Thin Film InN/Anatase Bilayers as a Substitute Dye/Semiconductor Interface for Solar Cells
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1102-LL04-02
Thin Film InN/Anatase Bilayers as a Substitute Dye/Semiconductor interface for Solar Cells Daniel Hoy, and Martin Kordesch Physics and Astronomy, Ohio University, Clippinger Labs RM 251B, Athens, OH, 45701 ABSTRACT The electronic properties of an InN/anatse-TiO2 bilayer, proposed as a replacement for the dye/semi-conductor interface in Dye Sensitized Solar Cell[1, 2], are measured. RF sputtered thin films of TiO2 and InN are used as the “dye” replacement. . Two types of InN film are prepared: polycrystalline samples deposited at high temperature, with an optical band gap of < 1 eV, and as-deposited (at least partially amorphous) samples with an optical band gap >1 eV. Energy Dispersive X-ray fluorescence, X-ray Diffraction, and Raman spectroscopy are used to characterize the samples. The sample resistance is measured in the dark and under illumination. The samples deposited at high temperature are crystalline and have a sheet resistivity ≈ 4 Ω/□, and display no photoconductivity. The partially amorphous samples have sheet resistivity of ≈ 500Ω/□. Since both types of InN films, including high quality (based on band gap) polycrystalline InN, do not show increased conductivity with light, we conclude that a solar cell based on an InN/TiO2 bilayer is not feasible. INTRODUCTION Experimental [1] and theoretical [2] literature suggest that the light harvesting component of the anatase based dye-sensitized solar cell (DSSC), typically a Ru-based dye molecule, could be replaced by a monolayer of InN. In [1], a “monolayer” of InN was deposited via chemical vapor deposition on a nanoparticulate anatase TiO2 substrate. The optical absorption of the sample was favorably compared with Grätzel’s “black dye”. Calculations [2] indicate that electrons from InN would be excited over a 1.7 eV band gap into the conduction band, from whence the electrons would be injected from the InN conduction band into the TiO2 conduction band via band gap overlap. This process of exciton creation and separation could form the basis of a modified DSSC. Wang and Lin [1] report that an InN monolayer on anatase TiO2 has a band gap energy that is different than both the literature values for the bulk and nanocrystalline band gap energies, implying that the band gap energy of InN could be modified by varying the thickness of InN deposited on the TiO2. Such tuneability could help optimize the efficiency of the DSSC. Also, by replacing the liquid dye/ TiO2 bond with a semi-conductor junction, the resistance of the sensitizer to oxidation by the electrolyte may be improved, allowing for greater cell stability. EXPERIMENTAL DETAILS All samples are deposited using radio frequency (RF) sputtering, using N2 gas at pressures around 12mTorr, 140 W. Some samples were heated (>400 ºC) with an incandescent lamp (120 W with reflector) during sputtering. Samples are deposited on glass, amorphous quartz, and ITO glass slides, depending on sample sputtering temperatures and conductivity measurements. Sputtered InN tends to take on two forms, depending on sam
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