Our group is focused on developing the new measurement science required to understand and control the chemical, physical and electrical properties of interfaces between electrical contacts and new electro-optical (active layer) materials, at nanometer length scales. Such studies underpin the development of emerging energy conversion technologies (solar cells, photoactive nanocrystalline materials for light-to-fuel formation), and new active layers for light emisison,
While reasonably efficient energy conversion and light emission platforms are becoming available, they are typyically expensive and not scalable. The challenges in creating massively scalable energy conversion technologies, that are inexpensive and easily integrated into other technology platforms, often lies in undertanding and controling composition, structure and energetics of the interfaces between electrical contacts and active layers. We show four examples here of our research emphasis:
Photoemission spectroscopies (UPS/XPS) used to characterize band edge energies and energetic dispersity in active layer/contact interfacial regions
High vacuum photoemission spectroscopies (UV- and X-ray - UPS/XPS) are used to characterize the band edge (frontier orbital) energies, with high enough sensitivity and energy resolution so as to be able to determine the extent of tailing of valence band states, well out into the bandgap region, as a function of molecular structure and molecular order at the contact/active layer interface.
Mapping electrical property heterogeneity with conductive tip atomic force microscopy (cAFM) at active layer/contact heterojunctions
We have recently shown that cAFM can be used to map the differences in electrical properties at the heterojunctions between prototype active layers and electrical contacts - in addition to imaging topology we take a current/voltage (J/V) curve at sub-micron x,y intervals, fit those J/V plots to conventional models of ohmic versus non-ohmic contacts, and create 2D full color maps which allow us to predict whether a contact pre-treatment or modification will lead to an efficient PV device response.
Creation of new charge selective interlayer materials which optimize collection of photo-generated electrical charges in thin film PV platforms
Large bandgap metal oxide thin films, such as TiO2 and ZnO can be used to enhance selectivity for electron collection in thin film PV platforms, but can be problematic to create in optimized physical, chemical and electrically active formats, as conformal thin films. We recently showed that we can deposit thin oxide layers with optimized performance, using either chemical vapor deposition (CVD) or (in collaboration with groups at Georgia Tech), atomic layer deposition, to achieve electron-selective layers, which are predicted to outperform those created by other deposition means - applicable across a wide range of organic and inorganic-hybrid (perovskite) active layers.
Contact doping of organic active layers - characterization using photoemission spectroscopies
We have recently shown that charge transfer (CT) reactions occur spontaneously at certain contact/donor polymer interfaces, that are relevant to the optimization of thin film PV platforms based on these materials. X-ray photoelectron spectroscopy in particular can be used to track these CT reactions using "reporter atoms" whose chemical shifts in binding energies allow for a quantitative monitoring of these reactions as a function of work function of the contact material.
“Determining Band-edge Energies and Morphology-dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy,” R. Clayton Shallcross, Yilong Zheng, S. Scott Saavedra, N.R. Armstrong, J. Amer. Chem. Soc. March, 2017 -- DOI: 10.1021/jacs.7b00516.
“Surface modification of indium-ˇ‐tin oxide with functionalized perylene diimides: Characterization of orientation, electron-ˇ‐transfer kinetics and electronic structure,” Yilong Zheng, Anthony J. Giordano, R. Clayton Shallcross, Sean R. Fleming, Stephen Barlow, Neal R. Armstrong, Seth R. Marder, S. Scott Saavedra, J. Phys. Chem. C. 120, 20040–20048 (2016) DOI: 10.1021/acs.jpcc.6b06812
“Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin Film Morphology and Device Performance,” Kai-Lin Ou,† Ramanan Ehamparam,† Gordon MacDonald,† Tobias Stubhan,‡ Xin Wu,† R. Clayton Shallcross,† Robin Richards,† Christoph J. Brabec,‡ S. Scott Saavedra,† and Neal R. Armstrong, ACS Applied Materials & Interfaces, 8, 19787-19798 (2016).
“Phosphonic acids for interfacial engineering of transparent conductive oxides,” Sergio A. Paniagua, Anthony J. Giordano, O’Neil L. Smith, Stephen Barlow, Hong Li, Neal R. Armstrong, Jeanne E. Pemberton, Jean-Luc Brédas, David Ginger, and Seth R. Marder, Chemical Reviews, 116, 7117–7158, (2016), DOI: 10.1021/acs.chemrev.6b00061.
“Overcoming Electrode Induced Losses in Organic Solar Cells by Tailoring a Quasi-Ohmic Contact to Fullerenes via Solution-Processed Alkali Hydroxide Layers,” Hong Zhang, R. Clayton Shallcross, Ning Li, Tobias Stubhan, Yi Hou, Wei Chen, Tayebeh Ameri, Mathieu Turbiez, Neal R. Armstrong, Christoph J. Brabec, Advanced Energy Materials, (2016), DOI: 10.1002/aenm.201502195
“Solution Processed Titanyl Phthalocyanines as Donors in Solar Cells: Photoresponse to 1000 nm,” Mayank Mayukh, Mariola R. Macech, Diogenes Placencia, Yu Cao, Neal R. Armstrong, and Dominic V. McGrath, ACS Applied Materials and Interfaces, 9, 8786-8800 (2015), DOI: 10.1021/acsami.5b05900.
