Erin L Ratcliff

Interests

Research

Printable electronic materials and devicesWebsite: http://ratcliff.faculty.arizona.edu/content/research-topics

Courses

Scholarly Contributions

Journals/Publications

• Chen, Z., & Ratcliff, E. L. (2022). Watching Polarons Move in the Energy and Frequency Domains Using Color Impedance Spectroscopy. Chemistry of Materials, 34(23), 10691-10700. doi:10.1021/acs.chemmater.2c02831
• Ratcliff, E. L., Anderson, M. A., Farahat, M. E., Martell, M., & Welch, G. C. (2022). New Perylene Diimide Ink for Interlayer Formation in Air-Processed Conventional Organic Photovoltaic Devices. ACS Applied Materials & Interfaces, 14(38), 43558-43567. doi:10.1021/acsami.2c12281
  More info

  Roll-to-roll coating of conventional organic photovoltaic architectures in air necessitates low work function, electron-harvesting interlayers as the top interface, termed cathode interlayers. Traditional materials based on metal oxides are often not compatible with coating in air and/or green solvents, require thermal annealing, and are limited in feasibility due to interactions with underlying layers. Alternatively, perylene diimide materials offer easily tunable redox properties, are amenable to air coating in green solvents, and are considered champion organic-based cathode interlayers. However, underlying mechanisms of the extraction of photogenerated electrons are less well understood. Herein, we demonstrate the utilization of two N-annulated perylene diimide materials, namely, PDIN-H and CN-PDIN-H, in air-processed conventional organic photovoltaic devices, using the now standard PM6:Y6 photoactive layer. The processing ink formulation using cesium carbonate as a processing agent to solubilize the perylene diimides in suitable green solvents (1-propanol and ethyl acetate) for uniform film formation using spin or slot-die coating on top of the photoactive layer is critical. Cesium carbonate remains in the film, creating hybrid organic/metal salt cathode interlayers. Best organic photovoltaic devices have power conversion efficiencies of 13.2% with a spin-coated interlayer and 13.1% with a slot-die-coated interlayer, superior to control devices using the classic conjugated polyelectrolyte PFN-Br as an interlayer (ca. 12.8%). The cathode interlayers were found to be semi-insulating in nature, and the device performance improvements were attributed to beneficial interfacial effects and electron tunneling through sufficiently thin layers. The efficiencies beyond 13% achieved in air-processed organic photovoltaic devices utilizing slot-die-coated cathode interlayers are among the highest reported so far, opening new opportunities for the fabrication of large-area solar cell modules.
• Ratcliff, E. L., Anderson, M. A., Yeager, S. M., Babiak, P., & Larson, B. W. (2022). ColorLab: Visualizing Color from Absorbance Spectra. ChemRxiv. doi:10.26434/chemrxiv-2022-g5v90
• Ratcliff, E. L., Armstrong, N. R., & De Keersmaecker, M. (2022). How Low Can You Go? Defect Quantification at the 10¹⁵ cm^–3 Level in Mixed-Cation Perovskites Using Differential Pulse Voltammetry. ACS Energy Letters, 7(11), 4017-4027. doi:10.1021/acsenergylett.2c02033
• Ratcliff, E. L., Armstrong, N. R., Du Hill, L., De Keersmaecker, M., Colbert, A. E., Hill, J. W., Placencia, D., & Boercker, J. E. (2022). Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces. Materials Horizons, 9(1), 471-481. doi:10.1039/d1mh01306h
• Ratcliff, E. L., Zhu, K., Larson, B. W., Yan, Y., Beard, M. C., Berry, J. J., Loo, Y., Parkin, S., Teeter, G., Schelhas, L. T., Stone, K. H., Mundt, L. E., Wang, X., Chen, X., Hill, L. D., Li, X., Uličná, S., Xiao, C., Dunfield, S. P., , Lu, H., et al. (2022). Metastable Dion-Jacobson 2D structure enables efficient and stable perovskite solar cells. Science, 375(6576), 71-76. doi:10.1126/science.abj2637
  More info

  The performance of three-dimensional (3D) organic-inorganic halide perovskite solar cells (PSCs) can be enhanced through surface treatment with 2D layered perovskites that have efficient charge transport. We maximized hole transport across the layers of a metastable Dion-Jacobson (DJ) 2D perovskite that tuned the orientational arrangements of asymmetric bulky organic molecules. The reduced energy barrier for hole transport increased out-of-plane transport rates by a factor of 4 to 5, and the power conversion efficiency (PCE) for the 2D PSC was 4.9%. With the metastable DJ 2D surface layer, the PCE of three common 3D PSCs was enhanced by approximately 12 to 16% and could reach approximately 24.7%. For a triple-cation–mixed-halide PSC, 90% of the initial PCE was retained after 1000 hours of 1-sun operation at ~40°C in nitrogen.
• Anderson, M. A., Larson, B. W., & Ratcliff, E. L. (2021). A Multi-modal Approach to Understanding Degradation of Organic Photovoltaic Materials. ACS Applied Materials & Interfaces, 13(37), 44641-44655.
• Anderson, M. A., Ratcliff, E. L., Welch, G. C., Munir, R., & Cieplechowicz, E. (2021). Zinc Oxide-Perylene Diimide Hybrid Electron Transport Layers for Air-Processed Inverted Organic Photovoltaic Devices. ACS Applied Materials & Interfaces, 13(41), 49096-49103. doi:10.1021/acsami.1c15251
  More info

  In this work, we report the formation of perylene diimide films, from green solvents, for use as electron transporting layers, when combined with ZnO, in inverted-type organic photovoltaics. A modified N-annulated PDI was functionalized with a tert-butyloxycarbonyl protecting group to solubilize the material, enabling solution processing from green solvents. Post-deposition treatment of films via thermal annealing cleaves the protecting group yielding the known PDIN-H material, rendering films solvent-resistant. The PDIN-H films were characterized by optical absorption spectroscopy, contact angle measurements, and atomic force microscopy. When used to modify the surface of ZnO in inverted-type organic photovoltaics (air-processed and tested) based on the PM6:Y6 and PTQ10:Y6 bulk-heterojunctions, the device power conversion efficiency increases from 9.8 to 11.0% and 7.2 to 9.8%, respectively.
• Biegel, C. M., & Kamat, P. V. (2022). Women Scientists at the Forefront of Energy Research: A Virtual Issue, Part 4. ACS Energy Letters, 7(1), 328-342.
• Cieplechowicz, E., Munir, R., Anderson, M. A., Ratcliff, E. L., & Welch, G. C. (2021). Zinc Oxide-Perylene Diimide Hybrid Electron Transport Layers for Air-Processed Inverted Organic Photovoltaic Devices. ACS Applied Materials & Interfaces, 13(41), 49096-49103.
• Colbert, A. E., Placencia, D., Ratcliff, E. L., Boercker, J. E., Lee, P., Aifer, E. H., & Tischler, J. G. (2021). Enhanced Infrared Photodiodes Based on PbS/PbClx Core/Shell Nanocrystals. ACS Applied Materials & Interfaces, 13(49), 58916-58926.
• De, K. M., Armstrong, N. R., & Ratcliff, E. L. (2021). Defect quantification in metal halide perovskites: the solid-state electrochemical alternative. Energy Environ. Sci., 14, 4840-4846.
• Du, H. L., De, K. M., Colbert, A. E., Hill, J. W., Placencia, D., Boercker, J. E., Armstrong, N. R., & Ratcliff, E. L. (2022). Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces. Mater. Horiz., 9, 471-481.
• Ratcliff, E. L., & Yu, S. (2021). Tuning Organic Electrochemical Transistor (OECT) Transconductance toward Zero Gate Voltage in the Faradaic Mode. ACS Applied Materials & Interfaces, 13(42), 50176-50186. doi:10.1021/acsami.1c13009
  More info

