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• 1. [Article] Fundamental Properties of Functional Zinc Oxide Nanowires Obtained by Electrochemical Method and Their Device Applications

  We report on the fundamental properties and device applications of semiconductor nanoparticles. ZnO nanowires and CdSe quantum dots were used, prepared, characterized, and assembled into novel light-emitting ...

  Citation × Citation Citation Abstract Copy Title: Fundamental Properties of Functional Zinc Oxide Nanowires Obtained by Electrochemical Method and Their Device Applications Author: Nadarajah, Athavan Year: 2012 We report on the fundamental properties and device applications of semiconductor nanoparticles. ZnO nanowires and CdSe quantum dots were used, prepared, characterized, and assembled into novel light-emitting diodes and solar cells. ZnO nanowire films were grown electrochemically using aqueous soluble chloride-based electrolytes as precursors at temperatures below 90° C. Dopants were added to the electrolyte in the form of chloride compounds, which are AlCl3, CoCl2, CuCl2, and MnCl2. The optical, magnetic, and structural properties of undoped and transition-metal-ion doped ZnO nanowires were explored. Our results indicate that the as-grown nanowire structures have considerable internal strain, resulting in clearly visible lattice distortions in bright and dark-field transmission electron micrographs. Photo and electroluminescence studies indicate that the strain-induced defects strongly dominate any dopant-related effects. However, annealing at moderate temperature as well as laser annealing induces strain relaxation and leads to dopant activation. Hence, the optical and electrical properties of the nanowires significantly improve, allowing these nanowires to become feasible for use in the fabrication of solar cell and LED devices. In addition, the magnetic impurities incorporated into our ZnO nanowires show superparamagnetic behavior at room-temperature, while Al-doped and undoped ZnO nanowires show no magnetic behavior. The electroluminescence (EL) is achieved from a vertical hybrid p-n junction LED arrangement consisting of a hole-conducting polymer and n-type ZnO nanowires, our group was the first to report this vertical nanowire-based LED in Könenkamp et al., 2004 [12]. The observed EL spectra show an ultraviolet excitonic emission peak and a broad defect-related emission band in the visible range. After annealing at 380° C, the defect related EL peak exhibits a characteristic shift to higher wavelengths, where the magnitude of the shift is dependent on the dopant type. Aluminum incorporation exhibited the most improved exciton related-emission, leading to the emergence of a narrow excitonic luminescence peak around 390 nm, which is close to the bandgap of ZnO. The comparison of spectra obtained from temperature-dependent photoluminescence (PL) measurements, before and after thermal annealing, also indicates that the optical activity of impurities changes noticeably upon annealing. The internal quantum efficiency for PL is measured to be as high as 16 percent for Al-doped samples annealed at 380° C. The PL measurements also show that the excitonic luminescence is preferentially guided, while the defect related emission is more isotropically emitted. The nanostructured heterojunction solar cell is designed such that thin CdSe quantum dot films are embedded between a ZnO nanowire film and a hole-conducting polymer layer. This arrangement allows for enhanced light absorption and an efficient collection of photogenerated carriers. Here, we present a detailed analysis of the pyridine solution and 1,2- ethanedithiol ligand exchange processes of the quantum dots, deposition processes of this quantum dot layer, the conformality of this layer on deeply nanostructured samples, and the effect of a surfactant-aided thermal annealing process. Annealing creates a structural conversion of the quantum dot layers into an extremely thin continuous poly-crystalline film, with typical grain diameters of 30-50 nm. This transition is accompanied by a loss of quantum confinement and a significant improvement of the charge transport in the CdSe layer. The combination of the solution and ligand exchange of CdSe quantum dots, as well as the deposition and optimized annealing processes of this quantum dot layer, resulted in solar cells with an open-circuit voltage up to 0.6 V, a short circuit