Publications by Michael Johnston


Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films

APL Materials American Institute of Physics 2 (2014) 081513-081513

C Wehrenfennig, M Liu, H Snaith, M Johnston, L Herz

The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3-xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3-xCl x exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion. © 2014 Author(s).


Homogeneous emission line broadening in the organo lead halide perovskite CH3NH3PbI3-xCl

journal of physical chemistry letters American Chemical Society 5 (2014) 1300-1306

C Wehrenfennig, M Liu, H Snaith, M Johnston, L Herz

The organic-inorganic hybrid perovskites methylammonium lead iodide (CH3NH3PbI3) and the partially chlorine-substituted mixed halide CH3NH3PbI3-xClx emit strong and broad photoluminescence (PL) around their band gap energy of ∼1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH3NH3PbI3-xClx using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration.


An ultrafast carbon nanotube terahertz polarisation modulator

Journal of Applied Physics American Institute of Physics 115 (2014) 203108-203108

CJ Docherty, S Stranks, SN Habisreutinger, H Joyce, L Herz, R Nicholas, M Johnston

We demonstrate ultrafast modulation of terahertz radiation by unaligned optically pumped single-walled carbon nanotubes. Photoexcitation by an ultrafast optical pump pulse induces transient terahertz absorption in nanowires aligned parallel to the optical pump. By controlling the polarisation of the optical pump, we show that terahertz polarisation and modulation can be tuned, allowing sub-picosecond modulation of terahertz radiation. Such speeds suggest potential for semiconductor nanowire devices in terahertz communication technologies.


Single GaAs/AlGaAs nanowire photoconductive terahertz detectors

2014 Conference on Optoelectronic and Microelectronic Materials and Devices, COMMAD 2014 (2014) 221-222

K Peng, P Parkinson, L Fu, Q Gao, N Jiang, YN Guo, F Wang, HJ Joyce, JL Boland, MB Johnston, HH Tan, C Jagadish

© 2014 IEEE. Photoconductive terahertz detectors based on single GaAs/AlGaAs core-shell nanowire have been designed and fabricated. The devices were characterised in a terahertz time-domain spectroscopy system, showing excellent sensitivity comparable to the standard bulk ion-implanted InP receiver, with a detection bandwidth of 0.1 ∼ 0.6 THz. Finite-difference time-domain simulations were performed to understand the origin of the narrow bandwidth of current detectors as well as optimize antenna designs to improve detector performance.


Solution deposition-conversion for planar heterojunction mixed halide perovskite solar cells

Advanced Energy Materials 4 (2014)

P Docampo, FC Hanusch, SD Stranks, M Döblinger, JM Feckl, M Ehrensperger, NK Minar, MB Johnston, HJ Snaith, T Bein

© 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Solution-deposited-converted perovskite solar cells are studied by converting PbI2planar films into the phase pure, mixed-halide perovskite (H3CNH3)PbI3-xClx. These solar cells exhibit very high photovoltaic performance and close to unity internal incident photon-to-electron conversion.


Formamidinium lead trihalide: A broadly tunable perovskite for efficient planar heterojunction solar cells

Energy and Environmental Science 7 (2014) 982-988

GE Eperon, SD Stranks, C Menelaou, MB Johnston, LM Herz, HJ Snaith

Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ∼1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV. We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm-2, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell. © 2014 The Royal Society of Chemistry.


Single GaAs/AlGaAs Nanowire Photoconductive Terahertz Detectors

2014 CONFERENCE ON OPTOELECTRONIC AND MICROELECTRONIC MATERIALS AND DEVICES (COMMAD 2014) (2014) 221-222

K Peng, P Parkinson, L Fu, Q Gao, N Jiang, Y-N Guo, F Wang, HJ Joyce, JL Boland, MB Johnston, HH Tan, C Jagadish


Special issue on terahertz science and technology Preface

JOURNAL OF PHYSICS D-APPLIED PHYSICS 47 (2014) ARTN 370301

MB Johnston


Formamidinium lead trihalide: A broadly tunable perovskite for efficient planar heterojunction solar cells

Energy and Environmental Science 7 (2014) 982-988

GE Eperon, SD Stranks, C Menelaou, MB Johnston, LM Herz, HJ Snaith

Perovskite-based solar cells have attracted significant recent interest, with power conversion efficiencies in excess of 15% already superceding a number of established thin-film solar cell technologies. Most work has focused on a methylammonium lead trihalide perovskites, with a bandgaps of ∼1.55 eV and greater. Here, we explore the effect of replacing the methylammonium cation in this perovskite, and show that with the slightly larger formamidinium cation, we can synthesise formamidinium lead trihalide perovskites with a bandgap tunable between 1.48 and 2.23 eV.We take the 1.48 eV-bandgap perovskite as most suited for single junction solar cells, and demonstrate long-range electron and hole diffusion lengths in this material, making it suitable for planar heterojunction solar cells. We fabricate such devices, and due to the reduced bandgap we achieve high short-circuit currents of >23 mA cm, resulting in power conversion efficiencies of up to 14.2%, the highest efficiency yet for solution processed planar heterojunction perovskite solar cells. Formamidinium lead triiodide is hence promising as a new candidate for this class of solar cell. © The Royal Society of Chemistry 2014.


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

Advanced Functional Materials 24 (2014) 668-677

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 2 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 TiO2 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 TiO2 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.


Electron mobilities approaching bulk limits in "surface-free" GaAs nanowires.

