Photon absorption by organic molecules with adequate HOMO-LUMO gap excites metastable electron-hole pairs, whose energy can be converted into electrical power by driving the photogenerated electrons and holes to the different electrodes of a solar cell. However, one of the facts that limit the efficiency of "plastic" solar cells is that photon absorption usually leads to the formation of tightly bound excitons which do not spontaneously dissociate into charge pairs. The hitherto most successful approach to promote dissociation is to use blends of phase-segregated electron-donor/electron-acceptor (D/A) molecules –the so-called bulk heterojunction concept. At the D/A interface, the difference in electron affinities drives the exciton dissociation by injecting free electrons (holes) into the electron-acceptor (electron-donor) areas. Provided that continuously connected paths between the interfaces and the electrodes exist, the free electrons and holes will be collected therein. From these considerations, an ideal morphology for optimum solar cell performance would then be composed of interdigitated donor-acceptor domains and elongated structures (to maximize D/A interface area) with typical diameters of no more than 10-20 nm (the typical diffusion length of an exciton, to suppress exciton radiative recombination events) connected to the electrodes.
In this work we describe Scanning Tunneling Microscopy (STM) experiments that show how monolayer-thick blends of the electron donor molecule 2-[9-(1,3-dithiol-2-ylidene)anthracen-10(9H)-ylidene]-1,3-dithiole (exTTF) with the electron acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) on a reconstructed Au(111) surface, segregate laterally into "nanostripes" whose width is of the order of the exciton diffusion length; it thus corresponds closely with the morphology for optimum solar cell performance. The reason for such a peculiar nano-scale morphology can be traced back to the different interactions between the two molecular species and the herringbone reconstruction of Au(111). Our results demonstrate the potential of atomistic studies about the growth of organic semiconductors to open new directions for the design and construction of highly-efficient organic electronic devices.
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