Browsing by Subject "Photovoltaic"
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Item Assembly and characterization of quantum-dot solar cells(2009-09) Leschkies, Kurtis SiegfriedEnvironmentally clean renewable energy resources such as solar energy have gained significant attention due to a continual increase in worldwide energy demand. A variety of technologies have been developed to harness solar energy. For example, photovoltaic (or solar) cells based on silicon wafers can convert solar energy directly into electricity with high efficiency, however they are expensive to manufacture, and thus unattractive for widespread use. As the need for low-cost, solar-derived energy becomes more dire, strategies are underway to identify materials and photovoltaic device architectures that are inexpensive yet efficient compared to traditional silicon solar cells. Nanotechnology enables novel approaches to solar-to-electric energy conversion that may provide both high efficiencies and simpler manufacturing methods. For example, nanometer-size semiconductor crystallites, or semiconductor quantum dots (QDs), can be used as photoactive materials in solar cells to potentially achieve a maximum theoretical power conversion efficiency which exceeds that of current mainstay solar technology at a much lower cost. However, the novel concepts of quantum dot solar cells and their energy conversion designs are still very much in their infancy, as a general understanding of their assembly and operation is limited. This thesis introduces various innovative and novel solar cell architectures based on semiconductor QDs and provides a fundamental understanding of the operating principles that govern the performance of these solar cells. Such effort may lead to the advancement of current nanotechnology-based solar power technologies and perhaps new initiatives in nextgeneration solar energy conversion devices. We assemble QD-based solar cells by depositing photoactive QDs directly onto thin ZnO films or ZnO nanowires. In one scheme, we combine CdSe QDs and singlecrystal ZnO nanowires to demonstrate a new type of quantum-dot-sensitized solar cell (QDSSC). An array of ZnO nanowires was grown vertically from a fluorine-doped-tinoxide conducting substrate and decorated with an ensemble of CdSe QDs, capped with mercaptopropionic acid. When illuminated with visible light, the CdSe QDs absorb photons and inject electrons into the ZnO nanowires. The morphology of the nanowires then provided these photoinjected electrons with a direct and efficient electrical pathway to the photoanode. When using a liquid electrolyte as the hole transport medium, our quantum-dot-sensitized nanowire solar cells exhibited short-circuit current densities up to 2.1 mA/cm2 and open-circuit voltages between 0.6–0.65 V when illuminated with 100 mW/cm2 of simulated AM1.5 light. Our QDSSCs also demonstrated internal quantum efficiencies as high as 50–60%, comparable to those reported for dye-sensitized solar cells made using similar nanowires. We found that the overall power conversion efficiency of these QDSSCs is largely limited by the surface area of the nanowires available for QD adsorption. Unfortunately, the QDs used to make these devices corrode in the presence of the liquid electrolyte and QDSSC performance degrades after several hours. Consequently, further improvements on the efficiency and stability of these QDSSCs required development of an optimal hole transport medium and a transition away from the liquid electrolyte. Towards improving the reliability of semiconductor QDs in solar cells, we developed a new type of all-solid-based solar cell based on heterojunctions between PbSe QDs and thin ZnO films. We found that the photovoltage obtained in these devices depends on QD size and increases linearly with the QD effective bandgap energy. Thus, these solar cells resemble traditional photovoltaic devices based on a semiconductor– semiconductor heterojunction but with the important difference that the bandgap energy of one of the semiconductors, and consequently the cell’s photovoltage, can be varied by changing the size of the QDs. Under simulated 100 mW/cm2 AM1.5 illumination, these QD-based solar cells exhibit short-circuit current densities as high as 15 mA/cm2 and open-circuit voltages up to 0.45 V, larger than that achieved with solar cells based on junctions between PbSe QDs and metal films. Moreover, we found that incident-photonto- current-conversion efficiency in these solar cells can be increased by replacing the ZnO films with a vertically-oriented array of single crystal ZnO nanowires, separated by distances comparable to the exciton diffusion length, and infiltrating this array with colloidal PbSe QDs. In this scheme, photogenerated excitons can encounter a donor– acceptor junction before they recombine. Thus, we were able to construct solar cells with thick QD absorber layers that were still capable of efficiently extracting charge despite short exciton or charge carrier diffusion lengths. When illuminated with the AM1.5 spectrum, these nanowire-based quantum-dot solar cells exhibited power conversion efficiencies approaching 2%, approximately three times higher than that achieved with thin film ZnO