Research Triangle – Research Experience for Undergraduates (RT-REU) on Hybrid Perovskite Materials

“On-Chip Hybrid Perovskite Single Crystals Semiconductors” (Prof. Aram Amassian, NC State)

Hybrid perovskite semiconductors have been shown to be easily processed into macroscopic single crystals (SCs) of high quality despite being grown from solution at low temperature, opening new opportunities for manufacturing high-performance, bespoke single-crystal semiconductors. While standalone SCs are easily synthesized, directly growing SCs on foreign substrates is considerably more important technologically but also particularly challenging due to the preponderance of heterogeneous nucleation. RT-REU students joining the Amassian lab will be exposed to all aspects of SC synthesis and formulation engineering and will benefit from the expert help of a graduate student and the PI’s encouragement to find creative ways of synthesizing SCs directly on surfaces. The Amassian lab has found that integrating REU students as full members of the research team in high value projects allows them to experience the life and pace of a researcher and provides a vested interest by the mentor and other lab members in the success of the REU student’s project. The student will be given the opportunity to choose between a project focusing on the synthesis side of SCs, such as by developing approaches for obtaining large SCs of new material formulations, or to focus instead on integration of SCs onto crystal-line substrates via direct printing techniques, followed by qualitative and quantitative characterization of crystals and their physical properties; alternatively, a student more inclined toward device fabrication will be given the opportunity to design new ways of patterning SC arrays on substrates toward their integration into optoelectronic or electronic devices. Students will be mentored daily and will participate in weekly group meetings to present their progress and receive feedback.

“Computational Discovery and Understanding of Tailored Hybrid Perovskites” (Prof. Volker Blum, Duke)

The Blum group focuses on quantum-mechanical, electronic structure based computational predictions of structure and properties of new materials, with a strong focus on developing new computational methods based on electronic structure theory. Through long-term, joint developments led by Blum, the group is connected to a large, international community of researchers, including the FHI-aims all-electron electronic structure code and the open-source “ELSI” infrastructure, which bridges several leading electronic structure software packages. Blum’s group is also involved in a separate NSF-funded project “HybriD3”, focused on broadening the materials space of new hybrid perovskites, as well as in a DoE Energy Frontier Research Center, “Center for Hybrid Organic-Inorganic Semiconductors for Energy” (CHOISE), focused on elucidating the physics of emergent phenomena such as spin, charge and light-matter interactions in new organic-inorganic hybrids. The undergraduate researcher in this project will use electronic structure methods to elucidate questions related to tailored structure (e.g. by incorporating chiral molecules to manipulate spin and chiroptical properties) and energy band structures of complex hybrid perovskites, using state-of the-art computational approaches that can nevertheless be learned by a sufficiently motivated undergraduate student. Mentoring will be provided by Blum as well as by a senior Ph.D. student or postdoctoral researcher in the group. The student will also be trained in using advanced computational research equipment effectively, e.g.supercomputers such as those accessible through the National Energy Research Supercomputing Center.

“CVD Growth, Conversion, and Passivation of Hybrid Perovskites” (Prof. James Cahoon, UNC)

Chemical vapor deposition (CVD) methods are widely used to synthesize thin film and nanostructured semiconductors, yet a robust method to synthesize hybrid perovskites by CVD has not been developed. The Cahoon group recently reported the vapor-liquid-solid (VLS) synthesis of lead halide nanowires and their conversion to hybrid perovskites using a CVD process. Hybrid perovskites such as MAPbI₃ are often formed by the intercalation of an organic halide with a lead halide, such as the introduction of methylammonium iodide (MAI) into a lead iodide (PbI₂) crystal lattice. For hybrid perovskites, these reactions are typically performed in the solution phase rather than vapor phase through a CVD type process, yet vapor-phase processes can potentially produce higher quality, pure materials and retain the morphology of the initial materials. RT-REU students will study the vapor-phase growth and conversion of lead halide and perovskite materials using a custom-built CVD in the Cahoon group. This one-of-a-kind CVD systems features vapor-phase precursors including tetraethyl lead, monomethylamine, HCl, HBr, and HI gases. These gases are safely introduced into a thermal reactor with pressure, temperature, and flow rates all under computer control. Using this system, researchers can study both the direct growth of hybrid perovskites and the conversion of lead halides to perovskite, as well as the vapor-phase passivation of perovskites using individual component gases. Studies will be performed on materials grown in-house as well as on single-crystals obtained from RT-REU collaborating groups (e.g. Huang, You). RT-REU students will learn the principles of CVD processes and characterization techniques available in RTNN facilities, including scanning and transmission electron microscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, and photoluminescence spectroscopy. They will develop skills in LabView and MatLab data collection and analysis and receive training on literature review, oral presentations, and manuscript preparation. In addition to RT-REU activities, students will participate in and present at sub-group and group meetings.

