Philosophiae Doctor - PhD / The rapid increase in global energy demand in recent decades coupled with the adverse environmental impact of conventional fuels has led to a high demand for alternative energy sources that are sustainable and efficient. Renewable solar energy technologies have received huge attention in recent decades with the aim of producing highly efficient, safe, flexible and robust solar cells to withstand harsh weather conditions. c-Si has been the material of choice in the development of conventional inorganic solar cells owing to it superior properties, abundance and higher efficiencies. However, the associated high costs of Si processing for solar cells have led to a gravitation towards alternative organic solar cells which are cheaper and easy to process even though they suffer from stability and durability challenges. In this work, combination of both inorganic and organic materials to form hybrid solar cells is one of the approaches adopted in order to address the challenges faced by solar cell development.

In order to become a viable alternative to silicon photovoltaics, dye-sensitized solar cells must overcome several issues primarily resulting from their use of a liquid electrolyte. Much research has gone into correcting these shortcomings by replacing the liquid electrolyte with solid-state hole-transport media. Using these solid-state materials brings new difficulties, such as completely filling the pores in the TiO₂nanostructure, and achieving good adhesion with the dye-coated TiO₂. A novel approach to addressing these difficulties is the inverse dye-sensitized solar cell design. In this method the devices are constructed in reverse order, with the solid-state hole-transport medium providing the nanostructure instead of the TiO₂. This allows new materials and methods to be used which may better address these issues. In this project, inverse dye-sensitized solar cells using polyaniline nanofibers as the hole transport medium were prepared and characterized. The devices were prepared on fluorine-doped tin oxide (FTO) coated glass electrodes. The first component was a dense spin-coated polyaniline blocking layer, to help prevent short circuiting of the devices. The second layer was a thin film of drop cast polyaniline nanofibers which acted as the hole transport medium and provided high surface area for the dye attachment. The dye used was 5,10,15,20-tetrakis (4-carboxyphenyl) porphyrin (TCPP), which was covalently attached to the nanofibers using a Friedel-Crafts acylation. Titania gel was then deposited into the pores of the nanofiber film by controlled hydrolysis of a titanium complex (Tyzor LA). A back electrode of TiO₂nanoparticles sintered on FTO was pressed on top to complete the devices. A typical device generated an open circuit voltage of 0.17 V and a closed circuit current of 5.7 nA/cm² while the highest open circuit voltage recorded for any variation on a device was 0.31 V and the highest short circuit current was 52 nA/cm² under AM 1.5 simulated olar spectrum at 100 mW/cm². Initially prepared devices did not generate a measureable photocurrent due to two materials flaws. The first was traced to the poorly developed conduction band of the titania gel, as deposited from Tyzor LA hydrolysis, resulting in poor electron conduction. This prevented the titania gel from efficiently functioning as the electron transport medium. A remedy was found in adding a layer of sintered anatase TiO₂nanoparticles on the back electrode to serve as the electron transport medium. However, this remedy does not address the issue of the inability of titania gel to efficiently transport electrons photogenerated deep in the nanofiber film to the back electrode. The second flaw was found to originate from fast recombination kinetics between electrons in TiO₂and holes in polyaniline. However, a positive feature was that the titania gel intended to be used as the electron transport medium was found to sufficiently insulate the interface such that the recombination rate slowed enough to allow generation of a measureable photocurrent. Electronic insulation was further enhanced by co-attaching decanoic acid onto the polyaniline nanofibers to fill in pinholes between the dye molecules. While these solutions were not ideal, they were intended to be diagnostic in nature and supplied critical information about the weak links in the device design, thus pointing the way toward improving device performance. Significant enhancements can be expected by addressing these issues in further detail.

