Publications


Control over crystal size in vapor deposited metal-halide perovskite films

ACS Energy Letters American Chemical Society (ACS) (2020) acsenergylett.0c00183

MU Rothmann, RDJ Oliver, HJ Snaith, CQ Xia, LM Herz, KB Lohmann, JB Patel, MB Johnston


CsPbBr 3 Nanocrystal Films: Deviations from Bulk Vibrational and Optoelectronic Properties

Advanced Functional Materials Wiley (2020) 1909904-1909904

AJ Ramadan, MV Kovalenko, MB Johnston, HJ Snaith, JB Patel, LM Herz, SG Motti, F Krieg


Trap States, Electric Fields, and Phase Segregation in Mixed-Halide Perovskite Photovoltaic Devices

ADVANCED ENERGY MATERIALS Wiley (2020) ARTN 1903488

LM Herz, AJ Knight, MB Johnston, HJ Snaith, JB Patel

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Mixed-halide perovskites are essential for use in all-perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed-halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed-halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge-carriers in the perovskite, a charge-neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA+ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge-carriers can be extracted from the iodide-rich low-bandgap regions of the phase-segregated perovskite formed under illumination, suggesting the existence of charge-carrier percolation pathways through grain boundaries where phase-segregation may occur.


Effect of ultraviolet radiation on organic photovoltaic materials and devices

ACS Applied Materials and Interfaces American Chemical Society 11 (2019) 21543-21551

J Patel, P Tiwana, N Seidler, GE Morse, OR Lozman, L Herz, M Johnston

Organic photovoltaics are a sustainable and cost-effective power-generation technology that may aid the move to zero-emission buildings, carbon neutral cities, and electric vehicles. While state-of-the-art organic photovoltaic devices can be encapsulated to withstand air and moisture, they are currently still susceptible to light-induced degradation, leading to a decline in the long-term efficiency of the devices. In this study, the role of ultraviolet (UV) radiation on a multilayer organic photovoltaic device is systematically uncovered using spectral filtering. By applying long-pass filters to remove different parts of the UV portion of the AM1.5G spectrum, two main photodegradation processes are shown to occur in the organic photovoltaic devices. A UV-activated process is found to cause a significant decrease in the photocurrent across the whole spectrum and is most likely linked to the deterioration of the charge extraction layers. In addition, a photodegradation process caused by UV-filtered sunlight is found to change the micromorphology of the bulk heterojunction material, leading to a reduction in photocurrent at high photon energies. These findings strongly suggest that the fabrication of inherently photostable organic photovoltaic devices will require the replacement of fullerene-based electron transporter materials with alternative organic semiconductors.


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


Tuning the circumference of six-porphyrin nanorings

Journal of the American Chemical Society American Chemical Society 141 (2019) 7965-7971

R Haver, L Tejerina, H-W Jiang, M Rickhaus, I Grübner, M Jirasek, L Herz, H Eggimann, H Anderson

<p>Most macrocycles are made from a simple repeat unit, resulting in high symmetry. Breaking this symmetry allows fine-tuning of the circumference, providing better control of the host–guest behavior and electronic structure. Here, we present the template-directed synthesis of two unsymmetrical cyclic porphyrin hexamers with both ethyne (C2) and butadiyne (C4) links, and we compare these nanorings with the symmetrical analogues with six ethyne or six butadiyne links. Inserting two extra carbon atoms into the smaller nanoring causes a spectacular change in binding behavior: the template affinity increases by a factor of 3 × 10<sup>9</sup>, to a value of ca. 10<sup>38</sup> M<sup>–1</sup>, and the mean effective molarity is ca. 830 M. In contrast, removing two carbon atoms from the largest nanoring results in almost no change in its template-affinity. The strain in these nanorings is 90–130 kJ mol<sup>–1</sup>, as estimated both from DFT calculation of homodesmotic reactions and from comparing template affinities of linear and cyclic oligomers. Breaking the symmetry has little effect on the absorption and fluorescence behavior of the nanorings: the low radiative rates that are characteristic of a circular delocalized S<sub>1</sub> excited state are preserved in the low-symmetry macrocycles.</p>


Heterogeneous photon recycling and charge diffusion enhance charge transport in quasi-2D lead-halide perovskite films

Nano Letters American Chemical Society 19 (2019) 3953-3960

S Motti, T Crothers, R Yang, Y Cao, R Li, M Johnston, J Wang, L Herz

<p>The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. Here we show that such effects are not detrimental to the charge transport for carefully processed films comprising a self-assembled thin layer of quasi-two-dimensional (2D) perovskite interfaced with a 3D MAPbI<sub>3</sub> perovskite layer. We apply a combination of time-resolved photoluminescence and photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited. Through modeling of the recorded dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective “heterogeneous photon recycling”. We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.</p>


Impurity tracking enables enhanced control and reproducibility of hybrid perovskite vapour deposition

ACS Applied Materials and Interfaces American Chemical Society 11 (2019) 28851-28857

J Borchert, I Levchuk, M Rothmann, L Snoek, R Haver, H Snaith, CJ Brabec, L Herz, M Johnston

Metal halide perovskite semiconductors have the potential to enable low-cost, flexible and efficient solar cells for a wide range of applications. Physical vapour deposition by co-evaporation of precursors is a method which results in very smooth and pin-hole-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) has proved extremely challenging. We show that the established method of controlling the evaporation-rate of MAI with quartz micro balances (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 MAI batch-to-batch 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 solar cell performance. Our results indicate that as long as precursor deposition rates are well controlled, physical vapour deposition will allow high solar cell device yields even if the purity of precursors change from run to run.


