Publications by Michael Johnston


Influence of interface morphology on hysteresis in vapor-deposited perovskite solar cells

Advanced Electronic Materials Wiley 3 (2016) 1600470-

JB Patel, J Wong-Leung, S Van Reenen, N Sakai, JTW Wang, ES Parrott, M Liu, HJ Snaith, LM Herz, M Johnston

Hysteresis in the current–voltage characteristics of vapor-deposited perovskite solar cells is shown to originate from an amorphous region of CH3NH3PbI3 at the interface with the device's electron transport layer. Interface engineering is used to produce highly crystalline perovskite material at this interface which results in hysteresis-free evaporated planar heterojunction solar cells.


Towards higher electron mobility in modulation doped GaAs/AlGaAs core shell nanowires

Nanoscale Royal Society of Chemistry 9 (2017) 7839-7846

JL Boland, G Tutuncuoglu, JQ Gong, S Conesa-Boj, CL Davis, LM Herz, A Fontcuberta i Morral, M Johnston

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation into novel electronic and optoelectronic devices. Advances in our understanding of doping mechanisms in nanowires and their influence on electron mobility and radiative efficiency are urgently required. Here, we investigate the electronic properties of n-type modulation doped GaAs/AlGaAs nanowires via optical pump terahertz (THz) probe spectroscopy and photoluminescence spectroscopy over the temperature range 5K-300K. We directly determine an ionisation energy of 6.7±0.5meV (T = 52K) for the Si donors that create the modulation doping in the AlGaAs shell. We further elucidate the temperature dependence of the electron mobility, photoconductivity lifetime and radiative efficiency, and determine the charge-carrier scattering mechanisms that limit electron mobility. We show that below the donor ionization temperature, charge scattering is limited by interactions with interfaces, leading to an excellent electron mobility of 4360±380cm2V-1s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220±130cm2V-1s-1. In addition, we show that the Si donors effectively passivate interfacial trap states in the nanowires, leading to prolonged photoconductivity lifetimes with increasing temperature, accompanied by an enhanced radiative efficiency that exceeds 10% at room temperature.


Semiconductor nanowires in terahertz photonics: From spectroscopy to ultrafast nanowire-based devices

2017 10th UK-Europe-China Workshop on Millimetre Waves and Terahertz Technologies, UCMMT 2017 (2017)

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

© 2017 IEEE. Nanowires show unique promise for a multitude of optoelectronic devices, ranging from solar cells to terahertz (THz) photonic devices. Here, we discuss how THz spectroscopy is guiding the development of such nanowire-based devices. As an example, we focus on developing nanowire-based THz polarization modulators.


Optoelectronics: Fast silicon photodiodes

Nature Photonics Nature Publishing Group 11 (2017) 268-269

MB Johnston

How much internet traffic did you generate today? Perhaps more than you realise given the increasing popularity of streaming audio or video content, “cloud” data storage, and social media. It is estimated that approximately 1 zettabyte (1021 bytes) of internet traffic was transmitted globally last year,1 which is the equivalent of about 360MB per day per person in the world. Much of the long distance, high volume internet traffic is transmitted via near infrared (NIR) light through optical fibre waveguides. At the end of the optical fibre the optical signal is turned into an electrical signal, typically for use in silicon based integrated circuits. However, presently most receivers for long distance optical fibre communications systems are based on photodiodes made from other semiconductors such as InxGa1-xAs, or Ge which are challenging and costly to integrate with silicon CMOS electronics on a single chip.


Photon Re-Absorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite

Nano Letters American Chemical Society 17 (2017) 5782-5789

TW Crothers, RL Milot, JB Patel, ES Parrott, J Schlipf, P Muller-Buschbaum, MB Johnston, L Herz

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH3NH3PbI3 are strongly affected by photon re-absorption which masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50nm and 533nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon re-absorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient, of value 6.8x10(-10)cm(3)s(-1), is common to all samples irrespective of film thickness. Hence we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples, with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, while thinner films or index-matched surrounding layers can reduce the possibility for photon re-absorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement.


Cs2InAgCl6: A new lead-free halide double perovskite with direct band gap.

