Publications by Michael Johnston

Effect of ultraviolet radiation on organic photovoltaic materials and devices

ACS Applied Materials and Interfaces American Chemical Society 11 (2019) 21543-21551

J Patel, P Tiwana, N Seidler, GE Morse, OR Lozman, L Herz, M Johnston

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.

Revealing the origin of voltage loss in mixed-halide perovskite solar cells

Energy and Environmental Science Royal Society of Chemistry (2019)

S Mahesh, RDJ Oliver, JM Ball, DP McMeekin, MB Johnston, P Nayak, H Snaith

The tunable bandgap of metal-halide perovskites has opened up the possibility of tandem solar cells with over 30% efficiency. Iodide-Bromide (I-Br) mixed-halide perovskites are crucial to achieve the optimum bandgap for such tandems. However, when the Br content is increased to widen the bandgap, cells fail to deliver the expected increase in open-circuit voltage (VOC). This loss in VOC has been attributed to photo-induced halide segregation. Here, we combine Fourier Transform Photocurrent Spectroscopy (FTPS) with detailed balance calculations to quantify the voltage loss expected from the halide segregation, providing a means to quantify the VOC losses arising from the formation of low bandgap iodide-rich phases during halide segregation. Our results indicate that, contrary to popular belief, halide segregation is not the dominant VOC loss mechanism in Br-rich wide bandgap cells. Rather, the loss is dominated by the relatively low initial radiative efficiency of the cells, which arises from both imperfections within the absorber layer, and at the perovskite/charge extraction layer heterojunctions. We thus identify that focussing on maximising the initial radiative efficiency of the mixed-halide films and devices is more important than attempting to suppress halide segeregation. Our results suggest that a VOC of up to 1.33 V is within reach for a 1.77 eV bandgap perovskite, even if halide segregation cannot be supressed

Heterogeneous photon recycling and charge diffusion enhance charge transport in quasi-2D lead-halide perovskite films

Nano Letters American Chemical Society 19 (2019) 3953-3960

S Motti, T Crothers, R Yang, Y Cao, R Li, M Johnston, J Wang, L Herz

<p>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 MAPbI<sub>3</sub> 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.</p>

Impurity tracking enables enhanced control and reproducibility of hybrid perovskite vapour deposition

ACS Applied Materials and Interfaces American Chemical Society 11 (2019) 28851-28857

J Borchert, I Levchuk, M Rothmann, L Snoek, R Haver, H Snaith, CJ Brabec, L Herz, M Johnston

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible and efficient solar cells for a wide range of applications. Physical vapour deposition by co-evaporation of precursors is a method which results in very smooth and pin-hole-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) has proved extremely challenging. We show that the established method of controlling the evaporation-rate of MAI with quartz micro balances (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 MAI batch-to-batch 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 solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapour deposition will allow high solar cell device yields even if the purity of precursors change from run to run.

Solution-processed all-perovskite multi-junction solar cells

Joule Elsevier 3 (2019) 387-401

D McMeekin, S Mahesh, N Noel, J Lim, M Klug, L Herz, J Warby, J Ball, M Johnston, H Snaith

Dual-source co-evaporation of low-bandgap FA1-xCsxSn1-yPbyI3 perovskites for photovoltaics

ACS Energy Letters American Chemical Society 4 (2019) 2748-2756

JM Ball, HC Sansom, J Patel, LM Herz, M Johnston, H Snaith, L Buizza, MT Klug, J Borchert

Electronic traps and phase segregation in lead mixed-halide Perovskite

ACS Energy Letters American Chemical Society 4 (2018) 75-84

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

An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related charge carrier recombination rate to ≲105s–1 in order for halide segregation to be sufficiently suppressed.

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

Nanoscale Royal Society of Chemistry 11 (2019) 14276

ES Parrott, J Patel, AA Haghighirad, H Snaith, L Herz, M Johnston

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 Role of a Tetrafluoroborate-Based Ionic Liquid at the n-Type Oxide/Perovskite Interface


NK Noel, SN Habisreutinger, B Wenger, Y-H Lin, F Zhang, JB Patel, A Kahn, MB Johnston, HJ Snaith

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

Energy and Environmental Science Royal Society of Chemistry 12 (2018) 169-176

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

Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction can complicate interpretation of data. Here we demonstrate a new technique to measure the long-range charge carrier mobility in such materials. We combine quasi-steady-state photo-conductivity measurements (electrical probe) with photo-induced transmission and reflection measurements (optical probe) to simultaneously evaluate the conductivity and charge carrier density. With this knowledge we determine the lateral mobility to be ∼2 cm2 V−1 s−1 for CH3NH3PbI3 (MAPbI3) polycrystalline perovskite films prepared from the acetonitrile/methylamine solvent system. Furthermore, we present significant differences in long-range charge carrier mobilities, from 2.2 to 0.2 cm2 V−1 s−1, between films of contemporary perovskite compositions prepared via different fabrication processes, including solution and vapour phase deposition techniques. Arguably, our work provides the first accurate evaluation of the long-range lateral charge carrier mobility in lead halide perovskite films, with charge carrier density in the range typically achieved under photovoltaic operation.