 “Charge Carrier Dynamics in 2D Hybrid Perovskites” (Prof. Gundogdu, NC State)

The REU student on this project will study optically excited carrier dynamics in various hybrid perovskite thin films using time resolved spectroscopy techniques existing in the Gundogdu lab. The goal of the research will be revealing exciton charge separation, relaxation, and recombination kinetics in designed hybrid perovskites structures. The student will work on 2D layered perovskites in which both the organic cation layer and the inorganic components are optically active. In other words, depending on the photon energy, optical excitations create carriers in organic, inorganic or in both of the component. The relative alignment of the electronic energy levels and the electronic coupling between the component materials leads to tunable electronic properties. As a result, this material system can be basis of many optoelectronic applications such as solid-state lighting and photovoltaics. The REU student will contribute to these studies by performing linear optical absorption and ultrafast differential transmission experiments. In the time resolved experiments a tunable laser source with 100 fs pulse out is used to excite the material, and a broadband probe pulse measures the population in different electronic states at various time delays. By tuning the excitation pulse across the absorption bands of the material, the student will selectively excite the organic or inorganic component of the material and investigate resulting dynamics. The outcome of these studies will be used to design perovskite based electronic materials.

“Fabrication of Perovskite Solar Cells” (Prof. Jinsong Huang, UNC)

In this project, a REU student will be trained to use the blading method to make high efficiency perovskite solar panels on either rigid or flexible indium tin oxide (ITO) substrates. The student will learn how thin-film solar cells work and the processes to make a thin film solar cell and a large-area solar module. The students will learn the advantages of photovoltaic cells based on hybrid perovskites, including simplicity to fabricate, low cost, high efficiency, and lightweight, by considering these properties in real applications. This project will also demonstrate the broader impact of hybrid perovskite materials by providing a clean renewable energy source.

“First-Principles DFT of Chemically-Substituted Perovskites” (Prof. Yosuke Kanai, UNC)

The Kanai group works on development and application of computational methods based on first-principles electronic structure theory for obtaining the understanding at the atomistic level. By applying large-scale density functional theory simulation methods, the REU student will work closely with graduate students and post-doctoral researchers to investigate electronic structure properties of organic-inorganic hybrid perovskites. The REU student will learn a wide range of scientific concepts in chemistry and solid states physics and also develop computational skills, including the use of Unix-operating environment and performing computer simulations of molecules and materials on modern massively-parallel computers.

“Structural fluctuations and atomic dynamics in hybrid perovskites” – Prof. Olivier Delaire (Duke)

Hybrid metal halide perovskites are attracting intense interest for their exceptional photovoltaic and optoelectronic performance, as well as unusual thermal properties. Yet, the origins of their long photocarrier lifetimes, defect tolerance, and unusual ultralow thermal conductivity remain poorly understood and controversial. Halide perovskites are unusually soft and anharmonic materials, and their atomic structure and dynamics are not well captured by conventional textbook models. Although they are crystalline materials, their atomic dynamics have been characterized as nearly liquid-like or “phonon glasses”. Studies from the Delaire group have established previously unknown collective fluctuations of the crystal structure in halide perovskites, which are more recently being found to occur in the entire family of these materials. Understanding the atomic structure (fluctuations) and dynamics is key to understand photocarrier-lattice coupling and material performance. In this project, the student will work with Raman spectroscopy, x-ray and neutron scattering techniques, with mentoring from PhD students, to characterize the atomic structure and dynamics in hybrid perovskites and shed light on these important scientific questions.

“Exploring Structure-Property Tunability in Crystalline and Glass-Forming Perovskites” (Prof. David Mitzi, Duke)

