Publications


Consolidation of the optoelectronic properties of CH3NH3PbBr3 perovskite single crystals.

Nature Communications Springer Nature 8 (2017) 590-

B Wenger, P Nayak, X Wen, SV Kesava, NK Noel, HJ Snaith

Ultralow trap densities, exceptional optical and electronic properties have been reported for lead halide perovskites single crystals; however, ambiguities in basic properties, such as the band gap, and the electronic defect densities in the bulk and at the surface prevail. Here, we synthesize single crystals of methylammonium lead bromide (CH3NH3PbBr3), characterise the optical absorption and photoluminescence and show that the optical properties of single crystals are almost identical to those of polycrystalline thin films. We observe significantly longer lifetimes and show that carrier diffusion plays a substantial role in the photoluminescence decay. Contrary to many reports, we determine that the trap density in CH3NH3PbBr3 perovskite single crystals is 1015 cm-3, only one order of magnitude lower than in the thin films. Our enhanced understanding of optical properties and recombination processes elucidates ambiguities in earlier reports, and highlights the discrepancies in the estimation of trap densities from electronic and optical methods.Metal halide perovskites for optoelectronic devices have been extensively studied in two forms: single-crystals or polycrystalline thin films. Using spectroscopic approaches, Wenger et al. show that polycrystalline thin films possess similar optoelectronic properties to single crystals.


A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells.

Science (New York, N.Y.) 358 (2017) 1192-1197

Y Hou, X Du, S Scheiner, DP McMeekin, Z Wang, N Li, MS Killian, H Chen, M Richter, I Levchuk, N Schrenker, E Spiecker, T Stubhan, NA Luechinger, A Hirsch, P Schmuki, H-P Steinrück, RH Fink, M Halik, HJ Snaith, CJ Brabec

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WO x )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WO x -doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.


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.


Large-area, highly uniform evaporated formamidinium lead triiodide thin-films for solar cells

ACS Energy Letters American Chemical Society 2 (2017) 2799-2804

J Borchert, R Milot, JB Patel, CL Davies, A Wright, LM Maestro, HJ Snaith, LM Hertz, M Johnston

Perovskite thin-film solar cells are one of the most promising emerging renewable energy technologies because of their potential for low-cost, large-area fabrication combined with high energy conversion efficiencies. Recently, formamidinium lead triiodide (FAPbI3) and other formamidinium (CH(NH2)2) based perovskites have been explored as interesting alternatives to methylammonium lead triiodide (MAPbI3) because they exhibit better thermal stability. However, at present a major challenge is the scale-up of perovskite solar cells from small test-cells to full solar modules. We show that coevaporation is a scalable method for the deposition of homogeneous FAPbI3 thin films over large areas. The method allows precise control over film thickness and results in highly uniform, pinhole-free layers. Our films exhibited a high charge-carrier mobility of 26 cm2 V–1s–1, excellent optical properties, and a bimolecular recombination constant of 7 × 10–11 cm3 s–1. Solar cells fabricated using these vapor-deposited layers within a regular device architecture produced stabilized power conversion efficiencies of up to 14.2%. Thus, we demonstrate that efficient FAPbI3 solar cells can be vapor-deposited, which opens up a pathway toward large-area stable perovskite photovoltaics.


Photon Reabsorption 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 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.


CF2-bridged C60 dimers and their optical transitions

ChemPhysChem Wiley 18 (2017) 3540-3543

P Dallas, S Zhou, S Cornes, H Niwa, Y Nakanishi, Y Kino, T Puchtler, R Taylor, GAD Briggs, H Shinohara, K Porfyrakis

Fullerene dyads bridged with perfluorinated linking groups have been synthesized through a modified arc-discharge procedure. The addition of Teflon inside an arc-discharge reactor leads to the formation of dyads, consisting of two C60 fullerenes bridged by -CF2- groups. The bridging groups consisting of electronegative atoms, lead to different energy levels and to new features in the photoluminescence spectrum. A suppression of the singlet oxygen photosensitization, indicated that the radiative decay from singlet to singlet state is favoured against the intersystem crossing singlet to triplet transition.


Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells.

Science advances 3 (2017) eaao4204-eaao4204

EM Sanehira, AR Marshall, JA Christians, SP Harvey, PN Ciesielski, LM Wheeler, P Schulz, LY Lin, MC Beard, JM Luther

We developed lead halide perovskite quantum dot (QD) films with tuned surface chemistry based on A-site cation halide salt (AX) treatments. QD perovskites offer colloidal synthesis and processing using industrially friendly solvents, which decouples grain growth from film deposition, and at present produce larger open-circuit voltages (VOC's) than thin-film perovskites. CsPbI3 QDs, with a tunable bandgap between 1.75 and 2.13 eV, are an ideal top cell candidate for all-perovskite multijunction solar cells because of their demonstrated small VOC deficit. We show that charge carrier mobility within perovskite QD films is dictated by the chemical conditions at the QD-QD junctions. The AX treatments provide a method for tuning the coupling between perovskite QDs, which is exploited for improved charge transport for fabricating high-quality QD films and devices. The AX treatments presented here double the film mobility, enabling increased photocurrent, and lead to a record certified QD solar cell efficiency of 13.43%.


