(4aw) Novel Thin Film Deposition Techniques to Accelerate Data-Driven Discovery and Optimization of Optoelectronic Hybrid Organic-Inorganic Materials

Despite recent advances in automation and computing, materials science remains rooted in 20^th century methods that rely heavily on laborious manual processing and characterization and human analysis of the resulting data. Consequently, it can take decades to bring technologies using new materials to market, rendering materials R&D risky to investors. A crucial bottleneck in materials discovery and optimization is the rate at which high-quality samples of different candidate compositions may be produced. Many optoelectronic technologies rely on materials that are processable as thin films, and there is a growing effort to develop high-throughput film deposition techniques that yield “libraries” comprising many compositions on a single substrate. However, it can be difficult to maintain consistent morphology across the entire range of compositions in a single library, complicating efforts to compare and optimize them. Furthermore, some of the most popular techniques for materials library fabrication, such as pulsed laser deposition and co-sputtering, are high-energy processes ill-suited for many promising softer materials, particularly those that are fully or partially organic. An increasingly prominent class of such materials is crystalline hybrid organic-inorganic compounds. In particular, the rise of halide perovskites as an active area of photovoltaics research over the past decade showcases these materials’ outstanding properties. Applications such as radiation emission and detection are of growing interest, as well as more cutting-edge technologies like neuromorphic computing, spintronics, and quantum information science. The untapped promise of this field is enormous due to the huge array of compounds that can be assembled from the wide ranges of possible organic and inorganic substructures, but the materials exploration challenges are equally vast. High-throughput thin film fabrication techniques compatible with these partially organic materials will be essential to accelerate the genesis of new hybrid material-based technologies. I aim to start a research group that will develop high-throughput fabrication and characterization techniques for hybrid material thin films, use them to assemble large datasets of material properties, construct machine learning models to optimize them and guide future cycles of experimentation, and use the knowledge gained to fabricate optoelectronic devices with outstanding performance. These techniques will be applied to burgeoning applications of hybrid materials, such as spintronics, light emission, and photovoltaics.

Prior Research. My previous work includes developing methods to process hybrid materials and machine learning models to forecast their properties. As a graduate student in the Mitzi group at Duke University, I pursued several projects focused on developing novel processing schemes to improve the functionality of hybrid perovskites. One of the most captivating projects concerned the use of resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) to deposit thin films of crystalline hybrid materials, in collaboration with the Stiff-Roberts group at Duke. We were the first researchers to deposit crystalline hybrid materials of any kind by RIR-MAPLE, obtaining device-quality films of the archetypal halide perovskite, methylammonium lead iodide.¹ Subsequently, we used RIR-MAPLE to deposit layered perovskite films containing large conjugated organic molecules, forming self-assembled tunable quantum wells.² These complex structures possess unique photophysical properties and offer exquisite control of excited-state dynamics, but they are exceedingly difficult to fabricate as thin films due to the chemical disparity between the organic and inorganic structural components. RIR-MAPLE makes them tractable by uniting the gentleness of solution processing with the fine control of vapor deposition. While at Duke, I also pursued the development of stable hybrid perovskite solar cell architectures, and a better understanding of the degradation mechanisms that can occur in these devices. From this work, I discovered several reactions between methylammonium lead iodide and semiconductors commonly used in thin film solar cell fabrication: CdS,³ NiO,⁴ and SnO₂.⁴ During my postdoctoral work in the Hillhouse group at the University of Washington, I continued my work on hybrid perovskite stability, but from a different angle: using data science to predict and understand degradation processes affecting hybrid perovskites. Using a unique instrument that collects photoconductivity, photoluminescence, and optical transmittance data in-situ from thin film samples exposed to controlled environmental conditions, my colleagues and I assembled a large body of perovskite degradation data. Using the initial observations of the evolution of perovskite films’ optoelectronic properties, we developed the world’s first predictive model of perovskite degradation, which forecasts the decay kinetics of carrier diffusion length, a key material parameter constraining photovoltaic performance.⁵

