Prof Laura Herz FRS

Abstract:

Organic-inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (∼7 μm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

Abstract:

A π-conjugated twelve-porphyrin tube is synthesized in 32% yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C-C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm(3). Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm(-1), reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Abstract:

One-dimensional semiconductors such as nanowires and nanotubes are attractive materials for incorporation in photovoltaic devices as they potentially offer short percolation pathways to charge-collecting contacts. We report the observation of free-electron lifetimes in TiO2 nanotubes of the order of tens of picoseconds. These lifetimes are surprisingly short compared to those determined in films of TiO2 nanoparticles. Samples of ordered nanotube arrays with several different tube wall thicknesses were fabricated by anodization and have been investigated by means of optical-pump-terahertz-probe (OPTP) spectroscopy, which allows measurement of transient photoinduced conductivity with picosecond resolution. Our results indicate a two-stage decay of the photoexcited electron population. We attribute the faster component to temporary immobilization of charge in shallow trap states, from which electrons can detrap again by thermal excitation. The slower component most likely reflects irreversible trapping in states deeper below the conduction band edge. Free-electron lifetimes associated with shallow trapping appear to be independent of the tube wall thickness and have very similar values for electrons directly photoexcited in the material and for those injected from an attached photoexcited dye. These results suggest that trap states are not predominantly located at the surface of the tubes. The effective THz charge-carrier mobility in the TiO2 nanotubes is determined (0.1-0.4 cm²/(Vs)) and found to be within the same range as carrier mobilities reported for TiO2 nanoparticles. Implications for the relative performance of these nanostructures in dye-sensitized solar cells are discussed.

Abstract:

Materials showing rapid intramolecular energy transfer and polarization switching are of interest for both their fundamental photophysics and potential for use in real-world applications. Here, we report two donor-acceptor-donor triad dyes based on perylene-bisimide subunits, with the long axis of the donors arranged either parallel or perpendicular to that of the central acceptor. We observe rapid energy transfer (<2 ps) and effective polarization control in both dye molecules in solution. A distributed-dipole Förster model predicts the excitation energy transfer rate for the linearly arranged triad but severely underestimates it for the orthogonal case. We show that the rapid energy transfer arises from a combination of through-bond coupling and through-space transfer between donor and acceptor units. As they allow energy cascading to an excited state with controllable polarization, these triad dyes show high potential for use in luminescent solar concentrator devices.

Abstract:

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.