Publications by Michael Johnston

Effect of Ultraviolet Radiation on Organic Photovoltaic Materials and Devices.

ACS applied materials & interfaces 11 (2019) 21543-21551

JB Patel, P Tiwana, N Seidler, GE Morse, OR Lozman, MB Johnston, LM Herz

Organic photovoltaics are a sustainable and cost-effective power-generation technology that may aid the move to zero-emission buildings, carbon neutral cities, and electric vehicles. While state-of-the-art organic photovoltaic devices can be encapsulated to withstand air and moisture, they are currently still susceptible to light-induced degradation, leading to a decline in the long-term efficiency of the devices. In this study, the role of ultraviolet (UV) radiation on a multilayer organic photovoltaic device is systematically uncovered using spectral filtering. By applying long-pass filters to remove different parts of the UV portion of the AM1.5G spectrum, two main photodegradation processes are shown to occur in the organic photovoltaic devices. A UV-activated process is found to cause a significant decrease in the photocurrent across the whole spectrum and is most likely linked to the deterioration of the charge extraction layers. In addition, a photodegradation process caused by UV-filtered sunlight is found to change the micromorphology of the bulk heterojunction material, leading to a reduction in photocurrent at high photon energies. These findings strongly suggest that the fabrication of inherently photostable organic photovoltaic devices will require the replacement of fullerene-based electron transporter materials with alternative organic semiconductors.

Heterogeneous Photon Recycling and Charge Diffusion Enhance Charge Transport in Quasi-2D Lead-Halide Perovskite Films.

Nano letters 19 (2019) 3953-3960

SG Motti, T Crothers, R Yang, Y Cao, R Li, MB Johnston, J Wang, LM Herz

The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. Here we show that such effects are not detrimental to the charge transport for carefully processed films comprising a self-assembled thin layer of quasi-two-dimensional (2D) perovskite interfaced with a 3D MAPbI3 perovskite layer. We apply a combination of time-resolved photoluminescence and photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited. Through modeling of the recorded dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective "heterogeneous photon recycling". We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.

Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition.

ACS applied materials & interfaces 11 (2019) 28851-28857

J Borchert, I Levchuk, LC Snoek, MU Rothmann, R Haver, HJ Snaith, CJ Brabec, LM Herz, MB Johnston

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Solution-Processed All-Perovskite Multi-junction Solar Cells

Joule 3 (2019) 387-401

DP McMeekin, S Mahesh, NK Noel, MT Klug, JC Lim, JH Warby, JM Ball, LM Herz, MB Johnston, HJ Snaith

© 2019 Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH 3 CN)/methylamine(CH 3 NH 2 ) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA 0.83 Cs 0.17 Pb(Br 0.7 I 0.3 ) 3 (1.94 eV) and MAPbI 3 (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb 0.75 Sn 0.25 I 3 (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells. Silicon-based solar cells are dominating today's solar energy market. However, their efficiencies will soon reach their maximum practical limit. Without any gains in efficiency, price reductions will become increasingly difficult to achieve. Tandem and multi-junction architectures can overcome this single-junction efficiency limit. Perovskite materials offer both band-gap tunability and solution processability. This unique combination of properties allows for fabrication of multi-junction solar cells using high-throughput deposition techniques such as blade coating, roll-to-roll, gravure coating or inkjet printing. However, these solar cells have yet to be fabricated using these deposition techniques due to difficulties in sequentially depositing these semiconductors. By utilizing an acetonitrile/methylamine-based solvent, we demonstrate the first monolithic all-perovskite multi-junction solar cells fabricated via solution processing of all active layers, apart from the electrodes. Perovskite solar cells can be processed using solution-based methods. Furthermore, perovskite solar cells can tune their band gap to absorb different portions of the solar spectrum. This property allows for fabrication of multi-junction solar cell, which can offer higher power conversion efficiencies than single-junction architecture. Here, we combine both features to fabricate the first solution-processed, monolithic, all-perovskite tandem and triple-junction solar cells.

Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite

ACS ENERGY LETTERS 4 (2019) 75-84

AJ Knight, AD Wright, JB Patel, DP McMeekin, HJ Snaith, MB Johnston, LM Herz

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films.

Nanoscale 11 (2019) 14276-14284

ES Parrott, JB Patel, A-A Haghighirad, HJ Snaith, MB Johnston, LM Herz

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2-320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

Elucidating the long-range charge carrier mobility in metal halide perovskite thin films


J Lim, MT Horantner, N Sakai, JM Ball, S Mahesh, NK Noel, Y-H Lin, JB Patel, DP McMeekin, MB Johnston, B Wenger, HJ Snaith

Raman Spectrum of the Organic-Inorganic Halide Perovskite CH3NH3PbI3 from First Principles and High-Resolution Low-Temperature Raman Measurements

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 21703-21717

MA Perez-Osorio, Q Lin, RT Phillips, RL Milot, LM Herz, MB Johnston, F Giustino

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires.

Nano letters 18 (2018) 3703-3710

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.

Temperature-Dependent Refractive Index of Quartz at Terahertz Frequencies


CL Davies, JB Patel, CQ Xia, LM Herz, MB Johnston

Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency


F Pulvirenti, B Wegner, NK Noel, G Mazzotta, R Hill, JB Patel, LM Herz, MB Johnston, MK Riede, HJ Snaith, N Koch, S Barlow, SR Marder

Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process.

Nature communications 9 (2018) 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.

Hybrid Perovskites: Prospects for Concentrator Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700792-

Q Lin, Z Wang, HJ Snaith, MB Johnston, LM Herz

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley-Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.

High irradiance performance of metal halide perovskites for concentrator photovoltaics (vol 3, pg 855, 2018)

NATURE ENERGY 3 (2018) 1013-1013

Z Wang, Q Lin, B Wenger, MG Christoforo, Y-H Lin, MT Klug, MB Johnston, LM Herz, HJ Snaith

Preface to Special Topic: Frontiers on THz photonic devices

APL PHOTONICS 3 (2018) ARTN 051501

S Atakaramians, M Johnston, W Padilla, R Mendis



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

The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films.

Advanced materials (Deerfield Beach, Fla.) 30 (2018) e1804506-

RL Milot, MT Klug, CL Davies, Z Wang, H Kraus, HJ Snaith, MB Johnston, LM Herz

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI3 ) thin films, whose background hole doping density is varied through SnF2 addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 1020 cm-3 , a strong Burstein-Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI3 (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (1020 cm-3 ), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 1019 cm-3 and below, the charge-carrier mobility increases with decreasing temperature according to ≈T-1.2 , suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 1018 cm-3 , charge-carrier mobilities reach a value of 67 cm2 V-1 s-1 at room temperature and 470 cm2 V-1 s-1 at 50 K. Intraexcitonic transitions observed in the THz-frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI3 , in agreement with the low bandgap energy exhibited by this perovskite.

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.

High irradiance performance of metal halide perovskites for concentrator photovoltaics

NATURE ENERGY 3 (2018) 855-861

Z Wang, Q Lin, B Wenger, MG Christoforo, Y-H Lin, MT Klug, MB Johnston, LM Herz, HJ Snaith

Highly Crystalline Methylammonium Lead Tribromide Perovskite Films for Efficient Photovoltaic Devices

ACS ENERGY LETTERS 3 (2018) 1233-1240

NK Noel, B Wenger, SN Habisreutinger, JB Patel, T Crothers, Z Wang, RJ Nicholas, MB Johnston, LM Herz, HJ Snaith