(4w) Device and Materials Physics of Emerging Semiconductors for Renewable Energy and Low-Cost Optoelectronics

To mitigate climate change, development of higher performance energy conversion devices is needed to hasten adoption of renewable energy and energy efficiency technologies. Organic and other emerging semiconducting materials have demonstrated exciting efficiency gains in a variety of at-market and laboratory devices. With these novel materials comes a variety of emergent material and device physics, requiring extensive study to understand and design next-generation energy conversion technologies. In particular, organic semiconductors thin films, dominant in high-area optoelectronics such as consumer displays, and their interfaces present rich photophysics, largely due to forming room-temperature stable excitons, not observed in more traditional semiconductors, i.e. silicon. Deeper study of emergent electronic and excitonic phenomena of novel semiconductor interfaces will reveal a plethora of processes that can be harnessed to improve performance of devices such as energy-efficient lighting, flexible or transparent solar cells, photodetectors and displays. Control over light-matter interactions in these and other materials will also enable devices for optical and quantum computing, distributed sensing, lasers for industry and medicine, and many more, all at low materials cost and operable at room-temperature.

In my post-doctoral research, I want to explore the nascent optoelectronic properties of semiconductors and their devices — to continue studying novel charge and exciton physics of emerging semiconductors. Building upon my experience with organic semiconductors, I am excited to gain broader experience with synthesis and characterization of materials such as metal-halide perovskites, transition-metal dichalcogenide monolayers, quantum dots and/or techniques such as ultrafast or pump-probe spectroscopies. I will do this using my knowledge and practice with semiconductor device fabrication and electrical and optical characterization techniques, all with the end goal of ushering in a cleaner, brighter future.

Research Experience:

I have focused on improving organic LEDs and solar cells by exploring exciton energetics and transport at organic semiconductor interfaces. I have used optical (steady-state and transient photoluminescence and UV-visible spectroscopies and spectroscopic ellipsometry), electrical (current and impedance measurements), and optoelectronic (current-luminance, photocurrent, and electroemission spectroscopy) measurements to study the unique energetic and transport properties of interfaces between different organic small-molecules. In one study, I used measurements of photoluminescence quenching and devices exhibiting space-charge limited current to explore nanoscale energy transport in organic semiconductor mixtures. In this study, I showed generality of charge-transfer state diffusion across material pairings and established materials selection rules for tuning the length scale of migration. Currently, I am studying the effect of electric field on the energy of photo- and electroluminescence from organic interfaces. This work identifies the Stark effect, which has previously been seen in more traditional semiconductors, but never clearly observed in organic materials, potentially expanding the range of applications for these materials. In addition to using vacuum thermal evaporation and spin coating to fabricate organic and inorganic thin films for materials and devices, I have experience in microfabrication techniques (photolithography, ALD, etc.) in both industrial and university fabs. I have additionally used air-free chemistry to synthesize nanoparticles and quantum dots, characterized them with transmission and scanning electron microscopy, and applied electrochemical techniques to study oxygen-reduction reaction catalysis and demonstrate a novel fuel cell form factor.

Education and Awards:

University of Minnesota, Minneapolis, MN (2017-Present)

PhD Candidate Chemical Engineering, Graduate Minor in Chemistry

Graduate Advisor: Russell J. Holmes, UMN Chemical Engineering & Materials Science

Massachusetts Institute of Technology, Cambridge, MA (2013-2017)

B.S. Chemical Engineering, Minor in Economics

Undergraduate Research Advisor: Yogesh Surendranath, MIT Chemistry

National Science Foundation- Graduate Research Fellow (2019)

Teaching Experience and Service:

Heat and Mass Transport (Undergraduate Chemical Engineering) (Fall 2016)

Wrote homework and exam problems and rubrics. Held office hours and proctored review sessions.

Numerical Methods (Undergraduate Chemical Engineering) (Spring 2018)

Held office hours, proctored exams, and graded homework and exams. Led two 1-hour recitation sessions.

Transport Phenomena (Graduate Chemical Engineering/Materials Science) (Fall 2019)

Held office hours, proctored exams, and graded homework and exams. Gave one 1-hour lecture.

UMN Science for All- Co-President (2018-Present)

Co-leading a team of ~60 graduate student mentors to plan and carry out monthly hands-on science experiments with ~160 middle-school students across four schools in Minneapolis-St. Paul with >50% under-represented minority enrollment.

Publications (5 total, 1 co-first author, 139 citations):

*contributed equally

[1] T. Zhang*; N. M. Concannon*; R. J. Holmes, “Migration of Charge-Transfer States at Organic Semiconductor Heterojunctions”, ACS Appl. Mater. Interfaces 2020, 12 (28), 31677-31686.

[2] J. S. Bangsund; J. R. Van Sambeek; N. M. Concannon; R. J. Holmes, “Sub–turn-on exciton quenching due to molecular orientation and polarization in organic light-emitting devices”, Science Advances 2020, 6 (32), eabb2659.

[3] C. A. Beaudette; J. T. Held; B. L. Greenberg; P. H. Nguyen; N. M. Concannon; R. J. Holmes; K. A. Mkhoyan; E. S. Aydil; U. R. Kortshagen, “Plasmonic nanocomposites of zinc oxide and titanium nitride”, J. Vac. Sci. & Technol. A 2020, 38 (4), 042404.

[4] B. Yan; N. M. Concannon; J. D. Milshtein; F. R. Brushett; Y. Surendranath, “A Membrane-Free Neutral pH Formate Fuel Cell Enabled by a Selective Nickel Sulfide Oxygen Reduction Catalyst”, Angew. Chemie. 2017, 129 (26), 7604-7607.

[5] J. M. Falkowski; N. M. Concannon; B. Yan; Y. Surendranath, “Heazlewoodite, Ni3S2: A Potent Catalyst for Oxygen Reduction to Water under Benign Conditions”, J. Am. Chem. Soc. 2015, 137 (25), 7978–7981.