current of ~15 mA/cm2, an external quantum efficiency of 70 percent, and an energy conversion efficiency of 3.4 percent. This 3.4 percent efficiency is presently one of the best efficiencies obtained for this type of device. Title: Fundamental Properties of Functional Zinc Oxide Nanowires Obtained by Electrochemical Method and Their Device Applications Author: Nadarajah, Athavan Year: 2012 We report on the fundamental properties and device applications of semiconductor nanoparticles. ZnO nanowires and CdSe quantum dots were used, prepared, characterized, and assembled into novel light-emitting diodes and solar cells. ZnO nanowire films were grown electrochemically using aqueous soluble chloride-based electrolytes as precursors at temperatures below 90° C. Dopants were added to the electrolyte in the form of chloride compounds, which are AlCl3, CoCl2, CuCl2, and MnCl2. The optical, magnetic, and structural properties of undoped and transition-metal-ion doped ZnO nanowires were explored. Our results indicate that the as-grown nanowire structures have considerable internal strain, resulting in clearly visible lattice distortions in bright and dark-field transmission electron micrographs. Photo and electroluminescence studies indicate that the strain-induced defects strongly dominate any dopant-related effects. However, annealing at moderate temperature as well as laser annealing induces strain relaxation and leads to dopant activation. Hence, the optical and electrical properties of the nanowires significantly improve, allowing these nanowires to become feasible for use in the fabrication of solar cell and LED devices. In addition, the magnetic impurities incorporated into our ZnO nanowires show superparamagnetic behavior at room-temperature, while Al-doped and undoped ZnO nanowires show no magnetic behavior. The electroluminescence (EL) is achieved from a vertical hybrid p-n junction LED arrangement consisting of a hole-conducting polymer and n-type ZnO nanowires, our group was the first to report this vertical nanowire-based LED in Könenkamp et al., 2004 [12]. The observed EL spectra show an ultraviolet excitonic emission peak and a broad defect-related emission band in the visible range. After annealing at 380° C, the defect related EL peak exhibits a characteristic shift to higher wavelengths, where the magnitude of the shift is dependent on the dopant type. Aluminum incorporation exhibited the most improved exciton related-emission, leading to the emergence of a narrow excitonic luminescence peak around 390 nm, which is close to the bandgap of ZnO. The comparison of spectra obtained from temperature-dependent photoluminescence (PL) measurements, before and after thermal annealing, also indicates that the optical activity of impurities changes noticeably upon annealing. The internal quantum efficiency for PL is measured to be as high as 16 percent for Al-doped samples annealed at 380° C. The PL measurements also show that the excitonic luminescence is preferentially guided, while the defect related emission is more isotropically emitted. The nanostructured heterojunction solar cell is designed such that thin CdSe quantum dot films are embedded between a ZnO nanowire film and a hole-conducting polymer layer. This arrangement allows for enhanced light absorption and an efficient collection of photogenerated carriers. Here, we present a detailed analysis of the pyridine solution and 1,2- ethanedithiol ligand exchange processes of the quantum dots, deposition processes of this quantum dot layer, the conformality of this layer on deeply nanostructured samples, and the effect of a surfactant-aided thermal annealing process. Annealing creates a structural conversion of the quantum dot layers into an extremely thin continuous poly-crystalline film, with typical grain diameters of 30-50 nm. This transition is accompanied by a loss of quantum confinement and a significant improvement of the charge transport in the CdSe layer. The combination of the solution and ligand exchange of CdSe quantum dots, as well as the deposition and optimized annealing processes of this quantum dot layer, resulted in solar cells with an open-circuit voltage up to 0.6 V, a short circuit current of ~15 mA/cm2, an external quantum efficiency of 70 percent, and an energy conversion efficiency of 3.4 percent. This 3.4 percent efficiency is presently one of the best efficiencies obtained for this type of device. Close