Journal of Physical Chemistry Letters American Chemical Society 2017 (2017) 772-778

G Volonakis, AA Haghighirad, RL Milot, WH Sio, MR Filip, B Wenger, MB Johnston, LM Herz, HJ Snaith, F Giustino

A2BB'X6 halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, Cs2BiAgX6 (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B(3+) and B(+) cations are In(3+) and Ag(+), respectively. Our first-principles calculations indicate that the hypothetical compounds Cs2InAgX6 (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize Cs2InAgCl6 and Cs2InAgBr6, and we succeed to form the hitherto unknown double perovskite Cs2InAgCl6. X-ray diffraction yields a double perovskite structure with space group Fm3̅m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of Cs2InAgX6 and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form Cs2InAg(Cl1-xBrx)6 for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.


Crystallization kinetics and morphology control of formamidinium-cesium mixed-cation lead mixed-halide perovskite via tunability of the colloidal precursor solution

Advanced Materials Wiley 29 (2017) 1-8

D McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, Marder, L Herz, HJ Snaith

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm(2) V(-1) s(-1) . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.


Band-tail recombination in hybrid lead iodide perovskite

Advanced Functional Materials Wiley (2017)

AD Wright, RL Milot, GE Eperon, HJ Snaith, L Johnston, MB Herz

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose bandstructure picture is moderated by a modest degree of energetic disorder.


Investigations of doping via optical pump terahertz-probe spectroscopy

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz (2017)

JL Boland, A Casadei, G Tutuncouglu, F Matteini, C Davies, F Gaveen, F Amaduzzi, HJ Joyce, LM Herz, A Fontcuberta I Morral, MB Johnston

© 2017 IEEE. Reliable doping in semiconductor nanowires is essential for the development of novel optoelectronic devices. Dopant incorporation within the nanowire can allow for optimisation of key optoelectronic properties, such as electron mobility and carrier lifetime. Thus, in-depth characterisation of doping mechanisms in semiconductor nanowires and their effect on the nanowire optoelectronics properties is crucial. However, extraction of the dopant concentration by conventional electrical methods remains difficult due to the associated challenges with fabricating lateral contacts onto the nanowire. In this work, we present a non-contact technique based on optical pump terahertz-probe spectroscopy for examining the extrinsic carrier concentration and optoelectronic properties of semiconductor nanowires. By extracting the temperature-dependent charge carrier dynamics, we show for the first time that the dopant activation energy and underlying scattering mechanisms affecting charge carrier mobility in these nanostructures can be determined via terahertz spectroscopy.


Unveiling the influence of pH on the crystallization of hybrid perovskites, felivering low voltage loss photovoltaics

Joule Cell Press 1 (2017) 328-343

N Noel, M Congiu, AJ Ramadan, S Fearn, DP McMeekin, JB Patel, MB Johnston, B Wenger, HJ Snaith

Impressive power conversion efficiencies coupled with the relative ease of fabrication have made perovskite solar cells a front runner for next-generation photovoltaics. Although perovskite films and optoelectronic devices have been widely studied, relatively little is known about the chemistry of the precursor solutions. Here, we present a study on the hydrolysis of N,N-dimethylformamide, correlating how pH changes related to its degradation affect the crystallization of MAPbI3xClx perovskite films. By careful manipulation of the pH, and the resulting colloid distribution in precursor solutions, we fabricate perovskite films with greatly improved crystallinity, which when incorporated into photovoltaic devices reproducibly yield efficiencies of over 18%. Extending this method to the mixed cation, mixed halide perovskite FA0.83MA0.17Pb(I0.83Br0.17)3, we obtain power conversion efficiencies of up to 19.9% and open-circuit voltages of 1.21 V for a material with a bandgap of 1.57 eV, achieving the lowest yet reported loss in potential from bandgap to a VOC of only 360 mV.


Single n+-i-n+ InP Nanowires for Highly Sensitive Terahertz Detection

Nanotechnology IOP Publishing Ltd 28 (2017) 125202-

K Peng, P Parkinson, Q Gao, J Boland, Z Li, F Wang, S Mokkapati, L Fu, M Johnston, HH Tan, C Jagadish

Developing single-nanowire terahertz (THz) electronics and employing them as sub-wavelength components for highly-integrated THz time-domain spectroscopy (THz-TDS) applications are a promising approach to achieve future low-cost, highly integrable and high-resolution THz tools, which are desirable in many areas spanning from security, industry, environmental monitoring and medical diagnostics to fundamental science. In this work, we present the design and growth of n<sup>+</sup>-i-n<sup>+</sup> InP nanowires. The axial doping profile of the n<sup>+</sup>-i-n<sup>+</sup> InP nanowires has been calibrated and characterized using combined optical and electrical approaches to achieve nanowire devices with low contact resistances, on which the highly-sensitive InP single-nanowire photoconductive THz detectors have been demonstrated. While the n<sup>+</sup>-i-n<sup>+</sup> InP nanowire detector has a only pA-level response current, it has a 2.5 times improved signal-to-noise ratio compared with the undoped InP nanowire detector and is comparable to traditional bulk THz detectors. This performance indicates a promising path to nanowire-based THz electronics for future commercial applications.


