Obadiah G. Reid

Research Associate

Renewable and Sustainable Energy Institute

University of Colorado, Boulder

Phone: 303-384-6588

Email: obadiah.reid@nrel.gov

obadiah.reid@colorado.edu

I study the conversion of light into electrical charge in functional electronic materials, with particular emphasis on how chemical and microscopic structure influence these dynamics. My present tool of choice is microwave conductivity, which I use to detect the generation, transport, and recombination of mobile charges without the need for a complete device structure. My work encompasses both continued development of the microwave conductivity technique, and fundamental research on photochemical energy conversion. I have a long-standing interest in organic electronic materials, but have more recently focused my attention on hybrid organic lead-halide perovskite compounds.

Microwave Conductivity

We use a wide variety of spectroscopic techniques to study photochemical energy conversion processes in our group, however microwave conductivity deserves special attention, as it is a unique tool in which we have world-leading expertise. Microwave conductivity is an ideal tool for anyone wishing to:

1. Resolve the dynamics of mobile charges in a semiconducting material with high sensitivity and nanosecond time resolution.

2. Measure the equilibrium conductivity and dielectric constant of a bulk material.

3. Study the above processes/properties in samples that are incompatible with device fabrication: solutions, powders, discontinuous films, etc.

4. Rapidly screen material formulations for potential photovoltaic performance.

Although transient microwave absorption measurements are widely used to study minority carrier lifetime in bulk inorganic semiconductors, instruments that can quantify photo-induced and equilibrium conductivity, and do so with sufficient sensitivity to study novel, weakly conducting materials are rare. Our system uses a well-defined sample geometry located inside microwave resonator to make this possible.

Diagram of the microwave conductivity setup in our lab.

1. Resolve the dynamics of mobile charges in a semiconducting material with high sensitivity and nanosecond time resolution.

2. Measure the equilibrium conductivity and dielectric constant of a bulk material.

3. Study the above processes/properties in samples that are incompatible with device fabrication: solutions, powders, discontinuous films, etc.

4. Rapidly screen material formulations for potential photovoltaic performance.

Two typical microwave resonators used for measuring solid (top) and solution/liquid (bottom) samples.

Depending on the sample geometry and properties, we can use this system to measure both photoconductivity, and equilibrium “dark” conductivity. In the first case we measure the transient change in microwave absorption in response to a laser pulse; in the latter we measure the absolute microwave absorption of the sample with no external perturbation. In cases where the quantum yield of charge generation is known, this allows us to calculate both the charge carrier mobility (at 9 GHz; sum of electron and hole contributions) and the equilibrium doping density of the sample.

Photochemistry of Organic Molecules

All life on earth is supported by photosynthesis. Light is absorbed by organic molecules, transferred to a catalytic reaction site, separated into charges, and converted to high-energy chemical fuel. At the heart of it all is the photochemistry of a relative handful of functional organic molecules, motivating our work on the fundamental properties of analogous systems. We focus on the initial stages of photochemical energy conversion: light absorption, energy transfer, charge separation, and how local structure influences these properties.

Charge Transfer in Low-Dielectric Environments

Organic photovoltaics are a nascent technology that crudely approximates photosynthesis, using organic molecules and polymers to capture solar energy and convert it to electric charge. The question is: how is this possible? Photosynthetic organisms use a well-defined cascade of energy and charge transfer steps to ensure stabilization of the charge separated state. Organic photovoltaic devices have no such feature, unless it exists adventitiously. Our work has revealed how the nanostructure of the conjugated polymers commonly used in these devices controls the generation of free charge, both in neat films, and in blends with an electron acceptor. In one specific test-case, we showed that without good intermolecular chain ordering, the generation of long-lived free charges shuts off and rapid (picosecond) geminate recombination is the result. Parellel work has shown that modulating the intermolecular coupling between the electron acceptor species has a similar effect. Ongoing work is investigating the generality of this observation, and whether it is inter- or intra-chain order that is most important.

Diagram illustrating the influence of interchain order on charge separation in organic photovoltaic blends. Ordered regions provide a delocalized charge transfer state that facilitates long-range charge separation.

Energy Flow Through Artificial Antennae Structures

Another recent thrust of our work as been to understand if artificial energy harvesting structures can be constructed that would emulate the light harvesting antennae present in natural photosynthetic complexes. Doing so could eliminate much of the complexity present in the best current organic solar cell devices. One candidate for this sort of structure is a multilayer cascade of energy harvesting molecules that harnesses electronic energy transfer to guide energy into an efficient reaction center. The diagram below illustrates this idea.

