Browsing by Author "Stiff-Roberts, Adrienne D"
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Item Open Access Design Rules for Complex Emulsion Targets in Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation of Polymer Thin Films(2021) Ferguson, SpencerPolymer thin films used in many modern technologies, such as light emitting diodes, solar cells, flexible electronics, and sensors, are fabricated in numerous ways. Of these methods, solution-based processing techniques are the most common with established commercial manufacturing and high throughput. However, these approaches face multiple disadvantages when applied to device heterostructures that are essential for optoelectronic devices. In contrast, vacuum-based techniques are amenable to multi-layer films with varying composition, regardless of polymer solubility. Resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) allows for the deposition of finely tuned polymer structures, such as uniquely graded heterostructures, which solution processing techniques cannot achieve. The primary contribution of this dissertation is to bring to light general design rules for complex emulsions that comprise the targets used in RIR-MAPLE to deposit polymer thin films containing crystalline domains.
Initially, design rules are discussed that lead to the formation of pinhole-free polymer films that also contain crystalline domains. This work addresses a fundamental challenge to promote the crystalline phase, as opposed to the more prevalent amorphous phase, while also maintaining high-quality film surfaces. These design rules are then expanded to include the effects of overall emulsion composition on the resulting emulsion morphology. In order to maximize the content of the crystalline polymer phase in thin films, the use of various surfactants (not yet applied to RIR-MAPLE emulsion targets) is investigated due to the unique interactions that occur, thereby determining the emulsion morphology. As a result, the impact of surfactant molecular structure on the resultant film properties is described for the first time. Finally, the design rules identified for complex emulsions are used in a fundamentally different emulsion chemistry to verify the extent to which these rules are generally applicable.
Item Open Access Dopant Incorporation in InAs/GaAs Quantum Dot Infrared Photodetectors(2009) Zhao, ZhiyaQuantum Dot Infrared Photodetectors (QDIPs) are important alternatives to conventional infrared photodetectors with high potential to provide required detector performance, such as higher temperature operation and multispectral response, due to the 3-D quantum confinement of electrons, discrete energy levels, and intrinsic response to perpendicular incident light due to selection rules. However, excessive dark current density, which causes QDIPs to underperform theoretical predictions, is a limiting factor for the advancement of QDIP technologies. The purpose of this dissertation research is to achieve a better understanding of dopant incorporation into the active region of QDIPs, which is directly related to dark current control and spectral response. From this dissertation research, doping related dipole fields are found to be responsible for excessive dark current in QDIPs.
InAs/GaAs QDIPs were grown using solid source molecular beam epitaxy (MBE) with different doping conditions. The QDIPs were optically characterized using photoluminescence and Fourier transform infrared (FT-IR) spectroscopy. Devices were fabricated using standard cleanroom fabrication procedures. Dark current and capacitance measurements were performed under different temperature to reveal electronic properties of the materials and devices. A novel scanning capacitance microscopy (SCM) technique was used to study the band structure and carrier concentration on the cross section of a quantum dot (QD) heterostructure. In addition, dark current modeling and bandstructure calculations were performed to verify and better understand experimental results.
Two widely used QDIP doping methods with different doping concentrations have been studied in this dissertation research, namely direct doping in InAs QD layer, and modulation doping in the GaAs barrier above InAs QD layer. In the SCM experiment, electron redistribution has been observed due to band-bending in the modulation-doping region, while there is no band-bending observed in directly doped samples. A good agreement between the calculated bandstructure and experimental results leads to better understanding of doping in QD structures. The charge filling process in QDs has been observed by an innovative polarization-dependent FT-IR spectroscopy. The red-shift of QD absorbance peaks with increasing electron occupation supports a miniband electronic configuration for high-density QD ensembles. In addition, the FT-IR measurement indicates the existence of donor-complex (DX) defect centers in Si-doped QDIPs. The existence of DX centers and related dipole fields have been confirmed by dark current measurements to extract activation energies and by photocapacitance quenching measurements.
With the understanding achieved from experimental results, a further improved dark current model has been developed based on the previous model originally established by Ryzhii and improved by Stiff-Roberts. In the model described in this dissertation, two new factors have been considered. The inclusion of background drift current originating from Si shallow donors in the low bias region results in excellent agreement between calculated and measured dark currents at different temperatures, which has not been achieved by previous models. A very significant effect has been observed in that dark current leakage occurs due to the dipole field caused by doping induced charge distribution and impact-ionized DX centers.
