Publications by Ashley Marshall

Dimethylammonium: An A‐site Cation for Modifying CsPbI3

Solar RRL Wiley (0) solr.202000599

AR Marshall, HC Sansom, MM McCarthy, JH Warby, OJ Ashton, B Wenger, HJ Snaith

A phosphine oxide route to formamidinium lead tribromide nanoparticles

Chemistry of Materials American Chemical Society (2020)

OJ Ashton, AR Marshall, JH Warby, B Wenger, HJ Snaith

We present the synthesis of formamidinium lead tribromide (FAPbBr3) perovskite nanocrystals through a phosphine oxide route, where, in comparison to more traditional syntheses, oleylamine is replaced with trioctylphosphine oxide (TOPO). This route has previously been shown to be successful for the inorganic cesium lead tribromide perovskite nanocrystals. We examine the interactions between the precursors via nuclear magnetic resonance spectroscopy (NMR). We confirm the existence of an interaction between FA-oleate and TOPO and use this to guide the optimisation of our synthesis. When the reaction is conducted at room temperature, we observe the formation of nanoparticles with high photoluminescence quantum yield (PLQY, ~70 %) at 2.39 eV (518 nm) with little ripening or size defocusing over time. Although we obtain narrow emission peaks, the crystals are irregular in shape, testament to the impact of the FA-oleate:TOPO interaction. Despite a drop in PLQY in the washed solutions, films made maintain a high PLQY of ~ 50 % at 2.33 eV (532 nm), which is fortuitously the ideal wavelength for the green emission channel in displays, and we demonstrate 532 nm electroluminescence in light emitting diodes with an EQE of 3.7 %.

Revealing factors influencing the operational stability of perovskite light-emitting diodes

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

JH Warby, B Wenger, AJ Ramadan, R Oliver, H Sansom, A Marshall, H Snaith

Light-emitting diodes (LEDs) made from metal halide perovskites have demonstrated external electroluminescent quantum efficiencies (EQEEL) in excess of 20%. However, their poor operational stability, resulting in lifetimes of only tens to hundreds of hours, needs to be dramatically improved prior to commercial use. There is little consensus in the community upon which factors limit the stability of these devices. Here, we investigate the role played by ammonium cations on the operational stability. We vary the amount of phenylethylammonium bromide, a widely used alkylammonium salt, that we add to a precursor solution of CsPbBr3 and track changes in stability and EQEEL. We find that while phenylethylammonium bromide is beneficial in achieving high efficiency, it is highly detrimental to operational stability. We investigate material properties and electronic characteristics before and after degradation and find that both a reduction in the radiative efficiency of the emitter and significant changes in current–voltage characteristics explain the orders of magnitude drop in the EQEEL, which we attribute to increased ionic mobility. Our results suggest that engineering new contacts and further investigation into materials with lower ionic mobility should yield much improved stability of perovskite LEDs.

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%.

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.

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.

Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells.

Science advances 3 (2017) eaao4204-eaao4204

EM Sanehira, AR Marshall, JA Christians, SP Harvey, PN Ciesielski, LM Wheeler, P Schulz, LY Lin, MC Beard, JM Luther

We developed lead halide perovskite quantum dot (QD) films with tuned surface chemistry based on A-site cation halide salt (AX) treatments. QD perovskites offer colloidal synthesis and processing using industrially friendly solvents, which decouples grain growth from film deposition, and at present produce larger open-circuit voltages (VOC's) than thin-film perovskites. CsPbI3 QDs, with a tunable bandgap between 1.75 and 2.13 eV, are an ideal top cell candidate for all-perovskite multijunction solar cells because of their demonstrated small VOC deficit. We show that charge carrier mobility within perovskite QD films is dictated by the chemical conditions at the QD-QD junctions. The AX treatments provide a method for tuning the coupling between perovskite QDs, which is exploited for improved charge transport for fabricating high-quality QD films and devices. The AX treatments presented here double the film mobility, enabling increased photocurrent, and lead to a record certified QD solar cell efficiency of 13.43%.

Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%

NATURE ENERGY 2 (2017) ARTN 17052

Y Yan, RW Crisp, J Gu, BD Chernomordik, GF Pach, AR Marshall, JA Turner, MC Beard

Quantum Dot Solar Cell Fabrication Protocols

CHEMISTRY OF MATERIALS 29 (2017) 189-198

BD Chernomordik, AR Marshall, GF Pach, JM Luther, MC Beard

Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics

Science American Association for the Advancement of Science 354 (2016) 92-95

A Swarnkar, AR Marshall, EM Sanehira, BD Chernomordik, DT Moore, JA Christians, T Chakrabarti, JM Luther

We show nanoscale phase stabilization of CsPbI3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI3 (α-CsPbI3)-the variant with desirable band gap-is only stable at high temperatures. We describe the formation of α-CsPbI3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.

Nongeminate radiative recombination of free charges in cation-exchanged PbS quantum dot films

CHEMICAL PHYSICS 471 (2016) 75-80

AR Marshall, MC Beard, JC Johnson

Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films.