Schematic of an energy transfer solar cell. Blue and green (higher energy) light are absorbed by outer layers and the energy is efficiently transferred to a well-defined heterointerface for separation into free charges. Figure credit: Al Hicks, NREL.

We have shown that this is theoretically possible in the simple classical limit of Förster theory, and provided detailed design rules for what molecular properties will be required to realize reasonable device efficiencies. Future work will explore what is possible beyond the classical limit, incorporating coherent energy transfer.

Hybrid Perovskites

Organic lead halide perovskites have transformed research on emerging photovoltaic technologies. They were first used as solar absorbers a few years ago, and have already overtaken most extant thin-film technologies in terms of efficiency. However, it still is not clear if these materials will have the stability needed to compete in the market. Thus, there is immense practical and scientific interest in understanding why they are such good photovoltaic materials so that more stable alternatives can be found.

My interest is in the fundamental properties of these materials, and particularly how their unique organic-inorganic hybrid structure confers their unique photovoltaic characteristics. To date, we have demonstrated the nature and importance of grain boundaries in thin-film materials, and we are pursuing work to understand how growth conditions influence the electronic properties in single-crystals.

Education

Ph.D. Chemistry, University of Washington (2010)

B.S. Chemistry, Pacific University (2004)

Professional Experience

Renewable and Sustainable Energy Institute, UCB (2014-present) | Research Associate

National Renewable Energy Laboratory (2010-2014) | Post Doc

University of Washington (2005-2010) | Graduate Research Assistant / Teaching Assistant

Pacific Northwest National Laboratory (02/2005 – 08/2005) | Limited Term Employee

Pacific University (08/2003 - 05/2004) | Undergraduate Research Assistant / Teaching Assistant

University of Arizona (06/2003 – 08/2003) | Undergraduate Research Assistant (REU)

Featured Publications

Google Scholar Publication Record

(1) Yang, M.; Li, Z.; Reese, M. O.; Reid, O. G.; Kim, D. H.; Siol, S. Perovskite Ink with Wide Processing Window for Scalable High-Efficiency Solar Cells. Nature Energy 2017, 2, 17038.

(2) Dolgopolova, E. A.; Brandt, A. J.; Ejegbavwo, O. Electronic Properties of Bimetallic Metal-Organic Frameworks (MOFs): Tailoring Density of Electronic States Through MOF Modularity. J. Am. Chem. Soc. 2017, 139, 5201–5209.

(3) Larson, B. W.; Reid, O. G.; Coffey, D. C.; Avdoshenko, S. M.; Popov, A. A.; Boltalina, O. V.; Strauss, S. H.; Kopidakis, N.; Rumbles, G. Inter‐Fullerene Electronic Coupling Controls the Efficiency of Photoinduced Charge Generation in Organic Bulk Heterojunctions. Adv. Energy Mater. 2016, 1601427.

(4) Nayak, P. K.; Moore, D. T.; Wenger, B.; Nayak, S.; Haghighirad, A. A.; Fineberg, A.; Noel, N. K.; Reid, O. G.; Rumbles, G.; Kukura, P.; et al. Mechanism for Rapid Growth of Organic–Inorganic Halide Perovskite Crystals. Nature Communications 2016, 7, 13303.

(5) Hughes, B. K.; Braunecker, W. A.; Bobela, D. C.; Nanayakkara, S. U.; Reid, O. G.; Johnson, J. C. Covalently Bound Nitroxyl Radicals in an Organic Framework. J. Phys. Chem. Lett. 2016, 7, 3660–3665.

(6) Reid, O. G.; Yang, M.; Kopidakis, N.; Zhu, K.; Rumbles, G. Grain-Size-Limited Mobility in Methylammonium Lead Iodide Perovskite Thin Films. ACS Energy Lett. 2016, 1, 561–565.

(7) Dowgiallo, A.-M.; Mistry, K. S.; Johnson, J. C.; Reid, O. G.; Blackburn, J. L. Probing Exciton Diffusion and Dissociation in Single-Walled Carbon Nanotube-C60 Heterojunctions. J. Phys. Chem. Lett. 2016, 7, 1794–1799.

