Publications


Direct Observation of Ultrafast Exciton Dissociation in Lead Iodide Perovskite by 2D Electronic Spectroscopy

ACS PHOTONICS 5 (2018) 852-860

A Jha, H-G Duan, V Tiwari, PK Nayak, HJ Snaith, M Thorwart, RJD Miller


Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3

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

JA Alexander-Webber, J Huang, J Beilsten-Edmands, P Čermák, C Drasar, RJ Nicholas, A Coldea

We report magnetotransport studies in thin (<100nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc ~3.5K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T<30K. 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 Lφ > 100nm indicating the presence of topologically protected surface states.


Multi-band magnetotransport in exfoliated thin films of CuxBi2Se3

Journal of Physics: Condensed Matter IOP Publishing 30 (2018) 155302

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

We report magnetotransport studies in thin (<100 nm) exfoliated films of CuxBi2Se3 and we detect an unusual electronic transition at low temperatures. Bulk crystals show weak superconductivity with Tc ∼ 3.5 K and a possible electronic phase transition around 200K. Following exfoliation, superconductivity is supressed and a strongly temperature dependent multi-band conductivity is observed for T < 30K. 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 Lφ > 100 nm indicating the presence of topologically protected surface states.


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.


The effect of ionic composition on acoustic phonon speeds in hybrid perovskites from Brillouin spectroscopy and density functional theory

JOURNAL OF MATERIALS CHEMISTRY C 6 (2018) 3861-3868

IV Kabakova, I Azuri, Z Chen, PK Nayak, HJ Snaith, L Kronik, C Paterson, AA Bakulin, DA Egger


The Route to Nanoscale Terahertz Technology: Nanowire-based Terahertz Detectors and Terahertz Modulators

2018 43RD INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ) (2018)

JL Boland, K Peng, S Baig, D Damry, P Parkinson, L Fu, HH Tan, C Jagadish, LM Herz, H Joyce, M Johnston, IEEE


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.


Naphthalenetetracarboxylic Diimide Derivatives: Molecular Structure, Thin Film Properties and Solar Cell Applications

Zeitschrift fur Physikalische Chemie (2018)

C Falkenberg, M Hummert, R Meerheim, C Schünemann, S Olthof, C Körner, MK Riede, K Leo

© 2018 Walter de Gruyter GmbH, Berlin/Boston 2018. The effciency of organic solar cells is not only determined by their absorber system, but also strongly dependent on the performance of numerous interlayers and charge transport layers. In order to establish new custom-made materials, the study of structure-properties relationships is of great importance. This publication examines a series of naphthalenetetracarboxylic diimide molecules (NTCDI) with varying side-chain length intended for the use as n-dopable electron transport materials in organic solar cells. While all compounds basically share very similar absorption spectra and energy level positions in the desired range, the introduction of alkyl chains has a large impact on thin film growth and charge transport properties: both crystallization and the increase of conductivity by molecular doping are suppressed. This has a direct influence on the series resistance of corresponding solar cells comprising an NTCDI derivative as electron transport material (ETM) as it lowers the power conversion efficiency to 1%. In contrast, using the side-chain free compound it is possible to achive an efficiency of 6.5%, which is higher than the efficiency of a comparable device comprising n-doped C60as standard ETM.


Femtosecond dynamics of photoexcited C60 films

Journal of Physical Chemistry Letters American Chemical Society (2018)

M Causa', IR Ramirez, JMF Hardigree, MK Riede, N Banerjii

The well-known organic semiconductor C60 is attracting renewed attention due to its centimetre-long electron diffusion length and high performance of solar cells containing 95% fullerene. Yet, its photophysical properties remain poorly understood. Here, 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 photo-dimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield depend strongly 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.


Spin-coated planar Sb2S3 hybrid solar cells approaching 5% efficiency.

Beilstein journal of nanotechnology 9 (2018) 2114-2124

P Kaienburg, B Klingebiel, T Kirchartz

Antimony sulfide solar cells have demonstrated an efficiency exceeding 7% when assembled in an extremely thin absorber configuration deposited via chemical bath deposition. More recently, less complex, planar geometries were obtained from simple spin-coating approaches, but the device efficiency still lags behind. We compare two processing routes based on different precursors reported in the literature. By studying the film morphology, sub-bandgap absorption and solar cell performance, improved annealing procedures are found and the crystallization temperature is shown to be critical. In order to determine the optimized processing conditions, the role of the polymeric hole transport material is discussed. The efficiency of our best solar cells exceeds previous reports for each processing route, and our champion device displays one of the highest efficiencies reported for planar antimony sulfide solar cells.


In situ simultaneous photovoltaic and structural evolution of perovskite solar cells during film formation

ENERGY & ENVIRONMENTAL SCIENCE 11 (2018) 383-393

M Alsari, O Bikondoa, J Bishop, M Abdi-Jalebi, LY Ozer, M Hampton, P Thompson, MT Horantner, S Mahesh, C Greenland, JE Macdonald, G Palmisano, HJ Snaith, DG Lidzey, SD Stranks, RH Frienda, S Lilliu


Figures of Merit Guiding Research on Organic Solar Cells

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 5829-5843

T Kirchartz, P Kaienburg, D Baran


Evidence of Nitrogen Contribution to the Electronic Structure of the CH3 NH3 PbI3 Perovskite.

