Aromaticity and Antiaromaticity in the Excited States of Porphyrin Nanorings


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.

Revealing the nature of photoluminescence emission in the metal-halide double perovskite Cs2AgBiBr6


SJ Zelewski, JM Urban, A Surrente, DK Maude, A Kuc, L Schade, RD Johnson, M Dollmann, PK Nayak, HJ Snaith, P Radaelli, R Kudrawiec, RJ Nicholas, P Plochocka, M Baranowski

Impurity Tracking Enables Enhanced Control and Reproducibility of Hybrid Perovskite Vapor Deposition.

ACS applied materials & interfaces 11 (2019) 28851-28857

J Borchert, I Levchuk, LC Snoek, MU Rothmann, R Haver, HJ Snaith, CJ Brabec, LM Herz, MB Johnston

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible, and efficient solar cells for a wide range of applications. Physical vapor deposition by co-evaporation of precursors is a method that results in very smooth and pinhole-free perovskite thin films and allows excellent control over film thickness and composition. However, for a deposition method to become industrially scalable, reproducible process control and high device yields are essential. Unfortunately, to date, the control and reproducibility of evaporating organic precursors such as methylammonium iodide (MAI) have proved extremely challenging. We show that the established method of controlling the evaporation rate of MAI with quartz microbalances (QMBs) is critically sensitive to the concentration of the impurities MAH2PO3 and MAH2PO2 that are usually present in MAI after synthesis. Therefore, controlling the deposition rate of MAI with QMBs is unreliable since the concentration of such impurities typically varies from one batch of MAI to another and even during the course of a deposition. However once reliable control of MAI deposition is achieved, we find that the presence of precursor impurities during perovskite deposition does not degrade the solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapor deposition will allow high solar cell device yields even if the purity of precursors changes from one run to another.

Photovoltaic solar cell technologies: analysing the state of the art


PK Nayak, S Mahesh, HJ Snaith, D Cahen

Giant Fine Structure Splitting of the Bright Exciton in a Bulk MAPbBr3 Single Crystal.

Nano letters (2019)

M Baranowski, K Galkowski, A Surrente, JM Urban, Ł Klopotowski, S Mackowski, DK Maude, R Ben Aich, K Boujdaria, M Chamarro, C Testelin, P Nayak, M Dollmann, HJ Snaith, RJ Nicholas, P Plochocka

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Since the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ~200μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure observed in perovskite nanocrystals.

Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide.

The journal of physical chemistry letters (2019) 6038-6047

KJ Savill, MT Klug, RL Milot, HJ Snaith, LM Herz

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain. Here we present ultrafast time-resolved photoluminescence measurements on formamidinum tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. Further, we do not find any persistent polarization anisotropy in FASnI3 within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.

Growth modes and quantum confinement in ultrathin vapour-deposited MAPbI3 films.

Nanoscale 11 (2019) 14276-14284

ES Parrott, JB Patel, A-A Haghighirad, HJ Snaith, MB Johnston, LM Herz

Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth of metal halide perovskites by this method at the current time. Here, we report the fabrication of MAPbI3 films with average thicknesses from 2-320 nm by co-evaporation. We analyze the film properties using X-ray diffraction, optical absorption and photoluminescence (PL) to provide insights into the nucleation and growth of MAPbI3 films on quartz substrates. We find that the perovskite initially forms crystallite islands of around 8 nm in height, which may be the cause of the persistent small grain sizes reported for evaporated metal halide perovskites that hinder device efficiency and stability. As more material is added, islands coalesce until full coverage of the substrate is reached at around 10 nm average thickness. We also find that quantum confinement induces substantial shifts to the PL wavelength when the average thickness is below 40 nm, offering dual-source vapour deposition as an alternative method of fabricating nanoscale structures for LEDs and other devices.

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.

Interfacial charge-transfer doping of metal halide perovskites for high performance photovoltaics


NK Noel, SN Habisreutinger, A Pellaroque, F Pulvirenti, B Wenger, F Zhang, Y-H Lin, OG Reid, J Leisen, Y Zhang, S Barlow, SR Marder, A Kahn, HJ Snaith, CB Arnold, BP Rand

Bimolecular recombination in methylammonium lead triiodide perovskite is an inverse absorption process.

Nature communications 9 (2018) 293-293

CL Davies, MR Filip, JB Patel, TW Crothers, C Verdi, AD Wright, RL Milot, F Giustino, MB Johnston, LM Herz

Photovoltaic devices based on metal halide perovskites are rapidly improving in efficiency. Once the Shockley-Queisser limit is reached, charge-carrier extraction will be limited only by radiative bimolecular recombination of electrons with holes. Yet, this fundamental process, and its link with material stoichiometry, is still poorly understood. Here we show that bimolecular charge-carrier recombination in methylammonium lead triiodide perovskite can be fully explained as the inverse process of absorption. By correctly accounting for contributions to the absorption from excitons and electron-hole continuum states, we are able to utilise the van Roosbroeck-Shockley relation to determine bimolecular recombination rate constants from absorption spectra. We show that the sharpening of photon, electron and hole distribution functions significantly enhances bimolecular charge recombination as the temperature is lowered, mirroring trends in transient spectroscopy. Our findings provide vital understanding of band-to-band recombination processes in this hybrid perovskite, which comprise direct, fully radiative transitions between thermalized electrons and holes.

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


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.

The Route to Nanoscale Terahertz Technology: Nanowire-based Terahertz Detectors and Terahertz Modulators


JL Boland, K Peng, S Baig, D Damry, P Parkinson, L Fu, HH Tan, C Jagadish, LM Herz, H Joyce, M Johnston, IEEE

Hybrid Perovskites: Prospects for Concentrator Solar Cells.

Advanced science (Weinheim, Baden-Wurttemberg, Germany) 5 (2018) 1700792-1700792

Q Lin, Z Wang, HJ Snaith, MB Johnston, LM Herz

Perovskite solar cells have shown a meteoric rise of power conversion efficiency and a steady pace of improvements in their stability of operation. Such rapid progress has triggered research into approaches that can boost efficiencies beyond the Shockley-Queisser limit stipulated for a single-junction cell under normal solar illumination conditions. The tandem solar cell architecture is one concept here that has recently been successfully implemented. However, the approach of solar concentration has not been sufficiently explored so far for perovskite photovoltaics, despite its frequent use in the area of inorganic semiconductor solar cells. Here, the prospects of hybrid perovskites are assessed for use in concentrator solar cells. Solar cell performance parameters are theoretically predicted as a function of solar concentration levels, based on representative assumptions of charge-carrier recombination and extraction rates in the device. It is demonstrated that perovskite solar cells can fundamentally exhibit appreciably higher energy-conversion efficiencies under solar concentration, where they are able to exceed the Shockley-Queisser limit and exhibit strongly elevated open-circuit voltages. It is therefore concluded that sufficient material and device stability under increased illumination levels will be the only significant challenge to perovskite concentrator solar cell applications.

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.

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


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

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.