  In this work, we investigate material design criteria for low-powered/self-powered and efficient organic electrochemical transistors (OECTs) to be operated in the faradaic mode (detection at the gate electrode occurs via electron transfer events). To rationalize device design principles, we adopt a Marcus-Gerischer perspective for electrochemical processes at both the gate and channel interfaces. This perspective considers density of states (DOS) for the semiconductor channel, the gate electrode, and the electrolyte. We complement our approach with energy band offsets of relevant electrochemical potentials that can be independently measured from transistor geometry using conventional electrochemical methods as well as an approach to measure electrolyte potential in an operating OECT. By systematically changing the relative redox property offsets between the redox-active electrolyte and semiconducting polymer channel, we demonstrate a first-order design principle that necessary gate voltage is minimized by good DOS overlap of the two redox processes at the gate and channel. Specifically, for p-type turn-on OECTs, the voltage-dependent, electrochemically active semiconductor DOS should overlap with the oxidant form of the electrolyte to minimize the onset voltage for transconductance. A special case where the electrolyte can be used to spontaneously dope the polymer via charge transfer is also considered. Collectively, our results provide material design pathways toward the development of simple, robust, power-saving, and high-throughput OECT biosensors.
• Ratcliff, E. L., Zhu, K., Zhou, Y., Padture, N. P., Berry, J. J., Anderson, M. A., Yang, M., Hao, J., Xiao, C., Wang, S., Dai, Z., Yadavalli, S. K., Hu, M., Gong, J., & Tong, J. (2021). High-performance methylammonium-free ideal-band-gap perovskite solar cells. Matter. doi:10.1016/j.matt.2021.01.003
• Tong, J., Gong, J., Hu, M., Yadavalli, S. K., Dai, Z., Zhang, F., Xiao, C., Hao, J., Yang, M., Anderson, M. A., Ratcliff, E. L., Berry, J. J., Padture, N. P., Zhou, Y., & Zhu, K. (2021). High-performance methylammonium-free ideal-band-gap perovskite solar cells. Matter, 4(4), 1365-1376.
• Yu, S., & Ratcliff, E. L. (2021). Tuning Organic Electrochemical Transistor (OECT) Transconductance toward Zero Gate Voltage in the Faradaic Mode. ACS Applied Materials & Interfaces, 13(42), 50176-50186.
• Yu, S., Sternberg, E., Runyon, J., Ratcliff, E. L., & Harris, J. K. (2021). Organic Electrochemical Transistors as Wearable, Human-Biochemistry Monitoring Technologies. Bulletin of the American Physical Society.
• Zhang, F., Park, S. Y., Yao, C., Lu, H., Dunfield, S. P., Xiao, C., Uličná, S., Zhao, X., Hill, L. D., Chen, X., Wang, X., Mundt, L. E., Stone, K. H., Schelhas, L. T., Teeter, G., Parkin, S., Ratcliff, E. L., Loo, Y., Berry, J. J., , Beard, M. C., et al. (2022). Metastable Dion-Jacobson 2D structure enables efficient and stable perovskite solar cells. Science, 375(6576), 71-76.
• Boyd, C. C., Shallcross, R. C., Moot, T., Kerner, R., Bertoluzzi, L., Onno, A., Kavadiya, S., Chosy, C., Wolf, E. J., Werner, J., Raiford, J. A., de Paula, C., Palmstrom, A. F., Yu, Z. J., Berry, J. J., Bent, S. F., Holman, Z. C., Luther, J. M., Ratcliff, E. L., , Armstrong, N. R., et al. (2020). Overcoming Redox Reactions at Perovskite-Nickel Oxide Interfaces to Boost Voltages in Perovskite Solar Cells. Joule, 4(8), 1759-1775.
• Brothers, M. C., Moore, D., St, L. M., Harris, J., Joseph, R. M., Ratcliff, E., Ruiz, O. N., Glavin, N., & Kim, S. S. (2020). Impact of Self-Assembled Monolayer Design and Electrochemical Factors on Impedance-Based Biosensing. SENSORS, 20(8).
• Farahat, M. E., Laventure, A., Anderson, M. A., Mainville, M., Tintori, F., Leclerc, M., Ratcliff, E. L., & Welch, G. C. (2020). Slot-Die-Coated Ternary Organic Photovoltaics for Indoor Light Recycling. ACS Applied Materials and Interfaces, 12(39), 43684-43693.
• Harris, J. K., & Ratcliff, E. L. (2020). Ion diffusion coefficients in poly(3-alkylthiophenes) for energy conversion and biosensing: Role of side-chain length and microstructure. Journal of Materials Chemistry C, 8(38), 13319-13327.
• Mundt, L. E., Tong, J., Palmstrom, A. F., Dunfield, S. P., Zhu, K., Berry, J. J., Schelhas, L. T., & Ratcliff, E. L. (2020). Surface-Activated Corrosion in Tin-Lead Halide Perovskite Solar Cells. ACS Energy Letters, 5(11), 3344-3351.
• Watts, K. E., Neelamraju, B., Moser, M., McCulloch, I., Ratcliff, E. L., & Pemberton, J. E. (2020). Thermally Induced Formation of HF4TCNQ-in F4TCNQ-Doped Regioregular P3HT. Journal of Physical Chemistry Letters, 11(16), 6586-6592.
• Arnold, S. P., Harris, J. K., Neelamraju, B., Rudolph, M., & Ratcliff, E. L. (2019). Microstructure-dependent electrochemical properties of chemical-vapor deposited poly(3,4-ethylenedioxythiophene) (PEDOT) films. SYNTHETIC METALS, 253, 26-33.
• Daviddi, E., Chen, Z., Massani, B. B., Lee, J., Bentley, C. L., Unwin, P. R., & Ratcliff, E. L. (2019). Nanoscale Visualization and Multiscale Electrochemical Analysis of Conductive Polymer Electrodes. ACS NANO, 13(11), 13271-13284.
• Harris, J. K., Neelamraju, B., & Ratcliff, E. L. (2019). Intersystem Subpopulation Charge Transfer and Conformational Relaxation Preceding in Situ Conductivity in Electrochemically Doped Poly(3-hexylthiophene) Electrodes. CHEMISTRY OF MATERIALS, 31(17), 6870-6879.
• Harris, J. K., Neelamraju, B., & Ratcliff, E. L. (2019). Intersystem Subpopulation Charge Transfer and Conformational Relaxation Preceding in Situ Conductivity in Electrochemically Doped Poly(3-hexylthiophene) Electrodes. Chemistry of Materials, 31(17), 6870-6879.
• Harris, J., Bickford, J., Cho, P., Coppock, M., Farrell, M., Holthoff, E., & Ratcliff, E. (2019). Approaching Single Molecule Sensing: Predictive Sweat Sensor Design for Ultra-Low Limits of Detection. CHEMICAL, BIOLOGICAL, RADIOLOGICAL, NUCLEAR, AND EXPLOSIVES (CBRNE) SENSING XX, 11010. doi:10.1117/12.2518543
• Ndione, P. F., Ratcliff, E. L., Dey, S. R., Warren, E. L., Peng, H., Holder, A. M., Lany, S., Gorman, B. P., Al-Jassim, M. M., Deutsch, T. G., Zakutayev, A., & Ginley, D. S. (2019). High-Throughput Experimental Study of Wurtzite Mn1-xZnxO Alloys for Water Splitting Applications. ACS OMEGA, 4(4), 7436-7447.
• Neelamraju, B., Ratcliff, E. L., Rudolph, M., Harris, J. K., & Arnold, S. P. (2019). Microstructure-dependent electrochemical properties of chemical-vapor deposited poly(3,4-ethylenedioxythiophene) (PEDOT) films. Synthetic Metals. doi:10.1016/j.synthmet.2019.04.022
  More info