• 2. [Article] Functional materials for advanced energy and electronic devices

  Advances in energy technologies and electronics have typically occurred through either heightened performance or cost reduction. This dissertation explores both routes through a series of fundamental material ...

  Citation × Citation Citation Abstract Copy Title: Functional materials for advanced energy and electronic devices Author: Flynn, Brendan T. Advances in energy technologies and electronics have typically occurred through either heightened performance or cost reduction. This dissertation explores both routes through a series of fundamental material studies that may contribute to the enabling of next generation devices. Solution based syntheses and deposition of chemical products offer a low cost alternative to conventional vacuum based methods, and is examined through the wet synthesis of CuZnSnS₄ light absorbing nanoparticles. The nanoparticles were synthesized by both microwave-assisted and continuous flow techniques, and at relatively modest temperatures with inexpensive solvents. Varying the initial concentration of precursors allowed a facile method to modify the nanoparticle composition. For the continuous flow system, consideration of fluid flow dynamics provided a means to control the generation of gas within the reactor to allow for improved mixing. Processing of as-deposited solution based films to form more functional variants was studied in further detail by way of the Hf(OH)[subscript 4−2x−2y](O₂)[subscript x](SO₄)[subscript y]·qH₂O (HafSOx) material system. Surface analytical techniques revealed that control of the catalytically active acid sites may be possible with optimization of water exposure and post-exposure annealing temperature of dehydrated films. Thermal and electron stimulated desorption of peroxide containing HafSOx films revealed complex surface dynamics. The total desorption cross sections of O₂ for 500-2000 eV incident electrons was ~10⁻¹⁶ cm². Interfacial formation and chemistry was investigated by evaporating platinum onto amorphous indium gallium zinc oxide, an increasingly common interface type found in memristors and other novel electronics. Metallic indium formed upon platinum deposition, and the interface had an unexpectedly low Schottky barrier height of 0.25 eV, likely as a result of the reactive interface. Finally, the surface properties of highly oriented LaFeO₃ films were characterized and modified through reductive annealing. Iron was found to exist in three oxidation states depending on the annealing temperature and location within the film. In addition, the concentration and types of oxygen species were also found to vary based on proximity to the surface and the thermal treatment. Together, the aforementioned studies are related by the potential manufacturing steps of inexpensive, innovative energy and electronic devices. These steps include solution synthesis and deposition, processing to form a functional film, interface formation, and adapting the film's surface properties to suit its intended application. Title: Functional materials for advanced energy and electronic devices Author: Flynn, Brendan T. Advances in energy technologies and electronics have typically occurred through either heightened performance or cost reduction. This dissertation explores both routes through a series of fundamental material studies that may contribute to the enabling of next generation devices. Solution based syntheses and deposition of chemical products offer a low cost alternative to conventional vacuum based methods, and is examined through the wet synthesis of CuZnSnS₄ light absorbing nanoparticles. The nanoparticles were synthesized by both microwave-assisted and continuous flow techniques, and at relatively modest temperatures with inexpensive solvents. Varying the initial concentration of precursors allowed a facile method to modify the nanoparticle composition. For the continuous flow system, consideration of fluid flow dynamics provided a means to control the generation of gas within the reactor to allow for improved mixing. Processing of as-deposited solution based films to form more functional variants was studied in further detail by way of the Hf(OH)[subscript 4−2x−2y](O₂)[subscript x](SO₄)[subscript y]·qH₂O (HafSOx) material system. Surface analytical techniques revealed that control of the catalytically active acid sites may be possible with optimization of water exposure and post-exposure annealing temperature of dehydrated films. Thermal and electron stimulated desorption of peroxide containing HafSOx films revealed complex surface dynamics. The total desorption cross sections of O₂ for 500-2000 eV incident electrons was ~10⁻¹⁶ cm². Interfacial formation and chemistry was investigated by evaporating platinum onto amorphous indium gallium zinc oxide, an increasingly common interface type found in memristors and other novel electronics. Metallic indium formed upon platinum deposition, and the interface had an unexpectedly low Schottky barrier height of 0.25 eV, likely as a result of the reactive interface. Finally, the surface properties of highly oriented LaFeO₃ films were characterized and modified through reductive annealing. Iron was found to exist in three oxidation states depending on the annealing temperature and location within the film. In addition, the concentration and types of oxygen species were also found to vary based on proximity to the surface and the thermal treatment. Together, the aforementioned studies are related by the potential manufacturing steps of inexpensive, innovative energy and electronic devices. These steps include solution synthesis and deposition, processing to form a functional film, interface formation, and adapting the film's surface properties to suit its intended application. Close

• 3. [Article] Continuous Microreactor-Assisted Solution Deposition for Scalable Production of CdS Films

  This is the publisher’s final pdf. The published article is copyrighted by The Electrochemical Society and can be found at: http://www.electrochem.org/.