MINERVA: A facility to study Microstructure and INterface Evolution in Realtime under VAcuum

Review of Scientific Instruments AIP Publishing 88 (2017) 103901-

C Nicklin, J Martinez Hardigree, A Warne, S Green, M Burt, J Naylor, A Dorman, D Wicks, S Din, MK Riede

A sample environment to enable real-time X-ray scattering measurements to be recorded during the growth of materials by thermal evaporation in vacuum is presented. The in-situ capabilities include studying microstructure development with time or during exposure to different environmental conditions, such as temperature and gas pressure. The chamber provides internal slits and a beam stop, to reduce the background scattering from the X-rays passing through the entrance and exit windows, together with highly controllable flux rates of the evaporants. Initial experiments demonstrate some of the possibilities by monitoring the growth of bathophenanthroline (BPhen), a common molecule used in organic solar cells and organic light emitting diodes, including the development of the microstructure with time and depth within the film. The results show how BPhen nanocrystal structures coarsen at room temperature under vacuum, highlighting the importance of using real time measurements to understand the as deposited pristine film structure and its development with time. More generally, this sample environment is versatile and can be used for investigation of structure-property relationships in a wide range of vacuum deposited materials and their applications in, for example, optoelectronic devices and energy storage.


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.


THz modulators and detectors based on semiconductor nanowires

2017 42nd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz) IEEE (2017)

M Johnston

Semiconductors nanowires have to potential to be building blocks for future nano-optoelectronic devices. We have recently demonstrated high performance THz photonic devices based on GaAs and InP nanowires. These include ultrafast optically switched modulators of THz radiation and single nanowire photoconductive detectors of THz pulses.


Charge-carrier dynamics in hybrid metal halide perovskites for photovoltaics and light emission

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

RL Milot, MB Johnston, LM Herz

© 2017 IEEE. Hybrid metal halide perovskite materials are showing great promise as the active layers of thin-film solar cells and as light emitters. Using optical-pump/THz-probe spectroscopy, we have investigated the charge-carrier dynamics and mobility in two-dimensional and tin-based hybrid metal halide perovskites for photovoltaic and light emission applications.


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.


The potential of multijunction perovskite solar cells

ACS Energy Letters American Chemical Society 2 (2017) 2506-2513

MT Hörantner, T Leijtens, ME Ziffer, GE Eperon, M Christoforo, HJ Snaith

Metal halide perovskite semiconductors offer rapid, low-cost deposition of solar cell active layers with a wide range of band gaps, making them ideal candidates for multijunction solar cells. Here, we combine optical and electrical models using experimental inputs to evaluate the feasible performances of all-perovskite double-junction (2PJ), triple-junction (3PJ), and perovskite-perovskite-silicon triple-junction (2PSJ) solar cells. Using parameters and design constraints from the current state-of-the-art generation of perovskite solar cells, we find that 2PJs can feasibly approach 32% power conversion efficiency, 3PJs can reach 33%, and 2PSJs can surpass 35%. We also outline pathways to improve light harvesting and demonstrate that it is possible to raise the performances to 34%, 37%, and 39% for the three architectures. Additionally, we discuss important future directions of research. Finally, we perform energy yield modeling to demonstrate that the multijunction solar cells should not suffer from reduced operational performances due to discrepancies between the AM1.5G and real-world spectrum over the course of a year.


Modulation of terahertz polarization on picosecond timescales using polymer-encapsulated semiconductor nanowires

Conference on Lasers and Electro-Optics (CLEO 2017) Optical Society of America 2017-January (2017) 1-2

J Boland, D Damry, HH Tan, C Jagadish, HJ Joyce, M Johnston

We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer.


Modulation of terahertz polarization on picosecond timescales using polymer-encapsulated semiconductor nanowires

2017 Conference on Lasers and Electro-Optics, CLEO 2017 - Proceedings 2017-January (2017) 1-2

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

© OSA 2017. We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer.


