About

Neal R. Armstrong is a Regents' Professor of Chemistry and Biochemistry at The University of Arizona, affiliated with the Department of Chemistry and Biochemistry. He also holds the position of Regents' Professor of Optical Sciences and serves as the Director of the Energy Frontier Research Center for Interface Science: Solar Electric Materials. His research interests include interface characterization and modification, new molecular materials for energy conversion, light emission, and sensing. He specializes in surface photoemission spectroscopies such as UPS and XPS, scanning probe microscopies, electrochemistry, and related fields. Dr. Armstrong earned his Ph.D. from the University of New Mexico in 1974 and his B.S. from the same institution in 1970.

Research topics

• Materials science
• Chemistry
• Chemical engineering
• Optoelectronics
• Nanotechnology
• Inorganic chemistry
• Physical chemistry
• Organic chemistry
• Crystallography
• Metallurgy

Selected publications

• Electrochemical reactions under reverse bias create additional mobile ions that enable hole tunneling in metal halide perovskite diodes

  arXiv (Cornell University) · 2026-04-20

  preprintOpen access

  Gradual reverse-bias breakdown in metal-halide perovskite diodes and solar cells is thought to originate from hole tunneling through steep bands in an ionic depletion region near the electron transport layer after positively charged iodine vacancies accumulate near the hole-transport layer (HTL). However, typical reported mobile ion concentrations near $1\times10^{17}$ cm$^{-3}$ are too small to quantitatively explain significant tunneling current densities and (Zener) breakdown observed near $-5$ V. Here, we show that inferred mobile ion concentrations increase by more than 100$\times$, to over $1\times10^{18}$ cm$^{-3}$, within just three minutes of reverse bias at $-6.0$ V in p-i-n perovskite diodes. We attribute the increase in mobile ion concentration to iodide oxidation and the resulting iodine vacancy creation which must be balanced by reduction reactions near the HTL. Thin and sub-optimal HTL coverage leads to direct contact between the transparent conducting electrode and perovskite and facilitates electron transfer and reduction, enabling the creation of even larger inferred mobile ion concentrations ($\sim1\times10^{19}$ cm$^{-3}$) and leading to faster degradation under reverse bias. This explains previous work that showed increased breakdown voltages and improved reverse-bias stability by implementing thick, uniform HTLs.

  PublisherDOI

• Electrochemical reactions under reverse bias create additional mobile ions that enable hole tunneling in metal halide perovskite diodes

  ArXiv.org · 2026-04-20

  articleOpen access

  Gradual reverse-bias breakdown in metal-halide perovskite diodes and solar cells is thought to originate from hole tunneling through steep bands in an ionic depletion region near the electron transport layer after positively charged iodine vacancies accumulate near the hole-transport layer (HTL). However, typical reported mobile ion concentrations near $1\times10^{17}$ cm$^{-3}$ are too small to quantitatively explain significant tunneling current densities and (Zener) breakdown observed near $-5$ V. Here, we show that inferred mobile ion concentrations increase by more than 100$\times$, to over $1\times10^{18}$ cm$^{-3}$, within just three minutes of reverse bias at $-6.0$ V in p-i-n perovskite diodes. We attribute the increase in mobile ion concentration to iodide oxidation and the resulting iodine vacancy creation which must be balanced by reduction reactions near the HTL. Thin and sub-optimal HTL coverage leads to direct contact between the transparent conducting electrode and perovskite and facilitates electron transfer and reduction, enabling the creation of even larger inferred mobile ion concentrations ($\sim1\times10^{19}$ cm$^{-3}$) and leading to faster degradation under reverse bias. This explains previous work that showed increased breakdown voltages and improved reverse-bias stability by implementing thick, uniform HTLs.

