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Abstract:
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.
