Ultrafast processes in organic semiconductors: From spectral and spatial dynamics in model systems towards heterostructures
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Abstract
Compared to conventional semiconductors, organic semiconductors host a number of
unique properties that give rise to a variety of interesting optical and electronic phenomena.
These include singlet fission and excimer formation, both of which are processes
highly desirable in certain technological applications in the realms of solar cells
and illuminants. Although extensively studied in various systems, the specific effects of
temperature and material quality on the above-mentioned kinetic processes are still unclear
in many cases. In order to properly exploit the aforementioned qualities of organic
semiconductors, detailed investigations of the underlying mechanisms are required. This
dissertation aims to address these aspects, using time- and polarization-resolved experimental
studies of model systems, complemented by theoretical calculations and kinetic
models.
The soft lattice in molecular crystals results in strong electron-phonon coupling, which
significantly affects the energetic relaxation dynamics. However, how these processes
take place microscopically is still unknown in many cases. In particular, part of this
thesis deals with the investigation of the phonon-driven relaxation pathways between
different excitonic states originating from the Davydov splitting in oligoacene single
crystals. Calculations are performed on pentacene and tetracene to compare their
relaxation dynamics. Complementary time- as well as polarization-resolved photoluminescence
(PL) and absorption measurements are conducted on a thin tetracene single
crystal at different temperatures to compare the Davydov peak characteristics with the
calculations. It is found that optically dark states play a vital role as intermediates in
the transition from the higher to the lower Davydov state. Additionally, in pentacene
an accumulation of excitons in a dark state is observed, leading to a phonon-bottleneck.
However, this seems to be absent in tetracene, most likely due to the lower energetic
separation between the two states compared to pentacene, making energetic relaxation
via electron-phonon scattering more efficient. The obtained model could thus be a
valuable tool for predicting the occurrence of phonon-bottlenecks in other oligoacene
systems. Likewise, the time- and polarization-resolved PL and absorption measurements
used provide a way to experimentally confirm the presence of this phenomenon
in a given system.
Tetracene is a material in which endothermic singlet fission is known to occur, along
with the reverse triplet-triplet annihilation process. The resulting interconversion between
singlet and triplet excitons has been demonstrated to have a significant impact on the total diffusion lengths, which could be beneficial for the design of organic solar
cells. However, the exact effect of temperature on the fission process is still unknown, as
is the role of crystal quality, which has been shown to significantly affect spectral signatures.
Part of this work focuses on investigating these factors, by performing spectrally-,
spatially- and time-resolved photoluminescence measurements on two tetracene single
crystals of different quality at varying temperatures. In order to elucidate the underlying
temporal population changes and to extract the kinetic parameters governing the
exciton diffusion and transition processes, a kinetic model is fit to the experimental
data. Crystal quality is shown to strongly affect spectral as well as spatial dynamics at
decreased temperatures, spectrally shifting the emission signature and modifying the
total singlet diffusion length. The model reveals that the temperature-activated step
for the process of singlet fission is the dissociation of the correlated triplet-pair. It also
suggests that a single radiative emission and fission constant is insufficient to explain
the observed dynamics in the pristine sample. Instead, the model must rely on multiple
species with different kinetic parameters to accurately reproduce the experimental
data. The study reveals crystal quality and temperature to act as possible turning
knobs for optimizing singlet fission in tetracene. Moreover, the derived kinetic model is
suitable for obtaining robust sets of parameters by simultaneously reproducing several
experimental datasets.
Even though organic semiconductors exhibit the unique qualities mentioned previously,
they also have some significant drawbacks compared to their inorganic counterparts,
one of which being their exciton transport properties. One way to remedy this issue is
to integrate organic semiconductors in a heterostructure with materials that complement
their weak points, such as monolayer transition metal dichalcogenides (TMDs).
Although theoretically a fascinating proposition, to date not much experimental research
has been performed on such systems. One reason for this is a lack of available
heterostructures with controlled orientation of the molecular lattice. The final part of
this thesis deals with the construction of a heterostructure consisting of an ionic organic
crystal, pyrenemethylammonium chloride, and a monolayer of tungsten disulphide via
physical stacking. The structure is studied using time- as well as polarization-resolved
and steady-state photoluminescence measurements at different temperatures to probe
for evidence of successful electronic coupling between the compounds. A significant
quenching of the PL of the organic crystal is observed in the heterostructure. Interestingly,
this trend is observed both for the mobile excitonic as well as the localized
excimer species. This, together with the emergence of a red-shifted, linearly polarized
emission, seemingly arising from an interfacial state, suggests the presence of efficient
electronic coupling. The study thus provides a method for the controlled fabrication of
organic/TMD heterostructures.
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Dates
Issued: 2025-09-26
Relations
Is based on: (DOI) 10.1002/ntls.20220040Is based on: (DOI) 10.1039/d4nr01086hIs based on: (DOI) 10.1039/d4lf00351a
Faculty
FB13:Physik
Language
en
Keywords
Exciton diffusionorganic semiconductorsHeterostructuresTetracenePyreneWS2
DFG-subjects
307-01 Experimentelle Physik der kondensierten Materie
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Muth, Dominik: Ultrafast processes in organic semiconductors: From spectral and spatial dynamics in model systems towards heterostructures. : 2025-09-26.
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This item has been published with the following license: In Copyright