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...

The pioneering work of Heeger, MacDiarmid, and Shirakawa, rewarded by the 2000 Nobel Prize in Chemistry, has paved the way for the development of the fields of plastic electronics and photonics. Functional organic molecular materials and conjugated oligomers or polymers now allow the low‐cost fabrication of thin films for insertion into new generations of electronic and optoelectronic devices. The performance of these devices relies on the understanding and optimization of several complementary processes (see sketch). Our goal is to discuss, from a theoretical standpoint, the electronic structure characteristics and interfacial properties that are of importance in all these areas. The concept of interface should be taken here in the microscopic sense, i.e., molecular interactions among two or several chains/molecules (of the same or of a different nature). Specifically, we will address the impact of interchain interactions within an organic layer on the transport and optical properties. These issues will therefore be more directly related to transistor and light‐emitting diode applications; however, in all instances, the aspects related to interfacial charge or energy transfer processes will dictate the ultimate performance of a material in a given device.

Quantum-chemical calculations coupled with a tight binding band model are used to study the charge carrier mobilities in oligoacene crystals. The transfer integrals for all nonzero interactions in four crystalline oligoacenes ͑naphthalene, anthracene, tetracene, and pentacene͒ were calculated, and then used to construct the excess electron and hole band structures of all four oligoacene crystals in the tight binding approximation. From these band structures, thermal-averaged velocityvelocity tensors in the constant-free-time and the constant-free-path approximations for all four materials were calculated at temperatures ranging from 2 to 500 K. The bandwidths for these oligoacenes were found to be of the order of 0.1-0.5 eV. Furthermore, comparison of the thermal-averaged velocity-velocity tensors with the experimental mobility data indicates that the simple band model is applicable for temperatures only up to about 150 K. A small-polaron band model is also considered, but the exponential band narrowing effect is found to be incompatible to experimental power law results.

Articles you may be interested inExcited state calculations on fluorene-based polymer blends: Effect of stacking orientation and solvation Theoretical and experimental studies of the opto-electronic properties of positively charged oligo(phenylene vinylene)s: Effects of chain length and alkoxy substitutionWe report a detailed quantum-chemical characterization of the electronic and optical properties of polyfluorene chains and compare them to those in copolymers containing alternating fluorene and benzothiadiazole or ethylenedioxythiophene units. The introduction of the comonomer can strongly modify the excitonic properties as well as the efficiency of charge-and energy-transfer processes. The choice of the comonomer is thus critical in targeting specific optical properties while maintaining good transport properties.