Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3D Nanoscale Strain Mapping

In recent years, halide perovskite materials have been used to make high performance solar cell and light-emitting devices. However, material defects still limit device performance and stability. Here, we use synchrotron-based Bragg Coherent Diffraction Imaging to visualise nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. We find significant strain heterogeneity within MAPbBr$_{3}$ (MA = CH$_{3}$NH$_{3}^{+}$) crystals in spite of their high optoelectronic quality, and identify both $\langle$100$\rangle$ and $\langle$110$\rangle$ edge dislocations through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, we uncover dramatic light-induced dislocation migration across hundreds of nanometres. Further, by selectively studying crystals that are damaged by the X-ray beam, we correlate large dislocation densities and increased nanoscale strains with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. Our results demonstrate the dynamic nature of extended defects and strain in halide perovskites and their direct impact on device performance and operational stability.Comment: Main text and Supplementary Information. Main text: 15 pages, 4 figures. Supplementary Information: 16 pages, 27 figures, 1 tabl

Building a bridge to medical school success: An analysis of the acculturation process of eight minority students in a post-baccalaureate, pre-medical program.

In this dissertation I investigated a group of underrepresented minority students enrolled in a compensatory program designed to support their attempts to become acculturated into a community of traditional medical students and eventually to enroll in medical school. These students had previously been unsuccessful in their attempts at admission to medical school, due to underpreparedness suggested by low MCAT scores and undergraduate grade point averages. I was particularly interested in the students' own identification of their underpreparedness, as well as the meanings they constructed about this underpreparedness. Given that the program provided a rich array of opportunities for students to remedy their lack of preparation, I was also interested in the extent and nature of the students' engagement with those opportunities. Finally, I wanted to determine whether students' preparedness, sociocultural identities, and use of program opportunities could be grouped together to characterize identifiable trajectories that students such as these might follow in their quest to become physicians. I followed all eight students enrolled in this compensatory program during the academic year 1996-1997. I collected the personalized study plans they composed as a response to reflection on their learning histories and preferences, conducted an observational study in one of their winter semester classes and collected the notes they took in that class, and interviewed each student twice during the year. Students varied in their ability to discuss the nature of their underpreparedness. Two were not willing to label themselves underprepared at all, two were so acutely aware of their lack of preparation that they felt inferior to their peers in the compensatory program, and four were able to frankly discuss their lack of preparation, provide rationales for it, and discuss how they intended to go about remedying that lack of preparation. Students also showed differential motivation and interest in using the opportunities provided by the program. Half of the eight students engaged with the program opportunities in thoughtful ways, motivated by metacognition of their own particular needs as potential medical students and strengths as learners. The other half were less consistent in their use of opportunities; for instance, some completed the requirements without engagement, while others rejected the opportunities without trying them. Three trajectories were identified that characterize students' movement toward preparation for and enrollment in medical school. The trajectories were based on the distance students had to traverse between the discourse of their home and family and that of the medical school culture, their ability to identify this distance and engage with opportunities to compensate for it, and their motivation and interest in becoming physicians. These findings suggest that compensatory programs provide valuable opportunities for students who are willing to seize these opportunities and use them judiciously. Medical schools may find that these opportunities can be provided for all medical students. Furthermore, my findings indicate that students who are not traditionally prepared can be successful in their quest to enter medical school, given time and support from the medical school, as well as their own motivation and desire to become physicians.Ph.D.EducationEthnic studiesHealth Sciences, EducationHealth and Environmental SciencesHigher educationSocial SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131205/2/9840537.pd

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3d Nanoscale Strain Mapping.

Funder: King Abdullah University of Science and Technology; doi: http://dx.doi.org/10.13039/501100004052Funder: U.S. Department of Energy; doi: http://dx.doi.org/10.13039/100000015Funder: Office of Science; doi: http://dx.doi.org/10.13039/100006132Funder: HORIZON EUROPE European Research Council; doi: http://dx.doi.org/10.13039/100019180Funder: China Scholarship Council; doi: http://dx.doi.org/10.13039/501100004543Funder: British Spanish SocietyFunder: Sir Richard Stapley Educational Trust; doi: http://dx.doi.org/10.13039/501100016406Funder: Rank Prize FundFunder: Winton Sustainability FundFunder: George and Lilian Schiff FoundationFunder: Ernest Oppenheimer Early Career FellowshipFunder: Schmidt Science FellowshipIn recent years, halide perovskite materials have been used to make high-performance solar cells and light-emitting devices. However, material defects still limit device performance and stability. Here, synchrotron-based Bragg coherent diffraction imaging is used to visualize nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. Significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals is found in spite of their high optoelectronic quality, and both 〈100〉 and 〈110〉 edge dislocations are identified through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, dramatic light-induced dislocation migration across hundreds of nanometers is uncovered. Further, by selectively studying crystals that are damaged by the X-ray beam, large dislocation densities and increased nanoscale strains are correlated with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. These results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability

Imaging Light-Induced Migration of Dislocations in Halide Perovskites with 3d Nanoscale Strain Mapping.

In recent years, halide perovskite materials have been used to make high performance solar cell and light-emitting devices. However, material defects still limit device performance and stability. Here, we use synchrotron-based Bragg Coherent Diffraction Imaging to visualise nanoscale strain fields, such as those local to defects, in halide perovskite microcrystals. We find significant strain heterogeneity within MAPbBr3 (MA = CH3 NH3 + ) crystals in spite of their high optoelectronic quality, and identify both 〈100〉 and 〈110〉 edge dislocations through analysis of their local strain fields. By imaging these defects and strain fields in situ under continuous illumination, we uncover dramatic light-induced dislocation migration across hundreds of nanometers. Further, by selectively studying crystals that are damaged by the X-ray beam, we correlate large dislocation densities and increased nanoscale strains with material degradation and substantially altered optoelectronic properties assessed using photoluminescence microscopy measurements. Our results demonstrate the dynamic nature of extended defects and strain in halide perovskites, which will have important consequences for device performance and operational stability. This article is protected by copyright. All rights reserved