  PublisherOA PDF

• Degradation Dynamics of Perovskite Solar Cells Under Fixed Reverse Current Injection

  ArXiv.org · 2026-03-20

  articleOpen access

  Previous studies of reverse-bias stability in perovskite solar cells have focused primarily on voltage controlled reverse-bias tests. Here we instead present an investigation of perovskite solar cell degradation under well-defined, constant reverse-current stress. We show that the choice of hole-transport layer dictates the dominant degradation pathway: cells using thick poly(triphenylamine) (PTAA) layers with better indium-doped tin oxide (ITO) coverage can tolerate high reverse bias but quickly undergo catastrophic breakdown under fixed reverse current near their one-sun maximum power-point. In contrast, cells modified with the phosphonic-acid interface layer MeO-2PACz, with poorer ITO coverage compared to PTAA, exhibit soft, gradual, and largely recoverable degradation, regardless of the shading conditions. For MeO-2PACz devices, degradation increases with both current magnitude and duration. Importantly, when normalized by injected charge (current times duration), lower currents applied over longer times cause more severe degradation than higher currents over shorter periods. Combining electrical measurements with spatially resolved photoluminescence imaging, we argue against shunt formation and instead support an ion- and charge-mediated interfacial electrochemical degradation mode.

  PublisherOA PDF

• Deposition-Dependent Coverage and Performance of Phosphonic Acid Interface Modifiers in Halide Perovskite Optoelectronics

  ACS Applied Materials & Interfaces · 2025-11-26

  articleCorresponding

  In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the method of deposition, specifically spin coating or prolonged dip coating; ITO surface treatment via HCl/FeCl3 etching; and use in combination with a second modifier, 1,6-hexylenediphosphonic acid. We demonstrate that varying these modification protocols can impact time-resolved photoluminescence carrier lifetimes and quasi-Fermi level splitting of perovskite films deposited onto the phosphonic acid-modified ITO. Ultraviolet photoelectron spectroscopy shows an increase in the effective work function after phosphonic acid modification and clear evidence for photoemission from carbazole functional groups at the ITO surface. We used X-ray photoelectron spectroscopy to probe differences in phosphonic acid coverage on the metal oxide contact and show that perovskite samples grown on ITO with the highest phosphonic acid coverage exhibit the longest carrier lifetimes. Finally, we establish that device performance follows these same trends. These results indicate that the reactivity, heterogeneity, and composition of the bottom contact help to control recombination rates and therefore power conversion efficiencies. ITO etching, prolonged deposition times for phosphonic acids via dip coating, and the use of a secondary, more hydrophilic bisphosphonic acid all contribute to improvements in surface coverage, carrier lifetime, and device efficiency. These improvements each have a positive impact, and we achieve the best results when all three strategies are implemented.

  PublisherDOI

• Vibrational Probe of Electrical Doping in N2200 and Fermi Level Alignment at Polymer Cathode/Metal Cocatalyst/Electrolyte Junction

  ChemRxiv · 2025-04-18

  preprintOpen access

  Hybrid (photo-) cathodes consisted of conjugated polymer and hydrogen evolution reaction (HER) co-catalysts are an emerging platform for low-cost solar fuels generation. The electron-accepting Poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′bithiophene)}, known as P(NDI2OD-T2) or N2200, is a promising material to serve as a conjugated polymer cathode or as the electron acceptor for bulk heterojunction photocathodes. Unlike inorganic semiconductor/metal junctions, much less is known about the energetic alignment of the conjugated polymer electrode/metal junctions, hindering rational development of this emerging class of electrodes. In this work, we investigate the electrical doping behavior in N2200 cathode and its Fermi level alignment with gold nanoparticles, which is used here as a model for hydrogen evolution metal cocatalyst. Through UV/visible, Raman and attenuated total-reflectance infrared spectroelectrochemistry, we observed the impact of electrical doping on the vibrational frequencies of neutral, polaron and dianion species in N2200, which suggests electron density changes within the corresponding NDI units. Upon one-electron reduction, C=O stretching frequency of the polaron unit shows a red shift by ~ 68 cm-1, indicating an increased electron density in the C=O bond. Additionally, the C=O stretching frequency of neutral units in the doped N2200 shows a minor red shift of ~ 5 cm-1, suggesting charge transfer from neighboring polaron units. Surface-enhanced Raman spectroscopy measurements of gold nanoparticle-functionalized N2200 electrode revealed that the Au Fermi level only shifts with that of the N2200 upon the polaron formation. This mechanistic study of the electron transfer from doped N2200 to the metal nanoparticles provides insight for the future design of the HER (photo)cathodes – the formal potential of the polymer polaron formation determines the behavior of the catalyst Fermi level and thus modulates the reaction capability.

