Browsing by Subject "thin film"

Abundant and widely available solar energy is one possible solution to the increasing demands for clean energy. The Thermodynamics and Sustainable Energy Laboratory (T-SEL) in Duke University has been dedicated to investigating methods to harness solar energy. Hybrid Solar System (HSS) is one of the promising methods to use solar energy, as it absorbs sunlight to produce hydrogen, which then can electrically power equipment through fuel cells. Hydrogen is produced through a biofuel reforming process, which occurs at a high temperature (over 700℃ for methane). Methods to design a high-temperature solar selective coating are investigated in this thesis.

The solar irradiance spectrum was assumed to be the same as Air Mass (AM) 1.5. A transfer-matrix method was adopted in this work to calculate the optical properties of the NREL #6, a design of nine-layer solar selective coating. Based on the design of NREL #6 coating, Differential Evolution (DE) algorithm was introduced to optimize this design. Two objective functions were considered: selectivity-oriented function and efficiency-oriented function, yielding the design of Revision #1 and Revision #2 respectively. The results showed a high selectivity (around 13) with low efficiency (66.6%) in Revision #1 and a high efficiency (82.6%) with moderate selectivity (around 9) in Revision #2.

Metal-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.

Advances in the development of miniaturized, autonomous general-purpose sensing systems for applications such as medical diagnostics, biological and chemical analysis, and point-of-care testing have driven the emergence of lab-on a-chip (LOC) systems, which integrate sample preparation and sensing. To realize LOC systems, enabling technologies are needed to carry out sample preparation and manipulation at the chip-scale, and sensing technologies that can be integrated with the chip-scale fluidic sample preparation platform.

The integration of sample preparation and sensing is key to LOC systems. Electrowetting-on-dielectric (EWD) fluidic technology enables digital droplet manipulation at the chip-scale with standard microfabrication manufacturing techniques, with the advantages of non-bulky systems, low cost and portability [1]. EWD system have been integrated with optical, mechanical and electrochemical sensing mechanisms to realize a miniaturized LOC [2]–[4].Fluorescence sensing is one of the most widely used types of analyte sensing for biochemical targets due to its high sensitivity and specificity [2]. Intensity-based fluorescence sensing provides fast, localized detection that can be correlated to the concentration of the test analyte, thus providing quantitative detection information. Most current microfluidic LOC systems that utilize fluorescent sensing use large external microscopes or bulky filters and lenses for extracting the quantitative information, thus compromising on the portability and cost-effectiveness. The realization of optical fluorescence sensing integrated directly into microfluidic-based LOC systems can enable the systems to become more self-contained, portable, and potentially low cost.

Fluorescence sensing is an effective method of identifying target species, and the integration of fluorescence sensing into a programmable digital microfluidic platform is the goal of the thesis work described herein. Silicon photodetectors (PDs) are an excellent choice for optical sensing due to their high responsivity at visible wavelengths, which corresponds to the emission wavelengths of many fluorophores. Additionally, using Si PDs enables the use of standard microfabrication processes that can be scaled for mass manufacturing. Furthermore, using thin film Si photodetectors provides the ability to be bonded to a chosen substrate for further system integration in small spaces where conventional photodetectors will not fit, without significantly altering the spatial configuration of the region where the sensing occurs.

This dissertation presents the design and fabrication of annular, thin film Si photodetectors heterogeneously bonded to a pyrex substrate for fluorescence sensing, with a longer-term goal of integration into an EWD to realize a LOC system. Herein we design, fabricate, and test the performance of PD fluorescence sensors with thin film Si PD testing, followed by PD integration into an EWD microfluidic system. This system is simple and low cost, because it does not utilize filters to block the optical pump beam selectively from the PD, but rather, uses a novel optical design to suppress the fluorescence pump signal with the bottom plate of the microfluidic system, so that the signal to noise ratio (SNR) of the fluorescence signal to the pump signal is high. This device has the potential to be applied as a miniature, non-invasive, multi-target sensor.

[Figure 1: Cross-section schematic of the integrated thin film Si fluorescence sensor with a bottom plate designed for integration with an EWD system. PD thickness not drawn to scale.]

The target EWD system for integration with the fluorescence sensor has a top plate and a bottom plate, with the thin film Si PD integrated onto the top plate. The PD sensor is a thin film annular silicon PD bonded to the pyrex top plate, as shown in Figure 1, which can also serve as a ground electrode for the microfluidic platform. The optical pump signal, delivered herein through an optical fiber, is coupled to the droplet under test (containing the fluorophore) through the pyrex, through a thin metallization on the pyrex to minimize stray light, and then through the aperture in the photodetector. Light delivery can also be carried out using an integrated optical (LED or laser) source with or without an optical waveguide. The droplet is sandwiched between the top plate and the bottom plate. Fluorophore concentrations ranging from 0.3 μM to 20 μM were tested with different bottom plate substrates and herein, we discuss high SNR detection for droplet sizes as low as 10 μL over a time period of microseconds. Next, the integration of the fluorescence sensor with a digital microfluidic system is discussed. Finally, the performance of the sensor is evaluated, followed by conclusions and thoughts for future work.

The 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.