  Abstract Conductive polymer electrodes hold exceptional promise in energy conversion technologies and bioelectronics due to the inherent mechanical flexibility and synthetic tunability of physical, chemical, and electronic properties. Solution-processing is favorable to retain low-cost but can often result in heterogeneity of physical and electronic structure due to non-conjugated side chains and polyionic dopants creating insulating domains. Such a complex landscape limits control and systematic understanding of fundamental properties including electrical and ionic transport and rates of electron transfer central to overall device efficiencies. Oxidative chemical vapor deposition (oCVD) offers a promising route to simultaneously synthesize and deposit conductive polymer films at low temperatures (
• Pemberton, J. E., Ratcliff, E. L., Neelamraju, B., & Watts, K. E. (2019). Stability of Charge Transfer States in F4TCNQ-doped P3HT. Chem. Mater., 31, 6986-6994. doi:10.1021/acs.chemmater.9b01549
• Ratcliff, E. L., Pemberton, J. E., Pfeilsticker, J. R., Owczarczyk, Z. R., Larson, B. W., Larsen, R. E., Ferguson, A. J., Braunecker, W. A., Anderson, M. A., Neelamraju, B., Tremolet de Villers, B. J., Nguyen, T., & Watts, K. E. (2019). Stability of push-pull small molecule donors for organic photovoltaics: spectroscopic degradation of acceptor endcaps on benzo[1,2-b: 4,5-b’]dithiophene cores. J. Mater. Chem. A, 7, 19984-19995. doi:10.1039/c9ta06310b
• Rudolph, M., Harris, J. K., & Ratcliff, E. L. (2019). Predicting limits of detection in real-time sweat-based human performance monitoring. SMART BIOMEDICAL AND PHYSIOLOGICAL SENSOR TECHNOLOGY XV, 11020.
• Rudolph, M., Harris, J. K., & Ratcliff, E. L. (2019). Printable transistors for wearable sweat sensing. SMART BIOMEDICAL AND PHYSIOLOGICAL SENSOR TECHNOLOGY XV, 11020.
• Watts, K. E., Neelamraju, B., Ratcliff, E. L., & Pemberton, J. E. (2019). Stability of Charge Transfer States in F(4)TCNQ-Doped P3HT. CHEMISTRY OF MATERIALS, 31(17), 6986-6994.
• Watts, K. E., Neelamraju, B., Ratcliff, E. L., & Pemberton, J. E. (2019). Stability of Charge Transfer States in F4TCNQ-Doped P3HT.
• Watts, K. E., Nguyen, T., de, V., Neelamraju, B., Anderson, M. A., Braunecker, W. A., Ferguson, A. J., Larsen, R. E., Larson, B. W., Owczarczyk, Z. R., Pfeilsticker, J. R., Pemberton, J. E., & Ratcliff, E. L. (2019). Stability of push-pull small molecule donors for organic photovoltaics: spectroscopic degradation of acceptor endcaps on benzo[1,2-b:4,5-b ']dithiophene cores. JOURNAL OF MATERIALS CHEMISTRY A, 7(34), 19984-19995.
• Neelamraju, B., Rudolph, M., & Ratcliff, E. L. (2018). Controlling the Kinetics of Charge Transfer at Conductive Polymer/Liquid Interfaces through Microstructure. JOURNAL OF PHYSICAL CHEMISTRY C, 122(37), 21210-21215.
• Neelamraju, B., Watts, K. E., Pemberton, J. E., & Ratcliff, E. L. (2018). Correlation of Coexistent Charge Transfer States in F(4)TCNQ-Doped P3HT with Microstructure. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 9(23), 6871-6877.
• Placencia, D., Lee, P., Tischler, J. G., & Ratcliff, E. L. (2018). Energy Level Alignment of Molybdenum Oxide on Colloidal Lead Sulfide (PbS) Thin Films for Optoelectronic Devices. ACS APPLIED MATERIALS & INTERFACES, 10(30), 24981-24986.
• Ratcliff, E. L., Pemberton, J. E., Watts, K. E., & Neelamraju, B. (2018). Correlation of Coexistent Charge Transfer States in F4TCNQ-Doped P3HT with Microstructure. J. Phys. Chem. Lett., 9, 6871-6877. doi:10.1021/acs.jpclett.8b03104
• Pemberton, J., Sang, L., Watts, K., Nguyen, T., & Ratcliff, E. (2017). Organic semiconductor interfaces in optoelectronic devices: Undesirable chemistry galore!. ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 253.
• Rudolph, M., & Ratcliff, E. L. (2017). Normal and inverted regimes of charge transfer controlled by density of states at polymer electrodes. NATURE COMMUNICATIONS, 8.
• Steirer, K. X., Ou, K. L., Armstrong, N. R., & Ratcliff, E. L. (2017). Critical Interface States Controlling Rectification of Ultrathin NiO- ZnO p-n Heterojunctions. ACS APPLIED MATERIALS & INTERFACES, 9(36), 31111-31118.
• McLachlan, M. A., & Ratcliff, E. L. (2016). Metal Oxide Heterointerfaces in Hybrid Electronic Platforms. ADVANCED MATERIALS, 28(20), 3801-3801.
• Ratcliff, E. L., Shallcross, R. C., & Armstrong, N. R. (2016). Introduction: Electronic Materials. CHEMICAL REVIEWS, 116(21), 12821-12822.
• Cowan, S. R., Li, J. V., Olson, D. C., & Ratcliff, E. L. (2015). Contact-Induced Mechanisms in Organic Photovoltaics: A Steady-State and Transient Study. ADVANCED ENERGY MATERIALS, 5(1).
• Fei, Z., Boufflet, P., Wood, S., Wade, J., Moriarty, J., Gann, E., Ratcliff, E. L., McNeill, C. R., Sirringhaus, H., Kim, J., & Heeney, M. (2015). Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-fluoro)thiophenes. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 137(21), 6866-6879.
• Lin, H., MacDonald, G. A., Shi, Y., Polaske, N. W., McGrath, D. V., Marder, S. R., Armstrong, N. R., Ratcliff, E. L., & Saavedra, S. S. (2015). Influence of Molecular Orientation on Charge-Transfer Processes at Phthalocyanine/Metal Oxide Interfaces and Relationship to Organic Photovoltaic Performance. JOURNAL OF PHYSICAL CHEMISTRY C, 119(19), 10304-10313.
• Oosterhout, S. D., Kopidakis, N., Owczarczyk, Z. R., Braunecker, W. A., Larsen, R. E., Ratcliff, E. L., & Olson, D. C. (2015). Integrating theory, synthesis, spectroscopy and device efficiency to design and characterize donor materials for organic photovoltaics: a case study including 12 donors. JOURNAL OF MATERIALS CHEMISTRY A, 3(18), 9777-9788.
• Shallcross, R. C., Stubhan, T., Ratcliff, E. L., Kahn, A., Brabec, C. J., & Armstrong, N. R. (2015). Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT). JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 6(8), 1303-1309.
• Steirer, K. X., Richards, R. E., Sigdel, A. K., Garcia, A., Ndione, P. F., Hammond, S., Baker, D., Ratcliff, E. L., Curtis, C., Furtak, T., Ginley, D. S., Olson, D. C., Armstrong, N. R., & Berry, J. J. (2015). Nickel oxide interlayer films from nickel formate-ethylenediamine precursor: influence of annealing on thin film properties and photovoltaic device performance. JOURNAL OF MATERIALS CHEMISTRY A, 3(20), 10949-10958.
• Braunecker, W. A., Oosterhout, S. D., Owczarczyk, Z. R., Kopidakis, N., Ratcliff, E. L., Ginley, D. S., & Olson, D. C. (2014). Semi-random vs Well-Defined Alternating Donor-Acceptor Copolymers. ACS MACRO LETTERS, 3(7), 622-627.
  More info

  The influence of backbone composition on the physical properties of donor-acceptor (D-A) copolymers composed of varying amounts of benzodithiophene (BDT) donor with the thienoisoindoledione (TID) acceptor is investigated. First, the synthesis of bis- and tris-BDT monomers is reported; these monomers are subsequently used in Stale copolymerizations to create well-defined alternating polymer structures with repeating (D-A), (D-D-A), and (D-D-D-A) units. For comparison, five semi-random D-A copolymers with a D:A ratio of 1.5, 2, 3, 4, and 7 were synthesized by reacting trimethyltin-functionalized BDT with various ratios of iodinated BDT and brominated TID. While the HOMO levels of all the resultant polymers are very similar, a systematic red shift in the absorbance spectra onset of the D-A copolymer films from 687 to 883 run is observed with increasing acceptor content, suggesting the LUMO can be fine-tuned over a range of 0.4 eV. When the solid-state absorbance spectra of well-defined alternating copolymers are compared to those of semi-random copolymers with analogous D:A ratios, the spectra of the alternating copolymers are significantly more red-shifted. Organic photovoltaic device efficiencies show that the semi-random materials all outperform the well-defined alternating copolymers, and an optimal D:A ratio of 2 produces the highest efficiency. Additional considerations concerning fine-tuning the lifetimes of the photoconductance transients of copolymer:fullerene films measured by time-resolved microwave conductivity are discussed. Overall, the results of this work indicate that the semi-random approach is a powerful synthetic strategy for fine-tuning the optoelectronic and photophysical properties of D-A materials for a number of systematic studies, especially given the ease with which the D:A ratios in the semi-random copolymers can be tuned.
• Cowan, S. R., Schulz, P., Giordano, A. J., Garcia, A., MacLeod, B. A., Marder, S. R., Kahn, A., Ginley, D. S., Ratcliff, E. L., & Olson, D. C. (2014). Chemically Controlled Reversible and Irreversible Extraction Barriers Via Stable Interface Modification of Zinc Oxide Electron Collection Layer in Polycarbazole-based Organic Solar Cells. ADVANCED FUNCTIONAL MATERIALS, 24(29), 4671-4680.
  More info

