Evolution of Hot Polaron States with a Nanosecond Lifetime in a Manganite Perovskite

Abstract

Understanding and controlling the relaxation process of optically excited charge carriers in solids with strong correlations is of great interest in the quest for new strategies to exploit solar energy. Usually, optically excited electrons in a solid thermalize rapidly on a femtosecond to picosecond timescale due to interactions with other electrons and phonons. New mechanisms to slow down thermalization will thus be of great significance for efficient light energy conversion, e.g., in photovoltaic devices. Ultrafast optical pump–probe experiments in the manganite Pr0.65Ca0.35MnO3, a photovoltaic, thermoelectric, and electrocatalytic material with strong polaronic correlations, reveal an ultraslow recombination dynamics on a nanosecond‐time scale. The nature of long living excitations is further elucidated by photovoltaic measurements, showing the presence of photodiffusion of excited electron–hole polaron pairs. Theoretical considerations suggest that the excited charge carriers are trapped in a hot polaron state. Escape from this state is possible via a slow dipole‐forbidden recombination process or via rare thermal fluctuations toward a conical intersection followed by a radiation‐less decay. The strong correlation between the excited polaron and the octahedral dynamics of its environment appears to be substantial for stabilizing the hot polaron.

Monitoring ultrafast metallization in LaCoO3 with femtosecond soft x-ray spectroscopy

“…Indeed, the same time constants τ 1 = 0.17 ps, τ 3 = 0.91 ps, τ 4 = 1.5 ps, and τ 5 = 45 ps with the same intensities are able to fit all delay scans. The first drop at short time delays [τ 1 = 0.17 ps] can be related to bleaching resulting from ultrafast carrier excitation 35 . Afterwards, a second broad minimum with almost the same intensity reduction is observed around the time delay τ 3 = 0.91 ps.…”

Contribution of Jahn-Teller and charge transfer excitations to the photovoltaic effect of manganite/titanite heterojunctions

“…Transitions between Mn3d e g states are commonly dipole forbidden. Taking into account the hybridiziation of O2p states in a molecular orbital picture, the minimum excitation energies can be attributed to the dipole-allowed transition from non-bonding Mn3d e g to antibonding σ type O2p-e g states that set in at the absorption maximum of the polaron transition ferromagnetically coupled JT polarons at 1 .6 eV [35] (see also figure S1 supplement section A). In CMO, the JT splitting is absent and we therefore expect that the only photovoltaic relevant excitation in CMO is the CT transition between O2p states and e g states, i.e.…”

Development of Ultrafast X-ray Free Electron Laser Tools in (Bio)Chemical Research

“…Figure 17.7 presents the structural mechanisms underlying an optimized all-over organic solar cell. Small atomic changes on the light-absorbing chromophore unit lead to a complete switching of its functional dynamics-from light absorbing solar cell devices [53][54][55][56][57][58][59] to a light emitting organic diode [54]. In another example the understanding of the crystallization processes of organic material out of time-resolved X-ray diffraction (TR-XRD) studies has been influenced by the optimization of the recycling process of molten PET bottles to ultra-hard polyethylene [56].…”

“…Starting from this introduction as a comprehensive summary of the basic principles, in the following, various FEL methods developed so far will be summarised, including the concept of filming chemical reactions in real time utilizing ultrafast high-flux X-ray sources [11][12][13][14][15][16], X-ray Diffraction and Crystallography for Condensed State Chemistry Studies-Crystallography with Ultrahigh Temporal and Ultrahigh Spatial Resolution [14, and Applications in organic electronics and energy research [14,[53][54][55][56][57][58][59]; from From Local to Global: Ultrafast Multidimensional Soft X-ray Spectroscopy and Ultrafast X-ray Diffraction Shake Their Hands [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76] and Applications in Bimolecular Reaction Studies and Photocatalysis [77][78][79][80] to Applications in Unimolecular Liquid Phase Reaction Dynamics [81][82][83][84][85][86][87][88]…”