Last but not least, QDIPs featuring the dipole interface doping (DID) method have been designed to reduce the dark current density without changing the activation energy (thus detection wavelength) of QDIPs. The DID samples involve an InAs QD layer directly-doped by Si, as well as Be doping in the GaAs barrier on both sides of the QD layer. The experimental result shows the dark current density has been significantly reduced by 104 times without any significant change to the corresponding activation energy. However, the high p-type doping in the GaAs barrier poses a challenge in that the Fermi level is reduced to be well below the QD energy states. High p-type doping is reported to reduce the dark current, photocurrent and the responsivity of the devices.
To conclude, it is significant to identify to effect of Si-induced defect centers on QDIP dark currents. The subsequent study reveals doping induced dipole fields can have significant effects on QDIP device performance, for example, causing charge leakage from QDs and reducing activation energy, thereby increasing dark current density. The DID approach developed in this work is a promising approach that could help address these issues by using controlled dipole fields to reduce dark current density without changing the minimum detectable energy of QDIPs.
Item Open Access Emulsion-Based RIR-MAPLE Deposition of Conjugated Polymers: Primary Solvent Effect and Its Implications on Organic Solar Cell Performance.(ACS Appl Mater Interfaces, 2016-08-03) Ge, Wangyao; Li, Nan K; McCormick, Ryan D; Lichtenberg, Eli; Yingling, Yaroslava G; Stiff-Roberts, Adrienne DEmulsion-based, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) has been demonstrated as an alternative technique to deposit conjugated polymer films for photovoltaic applications; yet, a fundamental understanding of how the emulsion target characteristics translate into film properties and solar cell performance is unclear. Such understanding is crucial to enable the rational improvement of organic solar cell (OSC) efficiency and to realize the expected advantages of emulsion-based RIR-MAPLE for OSC fabrication. In this paper, the effect of the primary solvent used in the emulsion target is studied, both experimentally and theoretically, and it is found to determine the conjugated polymer cluster size in the emulsion as well as surface roughness and internal morphology of resulting polymer films. By using a primary solvent with low solubility-in-water and low vapor pressure, the surface roughness of deposited P3HT and PCPDTBT polymer films was reduced to 10 nm, and the efficiency of P3HT:PC61BM OSCs was increased to 3.2% (∼100 times higher compared to the first MAPLE OSC demonstration [ Caricato , A. P. ; Appl. Phys. Lett. 2012 , 100 , 073306 ]). This work unveils the mechanism of polymer film formation using emulsion-based RIR-MAPLE and provides insight and direction to determine the best ways to take advantage of the emulsion target approach to control film properties for different applications.Item Open Access Foundational Studies of the Deposition of Metal-Halide Perovskite Thin Films by Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation(2020) Barraza, Enrique TomasMetal-halide perovskites (MHP) comprise a diverse family of crystalline materials whose optoelectronic properties have gathered significant interest recently. Their use in transistors, solar cells, light emitting diodes, and many other applications with significant real-world impacts has been enabled by synthesis techniques that can deposit high quality MHP thin films. The simple yet powerful chemistry involved in solution-processing techniques has allowed for MHP thin films to be deposited in a variety of different ways like spin-coating, inkjet printing, and doctor blading. However, solvent in these techniques can preclude the creation of advanced MHP structures like graded composition films or all-MHP heterojunctions. Additionally, the poor solubility of complex organic moieties in the polar solvents used in solution-processing of MHP materials could prevent the creation of MHP materials with unique photophysical properties. The development of vapor-processing techniques which circumvent the use of solvent to vaporize MHP precursors and deposit thin films has shown promise in addressing these concerns with solution-processing. However, the use of highly energetic precursor vaporization mechanisms has itself raised worries about its broad applicability.
In this dissertation, the deposition of MHP thin films using Resonant Infrared, Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE) is developed as an alternative to current MHP thin film deposition techniques. The ‘dry’ deposition of materials under dynamic vacuum and the use of a low energy infrared laser directly address shortcomings of solution- and vapor-processing techniques, respectively. Using the understanding of RIR-MAPLE developed by previous studies, a double solvent approach was first developed to solubilize and deposit MHP precursors in a manner which maintained the integrity of the resulting films. The viability of this baseline approach was confirmed through the creation of MHP solar cells with competitive performance and thin films of MHP materials with complex organic moieties that demonstrate unique photophysical properties. Subsequent studies of nuanced aspects in RIR-MAPLE deposition of MHP thin films helped develop an understanding of the process-structure-property relationships in play during RIR-MAPLE deposition and in post-processing of the resulting MHP thin films.