Halide perovskites represent a diverse collection of structures ranging from strictly inorganic to organic-containing and from zero- to three-dimensional connectivity within the underlying inorganic framework. These semiconductors offer an unprecedented opportunity for tailoring outstanding electronic characteristics for optoelectronic and spintronic devices, including a direct and tunable band gap, small electron/hole effective masses, defect tolerance (or resistance to non-radiative recombination), and various forms of spin splitting. Furthermore, this materials family can be processed cheaply and effectively using a number of simple solution- and vacuum-based approaches. One project within the group relates to understanding how organic cation choice during 2D perovskite synthesis determines the final structure type (e.g., effective dimensionality and connectivity of the metal halide octahedra, degree of distortion of the octahedra, overall symmetry of the structure) and whether computational approaches (e.g., machine learning coupled with molecular dynamics) can be used to predict the resulting structural and electronic characteristics. Such predictability would fundamentally change materials design within this important semiconductor family. A second possible project relates to a recent observation from our group that certain choices of organic cation can help to access a glassy state (i.e., lacking long-range order) for the hybrid 2D system. Since the amorphous (glassy) state has different physical properties from the crystalline state, ability to switch between the two states provides unique opportunities for optoelectronics and energy applications. The current project seeks to understand the structural basis of these glass-forming perovskites and how to control the kinetics of switching between the amorphous and crystalline states. Both projects will explore structure-property relationship using a combination of tools/approaches that may include crystal growth or film deposition, X-ray diffraction, optical and scanning electron microscopy, differential scanning calorimetry and optical spectroscopies.

 “Compositional Engineering in Perovskite Light Emitting Diodes” (Prof. Franky So, NC State)

The proposed research will be a study of quasi-2D perovskites to uncover their structural-property relationship in this class of materials for lasing operation. The objective is to gain an in-depth understanding of the effects of the perovskite composition, crystallinity, crystal dimensionality, nanoscale morphology on polaron formation, energy relaxation kinetics, and amplified spontaneous emission thresholds, and the outcome is to establish a set of perovskite material design rules leading to a big step forward toward the realization of room temperature CW operation lasers emitting in all colors in the visible spectrum. The REU student is expected to be involved in synthesis of quasi-2D perovskite thin films and their characterization. The characterization techniques include x-ray diffraction, atomic force microscopy, UV-VIS spectrophotometry, and photoluminescence measurements. 

 “Matrix-Assisted Pulsed Laser Evaporation of Perovskite” (Prof. Adrienne Stiff-Roberts, Duke)

A critically important requirement to realize the promise of hybrid perovskites as a semiconductor technology is the controlled and reproducible manufacture of thin films at scale. Thin-film deposition of heterogeneous systems comprising two or more materials with fundamentally different properties is a critical challenge to overcome because, traditionally, solution-phase processing of organic materials is not compatible with vapor-phase processing of inorganic materials. However, RIR-MAPLE is a hybrid deposition technique that is applicable to a wide range of organic and hybrid materials and is compatible with a variety of substrates. In this REU project, the overall goal is to explore the feasibility of RIR-MAPLE as a manufacturing system for hybrid perovskites. Some critical requirements will be investigated to enable an industrial-scale RIR-MAPLE process, including: in-situ monitoring and feedback to provide real-time information on film thickness and composition; extension of film thickness uniformity to larger areas for high-throughput manufacturing; and determination of maximum background pressures at which controlled film deposition can occur to relax vacuum requirements..

“Perovskite-Based Spin-optoelectronic and Terahertz devices” (Prof. Dali Sun, NC State)

Sun’s research group focuses on studying the spin-dependent optoelectronic and Terahertz (THz) device physics at GHz to THz timescale using 3D, 2D, and reduced dimensional hybrid perovskites materials. The REU students will be trained to operate a glove box integrated deposition system in Sun’s lab and will assist in fabricating hybrid perovskite-based spin-optoelectronic devices including light-emitting diodes and spin-transfer torque devices. The REU students will be trained to fabricate hybrid perovskite-based spintronic devices in a project that was recently funded by the DOE EFRC program in close collaboration with a variety of experts in the field of hybrid perovskites. Working with the senior graduate students in Sun’s group, the REU students will learn to develop LabVIEW programs and variable-temperature photoluminescence set up for characterizing the magneto-optical properties in various hybrid perovskite thin films and single crystals. 

“Novel Hybrid Perovskites with Tailored Organic Cations” (Prof. Wei You, UNC)

The Youlab is interested in advancing research and education in the area of novel organic/inorganic hybrid perovskites for fundamental understanding and a variety of applications. RT-REU student participants in the You lab will work with a graduate student and the PI to learn all aspects of hybrid perovskites, including synthesis, characterization of fundamental properties, and device fabrication and testing. Depending upon the REU student’s own research interest and research experience/background, the REU student will be supervised by the graduate student mentor on a specific project, either synthesis (organic and/or inorganic synthesis) oriented or device focused. The graduate student mentor will provide day-to-day mentoring with support from all members of the You lab, including the PI. The You lab has found this is the best strategy to support the REU student’s growth as an independent researcher while broadening his/her research experience through group meetings and interactions with all lab members. Moreover, due to the multidisciplinary nature of these projects, the REU student will have the opportunity to participate in project-focused group meetings to interact with researchers at other institutions (Duke and NC State).