150 research outputs found
Design and synthesis of nanocrystal heterostructures for optoelectronic applications
No full textThesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2008.Vita.Includes bibliographical references.Colloidal semiconductor nanocrystals can be used for a variety of optoelectronic applications including light emitting devices (LEDs) and photovoltaics. Their narrow emission spectra make them excellent fluorophors for use in red, green and blue emitting organic LEDs and have been shown to achieve external quantum efficiencies as high as 2.7%, 1.8% and 0.4% respectively. Better synthetic methods have produced nanocrystal emitters with higher quantum yield, boosting efficiency, while a better understanding of QD-OLED function has led to improved organic transport materials. These QD-OLED devices can also be redesigned using inorganic hole and electron transport materials to produce inorganic QD-LEDs (QD-ILEDs) with EQE as high as 0.1%. Inorganic transport layers are more robust to solvents and oxygen, and are expected to greatly increase the device lifetime of QD-LEDs over devices employing organic materials. New QD deposition techniques using an inorganic hole transport layer include inkjet printing and Langmuir-Shaeffer dip-coating. Greater synthetic control of the II-VI nanocrystals has also yielded type-II CdSe/CdTe nanobarbells capable of internal exciton separation for photovoltaic applications. Although efficient solar cells using this material could not be produced, the material has given us several insights into the physics and future designs of bulk heterojunction photovoltaic devices. Finally, nanocrystal heterostructures formed using J-aggregate dyes electrostatically bound to QDs, have shown potential for use in LCD or lasing device applications.by Jonathan E. HalpertPh.D
Nanostructured Inorganic Metal Halide Perovskites for Optoelectronic Applications
No full textSemiconductor quantumdots have proven to be promising materials for optoelectronic devices, such as light emitting devices (LEDs) and solar cells, due to their thin linewidth of emission, high photoluminescence quantum yield and high absorption coefficient. Over the last decade, perovskite crystals have gained significant attention due to their extraordinary optoelectronic properties. Therefore, perovskite nanocrystals combine the advantage of both crystalline perovskite and quantum dots. Here, we synthesised high quantum yield (50 - 80 %) monodispersed CsPbX₃ (X= Cl, Br, I) quantum dots, with tuneable emission spectra over the entire visible region, by a colloidal synthesis method. We have then successfully processed them to produce thin films as the emitting layer in an organic LED-type device architecture. Most importantly, we demonstrated field induced halide separation in mixed halide CsPb(Br/I)₃ NCs which is the reason color instability in these LEDs.
Perovskite nanocrystal LEDs were found to have low external quantum efficiency (EQE) due to their bulky ligands. As a result, Ruddlesden-Popper (RP) phase layered perovskite was investigated to increase the EQE over perovskite QD LEDs. As a result, we constructed RP perovskite phase CsPbX₃ LEDs with emission through the entire visible spectrum (460-700 nm). Colour tuning was achieved by taking advantage of both quantum confinement effect and halide mixing. The EQE of these LEDs outperformed the literature values in the blue and blue-green spectral regions, with relatively long life time.
We also invented a novel perovskite nanocrystals made from thalliumlead halide by replacing caesium with thallium. These materials are potential candidates for various optoelectronic applications. Size-, shape-, and composition- tuning in these nanocrystals were performed by varying the reaction conditions andmixing the halide composition. A weak confinementwas observed in these NCs. Additionally, we have shown the application of TlPbI₃ nanowires as photoconductors.
Collectively, this thesis includes the synthesis of various types of inorganic metal halide perovskite nanostructures followed by their implementation into working optoelectronic devices, specifically LEDs
Field-Driven Ion Migration and Color Instability in Red-Emitting Mixed Halide Perovskite Nanocrystal Light-Emitting Diodes
No full textPerovskite nanocrystals have shown great promise as the basis of a new family of nanocrystal light-emitting diodes (LEDs). However, the external quantum efficiency and color stability of these materials still lag behind those of well-established technologies. Producing stable efficient red emitters with electroluminescence (EL) in the "pure" red range of 620-650 nm is a particular challenge. Here we present mixed halide CsPbBr3-xXx (X = I or Cl) peNC organic LEDs using peNC emitters with photoluminescence across the visible region to produce LEDs displaying EL across the visible spectrum. By focusing on the yellow-orange to deep red (560-680 nm) visible regime, we present evidence that field-driven halide separation in CsPbBr3-xIx peNCs is responsible for the observed red-shifting and splitting of the EL peaks. Greater compositional stability is demonstrated to be the key to higher efficiency, long-lived devices for deep red-emitting mixed halide peNCs with higher compositional concentrations of iodide.</p
Charge transport in mixed CdSe and CdTe colloidal nanocrystal films
No full textWe report the influence of trap states on charge transport through films of mixed CdTe and CdSe nanocrystals (NCs) between lateral electrodes, through layered films of CdTe and CdSe NCs in a layered geometry, and through films of CdTe/CdSe nanobarbells in a layered geometry. We find that an electron trapping state on the surface of the CdTe NCs dominates the conduction in all devices studied. X-ray photoelectron spectroscopy and thermal activation studies implicate unpassivated or oxidized Te as the electron-trapping site.National Science Foundation (U.S.) (MRSEC program (Grant No. DMR-0819762))National Science Foundation (U.S.) (Award No. PHY-0646094)MIT/Army Institute for Soldier Nanotechnologies (Contract No. W911NF-07-D-004)United States. Dept. of Energy (Grant No. DE-FG36-08G018007
Single crystals of mixed Br/Cl and Sn-doped formamidinium lead halide perovskites via inverse temperature crystallization