ACS nano 10 (2016) 3302-3311

EM Miller, DM Kroupa, J Zhang, P Schulz, AR Marshall, A Kahn, S Lany, JM Luther, MC Beard, CL Perkins, J van de Lagemaat

We use a high signal-to-noise X-ray photoelectron spectrum of bulk PbS, GW calculations, and a model assuming parabolic bands to unravel the various X-ray and ultraviolet photoelectron spectral features of bulk PbS as well as determine how to best analyze the valence band region of PbS quantum dot (QD) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are commonly used to probe the difference between the Fermi level and valence band maximum (VBM) for crystalline and thin-film semiconductors. However, we find that when the standard XPS/UPS analysis is used for PbS, the results are often unrealistic due to the low density of states at the VBM. Instead, a parabolic band model is used to determine the VBM for the PbS QD films, which is based on the bulk PbS experimental spectrum and bulk GW calculations. Our analysis highlights the breakdown of the Brillioun zone representation of the band diagram for large band gap, highly quantum confined PbS QDs. We have also determined that in 1,2-ethanedithiol-treated PbS QD films the Fermi level position is dependent on the QD size; specifically, the smallest band gap QD films have the Fermi level near the conduction band minimum and the Fermi level moves away from the conduction band for larger band gap PbS QD films. This change in the Fermi level within the QD band gap could be due to changes in the Pb:S ratio. In addition, we use inverse photoelectron spectroscopy to measure the conduction band region, which has similar challenges in the analysis of PbS QD films due to a low density of states near the conduction band minimum.

The preparation and properties of carbon inverse opal papers using carbon fiber sheets as a framework


JC Lytle, JM Banbury, RA Blakney, MS Burke, RPA Clark, RD Fisher, SV Frederiksen, AR Marshall, MT McNally, ML Ostendorf, KN Serier, M Shiu, RE Toivola, CS Travers, ER Wright

Metal halide solid-state surface treatment for high efficiency PbS and PbSe QD solar cells.

Scientific reports 5 (2015) 9945-

RW Crisp, DM Kroupa, AR Marshall, EM Miller, J Zhang, MC Beard, JM Luther

We developed a layer-by-layer method of preparing PbE (E = S or Se) quantum dot (QD) solar cells using metal halide (PbI2, PbCl2, CdI2, or CdCl2) salts dissolved in dimethylformamide to displace oleate surface ligands and form conductive QD solids. The resulting QD solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. We find that the PbI2 treatment is the most successful in removing alkyl surface ligands and also replaces most surface bound Cl(-) with I(-). The treatment protocol results in PbS QD films exhibiting a deeper work function and band positions than other ligand exchanges reported previously. The method developed here produces solar cells that perform well even at film thicknesses approaching a micron, indicating improved carrier transport in the QD films. We demonstrate QD solar cells based on PbI2 with power conversion efficiencies above 7%.

Air-Stable and Efficient PbSe Quantum-Dot Solar Cells Based upon ZnSe to PbSe Cation-Exchanged Quantum Dots.

ACS nano 9 (2015) 8157-8164

S Kim, AR Marshall, DM Kroupa, EM Miller, JM Luther, S Jeong, MC Beard

We developed a single step, cation-exchange reaction that produces air-stable PbSe quantum dots (QDs) from ZnSe QDs and PbX2 (X = Cl, Br, or I) precursors. The resulting PbSe QDs are terminated with halide anions and contain residual Zn cations. We characterized the PbSe QDs using UV-vis-NIR absorption, photoluminescence quantum yield spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Solar cells fabricated from these PbSe QDs obtained an overall best power conversion efficiency of 6.47% at one sun illumination. The solar cell performance without encapsulation remains unchanged for over 50 days in ambient conditions; and after 50 days, the National Renewable Energy Laboratory certification team certified the device at 5.9%.

Exploration of Metal Chloride Uptake for Improved Performance Characteristics of PbSe Quantum Dot Solar Cells.

The journal of physical chemistry letters 6 (2015) 2892-2899

AR Marshall, MR Young, AJ Nozik, MC Beard, JM Luther

We explored the uptake of metal chloride salts with +1 to +3 metals of Na(+), K(+), Zn(2+), Cd(2+), Sn(2+), Cu(2+), and In(3+) by PbSe QD solar cells. We also compared CdCl2 to Cd acetate and Cd nitrate treatments. PbSe QD solar cells fabricated with a CdCl2 treatment are stable for more than 270 days stored in air. We studied how temperature and immersion times affect optoelectronic properties and photovoltaic cell performance. Uptake of Cd(2+) and Zn(2+) increase open circuit voltage, whereas In(3+) and K(+) increase the photocurrent without influencing the spectral response or first exciton peak position. Using the most beneficial treatments we varied the bandgap of PbSe QD solar cells from 0.78 to 1.3 eV and find the improved VOC is more prevalent for lower bandgap QD solar cells.

Multiple Exciton Generation Solar Cells: Effects of Nanocrystal Shape on Quantum Efficiency


AR Marshall, MC Beard, JM Luther, IEEE