Chemistry (Weinheim an der Bergstrasse, Germany) 24 (2018) 3539-3544

M Kot, K Wojciechowski, H Snaith, D Schmeißer

Despite fast development of hybrid perovskite solar cells, there are many fundamental questions related to the perovskite film which remain open. For example, there are contradicting theoretical reports on the role of the organic methylammonium cation (CH3 NH3+ ) in the methylammonium lead triiodide (CH3 NH3 PbI3 ) perovskite film. From one side it is reported that the organic cation does not contribute to electronic structure of the CH3 NH3 PbI3 film. From the other side, valence band maximum fluctuations, dependent on the CH3 NH3+ rotation, have been theoretically predicted. The resonant X-ray photoelectron spectroscopy results reported here show experimental evidence of nitrogen contribution to the CH3 NH3 PbI3 electronic structure. Moreover, the observed strong resonances of nitrogen with the I 5s and the Pb 5d-6s levels indicate that the CH3 NH3 PbI3 valence band is extended up to ≈18 eV below the Fermi energy, and therefore one should also consider these shallow core levels while modeling its electronic structure.


Optical Aharonov-Bohm Oscillations with Disorder Effects and Wigner Molecule in a Single GaAs/AlGaAs Quantum Ring

in NanoScience and Technology, (2018) 231-254

K Kyhm, HD Kim, R Okuyama, M Eto, KC Je, RA Taylor, G Nogues, LS Dang, AAL Nicholet, M Potemski, JS Kim, JD Song

© 2018, Springer International Publishing AG, part of Springer Nature. The optical Aharonov-Bohm effect in a single quantum ring is associated with disorder effects. In the presence of structure anisotropy, localisation, internal electric field, and impurity scattering, optical Aharonov-Bohm oscillations of an electron-hole pair become modulated. Additionally, provided that a strongly correlated exciton pair is formed in a single quantum ring similar to the Wigner molecule, novel oscillations can be observed for increasing magnetic field. In this case, the biexciton emission energy changes abruptly at transition magnetic fields with a fractional oscillation period compared to that of the exciton, the so-called fractional optical Aharonov-Bohm oscillations.


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 Lithuanian Academy of Sciences 58 (2018) 15-23

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

GaAs nanowires are promising candidates for advanced optoelectronic devices, despite their high surface recombination velocity and large surface-area-to-volume ratio, which renders them problematic for applications that require efficient charge collection and long charge-carrier lifetimes. Overcoating a bare GaAs nanowire core with an optimized larger-bandgap AlGaAs shell, followed by a capping layer of GaAs to prevent oxidation, has proven an effective way to passivate the nanowire surface and thereby improve electrical properties for enhanced device performance. However, it is difficult to quantify and distinguish the contributions between the nanowire core and cap layer when measuring the optoelectronic properties of a nanowire device. Here, we investigated the photoconductive terahertz (THz) response characteristics of single GaAs/AlGaAs/GaAs core–shell–cap nanowire detectors designed for THz time-domain spectroscopy. We present a detailed study of the contributions of the GaAs cap layer and GaAs core on the ultrafast optoelectronic performance of the detector. We show that both the GaAs cap and core contribute to the photoconductive signal in proportion to their relative volume in the nanowire. By increasing the cap volume ratio to above 90% of the total GaAs volume, a quasi-direct-sampling type photoconductive nanowire detector can be achieved that is highly desirable for low-noise and fast data acquisition detection.


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

Proceedings of SPIE Society of Photo-optical Instrumentation Engineers 10543 (2018)

HJ Joyce, C Uswachoke, SO Adeyemo, JL Boland, DA Damry, CL Davies, J Wong-Leung, HH Tan, C Jagadish, L Herz, M Johnston

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

Journal of Physical Chemistry Letters American Chemical Society 2018 (2017) 263-268

J Patel, Q Lin, O Zadvorna, C Davies, L Herz, M 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.


In-situ observation of stacking fault evolution in vacuum-deposited C60

Applied Physics Letters AIP Publishing 111 (2017) 233305

J Martinez Hardigree, IR Ramirez, G Mazzotta, C Nicklin, M Riede

We report an in-situ study of stacking fault evolution in C 60 thin films using grazing-incidence x-ray scattering (GIXS). A Williamson-Hall analysis of the main scattering features during growth of a 15 nm film on glass indicate lattice strain as high as 6% in the first 5 nm of the film, with a decrease to 2% beyond 8 nm thickness. Deformation stacking faults along the {220} plane are found to occur with 68% probability, and closely linked to the formation of a nanocrystalline powder-like film. Our findings, which capture monolayer-resolution growth, are consistent with previous work on crystalline and powder C60 films and provide a crystallographic context for the realtime study of organic semiconductor thin films.


Highly polarized electrically driven single-photon emission from a non-polar InGaN quantum dot

APPLIED PHYSICS LETTERS 111 (2017) ARTN 251108

CC Kocher, TJ Puchtler, JC Jarman, T Zhu, T Wang, L Nuttall, RA Oliver, RA Taylor

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