Publications


Impact of microstructure on the electron-hole interaction in lead halide perovskites

ENERGY & ENVIRONMENTAL SCIENCE 10 (2017) 1358-1366

AM Soufiani, Z Yang, T Young, A Miyata, A Surrente, A Pascoe, K Galkowski, M Abdi-Jalebi, R Brenes, J Urban, N Zhang, V Bulovic, O Portugall, Y-B Cheng, RJ Nicholas, A Ho-Baillie, MA Green, P Plochocka, SD Stranks


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.


Charge-Carrier Mobilities in Metal Halide Perovskites: Fundamental Mechanisms and Limits

ACS ENERGY LETTERS 2 (2017) 1539-1548

LM Herz


Polarisation-controlled single photon emission at high temperatures from InGaN quantum dots.

Nanoscale 9 (2017) 9421-9427

T Wang, TJ Puchtler, T Zhu, JC Jarman, LP Nuttall, RA Oliver, RA Taylor

Solid-state single photon sources with polarisation control operating beyond the Peltier cooling barrier of 200 K are desirable for a variety of applications in quantum technology. Using a non-polar InGaN system, we report the successful realisation of single photon emission with a g(2)(0) of 0.21, a high polarisation degree of 0.80, a fixed polarisation axis determined by the underlying crystallography, and a GHz repetition rate with a radiative lifetime of 357 ps at 220 K in semiconductor quantum dots. The temperature insensitivity of these properties, together with the simple planar epitaxial growth method and absence of complex device geometries, demonstrates that fast single photon emission with polarisation control can be achieved in solid-state quantum dots above the Peltier temperature threshold, making this system a potential candidate for future on-chip applications in integrated systems.


High-temperature performance of non-polar (11-20) InGaN quantum dots grown by a quasi-two-temperature method

PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS 254 (2017) ARTN 1600724

T Wang, TJ Puchtler, T Zhu, JC Jarman, RA Oliver, RA Taylor


Spectroscopic Insights into Carbon Dot Systems.

The journal of physical chemistry letters 8 (2017) 2236-2242

M Righetto, A Privitera, I Fortunati, D Mosconi, M Zerbetto, ML Curri, M Corricelli, A Moretto, S Agnoli, L Franco, R Bozio, C Ferrante

The controversial nature of the fluorescent properties of carbon dots (CDs), ascribed either to surface states or to small molecules adsorbed onto the carbon nanostructures, is an unresolved issue. To date, an accurate picture of CDs and an exhaustive structure-property correlation are still lacking. Using two unconventional spectroscopic techniques, fluorescence correlation spectroscopy (FCS) and time-resolved electron paramagnetic resonance (TREPR), we contribute to fill this gap. Although electron micrographs indicate the presence of carbon cores, FCS reveals that the emission properties of CDs are based neither on those cores nor on molecular species linked to them, but rather on free molecules. TREPR provides deeper insights into the structure of carbon cores, where C sp2 domains are embedded within C sp3 scaffolds. FCS and TREPR prove to be powerful techniques, characterizing CDs as inherently heterogeneous systems, providing insights into the nature of such systems and paving the way to standardization of these nanomaterials.


Trends in Perovskite Solar Cells and Optoelectronics: Status of Research and Applications from the PSCO Conference

ACS ENERGY LETTERS 2 (2017) 857-861

F De Angelis, D Meggiolaro, E Mosconi, A Petrozza, MK Nazeeruddin, HJ Snaith


Efficient ambient-air-stable solar cells with 2D-3D heterostructured butylammonium-caesium-formamidinium lead halide perovskites

NATURE ENERGY 2 (2017) ARTN 17135

Z Wang, Q Lin, FP Chmiel, N Sakai, LM Herz, HJ Snaith


Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites

ACS ENERGY LETTERS 2 (2017) 1621-1627

Z Yang, A Surrente, K Galkowski, A Miyata, O Portugall, RJ Sutton, AA Haghighirad, HJ Snaith, DK Maude, P Plochocka, RJ Nicholas


Single n+-i-n+ InP nanowires for highly sensitive terahertz detection.

Nanotechnology 28 (2017) 125202-

K Peng, P Parkinson, Q Gao, JL Boland, Z Li, F Wang, S Mokkapati, L Fu, MB Johnston, HH Tan, C Jagadish

Developing single-nanowire terahertz (THz) electronics and employing them as sub-wavelength components for highly-integrated THz time-domain spectroscopy (THz-TDS) applications is a promising approach to achieve future low-cost, highly integrable and high-resolution THz tools, which are desirable in many areas spanning from security, industry, environmental monitoring and medical diagnostics to fundamental science. In this work, we present the design and growth of n+-i-n+ InP nanowires. The axial doping profile of the n+-i-n+ InP nanowires has been calibrated and characterized using combined optical and electrical approaches to achieve nanowire devices with low contact resistances, on which the highly-sensitive InP single-nanowire photoconductive THz detectors have been demonstrated. While the n+-i-n+ InP nanowire detector has a only pA-level response current, it has a 2.5 times improved signal-to-noise ratio compared with the undoped InP nanowire detector and is comparable to traditional bulk THz detectors. This performance indicates a promising path to nanowire-based THz electronics for future commercial applications.


