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.


Raman Spectrum of the Organic-Inorganic Halide Perovskite CH3NH3PbI3 from First Principles and High-Resolution Low-Temperature Raman Measurements

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 21703-21717

MA Perez-Osorio, Q Lin, RT Phillips, RL Milot, LM Herz, MB Johnston, F Giustino


Surface modified fullerene electron transport layers for stable and reproducible flexible perovskite solar cells

NANO ENERGY 49 (2018) 324-332

S Song, R Hill, K Choi, K Wojciechowski, S Barlow, J Leisen, HJ Snaith, SR Marder, T Park


Perovskite quantum dot photovoltaic materials beyond the reach of thin films: Full-range tuning of a-site cation composition

ACS Nano American Chemical Society 12 (2018) 10327–10337-

A Hazarika, Q Zhao, EA Gaulding, JA Christians, B Dou, A Marshall, T Moot, JJ Berry, JC Johnson, JM Luther

We present a cation-exchange approach for tunable A-site alloys of cesium (Cs+) and formamidinium (FA+) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the complete range of Cs1–xFAxPbI3, unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs+ and FA+ is measured (∼0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs1–xFAxPbI3 materials, the quantum dot solar cells exhibit high open-circuit voltage (VOC) with a lower loss than the thin-film perovskite devices of similar compositions.


Hole Transport in Low-Donor-Content Organic Solar Cells.

The journal of physical chemistry letters (2018) 5496-5501

D Spoltore, A Hofacker, J Benduhn, S Ullbrich, M Nyman, O Zeika, S Schellhammer, Y Fan, I Ramirez, S Barlow, M Riede, SR Marder, F Ortmann, K Vandewal

Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C60, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well.


Temperature-dependent refractive index of quartz at terahertz frequencies

Journal of Infrared, Millimeter and Terahertz Waves Springer Verlag 39 (2018) 1236–1248-

CL Davies, JB Patel, CQ Xia, LM Herz, M Johnston

Characterisation of materials often requires the use of a substrate to support the sample being investigated. For optical characterisation at terahertz frequencies, quartz is commonly used owing to its high transmission and low absorption at these frequencies. Knowledge of the complex refractive index of quartz is required for analysis of time-domain terahertz spectroscopy and optical pump terahertz probe spectroscopy for samples on a quartz substrate. Here, we present the refractive index and extinction coefficient for α-quartz between 0.5 THz and 5.5 THz (17–183 cm^−1) taken at 10, 40, 80, 120, 160, 200 and 300 K. Quartz shows excellent transmission and is thus an ideal optical substrate over the THz band, apart from the region 3.9 ± 0.1 THz owing to a spectral feature originating from the lowest energy optical phonon modes. We also present the experimentally measured polariton dispersion of α-quartz over this frequency range.


Modification of the fluorinated tin oxide/electron-transporting material interface by a strong reductant and its effect on perovskite solar cell efficiency

Molecular Systems Design and Engineering Royal Society of Chemistry 3 (2018) 741-747

F Pulvirenti, B Wegner, NK Noel, G Mazzotta, R Hill, JB Patel, LM Herz, MB Johnston, MK Riede, HJ Snaith, N Koch, S Barlow

To date, the most efficient hybrid metal halide peroskite solar cells employ TiO2 as electron-transporting material (ETM), making these devices unstable under UV light exposure. Replacing TiO2 with fullerene derivatives has been shown to result in improved electronic contact and increased device lifetime, making it of interest to assess whether similar improvements can be achieved by using other organic semiconductors as ETMs. In this work, we investigate perylene-3,4:9,10-tetracarboxylic bis(benzimidazole) as a vacuum-processable ETM, and we minimize electron-collection losses at the electron-selective contact by depositing pentamethylcyclopentadienyl cyclopentadienyl rhodium dimer, (RhCp*Cp)2, on fluorinated tin oxide. With (RhCp*Cp)2 as an interlayer, ohmic contacts can be formed, there is interfacial doping of the ETM, and stabilized power conversion efficiencies of up to 14.2% are obtained.


Light controlled optical Aharonov–Bohm oscillations in a single quantum ring

Nano Letters American Chemical Society 18 (2018) 6188–6194-

H Kim, S Park, R Okuyama, K Kyhm, M Eto, R 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


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.


