Publications


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

ACS Energy Letters American Chemical Society (ACS) 5 (2020) 0c00183

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

Understanding and controlling grain growth in metal halide perovskite polycrystalline thin films is an important step in improving the performance of perovskite solar cells. We demonstrate accurate control of crystallite size in CH3NH3PbI3 thin films by regulating substrate temperature during vacuum co-deposition of inorganic (PbI2) and organic (CH3NH3I) precursors. Films co-deposited onto a cold (−2 °C) substrate exhibited large, micrometer-sized crystal grains, while films that formed at room temperature (23 °C) only produced grains of 100 nm extent. We isolated the effects of substrate temperature on crystal growth by developing a new method to control sublimation of the organic precursor, and CH3NH3PbI3 solar cells deposited in this way yielded a power conversion efficiency of up to 18.2%. Furthermore, we found substrate temperature directly affects the adsorption rate of CH3NH3I, thus impacting crystal formation and hence solar cell device performance via changes to the conversion rate of PbI2 to CH3NH3PbI3 and stoichiometry. These findings offer new routes to developing efficient solar cells through reproducible control of crystal morphology and composition.


Enhanced and Polarization Dependent Coupling for Photoaligned Liquid Crystalline Conjugated Polymer Microcavities

ACS Photonics American Chemical Society (ACS) (2020) acsphotonics.9b01596

RA Taylor, F Le Roux, DDC Bradley


Non-polar nitride single-photon sources

Journal of Optics IOP Publishing 22 (2020) 073001-073001

T Wang, RA Oliver, RA Taylor


Revealing Factors Influencing the Operational Stability of Perovskite Light-Emitting Diodes

ACS Nano American Chemical Society (ACS) (2020) acsnano.0c03516

JH Warby, B Wenger, AJ Ramadan, RDJ Oliver, HC Sansom, AR Marshall, HJ Snaith


Publisher Correction: Deciphering photocarrier dynamics for tuneable high-performance perovskite-organic semiconductor heterojunction phototransistors.

Nature communications 11 (2020) 2956-

Y-H Lin, W Huang, P Pattanasattayavong, J Lim, R Li, N Sakai, J Panidi, MJ Hong, C Ma, N Wei, N Wehbe, Z Fei, M Heeney, JG Labram, TD Anthopoulos, HJ Snaith

An amendment to this paper has been published and can be accessed via a link at the top of the paper.


Exciton efficiency beyond the spin statistical limit in organic light emitting diodes based on anthracene derivatives

Journal of Materials Chemistry C Royal Society of Chemistry (RSC) (2020)

N Sharma, MY Wong, D Hall, E Spuling, F Tenopala-Carmona, A Privitera, G Copley, DB Cordes, A Slawin, C Murawski, MC Gather, D Beljonne, Y Olivier, IDW Samuel, E Zysman-Colman

<p>We report two donor&ndash;acceptor (D&ndash;A) materials based on a cyanoanthracene acceptor paired with diphenylamine (<strong>DPAAnCN</strong>) and carbazole (<strong>CzAnCN</strong>) donor moieties. These compounds show hybrid locally excited (LE) charge-transfer (CT) excited states (HLCT), which we demonstrated through a combined photophysical and computational study. Vacuum-deposited organic light emitting diodes (OLEDs) using these HLCT emitters exhibit maximum external quantum efficiencies (EQE<sub>max</sub>) close to 6%, with impressive exciton utilization efficiency (<em>&Phi;</em><sub>s</sub>) of &gt;50%, far exceeding the spin statistic limit of 25%. We rule out triplet&ndash;triplet annihilation and thermally activated delayed fluorescence as triplet harvesting mechanisms along with horizontal orientation of emitters to enhance light outcoupling and, instead, propose a &ldquo;hot exciton&rdquo; channel involving the nearly isoenergetic T<sub>2</sub>&nbsp;and S<sub>1</sub>&nbsp;states.</p>


Ultrafast Charge Dynamics in Dilute-Donor versus Highly Intermixed TAPC:C60 Organic Solar Cell Blends.