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 American Chemical Society 18 (2018) 3703–3710-

S Sterzl, JL Boland, F Amaduzzi, H Potts, LM Herz, A Fontcuberta i Morral, M 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

Journal of Infrared, Millimeter and Terahertz Waves Springer Verlag 39 (2018) 1236–1248-

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

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17–183 cm^−1) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 ± 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.

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

Molecular Systems Design and Engineering Royal Society of Chemistry 3 (2018) 741-747

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

To date, the most efficient hybrid metal halide peroskite solar cells employ TiO2 as electron-transporting material (ETM), making these devices unstable under UV light exposure. Replacing TiO2 with fullerene derivatives has been shown to result in improved electronic contact and increased device lifetime, making it of interest to assess whether similar improvements can be achieved by using other organic semiconductors as ETMs. In this work, we investigate perylene-3,4:9,10-tetracarboxylic bis(benzimidazole) as a vacuum-processable ETM, and we minimize electron-collection losses at the electron-selective contact by depositing pentamethylcyclopentadienyl cyclopentadienyl rhodium dimer, (RhCp*Cp)2, on fluorinated tin oxide. With (RhCp*Cp)2 as an interlayer, ohmic contacts can be formed, there is interfacial doping of the ETM, and stabilized power conversion efficiencies of up to 14.2% are obtained.

Publisher Correction: High irradiance performance of metal halide perovskites for concentrator photovoltaics

Nature Energy Springer Nature America, Inc (2018)

Z Wang, Q Lin, B Wenger, M Christoforo, Y-H Lin, MT Klug, MICHAEL Johnston, LAURA Herz, HJ Snaith

© 2018, Springer Nature Limited. When this Article was originally published, an old version of the associated Supplementary Information file was uploaded. This has now been replaced.

Preface to special topic: frontiers on THz photonic devices

APL Photonics AIP Publishing 3 (2018) 051501-1

S Atakaramians, M Johnston, W Padilla, R Mendis

Terahertz (THz) photonic devices are now exploiting emerging materials systems, while novel device designs utilise plasmonic effects, nanophotinics, and metamaterials. The scope of this special topic highlights and reviews the recent cutting-edge THz photonic devices which have been revolutionised from the advances in the above research areas.

Impact of the organic cation on the optoelectronic properties of formamidinium lead triiodide

Journal of Physical Chemistry Letters American Chemical Society 9 (2018) 4502–4511-

CL Davies, J Borchert, RL Milot, CQ Xia, MB Johnston, H Kraus, L Herz

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI3 (thin film) and find remarkably different behavior across the β–γ phase transition compared with MAPbI3. While MAPbI3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI3 vary smoothly over a range of 100–160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI3 exhibits a clear T–0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron–phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T–1.5 dependence previously observed for MAPbI3. Finally, we directly observe intraexcitonic transitions in FAPbI3 at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

Interplay of structural and optoelectronic properties in formamidinium mixed tin-lead triiodide perovskites

Advanced Functional Materials Wiley 28 (2018)

ES Parrott, T Green, RL Milot, MB Johnston, HJ Snaith, LM Herz

Mixed lead-tin triiodide perovskites are promising absorber materials for low band-gap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb1-xSnxI3 perovskite are presented here as a function of lead:tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the band-gap and phase transition temperature. The large band-gap bowing parameter, a crucial element for the attainment of low band-gaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multi-phonon assisted process. These results highlight the importance of structure and composition for control of band-gap bowing and charge-carrier recombination mechanisms in low band-gap absorbers for all-perovskite tandem solar cells.

The effects of doping density and temperature on the optoelectronic properties of formamidinium tin triiodide thin films

Advanced Materials Wiley 30 (2018) 1804506-

RL Milot, MT Klug, C Davies, Z Wang, HAP Kraus, HJ Snaith, MB Johnston, LM Herz

Intrinsic and extrinsic optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI3) thin films, whose background hole doping density was 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 ~300meV beyond the band gap 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 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 cm2V-1s-1at room temperature and 470 cm2V-1s-1 at 50 K. Intra-excitonic transitions observed in the THz-frequency photoconductivity spectra at 5K reveal an exciton binding energy of only 3.1 meV for FASnI3, in agreement with the low bandgap energy exhibited by this perovskite.

High irradiance performance of metal halide perovskites for concentrator photovoltaics

Nature Energy Nature Publishing Group 3 (2018) 855–861-

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

Traditionally, III–V multi-junction cells have been used in concentrator photovoltaic (CPV) applications, which deliver extremely high efficiencies but have failed to compete with ‘flat-plate’ silicon technologies owing to cost. Here, we assess the feasibility of using metal halide perovskites for CPVs, and we evaluate their device performance and stability under concentrated light. Under simulated sunlight, we achieve a peak efficiency of 23.6% under 14 Suns (that is, 14 times the standard solar irradiance), as compared to 21.1% under 1 Sun, and measure 1.26 V open-circuit voltage under 53 Suns, for a material with a bandgap of 1.63 eV. Importantly, our encapsulated devices maintain over 90% of their original efficiency after 150 h aging under 10 Suns at maximum power point. Our work reveals the potential of perovskite CPVs, and may lead to new PV deployment strategies combining perovskites with low-concentration factor and lower-accuracy solar tracking systems.