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Organic semiconductors based on -conjugated oligomers and polymers constitute the active elements in new generations of plastic (opto)electronic devices. The performance of these devices depends largely on the efficiency of the charge-transport processes; at the microscopic level, one of the major parameters governing the transport properties is the amplitude of the electronic transfer integrals between adjacent oligomer or polymer chains. Here, quantum-chemical calculations are performed on model systems to address the way transfer integrals between adjacent chains are affected by the nature and relative positions of the interacting units. Compounds under investigation include oligothienylenes, hexabenzocoronene, oligoacenes, and perylene. It is shown that the amplitude of the transfer integrals is extremely sensitive to the molecular packing. Interestingly, in contrast to conventional wisdom, specific arrangements can lead to electron mobilities that are larger than hole mobilities, which is, for instance, the case of perylene. O rganic -conjugated materials offer remarkable potential as active elements in (opto)electronic devices that exploit their semiconducting properties, such as field-effect transistors (FETs; refs. 1-8), light-emitting diodes (LEDs; refs. 9-13), or photovoltaic and solar cells (14-17). Such devices are expected to be ultimately incorporated, for instance, into all-plastic integrated circuits for low-end and cheap electronics (7,8,18) and all-plastic light-emitting displays, where each pixel consists of an organic LED driven by an organic FET (19,20). In all of these applications, the efficiency of charge transport within the organic layer(s) plays a key role. In light-emitting diodes, it is desirable that the injected holes and electrons have large and similar mobilities to prevent electroluminescence quenching that can occur when charges recombine close to a metallic interface (21); high-charge mobilities favor recombination processes in the bulk (where charges can be confined further by means of organic-organic interfaces; ref. 22). In solar cells, the charges created upon photoexcitation of the active material have to be transported efficiently to be collected at the metallic contacts and stored under the form of electrical energy. Another challenge is to develop materials displaying high electron and hole mobilities in field-effect architectures to design complex organic circuits.The charge-transport properties critically depend on the degree of ordering of the chains in the solid state as well as on the density of chemical and͞or structural defects (23)(24)(25). This dependence explains why, over the last decades, the experimental characterization of the transport properties in organic thin films or crystals has led to results that vary with sample quality. Recently, the synthesis of ultra-pure single crystals of organic semiconductors such as oligoacenes has allowed Batlogg and coworkers (28) to demonstrate remarkable features that in many instances had been thought to be restricte...