Roland Hany 1 , Frank Nüesch 1 Jacques-Edouard Moser 2 1 Institution / Company Labortatory for Functional Polymers / Empa - Swiss Federal Laboratories for Materials Testing and Research 2 Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne 1 Address Überlandstrasse 129, CH-8600 Dübendorf 2 CH-1015 Lausanne, Switzerland 1 Telephone, E-mail, Homepage +41 44 823 47 40, frank.nueesch@empa.ch, http://www.empa.ch Project- / Contract Number CCEM-ThinPV project Part B Duration of the Project (from – to) 2007-2009 Date 1.4.2007 ABSTRACT Organic solar cells are currently subject to intense research efforts spurred by the promise of providing low cost devices to the growing photovoltaic market. Cyanine based materials are very strong light absorbers but have yet to prove their suitability for photovoltaics in terms of charge carrier transport. Photoinduced doping by oxygen demonstrates that charge carrier transport can be enhanced importantly in these materials, which may be a loophole to the charge carrier mobility limitation. When used in bilayer thin film photovoltaic devices, doped materials give rise to a ten-fold increase of the power conversion efficiency as compared to the pristine materials. Seite 119 von 288

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. Seite 120 von 288 Doping of cyanine solar cells: enhancing charge transport, B. Fan, F. Nüesch, Empa 2/3

Eidgenössisches Departement <strong>für</strong><br />

Umwelt, Verkehr, <strong>Energie</strong> und Kommunikation UVEK<br />

<strong>Bundesamt</strong> <strong>für</strong> <strong>Energie</strong> <strong>BFE</strong><br />

DOPING OF CYANINE SOLAR CELLS:<br />

ENHANCING CHARGE TRANSPORT<br />

Annual Report 2007<br />

Author and Co-Authors Bin Fan 1 , Roland Hany 1 , Frank Nüesch 1<br />

Jacques-Edouard Moser 2<br />

1<br />

Institution / Company<br />

Labortatory for Functional Polymers / Empa -<br />

Swiss Federal Laboratories for Materials Testing and Research<br />

2<br />

Photochemical Dynamics Group, Institute of Chemical Sciences<br />

and Engineering, Ecole Polytechnique Fédérale de Lausanne<br />

1<br />

Address<br />

Überlandstrasse 129, CH-8600 Dübendorf<br />

2<br />

CH-1015 Lausanne, Switzerland<br />

1<br />

Telephone, E-mail, Homepage +41 44 823 47 40, frank.nueesch@empa.ch, http://www.empa.ch<br />

Project- / Contract Number CCEM-ThinPV project Part B<br />

Duration of the Project (from – to) 2007-2009<br />

Date 1.4.2007<br />

ABSTRACT<br />

Organic solar cells are currently subject to intense research efforts spurred by the promise of providing<br />

low cost devices to the growing photovoltaic market. Cyanine based materials are very strong<br />

light absorbers but have yet to prove their suitability for photovoltaics in terms of charge carrier transport.<br />

Photoinduced doping by oxygen demonstrates that charge carrier transport can be enhanced<br />

importantly in these materials, which may be a loophole to the charge carrier mobility limitation. When<br />

used in bilayer thin film photovoltaic devices, doped materials give rise to a ten-fold increase of the<br />

power conversion efficiency as compared to the pristine materials.<br />

Seite 119 von 288

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