Organic light-emitting devices (OLEDs) are a promising technology for flat-panel displays and solid-state lighting. While OLED efficiencies have increased dramatically in recent years, further progress is complicated by the fact that the vast majority of organic materials are fluorescent and therefore emit only from molecular excited states ('excitons') with spin 0, or 'singlet' spin symmetry. Here, we demonstrate the ability to manipulate the fraction of excitons which form as singlets in fluorescent materials by altering the OLED structure. We insert a mixing layer that affects only charge-transfer (CT) states, which are the precursors to excitons. As a result, we triple the singlet fraction and the efficiency of the red fluorophore DCM2. We term fluorescence enhanced by CT spin mixing 'extrafluorescence', and show that its origin is in part an inversion of the usual energetic ordering of the singlet and triplet CT states.

Organic semiconductors (OSCs) have recently received significant attention for their potential use in photovoltaic, light emitting diode, and field effect transistor devices. Part of the appeal of OSCs is the disordered, amorphous nature of these materials, which makes them more flexible and easier to process than their inorganic counterparts. In addition to their technological applications, OSCs provide an attractive laboratory for examining the chemistry of heterogeneous systems. Because OSCs are both electrically and optically active, researchers have access to a wealth of electrical and spectroscopic probes that are sensitive to a variety of localized electronic states in these materials. In this Account, we review the basic concepts in first-principles modeling of the electronic properties of disordered OSCs. There are three theoretical ingredients in the computational recipe. First, Marcus theory of nonadiabatic electron transfer (ET) provides a direct link between energy and kinetics. Second, constrained density functional theory (CDFT) forms the basis for an ab initio model of the diabatic charge states required in ET. Finally, quantum mechanical/molecular mechanical (QM/MM) techniques allow us to incorporate the influence of the heterogeneous environment on the diabatic states. As an illustration, we apply these ideas to the small molecule OSC tris(8- hydroxyquinolinato)aluminum (Alq(3)). In films, Alq(3) can possess a large degree of short-range order, placing it in the middle of the order-disorder spectrum (in this spectrum, pure crystals represent one extreme and totally amorphous structures the opposite extreme). We show that the QM/MM recipe reproduces the transport gap, charge carrier hopping integrals, optical spectra, and reorganization energies of Alq(3) in quantitative agreement with available experiments. However, one cannot specify any of these quantities accurately with a single number. Instead, one must characterize each property by a distribution that reflects the influence of the heterogeneous environment on the electronic states involved. For example, the hopping integral between a given pair of Alq(3) molecules can vary by as much as a factor of 5 on the nanosecond timescale, but the integrals for two different pairs can easily differ by a factor of 100. To accurately predict mesoscopic properties such as carrier mobilities based on these calculations, researchers must account for the dynamic range of the microscopic inputs, rather than just their average values. Thus, we find that many of the computational tools required to characterize these materials are now available. As we continue to improve this computational toolbox, we envision a future scenario in which researchers can use basic information about OSC deposition to simulate device operation on the atomic scale. This type of simulation could allow researchers to obtain data that not only aids in the interpretation of experimental results but also guides the design of more efficient devices.

Because of their unique photophysical properties, organic semiconductors have shown great promise in both light-emitting devices (LEDs) and photovoltaic systems. In particular, the question of spin statistics looms large in these applications: the relative energetics and rates of formation for singlet versus triplet excited states can have a significant impact on device efficiency. In this Article, we study the singlet and triplet charge-transfer (CT) configurations that can be thought of as the immediate precursors to the luminescent states in organic LEDs. In particular, we find that the CT singlet-triplet energy gap (deltaE(ST)) of organic dyes and oligomers depends sensitively on both the material and the relative orientation of the donor/acceptor pair. Furthermore, in contrast with the commonly held view, we find that the singlet CT states nearly always lie energetically below the triplet CT states (deltaE(ST) < 0). This trend is attributed to two physical sources. First, the relatively close contact between the donor and acceptor leads to a strong kinetic exchange component that favors the singlet. Second, Coulombic attraction between the separated charges favors inner-sphere reorganization that brings the donor and acceptor closer together, further enhancing the kinetic exchange effect. We discuss the implications of these results on the design of organic LEDs.

Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for obtaining CT-exciton electronic couplings between CT states and excitons is presented. This method is applied to two organic heterodimers to obtain their CT-exciton coupling and adiabatic energy surfaces near their CT-exciton diabatic surface crossings. The results show the new method provides a new window into the role of CT states in exciton-exciton transitions within organic semiconductors.