The journal of physical chemistry letters (2020)

GJ Moore, M Causa', JF Martinez Hardigree, S Karuthedath, IR Ramirez, A Jungbluth, F Laquai, MK Riede, N Banerji

Elucidating the interplay between film morphology, photophysics, and device performance of bulk heterojunction (BHJ) organic photovoltaics remains challenging. Here, we use the well-defined morphology of vapor-deposited di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC):C60 blends to address charge generation and recombination by transient ultrafast spectroscopy. We gain relevant new insights to the functioning of dilute-donor (5% TAPC) fullerene-based BHJs compared to molecularly intermixed systems (50% TAPC). First, we show that intermolecular charge transfer (CT) excitons in the C60 clusters of dilute BHJs rapidly localize to Frenkel excitons prior to dissociating at the donor:acceptor interface. Thus, both Frenkel and CT excitons generate photocurrent over the entire fullerene absorption range. Second, we selectively monitor interfacial and bulk C60 clusters via their electro-absorption, demonstrating an energetic gradient that assists free charge generation. Third, we identify a fast (< 1 ns) recombination channel, whereby free electrons recombine with trapped holes on isolated TAPC molecules. This can harm the performance of dilute solar cells, unless the electrons are rapidly extracted in efficient devices.


Simple technique for determining the refractive index of phase-change materials using near-infrared reflectometry

Optical Materials Express The Optical Society 10 (2020) 1675-1675

E Gemo, S Kesava, C Ruiz De Galarreta, L Trimby, S García-Cuevas Carrillo, M Riede, A Baldycheva, A Alexeev, C Wright


CsI-antisolvent adduct formation in all-inorganic metal halide perovskites

Advanced Energy Materials Wiley 10 (2020) 1903365

T Moot, A Marshall, L Wheeler, S Habisreutinger, T Schloemer, CC Boyd, D Dikova, G Pach, M McGehee, A Hazarika, H Snaith, J Luther

The excellent optoelectronic properties demonstrated by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high‐performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead‐halide precursor salts. These adduct‐based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all‐inorganic perovskites such as CsPbI3. In this work, inorganic perovskite composition CsPbI3 is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A‐site cation salts. These CsI adducts control nucleation more so than the PbI2–dimethyl sulfoxide (DMSO) adduct and demonstrate how the A‐site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI–MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.


CsPbBr3 nanocrystal films: Deviations from bulk vibrational and optoelectronic properties

Advanced Functional Materials Wiley (2020) 1909904

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

Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr3 NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.


Preventing phase segregation in mixed-halide perovskites: a perspective

Energy & Environmental Science Royal Society of Chemistry (RSC) (2020)

AJ Knight, LM Herz

&lt;p&gt;Halide segregation represents a severe stability problem for certain mixed-halide perovskites. Here we explore a myriad of methods for mitigating halide segregation, including several largely unexplored approaches that show significant promise.&lt;/p&gt;


Efficiency enhancement of small molecule organic solar cells using hexapropyltruxene as an interface layer

JOURNAL OF MATERIALS CHEMISTRY C 8 (2020) 4909-4918

H Ye, SV Kesava, JFM Hardigree, RE Brown, G Mazzotta, R Warren, PJ Skabara, M Riede

This journal is © The Royal Society of Chemistry. The quenching of excitons in organic solar cells can play a significant role in limiting their power conversion efficiency (PCE). In this article, we investigate the effect of a thin layer of hexapropyltruxene inserted at the interface between the electron donor boron subphthalocyanine chloride (SubPc) and its underlying hole contact in planar heterojunction solar cells. We find that a 3.8 nm hexapropyltruxene interlayer between the molybdenum oxide (MoOx) hole contact and SubPc is sufficient to improve PCE in SubPc/C60 fullerene solar cells from 2.6% to 3.0%, a ∼20% performance improvement. While the absorption stays roughly the same, the comparison of external and internal quantum efficiencies reveals a significant increase in SubPc's contribution to the current for light with wavelengths between 520 and 600 nm. Microstructure and surface morphology assessed with in situ Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) and Atomic Force Microscopy (AFM), are evaluated alongside in situ spectroscopic ellipsometry, and photoluminescence measurements. The microstructural investigations demonstrate changes to the surface and bulk of SubPc grown atop a hexapropyltruxene interlayer indicating that the latter acts as a template layer in a similar way as MoOx. However, the improvement in PCE is found to be mainly via reduced exciton quenching at the MoOx contact with the insertion of the hexapropyltruxene layer.