Future Plans. I will synthesize my experience in advanced thin film deposition techniques for hybrid materials with my experience in automated data collection and machine learning to develop a program for characterizing, screening, and optimizing next-generation hybrid materials. A crucial first step will be developing and validating high-throughput techniques for deposition of hybrid materials libraries. Projects I am interested in pursuing once this capability is validated include discovery and optimization of chiral perovskite compositions for efficient spin valves and spin-LEDs; manipulating the emission properties of hybrid materials containing phosphorescent organic molecules; and targeting low-dimensionality hybrid material band gaps into the infrared regime by incorporating chalcogenide frameworks into the organic-inorganic crystal structure. Successful execution of these projects will establish a new blueprint for efficient, high-volume research in a new frontier of materials science. While I intend to cultivate scholars who are well-equipped for these multidisciplinary challenges within my group, I also look forward to the prospect of collaborating with researchers with complementary expertise, such as organic chemists, spectroscopists, and theorists.

Teaching Interests. Gaining a broad education in engineering and the physical sciences has been one of the most rewarding parts of my life, and I am invested in guiding future generations on equally edifying journeys. With my broad background in engineering, chemistry, and physics, I am most interested in teaching courses that exist at the nexus of these disciplines, such as thermodynamics and statistical mechanics, introductory courses in materials science and solid-state chemistry, or quantum mechanics. I would also be interested in developing upper-level courses on photovoltaics, advanced materials characterization, and the design and construction of custom experimental apparatus. In my view, there are three principal assets of a classroom STEM education: marketable skills, personal empowerment, and joy. Students are better able to retain knowledge and engage with difficult material when they have strong intrinsic motivation, and my courses will be designed to stoke their desire for active learning. Students will take informal surveys in the beginning of each course to tailor material to their interests where possible (e.g., worked examples), and periodically afterward to provide feedback on what is working and what is not. Hands-on experience from laboratory work and design projects will be an integral part of courses that I teach, as well as weekly design problems. These experiences are good representations of the sorts of problems students will encounter in their future careers and provide the opportunity for students to exercise critical thinking and judgment, as well as to document their reasoning.

Selected Publications.

(1) Dunlap-Shohl, W. A.;* Barraza, E. T.;* Barrette, A.; Gundogdu, K.; Stiff-Roberts, A. D.; Mitzi, D. B. MAPbI ₃ Solar Cells with Absorber Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation. ACS Energy Lett. 2018, 3, 270–275.

(2) Dunlap-Shohl, W. A.; Barraza, E. T.; Barrette, A.; Dovletgeldi, S.; Findik, G.; Dirkes, D. J.; Liu, C.; Jana, M. K.; Blum, V.; You, W.; Gundogdu, K.; Stiff-Roberts, A. D.; Mitzi, D. B. Tunable Internal Quantum Well Alignment in Rationally Designed Oligomer-Based Perovskite Films Deposited by Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation. Mater. Horiz. 2019, 6, 1707–1716.

(3) Dunlap-Shohl, W. A.; Younts, R.; Gautam, B.; Gundogdu, K.; Mitzi, D. B. Effects of Cd Diffusion and Doping in High-Performance Perovskite Solar Cells Using CdS as Electron Transport Layer. J. Phys. Chem. C 2016, 120 (30), 16437–16445.

(4) Dunlap-Shohl, W. A.; Li, T.; Mitzi, D. B. Interfacial Effects during Rapid Lamination within MAPbI ₃ Thin Films and Solar Cells. ACS Appl. Energy Mater. 2019, 2 (7), 5083–5093.

(5) Stoddard, R. J.;* Dunlap-Shohl, W. A.;* Qiao, H.; Meng, Y.; Kau, W. F.; Hillhouse, H. W. Forecasting the Decay of Hybrid Perovskite Performance Using Optical Transmittance or Reflected Dark-Field Imaging. ACS Energy Lett. 2020, 5 (3), 946–954.

* Equal credit