Publications by Michael Johnston


High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites

Advanced Materials (2013)

C Wehrenfennig, GE Eperon, MB Johnston, HJ Snaith, LM Herz


Dependence of dye regeneration and charge collection on the pore-filling fraction in solid-state dye-sensitized solar cells

Advanced Functional Materials (2013)

CT Weisspfennig, DJ Hollman, C Menelaou, SD Stranks, HJ Joyce, MB Johnston, HJ Snaith, LM Herz

Solid-state dye-sensitized solar cells rely on effective infiltration of a solid-state hole-transporting material into the pores of a nanoporous TiO network to allow for dye regeneration and hole extraction. Using microsecond transient absorption spectroscopy and femtosecond photoluminescence upconversion spectroscopy, the hole-transfer yield from the dye to the hole-transporting material 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) is shown to rise rapidly with higher pore-filling fractions as the dye-coated pore surface is increasingly covered with hole-transporting material. Once a pore-filling fraction of ≈30% is reached, further increases do not significantly change the hole-transfer yield. Using simple models of infiltration of spiro-OMeTAD into the TiO porous network, it is shown that this pore-filling fraction is less than the amount required to cover the dye surface with at least a single layer of hole-transporting material, suggesting that charge diffusion through the dye monolayer network precedes transfer to the hole-transporting material. Comparison of these results with device parameters shows that improvements of the power-conversion efficiency beyond ≈30% pore filling are not caused by a higher hole-transfer yield, but by a higher charge-collection efficiency, which is found to occur in steps. The observed sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory, predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls. From percolation theory it is predicted that, for standard mesoporous TiO with 20 nm pore size, the photocurrent should show no further improvement beyond an ≈83% pore-filling fraction. Solid-state dye-sensitized solar cells capable of complete hole transfer with pore-filling fractions as low as ∼30% are demonstrated. Improvements of device efficiencies beyond ∼30% are explained by a stepwise increase in charge-collection efficiency in agreement with percolation theory. Furthermore, it is predicted that, for a 20 nm pore size, the photocurrent reaches a maximum at ∼83% pore-filling fraction. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Optimizing the energy offset between dye and hole-transporting material in solid-state dye-sensitized solar cells

Journal of Physical Chemistry C 117 (2013) 19850-19858

CT Weisspfennig, MM Lee, J Teuscher, P Docampo, SD Stranks, HJ Joyce, H Bergmann, I Bruder, DV Kondratuk, MB Johnston, HJ Snaith, LM Herz

The power-conversion efficiency of solid-state dye-sensitized solar cells can be optimized by reducing the energy offset between the highest occupied molecular orbital (HOMO) levels of dye and hole-transporting material (HTM) to minimize the loss-in-potential. Here, we report a study of three novel HTMs with HOMO levels slightly above and below the one of the commonly used HTM 2,2′,7,7′- tetrakis(N,N-di-p-methoxyphenylamino)-9,9′- spirobifluorene (spiro-OMeTAD) to systematically explore this possibility. Using transient absorption spectroscopy and employing the ruthenium based dye Z907 as sensitizer, it is shown that, despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. We further demonstrate that the design of the HTM has an additional impact on the electronic density of states present at the TiO2 electrode surface and hence influences not only hole- but also electron-transfer from the sensitizer. These results provide insight into the complex influence of the HTM on charge transfer and provide guidance for the molecular design of new materials. © 2013 American Chemical Society.


Efficient planar heterojunction perovskite solar cells by vapour deposition

Nature Springer Science and Business Media LLC 501 (2013) 395-398

M Liu, MB Johnston, HJ Snaith


Transient terahertz spectroscopy of mono- and tri-layer CVD-grown MoS <inf>2</inf>

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz (2013)

CJ Docherty, HJ Joyce, LJ Li, MB Johnston

Molybdenum disulpide, a novel two-dimensional semiconductor, was studied using optical-pump terahertz-probe spectroscopy. Mono and trilayer samples grown by chemical vapour deposition were compared to reveal their dynamic electrical response. © 2013 IEEE.


Novel single-walled carbon nanotube: Dual polymer nanostructures

ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY 245 (2013)

RJ Nicholas, SD Stranks, B Dirks, C-K Yong, C Weisspfennig, MB Johnston, LM Herz


Electronic properties of GaAs, InAs and InP nanowires studied by terahertz spectroscopy.

