David Beljonne

The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors

Bands That Separate In organic photovoltaic devices, the charge carriers that form at the interface between donor and acceptor layers—the electrons and holes—form bound states called excitons. Efficient current generation requires some mechanism for their separation and for the movement of free carriers to the electrodes. Bakulin et al. (p. 1340, published online 23 February) studied a process in which the excitons created with an optical pulse were also subjected to infrared pulses. In polymer-blend devices, a three-step process was observed: The boundstate excitons diffused toward the donor-acceptor interface, formed a charge-transfer state, and then dissociated into free carriers. Bound excited charge carriers achieve long-range separation by promotion to delocalized band states. The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (<1 picosecond) via infrared (IR) optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modeling showed that the IR photons promote bound charge pairs to delocalized band states, similar to those formed just after singlet exciton dissociation, which indicates that such states act as the gateway for charge separation. Our results suggest that charge separation in efficient organic photoconversion systems occurs through hot-state charge delocalization rather than energy-gradient–driven intermolecular hopping.

Interchain Interactions in Organic π‐Conjugated Materials: Impact on Electronic Structure, Optical Response, and Charge Transport

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.

Beyond Förster Resonance Energy Transfer in Biological and Nanoscale Systems

After photoexcitation, energy absorbed by a molecule can be transferred efficiently over a distance of up to several tens of angstroms to another molecule by the process of resonance energy transfer, RET (also commonly known as electronic energy transfer, EET). Examples of where RET is observed include natural and artificial antennae for the capture and energy conversion of light, amplification of fluorescence-based sensors, optimization of organic light-emitting diodes, and the measurement of structure in biological systems (FRET). Forster theory has proven to be very successful at estimating the rate of RET in many donor-acceptor systems, but it has also been of interest to discover when this theory does not work. By identifying these cases, researchers have been able to obtain, sometimes surprising, insights into excited-state dynamics in complex systems. In this article, we consider various ways that electronic energy transfer is promoted by mechanisms beyond those explicitly considered in Forster RET theory. First, we recount the important situations when the electronic coupling is not accurately calculated by the dipole-dipole approximation. Second, we examine the related problem of how to describe solvent screening when the dipole approximation fails. Third, there are situations where we need to be careful about the separability of electronic coupling and spectral overlap factors. For example, when the donors and/or acceptors are molecular aggregates rather than individual molecules, then RET occurs between molecular exciton states and we must invoke generalized Forster theory (GFT). In even more complicated cases, involving the intermediate regime of electronic energy transfer, we should consider carefully nonequilibrium processes and coherences and how bath modes can be shared. Lastly, we discuss how information is obscured by various forms of energetic disorder in ensemble measurements and we outline how single molecule experiments continue to be important in these instances.

Role of Dimensionality on the Two‐Photon Absorption Response of Conjugated Molecules: The Case of Octupolar Compounds

A comparative study of the two-photon absorption (TPA) properties of octupolar compounds and their dipolar one-dimensional counterparts is presented on the basis of correlated quantum-chemical calculations. The roles of dimensionality and symmetry are first discussed on the basis of a simple exciton picture where the ground-state and excited-state wavefunctions of three-arm octupolar systems are built from a linear combination of the corresponding single-arm wavefunctions. This model predicts a factor of 3 increase in the TPA cross section in the limiting case of three independent charge-transfer pathways. When taking into account the full chemical structures of representative octupolar molecules, the results of the calculations indicate that a much larger enhancement associated with an increase in dimensionality and delocalization can be achieved when the core of the chromophore allows significant electronic coupling among the individual arms. These theoretical predictions are in agreement with the experimental determination of the TPA cross sections for crystal violet and the related compound, brilliant green, and suggest new strategies for the design of conjugated materials with large TPA cross sections.

Interchain vs. intrachain energy transfer in acceptor-capped conjugated polymers.

The energy-transfer processes taking place in conjugated polymers are investigated by means of ultrafast spectroscopy and correlated quantum-chemical calculations applied to polyindenofluorenes end-capped with a perylene derivative. Comparison between the time-integrated luminescence and transient absorption spectra measured in solution and in films allows disentangling of the contributions arising from intrachain and from interchain energy-migration phenomena. Intrachain processes dominate in solution where photoexcitation of the polyindenofluorene units induces a rather slow energy transfer to the perylene end moieties. In films, close contacts between chains favors interchain transport of the excited singlet species (from the conjugated bridge of one chain to the perylene unit of a neighboring one); this process is characterized by a 1-order-of-magnitude increase in transfer rate with respect to solution. This description is supported fully by the results of quantum-chemical calculations that go beyond the usual point-dipole model approximation and account for geometric relaxation phenomena in the excited state before energy migration. The calculations indicate a two-step mechanism for intrachain energy transfer with hopping along the conjugated chains as the rate-limiting step; the higher efficiency of the interchain transfer process is mainly due to larger electronic coupling matrix elements between closely lying chains.

Mechanisms for enhancement of two-photon absorption in donor–acceptor conjugated chromophores

Abstract Two-photon absorption (TPA) is a nonlinear optical process that is gaining increasing attention since it can be exploited in a number of optical applications. Here, on the basis of correlated quantum-chemical calculations, we investigate the structure–TPA property relationships for donor–acceptor π-conjugated compounds. These relationships provide strategies to design dyes with large TPA cross-sections for fundamental photon wavelengths in the desired 0.6–1.0 μm range of wavelengths.

Charge Separation in Localized and Delocalized Electronic States in Polymeric Semiconductors

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors, light-emitting diodes, photocells, and lasers. It is still not clear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons; in the latter they are delocalized and described as free electron–hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.

Singlet exciton fission in solution.

Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.

The nature of singlet excitons in oligoacene molecular crystals

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.

Green emission from poly(fluorene)s: The role of oxidation

Poly(fluorene)-type materials are widely used in polymer-based light emitting devices. In their pristine state, they emit in the deep blue spectral region. During operation there appears, however, an additional emission peak at around 2.3 eV. This observation has usually been attributed to aggregate or excimer formation. Recently, it has been shown that photo- and/or electro-oxidation of poly(fluorene) chains resulting in ketonic defects (i.e., formation of fluorenone groups) can also be held responsible for emission in that spectral region. In this contribution, we apply quantum-chemical techniques to gain a detailed understanding of the optical properties of poly(fluorene)s containing ketonic defects. In particular, we compare model systems for poly(fluorene) with their ketone-containing counterparts, focusing on the influence of excited-state localization effects. The results of the theoretical calculations are confirmed by experimental investigations on statistical copolymers of fluorene and 9-fluorenone.