High irradiance performance of metal halide perovskites for concentrator photovoltaics (vol 3, pg 855, 2018)

NATURE ENERGY 3 (2018) 1013-1013

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


Unravelling the improved electronic and structural properties of methylammonium lead iodide deposited from acetonitrile

Chemistry of Materials American Chemical Society 30 (2018) 7737–7743-

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

Perovskite-based photovoltaics are an emerging solar technology with lab scale device efficiencies of over 22 %, and significant steps are being made toward their commercialization. Conventionally high efficiency perovskite solar cells are formed from high boiling point, polar aprotic solvent solutions. Methylammonium lead iodide (CH3NH3PbI3) films can be made from a range of solvents and blends; however, the role the solvent system plays in determining the properties of the resulting perovskite films is poorly understood. Acetonitrile (ACN), in the presence of methylamine (MA), is a viable nontoxic solvent for fabrication of CH3NH3PbI3 photovoltaic devices with efficiencies >18 %. Herein we examine films prepared from ACN/MA and dimethylformamide (DMF) and scrutinize their physical and electronic properties using spectroscopy, scanning probe imaging, and ion scattering. Significant differences are observed in the chemistry and electronic structure of CH3NH3PbI3 films made with each solvent, ACN/MA produces films with superior properties resulting in more efficient photovoltaic devices. Here we present a holistic and complete understanding of a high performance perovskite material from an electronic, physical, and structural perspective and establish a robust toolkit with which to understand and optimize photovoltaic perovskites.


Room temperature InP/InGaAs nano-ridge lasers grown on silicon emitting at telecom-bands

Optica Optical Society of America 5 (2018) 918-923

Y Han, WK Ng, C Ma, Q Li, S Zhu, C Chan, KW Ng, S Lennon, R Taylor, KS Wong, KM Lau

Semiconductor nano-lasers grown on silicon and emitting at the telecom bands are advantageous ultra-compact coherent light sources for potential Si-based photonic integrated circuit applications. However, realizing room-temperature lasing inside nano-cavities at telecom bands is challenging and has only been demonstrated up to the E band. Here, we report on InP/InGaAs nano-ridge lasers with emission wavelengths ranging from the O, E, and S bands to the C band operating at room temperature with ultra-low lasing thresholds. Using a cycled growth procedure, ridge InGaAs quantum wells inside InP nano-ridges grown on patterned (001) Si substrates are designed as active gain materials. Room-temperature lasing at the telecom bands is achieved by transferring the InP/InGaAs nano-ridges onto a SiO2∕Si substrate for optical excitation. We also show that the operation wavelength of InP/InGaAs nano-lasers can be adjusted by altering the excitation power density and the length of the nano-ridges formed in a single growth run. These results indicate the excellent optical properties of the InP/InGaAs nano-ridges grown on (001) Si substrates and pave the way towards telecom InP/InGaAs nano-laser arrays on CMOS standard Si or silicon-on-insulator substrates.


Impact of the organic cation on the optoelectronic properties of formamidinium lead triiodide

Journal of Physical Chemistry Letters American Chemical Society 9 (2018) 4502–4511-

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


The Phosphine Oxide Route toward Lead Halide Perovskite Nanocrystals.

Journal of the American Chemical Society 140 (2018) 14878-14886

G Almeida, OJ Ashton, L Goldoni, D Maggioni, U Petralanda, N Mishra, QA Akkerman, I Infante, HJ Snaith, L Manna

We report an amine-free synthesis of lead halide perovskite (LHP) nanocrystals, using trioctylphosphine oxide (TOPO) instead of aliphatic amines, in combination with a protic acid (e.g., oleic acid). The overall synthesis scheme bears many similarities to the chemistry behind the preparation of LHP thin films and single crystals, in terms of ligand coordination to the chemical precursors. The acidity of the environment and hence the extent of protonation of the TOPO molecules tune the reactivity of the PbX2 precursor, regulating the size of the nanocrystals. On the other hand, TOPO molecules are virtually absent from the surface of our nanocrystals, which are simply passivated by one type of ligand (e.g., Cs-oleate). Furthermore, our studies reveal that Cs-oleate is dynamically bound to the surface of the nanocrystals and that an optimal surface coverage is critical for achieving high photoluminescence quantum yield. Our scheme delivers NCs with a controlled size and shape: only cubes are formed, with no contamination with platelets, regardless of the reaction conditions that were tested. We attribute such a shape homogeneity to the absence of primary aliphatic amines in our reaction environment, since these are known to promote the formation of nanocrystals with sheet/platelet morphologies or layered phases under certain reaction conditions. The TOPO route is particularly appealing with regard to synthesizing LHP nanocrystals for large-scale manufacturing, as the yield in terms of material produced is close to the theoretical limit: i.e., almost all precursors employed in the synthesis are converted into nanocrystals.