An ultrafast switchable terahertz polarization modulator based on III--V semiconductor nanowires

Nano Letters: a journal dedicated to nanoscience and nanotechnology American Chemical Society (2017)

MB Johnston, JL Boland, D Damry


Efficient and air-stable mixed-cation lead mixed-halide perovskite solar cells with n-doped organic electron extraction layers

Advanced Materials Wiley 29 (2016)

Z Wang, DP McMeekin, N Sakai, S van Reenen, K Wojciechowski, JB Patel, M Johnston, HJ Snaith

Air-stable doping of the n-type fullerene layer in an n-i-p planar heterojunction perovskite device is capable of enhancing device efficiency and improving device stability. Employing a (HC(NH2 )2 )0.83 Cs0.17 Pb(I0.6 Br0.4 )3 perovskite as the photoactive layer, glass-glass laminated devices are reported, which sustain 80% of their "post burn-in" efficiency over 3400 h under full sun illumination in ambient conditions.


Near-infrared and short-wavelength infrared photodiodes based on dye-perovskite composites

Advanced Functional Materials Wiley 27 (2017) 1702485

Q Lin, Z Wang, M Young, JB Patel, RL Milot, L Martinez Maestro, RR Lunt, HJ Snaith, MB Johnston, L Herz

Organohalide perovskites have emerged as promising light-sensing materials because of their superior optoelectronic properties and low-cost processing methods. Recently, perovskite-based photodetectors have successfully been demonstrated as both broadband and narrowband varieties. However, the photodetection bandwidth in perovskite-based photodetectors has so far been limited to the near-infrared regime owing to the relatively wide band gap of hybrid organohalide perovskites. In particular, short-wavelength infrared photodiodes operating beyond 1 μm have not yet been realized with organohalide perovskites. In this study, narrow band gap organic dyes are combined with hybrid perovskites to form composite films as active photoresponsive layers. Tuning the dye loading allows for optimization of the spectral response characteristics and excellent charge-carrier mobilities near 11 cm 2 V -1 s -1 , suggesting that these composites combine the light-absorbing properties or IR dyes with the outstanding charge-extraction characteristics of the perovskite. This study demonstrates the first perovskite photodiodes with deep near-infrared and short-wavelength infrared response that extends as far as 1.6 μm. All devices are solution-processed and exhibit relatively high responsivity, low dark current, and fast response at room temperature, making this approach highly attractive for next-generation light-detection techniques.


The influence of surfaces on the transient terahertz conductivity and electron mobility of GaAs nanowires

Journal of Physics D: Applied Physics IOP Publishing 50 (2017) 224001

H Joyce, P Parkinson, C Davies, J Boland, H Hoe Tan, C Jagadish, L Herz, M Johnston

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400–2100 cm2 V−1 s−1) and ultrashort charge carrier lifetimes (1–5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump–terahertz-probe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell–Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3  ×  106 cm s−1. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.


Photovoltaic mixed-cation lead mixed-halide perovskites: Links between crystallinity, photo-stability and electronic properties