“Investigation of Ultra-Thin Titania Films as Hole-Blocking Contacts for Organic Photovoltaics,” Hyungchul Kim, Kai-Lin Ou, Xin Wu, Paul F. Ndione, Joseph Berry, Yannick Lambert, Thierry Mélin, Neal R. Armstrong, and Samuel Graham Jr., Journal of Materials Chemistry A, (2015), DOI: 10.1039/c5ta04687d.
“Characterization of Band Edge Energetics and Rates of Charge Injection Processes in Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers,” Ramanan Ehamparam, Nicholas Pavlopoulos, Michael W. Liao, Lawrence J. Hill, Neal R. Armstrong, Jeffrey Pyun, S. Scott Saavedra, ACS Nano, in press, DOI: 10.1021/acsnano.5b01720
“Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal-Oxide Interfaces and Relationship to Organic Photovoltaic Performance,” Hsiao-Chu Lin, Gordon A. MacDonald, Yanrong Shi,‡ Nate W. Polaske, Dominic V. McGrath, Seth R. Marder, Neal R. Armstrong, Erin L. Ratcliff, S. Scott Saavedra, J. Phys. Chem. C, 119, 10304-10313 (2015) DOI: 10.1021/acs.jpcc.5b02971
“Quantification of Contact Doping Due to Fermi-Level Pinning at the Interface Between High Work Function Electrical Contacts and P3HT,” R. Clayton Shallcross, Tobias Stubhan, Erin L. Ratcliff, Antoine Kahn, Christoph J. Brabec, and Neal R. Armstrong, J. Phys. Chem. Lett., 6, 1303-1309 (2015), DOI: 10.1021/acs.jpclett.5b00444.
“Nickel oxide interlayer films from nickel formate-ethylenediamine precursor: Influence of annealing on thin film properties and photovoltaic device performance,” K. X. Steirer, R. E. Richards, A. K. Sigdel, A. Garcia, P. F. Ndione, S. Hammond, D. Baker, E. L. Ratcliff, C. Curtis, T. Furtak, D. S. Ginley, D. C. Olson, N. R. Armstrong, J. J. Berry, J. Materials Chemistry A, 3, 10949–10958, (2015), DOI: 10.1039/c5ta01379h.
“Pentafluorophenoxy Boron Subphthalocyanine (F5BsubPc) as a Multifunctional Material for Organic Photovoltaics,” Graham E. Morse, Jeremy L. Gantz, K. Xerxes Steirer, Neal R. Armstrong, and Timothy P. Bender, ACS Applied Mater. Int., 6, 1515-1524 (2014), DOI: 10.1021/am404179z
"Organic Solar Cells - At The Interface," Brian Zacher, Jeremy L. Gantz, Robin E. Richards, Neal R. Armstrong, Invited Commentary for the Journal of Physical Chemistry Letters, 4, 1949-1952, (2013), DOI: 10.1021/jz4009932.
"Energy level alignment and morphology of Ag and Au nanoparticle recombination contacts in tandem planar heterojunction solar cells," K. Xerxes Steirer, Gordon A. MacDonald, Selina Olthof, Jeremy Gantz, Erin L. Ratcliff, Antoine Kahn, Neal R. Armstrong, J. Phys. Chem. C., Invited (Ron Naaman Festschrift), (2013) DOI.org/10.1021/jp402672j
"Titanium Dioxide Electron-Selective Interlayers Created by Chemical Vapor Deposition for Inverted Configuration Organic Solar Cells," Kai-Lin Ou, Delvin Tadytin, K. Xerxes Steirer, Diogenes Placencia, Mike Nguyen, Paul Lee, and Neal R. Armstrong, J. Mater. Chem. A, 1, 6794-6803 (2013), DOI: 10.1039/C3TA10894E.
"Influence of Electrode Surface Composition and Energetics on Small-Molecule Organic Solar Cell Performance: Polar Versus Non-Polar Donors on ITO Contacts," Jeremy Gantz, Diogenes Placencia, Anthony Giordano, Seth R. Marder, Neal R. Armstrong, J. Phys. Chem. C., 117 1205-1216 (2013). DOI: 10.1021/jp307546v.
"Electrical Property Heterogeneity at Transparent Conductive Oxide/Organic Semiconductor Interfaces: Mapping Contact Ohmicity Using Conducting-Tip Atomic Force Microscopy," Gordon MacDonald, P. Alex Veneman, Diogenes Placencia, Neal R. Armstrong, ACS Nano, 6, 9623-9636, (2012). DOI: 10.1021/nn303043y.
"Electron Transfer Processes in Monolayer-Tethered Zinc Phthalocyanines: Characterization by Waveguide Spectroelectrochemistry, Voltammetry, and Potential-Modulated Attenuated Total Reflectance (PM-ATR)," Hsiao-Chu Lin, Nathan W. Polaske, Luis E. Oquendo, Matthew Gliboff, Kristina M. Knesting, Dennis Nordlund, David S. Ginger, Erin L. Ratcliff, Brooke M. Beam, Neal R. Armstrong, Dominic V. McGrath, S. Scott Saavedra, J. Phys. Chem. Lett., 3, 1154-1158, (2012).
