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 Cu Bi 2 Se 3

Journal of Physics: Condensed Matter 30 (2018) 155302-155302

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


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.


Cubic or Orthorhombic? Revealing the Crystal Structure of Metastable Black-Phase CsPbI<inf>3</inf>by Theory and Experiment

ACS Energy Letters 3 (2018) 1787-1794

RJ Sutton, MR Filip, AA Haghighirad, N Sakai, B Wenger, F Giustino, HJ Snaith

Copyright © 2018 American Chemical Society. Room-temperature films of black-phase cesium lead iodide (CsPbI3) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room-temperature black-phase CsPbI3in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin films of black-phase CsPbI3, fabricated either by high- or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI3and show that the orthorhombic polymorph is the most stable among its other perovskite polymorphs, but it remains less stable than the yellow nonperovskite polymorph.


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.


Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide.

The journal of physical chemistry letters (2018) 4502-4511

CL Davies, J Borchert, CQ Xia, RL Milot, H Kraus, MB Johnston, LM Herz

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI3 (thin film) and find remarkably different behavior across the β-γ phase transition compared with MAPbI3. While MAPbI3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI3 vary smoothly over a range of 100-160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI3 exhibits a clear T-0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron-phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T-1.5 dependence previously observed for MAPbI3. Finally, we directly observe intraexcitonic transitions in FAPbI3 at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.


Photophysical and Fluorescence Anisotropic Behavior of Polyfluorene β-Conformation Films.

The journal of physical chemistry letters 9 (2018) 364-372

M-N Yu, H Soleimaninejad, J-Y Lin, Z-Y Zuo, B Liu, Y-F Bo, L-B Bai, Y-M Han, TA Smith, M Xu, X-P Wu, DE Dunstan, R-D Xia, L-H Xie, DDC Bradley, W Huang

We demonstrate a systematic visualization of the unique photophysical and fluorescence anisotropic properties of polyfluorene coplanar conformation (β-conformation) using time-resolved scanning confocal fluorescence imaging (FLIM) and fluorescence anisotropy imaging microscopy (FAIM) measurements. We observe inhomogeneous morphologies and fluorescence decay profiles at various micrometer-sized regions within all types of polyfluorene β-conformational spin-coated films. Poly(9,9-dioctylfluorene-2,7-diyl) (PFO) and poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenylfluoren-2,7-diyl] (PODPF) β-domains both have shorter lifetime than those of the glassy conformation for the longer effective conjugated length and rigid chain structures. Besides, β-conformational regions have larger fluorescence anisotropy for the low molecular rotational motion and high chain orientation, while the low anisotropy in glassy conformational regions shows more rotational freedom of the chain and efficient energy migration from amorphous regions to β-conformation as a whole. Finally, ultrastable ASE threshold in the PODPF β-conformational films also confirms its potential application in organic lasers. In this regard, FLIM and FAIM measurements provide an effective platform to explore the fundamental photophysical process of conformational transitions in conjugated polymer.


Pronounced Side Chain Effects in Triple Bond-Conjugated Polymers Containing Naphthalene Diimides for n-Channel Organic Field-Effect Transistors.

ACS applied materials & interfaces 10 (2018) 12921-12929

S Nam, SG Hahm, D Khim, H Kim, T Sajoto, M Ree, SR Marder, TD Anthopoulos, DDC Bradley, Y Kim

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.


Modelling and simulation of photovoltaic module for micro inverter application

Proceedings - International Conference on Trends in Electronics and Informatics, ICEI 2017 2018-January (2018) 82-85

M Patel, H Surati, J Patel

© 2017 IEEE. This paper presents on a program developed in MATLAB/Simulink of photovoltaic module for micro inverter application. This program is based on mathematical equations and is defined through an equivalent circuit including a photocurrent source, and a diode. The developed program allows the prediction of PV module behaviour under different temperature and radiation. Effect of two environmental parameters of temperature and irradiance variations could be observed from simulated characteristics. The boost converter is to be used along with a Maximum Power Point Tracking topology. The MPPT is responsible for extracting the maximum possible power from the photovoltaic and feed it to the load via the boost converter which is used to steps up the voltage to required magnitude.


