5 research outputs found
Ultrafast photodynamics of ZnO solar cells sensitized with the organic indoline derivative D149
Thesis (PhD)--Stellenbosch University, 2014.ENGLISH ABSTRACT: The initial charge transfer from dye molecules' excited states to the conduction band of
a semiconductor, after absorption of visible light by the former, is critical to the performance
of Dye sensitized Solar Cells (DSC). In a ZnO-based DSC sensitized by the organic
indoline derivative D149, the dynamics associated with charge transfer are investigated
with femtosecond transient absorption spectroscopy. The time-resolved measurement of
the photo-initiated processes reveal electron transfer rates corresponding to excited state
lifetimes of 100s of fs, consistent with previously measured high absorbed photon to current
conversion efficiencies. The photo-electrode measured as an isolated system shows
decay times of bound electrons in excited states of the dye to be ~150 fs and shows
the subsequent emergence of absorption bands of the oxidized molecules. When the
I-/I-3 redox couple is added to the system, these excited state lifetimes change and are
found to be dependent on the cation in the electrolytic solution. Small cations like Li+
reduce the excited state lifetime to sub-100 fs, whilst larger cations like the organic tetrabutylammonium
result in longer lifetimes of 240 fs. The action of the electrolyte can be
observed by the reduced lifetime of the oxidized dye molecules' absorption bands. The
effect of operating parameters and changes in the production protocol of the DSC on the
primary charge injection are also investigated and reported on.AFRIKAANSE OPSOMMING: Die aanvanklike ladingsoordrag vanuit kleurstofmolekules' opgewekte toestande tot in
die leidingsband van 'n halfgeleier, na absorpsie van sigbare lig deur eersgenoemde, is
van kritiese belang vir die uitset van halfgeleier-gebaseerde sonkragselle wat met kleurstowwe
vir absorpsie verhoging, gebind is. In hierdie werk word hierdie proses en verwante
fotodinamika in die geval van 'n ZnO sonkragsel gekleur met indolien D149 ondersoek
d.m.v femtosekonde-tydopgelosde absorpsiespektroskopie. Hierdie metings onthul
elektron-oordragstempos wat ooreenstem met lewenstye van opgewekte toestande in die
orde van 100 fs. Hierdie is met voorheen-bepaalde hoë foton-tot-stroom omskakelingsdoeltreffendheid ooreenkomstig. Die foto-elektrode, as geïsoleerde sisteem beskou, toon
afvalstye van gebonde elektrone in opgewekte toestande van ~150 fs, en die gevolglike opkoms
van absorpsie deur geoksideerde molekules word waargeneem. As die I-/I-3 redoks
oplossing tot die sisteem bygevoeg word, verander die opgewekte toestande se afvalstye en
toon 'n katioon-afhanklikheid. Klein katioone soos Li+ verkort die afvalstye tot onder 100
fs, terwyl groter katioone soos die organiese tetra-butielammonium langer afvalstye (240
fs) tot gevolg het. Die werking van die elektrolitiese oplossing kan waargeneem word deur
die verkorte lewenstyd van die absorpsiebande wat aan die geoksideerde molekules toegeken
is. Die uitwerking van operasionele parameter asook veranderinge in die produksie
protokol op die primĂȘre ladingsoordrag word ondersoek en verslag daarop word gelewer
Energy and charge transfer dynamics in DNA based light harvesting antennae
DNA can serve as a versatile scaffold for chromophore assemblies. For example, light-harvesting
antennae have been realized by incorporating phenanthrene and pyrene building blocks into DNA
strands. It was shown that by exciting at 320 nm (absorption of phenanthrene), an emission at
450 nm is observed which corresponds to a phenanthrene-pyrene exciplex. The more
phenanthrenes are added into the DNA duplex, the higher is the fluorescence intensity with no
significant change in quantum yield. This shows that phenanthrene acts as a donor and efficiently
transfers the excitation energy to the pyrene. Up to now, the mechanism of this energy transfer
and exciplex formation is not known. Therefore, we first aim at studying the photo-cycle of such
DNA assemblies through transient absorption spectroscopy. Based on the results, we will explore
ways to manipulate the energy transfer by application of intense THz fields. Ground as well as
excited state Stark effect dynamics will be investigated
Tunable Lifetimes of Intramolecular Charge Separated States in Molecular Donor-Acceptor Dyads
We
report ultrafast transient UVâvis absorption and electrochemical
spectroscopies on the photoinduced charge separation dynamics in a
recently synthesized family of metal-free donorâacceptor systems,
where two redox-active molecules are fused into a compact and planar
structure upon annulation of a tetrathiafulvalene and a benzothiadiazole
as electron donor and acceptor, respectively. We found extraordinary
tunability of the lifetime of the photoinduced charge separation by
more than 2 orders of magnitude (from 6 to 900 ps) upon small changes
of the peripheral residues on the acceptor and the polarity of the
environment. Contrary to expectations, the lifetime of the charge
separation state decreases in more polar environments and with more
electronegative acceptors. This study proves that such fused donorâacceptor
systems give rise to a new class of electronically versatile materials
whose physicochemical properties can be tuned via a targeted substitution
or a suitable choice of local electrostatics
The interaction of photoexcited carbon nanodots with metal ions disclosed down to the femtosecond scale
Fluorescent carbon nanodots are a novel family of carbon-based nanoscale materials endowed with an outstanding combination of properties that make them very appealing for applications in nanosensing, photonics, solar energy harvesting and photocatalysis. One of the remarkable properties of carbon dots is their strong sensitivity to the local environment, especially to metal ions in solution. These interactions provide a testing ground for their marked photochemical properties, highlighted by many studies, and frequently driven by charge transfer events. Here we combine several optical techniques, down to femtosecond time resolution, to understand the interplay between carbon nanodots and aqueous metal ions such as CuÂČâș and ZnÂČâș. We find that copper inhibits the fluorescence of carbon dots through static and diffusional quenching mechanisms, and our measurements allow discriminating between the two. Ultrafast optical methods are then used to address the dynamics of copper-dot complexes, wherein static quenching takes place, and unveil the underlying complexity of their photocycle. We propose an initial increase of electronic charge on the surface of the dot, upon photo-excitation, followed by a partial electron transfer to the nearby ion, with 0.2 ps and 1.9 ps time constants, and finally a very fast (âȘ1 ps) non-radiative electronâhole recombination which brings the system back to the ground state. Notably, we find that the electron transfer stage is governed by an ultrafast water rearrangement around photo-excited dots, pointing out the key role of solvent interactions in the photo-physics of these systems
High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge
A new route to efficient generation of THz pulses with high-energy was demonstrated using semiconductor materials pumped at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and associated free-carrier absorption at THz frequencies. For pumping beyond the three-photon absorption edge, the THz generation efficiency for optical rectification of femtosecond laser pulses with tilted intensity front in ZnTe was shown to increase 3.5 times, as compared to pumping below the absorption edge. The four-photon absorption coefficient of ZnTe was estimated to be ÎČâ=(4±1)Ă10â»â” cmâ”/GWÂł. THz pulses with 14 ÎŒJ energy were generated with as high as 0.7% efficiency in ZnTe pumped at 1.7 ”m. It is shown that scaling the THz pulse energy to the mJ level by increasing the pump spot size and pump pulse energy is feasible