Organic, inorganic and hybrid semiconductors are a crucial ingredients to an increasing number of applications, such as solar cells, light-emitting devices, energy storage in batteries or fuel cells, and for chemical or biological sensing. In addition, the emergence of nanostructured systems has allowed the exploration of new fundamental science, as for example artificial light harvesting and photocurrent generation at interfaces, supramolecular self-assembly and an understanding of electron dynamics. Our group is particularly interested in increasing our fundamental understanding of electronic, lattice and nano-scale properties that govern these systems. Our experiments are largely based on a range of femtosecond spectroscopic methods to follow the dynamics of photoexcitations in a material. These techniques provide non-contact analytical tools to examine how molecular or hybrid materials may be optimized for implementation in applications.
A brief description of some of our recent work is given below; more details may be obtained from our publications. A listing of current research support may be found at the end of this page.
Figure: Clockwise from top left corner: Group photograph 2017; PL upconversion experiment; iCCD/TCSPC spectroscopy system; laser amplifier; optical-pump THz-probe photoconductivity experiment; time-resolved PL system.
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New organic-inorganic metal halide perovskite materials emerged
recently as active materials in photovoltaic cells exceeding power
conversion efficiencies of 25%. Our group has been at the forefront
of developing an understanding of the fundamental processes that
underpin the outstanding performance of these materials. For
example, we unravelled the
ultrafast dynamics governing the charge carrier motion and
recombination in these materials, high-lighting the role of
non-Langevin radiative charge-carrier recombination underpinning the exceptional
photovoltaic performance of these materials. We are currently
investigating a wide range of topics in this area, including the
fundamentals of electron-phonon coupling, charge-carrier mobility
and recombination, light emission, photon reabsorption, atomic microstructure and ion migration. These studies
feed directly into collaborative efforts aimed at addressing remaining
challenges in the creation of commercially available perovskite solar
cells, e.g. stability, band-gap tunability, lead-free perovskites,
trap-free materials, material morphology control and alternative device structures.
For recent reviews on this subject by our group, see: Energy and Environmental Science 13 (2020), p.2024. JPC Letters 10 (2018), p.6853. ACS Energy Letters 2 (2017), p.1539. Annu. Rev. Phys. Chem. 67 (2016), p.65. Acc. Chem. Res. 49 (2016), p. 146. Collaborators: Prof Henry Snaith, Prof Marina Filip and Prof Michael Johnston (Oxford Physics) Prof Feliciano Giustino (University of Texas) Prof Maksym Kovalenko (ETHZ) |
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Natural evolution very early on solved the problem of how to capture
light efficiently and use it to initiate the primary electron
transfer reactions of photosynthesis in a surprisingly efficient
manner. Synthetic molecules that mimic such biological
light-harvesting complexes should allow high-efficiency conversion
of solar light into energy in the form of electrical current, rather
than biological mass. We have recently shown that energy
delocalisation is extremely rapid (300fs – similar to natural
systems) in alkyne-linked Zn-porphyrin octamers that were curved
into a rigid ring by use of an octadentate template co-ordinating
with Zn. Our current work explores fully cyclic alkyne-linked
porphyrin chromophores that directly mimic natural light harvesting
systems. We are particularly interested in factors controlling
electronic delocalization in these systems and in the ultrafast
transfer of energy in nanostructured complexes comprising multiple chromophores.
Collaborators: Prof Harry Anderson (Oxford Chemistry). |
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Fully organic pi-conjugated molecular materials offer another solution
towards cheap and felxible light-harveting and light-emitting devices. They may be utilized in organic
photovoltaics which employ blends of electron and hole-transporting materials at whose
interface charges may dissociate, as a result of suitable energy
level alignment (type-II heterojunction). Charge transfer at such
interfaces occur on femtosecond timescales; as a result, ultrafast
spectroscopy has been an important activity for OPV development. The
Herz Group explores central aspects such as intermediate charge-transfer states at the interface between
electron and hole transporters, energy transfer, electronic delocalization and
lattice relaxation in organic semiconductors and the impact of morphology on charge
generation. A wide range of possible ingredients is explored,
including conjugated polymers and small molecules, and carbon nanotubes
as electron transporting materials.
In addition, the group explores molecules for Luminescent Solar
Concentrators (LSC) which are cheap plastic lightguides containing
fluorophores that absorb light and channel it to the edge where a highly efficient PV cell is placed to convert the light into electrical current. The Herz Group investigates ultrafast polarization switching and energy transfer in
coupled molecular assemblies and dyads for LSCs. Our aims here are also to
improve our current understanding of competing dynamic
energy-transfer processes, e.g. through-bond, distributed dipole
coupling and vibrationally activated mechanisms that permit
polarization switching leading to better-designed systems with low
light losses from the waveguide.