  Citation × Citation Citation Abstract Copy Title: Continuous Microreactor-Assisted Solution Deposition for Scalable Production of CdS Films Author: Palo, Daniel R., Chang, Chih-Hung, Su, Yu-Wei, Ramprasad, Sudhir, Paul, Brian K. This is the publisher’s final pdf. The published article is copyrighted by The Electrochemical Society and can be found at: http://www.electrochem.org/. Title: Continuous Microreactor-Assisted Solution Deposition for Scalable Production of CdS Films Author: Palo, Daniel R., Chang, Chih-Hung, Su, Yu-Wei, Ramprasad, Sudhir, Paul, Brian K. This is the publisher’s final pdf. The published article is copyrighted by The Electrochemical Society and can be found at: http://www.electrochem.org/. Close

• 4. [Article] Phthalimide-Based π-Conjugated Small Molecules with Tailored Electronic Energy Levels for Use as Acceptors in Organic Solar Cells

  The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide–thiophene–CORE–thiophene–phthalimide ...

  Citation × Citation Citation Abstract Copy Title: Phthalimide-Based π-Conjugated Small Molecules with Tailored Electronic Energy Levels for Use as Acceptors in Organic Solar Cells Author: Hendsbee, Arthur D., McAfee, Seth M., Sun, Jon-Paul, McCormick, Theresa M, Hill, Ian G., Welch, Gregory C. Year: 2015 The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide–thiophene–CORE–thiophene–phthalimide architecture. The CORE units utilized were phthalimide (M2), diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5), perylene diimide (M6), and difluorobenzothiadiazole (M7); they were specifically selected to progressively increase the electron affinity of the resulting compound. A small molecule with no core (M1) was synthesized for comparison. Each material was synthesized through optimized direct heteroarylation cross-coupling procedures using bench top solvents in air. Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV), differential scanning calorimetry (DSC), ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) were used to characterize each material. The use of various core acceptor building blocks with differing electron affinities resulted in the series M1–M7 having a range of energetically deep LUMO levels and a range of HOMO–LUMO gap energies. Meanwhile, the melting and crystallization temperatures of the molecules M1–M7 were also found to vary according to the change in central acceptor unit. Compounds M1–M7 were employed as acceptors in combination with either the polymeric donor P3HT or small molecule donor DTS(FBTTh2)2 to understand how the LUMO levels of each acceptor influences the open circuit voltage (Voc). It was found that, in general, Voc was only weakly related to the offset between the HOMO energy level of the donor and LUMO level of the acceptor used, with a Voc of up to 1.2 V being achieved for M1. Title: Phthalimide-Based π-Conjugated Small Molecules with Tailored Electronic Energy Levels for Use as Acceptors in Organic Solar Cells Author: Hendsbee, Arthur D., McAfee, Seth M., Sun, Jon-Paul, McCormick, Theresa M, Hill, Ian G., Welch, Gregory C. Year: 2015 The design, synthesis, and characterization of seven phthalimide-based organic π-conjugated small molecules are reported. The new materials are based on a phthalimide–thiophene–CORE–thiophene–phthalimide architecture. The CORE units utilized were phthalimide (M2), diketopyrrolopyrrole (M3), isoindigo (M4), naphthalene diimide (M5), perylene diimide (M6), and difluorobenzothiadiazole (M7); they were specifically selected to progressively increase the electron affinity of the resulting compound. A small molecule with no core (M1) was synthesized for comparison. Each material was synthesized through optimized direct heteroarylation cross-coupling procedures using bench top solvents in air. Combinations of UV-visible spectroscopy (UV-vis), cyclic voltammetry (CV), differential scanning calorimetry (DSC), ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) were used to characterize each material. The use of various core acceptor building blocks with differing electron affinities resulted in the series M1–M7 having a range of energetically deep LUMO levels and a range of HOMO–LUMO gap energies. Meanwhile, the melting and crystallization temperatures of the molecules M1–M7 were also found to vary according to the change in central acceptor unit. Compounds M1–M7 were employed as acceptors in combination with either the polymeric donor P3HT or small molecule donor DTS(FBTTh2)2 to understand how the LUMO levels of each acceptor influences the open circuit voltage (Voc). It was found that, in general, Voc was only weakly related to the offset between the HOMO energy level of the donor and LUMO level of the acceptor used, with a Voc of up to 1.2 V being achieved for M1. Close

• 5. [Article] Thin Film Solar Cells Using ZnO Nanowires, Organic Semiconductors and Quantum Dots

  A thin film organic/ inorganic hybrid solar cell was fabricated by incorporating ZnO nanowires, n- and p-type organic semiconductors and inorganic quantum dots. The basic cell design involved the electrodeposition ...

