Publications


Carbon Nanotubes in Perovskite Solar Cells

ADVANCED ENERGY MATERIALS 7 (2017) ARTN 1601839

SN Habisreutinger, RJ Nicholas, HJ Snaith


Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High-Efficiency Perovskite Solar Cells.

Small (Weinheim an der Bergstrasse, Germany) 13 (2017)

N Sakai, Z Wang, VM Burlakov, J Lim, D McMeekin, S Pathak, HJ Snaith

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.


The entangled triplet pair state in acene and heteroacene materials.

Nature communications 8 (2017) 15953-

CK Yong, AJ Musser, SL Bayliss, S Lukman, H Tamura, O Bubnova, RK Hallani, A Meneau, R Resel, M Maruyama, S Hotta, LM Herz, D Beljonne, JE Anthony, J Clark, H Sirringhaus

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.


Quasi-one-dimensional density of states in a single quantum ring.

Scientific reports 7 (2017) 40026-

H Kim, W Lee, S Park, K Kyhm, K Je, RA Taylor, G Nogues, LS Dang, JD Song

Generally confinement size is considered to determine the dimensionality of nanostructures. While the exciton Bohr radius is used as a criterion to define either weak or strong confinement in optical experiments, the binding energy of confined excitons is difficult to measure experimentally. One alternative is to use the temperature dependence of the radiative recombination time, which has been employed previously in quantum wells and quantum wires. A one-dimensional loop structure is often assumed to model quantum rings, but this approximation ceases to be valid when the rim width becomes comparable to the ring radius. We have evaluated the density of states in a single quantum ring by measuring the temperature dependence of the radiative recombination of excitons, where the photoluminescence decay time as a function of temperature was calibrated by using the low temperature integrated intensity and linewidth. We conclude that the quasi-continuous finely-spaced levels arising from the rotation energy give rise to a quasi-one-dimensional density of states, as long as the confined exciton is allowed to rotate around the opening of the anisotropic ring structure, which has a finite rim width.


Crystallization Kinetics and Morphology Control of Formamidinium-Cesium Mixed-Cation Lead Mixed-Halide Perovskite via Tunability of the Colloidal Precursor Solution.

Advanced materials (Deerfield Beach, Fla.) 29 (2017)

DP McMeekin, Z Wang, W Rehman, F Pulvirenti, JB Patel, NK Noel, MB Johnston, SR Marder, LM Herz, HJ Snaith

The meteoric rise of the field of perovskite solar cells has been fueled by the ease with which a wide range of high-quality materials can be fabricated via simple solution processing methods. However, to date, little effort has been devoted to understanding the precursor solutions, and the role of additives such as hydrohalic acids upon film crystallization and final optoelectronic quality. Here, a direct link between the colloids concentration present in the [HC(NH2 )2 ]0.83 Cs0.17 Pb(Br0.2 I0.8 )3 precursor solution and the nucleation and growth stages of the thin film formation is established. Using dynamic light scattering analysis, the dissolution of colloids over a time span triggered by the addition of hydrohalic acids is monitored. These colloids appear to provide nucleation sites for the perovskite crystallization, which critically impacts morphology, crystal quality, and optoelectronic properties. Via 2D X-ray diffraction, highly ordered and textured crystals for films prepared from solutions with lower colloidal concentrations are observed. This increase in material quality allows for a reduction in microstrain along with a twofold increase in charge-carrier mobilities leading to values exceeding 20 cm2 V-1 s-1 . Using a solution with an optimized colloidal concentration, devices that reach current-voltage measured power conversion efficiency of 18.8% and stabilized efficiency of 17.9% are fabricated.


Tailoring metal halide perovskites through metal substitution: influence on photovoltaic and material properties

ENERGY & ENVIRONMENTAL SCIENCE 10 (2017) 236-246

MT Klug, A Osherov, AA Haghighirad, SD Stranks, PR Brown, S Bai, JT-W Wang, X Dang, V Bulovic, HJ Snaith, AM Belcher


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

The Review of scientific instruments 88 (2017) 103901-

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


Band-Tail Recombination in Hybrid Lead Iodide Perovskite

ADVANCED FUNCTIONAL MATERIALS 27 (2017) ARTN 1700860

AD Wright, RL Milot, GE Eperon, HJ Snaith, MB Johnston, LM Herz


Unveiling the Influence of pH on the Crystallization of Hybrid Perovskites, Delivering Low Voltage Loss Photovoltaics