The effects of doping density and temperature on the optoelectronic properties of formamidinium tin triiodide thin films

Advanced Materials Wiley 30 (2018) 1804506-

RL Milot, MT Klug, C Davies, Z Wang, HAP Kraus, HJ Snaith, MB Johnston, LM Herz

Intrinsic and extrinsic optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI3) thin films, whose background hole doping density was varied through SnF2 addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 1020 cm-3, a strong Burstein-Moss effect increases absorption onset energies by ~300meV beyond the band gap energy of undoped FASnI3 (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (1020 cm-3), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 1019 cm-3 and below, the charge-carrier mobility increases with decreasing temperature according to ~T-1.2, suggesting it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2´1018 cm^-3, charge-carrier mobilities reach a value of 67 cm2V-1s-1at room temperature and 470 cm2V-1s-1 at 50 K. Intra-excitonic transitions observed in the THz-frequency photoconductivity spectra at 5K reveal an exciton binding energy of only 3.1 meV for FASnI3, in agreement with the low bandgap energy exhibited by this perovskite.


Meso-superstructured perovskite solar cells: Revealing the role of the mesoporous layer

Journal of Physical Chemistry C American Chemical Society 122 (2018) 21239–21247-

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

While perovskite solar cells (PSCs) have been developed with different device architectures, mesoporous devices have provided the highest power conversion efficiencies. In this work, the working mechanism of both positive-intrinsic-negative (p-i-n) and negative-intrinsic-positive (n-i-p) meso-superstructured (MSSC) PSCs, which include a thin interlayer of porous alumina at the bottom electrode, is explored. Interestingly, for both p-i-n and n-i-p architecture, the mesoporous configuration was more efficient than its planar counterpart. For MSSC SnO2-based n-i-p devices, that result was primarily due to an increase in Voc and Jsc, resulting from improved band alignment and filling of the electron trap states (n-doping at the SnO2/perovskite interface), which led to devices with 21.0% efficiency and 20.3% stabilized power output (SPO). Although MSSC NiOx-based p-i-n meso-superstructured devices were less efficient due to lower Voc, a slightly higher Jsc and fill factor improvement was achieved by the Al2O3 mesoporous layer, resulting in devices with 16.9% efficiency. Importantly, the electronic nature of the perovskite is dependent upon its physical confinement within a mesoporous scaffold. Therefore, either p- or n-type semiconductor/perovskite interfaces can be engineered by selectively modifying the semiconductor behavior with the introduction of an insulating mesoporous scaffold interlayer.


Atomic Layer Deposited Electron Transport Layers in Efficient Organometallic Halide Perovskite Devices

MRS Advances 3 (2018) 3075-3084

MM McCarthy, A Walter, SJ Moon, NK Noel, S O'Brien, ME Pemble, S Nicolay, B Wenger, HJ Snaith, IM Povey

Copyright © Materials Research Society 2018Â. Amorphous TiO2 and SnO2 electron transport layers (ETLs) were deposited by low-Temperature atomic layer deposition (ALD). Surface morphology and x-ray photoelectron spectroscopy (XPS) indicate uniform and pinhole free coverage of these ALD hole blocking layers. Both mesoporous and planar perovskite solar cells were fabricated based on these thin films with aperture areas of 1.04 cm2for TiO2 and 0.09 cm2and 0.70 cm2for SnO2. The resulting cell performance of 18.3 % power conversion efficiency (PCE) using planar SnO2 on 0.09 cm2and 15.3 % PCE using mesoporous TiO2 on 1.04 cm2active areas are discussed in conjunction with the significance of growth parameters and ETL composition.


High irradiance performance of metal halide perovskites for concentrator photovoltaics

Nature Energy Nature Publishing Group 3 (2018) 855–861-

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

Traditionally, III–V multi-junction cells have been used in concentrator photovoltaic (CPV) applications, which deliver extremely high efficiencies but have failed to compete with ‘flat-plate’ silicon technologies owing to cost. Here, we assess the feasibility of using metal halide perovskites for CPVs, and we evaluate their device performance and stability under concentrated light. Under simulated sunlight, we achieve a peak efficiency of 23.6% under 14 Suns (that is, 14 times the standard solar irradiance), as compared to 21.1% under 1 Sun, and measure 1.26 V open-circuit voltage under 53 Suns, for a material with a bandgap of 1.63 eV. Importantly, our encapsulated devices maintain over 90% of their original efficiency after 150 h aging under 10 Suns at maximum power point. Our work reveals the potential of perovskite CPVs, and may lead to new PV deployment strategies combining perovskites with low-concentration factor and lower-accuracy solar tracking systems.

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