Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CHNHPbBr, CH(NH)PbBr, and CsPbBr We observed hot fluorescence emission from energetic carriers with ~10-picosecond lifetimes in CHNHPbBr or CH(NH)PbBr, but not in CsPbBr The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit.

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.

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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...