(8) Ihly, R.; Mistry, K. S.; Ferguson, A. J.; Clikeman, T. T.; Larson, B. W.; Reid, O. G.; Boltalina, O. V.; Strauss, S. H.; Rumbles, G.; Blackburn, J. L. Tuning the Driving Force for Exciton Dissociation in Single-Walled Carbon Nanotube Heterojunctions. Nat. Chem. 2016, 8, 603–609.

(9) Ihly, R.; Dowgiallo, A.-M.; Yang, M.; Schulz, P.; Stanton, N. J.; Reid, O. G.; Ferguson, A. J.; Zhu, K.; Berry, J. J.; Blackburn, J. L. Efficient Charge Extraction and Slow Recombination in Organic–Inorganic Perovskites Capped with Semiconducting Single-Walled Carbon Nanotubes. Energy Environ. Sci. 2016, 9, 1439–1449.

(10) Feier, H. M.; Reid, O. G.; Pace, N. A.; Park, J.; Bergkamp, J. J.; Sellinger, A.; Gust, D.; Rumbles, G. Local Intermolecular Order Controls Photoinduced Charge Separation at Donor/Acceptor Interfaces in Organic Semiconductors. Adv. Energy Mater. 2016, 6, 1502176.

(11) Reid, O. G.; Rumbles, G. Resonance Energy Transfer Enables Efficient Planar Heterojunction Organic Solar Cells. J. Phys. Chem. C 2015, 120, 87–97.

(12) Crisp, R. W.; Callahan, R.; Reid, O. G.; Dolzhnikov, D. S.; Talapin, D. V.; Rumbles, G.; Luther, J. M.; Kopidakis, N. Photoconductivity of CdTe Nanocrystal-Based Thin Films: Te 2–Ligands Lead to Charge Carrier Diffusion Lengths Over 2 Μm. J. Phys. Chem. Lett. 2015, 6, 4815–4821.

(13) Ferguson, A. J.; Dowgiallo, A.-M.; Bindl, D. J.; Mistry, K. S.; Reid, O. G.; Kopidakis, N.; Arnold, M. S.; Blackburn, J. L. Trap-Limited Carrier Recombination in Single-Walled Carbon Nanotube Heterojunctions with Fullerene Acceptor Layers. Phys. Rev. B 2015, 91, 245311.

(14) Park, J.; Reid, O. G.; Rumbles, G. Photoinduced Carrier Generation and Recombination Dynamics of a Trilayer Cascade Heterojunction Composed of Poly(3-Hexylthiophene), Titanyl Phthalocyanine, and C 60. J. Phys. Chem. B 2015, 119, 7729–7739.

(15) Park, J.; Reid, O. G.; Blackburn, J. L.; Rumbles, G. Photoinduced Spontaneous Free-Carrier Generation in Semiconducting Single-Walled Carbon Nanotubes. Nat. Comm. 2015, 6, 1–8.

(16) OConnor, B. T.; Reid, O. G.; Zhang, X.; Kline, R. J.; Richter, L. J.; Gundlach, D. J.; DeLongchamp, D. M.; Toney, M. F.; Kopidakis, N.; Rumbles, G. Morphological Origin of Charge Transport Anisotropy in Aligned Polythiophene Thin Films. Adv. Funct. Mater. 2014, 24, 3422–3431.

(17) Marsh, H. S.; Reid, O. G.; Barnes, G.; Heeney, M.; Stingelin, N.; Rumbles, G. Control of Polythiophene Film Microstructure and Charge Carrier Dynamics Through Crystallization Temperature. J. Polym. Sci. B Polym. Phys. 2014, 52, 700–707.

(18) Buchaca-Domingo, E.; Ferguson, A. J.; Jameison, F. C.; McCarthy-Ward, T.; Shoaee, S.; Reid, O. G.; Madec, M.-B.; Pfannmöller, M.; Hermerschmidt, F.; Schröder, R. R.; et al. Additive-Assisted Supramolecular Manipulation of Polymer:Fullerene Blend Phase Morphologies and Its Influence on Photophysical Processes. Materials Horizons 2014, 1, 270–279.

(19) Reid, O. G.; Pensack, R. D.; Song, Y.; Scholes, G. D.; Rumbles, G. Charge Photogeneration in Neat Conjugated Polymers. Chem. Mater. 2013, 26, 561–575.

