Gold Nanolayers Embedded in Zinc Oxide for Large Area Flexible Photovoltaics

Abstract

ABSTRACTTransparent conducting ZnO/Au/ZnO thin film structures were grown by the magnetron sputtering technique on flexible polymer substrates. These films displayed a seven orders of magnitude drop in resistivity (200 to 5.2×10-5 Ω-cm) upon increase of the Au layer thickness from 0 nm to 12 nm. The sheet resistance also showed a substantial decrease to a value of 6.5 _/sq. These films displayed a photopically average transmittance between 75% and 85% depending upon the gold thickness, and a peak transmittance of up to 93%. The best Haacke figure of merit was 15.1×10-3 Ω−1. As the Au layer thickness was increased, the conduction changed from conduction through the substrate when the nanometal islands are small and far apart to activated tunneling between discontinuous islands, and finally to direct tunneling between larger islands and metallic conduction through a near-continuous layer. Optical transmission behavior of the films was described in terms of the Au’s absorption due to interband electronic transitions in the shorter visible wavelengths, and free carrier absorption losses at the longer red wavelengths. This was combined with the limitation of the mean free path in discontinuous films and the size-dependent dielectric constant of the Au particles that enhances absorption in the longer visible wavelengths.