Light management in optoelectronic devices

Abstract

This thesis presents studies on light management in optoelectronic devices. The broad aim of the thesis is to improve the efficiency of optoelectronic devices by optimised light usage. The studies emphasise the design and fabrication of nanostructures for optimised photon control. A key hypothesis guiding the research is that better designs can be achieved by ab initio identification of their desired Fourier properties. The specific devices studied are organic Distributed Feedback (DFB) lasers, organic solar cells and silicon solar cells. The impact of a substructured grating design capable of affording unprecedented control over the balance between feedback and output coupling in DFB organic lasers was investigated both experimentally and theoretically. It was found experimentally that such gratings can halve the threshold of organic DFB lasers. The reduction in the laser threshold is associated with reduced output coupling and higher feedback provided by the substructured gratings. The possibility of improving the efficiency of organic solar cells by trapping light into the absorbing medium was investigated. It was found that the low refractive index of the organic gain medium compromises the light trapping performance. It was found that strong absorption enhancement, however, can be achieved using plasmonic nanostructures. Finally, a novel design concept for light trapping in silicon solar cells is proposed. This design takes advantage of grating structures with long periods that are capable of providing broad-band light trapping, which is an important requirement for silicon solar cells. The design is based on a supercell that enables better light injection through manipulation of the grating’s Fourier properties. The design idea leads to the formation of quasi-random nanostructures that afford great versatility for photon control. Strong light trapping was achieved and characterised both theoretically and experimentally.