No full textHybrid organic–inorganic perovskite mixed halides of FAPbBr(3−x)Cl(x) and doped FAPb(1−x)Sn(x)Br(3) were synthesized using a generalized inverse temperature crystallization (ITC) method. With an appropriate choice of solvents and crystallization temperatures we show that large millimeter sized single crystals of these hybrid perovskites can be grown in a matter of hours to days using ITC. The structural and optical properties of these single crystals were characterized systematically. The mixed metal and mixed halide perovskites displayed a compositional bandgap tuneability in the region of 2.05 eV to 2.57 eV. The electrical properties of the perovskite single crystals were determined using a space-charge limited current (SCLC) method. The trap density determined from SCLC was between 10(9) and 10(11) cm(−3) for all perovskites which is exceptionally low. The mobility was found to increase by one order of magnitude on the addition of only 3% Sn for FAPb(1−x)Sn(x)Br(3) based perovskites which shows promise for enhancing the electrical properties. This demonstrates the generalizability of the ITC method to grow large high-quality perovskite single crystals with enhanced optical and electrical properties. In addition, it was observed for FAPbBr(3−x)Cl(x) based perovskites that initially degraded surfaces with suppressed PL emission could be repaired by using an anti-solvent treatment re-enabling the PL emission. Other perovskite compounds did not display any degraded surfaces and exhibited excellent stability in ambient conditions
Photo-Electrosensitive Memristor Using Oxygen Doping in HgTe Nanocrystal Films
No full textNanocrystal-based electronic devices with multiple functionalities offer one avenue toward novel passive and active electronic components. Here, we exhibit a planar and fully air-processed thin film device that demonstrates a photoinduced memristive behavior and can be used as a transistor, photodetector, or memory device. Following long-term (60 h) air exposure, unpackaged nanocrystal films develop reliable memristive characteristics in tandem with temperature, gate, and photoresponse. The on/off values of more than 50 are achieved, and the devices show long-term stability, producing repeatable metrics over days of measurement. The on/off behavior is shown to be dependent on the previous charge flow and carrier density, implying a memristive rather than switching behavior. These observations are described within a long-term trap-filling model. This work represents an advance in the integration of nanocrystal films into electronic devices, which may lead to the development of multifunctional electronic components.</p
300 nm Spectral Resolution in the Mid-Infrared with Robust, High Responsivity Flexible Colloidal Quantum Dot Devices at Room Temperature
No full textHgTe colloidal nanocrystals (NCs)
are used as the sensitizing layer
with a scalable, all air-processed patterning method to produce flexible
room temperature multicolor detectors operating in the mid-infrared
(MIR) spectral region. These devices demonstrate a “color”
sensitivity down to 300 nm in the MIR (∼10% of scale), with
superior responsivities for this class of device, up to 0.9 A/W, and
competitive specific detectivity up to 8 × 109 Jones
at 200 Hz and 300 K. Furthermore, these devices utilize a cheap and
robust substrate material that allows operation after deformation
up to 45° without degradation over many cycles. As such, this
offers a template for ultra low-cost MIR detectors with a performance
that rivals microbolometers, but with better measurement speed and
spectral sensitivity. As such, these devices showcase the advantages
of using colloidal NCs in MIR applications
300 nm Spectral Resolution in the Mid-Infrared with Robust, High Responsivity Flexible Colloidal Quantum Dot Devices at Room Temperature
No full textHgTe colloidal nanocrystals (NCs) are used as the sensitizing layer with a scalable, all air-processed patterning method to produce flexible room temperature multicolor detectors operating in the mid-infrared (MIR) spectral region. These devices demonstrate a "color" sensitivity down to 300 nm in the MIR (∼10% of scale), with superior responsivities for this class of device, up to 0.9 A/W, and competitive specific detectivity up to 8 × 109 Jones at 200 Hz and 300 K. Furthermore, these devices utilize a cheap and robust substrate material that allows operation after deformation up to 45° without degradation over many cycles. As such, this offers a template for ultra low-cost MIR detectors with a performance that rivals microbolometers, but with better measurement speed and spectral sensitivity. As such, these devices showcase the advantages of using colloidal NCs in MIR applications.</p
Room Temperature Mid-IR Detection through Localized Surface Vibrational States of SnTe Nanocrystals
No full textQuantum dots (QDs) are now well established as promising materials for room temperature mid-infrared (MIR) detection beyond 3 μm. Here, we have replaced commonly reported mercury based quantum dots with less toxic SnTe and PbSnTe. Inverse MIR detection at room temperature is demonstrated with planar, solution, and air-processed PbSnTe and SnTe QD devices. The detection mechanism is shown to be mediated by an interaction between MIR radiation and the vibrational stretches of adsorbed hydroxyl species. Devices are shown to possess mA/W responsivity via a reduction in conductance due to MIR irradiation and, unlike classic MIR photoconductors, are unaffected by visible wavelengths. As such, these devices offer the possibility of MIR thermal imaging that has an intrinsic solution to the blinding caused by higher energy light sources.</p
Recent advances in micro-/nano-structured hollow spheres for energy applications: From simple to complex systems
No full textHollow micro-/nano-structured materials are now playing an important role in cutting edge innovations for energy conversion and storage technologies such as solar cells, fuel cells, lithium ion batteries and super capacitors. These materials show great promise in addressing growing environmental concerns for cleaner power sources at a time of increasing global demand for energy. In this perspective, we show that complex multi-shelled micro-/nano-materials show significant material advantages in many applications over conventional simple hollow structures. We also summarize the vast array of synthetic strategies used to create multi-shelled hollow structures, and discuss the possible application of these novel materials for power generation and storage. Finally, the emergent challenges and future developments of multi-shelled hollow structures are further discussed.No Full Tex
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