The performance of semiconductor devices is fundamentally governed by chargecarrier dynamics within the active materials 1-6. While advances have been made towards understanding these dynamics under steady-state conditions, the importance of nonequilibrium phenomena and their effect on device performances remains elusive 7,8. In fact, the ballistic propagation of carriers is generally considered to not contribute to the mechanism of photovoltaics (PVs) and light emitting diodes (LEDs), as scattering rapidly disrupts such processes after carrier generation via photon absorption or electric injection 9. Here, we characterise the spatiotemporal dynamics of carriers immediately following photon absorption in organic-inorganic metal-halide perovskite films, using femtosecond transient absorption microscopy (fs-TAM) with 10 fs temporal resolution and 10 nm spatial localisation precision. We find that non-equilibrium carriers propagate ballistically over 150 nm within 20 fs after photon absorption. Our results suggest that in a typical perovskite PV device operating under standard conditions, a large fraction of

words)CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields and tuneable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperatureresolved photoluminescence (PL) and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that chargetransfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct timescales:initially from an ultrafast (~70 fs) electron transfer and then on longer timescales (~5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ~120 cm -1 which localizes them to the interface. Using low-temperature photoluminescence (PL) spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K the total PL quantum yield reaches close to unity, with an ~85 % contribution from CT emission and the remainder from an emissive subbandgap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ~50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximise the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g. by altering the relative core/crown size, are required.

We present a novel optical transient absorption and reflection microscope based on a diffraction-limited pump pulse in combination with a wide-field probe pulse, for the spatiotemporal investigation of ultrafast population transport in thin films. The microscope achieves a temporal resolution down to 12 fs and simultaneously provides sub-10 nm spatial accuracy. We demonstrate the capabilities of the microscope by revealing an ultrafast excited-state exciton population transport of up to 32 nm in a thin film of pentacene and by tracking the carrier motion in p-doped silicon. The use of few-cycle optical excitation pulses enables impulsive stimulated Raman microspectroscopy, which is used for in situ verification of the chemical identity in the 100–2000 cm–1 spectral window. Our methodology bridges the gap between optical microscopy and spectroscopy, allowing for the study of ultrafast transport properties down to the nanometer length scale.

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 106 m s−1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons

In quantum mechanics, entanglement is a powerful concept with applications in computing, cryptography, and chemical reactions to boost the efficiency of photovoltaic cells. An example of this in organic semiconductors is the coupling of localized triplet excitons into an overall spin-0, -1, or -2 configuration, termed the triplet-pair state. Here, we develop and apply methods to extract triplet pairs from the lowest excited singlet state in long conjugated molecules (polyenes) and understand the mechanism of this process for the aforementioned applications.

Ultrafast vibrational spectroscopy is employed to obtain real-time structural information on energy transport in double-walled light-harvesting nanotubes at room temperature, stabilized in a host matrix to mimic the rigid scaffolds of natural light-harvesting systems. We observe evidence of a low-frequency vibrational mode at 315 cm, which transfers excitons from the outer wall of the nanotubes to a crossing point through which energy transfer to the inner wall can occur. This mode is furthermore absent in solution phase. Importantly, the coherence of this mode is not transferred to the inner wall upon energy transfer and is only present on the outer wall's excited-state energy surface, highlighting that complete energy transfer between the outer and inner walls does not take place. Isolation of the individual walls of the nanotubes provides evidence that this mode corresponds to a supramolecular motion of the nanotubes. Our results emphasize the importance of the solid-state environment in modulating vibronic coupling and directing energy transfer in molecular light-harvesting systems.

We present a statistical analysis of femtosecond transient absorption microscopy applied to four different organic semiconductor thin films based on perylene-diimide (PDI). By achieving a temporal resolution of 12 fs with simultaneous sub-10 nm spatial precision, we directly probe the underlying exciton transport characteristics within 3 ps after photoexcitation free of model assumptions. Our study reveals sub-picosecond coherent exciton transport (12–45 cm2 s–1) followed by a diffusive phase of exciton transport (3–17 cm2 s–1). A comparison between the different films suggests that the exciton transport in the studied materials is intricately linked to their nanoscale morphology, with PDI films that form large crystalline domains exhibiting the largest diffusion coefficients and transport lengths. Our study demonstrates the advantages of directly studying ultrafast transport properties at the nanometer length scale and highlights the need to examine nanoscale morphology when investigating exciton transport in organic as well as inorganic semiconductors.

Heterostructured two-dimensional (2D) colloidal nanoplatelets are a class of material that have attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here using polarization-controlled, steadystate and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modelling and zero-field quantum beat measurements we find the brightexciton fine structure consists of two linearly polarized states with a fine structure splitting  50 μeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B -3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.