Nano letters American Chemical Society 14 (2014) 5989-5994

H Joyce, P Parkinson, N Jiang, CJ Docherty, Q Gao, HH Tan, C Jagadish, L Herz, M Johnston

Achieving bulk-like charge carrier mobilities in semiconductor nanowires is a major challenge facing the development of nanowire-based electronic devices. Here we demonstrate that engineering the GaAs nanowire surface by overcoating with optimized AlGaAs shells is an effective means of obtaining exceptionally high carrier mobilities and lifetimes. We performed measurements of GaAs/AlGaAs core-shell nanowires using optical pump-terahertz probe spectroscopy: a noncontact and accurate probe of carrier transport on ultrafast time scales. The carrier lifetimes and mobilities both improved significantly with increasing AlGaAs shell thickness. Remarkably, optimized GaAs/AlGaAs core-shell nanowires exhibited electron mobilities up to 3000 cm(2) V(-1) s(-1), reaching over 65% of the electron mobility typical of high quality undoped bulk GaAs at equivalent photoexcited carrier densities. This points to the high interface quality and the very low levels of ionized impurities and lattice defects in these nanowires. The improvements in mobility were concomitant with drastic improvements in photoconductivity lifetime, reaching 1.6 ns. Comparison of photoconductivity and photoluminescence dynamics indicates that midgap GaAs surface states, and consequently surface band-bending and depletion, are effectively eliminated in these high quality heterostructures.


Lead-free organic–inorganic tin halide perovskites for photovoltaic applications

Energy and Environmental Science Royal Society of Chemistry 7 (2014) 3061-3068

N Noel, S Stranks, A Abate, C Wehrenfennig, S Guarnera, AA Haghighirad, A Sadhanala, G Eperon, SK Pathak, M Johnston, A Petrozza, L Herz, H Snaith

Already exhibiting solar to electrical power conversion efficiencies of over 17%, organic-inorganic lead halide perovskite solar cells are one of the most promising emerging contenders in the drive to provide a cheap and clean source of energy. One concern however, is the potential toxicology issue of lead, a key component in the archetypical material. The most likely substitute is tin, which like lead, is also a group 14 metal. While organic-inorganic tin halide perovskites have shown good semiconducting behaviour, the instability of tin in its 2+ oxidation state has thus far proved to be an overwhelming challenge. Here, we report the first completely lead-free, CH3NH 3SnI3 perovskite solar cell processed on a mesoporous TiO2 scaffold, reaching efficiencies of over 6% under 1 sun illumination. Remarkably, we achieve open circuit voltages over 0.88 V from a material which has a 1.23 eV band gap.


Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite CH3NH3PbI3-xClx

Energy and Environmental Science Royal Society of Chemistry 7 (2014) 2269-2275

C Wehrenfennig, M Liu, HJ Snaith, MJ Johnston, LM Herz

We determine high charge-carrier mobilities ≥ 33 cm2 V−1 s−1 and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin's model for vapour-deposited CH3NH3PbI3−xClx using ultrafast THz spectroscopy. At charge-carrier densities below ∼1017 cm−3 intrinsic diffusion lengths are shown to approach 3 microns, limited by slow mono-molecular decay processes.


High charge carrier mobilities and lifetimes in organolead trihalide perovskites

Advanced Materials 26 (2014) 1584-1589

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

Organolead trihalide perovskites are shown to exhibit the best of both worlds: charge-carrier mobilities around 10 cm2 V-1 s -1 and low bi-molecular charge-recombination constants. The ratio of the two is found to defy the Langevin limit of kinetic charge capture by over four orders of magnitude. This mechanism causes long (micrometer) charge-pair diffusion lengths crucial for flat-heterojunction photovoltaics. © 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.


Measuring the electrical properties of semiconductor nanowires using terahertz conductivity spectroscopy

Proceedings of SPIE - The International Society for Optical Engineering 8923 (2013)

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

Accurately measuring the electronic properties of nanowires is a crucial step in the development of novel semiconductor nanowire-based devices. With this in mind, optical pump-terahertz probe (OPTP) spectroscopy is ideally suited to studies of nanowires: it provides non-contact measurement of carrier transport and dynamics at room temperature. OPTP spectroscopy has been used to assess key electrical properties, including carrier lifetime and carrier mobility, of GaAs, InAs and InP nanowires. The measurements revealed that InAs nanowires exhibited the highest mobilities and InP nanowires exhibited the lowest surface recombination velocity. © 2013 Copyright SPIE.


Fast electron trapping in anodized TiO<inf>2</inf> nanotubes

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

C Wehrenfennig, CM Palumbiny, L Schmidt-Mende, MB Johnston, HJ Snaith, LM Herz

We studied charge transport in anodized TiO2 nanotubes in the context of their application in dye-sensitized solar cells. Optical-pump-THz- probe spectroscopy revealed short free carrier lifetimes of about 15-30 ps, which we attribute to shallow trapping. © 2013 IEEE.


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.


Probing the critical electronic properties of III-V nanowires using optical pump-terahertz probe spectroscopy

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

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

Optical pump-terahertz probe spectroscopy was used to study the key electronic properties of GaAs, InAs and InP nanowires at room temperature. Of all nanowires studied, InAs nanowires exhibited the highest mobilities of 6000 cm2V-1s-1. InP nanowires featured the longest photoconductivity lifetimes and an exceptionally low surface recombination velocity of 170 cm/s. © 2013 IEEE.


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.

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