Synthesis, photophysical, electrochemical and electroluminescence studies of red emitting phosphorescent Ir(III) heteroleptic complexes

Journal of Chemical Sciences 129 (2017) 1391-1398

F Ali, PK Nayak, N Periasamy, N Agarwal

© 2017, Indian Academy of Sciences. Abstract: Five heteroleptic, cyclometalated (C∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline (piq) derivatives, Ir(acac)(piq)2, Ir(acac)(2,4-difluoro-piq)2, Ir(acac)(4-trifluoromethyl-piq)2, Ir(acac)(4-N,N-dimethyl-piq)2, Ir(acac)(4-acetyl-piq)2, were synthesized and characterized. The (C ∧N ) 2Ir(acac) complexes in toluene showed phosphorescence (λ max= 598 nm to 658 nm) with quantum yields (0.1 to 0.32) and microsecond lifetimes (0.43 to 1.9 μ s). The complexes were non-luminescent in thin films due to self-quenching but luminescent when lightly doped (5%) in a host organic material, 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP). The HOMO levels determined using cyclic voltammetric oxidation potentials were in the range −5.48 to −5.80 eV. Electroluminescence properties and performance of the Ir complexes doped in CBP (active layer) were studied in a multilayer (ITO/F4TCNQ/TPD/doped CBP/BCP/LiF/Al) organic light emitting device (OLED). The electroluminescense (EL) spectra of the device matched with the phosphorescent spectra of the Ir complexes. The turn-on voltage at ∼ 4.5 V, maximum brightness of 7600 cd/m 2 and current efficiency of ∼ 7.0 cd/A at a brightness of ∼ 100 cd/m 2 indicate that these are promising OLED materials. GRAPHICAL ABSTRACT: Synopsis. Heteroleptic, cyclometalated (C∧N) Iridium(III) complexes of acetylacetone (acac) and 1-phenyl-isoquinoline were synthesized and their photophysical, electrochemical and electroluminescence properties were studied. The OLED of Ir complex as emitting material showed turn-on voltage at ∼ 4.5 V, maximum brightness of 7600 cd/m 2 and current efficiency of ∼ 7.0 cd/A at a brightness of ∼ 100 cd/m 2.[Figure not available: see fulltext.].


Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures.

Scientific reports 7 (2017) 12067-12067

T Wang, TJ Puchtler, SK Patra, T Zhu, JC Jarman, RA Oliver, S Schulz, RA Taylor

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions.


Optical fabrication and characterisation of SU-8 disk photonic waveguide heterostructure cavities.

Optics express 25 (2017) 24615-24622

LP Nuttall, FSF Brossard, SA Lennon, BPL Reid, J Wu, J Griffiths, RA Taylor

In order to demonstrate cavity quantum electrodynamics using photonic crystal (PhC) cavities fabricated around self-assembled quantum dots (QDs), reliable spectral and spatial overlap between the cavity mode and the quantum dot is required. We present a method for using photoresist to optically fabricate heterostructure cavities in a PhC waveguide with a combined photolithography and micro-photoluminescence spectroscopy system. The system can identify single QDs with a spatial precision of ±25 nm, and we confirm the creation of high quality factor cavity modes deterministically placed with the same spatial precision. This method offers a promising route towards bright, on-chip single photon sources for quantum information applications.


Hybrid organic/inorganic perovskite-polymer nanocomposites: toward the enhancement of structural and electrical properties

Journal of Physical Chemistry Letters American Chemical Society 8 (2017) 5981-5986

A Privitera, M Righetto, M De Bastiani, F Carraro, M Rancan, L Armelao, G Granozzi, R Bozio, L Franco

Hybrid organic/inorganic perovskite nanoparticles (NPs) have garnered remarkable research attention because of their promising photophysical properties. New and interesting properties emerge after combining perovskite NPs with semiconducting materials. Here, we report the synthesis and investigation of a composite material obtained by mixing CH3NH3PbBr3 nanocrystals with the semiconducting polymer poly(3-hexylthiophene) (P3HT). By the combination of structural techniques and optical and magnetic spectroscopies we observed multiple effects of the perovskite NPs on the P3HT: (i) an enlargement of P3HT crystalline domains, (ii) a strong p-doping of the P3HT, and (iii) an enhancement of interchain order typical of H-aggregates. These observations open a new avenue toward innovative perovskite NP-based applications.


Organic molecule fluorescence as an experimental test-bed for quantum jumps in thermodynamics.

Proceedings. Mathematical, physical, and engineering sciences 473 (2017) 20170099-20170099

C Browne, T Farrow, OCO Dahlsten, RA Taylor, V Vlatko

We demonstrate with an experiment how molecules are a natural test bed for probing fundamental quantum thermodynamics. Single-molecule spectroscopy has undergone transformative change in the past decade with the advent of techniques permitting individual molecules to be distinguished and probed. We demonstrate that the quantum Jarzynski equality for heat is satisfied in this set-up by considering the time-resolved emission spectrum of organic molecules as arising from quantum jumps between states. This relates the heat dissipated into the environment to the free energy difference between the initial and final state. We demonstrate also how utilizing the quantum Jarzynski equality allows for the detection of energy shifts within a molecule, beyond the relative shift.

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