Publications by Ashley Marshall


Perovskite Quantum-Dot Photovoltaic Materials beyond the Reach of Thin Films: Full Range Tuning of A-site Cation Composition

ACS Nano American Chemical Society (2018)

A Hazarika, Q Zhao, A Gaulding, JA Christians, B Dou, AR MARSHALL, T Moot, JJ Berry, JC Johnson, JM Luther


Targeted Ligand-Exchange Chemistry on Cesium Lead Halide Perovskite Quantum Dots for High-Efficiency Photovoltaics.

Journal of the 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


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

JOURNAL OF MATERIALS CHEMISTRY A 4 (2016) 3494-3503

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


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.


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%.


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

2014 IEEE 40TH PHOTOVOLTAIC SPECIALIST CONFERENCE (PVSC) (2014) 1077-1079

AR Marshall, MC Beard, JM Luther, IEEE