Publications


Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings

(2019)

M Peeks, J Gong, K McLoughlin, T Kobatake, R Haver, L Herz, H Anderson

Aromaticity can be a useful concept for predicting the behavior of excited states. Here we show that π-conjugated porphyrin nanorings exhibit size-dependent excited-state global aromaticity and antiaromaticity, for rings containing up to eight porphyrin subunits, although they have no significant global aromaticity in their neutral singlet ground states. Applying Baird’s law, odd rings ([4n] π-electrons) are aromatic in their excited states, whereas the excited states of even rings ([4n+2] π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift in the T1 triplet state of each ring, which reveal the critical importance of the triplet delocalization to the emergence of excited-state aromaticity. The singlet excited states (S1) are explored by measurements of the radiative rate and fluorescence peak wavelength, revealing a subtle odd-even alternation as a function of ring size, consistent with symmetry-breaking in antiaromatic excited states.


How β-Phase Content Moderates Chain Conjugation and Energy Transfer in Polyfluorene Films

Journal of Physical Chemistry Letters American Chemical Society 10 (2019) 1729-1736

HJ Eggimann, F Le Roux, L Herz


Facile Synthesis of Stable and Highly Luminescent Methylammonium Lead Halide Nanocrystals for Efficient Light Emitting Devices.

Journal of the American Chemical Society (2019)

Y Hassan, OJ Ashton, JH Park, G Li, N Sakai, B Wenger, A-A Haghighirad, NK Noel, MH Song, BR Lee, RH Friend, HJ Snaith

Metal halide perovskites are promising candidates for use in light emitting diodes (LEDs), due to their potential for colour tuneable and high luminescence efficiency. While recent advances in perovskite-based light emitting diodes have resulted in external quantum efficiencies exceeding 12.4 % for the green emitters, and infrared emitters based on 3D/2D mixed dimensional perovskites have exceeded 20%, the external quantum efficiencies of the red and blue emitters still lag behind. A critical issue to date is creating highly emissive and stable perovskite emitters with the desirable emission band gap to achieve full-colour displays and white LEDs. Herein, we report the preparation and characterization of a highly luminescent and stable suspension of cubic-shaped methylammonium lead triiodide CH3NH3PbI3 perovskite nanocrystals, where we synthesise the nanocrystals via a ligand-assisted re-precipitation technique, using an acetonitrile/methylamine compound solvent system to solvate the ions, and toluene as the anti-solvent to induce crystallisation. Through tuning the ratio of the ligands, the ligand to toluene ratio, and the temperature of the toluene, we obtain a solution of CH3NH3PbI3 nanocrystals with a photoluminescence quantum yield exceeding 93%, and tuneable emission between 660 nm and 705 nm. We also achieved red emission at 635 nm by blending the nanocrystals with bromide salt and obtained perovskite-based light emitting diodes with maximum electroluminescent external quantum efficiency of 2.75%.


Structural and Optical Properties of Cs2AgBiBr6 Double Perovskite

ACS Energy Letters American Chemical Society (ACS) (2018) 299-305

L Schade, AD Wright, RD Johnson, M Dollmann, B Wenger, PK Nayak, D Prabhakaran, LM Herz, R Nicholas, HJ Snaith, PG Radaelli


Solution-Processed All-Perovskite Multi-junction Solar Cells

Joule 3 (2019) 387-401

DP McMeekin, S Mahesh, NK Noel, MT Klug, JC Lim, JH Warby, JM Ball, LM Herz, MB Johnston, HJ Snaith