devices constructed with the same amount of QDs. Supporting experiments using field-effect transistors made from the PbSe QDs as well as the sensitivity of these transistors to nitrogen and oxygen gas show that the solar cells described above are unlikely to be operating like traditional p–n heterojunction solar cells. All data, including significant improvements in both photocurrent and power conversion efficiency with increasing nanowire length, suggest that these photovoltaic devices operate as excitonic solar cells.Item Characterization of pi-conjugated polymers for transistor and photovoltaic applications(2012-12) Paulsen, Bryan D.pi-Conjugated polymers represent a unique class of optoelectronic materials. Being polymers, they are solution processable and inherently "soft" materials. This makes them attractive candidates for the production of roll-to-roll printed electronic devices on flexible substrates. The optical and electronic properties of pi-conjugated polymers are synthetically tunable allowing material sets to be tailored to specific applications. Two of the most heavily researched applications are the thin film transistor, the building block of electronic circuits, and the bulk heterojunction solar cell, which holds great potential as a renewable energy source. Key to developing commercially feasible pi-conjugated polymer devices is a thorough understanding of the electronic structure and charge transport behavior of these materials in relationship with polymer structure. Here this structure property relationship has been investigated through electrical and electrochemical means in concert with a variety of other characterization techniques and device test beds. The tunability of polymer optical band gap and frontier molecular orbital energy level was investigated in systems of vinyl incorporating statistical copolymers. Energy levels and band gaps are crucial parameters in developing efficient photovoltaic devices, with control of these parameters being highly desirable. Additionally, charge transport and density of electronic states were investigated in pi-conjugated polymers at extremely high electrochemically induced charge density. Finally, the effects of molecular weight on pi-conjugated polymer optical properties, energy levels, charge transport, morphology, and photovoltaic device performance was examined.Item Collection of Heat Loss in Photovoltaic System by Parallelly Connected Thermoelectric Network(2022-06) Erickson, JoelThe goal of this work is to increase solar cell efficiency by efficiently combining the electric power of a solar cell and a thermoelectric generator into a single two terminal hybrid device. This work presents a method of achieving this by dividing the thermoelectric generator into smaller thermoelectric generators, forming a parallelly connected network with them, and connecting this network in series with the solar cell. An equivalent circuit model was developed for this device scheme and compared with experimental data. The data show some support of the model, but fine evaluation of the model’s accuracy was hindered by limitations in the experimental setup. If thermoelectric generator efficiency increases in the future, it may become practical to combine thermoelectric generators with solar cells. Providing a method for combining the two power sources at the cellular level may be important for simplifying and improving systems that use these photovoltaic/thermoelectric hybrids.Item Growth and Characterization of Wide Bandgap CIAGS Solar Cell Material and Devices(2018-12) Hwang, SehyunIn this study, we present the development of copper-indium-aluminum-gallium-selenium (Cu(In1-x-yAlyGax)Se2, or CIAGS) as a wide bandgap top cell absorber for tandem photovoltaic (PV) applications. Realizing a tandem PV structure could lead to a breakthrough for high efficiency solar cells. CIAGS absorbers were grown in a single-step process using a custom-designed co-evaporation system under an ultra-high vacuum. The material properties of CIAGS thin films were analyzed in terms of grain morphology, elemental composition, and energy bandgap. The bandgap of CIAGS is tuned by controlling the elemental composition of group III elements. The relation between energy bandgap and elemental composition was empirically established for CIAGS absorbers with varying bandgaps. The CIAGS grown here targeted a bandgap of ~1.65 eV which is optimal for a tandem top cell. CIAGS solar cell devices were fabricated and characterized electrically by J-V measurements. The highest efficiency obtained was 12.8%, although the efficiency tends to decrease as the bandgap increases. Poor film adhesion or delamination is a major problem in wide bandgap CIAGS solar cells. Delamination occurs at the interface between the CIAGS absorber and the Mo back contact layer. We suggest two possible delamination mechanisms caused by interfacial molybdenum diselenide (MoSe2) in the wide bandgap CIAGS. The CIAGS/Mo interface was characterized mechanically (adhesion) and electrically (contact resistance). A TiN diffusion barrier to selenization improves the CIAGS/Mo interfacial adhesion and provides a potential solution to the delamination problem in the wide bandgap absorbers such as CIAGS.