Publications


Multi-band magnetotransport in exfoliated thin films of Cu x Bi2Se3.

Journal of physics. Condensed matter : an Institute of Physics journal 30 (2018) 155302-

JA Alexander-Webber, J Huang, J Beilsten-Edmands, P Čermák, Č Drašar, RJ Nicholas, AI Coldea

We report magnetotransport studies in thin (<100 nm) exfoliated films of Cu x Bi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with [Formula: see text] K and a possible electronic phase transition around 200 K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T  <  30 K. This transition between competing conducting channels may be enhanced due to the presence of electronic ordering, and could be affected by the presence of an effective internal stress due to Cu intercalation. By fitting to the weak antilocalisation conductivity correction at low magnetic fields we confirm that the low temperature regime maintains a quantum phase coherence length [Formula: see text] nm indicating the presence of topologically protected surface states.


High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires.

Nano letters (2018)

JL Boland, F Amaduzzi, S Sterzl, H Potts, LM Herz, A Fontcuberta I Morral, MB Johnston

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V-1 s-1 at 10 K.


Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3

Journal of Physics: Condensed Matter, 2018 Institute of Physics Publishing (2018)

JA Alexander-Webber, J Huang, J Beilsten-Edmands, P Cermak, C Drasar, RJ Nicholas, AI Coldea


Impact of Bi3+ Heterovalent Doping in Organic-Inorganic Metal Halide Perovskite Crystals.

Journal of the American Chemical Society 140 (2018) 574-577

PK Nayak, M Sendner, B Wenger, Z Wang, K Sharma, AJ Ramadan, R Lovrinčić, A Pucci, PK Madhu, HJ Snaith

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.


Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process.

Nature communications 9 (2018) 293-293

CL Davies, MR Filip, JB Patel, TW Crothers, C Verdi, AD Wright, RL Milot, F Giustino, MB Johnston, LM Herz

Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley-Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck-Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.


Present status and future prospects of perovskite photovoltaics.

Nature materials 17 (2018) 372-376

HJ Snaith


Temperature induced crossing in the optical bandgap of mono and bilayer MoS2 on SiO2.

Scientific reports 8 (2018) 5380-5380

Y Park, CCS Chan, RA Taylor, Y Kim, N Kim, Y Jo, SW Lee, W Yang, H Im, G Lee

Photoluminescence measurements in mono- and bilayer-MoS2 on SiO2 were undertaken to determine the thermal effect of the MoS2/SiO2 interface on the optical bandgap. The energy and intensity of the photoluminescence from monolayer MoS2 were lower and weaker than those from bilayer MoS2 at low temperatures, whilst the opposite was true at high temperatures above 200 K. Density functional theory calculations suggest that the observed optical bandgap crossover is caused by a weaker substrate coupling to the bilayer than to the monolayer.


Hybrid Perovskites: Prospects for Concentrator Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700792-

Q Lin, Z Wang, HJ Snaith, MB Johnston, LM Herz

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley-Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.


Femtosecond Dynamics of Photoexcited C60 Films.

The journal of physical chemistry letters 9 (2018) 1885-1892

M Causa', I Ramirez, JF Martinez Hardigree, M Riede, N Banerji

The well known organic semiconductor C60 is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C60) charge-transfer (CT) excitons in neat and diluted C60 films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.


Photocurrent Spectroscopy of Perovskite Solar Cells Over a Wide Temperature Range from 15 to 350 K.

The journal of physical chemistry letters 9 (2018) 263-268

JB Patel, Q Lin, O Zadvorna, CL Davies, LM Herz, MB Johnston

Solar cells based on metal halide perovskite thin films show great promise for energy generation in a range of environments from terrestrial installations to space applications. Here we assess the device characteristics of the prototypical perovskite solar cells based on methylammonium lead triiodide (CH3NH3PbI3) over a broad temperature range from 15 to 350 K (-258 to 77 °C). For these devices, we observe a peak in the short-circuit current density and open-circuit voltage at 200 K (-73 °C) with decent operation maintained up to 350 K. We identify the clear signature of crystalline PbI2 contributing directly to the low-temperature photocurrent spectra, showing that PbI2 plays an active role (beyond passivation) in CH3NH3PbI3 solar cells. Finally we observe a blue-shift in the photocurrent spectrum with respect to the absorption spectrum at low temperature (15 K), allowing us to extract a lower limit on the exciton binding energy of 9.1 meV for CH3NH3PbI3.


Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball.

Journal of the American Chemical Society 140 (2018) 5352-5355

J Cremers, R Haver, M Rickhaus, JQ Gong, L Favereau, MD Peeks, TDW Claridge, LM Herz, HL Anderson

We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings containing six and 10 porphyrin units. Fluorescence upconversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole three-dimensional π system in less than 0.3 ps if the nanoball is bound to its templates or over 2 ps if the nanoball is empty.