© 2019 Multi-junction device architectures can increase the power conversion efficiency (PCE) of photovoltaic (PV) cells beyond the single-junction thermodynamic limit. However, these devices are challenging to produce by solution-based methods, where dissolution of underlying layers is problematic. By employing a highly volatile acetonitrile(CH 3 CN)/methylamine(CH 3 NH 2 ) (ACN/MA) solvent-based perovskite solution, we demonstrate fully solution-processed absorber, transport, and recombination layers for monolithic all-perovskite tandem and triple-junction solar cells. By combining FA 0.83 Cs 0.17 Pb(Br 0.7 I 0.3 ) 3 (1.94 eV) and MAPbI 3 (1.57 eV) junctions, we reach two-terminal tandem PCEs of more than 15% (steady state). We show that a MAPb 0.75 Sn 0.25 I 3 (1.34 eV) narrow band-gap perovskite can be processed via the ACN/MA solvent-based system, demonstrating the first proof-of-concept, monolithic all-perovskite triple-junction solar cell with an open-circuit voltage reaching 2.83 V. Through optical and electronic modeling, we estimate the achievable PCE of a state-of-the-art triple-junction device architecture to be 26.7%. Our work opens new possibilities for large-scale, low-cost, printable perovskite multi-junction solar cells. Silicon-based solar cells are dominating today's solar energy market. However, their efficiencies will soon reach their maximum practical limit. Without any gains in efficiency, price reductions will become increasingly difficult to achieve. Tandem and multi-junction architectures can overcome this single-junction efficiency limit. Perovskite materials offer both band-gap tunability and solution processability. This unique combination of properties allows for fabrication of multi-junction solar cells using high-throughput deposition techniques such as blade coating, roll-to-roll, gravure coating or inkjet printing. However, these solar cells have yet to be fabricated using these deposition techniques due to difficulties in sequentially depositing these semiconductors. By utilizing an acetonitrile/methylamine-based solvent, we demonstrate the first monolithic all-perovskite multi-junction solar cells fabricated via solution processing of all active layers, apart from the electrodes. Perovskite solar cells can be processed using solution-based methods. Furthermore, perovskite solar cells can tune their band gap to absorb different portions of the solar spectrum. This property allows for fabrication of multi-junction solar cell, which can offer higher power conversion efficiencies than single-junction architecture. Here, we combine both features to fabricate the first solution-processed, monolithic, all-perovskite tandem and triple-junction solar cells.


Unravelling the key role of surface features behind facet-dependent photocatalysis of anatase TiO2

Chemical Communications Royal Society of Chemistry 55 (2019) 4415-4418

Y-K Peng, B Keeling, Y Li, J Zheng, T Chen, H-L Chou, TJ Puchtler, R Taylor, S Tsang

The high activity of nanocrystallites is commonly attributed to the terminal high-energy facets. However, we demonstrate that the high activity of the anatase TiO2(001) facet in photocatalytic H2 evolution is not due to its high intrinsic surface energy, but local electronic effects created by surface features on the facet.


Photovoltaic solar cell technologies: analysing the state of the art

Nature Reviews Materials Nature Research 4 (2019) 269-285

P Nayak, S Mahesh, HJ Snaith, D Cahen


Electronic Traps and Phase Segregation in Lead Mixed-Halide Perovskite

ACS Energy Letters (2018) 75-84

AJ Knight, AD Wright, JB Patel, DP McMeekin, HJ Snaith, MB Johnston, LM Herz

© 2018 American Chemical Society. An understanding of the factors driving halide segregation in lead mixed-halide perovskites is required for their implementation in tandem solar cells with existing silicon technology. Here we report that the halide segregation dynamics observed in the photoluminescence from CH3NH3Pb(Br0.5I0.5)3 is strongly influenced by the atmospheric environment, and that encapsulation of films with a layer of poly(methyl methacrylate) allows for halide segregation dynamics to be fully reversible and repeatable. We further establish an empirical model directly linking the amount of halide segregation observed in the photoluminescence to the fraction of charge carriers recombining through trap-mediated channels, and the photon flux absorbed. From such quantitative analysis we show that under pulsed illumination, the frequency of the modulation alone has no influence on the segregation dynamics. Additionally, we extrapolate that working CH3NH3Pb(Br0.5I0.5)3 perovskite cells would require a reduction of the trap-related charge carrier recombination rate to ≲105s-1 in order for halide segregation to be sufficiently suppressed.