  PublisherOA PDFDOI

• In Situ Electrochemistry of Buried Interfaces in Metal Halide Perovskites: Probing Energy Bands, Halide Redox Activity, and Kinetics

  Advanced Energy Materials · 2025-09-22 · 1 citations

  articleOpen access

  Abstract Control over charge injection and extraction processes across buried interfaces is fundamental for all (opto)electronic multilayer device platforms, necessitating detailed understanding of local structural and chemical differences that promote defect formation, distort energetic band‐edge alignments, and alter charge transport processes. Herein, the implementation of a low‐cost electroanalytical methodologies’ tool suite is described to quantitatively characterize buried interfaces and redox reactions in printable, mixed electrical–ionic, and redox‐active metal halide perovskites and a prototypical hole‐transporting nickel oxide (NiO x ) thin film. The objective is to demonstrate the power of electrochemical methodologies to improve the nanoscale understanding of complex interfaces within optoelectronic devices by providing case studies on how to: i) differentiate between electronic and chemical properties in NiO x contacts; ii) measure changes in reversibility of halide redox reactions via NiO x surface states; iii) assess energy alignment and charge transport across (modified) buried interfaces; and iv) quantify defects at buried interfaces that change with modifiers and differences in perovskite processing, including increasing defect concentrations when films are slot‐die‐coated versus spin‐cast. The collective approach addresses major challenges in understanding the precise energy landscape and interface reactivity under relevant electric fields that mimic operando conditions (away from equilibrium) and across length scales in thin film device formats.

  PublisherOA PDFDOI

• Activated Corrosion and Recovery in Lead Mixed-Halide Perovskites Revealed by Dynamic Near-Ambient Pressure X-ray Photoelectron Spectroscopy

  Journal of the American Chemical Society · 2025-02-27 · 8 citations

  articleOpen access

  /light catalyst is removed, postulated to be due to mobile halide species present within the film below XPS sampling depth. Small deviations in near-surface composition (<2%) of the perovskite are used to connect reaction rates to quantified, near-band edge donor and acceptor defect concentrations, demonstrating two energetically distinct sites are responsible for the redox process. Collectively, environmental flux and rate quantification are deemed critical for the future elucidation of chemical degradation processes in perovskites, where rate-dependent reaction pathways are expected to be very system dependent (environment and material).

  PublisherOA PDFDOI

• Deposition-Dependent Coverage and Performance of Phosphonic Acid Interface Modifiers in Halide Perovskite Optoelectronics

  ArXiv.org · 2025-06-24

  preprintOpen access

  In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the method of deposition, specifically spin coating or prolonged dip coating, ITO surface treatment via HCl/FeCl3 etching, and use in combination with a second modifier, 1,6-hexylenediphosphonic acid. We demonstrate that varying these modification protocols can impact time-resolved photoluminescence carrier lifetimes and quasi-Fermi level splitting of perovskite films deposited onto the phosphonic-acid-modified ITO. Ultraviolet photoelectron spectroscopy shows an increase in effective work function after phosphonic acid modification and clear evidence for photoemission from carbazole functional groups at the ITO surface. We use X-ray photoelectron spectroscopy to probe differences in phosphonic acid coverage on the metal oxide contact and show that perovskite samples grown on ITO with the highest phosphonic acid coverage exhibit the longest carrier lifetimes. Finally, we establish that device performance follows these same trends. These results indicate that the reactivity, heterogeneity, and composition of the bottom contact help to control recombination rates and therefore power conversion efficiencies. ITO etching, prolonged deposition times for phosphonic acids via dip coating, and the use of a secondary, more hydrophilic bis-phosphonic acid, all contribute to improvements in surface coverage, carrier lifetime, and device efficiency. These improvements each have a positive impact, and we achieve the best results when all three strategies are implemented.