Joule 1 (2017) 328-343

NK Noel, M Congiu, AJ Ramadan, S Fearn, DP McMeekin, JB Patel, MB Johnston, B Wenger, HJ Snaith

© 2017 Elsevier Inc. 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 MAPbI3−xClx 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. Metal halide perovskites have shown tremendous promise in optoelectronic devices and are of particular interest as absorber materials in solar cells, having achieved remarkable power conversion efficiencies in a staggeringly short period of time. Although improvements in deposition techniques have greatly increased the quality of perovskite films and have allowed perovskite solar cells to dominate the class of emerging photovoltaic technologies, relatively little focus has been placed on understanding the chemistry of the precursor solutions. Here, we elucidate how the hydrolysis and thermal decomposition of N,N-dimethylformamide, the most commonly used solvent for perovskites, has far-reaching effects on the crystallization and optoelectronic properties of perovskite films and show how controlling the degradation of this solvent allows us to achieve record low voltage losses in highly efficient perovskite solar cells. The degradation of N,N-dimethylformamide results in the formation of formic acid and dimethylamine. The changes in pH that occur as a result of this solvent degradation can be correlated to changes in the colloid concentration in perovskite precursor solutions. By tuning the pH and hence colloid concentration of these solutions, we improve the crystallization and optoelectronic quality of the perovskite films, resulting in solar cells with a record low loss in potential from bandgap to VOC of 360 mV.


Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%

NATURE ENERGY 2 (2017) ARTN 17052

Y Yan, RW Crisp, J Gu, BD Chernomordik, GF Pach, AR Marshall, JA Turner, MC Beard


A Nanophotonic Structure Containing Living Photosynthetic Bacteria.

Small (Weinheim an der Bergstrasse, Germany) 13 (2017)

D Coles, LC Flatten, T Sydney, E Hounslow, SK Saikin, A Aspuru-Guzik, V Vedral, JK-H Tang, RA Taylor, JM Smith, DG Lidzey

Photosynthetic organisms rely on a series of self-assembled nanostructures with tuned electronic energy levels in order to transport energy from where it is collected by photon absorption, to reaction centers where the energy is used to drive chemical reactions. In the photosynthetic bacteria Chlorobaculum tepidum, a member of the green sulfur bacteria family, light is absorbed by large antenna complexes called chlorosomes to create an exciton. The exciton is transferred to a protein baseplate attached to the chlorosome, before migrating through the Fenna-Matthews-Olson complex to the reaction center. Here, it is shown that by placing living Chlorobaculum tepidum bacteria within a photonic microcavity, the strong exciton-photon coupling regime between a confined cavity mode and exciton states of the chlorosome can be accessed, whereby a coherent exchange of energy between the bacteria and cavity mode results in the formation of polariton states. The polaritons have energy distinct from that of the exciton which can be tuned by modifying the energy of the optical modes of the microcavity. It is believed that this is the first demonstration of the modification of energy levels within living biological systems using a photonic structure.


In-situ observation of stacking fault evolution in vacuum-deposited C-60

APPLIED PHYSICS LETTERS 111 (2017) ARTN 233305

JFM Hardigree, IR Ramirez, G Mazzotta, C Nicklin, M Riede


Electrically tunable organic-inorganic hybrid polaritons with monolayer WS2.

Nature communications 8 (2017) 14097-

LC Flatten, DM Coles, Z He, DG Lidzey, RA Taylor, JH Warner, JM Smith

Exciton-polaritons are quasiparticles consisting of a linear superposition of photonic and excitonic states, offering potential for nonlinear optical devices. The excitonic component of the polariton provides a finite Coulomb scattering cross section, such that the different types of exciton found in organic materials (Frenkel) and inorganic materials (Wannier-Mott) produce polaritons with different interparticle interaction strength. A hybrid polariton state with distinct excitons provides a potential technological route towards in situ control of nonlinear behaviour. Here we demonstrate a device in which hybrid polaritons are displayed at ambient temperatures, the excitonic component of which is part Frenkel and part Wannier-Mott, and in which the dominant exciton type can be switched with an applied voltage. The device consists of an open microcavity containing both organic dye and a monolayer of the transition metal dichalcogenide WS2. Our findings offer a perspective for electrically controlled nonlinear polariton devices at room temperature.


