Publications


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.


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.


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.


Influence of Interface Morphology on Hysteresis in Vapor-Deposited Perovskite Solar Cells

ADVANCED ELECTRONIC MATERIALS 3 (2017) ARTN 1600470

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


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

Nanoscale 9 (2017) 7839-7846

JL Boland, G Tütüncüoglu, JQ Gong, S Conesa-Boj, CL Davies, LM Herz, A Fontcuberta I Morral, MB Johnston

Precise control over the electrical conductivity of semiconductor nanowires is a crucial prerequisite for implementation of these nanostructures 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 5 K-300 K. We directly determine an ionization energy of 6.7 ± 0.5 meV (T = 52 K) for the Si donors within the AlGaAs shell that create the modulation doping structure. 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 ± 380 cm2 V-1 s-1 at 5 K. Above the ionization temperature, polar scattering via longitudinal optical (LO) phonons dominates, leading to a room temperature mobility of 2220 ± 130 cm2 V-1 s-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.


OPTOELECTRONICS Fast silicon photodiodes

NATURE PHOTONICS 11 (2017) 268-269

MB Johnston


Large-Area, Highly Uniform Evaporated Formamidinium Lead Triiodide Thin Films for Solar Cells

ACS ENERGY LETTERS 2 (2017) 2799-2804

J Borchert, RL Milot, JB Patel, CL Davies, AD Wright, LM Maestro, HJ Snaith, LM Herz, MB Johnston


Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH3NH3PbI3 Perovskite.

Nano letters 17 (2017) 5782-5789

TW Crothers, RL Milot, JB Patel, ES Parrott, J Schlipf, P Müller-Buschbaum, MB Johnston, LM 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 reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, 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 reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10-10 cm3s-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, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. 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.

The journal of physical chemistry letters 8 (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 B3+ and B+ cations are In3+ 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 (Deerfield Beach, Fla.) 29 (2017)

DP McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, SR Marder, LM 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 cm2 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 27 (2017) ARTN 1700860

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


Unveiling the Influence of pH on the Crystallization of Hybrid Perovskites, Delivering Low Voltage Loss Photovoltaics

Joule 1 (2017) 328-343

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

© 2017 Elsevier Inc. 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 MAPbI 3−x Cl x 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 FA 0.83 MA 0.17 Pb(I 0.83 Br 0.17 ) 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 V OC of only 360 mV. Metal halide perovskites have shown tremendous promise in optoelectronic devices and are of particular interest as absorber materials in solar cells, having achieved remarkable power conversion efficiencies in a staggeringly short period of time. Although improvements in deposition techniques have greatly increased the quality of perovskite films and have allowed perovskite solar cells to dominate the class of emerging photovoltaic technologies, relatively little focus has been placed on understanding the chemistry of the precursor solutions. Here, we elucidate how the hydrolysis and thermal decomposition of N,N-dimethylformamide, the most commonly used solvent for perovskites, has far-reaching effects on the crystallization and optoelectronic properties of perovskite films and show how controlling the degradation of this solvent allows us to achieve record low voltage losses in highly efficient perovskite solar cells. The degradation of N,N-dimethylformamide results in the formation of formic acid and dimethylamine. The changes in pH that occur as a result of this solvent degradation can be correlated to changes in the colloid concentration in perovskite precursor solutions. By tuning the pH and hence colloid concentration of these solutions, we improve the crystallization and optoelectronic quality of the perovskite films, resulting in solar cells with a record low loss in potential from bandgap to V OC of 360 mV.


Modulation of Terahertz Polarization on Picosecond Timescales using Polymer-Encapsulated Semiconductor Nanowires

2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2017)

SA Baig, JL Boland, DA Damry, HH Tan, C Jagadish, HJ Joyce, MB Johnston, IEEE


An Ultrafast Switchable Terahertz Polarization Modulator Based on III-V Semiconductor Nanowires.

Nano letters 17 (2017) 2603-2610

SA Baig, JL Boland, DA Damry, HH Tan, C Jagadish, HJ Joyce, MB Johnston

Progress in the terahertz (THz) region of the electromagnetic spectrum is undergoing major advances, with advanced THz sources and detectors being developed at a rapid pace. Yet, ultrafast THz communication is still to be realized, owing to the lack of practical and effective THz modulators. Here, we present a novel ultrafast active THz polarization modulator based on GaAs semiconductor nanowires arranged in a wire-grid configuration. We utilize an optical pump-terahertz probe spectroscopy system and vary the polarization of the optical pump beam to demonstrate ultrafast THz modulation with a switching time of less than 5 ps and a modulation depth of -8 dB. We achieve an extinction of over 13% and a dynamic range of -9 dB, comparable to microsecond-switchable graphene- and metamaterial-based THz modulators, and surpassing the performance of optically switchable carbon nanotube THz polarizers. We show a broad bandwidth for THz modulation between 0.1 and 4 THz. Thus, this work presents the first THz modulator which combines not only a large modulation depth but also a broad bandwidth and picosecond time resolution for THz intensity and phase modulation, making it an ideal candidate for ultrafast THz communication.


Efficient and Air-Stable Mixed-Cation Lead Mixed-Halide Perovskite Solar Cells with n-Doped Organic Electron Extraction Layers.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

Z Wang, DP McMeekin, N Sakai, S van Reenen, K Wojciechowski, JB Patel, MB 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.


A low viscosity, low boiling point, clean solvent system for the rapid crystallisation of highly specular perovskite films

ENERGY & ENVIRONMENTAL SCIENCE 10 (2017) 145-152

NK Noel, SN Habisreutinger, B Wenger, MT Klug, MT Horantner, MB Johnston, RJ Nicholas, DT Moore, HJ Snaith


Near-Infrared and Short-Wavelength Infrared Photodiodes Based on Dye-Perovskite Composites

ADVANCED FUNCTIONAL MATERIALS 27 (2017) ARTN 1702485

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


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

JOURNAL OF PHYSICS D-APPLIED PHYSICS 50 (2017) ARTN 224001

HJ Joyce, SA Baig, P Parkinson, CL Davies, JL Boland, HH Tan, C Jagadish, LM Herz, MB Johnston


The 2017 terahertz science and technology roadmap

JOURNAL OF PHYSICS D-APPLIED PHYSICS 50 (2017) ARTN 043001

SS Dhillon, MS Vitiello, EH Linfield, AG Davies, MC Hoffmann, J Booske, C Paoloni, M Gensch, P Weightman, GP Williams, E Castro-Camus, DRS Cumming, F Simoens, I Escorcia-Carranza, J Grant, S Lucyszyn, M Kuwata-Gonokami, K Konishi, M Koch, CA Schmuttenmaer, TL Cocker, R Huber, AG Markelz, ZD Taylor, VP Wallace, JA Zeitler, J Sibik, TM Korter, B Ellison, S Rea, P Goldsmith, KB Cooper, R Appleby, D Pardo, PG Huggard, V Krozer, H Shams, M Fice, C Renaud, A Seeds, A Stoehr, M Naftaly, N Ridler, R Clarke, JE Cunningham, MB Johnston


Broadband Single-Nanowire Photoconductive Terahertz Detectors

2017 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2017)

K Peng, P Parkinson, Q Gao, JL Boland, Z Li, F Wang, YC Wenas, CL Davies, L Fu, MB Johnston, HH Tan, C Jagadish, IEEE

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