Polymer-Fullerene Bulk Heterojunction Solar Cells
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Polymer–Fullerene
Bulk Heterojunction Solar Cells
René A.J. Janssen, Jan C. Hummelen, and N. Serdar Sariciftci Abstract Nanostructured phase-separated blends, or bulk heterojunctions, of conjugated polymers and fullerene derivatives form a very attractive approach to large-area, solid-state organic solar cells. The key feature of these cells is that they combine easy processing from solution on a variety of substrates with good performance. Efficiencies of up to 5% in solar light have been achieved, and lifetimes are increasing to thousands of hours. Further improvements can be expected and some of the promising strategies towards that goal are presented in this article. Keywords: bulk heterojunctions, polymer–fullerene, nanostructure, photovoltaics, solar cells.
The Need for Bulk Heterojunctions In organic polymer-based photovoltaic devices, the primary effect upon exposure to solar light is a photoinduced electron transfer between donor- and acceptor-type semiconducting polymers or molecules, yielding a charge-separated state. This photoinduced electron transfer between donor and acceptor boosts the photogeneration of free charge carriers, as compared with the individual, pure materials, in which the formation of bound electron– hole pairs, or excitons, is generally favored. In combining electron-donating ( p-type) and electron-accepting (n-type) materials in the active layer of a solar cell, care must be taken that excitons created in either material can diffuse to the interface, to enable charge separation. Due to their short lifetime and low mobility, the diffusion length of excitons in organic semiconductors is limited to about 10 nm only. This imposes an important condition on efficient charge generation. Anywhere in the active layer, the distance to the interface should be on the order of the exciton diffusion length. Despite their high absorption coefficients, exceeding 105 cm1, a 20 nm double layer of donor and acceptor materials would not be optically dense, allowing most photons to pass
MRS BULLETIN • VOLUME 30 • JANUARY 2005
freely. The solution to this dilemma is elegantly simple.1,2 By simply mixing the pand n-type materials and relying on the intrinsic tendency of polymer materials to phase-separate on a nanometer dimension, junctions throughout the bulk of the material are created that ensure quantitative dissociation of photogenerated excitons, irrespective of the thickness. Polymer–fullerene solar cells were among the first to utilize this bulk heterojunction principle.1 However, this attractive solution poses a new challenge. Photogenerated charges must be able to migrate to the collecting electrodes through this intimately mixed blend. Because holes are transported by the p-type semiconductor and electrons by the n-type material, these materials should be preferably mixed into a bicontinuous, interpenetrating network in which inclusions, cul-de-sacs, or barrier layers are avoided. The close-to-ideal bulk heterojunction solar cell may look like the illustration in Figure 1.
Semiconductin
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