Mukul Dubey

TiO2 nanotube membranes on transparent conducting glass for high efficiency dye-sensitized solar cells

Crack-free TiO(2) nanotube (NT) membranes were obtained by short time re-anodization of a sintered TiO(2) NT array on Ti foil, followed by dilute HF etching at room temperature. The resulting freestanding TiO(2) membranes were opaque with a slight yellow color having one end open and another end closed. The membranes were then fixed on transparent fluorine-tin-oxide glass using a thin layer of screen-printed TiO(2) nanoparticles (NPs) as a binding medium. It was found that low temperature treatment of the resulting NT/NP film under appropriate pressure before sintering at 450 °C was critical for successful fixation of the NT membrane on the NP layer. The resulting films with open-ends of NT membranes facing the NP layer (open-ends down, OED, configuration) exhibited better interfacial contact between NTs and NPs than those with closed-ends facing the NP layer (closed-ends down, CED, configuration). The cells with an OED configuration exhibit higher external quantum efficiency, greater charge transfer resistance from FTO/TiO(2) to electrolyte, and better dye loading compared to CED configurations. The solar cells with the OED configuration gave 6.1% energy conversion efficiency under AM1.5G condition when the commercial N719 was used as a dye and I(-)/I(3)(-) as a redox couple, showing the promise of this method for high efficiency solar cells.

Electrophoretic deposited TiO(2) pigment-based back reflectors for thin film solar cells.

Highly reflective coatings with strong light scattering effect have many applications in optical components and optoelectronic devices. This work reports titanium dioxide (TiO(2)) pigment-based reflectors that have 2.5 times higher broadband diffuse reflection than commercially produced aluminum or silver based reflectors and result in efficiency enhancements of a single-junction amorphous Si solar cell. Electrophoretic deposition is used to produce pigment-based back reflectors with high pigment density, controllable film thickness and site-specific deposition. Electrical conductivity of the pigment-based back reflectors is improved by creating electrical vias throughout the pigment-based back reflector by making holes using an electrical discharge / dielectric breakdown approach followed by a second electrophoretic deposition of conductive nanoparticles into the holes. While previous studies have demonstrated the use of pigment-based back reflectors, for example white paint, on glass superstrate configured thin film Si solar cells, this work presents a scheme for producing pigment-based reflectors on complex shape and flexible substrates. Mechanical durability and scalability are demonstrated on a continuous electrophoretic deposition roll-to-roll system which has flexible metal substrate capability of 4 inch wide and 300 feet long.

Morphological and Photovoltaic Studies of TiO2 NTs for High Efficiency Solar Cells

Highly ordered nanostructures, especially TiO2 NTs, have attracted considerable research interest in recent years due to their diverse applications in photocatalysis, photonic crystals, sensors, batteries and photovoltaic devices. The photophysical, photochemical, electrical and surface properties of these nanostructured materials depend highly on their morphology because of the quantum size effect. Hence it is critical to study the effect of morphology of the ordered nanostructures for device applications. In this chapter we will only focus on the TiO2 NT morphology in context of their applications in dye-sensitized solar cells (DSCs).

The structure and photovoltaic property relationship of porphyrins for high efficiency solar cells

Several 5-(4-carboxyphenyl)-10,15,20-tris(phenyl) porphyrin derivatives with different substituents (NO<inf>2</inf>, CN, CH<inf>3</inf>, OCH<inf>3</inf>, CN, NO<inf>2</inf>, Cl, N(CH<inf>3</inf>)<inf>2</inf>, N(Ph)<inf>2</inf>) in the para-positions of phenyl groups are designed and synthesized for systematic study of structural effect on their photovoltaic performance in dye-sensitized solar cells (DSSCs). Density Functional Theory (DFT) level calculation indicates the decrease of energy levels of frontier molecular orbitals (HOMO and LUMO) when substituents change from NO<inf>2</inf>, CN, Cl, H, OCH<inf>3</inf>, CH<inf>3</inf>, N(CH<inf>3</inf>)<inf>2</inf> to N(Ph)<inf>2</inf>. Photovoltaic performances of these porphyrins in DSSCs show that introducing of electron donating groups in porphyrins and Zn²⁺ ion favors the energy conversion of sunlight to electricity.