  A spin-cast method is presented for the formation of phosphonic acid functionalized small molecule layers on solution-processed ZnO substrates for use as electron collecting interlayers in organic photovoltaics. Phosphonic acid interlayers modify the ZnO work function and the charge carrier injection barrier at its interface, resulting in systematic control of V-OC in inverted bulk heterojunction solar cells. Surface modification is shown to moderate the need for UV light-soaking of the ZnO contact layers. Lifetime studies (30 days) indicate stable and improved OPV performance over the unmodified ZnO contact, which show significant increases in charge extraction barriers and series resistance. Results suggest that enhanced stability using small molecule modifiers is due to partial passivation of the oxide surface to molecular oxygen adsorption. Surface passivation while maintaining work function control of a selective interlayer can be employed to improve net efficiency and lifetime of organic photovoltaic devices. The modified cathode work function modulates V-OC via static energetic barriers and modulates contact conductivity by creating reversible and irreversible S-shape current-voltage characteristics as a result of kinetic barriers to charge transport.
• Jenkins, J. L., Lee, P. A., Nebesny, K. W., & Ratcliff, E. L. (2014). Systematic electrochemical oxidative doping of P3HT to probe interfacial charge transfer across polymer-fullerene interfaces. JOURNAL OF MATERIALS CHEMISTRY A, 2(45), 19221-19231.
  More info

  This work demonstrates the detection and control of interfacial charge transfer across polymer-fullerene interfaces relevant to organic electronic platforms, including solar cells and photodetectors. Electrochemical deposition of poly(3-hexylthiophene) (e-P3HT) and subsequent electrochemical oxidation to systematically vary the fraction of oxidized thiophene (e-P3HT(+)) was used to form donor polymer films. The fullerene electron acceptor C-60 was vacuum deposited onto the e-P3HT, and interfacial interactions were monitored with optical and photoelectron spectroscopy. Charge redistribution (sub-stoichiometric or even stoichiometric electron transfer) from e-P3HT to C-60 was observed when the initial fraction of e-P3HT+ was low, as evidenced by the formation of new polaronic species and simultaneous n-doping of the C-60. These charge transfer results are expected to impact interfacial rates of free carrier generation and recombination, as well as competing charge transport processes, particularly in thin film devices (
• Li, H., Ratcliff, E. L., Sigdel, A. K., Giordano, A. J., Marder, S. R., Berry, J. J., & Bredas, J. (2014). Modification of the Gallium-Doped Zinc Oxide Surface with Self-Assembled Monolayers of Phosphonic Acids: A Joint Theoretical and Experimental Study. ADVANCED FUNCTIONAL MATERIALS, 24(23), 3593-3603.
  More info

  Gallium-doped zinc oxide (GZO) surfaces, both bare and modified with chemisorbed phosphonic acid (PA) molecules, are studied using a combination of density functional theory calculations and ultraviolet and X-ray photoelectron spectroscopy measurements. Excellent agreement between theory and experiment is obtained, which leads to an understanding of: i) the core-level binding energy shifts of the various oxygen atoms belonging to different surface sites and to the phosphonic acid molecules; ii) the GZO work-function change upon surface modification, and; iii) the energy level alignments of the frontier molecular orbitals of the PA molecules with respect to the valence band edge and Fermi level of the GZO surface. Importantly, both density of states calculations and experimental measurements of the valence band features demonstrate an increase in the density of states and changes in the characteristics of the valence band edge of GZO upon deposition of the phosphonic acid molecules. The new valence band features are associated with contributions from surface oxygen atoms near a defect site on the oxide surface and from the highest occupied molecular orbitals of the phosphonic acid molecules.
• Saavedra, S. S., McGrath, D. V., Armstrong, N. R., Ratcliff, E. L., Beam, B. M., Ratcliff, E. L., Ginger, D. S., Nordlund, D., Knesting, K. M., Gliboff, M., Oquendo, L. E., Polaske, N. W., & Lin, H. (2012). Electron Transfer Processes in Zinc Phthalocyanine-Phosphonic Acid Monolayers on ITO: Characterization of Orientation and Charge Transfer Kinetics By Waveguide Spectroelectrochemistry. J. Phys. Chem. Lett, 3, 1154-1158. doi:http://dx.doi.org/10.1021/jz3002426
  More info

  Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with ks,app = 2 × 102 s–1, independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, ks,app = 2 × 103 s–1, whereas for upright Pc aggregates, ks,app = 7 × 102 s–1. The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells.
• Gliboff, M., Sang, L., Knesting, K. M., Schalnat, M. C., Mudalige, A., Ratcliff, E. L., Li, H., Sigdel, A. K., Giordano, A. J., Berry, J. J., Nordlund, D., Seidler, G. T., Bredas, J., Marder, S. R., Pemberton, J. E., & Ginger, D. S. (2013). Orientation of Phenylphosphonic Acid Self-Assembled Monolayers on a Transparent Conductive Oxide: A Combined NEXAFS, PM-IRRAS, and DFT Study. LANGMUIR, 29(7), 2166-2174.
  More info

  Self-assembled monolayers (SAMs) of dipolar phosphonic acids can tailor the interface between organic semiconductors and transparent conductive oxides. When used in optoelectronic devices such as organic light emitting diodes and solar cells, these SAMs can increase current density and photovoltaic performance. The molecular ordering and conformation adopted by the SAMs determine properties such as work function and wettability at these critical interfaces. We combine angle-dependent near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) to determine the molecular orientations of a model phenylphosphonic acid on indium zinc oxide, and correlate the resulting values with density functional theory (DFT). We find that the SAMs are surprisingly well-oriented, with the phenyl ring adopting a well-defined tilt angle of 12-16 degrees from the surface normal. We find quantitative agreement between the two experimental techniques and density functional theory calculations. These results not only provide a detailed picture of the molecular structure of a technologically important class of SAMs, but also resolve a long-standing ambiguity regarding the vibrational-mode assignments for phosphonic acids on oxide surfaces, thus improving the utility of PM-IRRAS for future studies.
• Matz, D. L., Ratcliff, E. L., Meyer, J., Kahn, A., & Pemberton, J. E. (2013). Deciphering the Metal-C-60 Interface in Optoelectronic Devices: Evidence for C-60 Reduction by Vapor Deposited Al. ACS APPLIED MATERIALS & INTERFACES, 5(13), 6001-6008.
  More info

  The formation of interfacial midgap states due to the reduction of bucluninsterfullerene (C-60) to amorphous carbon upon subsequent vapor deposition of Al is confirmed using Raman spectroscopy and X-ray, ultraviolet, and inverse photoemission spectroscopies. We demonstrate that vapor deposition of Al results in n-type doping of C-60 due to an electron transfer from Al to the LUMO of C-60, resulting in the formation of midgap states near the C-60 Fermi level. Raman spectroscopy in ultrahigh vacuum clearly identifies the presence of the C-60 anion radical (C-60(center dot-)) as well as amorphous carbon created by further degradation of C-60(center dot-). In contrast, the interface formed by vapor deposition of Ag shows only a slight Ag/C-60 interfacial charge displacement with no evidence for complete metal-to-C-60 electron transfer to form the anion radical or its further degradation products. These results confirm previous speculations of metal-induced chemical damage of C-60 films after Al deposition, which is widely suspected of decreasing charge collection efficiency in OPVs, and provide key insight into charge collection at metal/organic interfaces in such devices.
• Ndione, P. F., Garcia, A., Widjonarko, N. E., Sigdel, A. K., Steirer, K. X., Olson, D. C., Parilla, P. A., Ginley, D. S., Armstong, N. R., Richards, R. E., Ratcliff, E. L., & Berry, J. J. (2013). Highly-Tunable Nickel Cobalt Oxide as a Low-Temperature P-Type Contact in Organic Photovoltaic Devices. ADVANCED ENERGY MATERIALS, 3(4), 524-531.
  More info

  We report on the investigation of nickel cobalt oxide (NixCo3xO4) thin films grown by pulsed laser deposition as hole-transport interlayers (HTL) in organic photovoltaic (OPV) devices. Films of 7 nm thickness were grown under various oxygen deposition pressures (pO2) in the range of 2200 mTorr. We explore both bulk and surface properties of these thin films. The workfunction (phi) for each of the films was statistically similar (approximate to 4.7 eV), regardless of pO2. There was not a strong dependence of the power conversion efficiency () on the conductivities of the HTLs varying between 0.009 - 10 S/cm. The observed differences in OPV efficiencies (ranging from 1.16 to 2.46%) were correlated to the near surface chemical composition of the NixCo3xO4 HTL, as observed by differences in the relative surface hydroxyl concentration. The critical role of the near-surface composition of the HTL at the HTL/organic interface was further explored by modifying the hydroxyl concentration using an oxygen plasma treatment. This treatment mitigated the impact of surface hydroxyl coverage, demonstrating either identical or increased values for phi and , regardless of initial pO2 in the creation of the NixCo3xO4 HTL. To further explore this we also employed a phosphonic acid surface modification agent on the HTL, increasing phi to 5.2 eV producing the best value of 3.4%, equivalent to the PEDOT:PSS control devices. These results indicate that nickel cobalt oxide is a promising p-type oxide for carrier-selective interlayers in organic solar cells; however, for this to be fully realized the specific surface chemistry at the oxide/polymer interface must be controlled to increase phi and optimize device performance.
• Ratcliff, E. L., Bakus, R. C., Welch, G. C., van der Poll, T. S., Garcia, A., Cowan, S. R., MacLeod, B. A., Ginley, D. S., Bazan, G. C., & Olson, D. C. (2013). Formation of interfacial traps upon surface protonation in small molecule solution processed bulk heterojunctions probed by photoelectron spectroscopy. JOURNAL OF MATERIALS CHEMISTRY C, 1(39), 6223-6234.
  More info

