The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline tetracene lattice satisfactorily accounts for the experimental observations. It also reveals the critical roles of the charge transfer states and the high dephasing rates in ensuring the ultrafast formation of multiexciton states. In addition, we address the origins of microscopic relaxation and dephasing rates, and adopt these rates in a quantum master equation description. We show the need to take the theoretical effort one step further in the near future by combining high-level electronic structure calculations with accurate quantum relaxation dynamics for large systems.

Singlet fission, the splitting of a singlet exciton into two triplet excitons in molecular materials, is interesting not only as a model many-electron problem, but also as a process with potential applications in solar energy conversion. Here we discuss limitations of the conventional four-electron and molecular dimer model in describing singlet fission in crystalline organic semiconductors, such as pentacene and tetracene. We emphasize the need to consider electronic delocalization, which is responsible for the decisive role played by the Mott-Wannier exciton, also called the charge transfer (CT) exciton, in mediating singlet fission. At the strong electronic coupling limit, the initial excitation creates a quantum superposition of singlet, CT, and triplet-pair states, and we present experimental evidence for this interpretation. We also discuss the most recent attempts at translating this mechanistic understanding into design principles for CT state-mediated intramolecular singlet fission in oligomers and polymers.

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

The absorption of a photon usually creates a singlet exciton (S) in molecular systems, but in some cases S may split into two triplets (2×T) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair (TT) state. Here, we probe the(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive (TT) state, which decays to 2×T with a time constant of 270 fs. However, the decay of S and the formation of (TT) occur on different timescales of 180 fs and<50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S→(TT) rate process responsible for the 180 fs timescale, S may couple coherently to a vibronically excited (TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

How an electron-hole pair escapes the Coulomb potential at a donor/acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast timescales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor/vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.Charge generation in organic photovoltaic (OPV) devices is contingent upon the dissociation of excitons into charge-separated states across donor/acceptor (D/A) interfaces, a process that can occur on femtosecond timescales with near unity quantum efficiency [1][2][3]. However, such rapid formation of charge-separated states appears contrary to the excitonic nature of the materials that comprise organic solar cells [4]. Given their low dielectric constants, it is not immediately obvious how the electron-hole pair is able to escape the poorly screened Coulomb potential that can give rise to charge transfer excitons (CTEs) with binding energies an order of magnitude higher than thermal energy at room temperature [5][6][7][8][9]. Recent experimental [10][11][12][13][14] and theoretical [15][16][17] studies suggest that electronic delocalization enables the electron-hole pair to escape the CTE trap and promotes long-range charge separation, in agreement with the Onsager model for ionization in solution [18]. Excess energy from the offset in energy levels at the donor/acceptor interface [10][11][12][13][14][15][16][17] or from the initial excitation photon [19,20] is believed to assist long-range charge separation. The electron-hole pair in the CTE trap can also dissociate with the help of an additional photoexcitation step [ 21 ]. Despite this progress, the exact mechanism of long-range electron-hole pair formation at donor/acceptor interfaces remains Phys. Rev. Lett. submitted. 2 poorly defined and there are also seemingly contradicting findings of photo-carrier generation from low energy CTEs that are supposedly trapped [22][23][24]. In the latter case, there is likely a potential energy gradient that counters the Coulomb potential [25][26][27][28]. One universally present driving force for photo-carrier generation, which may be partially responsible for the initial longrange charge separation or the subsequent escape from the CTE trap, may be the entropic gain with increasing electron-hole separation [1,29 ]. However, there has been little experiment evidence for this proposal.Here we provide the first direct time domain view of entropy-driven charge separation using the model system of CTEs at a molecular semiconductor/vacuum interface. In this system, a molecular semiconductor is the donor and the free-electron...