11:10 - 11:35InvitedID: 117
EXCITON POLARONS IN TWO-DIMENSIONAL LEAD-HALIDE PEROVSKITES
1Georgia Institute of Technology, Atlanta, United States of America; 2Center for Nano Science and Technology@PoliMi, Istituto Italiano di Tecnologia, Milan, Italy
Hybrid organic/inorganic semiconductors feature complex lattice dynamics due to the ‘softness’ arising from non-covalent bonds between molecular moieties and the inorganic network, and due to the ionic character of the crystal. Such complex lattice motion has profound consequences on the fundamental character of primary photoex- citations with respect to purely covalent semiconductor crystals. In this work, we establish that this dynamic structural complexity gives rise to the coexistence of di- verse excitonic resonances in a prototypical two-dimensional lead iodide perovskite, each with a distinct degree of polaronic character. By means of high-resolution res- onant impulsive stimulated Raman spectroscopy, we address the coupling of both charge carriers and excitons to low-frequency optical phonons (those with frequency ≤50 cm−1). Resonant photoexcitation results in vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarri- ers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various phonons involving motion in the lead-iodide layers. We thus conclude that different excitons induce specific lattice reorganizations that are distinct from those involving charge carriers, which are signatures of polaronic bind- ing. Our conclusions provide a novel perspective of the energetic/configurational land- scape involving globally neutral primary photoexcitations in a broad class of emerging hybrid semiconductor materials.
11:35 - 11:50ID: 159
FARADAY EFFECT IN LEAD HALIDE PEROVSKITES
1University of Sydney, Sydney, Australia; 2Monash University, Melbourne, Australia
Interest in hybrid organic-inorganic perovskites has surged after the rapid improvement in perovskite photovoltaics. Perovskites have also found application in other optoelectronic devices, including lasers, light-emitting diodes, and photodetectors. Recently, the magneto-optic properties of perovskites have been explored, such as Rashba splitting, spin lifetime, and spin-dependent optical selection rules. We now investigate the effect of a magnetic field on the optical rotation of perovskite single crystals (SC). By incorporating the crystal into a magnetic field, we are able to measure the Faraday effect for a series of lead halide perovskites. We investigate perovskites with different halogens, cations, and dimensionalities, building a correlation between degree of optical rotation and perovskite structure. This allows intuition toward synthesizing the next-generation of perovskite materials for magneto-optic effects. Strikingly, we observe rotations of up to 20°(T mm)-1 in non-absorbing regions, competing with some commercially available materials currently on the market. We demonstrate further the Faraday effect of hybrid organic-inorganic perovskites by building a SC perovskite optical isolator, where we achieve substantial suppression of back-reflected light.
11:50 - 12:05ID: 193
LONG-RANGE BALLISTIC PROPAGATION OF CARRIERS IN ORGANIC-INORGANIC METAL-HALIDE PEROVSKITE THIN FILMS
1University of Cambridge, Cambridge, United Kingdom; 2Unviersity of Oxford, Oxford, United Kingdom
Organic-inorganic metal-halide perovskites have attracted great attention in recent years due to their remarkable semiconductor properties. Although great advances have been made towards understanding carrier dynamics of these materials, the importance of non-equilibrium phenomena and their effect on device performances remains elusive. In fact, it is generally considered that coherent wavelike motion of carriers, i.e. ballistic transport, does not participate in the operational mechanism of photovoltaics, since scattering rapidly disrupts such processes after carrier generation via photon absorption or injection from electrodes.
Besides, the majority of studies have been carried out under steady-state conditions or quasi steady-state condition via indirect means such as photoluminescence quenching. Accordingly, there has been an increasing demand for direct experimental method to investigate carrier transport dynamics. A few recent studies resolved diffusive motion of carriers on ps timescales thanks to a newly developed time-resolved microscopy technique. However, these were insensitive to the ultrafast spatial dynamics of non-equilibrium carriers due to limited temporal resolution and spatial localization precision (300 fs and 50 nm). 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) capable of simultaneous 10 fs temporal resolution and 10 nm spatial localisation precision
The mean squared displacement profile reveals that non-equilibrium carriers propagate 150 nm within 20 fs after photon absorption, signifying wavelike carrier propagation directly after photogeneration. Our results suggest that approximately ~25% of carriers generated in a typical perovskite PV device, operating under standard conditions, reach the charge collection layers ballistically. The ballistic transport distance appears to be limited by energetic disorder within the materials, likely due to disorder induced localisation of the wavefunctions. This provides a direct route towards optimisation of the ballistic transport distance via improvements in materials and processing to minimise energetic disorder.
12:05 - 12:20ID: 233
SPIN-POLARISED EXCITON STATE SPLITTING AND POPULATION DYNAMICS IN 2D METAL-HALIDE PEROVSKITES
University of Cambridge, Cambridge, United Kingdom
Polarised exciton spin populations are generated in 2D metal-halide perovskites by circularly polarised optical excitation. These self-assembled quantum well structures of high performance, defect tolerant semiconductors are broadly tuneable through chemical composition. Here, we use ultrafast circularly polarised broadband transient absorption spectroscopy to study the effect of perovskite composition on the polarisation and dynamics of exciton spin-states in two-dimensional (2D) Ruddlesden-Popper perovskites. We spectrally resolve the dynamic circular dichroism from a photoinduced polarisation in the secondary total angular momentum quantum number. This dichroism is used to determine the impact of the monovalent cation’s dipole moment on the exciton spin-state depolarisation time at room temperature.