Energy and Environmental Science Royal Society of Chemistry 10 (2016) 361-369

W Rehman, DP McMeekin, JB Patel, RL Milot, MB Johnston, HJ Snaith, LM Herz

<p> Lead mixed halide perovskites are highly promising semiconductors for both multi-junction photovoltaic and light emitting applications due to their tunable band gaps, with emission and absorption energies spanning the UV-visible to near IR regions. However, many such perovskites exhibit unwanted halide segregation under photoillumination, the cause of which is still unclear. In our study, we establish crucial links between crystal phase stability, photostability and optoelectronic properties of the mixed-cation lead mixed-halide perovskite CsyFA(1-y)Pb(BrxI(1-x))3. We demonstrate a region for caesium content between 0.10 &lt; y &lt; 0.30 which features high crystalline quality, long chargecarrier lifetimes and high charge-carrier mobilities. Importantly, we show that for such high-quality perovskites, photoinduced halide segregation is strongly suppressed, suggesting that high crystalline quality is a prerequisite for good optoelectronic quality and band gap stability. We propose that regions of short-range crystalline order aid halide segregation, possibly by releasing lattice strain between iodide rich and bromide rich domains. For an optimized caesium content, we explore the orthogonal halide-variation parameter space for Cs0.17FA0.83Pb(BrxI(1-x))3 perovskites. We demonstrate excellent charge-carrier mobilities (11-40 cm2 V^−1 s^−1) and diffusion lengths (0.8 - 4.4 µm) under solar conditions across the full iodide-bromide tuning range. Therefore, the addition of caesium yields a more photostable perovskite system whose absorption onsets can be tuned for bandgap-optimized tandem solar cells.</p>


The 2017 terahertz science and technology roadmap

Journal of Physics D: Applied Physics IOP Publishing 50 (2017)

SS Dhillon, MS Vitiello, EH Linfield, AG Davies, MC Hoffmann, J Booske, GP Williams, J Grant, S Lucyszyn, M Kuwata-Gonokami, K Konishi, M Koch, AG Markelz, J Axel Zeitler, J Sibik, KB Cooper, R Appleby, D Pardo, PG Huggard, H Shams, M Naftaly, N Ridler, R Clarke, JE Cunningham, M Johnston

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz-30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.


Broadband single-nanowire photoconductive Terahertz detectors

CLEO: Science and Innovations Optical Society of America (2017)
Part of a series from Conference on Lasers and Electro-Optics

K Peng, P Parkinson, Q Gao, J Boland, Z Li, F Wang, YC Wenas, CL Davies, L Fu, M Johnston, HH Tan, C Jagadish, I Ieee

Broadband photoconductive terahertz detectors based on undoped InP single nanowires were demonstrated. By further design and growth of an axial n+-i-n+ structure to reduce the contact resistance, highly-sensitive n+-i-n+ InP single-nanowire terahertz detectors were achieved.


Increased photoconductivity lifetimes in GaAs nanowires via n-type and p-type shell doping

41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2016 IEEE 2016-November (2016)

JL Boland, A Casadei, G Tutuncouglu, F Matteini, C Davies, F Gaveen, F Amaduzzi, HJ Joyce, LM Herz, A Fontcuberta i Morral, M Johnston

Reliable doping in GaAs nanowires is essential for the development of novel optoelectronic devices. Previously, GaAs nanowires have been shown to exhibit extremely short photoconductivity lifetimes of a few picoseconds due to their high surface recombination velocity, which is detrimental for nanowire devices, such as solar cells and nanowire lasers. Here, we show that, by exploiting engineered band-bending via selective doping, this parasitic surface recombination can be reduced. We utilise non-contact time-resolved terahertz spectroscopy to characterise the doping efficiency in n-type and p-type doped GaAs nanowire8 and show high carrier concentrations of the order of 1018 cm-3. The carrier lifetimes were increased by an order of magnitude from 0.13ns for undoped to 3.8ns and 2.5ns for n-doped and p-doped GaAs nanowires respectively; showing that surface recombination is greatly suppressed as a result of shell doping. We also present a novel effect of p-doping in GaAs nanowires: a rapid decay in photoconductivity within 25ps after photoexcitation. This fast decay is attributed to rapid electron trapping at the nanowire surface due to doping related band bending. Thus, we demonstrate the advantages of selective doping for enhancement of desirable transport properties in GaAs nanowires, as well as highlighting terahertz spectroscopy as a reliable technique for characterising doped GaAs nanowires1.


Extracting the key electrical properties of semiconductors using optical pump terahertz probe spectroscopy

41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2016 IEEE 2016-November (2016)

M Johnston

We have used optical-pump-terahertz-probe spectroscopy (OPTPS) to study a range of novel of semiconductors including III-V nanowires and metal halide perovskites. We show that OPTPs allows key figures of merit to be extracted in a non-contact manner, including charge mobility, surface recombination velocity, and doping density. Furthermore, the technique allows charge recombination dynamics to be followed on a picosecond time-scale. This knowledge is useful in the design of new optoelectronic devices from lasers to solar cells as well as for the development and optimisation of new semiconductors.

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