Charge-Carrier Trapping Dynamics in Bismuth-Doped Thin Films of MAPbBr3 Perovskite.

The journal of physical chemistry letters American Chemical Society (ACS) 11 (2020) 3681-3688

AM Ulatowski, AD Wright, B Wenger, LRV Buizza, SG Motti, HJ Eggimann, KJ Savill, J Borchert, HJ Snaith, MB Johnston, LM Herz

Successful chemical doping of metal halide perovskites with small amounts of heterovalent metals has attracted recent research attention because of its potential to improve long-term material stability and tune absorption spectra. However, some additives have been observed to impact negatively on optoelectronic properties, highlighting the importance of understanding charge-carrier behavior in doped metal halide perovskites. Here, we present an investigation of charge-carrier trapping and conduction in films of MAPbBr3 perovskite chemically doped with bismuth. We find that the addition of bismuth has no effect on either the band gap or exciton binding energy of the MAPbBr3 host. However, we observe a substantial enhancement of electron-trapping defects upon bismuth doping, which results in an ultrafast charge-carrier decay component, enhanced infrared emission, and a notable decrease of charge-carrier mobility. We propose that such defects arise from the current approach to Bi-doping through addition of BiBr3, which may enhance the presence of bromide interstitials.


Elucidating the Role of a Tetrafluoroborate-Based Ionic Liquid at the n-Type Oxide/Perovskite Interface

ADVANCED ENERGY MATERIALS 10 (2020) ARTN 1903231

NK Noel, SN Habisreutinger, B Wenger, Y-H Lin, F Zhang, JB Patel, A Kahn, MB Johnston, HJ Snaith


Filamentary high-resolution electrical probes for nanoengineering

Nano Letters American Chemical Society (2020)

E Soh, G Syed, G Mazzotta, BF Porter, MK Riede, R Nicholas, JS Kim, H Bhaskaran

Confining electric fields to a nanoscale region is challenging yet crucial for applications such as high resolution probing of electrical properties of materials and electric-field manipulation of nanoparticles. State-of-the-art techniques involving atomic force microscopy typically have a lateral resolution limit of tens of nanometers due to limitations in the probe geometry and stray electric fields that extend over space. Engineering the probes is the most direct approach to improving this resolution limit. However, current methods to fabricate high-resolution probes, which can effectively confine the electric fields laterally involve expensive and sophisticated probe manipulation, which has limited the use of this approach. Here, we demonstrate that nanoscale phase switching of configurable thin films on probes can result in high-resolution electrical probes. These configurable coatings can be both germanium-antimony-tellurium (GST) as well as amorphous-carbon, materials known to undergo electric field-induced non-volatile, yet reversible switching. By forming a localized conductive filament through phase transition, we demonstrate a spatial resolution of electrical field beyond the geometrical limitations of commercial platinum probes (i.e. an improvement of ~48%). We then utilize these confined electric fields to manipulate nanoparticles with single nanoparticle precision via dielectrophoresis. Our results advance the field of nanomanufacturing and metrology with direct applications for pick and place assembly at the nanoscale.


