Publications by Michael Johnston

Control over crystal size in vapor deposited metal-halide perovskite films

ACS Energy Letters American Chemical Society (ACS) 5 (2020) 0c00183

KB Lohmann, JB Patel, MU Rothmann, CQ Xia, RDJ Oliver, LM Herz, HJ Snaith, MB Johnston

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.

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

Energy and Environmental Science Royal Society of Chemistry (2019)

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

Three-dimensional cross-nanowire networks recover full terahertz state

Science American Association for the Advancement of Science 368 (2020) 510-513

K Peng, D Jevtics, F Zhang, S Sterzl, D Damry, M Rothmann, B Guilhabert, MJ Strain, HH Tan, LM Herz, L Fu, MD Dawson, A Hurtado, C Jagadish, M Johnston

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

A piperidinium salt stabilizes efficient metal-halide perovskite solar cells.

Science (New York, N.Y.) Nature Research 369 (2020) 96-102

Y-H Lin, N Sakai, P Da, J Wu, HC Sansom, AJ Ramadan, S Mahesh, J Liu, RDJ Oliver, J Lim, L Aspitarte, K Sharma, P Madhu, AB Morales-Vilches, PK Nayak, S Bai, F Gao, CRM Grovenor, MB Johnston, JG Labram, JR Durrant, JM Ball, B Wenger, B Stannowski, HJ Snaith

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.

Charge-carrier trapping dynamics in bismuth-doped thin films of MAPbBr3 perovskite

Journal of Physical Chemistry Letters American Chemical Society (ACS) 11 (2020) 3681-3688

AM Ulatowski, AD Wright, B Wenger, LRV Buizza, SG Motti, HJ Eggimann, KJ Savill, J Borchert, HJ Snaith, MB Johnston, LM Herz

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials.

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

Trap states, electric fields, and phase segregation in mixed-halide perovskite photovoltaic devices

Advanced Energy Materials Wiley 10 (2020) 1903488

A Knight, J Patel, H Snaith, M Johnston, L Herz

Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.

Three-dimensional cross-nanowire networks recover full terahertz state.

Science (New York, N.Y.) 368 (2020) 510-513

K Peng, D Jevtics, F Zhang, S Sterzl, DA Damry, MU Rothmann, B Guilhabert, MJ Strain, HH Tan, LM Herz, L Fu, MD Dawson, A Hurtado, C Jagadish, MB Johnston

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.

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, M Johnston, L 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 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, L Snoek, M Rothmann, 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.

Charge-carrier dynamics, mobilities and diffusion lengths of 2D-3D hybrid butylammonium-caesium-formamidinium lead halide perovskites

Advanced Functional Materials Wiley (2019)

L Buizza, T Crothers, Z Wang, P Jay, R Milot, H Snaith, M Johnston, L Herz

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature‐ and humidity‐induced instability. Materials that combine high charge‐carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BAx(FA0.83Cs0.17)1‐xPb(I0.6Br0.4)3 is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap‐mediated charge‐carrier recombination and enhancing charge‐carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge‐carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge‐carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials.

Solution-processed all-perovskite multi-junction solar cells

Joule Elsevier 3 (2019) 387-401

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

Unveiling Temperature-Dependent Scattering Mechanisms in Semiconductor Nanowires Using Optical-Pump Terahertz-Probe Spectroscopy

International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2019-September (2019)

JL Boland, F Amaduzzi, S Sterzl, H Potts, G Tutuncuoglu, LM Herz, AFI Morral, MB Johnston

© 2019 IEEE. Optical-pump terahertz-probe (OPTP) spectroscopy is a powerful, non-contact tool for extracting the electrical conductivity within a material. In this work, we show how OPTP spectroscopy can be used to extract the temperature-dependent electron mobility and photoconductivity lifetime within semiconductor nanowires (NWs), in order to reveal the underlying scattering mechanisms governing carrier transport in these materials.

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

Electronic traps and phase segregation in lead mixed-halide Perovskite

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

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

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, M Johnston, L 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

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

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

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

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.