Infrared light management using a nanocrystalline silicon oxide interlayer in monolithic perovskite/silicon heterojunction tandem solar cells with efficiency above 25%

Advanced Energy Materials Wiley 9 (2019) 1803241

L Mazzarella, Y-H Lin, S Kirner, Morales-Vilches, L Korte, S Albrecht, E Crossland, B Stannowski, C Case, H Snaith, R Schlatmann

Perovskite/silicon tandem solar cells are attractive for their potential for boosting cell efficiency beyond the crystalline silicon (Si) single-junction limit. However, the relatively large optical refractive index of Si, in comparison to that of transparent conducting oxides and perovskite absorber layers, results in significant reflection losses at the internal junction between the cells in monolithic (two-terminal) devices. Therefore, light management is crucial to improve photocurrent absorption in the Si bottom cell. Here it is shown that the infrared reflection losses in tandem cells processed on a flat silicon substrate can be significantly reduced by using an optical interlayer consisting of nanocrystalline silicon oxide. It is demonstrated that 110 nm thick interlayers with a refractive index of 2.6 (at 800 nm) result in 1.4 mA cm − ² current gain in the silicon bottom cell. Under AM1.5G irradiation, the champion 1 cm 2 perovskite/silicon monolithic tandem cell exhibits a top cell + bottom cell total current density of 38.7 mA cm −2 and a certified stabilized power conversion efficiency of 25.2%.


Solubilization of Carbon Nanotubes with Ethylene-Vinyl Acetate for Solution-Processed Conductive Films and Charge Extraction Layers in Perovskite Solar Cells.

ACS Appl Mater Interfaces (2018)

G Mazzotta, M Dollmann, SN Habisreutinger, MG Christoforo, Z Wang, HJ Snaith, MK Riede, RJ Nicholas

Carbon nanotube (CNT) solubilization via non-covalent wrapping of conjugated semiconducting polymers is a common technique used to produce stable dispersions for depositing CNTs from solution. Here, we report the use of a non-conjugated insulating polymer, ethylene vinyl acetate (EVA), to disperse multi- and single-walled CNTs (MWCNT and SWCNT) in organic solvents. We demonstrate that despite the insulating nature of the EVA, we can produce semitransparent films with conductivities of up to 34 S/cm. We show, using photoluminescence spectroscopy, that the EVA strongly binds to individual CNTs, thus making them soluble, preventing aggregation, and facilitating the deposition of high-quality films. To prove the good electronic properties of this composite, we have fabricated perovskite solar cells using EVA/SWCNTs and EVA/MWCNTs as selective hole contact, obtaining power conversion efficiencies of up to 17.1%, demonstrating that the insulating polymer does not prevent the charge transfer from the active material to the CNTs.


In-depth optical characterization of poly(3-hexylthiophene) after formation of nanosecond laser-induced periodic surface structures.

Nanoscale 11 (2019) 7567-7571

SY Son, G Kang, M Kim, J Lee, T Kim, T Park, J Lim

Herein, poly(3-hexylthiophene) films with periodic wavy surface structures are generated upon laser irradiation at a wavelength of 530 nm using a pulse duration of 5 ns and a repetition frequency of 10 Hz. The optical properties of the films irradiated with 1200, 3000, and 6000 pulses, respectively, are studied using various techniques.


Tuning the ambipolar behaviour of organic field effect transistors via band engineering

AIP ADVANCES 9 (2019) ARTN 035202

PR Warren, JFM Hardigree, AE Lauritzen, J Nelson, M Riede


Elucidating the long-range charge carrier mobility in metal halide perovskite thin films

Energy & Environmental Science Royal Society of Chemistry (RSC) (2018)

J Lim, M Hoerantner, N Sakai, JM Ball, S Mahesh, NK Noel, Y-H Lin, J Patel, D McMeekin, MB Johnston, B Wenger, H Snaith

<p>Many optoelectronic properties have been reported for lead halide perovskite polycrystalline films. However, ambiguities in the evaluation of these properties remain, especially for long-range lateral charge transport, where ionic conduction...</p>


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


How lattice dynamics moderate the electronic properties of metal-halide perovskites

Journal of Physical Chemistry Letters American Chemical Society 9 (2018) 6853–6863-