DISTINGUISHING CAP AND CORE CONTRIBUTIONS TO THE PHOTOCONDUCTIVE TERAHERTZ RESPONSE OF SINGLE GaAs BASED CORE-SHELL-CAP NANOWIRE DETECTORS

LITHUANIAN JOURNAL OF PHYSICS 58 (2018) 15-23

K Peng, P Parkinson, L Fu, Q Gao, J Boland, Y-N Guo, N Jian, HH Tan, MB Johnston, C Jagadish


Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin–Lead Triiodide Perovskites

Advanced Functional Materials 28 (2018)

ES Parrott, T Green, RL Milot, MB Johnston, HJ Snaith, LM Herz

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Mixed lead–tin triiodide perovskites are promising absorber materials for low bandgap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb1−xSnxI3perovskite are presented here as a function of lead:tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the bandgap and phase transition temperature. The large bandgap bowing parameter, a crucial element for the attainment of low bandgaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, monoexponential recombination that is almost temperature-independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multiphonon assisted process. These results highlight the importance of structure and composition for control of bandgap bowing and charge-carrier recombination mechanisms in low bandgap absorbers for all-perovskite tandem solar cells.


Engineering III-V nanowires for optoelectronics: From epitaxy to terahertz photonics

Proceedings of SPIE - The International Society for Optical Engineering 10543 (2018)

HJ Joyce, C Uswachoke, SA Baig, SO Adeyemo, JL Boland, DA Damry, CL Davies, J Wong-Leung, HH Tan, C Jagadish, LM Herz, MB Johnston

© COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only. Nanowires show unique promise as nanoscale building blocks for a multitude of optoelectronic devices, ranging from solar cells to terahertz photonic devices. We will discuss the epitaxial growth of these nanowires in novel geometries and crystallographic phases, and the use of terahertz conductivity spectroscopy to guide the development of nanowire-based devices. As an example, we will focus on the development of nanowire-based polarization modulators for terahertz communications systems.


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.


Enhanced photovoltage for inverted planar heterojunction perovskite solar cells.

Science (New York, N.Y.) 360 (2018) 1442-1446

D Luo, W Yang, Z Wang, A Sadhanala, Q Hu, R Su, R Shivanna, GF Trindade, JF Watts, Z Xu, T Liu, K Chen, F Ye, P Wu, L Zhao, J Wu, Y Tu, Y Zhang, X Yang, W Zhang, RH Friend, Q Gong, HJ Snaith, R Zhu

The highest power conversion efficiencies (PCEs) reported for perovskite solar cells (PSCs) with inverted planar structures are still inferior to those of PSCs with regular structures, mainly because of lower open-circuit voltages (Voc). Here we report a strategy to reduce nonradiative recombination for the inverted devices, based on a simple solution-processed secondary growth technique. This approach produces a wider bandgap top layer and a more n-type perovskite film, which mitigates nonradiative recombination, leading to an increase in Voc by up to 100 millivolts. We achieved a high Voc of 1.21 volts without sacrificing photocurrent, corresponding to a voltage deficit of 0.41 volts at a bandgap of 1.62 electron volts. This improvement led to a stabilized power output approaching 21% at the maximum power point.


Linearly polarized photoluminescence of InGaN quantum disks embedded in GaN nanorods.

Scientific reports 8 (2018) 8124-8124

Y Park, CCS Chan, L Nuttall, TJ Puchtler, RA Taylor, N Kim, Y Jo, H Im

We have investigated the emission from InGaN/GaN quantum disks grown on the tip of GaN nanorods. The emission at 3.21 eV from the InGaN quantum disk doesn't show a Stark shift, and it is linearly polarized when excited perpendicular to the growth direction. The degree of linear polarization is about 39.3% due to the anisotropy of the nanostructures. In order to characterize a single nanostructure, the quantum disks were dispersed on a SiO2 substrate patterned with a metal reference grid. By rotating the excitation polarization angle from parallel to perpendicular relative to the nanorods, the variation of overall PL for the 3.21 eV peak was recorded and it clearly showed the degree of linear polarization (DLP) of 51.5%.


Host Exciton Confinement for Enhanced Forster-Transfer-Blend Gain Media Yielding Highly Efficient Yellow-Green Lasers

ADVANCED FUNCTIONAL MATERIALS 28 (2018) ARTN 1705824

Q Zhang, J Liu, Q Wei, X Guo, Y Xu, R Xia, L Xie, Y Qian, C Sun, L Lueer, J Cabanillas-Gonzalez, DDC Bradley, W Huang


Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

OPTICAL MATERIALS 78 (2018) 365-369

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


Highly Crystalline Methylammonium Lead Tribromide Perovskite Films for Efficient Photovoltaic Devices

ACS ENERGY LETTERS 3 (2018) 1233-1240

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

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