ORGANICPV
Scanning nanostructures
Organic solar cells hold promise as an economical means of harvesting solar energy due to their ease of production and processing. However, the efficiency of such devices is currently below that required for widespread adoption. Rajiv Giridharagopal,
Guozheng Shao, Chris Groves, and David S. Ginger of the Department of Chemistry at the University of Washington review the instrumental issues associated
with the application of scanning probe microscopy techniques that have been shown to be useful in the study of nanostructured organic solar cells.
alternative technology for converting sunlight into electricity. OPVs are potentially very inexpensive to process, highly scalable in terms of manufacturing, and compatible with mechanically flexible substrates. In an OPV device, semiconducting polymers or small organic molecules are used to accomplish the functions of collecting solar photons, converting the photons to electrical charges, and transporting the charges to an external circuit as a useable current.1-3
O
At present, the most intensely-studied and highest- performing OPV systems are those that employ bulk heterojunction (or BHJ) blends as the active layer, with NREL-certified power conversion efficiencies improving seemingly monthly, and currently standing at 6.77%.4
In a bulk
heterojunction blend, the donor and acceptor material are typically mixed in solution, and the mixture is then coated on the substrate to form the active layer. The donor/acceptor pair can consist of
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rganic photovoltaic (OPV) materials are an emerging
two different conjugated polymers, but it is often a conjugated polymer (donor) and a soluble fullerene derivative (acceptor).5, 6
Figure 1a shows a typical BHJ-based OPV device architecture.
Despite the advances of the last few years, the efficiencies of OPVs are still below the level needed for industrial viability. The path towards improved OPV efficiency appears straightforward and researchers are actively working on goals such as better coverage of the solar spectrum to increase current and tailored energy levels of the donors and acceptors to gain higher open circuit voltages. However, these otherwise straightforward problems in materials synthesis are complicated by the fact that the texture, or morphology, of the donor acceptor blend – which is sensitive to the exact conditions of how the blend was processed into a thin film – has a dramatic effect on the performance of OPVs.1
The importance of
morphology arises from the competing demands of a number of microscopic processes. First, when
The illustration in
www.solar-pv-management.com Issue II 2010
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