Publications


The Structure of VOPc on Cu(111): Does V=O Point Up, or Down, or Both?

Journal of Physical Chemistry C (2018)

PJ Blowey, RJ Maurer, LA Rochford, DA Duncan, JH Kang, DA Warr, AJ Ramadan, TL Lee, PK Thakur, G Costantini, K Reuter, DP Woodruff

© 2018 American Chemical Society. The local structure of the nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc), adsorbed on Cu(111) at a coverage of approximately one-half of a saturated molecular layer, has been investigated by a combination of normal-incidence X-ray standing waves (NIXSW), scanned-energy mode photoelectron diffraction (PhD), and density-functional theory (DFT), complemented by scanning tunnelling microscopy (STM). Qualitative assessment of the NIXSW data clearly shows that both "up" and "down" orientations of the molecule (with V=O pointing out of, and into, the surface) must coexist on the surface. O 1s PhD proves to be inconclusive regarding the molecular orientation. DFT calculations, using two different dispersion correction schemes, show good quantitative agreement with the NIXSW structural results for equal co-occupation of the two different molecular orientations and clearly favor the many body dispersion (MBD) method to deal with long-range dispersion forces. The calculated relative adsorption energies of the differently oriented molecules at the lowest coverage show a strong preference for the "up" orientation, but at higher local coverages, this energetic difference decreases, and mixed orientation phases are almost energetically equivalent to pure "up"-oriented phases. DFT-based Tersoff-Hamann simulations of STM topographs for the two orientations cast some light on the extent to which such images provide a reliable guide to molecular orientation.


Direct Observation of Ultrafast Exciton Dissociation in Lead Iodide Perovskite by 2D Electronic Spectroscopy

ACS PHOTONICS 5 (2018) 852-860

A Jha, H-G Duan, V Tiwari, PK Nayak, HJ Snaith, M Thorwart, RJD Miller


High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires.

Nano letters 18 (2018) 3703-3710

JL Boland, F Amaduzzi, S Sterzl, H Potts, LM Herz, A Fontcuberta I Morral, MB 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.


Nitride Single Photon Sources

2018 IEEE PHOTONICS CONFERENCE (IPC) (2018)

T Zhu, JC Jarman, CX Ren, F Tang, CC Kocher, TJ Puchtler, BPL Reid, T Wang, SK Patra, S Schulz, RA Taylor, RA Oliver


Photonic molecules defined by SU-8 photoresist strips on a photonic crystal waveguide.

Optics express 26 (2018) 32332-32345

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

We present experimental and numerical investigations of photonic molecules obtained from laser patterned SU-8 photoresist strips on photonic crystal waveguides. Properties of cavities defined by a single strip are investigated and we show that two adjacent strips on a waveguide form a pair of optically coupled cavities. Simulation results and micro-photoluminescence mapping measurements demonstrate that the coupling strength is tunable by controlling the separation between the strips. Confocal mapping with decoupled collection and excitation points is used to explicitly show coupling between two cavities of a photonic molecule.


Present status and future prospects of perovskite photovoltaics.

Nature materials 17 (2018) 372-376

HJ Snaith


Naphthalenetetracarboxylic Diimide Derivatives: Molecular Structure, Thin Film Properties and Solar Cell Applications

Zeitschrift fur Physikalische Chemie (2018)

C Falkenberg, M Hummert, R Meerheim, C Schünemann, S Olthof, C Körner, MK Riede, K Leo

© 2018 Walter de Gruyter GmbH, Berlin/Boston 2018. The effciency of organic solar cells is not only determined by their absorber system, but also strongly dependent on the performance of numerous interlayers and charge transport layers. In order to establish new custom-made materials, the study of structure-properties relationships is of great importance. This publication examines a series of naphthalenetetracarboxylic diimide molecules (NTCDI) with varying side-chain length intended for the use as n-dopable electron transport materials in organic solar cells. While all compounds basically share very similar absorption spectra and energy level positions in the desired range, the introduction of alkyl chains has a large impact on thin film growth and charge transport properties: both crystallization and the increase of conductivity by molecular doping are suppressed. This has a direct influence on the series resistance of corresponding solar cells comprising an NTCDI derivative as electron transport material (ETM) as it lowers the power conversion efficiency to 1%. In contrast, using the side-chain free compound it is possible to achive an efficiency of 6.5%, which is higher than the efficiency of a comparable device comprising n-doped C60as standard ETM.


In situ simultaneous photovoltaic and structural evolution of perovskite solar cells during film formation

ENERGY & ENVIRONMENTAL SCIENCE 11 (2018) 383-393

M Alsari, O Bikondoa, J Bishop, M Abdi-Jalebi, LY Ozer, M Hampton, P Thompson, MT Horantner, S Mahesh, C Greenland, JE Macdonald, G Palmisano, HJ Snaith, DG Lidzey, SD Stranks, RH Frienda, S Lilliu


Perovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic Structure

ACS ENERGY LETTERS 3 (2018) 869-874

A Karani, L Yang, S Bai, MH Futscher, HJ Snaith, B Ehrler, NC Greenham, D Di


Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide.

The journal of physical chemistry letters 9 (2018) 4502-4511

CL Davies, J Borchert, CQ Xia, RL Milot, H Kraus, MB Johnston, LM 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.


