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.


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


Light Controlled Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

Nano letters 18 (2018) 6188-6194

H Kim, S Park, R Okuyama, K Kyhm, M Eto, RA Taylor, G Nogues, LS Dang, M Potemski, K Je, J Kim, J Kyhm, J Song

We found that optical Aharonov-Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm-2, the optical Aharonov-Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm-2), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.


Aligned and Graded Type-II Ruddlesden-Popper Perovskite Films for Efficient Solar Cells

ADVANCED ENERGY MATERIALS 8 (2018) ARTN 1800185

J Qing, X-K Liu, M Li, F Liu, Z Yuan, E Tiukalova, Z Yan, M Duchamp, S Chen, Y Wang, S Bai, J-M Liu, HJ Snaith, C-S Lee, TC Sum, F Gao


The effect of ionic composition on acoustic phonon speeds in hybrid perovskites from Brillouin spectroscopy and density functional theory

JOURNAL OF MATERIALS CHEMISTRY C 6 (2018) 3861-3868

IV Kabakova, I Azuri, Z Chen, PK Nayak, HJ Snaith, L Kronik, C Paterson, AA Bakulin, DA Egger


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


Balancing Charge Carrier Transport in a Quantum Dot P-N Junction toward Hysteresis-Free High-Performance Solar Cells.

ACS energy letters 3 (2018) 1036-1043

Y Cho, B Hou, J Lim, S Lee, S Pak, J Hong, P Giraud, A-R Jang, Y-W Lee, J Lee, JE Jang, HJ Snaith, SM Morris, JI Sohn, S Cha, JM Kim

In a quantum dot solar cell (QDSC) that has an inverted structure, the QD layers form two different junctions between the electron transport layer (ETL) and the other semiconducting QD layer. Recent work on an inverted-structure QDSC has revealed that the junction between the QD layers is the dominant junction, rather than the junction between the ETL and the QD layers, which is in contrast to the conventional wisdom. However, to date, there have been a lack of systematic studies on the role and importance of the QD heterojunction structure on the behavior of the solar cell and the resulting device performance. In this study, we have systematically controlled the structure of the QD junction to balance charge transport, which demonstrates that the position of the junction has a significant effect on the hysteresis effect, fill factor, and solar cell performance and is attributed to balanced charge transport.


Mitigating the photocurrent persistence of single ZnO nanowires for low noise photodetection applications.

Nanotechnology (2018)

J-P Girard, L Giraudet, S Kostcheev, B Bercu, TJ Puchtler, R Taylor, C Couteau

In this work, we investigate the optoelectronic properties of zinc oxide (ZnO) nanowires, which are good candidates for applications based on integrated optics. Single ZnO nanowire photodetectors were fabricated with ohmic contacts. By taking current transient measurements in different atmospheres (oxygen, air, vac- uum and argon), and at various temperatures, we point out the importance of surface effects on the electrical behaviour. Results confirm that oxygen chemisorption is responsible for the existence of a high photocon- ductive gain in these devices, and for the first time a two step process in the photocurrent rise transient is reported. A maximum gain of G = 7.8 × 10^7 is achieved. However, under certain conditions, the persistence of the photocurrent can last up to several hours and as such may prevent the device from operating at useful rates. From a knowledge of the photocurrent response mechanisms, we establish a method to restore the pho- todetector to its initial state, with very low dark current, by applying an appropriate gate voltage sequence. This advances the state of the art for these detectors towards commercial applications.


Unravelling the Improved Electronic and Structural Properties of Methylammonium Lead Iodide Deposited from Acetonitrile

CHEMISTRY OF MATERIALS 30 (2018) 7737-7743

AJ Ramadan, NK Noel, S Fearn, N Young, M Walker, LA Rochford, HJ Snaith


Femtosecond Dynamics of Photoexcited C60 Films.

The journal of physical chemistry letters 9 (2018) 1885-1892

M Causa', I Ramirez, JF Martinez Hardigree, M Riede, N Banerji

The well known organic semiconductor C60 is attracting renewed attention due to its centimeter-long electron diffusion length and high performance of solar cells containing 95% fullerene, yet its photophysical properties remain poorly understood. We elucidate the dynamics of Frenkel and intermolecular (inter-C60) charge-transfer (CT) excitons in neat and diluted C60 films from high-quality femtosecond transient absorption (TA) measurements performed at low fluences and free from oxygen or pump-induced photodimerization. We find from preferential excitation of either species that the CT excitons give rise to a strong electro-absorption (EA) signal but are extremely short-lived. The Frenkel exciton relaxation and triplet yield strongly depend on the C60 aggregation. Finally, TA measurements on full devices with applied electric field allow us to optically monitor the dissociation of CT excitons into free charges for the first time and to demonstrate the influence of cluster size on the spectral signature of the C60 anion.


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.


Targeted Ligand-Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics.

Journal of the American Chemical Society 140 (2018) 10504-10513

LM Wheeler, EM Sanehira, AR Marshall, P Schulz, M Suri, NC Anderson, JA Christians, D Nordlund, D Sokaras, T Kroll, SP Harvey, JJ Berry, LY Lin, JM Luther

The ability to manipulate quantum dot (QD) surfaces is foundational to their technological deployment. Surface manipulation of metal halide perovskite (MHP) QDs has proven particularly challenging in comparison to that of more established inorganic materials due to dynamic surface species and low material formation energy; most conventional methods of chemical manipulation targeted at the MHP QD surface will result in transformation or dissolution of the MHP crystal. In previous work, we have demonstrated record-efficiency QD solar cells (QDSCs) based on ligand-exchange procedures that electronically couple MHP QDs yet maintain their nanocrystalline size, which stabilizes the corner-sharing structure of the constituent PbI64- octahedra with optoelectronic properties optimal for solar energy conversion. In this work, we employ a variety of spectroscopic techniques to develop a molecular-level understanding of the MHP QD surface chemistry in this system. We individually target both the anionic (oleate) and cationic (oleylammonium) ligands. We find that atmospheric moisture aids the process by hydrolysis of methyl acetate to generate acetic acid and methanol. Acetic acid then replaces native oleate ligands to yield QD surface-bound acetate and free oleic acid. The native oleylammonium ligands remain throughout this film deposition process and are exchanged during a final treatment step employing smaller cations-namely, formamidinium. This final treatment has a narrow processing window; initial treatment at this stage leads to a more strongly coupled QD regime followed by transformation into a bulk MHP film after longer treatment. These insights provide chemical understanding to the deposition of high-quality, electronically coupled MHP QD films that maintain both quantum confinement and their crystalline phase and attain high photovoltaic performance.


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


Engineering III-V nanowires for optoelectronics: from epitaxy to terahertz photonics

QUANTUM DOTS AND NANOSTRUCTURES: GROWTH, CHARACTERIZATION, AND MODELING XV 10543 (2018) UNSP 105430I

HJ Joyce, C Uswachoke, SA Baig, SO Adeyemo, JL Boland, DA Damry, CL Davies, J Wong-Leung, HH Tan, C Jagadish, LM Herz, MB Johnston


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


High irradiance performance of metal halide perovskites for concentrator photovoltaics

NATURE ENERGY 3 (2018) 855-861

Z Wang, Q Lin, B Wenger, MG Christoforo, Y-H Lin, MT Klug, MB Johnston, LM Herz, HJ Snaith

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