Following these baseline studies, unique precursor delivery schemes were developed to demonstrate the versatility of RIR-MAPLE. These schemes were shown to reliably deposit continuous films of MHP materials despite differences in the state of precursors during deposition and crystallization. Finally, a comprehensive study of MHP film formation mechanisms during RIR-MAPLE deposition was undertaken. These experiments categorically described the wetting, nucleation, diffusion, and accumulation essential to MHP film development during RIR-MAPLE deposition. Overall, this work demonstrates some of the most promising aspects of the RIR-MAPLE deposition technique and develops the candidacy of RIR-MAPLE as an MHP thin film technique uniquely positioned to address the shortcomings of other currently established methods.
Item Open Access Investigation of 2D Hybrid Organic-Inorganic Perovskite Thin Films Deposited by RIR-MAPLE for Heterostructure Integration(2023) Phillips, Niara ElyssaThe power conversion efficiency of perovskite solar cells has increased significantly in the past 10 years from around 13% to over 25%. However, the most common perovskite absorber materials, three-dimensional (3D) perovskites, are challenged by moisture stability which hinders their more widespread commercialization. One approach to increase moisture stability is to incorporate a layer of hydrophobic ligands on top of the absorber layer in the form of a two-dimensional (2D) perovskites, thereby forming a 2D-on-3D heterostructure, however there are significant processing challenges. This dissertation conducts a careful investigation of n = 1 2D perovskite thin films deposited using a technique called resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE) to better understand and then improve the heterostructure. The major conclusions of this work are that any halide mixing likely occurs in the 3D layer only at the heterostructure interface due to the site preferences that bromine and iodine have in the octahedra. Several supplemental processing parameters – deposition scheme, growth temperature, and solvent ratio (DMSO:MEG) – were used to successfully increase the average grain size, increase the amount of vertically oriented grains, modify the morphology, and decrease the Stokes shift in (PEA)2PbI4 thin films. Ultimately, out-of-plane conductivity in (PEA)2PbI4 thin films was successfully improved using the sequential deposition scheme, elevated growth temperature, and decreased amount of matrix solvent. The structural improvements and improved out-of-plane conductivity were also demonstrated for the heterostructure when modified processing conditions were used to deposit the 2D layer. The process-structure-property relationships investigated in this work serve as guidelines for tailoring 2D-on-3D heterostructures.
Item Open Access Matrix-Assisted Pulsed Laser Evaporation of Conjugated Polymer and Hybrid Nanocomposite Thin Films: A Novel Deposition Technique for Organic Optoelectronic Devices(2011) Pate, Ryan JaredThis dissertation develops a novel application of the resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) technique toward the end goal of conjugated-polymer-based optoelectronic device fabrication. Conjugated polymers are attractive materials that are being investigated in the development of efficient optoelectronic devices due to their inexpensive material costs. Moreover, they can easily be combined with inorganic nanomaterials, such as colloidal quantum dots (CQDs), so as to realize hybrid nanocomposite-based optoelectronic devices with tunable optoelectronic characteristics and enhanced desirable features. One of the most significant challenges to the realization of optimal conjugated polymer-CQD hybrid nanocomposite-based optoelectronics has been the processes by which these materials are deposited as thin films, that is, conjugated polymer thin film processing techniques lack sufficient control so as to maintain preferred optoelectronic device behavior. More specifically, conjugated-polymer-based optoelectronics device operation and efficiency are a function of several attributes, including surface film morphology, internal polymer chain morphology, and the distribution and type of nanomaterials in the film bulk. Typical conjugated-polymer thin-film fabrication methodologies involve solution-based deposition, and the presence of the solvent has a deleterious impact, resulting in films with poor charge transport properties and subsequently poor device efficiencies. In addition, many next-generation conjugated polymer-based optoelectronics will require multi-layer device architectures, which can be difficult to achieve using traditional solution processing techniques. These issues direct the need for the development of a new polymer thin film processing technique that is less susceptible to solvent-related polymer chain morphology problems and is more capable of achieving better controlled nanocomposite thin films and multi-layer heterostructures comprising a wide range of materials. Therefore, this dissertation describes the development of a new variety of RIR-MAPLE that uses a unique target emulsion technique to address the aforementioned challenges.
The emulsion-based RIR-MAPLE technique was first developed for the controlled deposition of the conjugated polymers poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and poly[2-methoxy-5-(2'ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene] (MEH-CN-PPV) into homogenous thin films. Therein, it was identified that target composition had the most significant influence on film surface morphology, and by tuning the concentration of hydroxyl bonds in the target bulk, the laser-target absorption depth could be tuned so as to yield more or less evaporative deposition, resulting in films with tunable surface morphologies and optical behaviors.
