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Ultrafast dynamics and energy loss channels at a hybrid organic inorganic interface


Foglia,  Laura
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Foglia, L. (2015). Ultrafast dynamics and energy loss channels at a hybrid organic inorganic interface. PhD Thesis, Technische Universität, Berlin.

Cite as: http://hdl.handle.net/11858/00-001M-0000-0029-4BE4-3
Ultrafast dynamics and energy loss channels at a hybrid organic inorganic interface Hybrid inorganic organic systems (HIOS) promise to lead to a new generation of lightharvesting and emitting devices that combine high carrier mobilities and charge injection (or ejection) efficiency with strong light matter coupling and wide tunability. The efficiency of hybrid devices relies on the occurrence of charge or energy transfer processes at the interface before a significant amount of excess energy is lost in competing processes. The understanding of the relative balance of energy loss mechanisms and their timescales is thus a fundamental aspect in the design of such heterojunctions. This thesis investigates these relaxation mechanisms in the model HIOS formed by the spirobifluorene derivative 2,7-bis(biphenyl-4-yl)-2’,7’-ditertbutyl- 9,9’-spirobifluorene (SP6) and the inorganic semiconductor ZnO with complementary time-resolved optical techniques, time-resolved photoluminescence (tr-PL) and time-resolved excited state transmission (tr-EST), that access the excited state dynamics in the bulk of the system on a femtosecond timescale. Additionally, a novel non-linear optical technique, timeresolved electronic sum-frequency generation (tr-eSFG) spectroscopy, is applied, for the first time, to the study of a solid state system. tr-eSFG is based on a second order optical effect that arises where inversion symmetry is broken. Therefore, it is potentially an interface-specific technique that allows for the spectroscopy of interfacial electronic states in real devices, in which the active interface is buried under layers of matter. This study shows that at high excitation densities, the transient optical properties of ZnO are strongly affected by the photoinduced depletion of in-gap states (IGS) which act as traps for excited electrons in the conduction band (CB), leading to ultrafast decay of the photoluminescence (PL) response. Since the trapping mechanism is a second order process, i.e. it requires the absorption of two photons to occur, lower excitation density reduces the influence of IGS on the charge carrier lifetime and dynamics and, indeed, exciton formation is observed within hundreds of picoseconds. The tr-EST of SP6 shows the formation of two excitonic states of comparable lifetime localized on the two π-systems of the molecule, X6P and X2P , which are populated after the initial vibrational relaxation. Additionally, a triplet state is efficiently populated by intersystem crossing (ISC). Only the X6P excitons decay via radiative recombination and charge separation (CS). The CS efficiency decreases with increasing temperature due to exciton scattering events that reduce the exciton lifetime and thus shorten the diffusion length. The X2P excitons, instead, which are identified as intramolecular charge transfer excitons, decay exclusively via ISC and constitute the main loss channel in the hybrid system. The presented results show that the dominant energy loss channels in both semiconductors and at the hybrid interface are related to the presence of long-lived, strongly-localized excited states, such as defect-related IGS or charge transfer and triplet excitons. These states act as electron or exciton traps and limit the probability of radiative recombination or charge separation at the interface. Remarkably, despite the long lifetime of these trap states, the respective relaxation pathway is determined by ultrafast processes, either already during the photoexcitation or the initial vibrational relaxation phase. These findings suggest that alternative excitation schemes are likely to increase the efficiency of the hybrid system.