Thesis (Ph.D.)--Bowling Green State University, 2009. / Document formatted into pages; contains xix, 184 p. : ill. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 76-82).

Solar cells are at the frontier of renewable energy technologies. Photovoltaic energy is clean, reusable, can be used anywhere in our solar system and can be very well integrated with power distribution grids and advanced technological systems. Thin film solar cells are a class of solar cells that offer low material cost, efficient fabrication process and compatibility with advanced electronics. However, as of now, the conversion efficiency of thin film solar cells is inferior to that of thick crystalline cells. Research efforts to improve the performance bottlenecks of thin film solar cells are highly motivated. A class of techniques towards this goal is called light trapping methods, which aims at improving the spectral absorptivity of a thin film cell by using surface texturing. The precise mathematical and physical characterization of these techniques is very challenging. This dissertation proposes a numerical and computational framework to optimize, design, and fabricate efficient light trapping structures in thin film solar cells, as well as methods to verify the fabricated designs. The numerical framework is based on the important "inverse optimization" technique, which is very is widely applicable to engineering design problems. An overview of the state-of-the-art thin film technology and light trapping techniques is presented in this thesis. The inverse problem is described in details with numerous examples in engineering applications, and is then applied to light trapping optimization. The proposed designs are studied for sensitivity analysis and fabrication error, as other aspects of the proposed computational framework. At the end, reports of fabrication, measurement and verification of some of the proposed designs are presented. / text

Zinc oxide (ZnO) is a wide band-gap semiconductor that is of interest for application in dye sensitized solar cells (DSSCs) because of similarity of its properties to TiO2. Unlike TiO2, ZnO can readily be grown in a wide variety of morphologies, using inexpensive, simple, and low temperature methods. Recent research on ZnO-based DSSCs focuses on modifying the ZnO layer morphology in order to maximize surface area, and enhance the electron collection by providing fast electron transport. It is expected that the improvement in cell performance by morphology modification is due to higher dye loading, increased electron lifetime and fast electron transport. However, ZnO properties may be affected by various synthesis methods. It is difficult to make a conclusion whether the change of performance are attributed to change of morphology or a change in the defect types and/ or defect concentrations. In this study, the influence of ZnO nanoparticle properties on cell performance has been investigated. Commercial ZnO nanoparticles with different sizes and optical properties were utilized. It was found that there is a complex relationship between native defects, dye loading, charge transport and photovoltaic performance. In particular, the presence of non-radiative defects was found to be detrimental to photovoltaic performance. In addition, with the similar defect emission intensities, sample exhibiting orange-red defect emission showed better performance than the samples emitting green defect emission. Nanoparticle properties and their relationship between dye adsorption, electron injection, electron lifetime and electron transport, and photovoltaic performance will be discussed. / published_or_final_version / Physics / Master / Master of Philosophy

Colloidal nanocrystal assemblies offer an attractive opportunity for designer metamaterials. The ability to permute chemical composition, size, shape, and arrangement of nanocrystals leads to an astounding number of unique materials properties that find use in an extensive array of applications---ranging from solar cells to medicine. However, to take full advantage of these materials in useful applications, the nature of their assembly and their behavior under external stimuli must be well understood. Additionally, the assembly of colloidal nanocrystals into thin films provides a promising pathway to the solution-processing of inorganic materials that are prohibitively too expensive and/or difficult to deposit by conventional methods. Nanocrystal superlattices (NCSLs) of sterically stabilized nanocrystals were assembled by slow evaporation of colloidal dispersions on various substrates. Detailed analysis of the NCSL structures was carried out using transmission and scanning electron microscopy (TEM and SEM) and small-angle x-ray scattering (SAXS). Body-centered cubic (bcc) NCSLs, in particular, were studied in detail and ligand packing frustration was proposed as a significant driving force for their assembly. The behavior of NCSLs was also studied by SAXS under mild heating and solvent vapor exposure revealing several remarkable order-order, order-disorder, and amorphous-crystalline structural transitions. Colloidal Cu(In [subscript 1-x] Ga [subscript x])Se₂ (CIGS) nanocrystals were synthesized by arrested precipitation and formulated into inks. These inks were spray deposited into thin films under ambient conditions to serve as the active light absorbing material in printed low-cost photovoltaic (PV) devices. These devices, which were fabricated without the need for high temperature processes, have achieved power conversion efficiencies above 3 % under AM1.5 illumination. While the efficiencies of these devices are still too low for commercial viability, this work does provide a proof of concept that reasonable efficient solar cells can be created with a low-cost printable process using nanocrystal inks. Since high temperatures are not used to form the light-absorbing layer, nanocrystal-based solar cells were built on flexible light weight plastic substrates. The main obstacle to achieving high power conversation efficiencies was found to be the ability to extract the photo induced charge carriers. Nanocrystal films suffer from poor transport that leads to high recombination rates in thicker films. To date, the best efficiencies have been achieved with thin light absorber layers that only absorb a fraction of the incident light. / text