  PublisherOA PDFDOI

• Extremely Long-Lived Charge Donor States Formed by Visible Irradiation of Quantum Dots

  ACS Nano · 2024-08-20 · 15 citations

  article

  Using cyclic voltammetry under illumination, we recently demonstrated that CdS quantum dots (QDs) form charge donor states that live for at least several minutes after illumination ends, ∼12 orders of magnitude longer than expected for free carriers. This time scale suggests that the conventionally accepted mechanism of charge transfer, wherein charges directly transfer to an acceptor following exciton dissociation, cannot be complete. Because of these long time scales, this unconventional pathway is not readily observed using time-resolved spectroscopy to probe charge transfer dynamics. Here, we investigated the chemical nature of these charge donor states using cyclic voltammetry under illumination coupled with NMR spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and optical spectroscopy. Our data reveal that charges are stored locally rather than as free carriers, and the number of charges stored is dependent on the QD surface ligation and stoichiometry. Altogether, our results confirm that electrons are stored at ligated surface Cd, these sites are competent charge donors, and this storage is charge balanced by X-type ligand desorption. We found that charge storage occurs in every QD system studied, including CdS, CdSe, and InP capped with carboxylate and phosphonate ligands.

  PublisherDOI

• Extremely long-lived charge donor states formed by visible irradiation of quantum dots

  ChemRxiv · 2024-04-23 · 2 citations

  preprintOpen access

  Using cyclic voltammetry under illumination, we recently demonstrated that CdS quantum dots (QDs) form charge donor states that live for at least several minutes after illumination ends, ~12 orders of magnitude longer than expected for free carriers. This timescale suggests that the conventionally accepted mechanism of charge transfer, wherein charges directly transfer to an acceptor following exciton dissociation, cannot be complete. Because of these long timescales, this unconventional pathway is not readily observed using time-resolved spectroscopy to probe charge transfer dynamics. Here, we investigated the chemical nature of these charge donor states using cyclic voltammetry under illumination coupled with NMR spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and optical spectroscopy. Our data reveal that charges are stored locally rather than as free carriers, and the number of charges stored is dependent on the QD surface ligation and stoichiometry. Altogether, our results confirm that electrons are stored at ligated surface Cd, these sites are competent charge donors, and this storage is charge balanced by X-type ligand desorption. We found that charge storage occurs in every QD system studied, including CdS, CdSe, and InP capped with carboxylate and phosphonate ligands.

  PublisherOA PDFDOI

Recent grants

• Ultrathin Film Molecular Electronic Materials: Self-Organizing Discotic Mesophases and Organic Heterojunctions

  NSF · $860k · 2005–2009

• Characterization and Control of Structure, Energetics and Electrical Properties at Interfaces between Perovskite Active Layers and Charge-Collection Electrodes

  NSF · $404k · 2015–2018

Frequent coauthors

• Seth R. Marder

  71 shared

• S. Scott Saavedra

  University of Arizona

  69 shared

• Bernard Kippelen

  Georgia Institute of Technology

  66 shared

• Erin L. Ratcliff

  52 shared

• Kenneth W. Nebesny

  University of Arizona

  49 shared

• Stephen Barlow

  University of Colorado Boulder

  41 shared

• Ghassan E. Jabbour

  University of Ottawa

  41 shared

• Jeffrey Pyun

  University of Arizona

  40 shared

Labs

• Quantum Optomechanics LabPI