Near-strain-free GaN/AlGaN narrow line width UV light emission with very stable wavelength on excitation power by using superlattices

ACS Applied Electronic Materials American Chemical Society 2 (2020) 571-579

M Li, F Chen, C Kocher, H Zhang, S Li, F Huang, J Zhang, RA Taylor

<p>Because of the strong strain in nitrides, superlattice layers have been used to release the strain in the QW and reduce the quantum confined Stark effect. However, few reports discuss comprehensively the strain relaxation behavior and optical performance of a GaN/AlGaN single quantum well (QW) with inserted GaN/AlGaN superlattices (SLs). In this work, we examined a group of graded Al content GaN/Al<sub><em>x</em></sub>Ga<sub>1&ndash;<em>x</em></sub>N SL layers under the GaN/Al<sub>0.3</sub>Ga<sub>0.7</sub>N single QW grown on&nbsp;<em>c</em>-plane sapphire. Both the excitation power and temperature dependence of the time-integrated micro-photoluminescence (&mu;-PL) and time-resolved &mu;-PL were measured. The samples exhibited very narrow UV emission and had almost unchanged emission wavelength and stable line width behavior with excitation power as well as &ldquo;S-shape&rdquo; and weak &ldquo;W-shape&rdquo; characteristics with temperature due to the localization. The temperature-dependent PL lifetime was measured from 5 to 300 K, and the relatively fast recombination lifetime of the two samples was examined. Micro-Raman spectroscopy was also conducted to probe the strain state. All the results showed that adopting SLs around the QW structure produced a much more stable and desirable performance, which can be attributed to an effective relaxation of the strain in the QW.</p>


Optical shaping of the polarization anisotropy in a laterally coupled quantum dot dimer.

Light, science & applications 9 (2020) 100-

H Kim, K Kyhm, RA Taylor, JS Kim, JD Song, S Park

We find that the emission from laterally coupled quantum dots is strongly polarized along the coupled direction [1 1¯ 0], and its polarization anisotropy can be shaped by changing the orientation of the polarized excitation. When the nonresonant excitation is linearly polarized perpendicular to the coupled direction [110], excitons (X1 and X2) and local biexcitons (X1X1 and X2X2) from the two separate quantum dots (QD1 and QD2) show emission anisotropy with a small degree of polarization (10%). On the other hand, when the excitation polarization is parallel to the coupled direction [1 1¯ 0], the polarization anisotropy of excitons, local biexcitons, and coupled biexcitons (X1X2) is enhanced with a degree of polarization of 74%. We also observed a consistent anisotropy in the time-resolved photoluminescence. The decay rate of the polarized photoluminescence intensity along the coupled direction is relatively high, but the anisotropic decay rate can be modified by changing the orientation of the polarized excitation. An energy difference is also observed between the polarized emission spectra parallel and perpendicular to the coupled direction, and it increases by up to three times by changing the excitation polarization orientation from [110] to [1 1¯ 0]. These results suggest that the dipole-dipole interaction across the two separate quantum dots is mediated and that the anisotropic wavefunctions of the excitons and biexcitons are shaped by the excitation polarization.


Trap states, electric fields, and phase segregation in mixed-halide perovskite photovoltaic devices

Advanced Energy Materials Wiley 10 (2020) 1903488

A Knight, J Patel, H Snaith, M Johnston, L Herz

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.


Three-dimensional cross-nanowire networks recover full terahertz state.

Science (New York, N.Y.) 368 (2020) 510-513

K Peng, D Jevtics, F Zhang, S Sterzl, DA Damry, MU Rothmann, B Guilhabert, MJ Strain, HH Tan, LM Herz, L Fu, MD Dawson, A Hurtado, C Jagadish, MB Johnston

Terahertz radiation encompasses a wide band of the electromagnetic spectrum, spanning from microwaves to infrared light, and is a particularly powerful tool for both fundamental scientific research and applications such as security screening, communications, quality control, and medical imaging. Considerable information can be conveyed by the full polarization state of terahertz light, yet to date, most time-domain terahertz detectors are sensitive to just one polarization component. Here we demonstrate a nanotechnology-based semiconductor detector using cross-nanowire networks that records the full polarization state of terahertz pulses. The monolithic device allows simultaneous measurements of the orthogonal components of the terahertz electric field vector without cross-talk. Furthermore, we demonstrate the capabilities of the detector for the study of metamaterials.


Toward understanding space-charge limited current measurements on metal halide perovskites

ACS Energy Letters American Chemical Society 5 (2020) 376-384

E Duijnstee, JM Ball, VM Le Corre, LJA Koster, HJ Snaith, J Lim

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