Nanotechnology 24 (2013) 214006-

HJ Joyce, CJ Docherty, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston

We have performed a comparative study of ultrafast charge carrier dynamics in a range of III-V nanowires using optical pump-terahertz probe spectroscopy. This versatile technique allows measurement of important parameters for device applications, including carrier lifetimes, surface recombination velocities, carrier mobilities and donor doping levels. GaAs, InAs and InP nanowires of varying diameters were measured. For all samples, the electronic response was dominated by a pronounced surface plasmon mode. Of the three nanowire materials, InAs nanowires exhibited the highest electron mobilities of 6000 cm² V⁻¹ s⁻¹, which highlights their potential for high mobility applications, such as field effect transistors. InP nanowires exhibited the longest carrier lifetimes and the lowest surface recombination velocity of 170 cm s⁻¹. This very low surface recombination velocity makes InP nanowires suitable for applications where carrier lifetime is crucial, such as in photovoltaics. In contrast, the carrier lifetimes in GaAs nanowires were extremely short, of the order of picoseconds, due to the high surface recombination velocity, which was measured as 5.4 × 10⁵  cm s⁻¹. These findings will assist in the choice of nanowires for different applications, and identify the challenges in producing nanowires suitable for future electronic and optoelectronic devices.


Efficient planar heterojunction perovskite solar cells by vapour deposition

Nature 501 (2013) 395-398

M Liu, MB Johnston, HJ Snaith

Many different photovoltaic technologies are being developed for large-scale solar energy conversion. The wafer-based first-generation photovoltaic devices have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures. © 2013 Macmillan Publishers Limited. All rights reserved.


Direct observation of charge-carrier heating at WZ-ZB InP nanowire heterojunctions.

Nano Lett 13 (2013) 4280-4287

CK Yong, J Wong-Leung, HJ Joyce, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.


Chapter 10: Pump-probe spectroscopy at terahertz frequencies

Springer Series in Optical Sciences 171 (2013) 251-271

MB Johnston, J Lloyd-Hughes

Optical-pump-terahertz-probe spectroscopy is a technique that can measure directly the conductivity of photoexcited carriers on a picosecond timescale. In this chapter, we introduce the technique and data analysis, and discuss suitable models of the interaction between terahertz radiation and quasiparticles in materials. We then review some recent studies of inorganic and organic semiconductors and nanomaterials. © Springer-Verlag Berlin Heidelberg 2013.


Ultralow surface recombination velocity in InP nanowires probed by terahertz spectroscopy.

Nano Lett 12 (2012) 5325-5330

HJ Joyce, J Wong-Leung, C-K Yong, CJ Docherty, S Paiman, Q Gao, HH Tan, C Jagadish, J Lloyd-Hughes, LM Herz, MB Johnston

Using transient terahertz photoconductivity measurements, we have made noncontact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1 ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170 cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.


The origin of an efficiency improving "light soaking" effect in SnO <inf>2</inf> based solid-state dye-sensitized solar cells

Energy and Environmental Science 5 (2012) 9566-9573

P Tiwana, P Docampo, MB Johnston, LM Herz, HJ Snaith

We observe a strong "light-soaking" effect in SnO 2 based solid-state dye-sensitized solar cells (SDSCs). Both with and without the presence of UV light, the device's short-circuit photocurrent and efficiency increase significantly over 20-30 minutes, until steady-state is achieved. We demonstrate that this is not due to improved charge collection and investigate the charge generation dynamics employing optical-pump terahertz-probe spectroscopy. We observe a monotonic speeding-up of the generation of free-electrons in the SnO 2 conduction band as a function of the light-soaking time. This improved charge generation can be explained by a positive shift in the conduction band edge or, alternatively, an increase in the density of states (DoS) at the energy at which photoinduced electron transfer occurs. To verify this hypothesis, we perform capacitance and charge extraction measurements which indicate a shift in the surface potential of SnO 2 of up to 70 mV with light soaking. The increased availability of states into which electrons can be transferred justifies the increase in both the charge injection rate and ensuing photocurrent. The cause for the shift in surface potential is not clear, but we postulate that it is due to the photoinduced charging of the SnO 2 inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. Understanding temporally evolving processes in DSCs is of critical importance for enabling this technology to operate optimally over a prolonged period of time. This work specifically highlights important changes that can occur at the dye-sensitized heterojunction, even without direct light absorption in the metal oxide. © 2012 The Royal Society of Chemistry.


Strong carrier lifetime enhancement in GaAs nanowires coated with semiconducting polymer.

Nano Lett 12 (2012) 6293-6301

CK Yong, K Noori, Q Gao, HJ Joyce, HH Tan, C Jagadish, F Giustino, MB Johnston, LM Herz

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors.