  This work expands on the recently reported protonation of the donor molecule 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(4-(5'-hexyl-[2,2'-bithiophen]-5-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine) (d-DTS(PTTh2)(2)) by the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) interlayer to include an electrostatic picture of interfacial energetic states. Ultraviolet photoemission spectroscopy results initially suggested favorable band level alignment for hole extraction between d-DTS(PTTh2)(2) and PEDOT: PSS. However photovoltaic device performance yields a low fill factor and photovoltage, indicative of poor hole-extraction at the hole-collecting interface, relative to the nickel oxide device. Further investigation into the interfacial composition via theory and X-ray photoelectron studies of both the interface and a control system of d-DTS(PTTh2)(2) reacted with p-toluenesulfonic acid verify the presence of a chemically unique species at the interface arising from protonation reaction with the residual acidic protons present in PEDOT:PSS that was masked in the UPS experiment. From these results, the energy band diagram is re-interpreted to account for the interfacial chemical reaction and modified interfacial density of states. Additionally, the detrimental protonation reaction is avoided when the pyridyl[1,2,5]thiadiazole acceptor unit was replaced with a 5-fluorobenzo[c][1,2,5]thiadiazole acceptor unit, which shows no such reaction with the PEDOT: PSS substrate. These results indicate the necessity of using a large analytical toolkit to elucidate the energetics and mechanisms of buried interfaces that will impact dynamics of hole collection.
• Ratcliff, E. L., Garcia, A., Paniagua, S. A., Cowan, S. R., Giordano, A. J., Ginley, D. S., Marder, S. R., Berry, J. J., & Olson, D. C. (2013). Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity. ADVANCED ENERGY MATERIALS, 3(5), 647-656.
  More info

  The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) in bulk heterojunction organic photovoltaics is examined for poly(N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole) (PCDTBT) and [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the VOC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J0. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J0 and increased shunt resistance, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC in bulk heterojunction organic photovoltaics.
• Steirer, K. X., MacDonald, G. A., Olthof, S., Gantz, J., Ratcliff, E. L., Kahn, A., & Armstrong, N. R. (2013). Energy Level Alignment and Morphology of Ag and Au Nanoparticle Recombination Contacts in Tandem Planar Heterojunction Solar Cells. JOURNAL OF PHYSICAL CHEMISTRY C, 117(43), 22331-22340.
  More info

  We present the interface characterization of vacuum-deposited metal nanoparticle recombination layers (Ag, Au; 1 nm equivalent thickness) at donor/acceptor heterojunctions comprising copper phthalocyanine (CuPc) and C-60 as model interfaces for tandem planar heterojunction organic photovoltaics (TOPVs). We compare the extent to which voltage doubling occurs using these two metal recombination contacts (RC) in TOPVs (ITO/CuPc/C-60/(Ag,Au)/CuPc/C-60/BCP/Al) and correlate the differences with energetic dissimilarities revealed by UV-photoemission (UPS) and inverse photoemission (IPES) spectroscopies, and morphology as revealed by atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). Ag interlayer RCs produce the expected voltage doubling in the open-circuit voltage (V-OC) for the TOPV, whereas Au RCs showed poor voltage addition. Significant shifts in ionization potential and electron affinity and shifts in local work function were observed for C-60/metal heterojunctions and for heterojunctions based on C-60/metal/C-60 and for C-60/metal/CuPc, with clear evidence for partial charge redistribution between C-60 and Ag nanoparticles. AFM and FE-SEM images revealed discrete Ag nanoparticles at the C-60 interface, whereas Au/C-60 heterojunctions consisted of more uniform Au thin films that wet the C-60 surface and penetrated below the surface. These studies point to the need for careful control of both electronic and morphological properties of thin RCs in emerging tandem organic solar cell technologies.
• Garcia, A., Welch, G. C., Ratcliff, E. L., Ginley, D. S., Bazan, G. C., & Olson, D. C. (2012). Improvement of Interfacial Contacts for New Small-Molecule Bulk-Heterojunction Organic Photovoltaics. ADVANCED MATERIALS, 24(39), 5368-5373.
• Lin, H., Polaske, N. W., Oquendo, L. E., Gliboff, M., Knesting, K. M., Nordlund, D., Ginger, D. S., Ratcliff, E. L., Beam, B. M., Armstrong, N. R., McGrath, D. V., & Saavedra, S. S. (2012). Electron-Transfer Processes in Zinc Phthalocyanine Phosphonic Acid Monolayers on ITO: Characterization of Orientation and Charge-Transfer Kinetics by Waveguide Spectroelectrochemistry. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 3(9), 1154-1158.
  More info

  Using a monolayer of zinc phthalocyanine (ZnPcPA) tethered to indium tin oxide (ITO) as a model for the donor/transparent conducting oxide (TCO) interface in organic photovoltaics (OPVs), we demonstrate the relationship between molecular orientation and charge-transfer rates using spectroscopic, electrochemical, and spectroelectrochemical methods. Both monomeric and aggregated forms of the phthalocyanine (Pc) are observed in ZnPcPA monolayers. Potential-modulated attenuated total reflectance (PM-ATR) measurements show that the monomeric subpopulation undergoes oxidation/reduction with k(s,app) = 2 x 10(2) s(-1), independent of Pc orientation. For the aggregated ZnPcPA, faster orientation-dependent charge-transfer rates are observed. For in-plane-oriented Pc aggregates, k(s,app) = 2 X 10(3) s(-1), whereas for upright Pc aggregates, k(s,app) = 7 X 10(2) s(-1) The rates for the aggregates are comparable to those required for redox-active interlayer films at the hole-collection contact in organic solar cells.
• MacLeod, B. A., Horwitz, N. E., Ratcliff, E. L., Jenkins, J. L., Armstrong, N. R., Giordano, A. J., Hotchkiss, P. J., Marder, S. R., Campbell, C. T., & Ginger, D. S. (2012). Built-In Potential in Conjugated Polymer Diodes with Changing Anode Work Function: Interfacial States and Deviation from the Schottky-Mott Limit. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 3(9), 1202-1207.
  More info

  We use electroabsorption spectroscopy to measure the change in built-in potential (V-BI) across the polymer photoactive layer in diodes where indium tin oxide electrodes are systematically modified using dipolar phosphonic acid self-assembled monolayers (SAMs) with various dipole moments. We find that V-BI scales linearly with the work function (Phi) of the SAM-modified electrode over a wide range when using a solution-coated poly(p-phenylenevinylene) derivative as the active layer. However, we measure an interfacial parameter of S = e Delta V-BI/Delta Phi < 1, suggesting that these ITO/SAM/polymer interfaces deviate from the Schottky-Mott limit, in contrast to what has previously been reported for a number of ambient processed organic-on-electrode systems. Our results suggest that the energetics at these ITO/SAM/polymer interfaces behave more like metal/organic interfaces previously studied in UHV despite being processed from solution.
• Ratcliff, E. L., Meyer, J., Steirer, K. X., Armstrong, N. R., Olson, D., & Kahn, A. (2012). Energy level alignment in PCDTBT:PC70BM solar cells: Solution processed NiOx for improved hole collection and efficiency. ORGANIC ELECTRONICS, 13(5), 744-749.
  More info

  Solution-based NiOx outperforms PEDOT:PSS in device performance and stability when used as a hole-collection layer in bulk-heterojunction (BHJ) solar cells formed with poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) and PC70BM. The origin of the enhancement is clarified by studying the interfacial energy level alignment between PCDTBT or the 1: 4 blended heterojunctions and PEDOT: PSS or NiOx using ultraviolet and inverse photoemission spectroscopies. The 1.6 eV electronic gap of PEDOT: PSS and energy level alignment with the BHJ result in poor hole selectivity of PEDOT:PSS and allows electron recombination at the PEDOT: PSS/BHJ interface. Conversely, the large band gap (3.7 eV) of NiOx and interfacial dipole (>= 0.6 eV) with the organic active layer leads to a hole-selective interface. This interfacial dipole yields enhanced electron blocking properties by increasing the barrier to electron injection. The presence of such a strong dipole is predicted to further promote hole collection from the organic layer into the oxide, resulting in increased fill factor and short circuit current. An overall decrease in recombination is manifested in an increase in open circuit voltage and power conversion efficiency of the device on NiOx versus PEDOT: PSS interlayers. (C) 2012 Elsevier B.V. All rights reserved.
• Ratcliff, E. L., Sigdel, A. K., Macech, M. R., Nebesny, K., Lee, P. A., Ginley, D. S., Armstrong, N. R., & Berry, J. J. (2012). Surface composition, work function, and electrochemical characteristics of gallium-doped zinc oxide. THIN SOLID FILMS, 520(17), 5652-5663.
  More info

