Category Thin Film Solar Cells Fabrication, Characterization and Applications

Donor-acceptor ‘double cable’ polymers

The above morphology studies demonstrated that although the preparation of bulk heterojunc­tion solar cells is very simple, the proper control of the processing parameters is complex, and involves detailed studies of the influence of many factors, incl., the choice of solvent, the


Figure 10.7 AFM images of MDMO-PPV:PCBM 1:4 ratio films cast from toluene solutions with increasing poymer content. Reprinted with permission from Hoppe et al, Adv. Funct. Mater, 14, 1005, Nanoscale morphology of conjugated polymer/fullerene-based bulk-heterojunction solar cells. Copyright (2004) by Wiley-VCH.

concentration of the solution, the weight ratio of the active components, etc. The optimum set of parameters may obviously change with different material combinations...

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Nanomorphology-property relations Investigation of the nanomorphology

The power conversion efficiency of bulk heterojunction solar cells based on an MDMO – PPV:PCBM mixture of 1:4 weight % is improved from approximately 1 % (AM1.5) to 2.5 % by simply changing the solvent from which the active layer has been cast [24]. Atomic force microscopy (AFM) investigations showed that rather large (i. e., 100-200 nm) clusters are formed in toluene cast films, while a smooth surface morphology was observed for chloroben­zene cast films. Transmission electron microscopy (TEM) and AFM studies [39] assigned the large clusters to a PCBM rich phase...

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10.1.1 Operational principles

The concept of a bulk heterojunction is shown in Figure 10.3 together with a band structure displaying the highest occupied molecular orbital (HOMO) and the lowest unoccupied molec­ular orbital (LUMO) of the electron donor and electron acceptor, and the work function of the electrodes. The chemical structure of some of the materials commonly used in organic solar cells is shown in Figure 10.4.

The charge carriers are generated by photoinduced electron transfer. For efficient charge generation, an exciton photogenerated anywhere in the blend has to reach an acceptor interface within its lifetime; therefore the magnitude of the maximum allowed phase separation is de­termined by the exciton diffusion length. For intimately mixed blends, experiment has shown

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In n conjugated polymers, the semiconducting properties are accompanied by the attractive mechanical properties and processing advantages of conventional polymers [7]. Fullerenes [13] as new forms of carbon are another interesting class of functional materials. Buckmin­sterfullerene (Сбо) is a strong electron acceptor capable of taking as many as six electrons, and forms charge transfer salts with a variety of strong donors.

Mixing Сб0 with semiconducting polymers, no significant ground state interaction is ob­served. Upon photoexcitation of the conjugated polymer donor, a cascade of reactions is initi­ated leading to charge separation as schematically illustrated by the following equations [14]:

D + A ^ 1,3 D* + A (excitation on donor) (10.1)

1,3D* + A ^ 1,3 (D ■■■ A)* (excitation...

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Charge Transport and Recombination in Donor-Acceptor Bulk Heterojunction Solar Cells

A. J. Mozer and N. S. Sariciftci

Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, Linz, Austria


The idea of utilizing organic materials for solar energy conversion has been the subject of fascinating research for several decades. Organic materials are typically inexpensive, easily processable and their functionality can be tailored by molecular design and chemical synthesis. The most important functionality is the highly polarizable n electron systems of organic molecules. Although the electronic structure of organic materials is different from typical inorganic crystalline materials, the delocalized n electron systems assign material properties that are traditionally assigned to inorganic semiconductors, e. g...

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Reaching much beyond 12 % conversion efficiency for DSC, by relying mainly on panchro­matic and IR absorbing dyes or surface modifications, will require enhanced light collection in the 700-900 nm region. An alternative and promising approach will be the use of a tandem concept, where the top and bottom cells are judiciously chosen to absorb complimentary com­ponents of the available light including the infrared region. Such a device was recently tried in our laboratory and obtained 15 % conversion efficiency. A preliminary report on this work was given at the 2005 European conference in Barcelona and we shall disclose further details shortly.

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First large scale field tests and commercial developments

Over recent years industrial interest in the dye sensitized solar cell has surged and the devel­opment of commercial products is progressing rapidly. A number of industrial corporations, such as Konarka (www. konarkatech. com) in the USA, Aisin Seiki in Japan, and well as RWE in Germany and Solaronix in Switzerland are actively pursuing the development of modules, both flexible and glass based. Particularly interesting are applications in building integrated photovoltaic elements such as electric power producing glass tiles. The Australian company Sustainable Technologies International (www. sta. com. au) has produced such tiles on a large scale for field testing and several buildings have been equipped with a wall of this type...

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Recent experimental results on dye sensitized solar cell stability

Many long term tests have been performed with the N3 type ruthenium complexes confirming the extraordinary stability of these charge transfer sensitizers. For example, a European con­sortium financed under the Joule program [42] has confirmed cell photocurrent stability over 10 000 hours of light soaking at 2.5 sun equivalents corresponding to ca. 56 million turnovers of the dye without any significant degradation. These results corroborate the projections from the kinetic considerations made above. A more difficult task has been to reach stability under prolonged stress at higher temperatures, i. e. 80-85 °C...

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