Next, the internal morphologies of emulsion-based RIR-MAPLE-deposited MEH-PPV thin films were investigated by measuring their hole drift mobilities using the time-of-flight (TOF) photoconductivity method in the context of amorphous materials disorder models (Bässler's Gaussian Disorder model and the Correlated Disorder model) in order to provide a quantitative measure of polymer chain packing. The polymer chain packing of the RIR-MAPLE-deposited films was demonstrated to be superior and more conducive to charge transport in comparison to spin-cast and drop-cast MEH-PPV films, yielding enhanced hole mobilities.
The emulsion-based RIR-MAPLE technique was also developed for the deposition of different classes of inorganic nanoparticles, namely un-encapsulated nanoparticles and ligand-encapsulated nanoparticles. These different classes of nanoparticles were identified to have different film growth regimes, such that either rough or smooth films were obtained, respectively. The ligand-encapsulated nanoparticles were then co-deposited with MEH-PPV as conjugated polymer-CQD hybrid nanocomposites, wherein the distributions of the constituent materials in the film bulk were identified to be tunable, from homogeneous to highly clustered. The RIR-MAPLE deposition regime determined the said distributions, that is, if the polymer and CQDs were sequentially deposited from a sectioned target or simultaneously deposited from a single target, respectively. The homogeneous conjugated polymer-CQD nanocomposites were also investigated in terms of their charge transport properties using the TOF photoconductivity technique, where it was identified that despite the enhanced dispersion of CQDs in the film bulk, the presence of a high concentration of CQDs degraded hole drift mobility, which indicates that special considerations must be taken when incorporating CQDs into conjugated-polymer-based nanocomposite optoelectronics.
Finally, the unique capability of RIR-MAPLE to enable novel conjugated polymer-based optical heterostructures and optoelectronic devices was evaluated by the successful demonstration of a conjugated polymer-based distributed Bragg reflector (DBR), a plasmonic absorption enhancement layer, and a conjugated polymer-based photovoltaic solar cell featuring a novel electron-transporting layer. These optical heterostructures and optoelectronic devices demonstrate that all of the constituent polymer and nanocomposite layers have controllable thicknesses and abrupt interfaces, thereby confirming the capability of RIR-MAPLE to achieve multi-layer, conjugated polymer-based heterostructures and device architectures that are appropriate for enhancing specific desired optical behaviors and optoelectronic device efficiencies.
Item Open Access Organic/Inorganic Hybrid Nanocomposite Infrared Photodetection by Intraband Absorption(2011) Lantz, Kevin RichardThe ability to detect infrared radiation is vital for a host of applications that include optical communication, medical diagnosis, thermal imaging, atmospheric monitoring, and space science. The need to actively cool infrared photon detectors increases their operation cost and weight, and the focus of much recent research has been to limit the dark current and create room-temperature infrared photodetectors appropriate for mid-to-long-wave infrared detection. Quantum dot infrared photodetectors (QDIPs) provide electron quantum confinement in three dimensions and have been shown to demonstrate high temperature operation (T>150 K) due to lower dark currents. However, these inorganic devices have not achieved sensitivity comparable to state-of-the-art photon detectors, due in large part to the inability to control the uniformity (size and shape) of QDs during strained-layer epitaxy.
The purpose of this dissertation research was to investigate the feasibility of room-temperature infrared photodetection that could overcome the shortfalls of QDIPs by using chemically synthesized inorganic colloidal quantum dots (CQDs). CQDs are coated with organic molecules known as surface ligands that prevent the agglomeration of dots while in solution. When CQDs are suspended in a semiconducting organic polymer, these materials are known as organic/inorganic hybrid nanocomposites. The novel approach investigated in this work was to use intraband transitions in the conduction band of the polymer-embedded CQD for room-temperature photodetection in the mid-wave, and possibly long-wave, infrared ranges. Hybrid nanocomposite materials promise room-temperature operation due to: (i) large bandgaps of the inorganic CQDs and the semiconducting polymer that reduce thermionic emission; and (ii) low dark current due to the three-dimensional electron confinement in the CQD and low carrier mobility in the semiconducting polymer. The primary material system investigated in this research was CdSe CQDs embedded in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-(1-cyanovinylene)phenylene] (MEH-CN-PPV).