  Gallium-doped zinc oxide (GZO) possesses the electric conductivity, thermal stability, and earth abundance to be a promising transparent conductive oxide replacement for indium tin oxide electrodes in a number of molecular electronic devices, including organic solar cells and organic light emitting diodes. The surface chemistry of GZO is complex and dominated by the hydrolysis chemistry of ZnO, which influences the work function via charge transfer and band bending caused by adsorbates. A comprehensive characterization of the surface chemical composition and electrochemical properties of GZO electrodes is presented, using both solution and surface adsorbed redox probe molecules. The GZO surface is characterized using monochromatic X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy after the following pretreatments: (i) hydriodic acid etch, (ii) potassium hydroxide etch, (iii) RF oxygen plasma etching, and (iv) high-vacuum argon-ion sputtering. The O is spectra for the GZO electrodes have contributions from the stoichiometric oxide lattice, defects within the lattice, hydroxylated species, and carbonaceous impurities, with relative near-surface compositions varying with pretreatment. Solution etching procedures result in an increase of the work function and ionization potential of the GZO electrode, but yield different near surface Zn:Ga atomic ratios, which significantly influence charge transfer rates for a chemisorbed probe molecule. The near surface chemical composition is shown to be the dominant factor in controlling surface work function and significantly influences the rate of electron transfer to both solution and tethered probe molecules. (C) 2012 Elsevier B.V. All rights reserved.
• Widjonarko, N. E., Ratcliff, E. L., Perkins, C. L., Sigdel, A. K., Zakutayev, A., Ndione, P. F., Gillaspie, D. T., Ginley, D. S., Olson, D. C., & Berry, J. J. (2012). Sputtered nickel oxide thin film for efficient hole transport layer in polymer-fullerene bulk-heterojunction organic solar cell. THIN SOLID FILMS, 520(10), 3813-3818.
  More info

  Bulk-heterojunction (BHJ) organic photovoltaics (OPV) are very promising thin film renewable energy conversion technologies due to low production cost by high-throughput roll-to-roll manufacturing, an expansive list of compatible materials, and flexible device fabrication. An important aspect of OPV device efficiency is good contact engineering. The use of oxide thin films for this application offers increased design flexibility and improved chemical stability. Here we present our investigation of radio frequency magnetron sputtered nickel oxide (NiOx) deposited from oxide targets as an efficient, easily scalable hole transport layer (HTL) with variable work-function, ranging from 4.8 to 5.8 eV. Differences in HTL work-function were not found to result in statistically significant changes in open circuit voltage (V-oc) for poly(3-hexylthiophene):[6,6]-phenyl-C-61-butyric acid methyl ester (P3HT:PCBM) BHJ device. Ultraviolet photoemission spectroscopy (UPS) characterization of the NiOx film and its interface with the polymer shows Fermi level alignment of the polymer with the NiOx film. UPS of the blend also demonstrates Fermi level alignment of the organic active layer with the HTL, consistent with the lack of correlation between V-oc and HTL work-function. Instead, trends in j(sc), V-oc, and thus overall device performance are related to the surface treatment of the HTL prior to active layer deposition through changes in active layer thickness. (C) 2011 Elsevier B.V. All rights reserved.
• Polaske, N. W., Lin, H., Tang, A., Mayukh, M., Oquendo, L. E., Green, J. T., Ratcliff, E. L., Armstrong, N. R., Saavedra, S. S., & McGrath, D. V. (2011). Phosphonic Acid Functionalized Asymmetric Phthalocyanines: Synthesis, Modification of Indium Tin Oxide, and Charge Transfer. LANGMUIR, 27(24), 14900-14909.
  More info

  Metalated and free-base A(3)B-type asymmetric phthalocyanines (Pcs) bearing, in the asymmetric quadrant, a flexible alkyl linker of varying chain lengths terminating in a phosphonic acid (PA) group have been synthesized. Two parallel series of asymmetric Pc derivatives bearing aryloxy and arylthio substituents are reported, and their synthesis and characterization through NMR, combustion analysis, and MALDI-MS are described. We also demonstrate the modification of indium tin oxide (ITO) substrates using the PA functionalized asymmetric Pc derivatives and monitoring their electrochemistry. The PA functionalized asymmetric Pcs were anchored to the ITO surface through chemisorption and their electrochemical properties characterized using cyclic voltammetry to investigate the effects of PA structure on the thermodynamics and kinetics of charge transfer. Ionization energies of the modified ITO surfaces were measured using ultraviolet photoemission spectroscopy.
• Ratcliff, E. L., Meyer, J., Steirer, K. X., Garcia, A., Berry, J. J., Ginley, D. S., Olson, D. C., Kahn, A., & Armstrong, N. R. (2011). Evidence for near-Surface NiOOH Species in Solution-Processed NiOx Selective Interlayer Materials: Impact on Energetics and the Performance of Polymer Bulk Heterojunction Photovoltaics. CHEMISTRY OF MATERIALS, 23(22), 4988-5000.
  More info

  The characterization and implementation of solution-processed, wide bandgap nickel oxide (NiOx) hole-selective interlayer materials used in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) are discussed. The surface electrical properties and charge selectivity of these thin films are strongly dependent upon the surface chemistry, band edge energies, and midgap state concentrations, as dictated by the ambient conditions and film pretreatments. Surface states were correlated with standards for nickel oxide, hydroxide, and oxyhydroxide components, as determined using monochromatic X-ray photoelectron spectroscopy. Ultraviolet and inverse photoemission spectroscopy measurements show changes in the surface chemistries directly impact the valence band energies. O-2-plasma treatment of the as-deposited NiOx films was found to introduce the dipolar surface species nickel oxyhydroxide (NiOOH), rather than the p-dopant Ni2O3, resulting in an increase of the electrical band gap energy for the near-surface region from 3.1 to 3.6 eV via a vacuum level shift. Electron blocking properties of the as-deposited and O-2-plasma treated NiOx films are compared using both electron-only and BHJ devices. O-2-plasma-treated NiOx interlayers produce electron-only devices with lower leakage current and increased turn on voltages. The differences in behavior of the different pretreated interlayers appears to arise from differences in local density of states that comprise the valence band of the NiOx interlayers and changes to the band gap energy, which influence their hole-selectivity. The presence of NiOOH states in these NiOx films and the resultant chemical reactions at the oxide/organic interfaces in OPVs is predicted to play a significant role in controlling OPV device efficiency and lifetime.
• Ratcliff, E. L., Zacher, B., & Armstrong, N. R. (2011). Selective Inter layers and Contacts in Organic Photovoltaic Cells. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 2(11), 1337-1350.
  More info

  Organic photovoltaic cells (OPVs) are promising solar electric energy conversion systems with impressive recent optimization of active layers. OPV optimization must now be accompanied by the development of new charge-selective contacts and interlayers. This Perspective considers the role of interface science in energy harvesting using OPVs, looking back at early photoelectrochemical (photogalvanic) energy conversion platforms, which suffered from a lack of charge carrier selectivity. We then examine recent platforms and the fundamental aspects of selective harvesting of holes and electrons at opposite contacts. For blended heterojunction OPVs, contact/interlayer design is especially critical because charge harvesting competes with recombination at these same contacts. New interlayer materials can modify contacts to both control work function and introduce selectivity and chemical compatibility with nonpolar active layers and add thermodynamic and kinetic selectivity to charge harvesting. We briefly discuss the surface and interface science required for the development of new interlayer materials and take a look ahead at the challenges yet to be faced in their optimization.
• Steirer, K. X., Ndione, P. F., Widjonarko, N. E., Lloyd, M. T., Meyer, J., Ratcliff, E. L., Kahn, A., Armstrong, N. R., Curtis, C. J., Ginley, D. S., Berry, J. J., & Olson, D. C. (2011). Enhanced Efficiency in Plastic Solar Cells via Energy Matched Solution Processed NiOx Interlayers. ADVANCED ENERGY MATERIALS, 1(5), 813-820.
  More info

