| Organic Solar Cells Based on Self-Organized Bulk Heterojunctions |
| Organic semiconductors comprise a remarkable class of materials with various opto-electronic applications. The excitonic nature of organic semiconductors imposes constraints on the design of organic solar cells. As the intrinsic photocarrier yield is weak, the photocurrent in organic solar cells is generated by a photoinduced charge transfer reaction between adjacent chromophores. Obtaining eficient charge generation and charge transport is challenging as optimizing the former has often meant sacrificing the latter. The most efficient charge generation is obtained by mixing the donor and acceptor—a bulk heterojunction. Intimate contact results in complete charge transfer, but interferes with charge transport due to recombination and trapping. The ultimate goal is a heterogeneous blend that provides a large interfacial area between separate domain of the donor and acceptor domains and percolation pathways for charge carrier transport. Self organization of organic semi¬conductors has enabled to the fabrication of organic solar cells with such a morphology. Progress in organic cells will be reviewed with particular emphasis on self-organization of the bulk heterojunction. The first bulk heterojunction cells were prepared by co-sublimation of the donor and acceptor or by spin-casting a blend of two different polymers. These cells had relatively low efficiency, primarily due to a morphology that was not conducive to charge transport. Advances in materials, particularly and processing have led to cells with the desired morphology. Devices with several percent efficiency were obtained from poly(para-phenylene vinylene) (PPV) and a functionalized derivative of C60 (PC61BM), increasing to five percent when using polythiophene derivatives and even higher efficiencies from new, low-gap polymers. Progress in planar heterojunction cells with a co-sublimed charge generation layer will also be discussed. Biosketch. Paul Lane received a Bachelor of Arts cum laude in Physics and Mathematics from Macalester College in 1986 and a Ph.D. in Experimental Condensed Matter Physics from Iowa State University in 1994. His research experience includes post-doctoral research at the University of Utah, a lectureship at the University of Sheffield (U.K.), and the electro-optics group at the Draper Laboratory. Dr. Lane joined the Naval Research Laboratory in 2003 and is a member of the Quantum Electronics Section. His current research interest focus on optical spectroscopy of organic semiconductors and opto-electronic devices. |