Matthew W. Smith

Inorganic–Organic Hybrid Solar Cell: Bridging Quantum Dots to Conjugated Polymer Nanowires

Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.

FIRE: a near-infrared cross-dispersed echellette spectrometer for the Magellan telescopes

FIRE (the Folded-port InfraRed Echellette) is a prism cross-dispersed infrared spectrometer, designed to deliver singleobject R=6000 spectra over the 0.8-2.5 micron range, simultaneously. It will be installed at one of the auxiliary Nasmyth foci of the Magellan 6.5-meter telescopes. FIRE employs a network of ZnSe and Infrasil prisms, coupled with an R1 reflection grating, to image 21 diffraction orders onto a 2048 × 2048, HAWAII-2RG focal plane array. Optionally, a user-controlled turret may be rotated to replace the reflection grating with a mirror, resulting in a singleorder, longslit spectrum with R ~ 1000. A separate, cold infrared sensor will be used for object acquisition and guiding. Both detectors will be controlled by cryogenically mounted SIDECAR ASICs. The availability of low-noise detectors motivates our choice of spectral resolution, which was expressly optimized for Magellan by balancing the scientific demand for increased R with practical limits on exposure times (taking into account statistics on seeing conditions). This contribution describes that analysis, as well as FIREs optical and opto-mechanical design, and the design and implementation of cryogenic mechanisms. Finally, we will discuss our data-flow model, and outline strategies we are putting in place to facilitate data reduction and analysis.

Minimizing high spatial frequency residual error in active space telescope mirrors

The trend in future space telescopes is towards larger apertures, which provide increased sensitivity and improved angular resolution. Lightweight, segmented, rib-stiffened, actively controlled primary mirrors are an enabling technology, permitting large aperture telescopes to meet the mass and volume restrictions imposed by launch vehicles. Such mirrors, however, are limited in the extent to which their discrete surface-parallel electrostrictive actuators can command global prescription changes. Inevitably some amount of high spatial frequency residual error is added to the wavefront due to the discrete nature of the actuators. A parameterized finite element mirror model is used to simulate this phenomenon and determine designs that mitigate high spatial frequency residual errors in the mirror surface figure. Two predominant residual components are considered: dimpling induced by embedded actuators and print-through induced by facesheet polishing. A gradient descent algorithm is combined with the parameterized mirror model to allow rapid trade space navigation and optimization of the mirror design, yielding advanced design heuristics formulated in terms of minimum machinable rib thickness. These relationships produce mirrors that satisfy manufacturing constraints and minimize uncorrectable high spatial frequency error.

The REgolith X-Ray Imaging Spectrometer (REXIS) for OSIRIS-REx: Identifying Regional Elemental Enrichment on Asteroids

The OSIRIS-REx Mission was selected under the NASA New Frontiers program and is scheduled for launch in September of 2016 for a rendezvous with, and collection of a sample from the surface of asteroid Bennu in 2019. 101955 Bennu (previously 1999 RQ36) is an Apollo (near-Earth) asteroid originally discovered by the LINEAR project in 1999 which has since been classified as a potentially hazardous near-Earth object. The REgolith X-Ray Imaging Spectrometer (REXIS) was proposed jointly by MIT and Harvard and was subsequently accepted as a student led instrument for the determination of the elemental composition of the asteroids surface as well as the surface distribution of select elements through solar induced X-ray fluorescence. REXIS consists of a detector plane that contains 4 X-ray CCDs integrated into a wide field coded aperture telescope with a focal length of 20 em for the detection of regions with enhanced abundance in key elements at 50 m scales. Elemental surface distributions of approximately 50-200 m scales can be detected using the instrument as a simple collimator. An overview of the observation strategy of the REXIS instrument and expected performance are presented here.

Enhancing undergraduate education in aerospace engineering and planetary sciences at MIT through the development of a CubeSat mission

CubeSats are a class of nanosatellites that conform to a standardized 10 cm x 10 cm x 10 cm, 1 kg form factor. This miniaturization, along with a standardized deployment device for launch vehicles, allows CubeSats to be launched at low cost by sharing the trip to orbit with other spacecraft. Part of the original motivation for the CubeSat platform was also to allow university students to participate more easily in space technology development and to gain hands-on experience with flight hardware. The Department of Aeronautics and Astronautics along with the Department of Earth, Atmospheric, and Planetary Studies (EAPS) at the Massachusetts Institute of Technology (MIT) recently completed a three semester-long course that uses the development of a CubeSat-based science mission as its core teaching method. Serving as the capstone academic experience for undergraduates, the goal of this class is to design and build a CubeSat spacecraft that serves a relevant science function, such as the detection of exoplanets transiting nearby stars. This project-based approach gives students essential first hand insights into the challenges of balancing science requirements and engineering design. Students are organized into subsystem-specific teams that refine and negotiate requirements, explore the design trade space, perform modeling and simulation, manage interfaces, test subsystems, and finally integrate prototypes and flight hardware. In this work we outline the heritage of capstone design/build classes at MIT, describe the class format in greater detail, and give results on the ability to meet learning objectives using this pedagogical approach.

Minimizing actuator-induced errors in active space telescope mirrors

The trend in future space telescopes points toward increased primary mirror diameter, which improves resolution and sensitivity. However, given the constraints on mass and volume deliverable to orbit by current launch vehicles, creative design solutions are needed to enable increased mirror size while keeping mass and volume within acceptable limits. Lightweight, segmented, rib-stiffened, actively controlled primary mirrors have emerged as a potential solution. Embedded surface-parallel actuators can be used to change the mirror prescription onorbit, lowering mirror mass overall by enabling lighter substrate materials such as silicon carbide (SiC) and relaxing manufacturing constraints. However, the discrete nature of the actuators causes high spatial frequency residual errors when commanding low-order prescription changes. A parameterized finite element model is used to simulate actuator-induced residual error and investigate design solutions that mitigate this error source. Judicious specification of mirror substrate geometry and actuator length is shown to reduce actuator-induced residual while keeping areal density constant. Specifically, a sinusoidally-varying rib shaping function is found to increase actuator influence functions and decrease residual. Likewise, longer actuators are found to offer reduced residual. Other options for geometric shaping are discussed, such as rib-to-facesheet blending and the use of two dimensional patch actuators.