Nonspiro, Fluorene-Based, Amorphous Hole Transporting Materials for Efficient and Stable Perovskite Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700811-

Š Daškevičiū Tė, N Sakai, M Franckevičius, M Daškevičienė, A Magomedov, V Jankauskas, HJ Snaith, V Getautis

Novel nonspiro, fluorene-based, small-molecule hole transporting materials (HTMs) V1050 and V1061 are designed and synthesized using a facile three-step synthetic route. The synthesized compounds exhibit amorphous nature with a high glass transition temperature, a good solubility, and decent thermal stability. The planar perovskite solar cells (PSCs) employing V1050 generated an excellent power conversion efficiency of 18.3%, which is comparable to 18.9% obtained with the state-of-the-art Spiro-OMeTAD. Importantly, the devices based on V1050 and V1061 show better stability compared to devices based on Spiro-OMeTAD when aged without any encapsulation under uncontrolled humidity conditions (relative humidity around 60%) in the dark and under continuous full sun illumination.


Key Tradeoffs Limiting the Performance of Organic Photovoltaics

Advanced Energy Materials (2018)

I Ramirez, M Causa', Y Zhong, N Banerji, M Riede

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. 2017 saw the publication of several new material systems that challenge the long-held notion that a driving force is necessary for efficient exciton dissociation in organic photovoltaics (OPVs) and that a loss of ≈0.6 eV between the energy of the charge transfer state E ct and the energy corresponding to open circuit is general. In light of these developments, the authors combine insights from device physics and spectroscopy to review the two key tradeoffs limiting OPV performances. These are the tradeoff between the charge carrier generation efficiency and the achievable open circuit voltage (V oc ) and the tradeoff between device thickness (light absorption) and fill factor. The emergence of several competitive nonfullerene acceptors (NFAs) is exciting for both of these. The authors analyze what makes these materials compare favorably to fullerenes, including the potential role of molecular vibrations, and discuss both design criteria for new molecules and the achievable power conversion efficiencies.


DISTINGUISHING CAP AND CORE CONTRIBUTIONS TO THE PHOTOCONDUCTIVE TERAHERTZ RESPONSE OF SINGLE GaAs BASED CORE-SHELL-CAP NANOWIRE DETECTORS

LITHUANIAN JOURNAL OF PHYSICS 58 (2018) 15-23

K Peng, P Parkinson, L Fu, Q Gao, J Boland, Y-N Guo, N Jian, HH Tan, MB Johnston, C Jagadish


Interplay of Structural and Optoelectronic Properties in Formamidinium Mixed Tin-Lead Triiodide Perovskites

ADVANCED FUNCTIONAL MATERIALS 28 (2018) ARTN 1802803

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


New Generation Hole Transporting Materials for Perovskite Solar Cells: Amide-Based Small-Molecules with Nonconjugated Backbones

ADVANCED ENERGY MATERIALS 8 (2018) ARTN 1801605

ML Petrus, K Schutt, MT Sirtl, EM Hutter, AC Closs, JM Ball, JC Bijleveld, A Petrozza, T Bein, TJ Dingemans, TJ Savenije, H Snaith, P Docampo


Meso-Superstructured Perovskite Solar Cells: Revealing the Role of the Mesoporous Layer

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 21239-21247

D Ramirez, K Schutt, JF Montoya, S Mesa, J Lim, HJ Snaith, F Jaramillo


Exciton-Dominated Core-Level Absorption Spectra of Hybrid Organic-Inorganic Lead Halide Perovskites.

The journal of physical chemistry letters 9 (2018) 1852-1858

C Vorwerk, C Hartmann, C Cocchi, G Sadoughi, SN Habisreutinger, R Félix, RG Wilks, HJ Snaith, M Bär, C Draxl

In a combined theoretical and experimental work, we investigate X-ray absorption near-edge structure spectroscopy of the I L3 and the Pb M5 edges of the methylammonium lead iodide (MAPbI3) hybrid inorganic-organic perovskite and its binary phase PbI2. The absorption onsets are dominated by bound excitons with sizable binding energies of a few hundred millielectronvolts and pronounced anisotropy. The spectra of both materials exhibit remarkable similarities, suggesting that the fingerprints of core excitations in MAPbI3 are essentially given by its inorganic component, with negligible influence from the organic groups. The theoretical analysis complementing experimental observations provides the conceptual insights required for a full characterization of this complex material.


Template-Directed Synthesis of a Conjugated Zinc Porphyrin Nanoball.

Journal of the American Chemical Society 140 (2018) 5352-5355

J Cremers, R Haver, M Rickhaus, JQ Gong, L Favereau, MD Peeks, TDW Claridge, LM Herz, HL Anderson

We report the template-directed synthesis of a π-conjugated 14-porphyrin nanoball. This structure consists of two intersecting nanorings containing six and 10 porphyrin units. Fluorescence upconversion spectroscopy experiments demonstrate that electronic excitation delocalizes over the whole three-dimensional π system in less than 0.3 ps if the nanoball is bound to its templates or over 2 ps if the nanoball is empty.


Carrier confinement effects of InxGa1-xN/GaN multi quantum disks with GaN surface barriers grown in GaN nanorods

OPTICAL MATERIALS 78 (2018) 365-369

Y Park, CCS Chan, RA Taylor, N Kim, Y Jo, SW Lee, W Yang, H Im


Highly Crystalline Methylammonium Lead Tribromide Perovskite Films for Efficient Photovoltaic Devices

ACS ENERGY LETTERS 3 (2018) 1233-1240

NK Noel, B Wenger, SN Habisreutinger, JB Patel, T Crothers, Z Wang, RJ Nicholas, MB Johnston, LM Herz, HJ Snaith

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