Cu₂S/CdS solar cells have been studied extensively for the past two decades due to their potentially high efficiencies per unit cost. The operation and characteristics of Cu₂S/CdS solar cells are fairly well understood. However, the properties of the newer Cu₂S/ZnCdS cell type are not well understood. The main goals of this thesis were to compare Cu₂S/CdS and Cu₂S/ZnCdS cells using Cu₂S/CdS cells as a reference, and to understand the operation and properties of Cu₂S/ZnCdS cells in order to improve cell performance. Four different measurements were used in this research to achieve these goals. They were; electrical, spectral, capacitance and deep trap measurements. I-V measurements give important electrical parameters of the cells; cell efficiency, fill factor, short circuit current, open circuit voltage, shunt resistance and series resistance are reported. From a In(I_SC) versus V_OC measurement, the diode factor, A, was found to be about 1 for Cu₂S/CdS, Cu₂S/Zn_0.11Cd_0.89S, and about 1.2 for Cu₂S/Zn_0.25Cd_0.75S cells. The relation between In(J_oo) (current density) and ϕ (potential barrier height) is linear for both types of cells. The slope of this linear relationship increases as the content of Zn increases in Zn_xCd_1-xS. Under air mass 1 (100 mW/cm²) illumination, it was found that V_OC decays and capacitance increases for Cu₂S/ZnCdS cells. This is attributed to electron relaxation from deep traps near the junction. Spectral response with and without bias light were measured for both Cu₂S/CdS and Cu₂S/ZnCdS cells. White and blue bias light enhance the spectral response, while red bias light quenches the response. This is attributed to ionization and filling of deep traps near the junction. Capacitance measurements on both cell types show that 1/C² versus voltage is quite flat, which indicates the existence of an i-layer (insulation layer) in the CdS or ZnCdS near the junction. Three methods–photocapacitance, space-charge-limited current, and thermally stimulated. current techniques–were used for deep trap measurements. Photocapacitance measurements indicate one deep donor energy and two deep acceptor energy levels. These trap energies become larger as the content of Zn in ZnCdS increases. Space-charge-limited current measurements give a trap density of the order of 10¹⁶ cm³ for both cell types. The shallow energy trap is found to be 0.26 eV below the conduction band edge of CdS. The occurrence of a current-saturated region for Cu₂S/ZnCdS is attributed to the filling of the interface traps near the junction. Thermally stimulated current measurements give two energy levels below the conduction band of CdS; 0.05 eV and 0.26 eV. From the above results, several differences between the Cu₂S/CdS and the Cu₂S/ZnCdS cells can be seen. The Cu₂S/ZnCdS cells show stronger red quenching, smaller electron lifetime at the interface near the junction, and deeper traps than the Cu₂S/CdS cells. These differences can account for the decline of I_SC and the V_OC decay. The smaller I_SC for the Cu₂S/ZnCdS cells can also possibly result from smaller electron lifetime at the interface, larger interface recombination velocity, different deep trap levels, and enhanced Zn concentration near the junction. The V_OC decay for the Cu₂S/ZnCdS cells is mostly due to long decay of charge. Longer decay could be attributed to deeper traps. / Ph. D.

Quantum dot sensitized solar cells (QDSCs) constitute one of the most promising low-cost solutions that are explored for the world’s needs of clean and renewable energy. Efficient, low-toxic and stable QDSCs for large-scale applications have formed the subject for the solar cell research during recent years. This circumstance also forms the motivation for this thesis, where the results of my studies to improve the efficiency and stability of green QDSCs are presented and discussed. The surface condition of quantum dots (QDs) is always crucial to the performance of QDSCs, since surface ligands can influence the loading amount of QDs, and that the surface defects can induce charge recombination in the solar cells. In this thesis work, a hybrid passivation approach was firstly utilized to improve the photovoltaic performance of CdSe QDs. After hybrid passivation by MPA and iodide ions, the loading efficiency of the QDs was increased with the ligands of MPA, and the surface defects on the QDs were reduced by the iodide ions, both contributing to an enhancement in the efficiency of the CdSe based QDSCs. This hybrid passivation strategy was then employed for low-toxic CuInS2 QDs, and was also demonstrated as an effective way to modify the surface state of the CuInS2 QDs and improve the performance of the QDSCs based on CuInS2 QDs.   To improve the stability of the QDSCs, solid state quantum dot sensitized solar cells (ss-QDSCs) based on CuInS2 QDs were investigated. In addition to the hybrid passivation, increasing the pore size of the TiO2 active layer and changing the composition of the CuInS2 QDs were also found to be useful approaches to improve the performance of the ss-QDSCs based on CuInS2 QDs. Furthermore, for the most used hole transport material- Spiro-OMeTAD- in solid state solar cells, silver bis(trifluoromethanesulfonyl)imide was shown to be an effective p-type dopant to increase its conductivity and to improve the performance of the solar cells based on it. / <p>QC 20160516</p>