Collaborators: Prof Albert Schenning (Technical University of Eindhoven) Dr Priti Tiwana, Dr Owen Lozman (Merck Chemicals, Southampton) Prof Robin Nicholas (Oxford Physics) |
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Inorganic semiconductor nanowires show tremendous potential as
ultrafast switching devices and ultrafast optoelectronic devices
such as fast photodetectors or modulators for use in very high
bandwidth communications systems. We have investigated the ultrafast
charge carrier dynamics in such nanowires using a variety of
all-optical non-contact techniques, such as photoluminescence
up-conversion and optical-pump terahertz-probe transient
spectroscopy. We demonstrated that the peak conductivity of charge
carriers injected into GaAs nanowires can be as high as 1/3 of the
conductivity in the bulk. However, we find that the large surface
area to volume ratio for these wires leads to an ultrafast
conductivity decay as charges are trapped at surface defect
traps. In further work we demonstrate that such defect states can be
passivated effectively e.g. through overcoating with higher-bandgap
organic semiconductors, or by surface delta-doping from an organic
polymer overcoat. Our current activities are focused on exploring
effective doping in nanowires, which is a challenge in materials
with such high surface-to-volume ratio.
Collaborators: Prof Michael Johnston (Oxford Physics) and Prof Chennupati Jagadish (Australian National University) |
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Organic-inorganic hybrid cells based on dye-sensitised metal-oxide particles have recently entered commercial production. Such dye-sensitized solar cells (DSCs) contain a monolayer of light-absorbing dye (the sensitizer) adsorbed to the surface of a mesoporous electron-transporting metal oxide, usually fabricated from sintered nanoparticles. Light absorption promotes an electron to the dye excited state, with subsequent electron transfer into, and transport through the metal-oxide to the collection point. The dye is also in contact with hole-transporter which regenerates the oxidized dye molecules and completes the circuit. Using ultrafast spectroscopic techniques we are able to probe individually the processes leading from light absorption to photocurrent generation. From such measurements we can assess how performance is affected by certain changes in material system design, e.g. electrode composition and nanostructure, light-soaking effects or inclusion of interlayers and additives.
Collaborators: Prof Henry Snaith and Prof Michael Johnston (Oxford Physics), Prof Lukas Schmidt-Mende (University of Konstanz) |
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Self-assembly has fast become a staple tool used in the control of complex supramolecular systems. When governed by molecular recognition, self-assembly can produce systems from solution with specific architectures and properties that are determined by intermolecular interactions such as hydrogen bonding, hydrophobic, hydrophilic, and pi-pi interactions. We investigate how the arrangement of chromophores in such supramolecular architectures affects the transfer of energy within the system. For example, we recently demonstrated that efficient energy transfer, in excess of that expected from simple Foerster calculations, is feasible even for long DNA-templated assemblies of pi-stacked conjugated chromophores. In another recent study, we have investigated the effects of phase segregation on the excitation transfer in nanoparticles designed for biological sensing applications.
Collaborators: Prof Bert Meijer, Prof Albert Schenning (Technical University of Eindhoven) |
L .M. Herz, M. B. Johnston, N. Noel, Exploring intrinsic quantum confinement in metal halide perovskites, Leverhulme Trust Research Grant RPG-2022-272 (1 Aug 2023 – 1 Apr 2027)
M. B. Johnston, H. Kraus, L. M. Herz, Ultrafast Terahertz Polarimetry Enabled by Semiconductor Nanowire Sensors
, EPSRC grant EP/W018489/1 (1 Sept 2022 – 31 Aug 2026)
L. M. Herz, H. J. Snaith, M. B. Johnston, M.R. Filip, Metal
halide semiconductors: materials discovery beyond ABX3 perovskites
, EPSRC grant EP/V010840/1 (1 March 2021 – 31 Aug 2024)
H.J.S. Snaith, C. Case and L.M. Herz, All-perovskite multi-junction solar cells, EPSRC grant EP/S004947/1/1 (1 Nov 2018 - 31 Oct 2023)
P Stavrinou, N. Stingelin, M. Heeney, S.C. Dunn and L.M. Herz, EPSRC
Centre for Doctoral Training in Plastic Electronic Materials, EPSRC
grant EP/L016702/1 (1 April 2014 - 31 July 2023)
Laura Herz
(laura.herz@physics.ox.ac.uk)