Highly Crystalline Methylammonium Lead Tribromide Perovskite Films for Efficient Photovoltaic Devices

ACS Energy Letters (2018)

NK Noel, B Wenger, SN Habisreutinger, JB Patel, T Crothers, Z Wang, RJ Nicholas, MB Johnston, LM Herz, HJ Snaith

© 2018 American Chemical Society. The rise of metal-halide perovskite solar cells has captivated the research community, promising to disrupt the current energy landscape. While a sizable percentage of the research done on this class of materials has been focused on the neat, and iodide-rich perovskites, bromide-based perovskites can deliver substantially higher voltages due to their relatively wide bandgaps of over 2 eV. The potential for efficient, high-voltage devices makes materials such as these incredibly attractive for multi-junction PV applications. Here, we use the acetonitrile/methylamine solvent system to deposit smooth, highly crystalline films of CH 3 NH 3 PbBr 3 . By using choline chloride as a passivating agent for these films, we achieve photoluminescence quantum efficiencies of up to 5.5%, and demonstrate charge-carrier mobilities of 17.8 cm 2 /Vs. Incorporating these films into photovoltaic devices, we achieve scanned power conversion efficiencies of up to 8.9%, with stabilized efficiencies of 7.6%, providing a simple route to realizing efficient, high-voltage CH3NH3PbBr3 planar-heterojunction devices.


Exciton Diffusion Length and Charge Extraction Yield in Organic Bilayer Solar Cells.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

B Siegmund, MT Sajjad, J Widmer, D Ray, C Koerner, M Riede, K Leo, IDW Samuel, K Vandewal

A method for resolving the diffusion length of excitons and the extraction yield of charge carriers is presented based on the performance of organic bilayer solar cells and careful modeling. The technique uses a simultaneous variation of the absorber thickness and the excitation wavelength. Rigorously differing solar cell structures as well as independent photoluminescence quenching measurements give consistent results.


OPTOELECTRONICS Fast silicon photodiodes

NATURE PHOTONICS 11 (2017) 268-269

MB Johnston


Charge-Carrier Mobilities in Metal Halide Perovskites: Fundamental Mechanisms and Limits

ACS ENERGY LETTERS 2 (2017) 1539-1548

LM Herz


Investigating the Role of 4-Tert Butylpyridine in Perovskite Solar Cells

ADVANCED ENERGY MATERIALS 7 (2017) ARTN 1601079

SN Habisreutinger, NK Noel, HJ Snaith, RJ Nicholas


Reproducible Planar Heterojunction Solar Cells Based on One-Step Solution-Processed Methylammonium Lead Halide Perovskites

CHEMISTRY OF MATERIALS 29 (2017) 462-473

S Bai, N Sakai, W Zhang, Z Wang, JT-W Wang, F Gao, HJ Snaith


V-Shaped Hole-Transporting TPD Dimers Containing Troger's Base Core

JOURNAL OF PHYSICAL CHEMISTRY C 121 (2017) 10267-10274

T Braukyla, N Sakai, M Daskeviciene, V Jankauskas, E Kamarauskas, R Komskis, T Malinauskas, S Jursenas, HJ Snaith, V Getautis

V-shaped hole transporting materials based on N,N,N′,N′-tetraarylbenzidine (TPD)-type moieties conjoined by Tröger’s base core were synthesized and investigated. These hole transporting materials were obtained by a three-step synthetic method, are fully amorphous, and demonstrate high glass transition temperatures and good thermal and morphological stability. Relatively high charge mobility (up to 0.036 cm2 V –1 s–1) was measured in these hole transporting materials, exceeding that of corresponding methyl and methoxy substituted TPD analogues without TB core by more than 2 orders of magnitude. Determined ionization potential and charge mobility values permit use of the synthesized compounds as hole transporting materials in fabrication of perovskite solar cells.


Two-Dimensional Excitonic Photoluminescence in Graphene on a Cu Surface.

ACS nano 11 (2017) 3207-3212

Y Park, Y Kim, CW Myung, RA Taylor, CCS Chan, BPL Reid, TJ Puchtler, RJ Nicholas, LT Singh, G Lee, C-C Hwang, C-Y Park, KS Kim

Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on a Cu(111) surface, which shows an unexpected and remarkably sharp strong emission near 3.16 eV (full width at half-maximum ≤3 meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue shift, displaying the characteristic negative thermal coefficient of graphene. The observed PL originates from the significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene π and Cu d orbitals of the first and second Cu layers at a shifted saddle point 0.525(M+K) of the Brillouin zone. This finding provides a pathway to engineering optoelectronic graphene devices, while maintaining the outstanding electrical properties of graphene.


An Ultrafast Switchable Terahertz Polarization Modulator Based on III-V Semiconductor Nanowires.

Nano letters 17 (2017) 2603-2610

SA Baig, JL Boland, DA Damry, HH Tan, C Jagadish, HJ Joyce, MB Johnston

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.

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