This thesis presents the design and synthesis of nanowires (NW) with targeted and tunable optical properties. Moreover, we show how single and assembled NW devices can enable new photovoltaic (PV) and photonic platforms. Beginning with an investigation of axially modulated p-i-n junction NWs, we established several fundamental parameters dictating solar cell performance at the nanoscale and demonstratred the first series integration of multiple solar cells on a single NW. Thereafter, implementation of the first silicon NW photovoltaic device with radially modulated p-n junctions showed that power conversion efficiencies of 3-4% are attainable from a nanoscale architecture, exceeding efficiencies for many organic and hybrid organic-inorganic solar cells. Despite these achievements, the poor electrical characteristics and insufficient control over absorption properties characterizing the aforementioned devices would limit the promise of silicon NWs for next generation solar cells. We overcome these limitations with a class of polymorphic core/multi-shell silicon NWs with highly-crystalline hexagonally-faceted shells and embedded coaxial p/i/n junctions. NW PV devices 200-300 nm in diameter exhibit open-circuit voltages of 0.5 V and fill-factors of 73% under one-sun solar illumination. Single-NW wavelength-dependent photocurrent measurements agree quantitatively with FDTD simulations. Synthetic manipulation of NW size and morphology drives tuning of optical resonances such that optimized structures can yield current densities double those for films of comparable thickness. Further optimized NW devices achieve current densities of 17 mA/cm2 and power conversion efficiencies of 6%. We also present steps toward rational assembly of larger-scale NW PV arrays. Parallel integration of NWs preserves PV metrics while assembly of vertically-stacked NWs yields current densities of \(25 mA/cm^2\) and projected efficiencies of ~15% for \(1 \mu m\) thick assemblies. Finally, we present the first ever NW material possessing 3 degrees of structural freedom, thus expanding the NW "structome." Such NWs were achieved through the first demonstration of facet selective growth of silicon and germanium in the gas phase. Photonic devices based on this new material present intriguing optical properties, including selective attenuation, enhancement, and wavelength tunability of resonant cavity modes. / Chemistry and Chemical Biology

Metallic nanostructures such as nanoparticles, nanowires and nanoapertures exhibit extraordinary optical properties in absorption, scattering and transmission of electromagnetic radiation due to the excitation of surface plasmons. This allows them to provide applications in converting photon energy to other forms of energy such as heat, mechanical work and electricity in a more efficient or controlled manner. When incorporated into an amorphous silicon thin film solar cell, nanoparticles were found to substantially increase the light absorption in the photoactive layer within certain wavelength range. The mechanism of this optical absorption was studied using three-dimensional finite element method. It was found that intensified Fabry-Perot resonance in the active layer due to the addition of the nanostructures and enhanced light scattering by the plasmonic nanostructures were both responsible for this phenomenon. Interestingly, higher absorption only occurs at wavelength range outside the surface plasmons resonance of the nanostructures. A further study on the absorption of the nanoparticles themselves revealed that enhanced near field associated with the SP resonance of particles causes extraordinary energy dissipation in the particles, resulting in decreased light scattering. Strong power dissipation accompanied with the surface plasmons resonance becomes desirable when nanostructures are used as heat generator. Using the new technique of three-dimensional localization of the metallic nanoparticles on polymer microstructures, wavelength dependent controlling of a light-driven microactuator was achieved by selectively coating it with nanoparticles of different materials. Another important plasmonic nanostructure is the subwavelength hole arrays perforated on a metal film. The optical transmission through these nanometer scaled apertures whose dimensions are smaller than the wavelength of the incident light can be several orders of magnitude larger than expected. Based on this property, a novel tandem solar cell structure was proposed. A metal film perforated with periodic subwavelength hole arrays was inserted in a tandem solar cell as a light transmittable intermediate common electrode for the top and the bottom cell. The perforated electrode removes the current matching restriction in conventional tandem cells and allows active materials with different energy conversion and charge transport mechanisms to be combined in the same device. If used in a multi-junction solar cell, the new design can also save the power loss across the tunnel junction. The perforated intermediate electrode was modeled and its optical performance in the tandem solar cell was investigated. / text

