3 research outputs found
Molecule-like CdSe Nanoclusters Passivated with Strongly Interacting Ligands: Energy Level Alignment and Photoinduced Ultrafast Charge Transfer Processes
Semiconductor nanoclusters (SCNCs)
are promising electronic materials
for use in solid-state device fabrication, where device efficiency
is strongly controlled by charge generation and transfer from SCNCs
to their surroundings. In this paper we report the excited-state dynamics
of molecule-like 1.6 nm diameter CdSe SCNCs, which are passivated
with the highly conjugated ligand phenyldithiocarbamate. Femtosecond
transient absorption studies reveal subpicosecond hole transfer (Ï
â 0.9 ps) from a SCNC to its ligand shell based on strong electronic
interaction and hole delocalization, and subpicosecond hot electron
transfer (Ï â 0.2 ps) to interfacial states created by
charge separation. A series of control experiments were performed
by varying SCNC size (1.6 nm vs 2.9 nm) and photon energy of the pump
laser (388 nm vs 490 nm) as well as addition of electron quencher
(benzoquinone) and hole quencher (pyridine), which rules out alternative
mechanisms and confirms the critical role of energy level alignment
between the SCNC and its passivating ligands. Understanding such charge
carrier transfer dynamics across the SCNCâorganic molecule
interface is very important to various physical phenomena such as
hot carrier relaxation and multiple exciton generation, which together
could aid in the design of high-efficiency solar cells and photocatalysts
Photocatalysts Based on Cobalt-Chelating Conjugated Polymers for Hydrogen Evolution from Water
Developing
photocatalytic systems for water splitting to generate
oxygen and hydrogen is one of the biggest chemical challenges in solar
energy utilization. In this work, we report the first example of heterogeneous
photocatalysts for hydrogen evolution based on in-chain cobalt-chelating
conjugated polymers. Two conjugated polymers chelated with earth-abundant
cobalt ions were synthesized and found to evolve hydrogen photocatalytically
from water. These polymers are designed to combine functions of the
conjugated backbone as a light-harvesting antenna and electron-transfer
conduit with the in-chain bipyridyl-chelated transition metal centers
as catalytic active sites. In addition, these polymers are soluble
in organic solvents, enabling effective interactions with the substrates
as well as detailed characterization. We also found a polymer-dependent
optimal cobalt chelating concentration at which the highest photocatalytic
hydrogen production (PHP) activity can be achieved
Molecular Structure Controlled Transitions between Free-Charge Generation and Trap Formation in a Conjugated Copolymer Series
Rational
design strategies for controlling the energetics of conjugated
âdonorâacceptorâ copolymers are ubiquitous in
the literature, as they allow for simple energy-level tuning strategies
to be employed for photovoltaic and transistor applications. Utilizing
the recently reported <b>PTR<i>n</i></b> series of
conjugated polymers closely related to the widely implemented material <b>PTB7</b>, we investigate the effect of local copolymer block energetics
on the generation of transient excitonic and charge carrier species.
It is clearly demonstrated that local copolymer block energetics play
a much larger role than is apparent from simple energy-level tuning
arguments, and drastically affect the ultrafast generation of free-charge
carrier and trap state populations. Specifically, we observe an almost
complete reversal in the efficient generation of free-charge in <b>PTB7</b> to the ultrafast creation of a high percentage of trapped
pseudo charge-transfer states. The implications of this secondary
effect of âdonorâacceptorâ energy level tuning
are discussed, along with strategies for avoiding the generation of
trap states in âdonorâacceptorâ copolymers