Integration of optically random media into optoelectronic devices

Laburpena

The work presented in this thesis addresses the phenomena of light absorption and emission enhancement in optically random media and their implementation in optoelectronic devices, such as light-emitting devices (LEDs) and dye-sensitized solar cells (DSSCs). Whereas periodic structures have demonstrated and unprecedented control over light propagation, optically disordered structures presenting a random variation of the refractive index can also significantly alter light transport. In this work, the fabrication of a novel optically random medium coined Mie glass, consisting of a porous transparent matrix in the shape of a film integrating a random distribution of high refractive index scattering centres of controlled size and shape is proposed and realised. Specifically, three Mie glasses, which comprise a porous TiO2 matrix, a highly porous SiO2 matrix and a porous matrix of a luminescent material, integrating nanocrystalline monodisperse TiO2 spheres, for which intense scattering strength is expected, are fabricated and their optical properties characterised. An analysis of their optical response demonstrate that light propagation in them can be predicted by means of Mie formalism, that is, taking into account light scattering by an individual particle, thus proving their behaviour as a solid version of particle suspensions in a liquid medium typically employed in the field of random lasing. As a consequence, the Mie glass enables a design of their scattering properties prior to fabrication through the conditions of the included optical disorder, thus allowing tailoring their optical response. As a part of the fabrication process, a procedure for the preparation of films of controlled ultralow refractive index films and the preparation of luminescent nanophosphor-based films are addressed. Furthermore, a procedure for the fabrication of a self-standing flexible version of those Mie glasses based on high refractive index matrices is developed and described. After demonstrating a controlled enhancement of light absorption by the Mie glass based on a mesoporous TiO2 matrix, the material typically employed as photoanode in DSSCs, sensitised with an absorbing dye due to phenomena of light scattering, it is integrated as electrode into bifacial DSSCs. A full optoelectrical characterisation demonstrates an enhanced performance of these devices, which offer control through the size and concentration of the included scattering centres. This material also displays a controlled improvement of its photoluminescence when infiltrated with organic molecules of an emitting dye, thus proving its potential for colour conversion applications. Eventually, an analysis of the photoluminescence of the Mie glass based on a nanophosphor matrix demonstrates a tunable enhancement of their emission intensity as a consequence of a controlled process of improved out-coupling of the generated light from the material, which is proved valid for either rigid or flexible self-standing films. Such material presents the potential to be employed as a low-cost efficient colour conversion layer in LEDs. This thesis combines theoretical methods with experimental preparation and characterisation approaches enabling an in-depth understanding of light propagation in optically random media and the effect of their integration into bifacial DSSCs and implementation as colour converters for light-emission applications.