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.


Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

NANOPHOTONICS 6 (2017) 1175-1183

T Wang, TJ Puchtler, SK Patra, T Zhu, M Ali, TJ Badcock, T Ding, RA Oliver, S Schulz, RA Taylor


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.


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.].


How to Avoid Artifacts in Surface Photovoltage Measurements: A Case Study with Halide Perovskites.

The journal of physical chemistry letters 8 (2017) 2941-2943

I Levine, G Hodes, HJ Snaith, PK Nayak


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


Tuning Biocompatible Block Copolymer Micelles by Varying Solvent Composition: Core/Corona Structure and Solvent Uptake

Macromolecules 50 (2017) 4322-4334

TJ Cooksey, A Singh, KM Le, S Wang, EG Kelley, L He, S Vajjala Kesava, ED Gomez, BE Kidd, LA Madsen, ML Robertson

© 2017 American Chemical Society. Block copolymer micelles enable the formation of widely tunable self-assembled structures in liquid phases, with applications ranging from drug delivery to personal care products to nanoreactors. In order to understand fundamental aspects of micelle assembly and dynamics, the structural properties and solvent uptake of biocompatible poly(ethylene oxide-b-ϵ-caprolactone) (PEO-PCL) diblock copolymers in deuterated water (D 2 O)/tetrahydrofuran (THF-d 8 ) mixtures were investigated with a combination of small-angle neutron scattering, nuclear magnetic resonance, and transmission electron microscopy. PEO-PCL block copolymers, of varying molecular weight yet constant block ratio, formed spherical micelles through a wide range of solvent compositions. Varying the solvent composition from 10 to 60 vol % THF-d 8 in D 2 O/THF-d 8 mixtures was a convenient means of varying the core-corona interfacial tension in the micelle system. An increase in THF-d 8 content in the bulk solvent increased the solvent uptake within the micelle core, which was comparable for the two series, irrespective of the polymer molecular weight. Whereas the smaller molecular weight micelle series exhibited a decrease in aggregation number with increasing THF-d 8 content in the bulk solvent, as anticipated due to changes in the core-corona interfacial tension, the aggregation number of the larger molecular weight series was surprisingly invariant with bulk solvent composition. Differences in the dependencies of the micelle size parameters (core radius and overall micelle radius) on the solvent composition originated from the differing trends in aggregation number for the two micelle series. Incorporation of the known unimer content determined from NMR (described in the companion paper), and directly accounting for impacts of solvent swelling of the micelle core on the neutron scattering length density of the core, allowed refinement of and increased confidence in extracted micelle parameters. In summary, the two micelle series showed similar solvent uptake that was independent of the polymer molecular weight yet significantly different dependencies of their aggregation number and size parameters on the solvent composition.


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


Interplay between many body effects and Coulomb screening in the optical bandgap of atomically thin MoS2.

Nanoscale 9 (2017) 10647-10652

Y Park, SW Han, CCS Chan, BPL Reid, RA Taylor, N Kim, Y Jo, H Im, KS Kim

Due to its unique layer-number dependent electronic band structure and strong excitonic features, atomically thin MoS2 is an ideal 2D system where intriguing photoexcited-carrier-induced phenomena can be detected in excitonic luminescence. We perform micro-photoluminescence (PL) measurements and observe that the PL peak redshifts nonlinearly in mono- and bi-layer MoS2 as the excitation power is increased. The excited carrier-induced optical bandgap shrinkage is found to be proportional to n4/3, where n is the optically-induced free carrier density. The large exponent value of 4/3 is explicitly distinguished from a typical value of 1/3 in various semiconductor quantum well systems. The peculiar n4/3 dependent optical bandgap redshift may be due to the interplay between bandgap renormalization and reduced exciton binding energy.


Electron injection and scaffold effects in perovskite solar cells.

Journal of materials chemistry. C 5 (2017) 634-644

M Anaya, W Zhang, BC Hames, Y Li, F Fabregat-Santiago, ME Calvo, HJ Snaith, H Míguez, I Mora-Seró

In spite of the impressive efficiencies reported for perovskite solar cells (PSCs), key aspects of their working principles, such as electron injection at the contacts or the suitability of the utilization of a specific scaffold layer, are not yet fully understood. Increasingly complex scaffolds attained by the sequential deposition of TiO2 and SiO2 mesoporous layers onto transparent conducting substrates are used to perform a systematic characterization of both the injection process at the electron selective contact and the scaffold effect in PSCs. By forcing multiple electron injection processes at a controlled sequence of perovskite-TiO2 interfaces before extraction, interfacial injection effects are magnified and hence characterized in detail. An anomalous injection behavior is observed, the fingerprint of which is the presence of significant inductive loops in the impedance spectra with a magnitude that correlates with the number of interfaces in the scaffold. Analysis of the resistive and capacitive behavior of the impedance spectra indicates that the scaffolds could hinder ion migration, with positive consequences such as lowering the recombination rate and implications for the current-potential curve hysteresis. Our results suggest that an appropriate balance between these advantageous effects and the unavoidable charge transport resistive losses introduced by the scaffolds will help in the optimization of PSC performance.

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