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First, we provide an overview of the optical and electronic processes that take place in a solid-state organic solar cell, which we define as a cell in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules; this discussion is also meant to set the conceptual framework in which many of the contributions to this Special Issue on Photovoltaics can be viewed. We successively turn our attention to (i) optical absorption and exciton formation, (ii) exciton migration to the donor-acceptor interface, (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor and electrons in the acceptor, (iv) charge-carrier mobility, and (v) charge collection at the electrodes.For each of these processes, we also describe the theoretical challenges that need to be overcome to gain a comprehensive understanding at the molecular level. Finally, we highlight recent theoretical advances, in particular regarding the determination of the energetics and dynamics at organic-organic interfaces, and underline that the right balance needs to be found for the optimization of material parameters that often result in opposite effects on the photovoltaic performance.

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...

A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots, metallic and semiconducting nanoparticles, carbon nanotubes, and single molecules. Recently, transport has been shown to be also influenced by spin--through the Kondo effect--for both nanotubes and single molecules, as well as by vibrational fine structure. Here we describe a single-electron transistor where the electronic levels of a single pi-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.

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.

The rates for up-conversion intersystem crossing (UISC) from the T state to the S state are calculated for a series of organic emitters with an emphasis on thermally activated delayed fluorescence (TADF) materials. Both the spin-orbit coupling and the energy difference between the S and T states (ΔE) are evaluated, at the density functional theory (DFT) and time-dependent DFT levels. The calculated UISC rates and ΔE values are found to be in good agreement with available experimental data. Our results underline that small ΔE values and sizable spin-orbit coupling matrix elements have to be simultaneously realized in order to facilitate UISC and ultimately TADF. Importantly, the spatial separation of the highest occupied and lowest unoccupied molecular orbitals of the emitter, a widely accepted strategy for the design of TADF molecules, does not necessarily lead to a sufficient reduction in ΔE; in fact, either a significant charge-transfer (CT) contribution to the T state or a minimal energy difference between the local-excitation and charge-transfer triplet states is required to achieve a small ΔE. Also, having S and T states of a different nature is found to strongly enhance spin-orbit coupling, which is consistent with the El-Sayed rule for ISC rates. Overall, our results indicate that having either similar energies for the local-excitation and charge-transfer triplet states or the right balance between a substantial CT contribution to T and somewhat different natures of the S and T states, paves the way toward UISC enhancement and thus TADF efficiency improvement.

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