To make solar energy mainstream, lower-cost and more efficient power generation is key. A lot of effort in the silicon photovoltaic industry has gone into using fewer raw materials (i.e., silicon) and using more inexpensive processing techniques and materials to reduce cost. Utilizing thinner substrates not only reduces cost, but improves cell efficiency provided both front and back surfaces are well-passivated. In the current work, a kerf-less process is developed in which ultra-thin (~25 [mu]m), flexible mono-crystalline silicon substrates can be obtained through an exfoliation technique from a thicker parent wafer. These substrates, when exfoliated, have thick metal backing which provides mechanical support to the thin silicon and enables ease of processing of the substrates for device fabrication. Optical, electrical, and reliability characterization studies for completed cells show this technology’s compatibility with a heterojunction solar cell process flow. Building on the promising results achieved on exfoliated substrates, further optimization work was carried out. Namely, an improved cleaning process was developed to remove front surface contamination on textured surfaces of exfoliated, flexible mono-crystalline silicon. This process is very effective at cleaning metallic and organic residues, without introducing additional contamination or degrading the supporting back metal used for ultra-thin substrate handling. Spectroscopic studies were performed to qualitatively and quantitatively understand the efficacy of different cleaning procedures in order to develop the new cleaning process. Results of the spectroscopic studies were further supported by comparing the electrical performance of cells fabricated with different cleans. To replace silver as contact metal with a cheaper substitute like nickel or copper, patterning and etching processes are generally used. A low-cost alternative is proposed, where a reusable shadow mask with a metal grid pattern is kept in contact with the surface of the substrate in a plasma-enhanced chemical vapor deposition chamber during silicon nitride deposition. This leaves a patterned silicon surface for selective metal growth by direct electro-deposition. The viability of this process flow is demonstrated by fabricating diffused junction n[superscript+]pp[superscript+] monofacial and bifacial cells and electrically characterizing them. Investigation of the factors limiting the efficiency of the cells was carried out by lifetime measurement experiments. / text

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Approved for public release; distribution is unlimited. / The Naval Postgraduate School Spacecraft Architecture and Technology Demonstration Satellite, NPSAT1, launching in the fall of 2006, will include a system to measure the performance of new experimental triplejunction solar cells. The measuring circuit in the Solar Cell Measurement System (SMS) is based on a circuit developed at the Naval Postgraduate School many years ago. It will trace the cells' current-voltage (I-V) curves while in orbit. The SMS consists of a radiation-hardened microcontroller that uses a radiation-hardened FPGA to monitor a collection of sensors. A current-sink circuit is used to measure the current and voltage on the test cells. Prior to launch, extensive testing is being performed on the system to ensure proper operation. The tests consist of subjecting solar cells and the measuring circuit electronics under conditions modeling the space environment while taking cell measurements. This thesis presents the mission information, system design, test setup, and test results of the SMS measuring circuit. / Ensign, United States Naval Reserve

An optical, contact-free method for measuring minority carrier diffusion lengths is developed and demonstrated for a range of semiconductor materials used in high efficiency triple junction solar cells. This method uses a Scanning Electron Microscope (SEM) coupled with an optical microscope. The diffusion lengths, combined with minority carrier lifetime measured via time-resolved photoluminescence, allow for the computation of minority charge carrier mobility. The technique uses images to extract diffusion length measurements from GaAs, InGaAs, and InGaP heterostructures at different SEM beam energies and probe currents. Excellent correlation between measurements shows the reproducibility of this technique. Diffusion lengths from 2-63 microns have been measured in a variety of GaAs, InGaAs, and InGaP samples. Effects of alloy ordering, doping, and lattice matching have been investigated. Several areas for further research are offered, including detailed radiationdamage mapping of solar cell layers. Further anisotropic studies of the solar cell layers are suggested to investigate the directional dependence of diffusion length within the InGaP heterostructures. Finally, new and emerging solar cell materials would benefit from this technique, allowing for the complete characterization of minority charge transport properties before growing an entire solar cell.

Approved for public release; distribution is unlimited. / Many improvements have been made in the design and manufacture of high efficiency solar cells. The need to understand the behavior of these new types of solar cells is crucial to the procurement of future space systems, both commercial and military. This thesis studies the results of irradiating three commonly used solar cells with 30 MeV electrons using the Naval Postgraduate School Linear Accelerator. A comparison of the performance characteristics of the three cells is made using commonly accepted parameters and notes the differences in failure mode. Additionally, the affect of current annealing is investigated. / Lieutenant, United States Navy

Wong Yuen-fai, Dick. / Includes bibliographical references / Thesis (M.Ph.)--Chinese University of Hong Kong, 1985