  We show enhanced efficiency and stability of a high performance organic solar cell (OPV) when the work-function of the hole collecting indium-tin oxide (ITO) contact, modified with a solution-processed nickel oxide (NiOx) hole-transport layer (HTL), is matched to the ionization potential of the donor material in a bulk-heterojunction solar cell. Addition of the NiOx HTL to the hole collecting contact results in a power conversion efficiency (PCE) of 6.7%, which is a 17.3% net increase in performance over the 5.7% PCE achieved with a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) HTL on ITO. The impact of these NiOx films is evaluated through optical and electronic measurements as well as device modeling. The valence and conduction band energies for the NiOx HTL are characterized in detail through photoelectron spectroscopy studies while spectroscopic ellipsometry is used to characterize the optical properties. Oxygen plasma treatment of the NiOx HTL is shown to provide superior contact properties by increasing the ITO/NiOx contact work-function by 500 meV. Enhancement of device performance is attributed to reduction of the band edge energy offset at the ITO/NiOx interface with the poly(N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4', 7'-di-2-thienyl-2', 1', 3'-benzothidiazole) (PCDTBT):[6,6]-phenyl-C61 butyric acid methyl ester PCBM and [6,6]-phenyl-C71 butyric acid methyl ester (PC70BM) active layer. A high work-function hole collecting contact is therefore the appropriate choice for high ionization potential donor materials in order to maximize OPV performance.
• Kim, B. Y., Ratcliff, E. L., Armstrong, N. R., Kowalewski, T., & Pyun, J. (2010). Ferrocene Functional Polymer Brushes on Indium Tin Oxide via Surface-Initiated Atom Transfer Radical Polymerization. LANGMUIR, 26(3), 2083-2092.
  More info

  The synthesis and electrochemical characterization of ferrocene functional polymethacrylate brushes oil indium tin oxide (ITO) electrodes using surface-initiated atom transfer radical polymerization (SI-ATRP) is reported. SI-ATRP of ferrocene-containing methacrylate (FcMA) monomers from a phosphonic acid initiator-modified ITO substrate yielded well-defined homo- and block (co)polymer brushes of varying molar mass (4,000 to 37,000 g/mol). Correlation of both electrochemical properties and brush thicknesses confirmed controlled SI-ATRP from modified ITO surfaces. The preparation of block copolymer brushes with varying sequences of FcMA segments was conducted to interrogate the effects of spacing from the ITO electrode surface on the electrochemical properties of a tethered electroactive film.
• Ratcliff, E. L., Lee, P. A., & Armstrong, N. R. (2010). Work function control of hole-selective polymer/ITO anode contacts: an electrochemical doping study. JOURNAL OF MATERIALS CHEMISTRY, 20(13), 2672-2679.
  More info

  We present a novel method for electrodeposition of ultra-thin films of poly-3-hexylthiophene (e-P3HT) on chemically modified indium-tin oxide (ITO) electrodes, to produce a hole-selective contact with an easily tuned work function (Phi), as demonstrated by a combination of spectroelectrochemistry and ultraviolet photoemission spectroscopy (UPS). Selective contacts for optimized charge injection have become essential components for both thin film organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). Electrochemically doped e-P3HT thin films, using counter ions such as PF6- do not suffer from stability issues associated with more "acidic'' polymer layers (e.g. PEDOT:PSS). By controlling the oxidation state of the e-P3HT film via electrochemical doping we control the charge density within the film, resulting in an increase in work function with an increase in degree of oxidation. The method of electrochemical formation and doping of the e-P3HT film, using either constant potential step (CA) versus pulsed-potential step (PPS) protocols, has a significant secondary impact on the work function, as a result of the interface dipole effects from entrapment of these counter ions in the near-surface region of the polymer film. These results have significance for the performance of both OLEDs and OPVs built on these doped e-P3HT layers.
• Ratcliff, E. L., Veneman, P. A., Simmonds, A., Zacher, B., Huebner, D., Saavedra, S. S., & Armstrong, N. R. (2010). A Planar, Chip-Based, Dual-Beam Refractometer Using an Integrated Organic Light-Emitting Diode (OLED) Light Source and Organic Photovoltaic (OPV) Detectors. ANALYTICAL CHEMISTRY, 82(7), 2734-2742.
  More info

  We present a simple chip-based refractometer with a central organic light-emitting diode (OLED) light source and two opposed organic photovoltaic (OPV) detectors on an internal reflection element (IRE) substrate, creating a true dual-beam sensor platform. For first-generation platforms, we demonstrate the use of a single heterojunction OLED based on electroluminescence from an Alq(3)/TPD heterojunction (tris-(8-hydroxyquinoline)-aluminum/N,N'-bis(3-methylpheny1)-N,N'-diphenyl-benzidine) and light detection with planar heterojunction pentacene/C(60) OPVs. The sensor utilizes the considerable fraction of emitted light from conventional thin-film OLEDs that is coupled into guided modes in the IRE, instead of into the forward (display) direction. A ray-optics description is used to describe light throughput and efficiency-limiting factors for light coupling from the OLED into the substrate modes, light traversing through the IRE substrate, and light coupling into the OPV detectors. The arrangement of the OLED at the center of the chip provides for two sensing regions: a "sample" channel and a "reference" channel, with detection of light by independent OPV detectors. This configuration allows for normalization of the sensor response against fluctuations in OLED light output, stability, and local fluctuations (temperature) that might influence sensor response. The dual-beam configuration permits significantly enhanced sensitivity to refractive index changes, relative to single-beam protocols, and is easily integrated into a field-portable instrumentation package. Changes in refractive index (SRI) between 10(-2) and 10(-3) RI units could be detected for single beam operation, with sensitivity increased to Delta RI approximate to 10(-4) RI units when the dual-beam configuration is employed.
• Keng, P. Y., Shim, I., Kim, B. Y., Sahoo, R., Veneman, P. E., Armstrong, N. R., Yoo, H., Pemberton, J. E., Bull, M. M., Griebel, J. J., Ratcliff, E. L., Nebesny, K. W., Pyun, D., Keng, P. Y., Shim, I., Kim, B. Y., Sahoo, R., Veneman, P. E., Armstrong, N. R., , Yoo, H., et al. (2009). Colloidal Polymerization of Polymer Coated Ferromagnetic Nanoparticles into Cobalt Oxide Nanowires. ACS Nano, 3, 3143-3157.
• Ratcliff, E. L., Jenkins, J. L., Nebesny, K., & Armstrong, N. R. (2008). Electrodeposited, "textured" poly(3-hexyl-thiophene) (e-P3HT) films for photovoltaic applications. CHEMISTRY OF MATERIALS, 20(18), 5796-5806.
  More info

  Organic photovoltaic devices have been created on activated and modified ITO electrodes from electrodeposited poly(3-hexylthiophene) (e-P3HT) donor layers, using pulsed-potential-step (PPS) electrodeposition protocols. PPS electrodeposition uses a series of potential steps of diffusion-controlled e-P3HT deposition, alternated with rest periods where no deposition occurs and the diffusion layer region near the electrode/solution interface refills with thiophene monomer. To create the most photoactive e-P3HT films, a "carpet" layer of polymer was first deposited using dual step chronoamperometry, to create a smooth, pinhole-free film on the ITO electrode. PPS electrodeposition was subsequently used to electrodeposit additional polymer and texture the e-P3HT surface, as revealed by both AFM and SEM. The extent of doping of the polymer film was controlled by the last applied rest potential and monitored by anion incorporation into the e-P3HT film using X-ray photoelectron spectroscopy (XPS). Textured and electrochemically doped e-P3HT films were used as the donor layer in photovoltaic devices, using vacuum deposited C(60) as the electron acceptor/electron transport layer: (ITO/e-P3HT/C(60)/BCP/Al). The performance of these ultrathin OPVs was markedly dependent upon the degree of electrochemical doping of the P3HT layers. The best OPV performance was obtained for e-P3HT films with an average doping level (ratio of oxidized to reduced thiophene units) of approximately 35%, as estimated by XPS. At 100 mW/cm(2) white light illumination, optimized devices give a V(OC) similar to 0.5 V and a maximum J(SC) similar to 3 mA/cm(2), with series resistance (R(S)) below 1 Omega.cm(2), shunt resistance (R(P)) in excess of 160 k Omega.cm(2), fill-L factors (FF) of approximately 0.65, and an overall power conversion efficiency of approximately 1%. These results demonstrate the promise of electrochemical protocols for the creation of a variety of hybrid energy conversion materials.
• Ratcliff, E. L., & Hillier, A. C. (2007). Directed Electrodeposition of polymer films using spatially controllable electric field gradients. LANGMUIR, 23(19), 9905-9910.
  More info

  We report a method for the directed electrodeposition of polymer films in various patterns using spatially controllable electric field gradients. One- and two- dimensional surface electric field gradients were produced by applying different potential values at spatially distinct locations on an electrode surface. Variations in the resulting local electrochemical potentials were used to spatially manipulate the rate of electrodeposition of several polymers. By controlling the electric field gradient in the presence of sequentially varying deposition solutions, complex polymer patterns could be produced. One-dimensional structures consisting of alternating bands of polyaniline and either poly(phenylene) oxide or poly(aminophenylene) oxide were produced, as well as more complex two-dimensional structures. Film characterization was achieved through optical imaging, UV-vis spectroscopy, and ellipsometry. Results indicate that this directed deposition technique is a simple strategy to create complex, millimeter-sized surface patterns of electrodeposited materials.

