Guided optical modes in randomly textured ZnO thin films imaged by near-field scanning optical microscopy

摘要

We report on near-field optical imaging of light transmission through textured zinc oxide (ZnO) thin films grown on glass substrates. Such ZnO layers are commonly used as a transparent conducting oxide for the front contacts in thin-film solar cells. To increase the quantum efficiency of such solar cells, the ZnO surface is randomly textured, resulting in a higher light scattering and promoting light trapping. Here, we study these phenomena microscopically by imaging the local optical mode profiles in such thin films. Our results give clear evidence for the interconversion of propagating and guided evanescent modes. Such information, which cannot be extracted from more conventional far-field optical studies, may prove helpful in further optimizing the efficiencies of thin-film optoelectronic devices.