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46639-G10
Structure-Performance Correlations in Nanocolumn-Array Supported Nanoparticle Films for Solar Energy Conversions

Ming Su, University of Central Florida

Light deflection is an essential way to increase the optical paths of light for solar cells. Making ordered structures that can deflect incoming light in a controlled manner is important to enhance light absorption by functional materials. We have fabricated three-dimensionally ordered glass micro-mirrors by combining glass fiber drawing and selective chemical etching. The geometry of the glass spikes structures can be controlled by changing drawing or etching conditions: the cone angles of glass structures can be controlled over a wide range from 5 to 140 degree. The light deflection abilities of the structures are studied by measuring the transmission and reflection spectra: a normal incident light is scattered by micromirrors, the intensities of transmitted (parallel) and reflected (antiparallel) lights are measured. Deflection efficiencies are then derived from the transmitted and reflected lights. The deflection efficiencies of all the glass micro-mirrors depend on the cone angle and the etching conditions. The micromirrors with cone angle 60° deflect light strongly, which corresponds to an etching time of 120 min in an etchant that has 4% HF and 2.5% BOE. We have coated the micro-mirror array with a thin film of fluorine doped tin oxide by plasmon enhanced chemical vapor deposition. The composition and the structure of the thin film coated glass have been studied by SEM, X-ray diffraction (XRD) and energy dispersive X-ray diffraction (EDX). The transmission and reflection are 2% and 1% at 600 nm. The electric conductivities are measured by a sourcemeter and the current verse voltage curve of the film shows the conductivity of 160 Ω/cm². An undergraduate student (Michelle Garcia) has worked on the project by making glass mirrors and measuring the optical properties.
We have made dye sensitized solar cells on the tin oxide film by coating a thin film of titanium oxide nanoparticles (TiO₂). Briefly, a particle suspension is made by incrementally adding 20ml of water to 12g of colloidal TiO₂ powder (Degussa P25). The suspension is then coated on the tin oxide film by spin coating. The film is allowed to dry in air and then sintered in furnace at 450 ^oC for 0.5 hour. After slowly cooling down to room temperature, the sample is soak in 300 µM cis-bis(4,4-dicarboxy-2,2-bipyridine) dithiocyanato ruthenium(II) (N3 Dye) solution for 1 hour. The counter electrode is made by coating carbon thin film on a conductive indium tin oxide (ITO) coated flat glass. In order to assemble a solar cell, carbon-coated ITO glass is placed on the bottom and the carbon film faces up. The dye-stained TiO₂ is placed on the top and the dye-stained TiO₂ side faces the carbon film. The electrolyte is filled between two electrodes by capillary action. The measurement is done by using two digital multimeters under a 75W incandesce lamp at a distance of 10 cm. The current versus voltage curves of the device in the dark or under the light are collected. We have estimated the efficiency curve by changing the external load applied on the solar cell. The open circuit voltage (V_oc) is 55 mV, and the short circuit current density (J_sc) is 5µA/cm². Since much energy from the lamp is dissipated as heat energy, we do not derive the effective efficiency of the solar cell. We are now working with a scientist at Florida Solar Energy Center to measure the efficiency of the solar cells. In the past year, we have published a paper in Advanced Materials (in press) titled as “Engineering three dimensional micro-mirror arrays by fiber-drawing nanomanufacturing for solar energy conversion”.

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