L Herz

Metal-halide perovskites have emerged as highly promising semiconductors with excellent optoelectronic properties. This Perspective outlines how the dynamic response of the ionic lattice affects key electronic properties such as exciton binding energies and charge-carrier mobilities in hybrid perovskites. Such links are shown to derive from the frequency-dependence of the dielectric function, which is governed by contributions from electronic interband transitions, polar vibrations of the metal-halide sublattice, organic cation collective reorientations, and ionic movement. The influence of each of these contributions to charge-carrier screening and carrier–lattice interactions is discussed, which allows for general trends with material composition to be revealed. Overall, this Perspective highlights the challenges and questions arising from the peculiar combination of a soft polar metal-halide sublattice interspersed with rotationally mobile dipolar molecules that is encountered in hybrid metal-halide perovskites.


How Contact Layers Control Shunting Losses from Pinholes in Thin-Film Solar Cells

JOURNAL OF PHYSICAL CHEMISTRY C 122 (2018) 27263-27272

P Kaienburg, P Hartnage, BE Pieters, J Yu, D Grabowski, Z Liu, J Haddad, U Rau, T Kircharte


Impact of Bi3+ Heterovalent Doping in Organic-Inorganic Metal Halide Perovskite Crystals.

Journal of the American Chemical Society 140 (2018) 574-577

PK Nayak, M Sendner, B Wenger, Z Wang, K Sharma, AJ Ramadan, R Lovrinčić, A Pucci, PK Madhu, HJ Snaith

Intrinsic organic-inorganic metal halide perovskites (OIHP) based semiconductors have shown wide applications in optoelectronic devices. There have been several attempts to incorporate heterovalent metal (e.g., Bi3+) ions in the perovskites in an attempt to induce electronic doping and increase the charge carrier density in the semiconductor. It has been reported that inclusion of Bi3+ decreases the band gap of the material considerably. However, contrary to the earlier conclusions, despite a clear change in the appearance of the crystal as observed by eye, here we show that the band gap of MAPbBr3 crystals does not change due the presence of Bi3+ in the growth solution. An increased density of states in the band gap and use of very thick samples for transmission measurements, erroneously give the impression of a band gap shift. These sub band gap states also act as nonradiative recombination centers in the crystals.


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

Optics Express Optical Society of America 26 (2018) 32332-32345

SA Lennon, FSF Brossard, LP Nuttall, J Wu, J Griffiths, R 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.


Carbon nanotubes for quantum dot photovoltaics with enhanced light management and charge transport

ACS Photonics American Chemical Society 5 (2018) 4854–4863-

Y Tazawa, S Habisreutinger, N Zhang, DAF Gregory, G Nagamine, S Kesava, G Mazzotta, H Assender, MK Riede, L Padilha, RJ Nicholas, AAR Watt

Colloidal quantum dot (CQD)-based photovoltaics are an emerging low-cost solar cell technology with power conversion efficiencies exceeding 10%, i.e., high enough to be interesting for commercialization. Well-controlled and understood charge carrier transport through the device stack is required to make the next step in efficiency improvements. In this paper, polymer-wrapped single-walled carbon nanotube (SWNT) films embedded in an insulating poly(methyl methacrylate) (PMMA) matrix and capped by a thermally evaporated Au electrode are investigated as a composite hole transport layer and optical spacer. Employing transient absorption spectroscopy we show that the SWNTs enhance the charge transfer rate from CQD to CQD, ZnO, or SWNT. In order to pinpoint the underlying mechanism for the improvement, we investigate the energetics of the junction by measuring the relative alignment of the band edges, using Kelvin probe and cyclic voltammetry. Measuring the external quantum efficiency and absorption we find that the improvement is not mainly from electronic improvements but from enhanced absorption of the CQD absorber. We demonstrate experimentally and theoretically, by employing a transfer-matrix model, that the transparent PMMA matrix acts as an optical spacer, which leads to an enhanced absorption in the absorber layer. With these electronic and optical enhancements, the efficiency of the PbS CQD solar cells improved from 4.0% to 6.0%.


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

Pages