Structural and optical properties of Cs2AgBiBr6 double perovskite

ACS Energy Letters American Chemical Society 4 (2018) 299-305

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

We present a comprehensive study of the relationship between the crystal structure and optoelectronic properties of the double perovskite Cs2AgBiBr6, which has emerged as a promising candidate for photovoltaic devices. On the basis of single-crystal/powder X-ray diffraction and neutron powder diffraction, we have revealed the presence of a structural phase transition at Ts ≈ 122 K between the room-temperature cubic structure (space group Fm3̅m) and a new low-temperature tetragonal structure (I4/m). From reflectivity measurements we found that the peak exciton energy Eex ≈ 2.85 eV near the direct gap shifts proportionally to the tetragonal strain, which is consistent with the Eex being primarily controlled by a rotational degree of freedom of the crystal structure, thus by the angle Bi−Ag−Br. We observed the time-resolved photoluminescence kinetics and we found that, among the relaxation channels, a fast one is mainly present in the tetragonal phase, suggesting that its origin may lie in the formation of tetragonal twin domains.


Charge-carrier dynamics, mobilities and diffusion lengths of 2D-3D hybrid butylammonium-caesium-formamidinium lead halide perovskites

Advanced Functional Materials Wiley (2019)

L Buizza, T Crothers, Z Wang, P Jay, R Milot, H Snaith, M Johnston, L Herz

Perovskite solar cells (PSCs) have improved dramatically over the past decade, increasing in efficiency and gradually overcoming hurdles of temperature‐ and humidity‐induced instability. Materials that combine high charge‐carrier lifetimes and mobilities, strong absorption, and good crystallinity of 3D perovskites with the hydrophobic properties of 2D perovskites have become particularly promising candidates for use in solar cells. In order to fully understand the optoelectronic properties of these 2D–3D hybrid systems, the hybrid perovskite BAx(FA0.83Cs0.17)1‐xPb(I0.6Br0.4)3 is investigated across the composition range 0 ≤ x ≤ 0.8. Small amounts of butylammonium (BA) are found that help to improve crystallinity and appear to passivate grain boundaries, thus reducing trap‐mediated charge‐carrier recombination and enhancing charge‐carrier mobilities. Excessive amounts of BA lead to poor crystallinity and inhomogeneous film formation, greatly reducing effective charge‐carrier mobility. For low amounts of BA, the benevolent effects of reduced recombination and enhanced mobilities lead to charge‐carrier diffusion lengths up to 7.7 µm for x = 0.167. These measurements pave the way for highly efficient, highly stable PSCs and other optoelectronic devices based on 2D–3D hybrid materials.


Aromaticity and antiaromaticity in the excited states of porphyrin nanorings

Journal of Physical Chemistry Letters American Chemical Society 10 (2019) 2017-2022

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

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 rule, even rings ([4 n] π-electrons) are aromatic in their lowest excited states, whereas the lowest excited states of odd rings ([4 n + 2] π-electrons) are antiaromatic. These predictions are borne out by density functional theory (DFT) studies of the nucleus-independent chemical shift (NICS) 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.


Solution-processed all-perovskite multi-junction solar cells

Joule Elsevier 3 (2019) 387-401

D McMeekin, S Mahesh, N Noel, J Lim, M Klug, L Herz, J Warby, J Ball, M Johnston, H Snaith


Dual-source co-evaporation of low-bandgap FA1-xCsxSn1-yPbyI3 perovskites for photovoltaics

ACS Energy Letters American Chemical Society 4 (2019) 2748-2756

L Buizza, JM Ball, HC Sansom, Farrar, MT Klug, J Borchert, J Patel, LM Herz, M Johnston, H Snaith


Charge-carrier cooling and polarization memory loss in formamidinium tin triiodide

Journal of Physical Chemistry Letters American Chemical Society 10 (2019) 6038-6047

K Savill, M Klug, RL Milot, H Snaith, L Herz

<p>Combination of a cryogenic ion-trap machine, operated at 4.7 K, with the free-electron-laser FELIX allows the first experimental characterization of the unusually bright antisymmetric stretch (ν<sub>3</sub>) and π-bending (ν<sub>2</sub>) fundamentals of the He–X<sup>+</sup>–He (X = H, D) chromophore of the in situ prepared HHe<sub><em>n</em></sub><sup>+</sup> and DHe<sub><em>n</em></sub><sup>+</sup> (<em>n</em> = 3–6) complexes. The band origins obtained are fully supported by first-principles quantum-chemical computations, performed at the MP2, the CCSD(T), and occasionally the CCSDTQ levels employing extended basis sets. Both the experiments and the computations are consistent with structures for the species with <em>n</em> = 3 and 6 being of T-shaped <em>C</em><sub>2<em>v</em></sub> and of <em>D</em><sub>4<em>h</em></sub> symmetry, respectively, while the species with <em>n</em> = 4 are suggested to exhibit interesting dynamical phenomena related to large-amplitude motions.</p>


Electronic traps and phase segregation in lead mixed-halide Perovskite

ACS Energy Letters American Chemical Society 4 (2018) 75-84

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

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.


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

Nanoscale Royal Society of Chemistry 11 (2019) 14276

ES Parrott, J Patel, AA Haghighirad, H Snaith, L Herz, M Johnston

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.


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.


Publisher Correction: High irradiance performance of metal halide perovskites for concentrator photovoltaics

Nature Energy Springer Nature America, Inc (2018)

Z Wang, Q Lin, B Wenger, M Christoforo, Y-H Lin, MT Klug, MICHAEL Johnston, LAURA Herz, HJ Snaith

© 2018, Springer Nature Limited. When this Article was originally published, an old version of the associated Supplementary Information file was uploaded. This has now been replaced.


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.

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