"Characterization of surface composition and energetics of solution processed NiOx selective interlayer materials for use in polymer bulk heterojunction organic photovoltaics," Erin L. Ratcliff, Jens Meyer, K. Xerxes Steirer, Andres Garcia, Joseph J. Berry, David S. Ginley, Dana C. Olson, Antoine Kahn, Neal R. Armstrong, Chemistry of Materials, 23, 4988-5000 (2011).
"Modeling Nanometer-Scale Heterogeneity in the Electrical Properties of Contacts in Organic Solar Cells," Brian Zacher, Neal R. Armstrong, J. Phys. Chem. C, 115, 25496-25507, (2011).
Highly Photoactive Titanyl Phthalocyanine Polymorphs as Textured Donor Layers in Organic Solar Cells," Diogenes Placencia, Weining Wang, Jeremy Gantz, Judith Jenkins, Neal R. Armstrong, J. Phys. Chem. C, 115, 18873-18884 (2011).
"The Modification of Indium Tin Oxide with Phosphonic Acids: Mechanism of Binding, Tuning of Surface Properties, and Potential for use in Organic Electronic Applications," Peter Hotchkiss, Simon Jones, Sergio Paniagua, Bernard Kippelen, Neal R. Armstrong, Asha Sharma, Seth Marder, Accounts of Chemical Research, 45, 337-346 (2012). DOI: 10.1021/ar200119g.
"Selective Interlayers and Contacts in Organic Photovoltaic Cells," Erin L. Ratcliff, Brian Zacher, and Neal R. Armstrong, J. Phys. Chem. Letters - Perspective - Invited, J. Phys. Chem. Lett., 2, 1337-1350 (2011).
"Enhanced efficiency in plastic solar cells via energy matched solution processed NiOx Interlayers," K. Xerxes Steirer, Paul F. Ndione, N. Edwin Widjonarko, Matthew T. Lloyd, Jens Meyer, Erin Ratcliff, Antoine Kahn, Neal R. Armstrong, Calvin J. Curtis, David S. Ginley, Joseph J. Berry, and Dana C. Olson, Advanced Energy Materials, 1, 813-820 (2011).
"Potential-modulated attenuated total reflectance (PMATR) characterization of charge injection processes in monolayer-tethered CdSe nanocrystals, Zeynep Ozkan Araci, Clayton R. Shallcross, Neal R. Armstrong and S. Scott Saavedra, J. Phys. Chem. Letters, 1, 1900-1905, (2010)
"Photoemission spectroscopy of tethered CdSe nanocrystals: Shifts in ionization potential and local vacuum level as a function of nanocrystal capping ligand," Andrea Munro, Brian Zacher, Amy Graham, Neal R. Armstrong, ACS Applied Materials and Interfaces, 2, 863-869, (2010).
"A Planar, Chip-Based, Dual-Beam Refractometer Using an Integrated Organic Light Emitting Diode (OLED) Light Source and Organic Photovoltaic (OPV) Detectors," Erin L. Ratcliff, P. Alex Veneman, Adam Simmonds, Brian Zacher, Daniel Huebner, S. Scott Saavedra, Neal R. Armstrong, Analytical Chemistry, 82, 2734-2742, (2010).
"Work function control of hole-selective polymer/ITO anode contacts: an electrochemical doping study," Erin L. Ratcliff, Paul A. Lee, and Neal R. Armstrong, J. Mater. Chem. (invited for themed issue on Interface Engineering of Organic and Molecular Electronics,) 20, 2672-2679 (2010).
"Efficient CdSe Nanocrystal Diffraction Gratings Prepared by Microcontact Molding," R. Clayton Shallcross, Gulraj S. Chawla, F. Saneeha Marikkar, Stephanie Tolbert, Jeffrey Pyun, Neal R. Armstrong, ACS Nano, 3, 3629-3637 (2009).
"Photoelectrochemical Processes in Polymer-Tethered CdSe Nanocrystals, R. Clayton Shallcross, Gemma D. D'Ambruoso, Jeffrey Pyun, Neal R. Armstrong, J. Amer. Chem. Soc., 132, 2622-2632, (2010).
"Tuning the Effective Work Function of Gold and Silver Using ω-Functionalized Alkanethiols: Varying Surface Composition through Dilution and Choice of Terminal Groups," Dana M. Alloway, Amy L. Graham, Xi Yang, Anoma Mudalige, Ramon Colorado, Jr., Vicki H. Wysocki , Jeanne E. Pemberton, T. Randall Lee, Ronald J. Wysocki, Neal R. Armstrong, J. Phys. Chem. C, 113, 20328-20334 (2009).
"Oxide Contacts in Organic Photovoltaics: Characterization and Control of Near-SurfaceComposition in Indium-Tin Oxide (ITO) Electrodes," Neal R. Armstrong, P. Alex Veneman, Diogenes Placencia, Erin Ratcliff, Michael Brumbach, Accounts Chemical Research (Invited), 42, 1748-1757 (2009).