Terahertz properties of graphene

Journal of Infrared, Millimeter, and Terahertz Waves 33 (2012) 797-815

CJ Docherty, MB Johnston

Graphene has proved itself as being unique in terms of fundamental physics, and of particular importance for post-silicon electronics. Research into graphene has divided into two branches, one probing the remarkable electronic and optical properties of graphene, and the other pursuing technologically viable forms of the material. Terahertz time domain spectroscopy (THz TDS) is a powerful tool for both, able to characterise the free carrier response of graphene and probe the inter and intraband response of excited carriers with sub-ps time resolution. We review THz TDS and related THz measurements of graphene. © Springer Science+Business Media, LLC 2012.


Noncontact measurement of charge carrier lifetime and mobility in GaN nanowires.

Nano Lett 12 (2012) 4600-4604

P Parkinson, C Dodson, HJ Joyce, KA Bertness, NA Sanford, LM Herz, MB Johnston

The first noncontact photoconductivity measurements of gallium nitride nanowires (NWs) are presented, revealing a high crystallographic and optoelectronic quality achieved by use of catalyst-free molecular beam epitaxy. In comparison with bulk material, the NWs exhibit a long conductivity lifetime (>2 ns) and a high mobility (820 ± 120 cm(2)/(V s)). This is due to the weak influence of surface traps with respect to other III-V semiconducting NWs and to the favorable crystalline structure of the NWs achieved via strain-relieved growth.


Quantitative measurement of a 3-component mixture based on THz spectra

Proceedings of SPIE - The International Society for Optical Engineering 8330 (2012)

Z Li, Z Zhang, X Zhao, H Su, F Yan, K Dunn, MB Johnston

Quantitative measurement based on THz absorption spectrum is of great importance in THz applications. Several researchers have worked on it and gained some achievements, but most of them explored pure component or no more than 2-component s samples. In this paper, a mixture sample consisting of Glutamine, Histidine and Threonine is investigated in the frequency range from 0.3 to 2.6 THz. The quantitative measurement principle is the Lambert-Beer's Law which have been accepted in infrared and visible spectra. Our experiments show the validation of the law in THz region. A Least-Mean-Square algorithm is adopted and measurement errors of Glutamine, Histidine and Threonine are 17.60%, 4.44% and 2.59%.© 2012 SPIE.


Nanoengineering coaxial carbon nanotube-dual-polymer heterostructures.

ACS Nano 6 (2012) 6058-6066

SD Stranks, C-K Yong, JA Alexander-Webber, C Weisspfennig, MB Johnston, LM Herz, RJ Nicholas

We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely, poly(3-hexylthiophene) (P3HT) and poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT). Using absorption spectroscopy and steady-state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimize both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures.


Ultrafast dynamics of exciton formation in semiconductor nanowires.

Small 8 (2012) 1725-1731

CK Yong, HJ Joyce, J Lloyd-Hughes, Q Gao, HH Tan, C Jagadish, MB Johnston, LM Herz

The dynamics of free electron-hole pairs and excitons in GaAs-AlGaAs-GaAs core-shell-skin nanowires is investigated using femtosecond transient photoluminescence spectroscopy at 10 K. Following nonresonant excitation, a bimolecular interconversion of the initially generated electron-hole plasma into an exciton population is observed. This conducting-to-insulating transition appears to occur gradually over electron-hole charge pair densities of 2-4 × 10(16) cm(-3) . The smoothness of the Mott transition is attributed to the slow carrier-cooling during the bimolecular interconversion of free charge carriers into excitons and to the presence of chemical-potential fluctuations leading to inhomogeneous spectral characteristics. These results demonstrate that high-quality nanowires are model systems for investigating fundamental scientific effects in 1D heterostructures.


Environment induced variation in the photoconductivity of graphene observed by terahertz spectroscopy

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz (2012)

CJ Docherty, CT Lin, HJ Joyce, RJ Nicholas, LJ Li, MB Johnston

Chemical vapour deposition (CVD) grown graphene sheets were investigated using optical-pump terahertz-probe spectroscopy, revealing a dramatic variation in the photoinduced terahertz conductivity of graphene in different atmospheres. © 2012 IEEE.


Simulation of fluence-dependent photocurrent in terahertz photoconductive receivers

Semiconductor Science and Technology 27 (2012)

E Castro-Camus, MB Johnston, J Lloyd-Hughes

A semi-classical Monte Carlo simulation of carrier dynamics in photoconductive detectors of terahertz (THz) radiation is presented. We have used this simulation to elucidate the importance of carrier trapping in the operation of photoconductive detectors. Simulations of the detection of single-cycle THz pulses by photoconductive antennas based on GaAs with trap densities between 2 × 10 17 and 2 × 10 18 cm 3 are presented. We show that the high frequency (>1 THz) spectral response of photoconductive devices decreases with increasing excitation fluence. Our simulations reveal that this effect is a direct consequence of the saturation of trapping centres © 2012 IOP Publishing Ltd.

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