My research on metal oxide electron-harvesting interlayers for organic solar cells was focused as three interrelated projects in this dissertation: i) development of a chemical vapor deposition (CVD) system for titanium dioxide (TiO₂) film; ii) an electrochemical methodology to evaluate ZnO thin film charge (hole) blocking ability; iii) the effects of plasma modifications on sol-gel ZnO and sol-gel ZnO/organic (active layer) interfaces. In i), we showed that nanoscale (12-36 nm) CVD TiO₂ film deposited at 210 °C from our system obtains properties of conformal growth with ITO substrate, superior hole blocking ability, stoichiometric metal to oxide ratio, and close energetic alignment with electron acceptors, e.g., fullerenes. The introduction of CVD TiO₂ film as an electron transport layer (ETL) into organic solar cell significantly improves its J-V characteristics compared to bare ITO electrode. The optimum TiO₂ thickness in the OPV device applications was found to be 24 nm with a high fill factor (0.58) and power conversion efficiency (3.7%) obtained. In ii), simple electrochemical methods, i.e., cyclic voltammetry, impedance spectroscopy have been used to evaluate sol-gel derived ZnO (sg-ZnO) and sputtered ZnO (sp-ZnO) porosity and pinhole density. We showed that sg-ZnO with high surface area porous structure allows the probe molecules and poly-thiophene (P3HT) thin layer to direct contact ITO substrate, whereas sp-ZnO with dense structural property efficiently eliminates the probe molecule diffusion and the penetration of P3HT layer to ITO substrate. This electrochemical property difference also directly reflects on the device shunt resistance (Rp), where we observed larger leakage current for the devices using sg-ZnO than that of devices using sp-ZnO. We envision these simple electrochemical characterizations can be applied into other similar metal oxide interlayers as well as on flexible TCO substrates, in which pinholes and physical imperfections, e.g., cracking may occur after multiple bending processes. In iii), we demonstrated low power (10.5 watts) radio frequency (RF) O₂ and Ar plasma treatments have significant impacts on sg-ZnO near-surface chemical compositions, which in turn influence the onset potential of sg-ZnO electron injection from the underlying ITO substrate and its energetic alignment with electron acceptors, e.g., C₆₀. Using UPS, we found the presence of localized mid-gap states near the Fermi-level (Ef) of sg-ZnO, which induces the most favorable band bending and the largest vacuum level shift due to significant electron transfer from sg-ZnO to C₆₀. As a result, the resultant solar cells show the best device performance. Upon the plasma treatments, the passivation effects eliminate the mid-gap state. Therefore, we observed less degree of band bending at ZnO/C₆₀ interface and poorer device performance for the plasma treated sg-ZnO. The study demonstrates the importance of oxide/organics interface in operations of organic solar cells and provides a modification method to tune surface properties of oxide materials which can apparently be applied in other organic electronic devices, e.g., field effect transistors (FETs), organic light emitting diodes (OLEDs), etc.

ZnO nanowire arrays (NWAs) and Al-doped ZnO (AZO) cathodes were applied in hybrid organic / inorganic solar cells for lower-cost solar energy. Parameters for the electrodeposition of ZnO NWAs and the fabrication of NWA-free baseline devices were systematically optimized using ITO as the cathodes. High efficiencies of up to 5.4% were achieved. Incorporation of the ZnO NWAs into the baseline devices significantly reduced their efficiencies due to possible shorting in the active layer. Devices fabricated using AZO cathodes were characterized. The AZO-based devices achieved efficiencies of up to ~4.8%, showing promising results for the application of AZO as an ITO alternative. Formation of numerous large nanoplatelets was observed during the electrodeposition of ZnO NWAs on AZO cathodes. The NWAs grown on AZO cathodes were also non-uniform. Future studies were proposed to address the issues with incorporation of ZnO NWAs in hybrid solar cells and their combination with AZO cathodes.

ZnO nanowire arrays (NWAs) and Al-doped ZnO (AZO) cathodes were applied in hybrid organic / inorganic solar cells for lower-cost solar energy. Parameters for the electrodeposition of ZnO NWAs and the fabrication of NWA-free baseline devices were systematically optimized using ITO as the cathodes. High efficiencies of up to 5.4% were achieved. Incorporation of the ZnO NWAs into the baseline devices significantly reduced their efficiencies due to possible shorting in the active layer. Devices fabricated using AZO cathodes were characterized. The AZO-based devices achieved efficiencies of up to ~4.8%, showing promising results for the application of AZO as an ITO alternative. Formation of numerous large nanoplatelets was observed during the electrodeposition of ZnO NWAs on AZO cathodes. The NWAs grown on AZO cathodes were also non-uniform. Future studies were proposed to address the issues with incorporation of ZnO NWAs in hybrid solar cells and their combination with AZO cathodes.