(20) Reid, O. G.; Rumbles, G. Quantitative Transient Absorption Measurements of Polaron Yield and Absorption Coefficient in Neat Conjugated Polymers. J. Phys. Chem. Lett. 2013, 4, 2348–2355.

(21) Treat, N. D.; Nekuda Malik, J. A.; Reid, O. G.; Yu, L.; Shuttle, C. G.; Rumbles, G.; Hawker, C. J.; Chabinyc, M. L.; smith, P.; Stingelin, N. Microstructure Formation in Molecular and Polymer Semiconductors Assisted by Nucleation Agents. Nat. Mater. 2013, 12, 628–633.

(22) Ruddy, D. A.; Reid, O. G.; Leonard, B. M.; Pylypenko, S.; Neale, N. R. Non-Aqueous Thermolytic Route to Oxynitride Photomaterials Using Molecular Precursors Ti(OtBu)4 and NMo(OtBu)3. J. Mater. Chem. A 2013, 1, 14066–14070.

(23) Reid, O. G.; Malik, J. A. N.; Latini, G.; Dayal, S.; Kopidakis, N.; Silva, C.; Stingelin, N.; Rumbles, G. The Influence of Solid‐State Microstructure on the Origin and Yield of Long‐Lived Photogenerated Charge in Neat Semiconducting Polymers. J. Polym. Sci. B Polym. Phys. 2012, 50, 27–37.

(24) Giridharagopal, R.; Rayermann, G. E.; Shao, G.; Moore, D. T.; Reid, O. G.; Tillack, A. F.; Masiello, D. J.; Ginger, D. S. Submicrosecond Time Resolution Atomic Force Microscopy for Probing Nanoscale Dynamics. Nano Lett. 2012, 12, 893–898.

(25) Reid, O. G.; Xin, H.; Jenekhe, S. A.; Ginger, D. S. Nanostructure Determines the Intensity-Dependence of Open-Circuit Voltage in Plastic Solar Cells. J. Appl. Phys. 2010, 108, 084320.

(26) Reid, O. G.; Rayermann, G. E.; Coffey, D. C.; Ginger, D. S. Imaging Local Trap Formation in Conjugated Polymer Solar Cells: a Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging. J. Phys. Chem. C 2010, 114, 20672–20677.

(27) Groves, C.; Reid, O. G.; Ginger, D. S. Heterogeneity in Polymer Solar Cells: Local Morphology and Performance in Organic Photovoltaics Studied with Scanning Probe Microscopy. Accounts Chem. Res. 2010, 43, 612–620.

(28) Rodovsky, D. B.; Reid, O. G.; Pingree, L. S. C.; Ginger, D. S. Concerted Emission and Local Potentiometry of Light-Emitting Electrochemical Cells. Acs Nano 2010, 4, 2673–2680.

(29) Xin, H.; Reid, O. G.; Ren, G.; Kim, F. S.; Ginger, D. S.; Jenekhe, S. A. Polymer Nanowire/Fullerene Bulk Heterojunction Solar Cells: How Nanostructure Determines Photovoltaic Properties. Acs Nano 2010, 4, 1861–1872.

(30) Pingree, L. S. C.; Reid, O. G.; Ginger, D. S. Electrical Scanning Probe Microscopy on Active Organic Electronic Devices. Adv. Mater. 2009, 21, 19–28.

(31) Pingree, L. S. C.; Reid, O. G.; Ginger, D. S. Imaging the Evolution of Nanoscale Photocurrent Collection and Transport Networks During Annealing of Polythiophene/Fullerene Solar Cells. Nano Lett. 2009, 9, 2946–2952.

(32) Reid, O. G.; Munechika, K.; Ginger, D. S. Space Charge Limited Current Measurements on Conjugated Polymer Films Using Conductive Atomic Force Microscopy. Nano Lett. 2008, 8, 1602–1609.

(33) Coffey, D. C.; Reid, O. G.; Rodovsky, D. B.; Bartholomew, G. P.; Ginger, D. S. Mapping Local Photocurrents in Polymer/Fullerene Solar Cells with Photoconductive Atomic Force Microscopy. Nano Lett. 2007, 7, 738–744.

(34) Reid, O. G.; Johnson, K. Effects of Ph on Structure in Acid and Base Terminated Thioaromatic Self-Assembled Monolayers on Gold Substrates. Abstr Pap Am Chem S 2004, 227, U483–U483.

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