Mapping the Diffusion Tensor in Microstructured Perovskites

Understanding energy transport in semiconductors is critical for design of electronic and optoelectronic devices. Semiconductor material properties such as charge carrier mobility or diffusion length are measured in bulk crystals and determined using models that describe transport behavior in homogeneous media, where structural boundary effects are minimal. However, most emerging semiconductors exhibit microscale heterogeneity. Therefore, experimental techniques with high spatial resolution paired with models that capture anisotropy and domain boundary behavior are needed. We develop a diffusion tensor-based framework to analyze experimental photoluminescence (PL) diffusion maps accounting for material microstructure. Specifically, we quantify both carrier transport and recombination in single crystal and polycrystalline lead halide perovskites by globally fitting diffusion maps, with spatial, temporal, and PL intensity data. We reveal a 29% difference in principal diffusion coefficients and alignment between electronically coupled grains for CH3NH3PbI3 polycrystalline films. This framework allows for understanding and optimizing anisotropic energy transport in heterogeneous materials.Comment: 47 pages, 19 figure

Acetonitrile based single step slot-die compatible perovskite ink for flexible photovoltaics

The demonstration of photovoltaic devices with high power conversion efficiencies using low cost perovskite materials hints at the possibility of dramatically lowering the cost of solar energy. Key to further exploiting the potential of these materials is developing rapid processing techniques that can be used to deliver lower cost high throughput manufacture. This work details the development of low viscosity rapid drying perovskite formulations designed to give high quality solar films when slot-die coated on flexible roll-to-roll compatible substrates. A single step slot-die compatible perovskite ink based on an acetonitrile/methylamine solvent system utilizing a chloride additive is developed, resulting in large area perovskite films from slot-die coating under ambient conditions. The drying conditions for the perovskite film are optimized and fast (<10 min), low temperature (<120 °C) drying of slot-die coated films on flexible substrates are demonstrated and result in high performance devices

Scalable Perovskite Thin-Film Photovoltaics

In recent years lead halide based thin-film perovskites have emerged as a promising direct bandgap semiconductor for light absorption and emission applications. This is in part due to their compositional tunability and potential alloying that allows their color to be tuned from the near-IR through the visible spectrum. Additionally, these materials are moderately soluble, opening the door to solution processing methods rather than traditional physical or chemical vapor-based growth. For photovoltaic applications, the thin-film devices created using perovskite materials are thin relative to silicon wafer-based counterparts and therefore have the potential to be used in mechanically flexible cell architectures. This allows for high-speed roll-to-roll printing and coating processes as pathways for large scale manufacturing. Here, the challenges of scaling these materials are discussed from multiple vantage points keeping end slot-die manufacturing in mind. First, as these are ink-based materials, governmental regulations on the use of solvents is considered and a technoeconomic model is created to guide manufacturing scale-up development. Second, the solubility limits of these materials are determined and novel ligand based multicomponent inks are developed that fit within the economic limits of regulation. Lastly, a novel ink-based recrystallization method is presented that is capable of accessing all industrially relevant stable perovskite compositions with minimal post-annealing requirements. We use these inks and recrystallization methods on both lab scale and larger area slot-die coating techniques for high-efficiency photovoltaic devices.Ph.D

Smoothing silver nanowires for optoelectronic applications

Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 113-118).Silver nanowire meshes have shown potential for becoming a more robust, flexible alternative to traditional ceramic and brittle metal oxides. However, the current methods employed at making these films are not scalable and the high aspect ratios limit their final use. Spray coating is a technique that is widely used in many industries and has proven effective in many coating applications but continues to be limited by drying defects when used traditionally. A simple modification; pulsed spraying can decouple rheology and chemistry and allow for deposition of more uniform silver nanowire mesh films. Additional processing methods, such as metal mesh compression and matrix infilling with a CVD deposited plastic, Parylene-C, can yield a composite that solves roughness issues present with bare films. By combining all of these methods, a silver nanowire-parylene composite can be fabricated using fully scalable techniques ready for sheet-to-sheet or roll-to-roll processing. The composite is flexible, optically transparent, surface smooth, atmospherically stable and conductive, proving itself as a potential replacement for traditional transparent electrodes.by Richard Michael Steuben Swartwout.S.M

Demonstrating group selection: A comment on Janet Landa’s ‘The bioeconomics of homogenous middleman groups as adaptive units’

Cooperation, Cultural evolution, Group selection,

Building Smaller, Cheaper, Faster Satellites within the Constraints of an Academic Environment

Student driven satellite projects are working under the constraints of extremely limited budgets, short development times due to student turnover, limited technical expertise, and other academic time commitment pressures. These are issues prevalent throughout the university system, but many programs do not directly address these realities. So how can educational programs promote a smaller, cheaper, faster philosophy and at the same time effectively realize its potential? This question was asked of the students in Stanford University\u27s Satellite Systems Development Laboratory (SSDL). In response, the Satellite QUIck Research Testbed (SQUIRT) program was developed to give students the opportunity to participate in the entire lifecycle of a satellite development. This paper will summarize the technical solutions they have achieved, given the environmental constraints the program is under. In particular, the issues addressed are those relevant to the subsystems which the authors manage: Communications, Thermal Control, and Attitude Determination & Control. Each subsystem section will describe its goals, constraints, and technical approaches, as well as the integral role of industry. It is hoped that this research and approach is transportable to other universities that wish to build their own small satellites. The successful matriculation of such smaller, cheaper, faster programs are not only fantastic educational opportunities for young engineers and future program managers, but eventually will prove beneficial to industry as this paradigm becomes the competitive norm

Christian Religious Badges Instill Trust in Christian and Non-Christian Perceivers

We conducted 4 experiments to examine how people incorporate visual information about strangers' religious identities-religious badges-into their decisions about how much to trust them. Experiment 1 revealed that Christian and non-Christian participants were more trusting (as measured by self-report) of targets who wore a religious badge associated with Christianity (Ash Wednesday ashes) than toward targets who did not wear such a badge. Experiment 2 replicated Experiment 1 and also revealed that the effects of Ash Wednesday ashes on Christians' and non-Christians' trust extended to a behavioral measure of trust (i.e., monetary allocations in a multiplayer trust game). Experiment 3 replicated Experiments 1 and 2 with a different religious badge (a necklace with the Christian cross on it). Experiment 4 ruled out a potential confound. Consistent with a stereotype interpretation, these results suggest that U.S. students regard visual cues to people's espousal of Christian religious beliefs as signals of their trustworthiness