Light scattering behavior of oxide nanoparticles

Titanium dioxide (TiO2) has high optical refractive index and is transparent to visible light. No strong reflection is expected from a single layer of continuous TiO2 film. In this work, we report the unusually high reflection from a thick layer of TiO2 nanoparticles. TiO2 nanoparticles of different sizes (~ 21 nm and ~ 400 nm) were deposited on indium tin oxide (ITO) coated glass substrate by electrophoretic process in atmosphere. It was observed that the film characteristics such as morphology and thickness were affected by the electrophoretic process conditions like voltage, deposition time, and electrolyte concentration. The electrolyte had the most significant effect on the deposition rate of the nanoparticles. The film thickness was proportional to the deposition time, which in turn determined the diffused reflectance of the TiO2 particle films. It was observed that the 400 nm TiO2 nanoparticle films lead to much stronger light scattering as compared to the 21 nm particles. The diffuse reflectance was compared to sputtering deposited Ag/ZnO back reflectors that are used in thin film solar cells. The TiO2 nanoparticle film showed higher reflectance, making it a potential candidate to replace the unstable Ag/ZnO back reflector.

Calculating electronic properties of the Si:SiO 2 interface using density functional theory with periodical boundary condition

Si:SiO2 core-shell nanoparticles are ideal for photovoltaic applications due to their stability and easily tunable optical properties. Investigations with ab initio methods have the potential to lead to a better understanding of the electronic properties of these materials. Using density functional theory, we calculated the density of states, absorption spectra, and partial charge densities of a model interface composed of a pure silicon portion sandwiched between SiO2 layers with a formula of Si264O160. Quantum confinement was observed in the pure Si portion, indicating that SiO2 serves as an insulating barrier to charge delocalization within the interface. This provides theoretical evidence that tuning the size of the nanoparticles and the thickness of the silicon oxide layer can affect the electronic properties.

Oxygen induced limitation on grain growth in RF sputtered Indium tin oxide thin films

Indium tin oxide (ITO) thin films were deposited onto glass substrates by RF magnetron sputtering to study variation of grain growth in pure argon and 99% argon plus 1% oxygen at different substrate temperatures. The average grain size increased with the increasing substrate temperature in pure argon. However, in oxygen presence environment the grain growth is limited at above 150°C. The films optoelectronic properties were evaluated. It was found that 200 nm ITO films prepared at 220 °C substrate temperature in pure argon possessed optimum sheet resistance of 10 Ω/sq. The transmittance of ITO films was enhanced with increasing the substrate temperature in pure argon but limited by the presence of excess oxygen.

Aqueous electrolyte system for porous silicon using electrochemical anodization

Electrochemical anodization on n-type silicon was performed with silicon as anode and platinum as cathode in a weak-acid aqueous electrolyte containing orthophosphoric acid and ammonium fluoride. Anodization was carried out in two different modes, i.e., potentiostatic and galvanostatic with the voltage and current density of 5-80 V and 10-50 mA/cm2 respectively. Anodization time was varied from 5 min to 10 hours. Morphology of the anodized samples was studied using scanning electron microscopy (SEM), revealing the formation of pores with uniform distribution throughout the silicon substrate. The pore size and density were controllable by varying the anodization voltage and current. Results showed that, in a broad spectrum range of 400-1100 nm, the total reflectance (sum of diffused and specular reflectance) of the porous silicon was about 10% compared to >30% of the as-received silicon wafer. The porous silicon is promising for solar cell applications due to the low reflection loss.

Quantitative evaluation of texture characteristics of solar cell back reflectors

A quantitative characterization technique was developed to study the surface morphology of solar cell back reflector. The texture characteristics of back reflectors have strong effects on solar cell efficiency. While average peak height and roughness are directly available from atomic force microscopy (AFM) analysis, several key parameters are missing. These parameters include average peak angle, peak angle distribution, and peak height distribution. This work demonstrates a numerical scheme to quantify these texture parameters. To do this, we developed a program that calculates the peak height and angle from the original AFM data. The output results establish quantitative relationship between the back reflector process parameters and the texture characteristics.

A numerical scheme to quantify the texture characteristics of sputtered aluminum thin film back reflectors

Texture angle and texture height are critical parameters that determine the performance of solar cell back reflectors. While average peak height and roughness are directly available from atomic force microscopy (AFM) analysis, several key parameters are missing. These parameters include average peak angle, peak angle distribution, and peak height distribution. In this work, a numerical scheme was developed to characterize the surface morphology of solar cell back reflectors. First part of this work demonstrated a numerical method to quantify the texture parameters by identifying relevant peak and valley points from 3D surface morphology data, such as AFM scan data. Peak angle and peak height filters were introduced into the numerical code to eliminate noises. In the second part, the program was utilized to systematically study the effects of sputtering deposition parameters on the morphology of thin film aluminum back reflectors. The processed data clearly indicated the existence of multiple factors that ...