Proceedings Publications

• Harris, J., Bickford, J., Cho, P., Coppock, M., Farrell, M., Holthoff, E., & Ratcliff, E. L. (2019, 2019). Approaching single molecule sensing: Predictive sweat sensor design for ultra-low limits of detection. In Approaching single molecule sensing: Predictive sweat sensor design for ultra-low limits of detection, 11010.
• Rudolph, M., Harris, J. K., & Ratcliff, E. L. (2019, 2019). Predicting limits of detection in real-time sweat-based human performance monitoring. In Predicting limits of detection in real-time sweat-based human performance monitoring, 11020.
• Rudolph, M., Harris, J. K., & Ratcliff, E. L. (2019, 2019). Printable transistors for wearable sweat sensing. In Printable transistors for wearable sweat sensing, 11020.
• Ratcliff, E. L., Garcia, A., Cowan, S. R., Meyer, J., Steirer, K. X., Paniagua, S. A., Giordano, A., Marder, S., Armstrong, N. R., Kahn, A., Ginley, D., Olson, D. C., & , . (2012). Understanding Energy Level Alignment in PCDTBT:PC70BM Solar Cells. In 2012 38TH IEEE PHOTOVOLTAIC SPECIALISTS CONFERENCE (PVSC), 3377-3379.
  More info

  Blended heterojunctions continue to improve in overall efficiency, with current power conversion efficiencies (PCE) at >7%, with interfaces being a dominating factor in improving PCE. Charge separation is dictated by energetic alignment between the donor and acceptor. Selective interlayers are used to preferentially harvest one charge via either thermodynamic or kinetic pathways. Energetic level alignment between the blend and the charge collection interfaces can impact the open circuit voltage (V-OC) through both loss mechanisms and poor choice of interlayers can result in less than ideal charge collection efficiencies. Here we report on the surface work function characterization of interlayers and phosphonic acid modifiers using ultraviolet photoelectron spectroscopy and the measured energetic level alignment between the interlayers and PCDTBT: PC70BM BHJs. We will address the overall charge extraction from the bulk heterojunction of PCDTBT: PC70BM and subsequent impact on V-OC, J(SC), and PCE.
• Jenkins, J. L., Ratcliff, E. L., Shallcross, R. C., Head, J. L., Armstrong, N. R., & Tsakalakos, L. (2008). Photovoltaic devices created from electrodeposited, "nano-textured" poly(thiophene) films. In NANOSCALE PHOTONIC AND CELL TECHNOLOGIES FOR PHOTOVOLTAICS, 7047.
  More info

  Polythiophene films can be electrodeposited on modified ITO substrates, textured to increase their active surface area, doped to enhance charge transport, and then interfaced with C(60) thin films to create "planar heterejunction" photovoltaic devices with power conversion efficiencies up to 1%. Preliminary results indicate that these electrodeposited films (e-P3HT) modified with appropriate ligands can serve as hosts for semi-conducting nanoparticles (CdSe NPs). These NPs may ultimately extend the device spectral sensitivity into the red and near-IR spectral regions.

Reviews

• Armstrong, N. R., Veneman, P. A., Ratcliff, E., Placencia, D., & Brumbach, M. (2009. Oxide Contacts in Organic Photovoltaics: Characterization and Control of Near-Surface Composition in Indium-Tin Oxide (ITO) Electrodes(pp 1748-1757).
  More info

  The recent improvements in the power conversion efficiencies of organic photovoltaic devices (OPVs) promise to make these technologies increasingly attractive alternatives to more established photovoltaic technologies. OPVs typically consist of photoactive layers 20-100 nm thick sandwiched between both transparent oxide and metallic electrical contacts. Ideal OPVs rely on ohmic top and bottom contacts to harvest photogenerated charges without compromising the power conversion efficiency of the OPV. Unfortunately, the electrical contact materials (metals and metal oxides) and the active organic layers in OPVs are often incompatible and may be poorly optimized for harvesting photogenerated charges. Therefore, further optimization of the chemical and physical stabilities of these metal oxide materials with organic materials will be an essential component of the development of OPV technologies. The energetic and kinetic barriers to charge injection/collection must be minimized to maximize OPV power conversion efficiencies.
• Armstrong, N. R., Wang, W., Alloway, D. M., Placencia, D., Ratcliff, E., & Brumbach, M. (2009. Organic/Organic' Heterojunctions: Organic Light Emitting Diodes and Organic Photovoltaic Devices(pp 717-731).
  More info

  Heterojunctions created from thin films of two dissimilar organic semiconductor materials [organic/organic' (O/O') heterojunctions] are an essential component of organic light emitting diode displays and lighting systems (OLEDs, PLEDs) and small molecule or polymer-based organic photovoltaic (solar cell) technologies (OPVs). O/O' heterojunctions are the site for exciton formation in OLEDs, and the site for exciton dissociation and photocurrent production in OPVs. Frontier orbital energy offsets in O/O' heterojunctions establish the excess free energy controlling rates of charge recombination and formation of emissive states in OLEDs and PLEDs. These energy offsets also establish the excess free energy which controls charge separation and the short-circuit photocurrent (J(SC)) in OPVs, and set the upper limit for the open-circuit photopotential (V(OC)). We review here how these frontier orbital energy offsets are determined using photo-emission spectroscopies, how these energies change as a function of molecular environment, and the influence of interface dipoles on these frontier orbital energies. Recent examples of heterojunctions based on small molecule materials are shown, emphasizing those heterojunctions which are of interest for photovoltaic applications. These include heterojunctions of perylenebisimide dyes with trivalent metal phthalocyanines, and heterojunctions of titanyl phthalocyanine with C(60), and with pentacene. Organic solar cells comprised of donor/acceptor pairs of each of these last three materials confirm that the V(OC) scales with the energy offsets between the HOMO of the donor and LUMO of the acceptor (E(HOMOD)-E(LUMOA)).
• Keng, P. Y., Kim, B. Y., Shim, I., Sahoo, R., Veneman, P. E., Armstrong, N. R., Yoo, H., Pemberton, J. E., Bull, M. M., Griebel, J. J., Ratcliff, E. L., Nebesny, K. G., & Pyun, J. (2009. Colloidal Polymerization of Polymer-Coated Ferromagnetic Nanoparticles into Cobalt Oxide Nanowires(pp 3143-3157).
  More info

  The preparation of polystyrene-coated cobalt oxide nanowires is reported via the colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Using a combination of dipolar nanoparticle assembly and a solution oxidation of preorganized metallic colloids, interconnected nanoparticles of cobalt oxide spanning micrometers in length were prepared, The colloidal polymerization of PS-CoNPs into cobalt oxide (CoO and Co(3)O(4)) nanowires was achieved by bubbling O(2) into PS-CoNP dispersions in 1,2-dichlorobenzene at 175 degrees C. Calcination of thin films of PS-coated cobalt oxide nanowires afforded Co(3)O(4) metal oxide materials. Transmission electron microscopy (TEM) revealed the formation of interconnected nanoparticles; of cobalt oxide with hollow inclusions, arising from a combination of dipolar assembly of PS-CoNPs and the nanos(ale Kirkendall effect in the oxidation reaction. Using a wide range of spectroscopic and electrochemical characterization techniques, we demonstrate that cobalt oxide nanowires prepared via this novel methodology were electroactive with potential applications as nanostructured electrodes for energy storage.

Others

• Gliboff, M., Sang, L., Knesting, K. M., Schalnat, M. C., Mudalige, A., Ratcliff, E. L., Li, H., Sigdel, A. K., Giordano, A. J., Berry, J. J., Nordlund, D., Seidler, G. T., Bredas, J., Marder, S. R., Pemberton, J. E., & Ginger, D. S. (2013, MAR 26). Orientation of Phenylphosphonic Acid Self-Assembled Mono layers on a Transparent Conductive Oxide: A Combined NEXAFS, PM-IRRAS, and DFT Study (vol 29, pg 2166, 2013). LANGMUIR.