Studies on Optically Induced DC-Voltage in Thin Film Structures

Abstract

Conversion of light into direct current is important for applications ranging from energy conversion to photodetection. However, there are still many challenges to reach high conversion efficiency and broad spectral coverage. Overall, photodetection through conventional procedures is based on light absorption by a material with a matching bandgap. This traditional approach limits the range of wavelengths that can be detected, it is not sensitive to polarization, and loses accuracy in the infrared range because of thermal noise. In order to design and optimize a photodetector that can overcome those limitations, I first built an instrument to evaluate the functionality of the proposed devices. Different devices and structures were fabricated and tested by means of photodetection in order to clearly identify the origin of the detected photovoltages. Further investigations on rectenna configurations and nonlinear optical rectification process showed promising results for photovoltage generation in a broadband scheme. A photodetector that combines polarization sensitivity and circularly polarized light sensitivity in the near infrared region was fabricated using an ITO-Au hybrid layer. Furthermore, the sensitivity of the device was significantly increased by adding a poled molecular-glass film in a capacitor configuration. The resulting device is capable of detecting light below the ITO-bandgap at ambient temperature without any bias voltage. It does not rely on the photoelectric effect, which is at the origin of the photovoltaic effect in semiconductor devices. It works based on hot electron emission in plasmonic nanostructures. This photodetector, which is amenable to large-area fabrication, can be integrated with other nanophotonic and nanoplasmonic structures for operation at telecom wavelengths. I then show how an array of aligned plasmonic nanorods covalently coupled to molecular rectifiers can also be used as optical nanoantennas to harvest the light and convert it into a DC-potential difference, which may be practical for energy production. I discuss the design, rectification processes, and propos two antenna fabrication procedures: electrochemical deposition and e-beam lithography.