Photoluminescence (PL) spectroscopy of MEH-CN-PPV thin films was conducted to determine the dependence of polymer morphology on deposition method in order to identify a reliable device fabrication technique. Three different deposition methods were investigated: drop-casting and spin-casting, which are solution-based; and matrix-assisted pulsed laser evaporation (MAPLE), which is a vacuum-based method that gently evaporates polymers (or hybrid nanocomposites) and limits substrate exposure to solvents. It was found that MAPLE deposition provides repeatable control of the thin film morphology and thickness, which is important for nanocomposite device optimization.
Ultra-fast PL spectroscopy of MEH-CN-PPV/CdSe thin films was investigated to determine the charge generation and relaxation dynamics in the hybrid nanocomposite thin films. The mathematical fitting of time-integrated and time-resolved PL provided a rigorous and unique model of the charge dynamics, which enabled calculation of the radiative and non-radiative decay lifetimes in the polymer and CQD. These results imply that long-lived electrons exist in the conduction band of the CQD, which demonstrate that it should be possible to generate a mid- to long-wave infrared photocurrent based on intraband transitions. In fact, room-temperature, intraband, mid-infrared absorption was measured in thin films of MEH-CN-PPV/CdSe hybrid nanocomposites by Fourier transform infrared (FTIR) absorbance spectroscopy. In addition, the hybrid nanocomposite confined energy levels and corresponding oscillator strengths were calculated in order to model the absorption spectrum. The calculated absorption peaks agree well with the measured peaks, demonstrating that the developed computer model provides a useful design tool for determining the impact of important materials system properties, such as CQD size, organic surface ligand material choice, and conduction band offset due to differences in CQD and polymer electron affinities.
Finally, a room-temperature, two terminal, hybrid nanocomposite mid-infrared photoconductor based on intraband transitions was demonstrated by FTIR spectral response measurements, measuring a spectral responsivity peak of 4.32 µA/W at 5.5µm (5 volts), and calibrated blackbody spectral photocurrent measurements, measuring a spectral responsivity peak of 4.79 µA/W at 5.7 µm (22 volts). This device characterization demonstrated that while the novel approach of intraband infrared photodetection in hybrid nanocomposites is feasible, significant challenges exist related to device fabrication and operation. Future work is proposed that could address some of these important issues.
Item Open Access Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation: Advancing Methodology and Elucidating Mechanisms for Precise Control of Film Morphology and Composition(2023) Zhang, BuangThe aim of this dissertation is to advance the understanding and methodology of resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), with a focus on the surfactant chemistry in organic emulsions, the fine modulation of film composition, and the development of novel deposition strategies combining both emulsion-based and solution-based target chemistries. RIR-MAPLE is a special thin-film deposition technique that extends the capabilities of the traditional pulsed laser deposition process to deposit a wide range of materials, including sensitive organic polymers (emulsion-based targets) and emerging hybrid perovskite materials (solution-based targets). It utilizes a pulsed laser, typically in the infrared range, which resonates with the vibrational modes of the matrix solvent molecules in a frozen target that comprises the material of interest dissolved in a primary solvent. The energy absorbed by the matrix solvent results in its sublimation, and kinetic energy is transferred to the solute (the material to be deposited), leading to solute ejection from the target without damaging sensitive organic materials. Although extensive research has previously been conducted to decipher the complexities of the RIR-MAPLE deposition process, several issues remain unresolved, and the full potential of this innovative technique has yet to be harnessed. This study addresses and investigates pivotal challenges inherent to RIR-MAPLE, paving the way for its application in a broader spectrum of future technological advancements.First, the deposition of organic material and its corresponding emulsion chemistry, specifically surfactant usage, was studied to determine predictive correlations with emulsified particle morphology and thin film properties. Consequently, the impact of surfactant choice on micelle density and the non-bonded interaction amongst disparate components was first documented and substantiated by the demonstration of blue organic light-emitting diodes (OLEDs). Second, an investigation was conducted to understand mechanisms for achieving fine control of composition when two organic materials are blended in a single film. An OLED exhibiting broadband emission due to two different polymer constituents was fabricated to demonstrate the precise control achievable over blended compositions. Consequently, it was discerned that the two organic materials remained contained within their respective emulsified particles and sequentially deposited atop one another without influencing the preceding deposited material. Third, the versatility of RIR-MAPLE was investigated by depositing films using solution-based and emulsion-based targets demonstrated by hybrid organic-inorganic perovskite (HOIP)-organic macromolecule nanocomposite films. A general approach for depositing such mixed nanocomposites was developed for RIR-MAPLE for the first time, subsequently unveiling the interactions between the organic macromolecule and the hybrid perovskite and achieving a fundamentally different compositional range and film morphology compared to other nanocomposite approaches.