  Citation × Citation Citation Abstract Copy Title: Thin Film Solar Cells Using ZnO Nanowires, Organic Semiconductors and Quantum Dots Author: VanSant, Kaitlyn Year: 2007 A thin film organic/ inorganic hybrid solar cell was fabricated by incorporating ZnO nanowires, n- and p-type organic semiconductors and inorganic quantum dots. The basic cell design involved the electrodeposition of ZnO nanowires grown on a substrate coated with a transparent conductive oxide. The ZnO nanowires were coated with a thin layer of an organic n-type material, followed by a deposition of inorganic quantum dots. A p-type polymer layer was subsequently deposited and the sample was then contacted with gold to form a quantum dot layer sandwiched between a p-n junction of organic conductive materials. Various materials and processing methods were adjusted, using I-V characteristics, photovoltage and/ or photocurrent measurements to determine the performance of the cell. Each constituent material in the basic device design was evaluated in terms of its contribution to the sample characteristics. A variety of deposition techniques were investigated to obtain homogeneous layers. Different annealing procedures were explored with the intent of balancing the time and temperatures required for electrical activation with material constraints such as tendency towards oxidation and low melting points. The effect of time on the sample characteristics was also observed. The evaluation primarily includes data for samples that led to design modifications aimed at improving both electrical properties and quantum efficiencies. This research led to the development of a hybrid solar cell sensitized by the addition of quantum dots. The organic semiconductors were used to form a p-n junction, and the p-type polymer also served as an active absorber layer. The quantum dots were used as the inorganic absorber fayer, and the results show that the range of optical absorption in the cell can be modified by adjusting particle size. In addition, the ZnO nanowires appear to improve charge transfer, when used with materials that have favorable band offsets. Title: Thin Film Solar Cells Using ZnO Nanowires, Organic Semiconductors and Quantum Dots Author: VanSant, Kaitlyn Year: 2007 A thin film organic/ inorganic hybrid solar cell was fabricated by incorporating ZnO nanowires, n- and p-type organic semiconductors and inorganic quantum dots. The basic cell design involved the electrodeposition of ZnO nanowires grown on a substrate coated with a transparent conductive oxide. The ZnO nanowires were coated with a thin layer of an organic n-type material, followed by a deposition of inorganic quantum dots. A p-type polymer layer was subsequently deposited and the sample was then contacted with gold to form a quantum dot layer sandwiched between a p-n junction of organic conductive materials. Various materials and processing methods were adjusted, using I-V characteristics, photovoltage and/ or photocurrent measurements to determine the performance of the cell. Each constituent material in the basic device design was evaluated in terms of its contribution to the sample characteristics. A variety of deposition techniques were investigated to obtain homogeneous layers. Different annealing procedures were explored with the intent of balancing the time and temperatures required for electrical activation with material constraints such as tendency towards oxidation and low melting points. The effect of time on the sample characteristics was also observed. The evaluation primarily includes data for samples that led to design modifications aimed at improving both electrical properties and quantum efficiencies. This research led to the development of a hybrid solar cell sensitized by the addition of quantum dots. The organic semiconductors were used to form a p-n junction, and the p-type polymer also served as an active absorber layer. The quantum dots were used as the inorganic absorber fayer, and the results show that the range of optical absorption in the cell can be modified by adjusting particle size. In addition, the ZnO nanowires appear to improve charge transfer, when used with materials that have favorable band offsets. Close

• 6. [Article] Metastability of copper indium gallium diselenide polycrystalline thin film solar cell devices

  xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.

  Citation × Citation Citation Abstract Copy Title: Metastability of copper indium gallium diselenide polycrystalline thin film solar cell devices Author: Lee, Jinwoo, 1973- Year: 2009, 2008 xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. Title: Metastability of copper indium gallium diselenide polycrystalline thin film solar cell devices Author: Lee, Jinwoo, 1973- Year: 2009, 2008 xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. Close