Structure-Activity Correlations for Bronsted Acid, Lewis Acid, and Photocatalyzed Reactions of Exfoliated Crystalline Niobium Oxides

CHEMCATCHEM 9 (2017) 144-154

Y Koito, GJ Rees, JV Hanna, MMJ Li, Y-K Peng, T Puchtler, R Taylor, T Wang, H Kobayashi, IF Teixeira, MA Khan, HT Kreissl, SCE Tsang


Close-Packed Spherical Morphology in an ABA Triblock Copolymer Aligned with Large-Amplitude Oscillatory Shear

Macromolecules 49 (2016) 4875-4888

S Wang, R Xie, S Vajjala Kesava, ED Gomez, EW Cochran, ML Robertson

© 2016 American Chemical Society. A microphase-separated poly(styrene-b-(lauryl-co-stearyl acrylate)-b-styrene) (SAS) triblock copolymer exhibiting a disordered spherical microstructure with randomly oriented grains was aligned through the application of large-amplitude oscillatory shear (LAOS) at a temperature below the order-disorder transition temperature of the triblock copolymer, yet above the glass transition temperature of the polystyrene spherical domains. The thermoplastic elastomeric behavior of the SAS triblock copolymer provided a convenient means to observe the aligned morphology. Following application of LAOS, the specimen was quenched to room temperature (below the glass transition temperature of polystyrene), and small-angle X-ray scattering data were obtained in the three principal shear directions: shear gradient, velocity, and vorticity directions. The analysis revealed that the SAS triblock copolymer formed coexisting face-centered cubic and hexagonally close-packed spherical microstructures. The presence of a close-packed microstructure is in stark contrast to an extensive body of literature on sphere-forming bulk block copolymers that favor body-centered cubic systems under quiescent conditions and under shear. The aligned microstructure observed in this bulk block copolymer was reminiscent of that observed in various spherical soft material systems such as colloidal spheres, sphere-forming block copolymer solutions, and star polymer solutions. The highly unanticipated observation of close-packed spherical microstructures in a neat block copolymer under shear is hypothesized to originate from the dispersity of the block copolymer.


Nongeminate radiative recombination of free charges in cation-exchanged PbS quantum dot films

CHEMICAL PHYSICS 471 (2016) 75-80

AR Marshall, MC Beard, JC Johnson


Efficient perovskite solar cells by metal ion doping

ENERGY & ENVIRONMENTAL SCIENCE 9 (2016) 2892-2901

JT-W Wang, Z Wang, S Pathak, W Zhang, DW deQuilettes, F Wisnivesky-Rocca-Rivarola, J Huang, PK Nayak, JB Patel, HAM Yusof, Y Vaynzof, R Zhu, I Ramirez, J Zhang, C Ducati, C Grovenor, MB Johnston, DS Ginger, RJ Nicholas, HJ Snaith


Enhanced Efficiency and Stability of Perovskite Solar Cells Through Nd-Doping of Mesostructured TiO2

ADVANCED ENERGY MATERIALS 6 (2016) ARTN 1501868

B Roose, KC Godel, S Pathak, A Sadhanala, JPC Baena, BD Wilts, HJ Snaith, U Wiesner, M Gratzel, U Steiner, A Abate


Broadband Phase-Sensitive Single InP Nanowire Photoconductive Terahertz Detectors.

Nano letters 16 (2016) 4925-4931

K Peng, P Parkinson, JL Boland, Q Gao, YC Wenas, CL Davies, Z Li, L Fu, MB Johnston, HH Tan, C Jagadish

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for materials characterization and imaging. A trend toward size reduction, higher component integration, and performance improvement for advanced THz-TDS systems is of increasing interest. The use of single semiconducting nanowires for terahertz (THz) detection is a nascent field that has great potential to realize future highly integrated THz systems. In order to develop such components, optimized material optoelectronic properties and careful device design are necessary. Here, we present antenna-optimized photoconductive detectors based on single InP nanowires with superior properties of high carrier mobility (∼1260 cm(2) V(-1) s(-1)) and low dark current (∼10 pA), which exhibit excellent sensitivity and broadband performance. We demonstrate that these nanowire THz detectors can provide high quality time-domain spectra for materials characterization in a THz-TDS system, a critical step toward future application in advanced THz-TDS system with high spectral and spatial resolution.


Gain Spectroscopy and Tunable Single Mode Lasing of Solution-Based Quantum Dots and Nanoplatelets Using Tunable Open Microcavities

2016 CONFERENCE ON LASERS AND ELECTRO-OPTICS (CLEO) (2016)

RK Patel, AAP Trichet, DM Coles, PR Dolan, SM Fairclough, SCE Tsang, MA Leontiadou, DJ Binks, E Jang, H Jang, RA Taylor, S Christodoulou, I Moreels, JM Smith, IEEE

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