Light trapping in thin film solar cells using photonic engineering device concepts

In this era of uncertainty concerning future energy solutions, strong reservations have arisen over the continued use and pursuit of fossil fuels and other conventional sources of energy. Moreover, there is currently a strong and global push for the implementation of stringent measures, in order to reduce the amount of green house gases emitted by every nation. As a consequence, there has emerged a sudden and frantic rush for new renewable energy solutions. In this world of renewable energy technologies is where we find photovoltaic (PV) technology today. However, as is, there are still many issues that need to be addressed before solar energy technologies become economically viable and available to all people, in every part of the world.;This renewed interest in the development of solar electricity, has led to the advancement of new avenues that address the issues of cost and efficiency associated with PV. To this end, one of the prominent approaches being explored is thin film solar cell (TFSC) technology, which offers prospects of lower material costs and enables larger units of manufacture than conventional wafer based technology. However, TFSC technologies suffer from one major problem; they have lower efficiencies than conventional wafer based solar cell technologies. This lesser efficiency is based on a number of reasons, one of which is that with less material, there is less volume for the absorption of incident photons. This shortcoming leads to the need for optical light trapping; which is concerned with admitting the maximum amount of light into the solar cell and keeping the light within the structure for as long as possible.;In this thesis, I present the fundamental scientific ideas, practice and methodology behind the application of photonic engineering device concepts to increase the light trapping capacity of thin film solar cells. In the introductory chapters, I develop the basic ideas behind light trapping in a sequential manner, where the effects of the inclusion of various structures on the front and back surfaces of solar cells are examined. This framework is then adapted as a basis for the development of more advanced topics, such as the inclusion of micro and nano scale surface textures, diffraction gratings and photonic bandgap structures. Analyses of the effects of these light trapping structures is undertaken using performance metrics, such as the short circuit current characteristics and a band-edge enhancement factor, which all serve to quantitatively demonstrate the effects of the optical enhancements.;I begin this thesis with an investigation of one dimensional photonic crystals, which are used as selective light filters between vertically stacked tandem multi-junction solar cells. These ideas are then further developed for single junction stand alone thin film solar cells, where the optical enhancement is shown to be very significant. A further investigation on the application of engineered photonic crystal materials as angular selective light filters is then presented; these filters are shown to overcome the physical limitations of light trapping that are imposed by the optical properties of materials; specifically limitations associated with total internal reflection.;In the next part of this thesis, I present a fundamental redesign approach to multiple period distributed Bragg reflectors (DBR's) and their applications to solar cell light trapping. As it turns out, multiple period DBR's, which are required for high back surface reflectance -- which is especially necessary in thin film solar cells -- present formidable challenges in terms of cost and complexity when considered for high volume manufacturing. To this end, I show that when a single period DBR is combined with a phase matching and metallic layer, the combined structure can achieve high back surface reflectance that is comparable to that of a DBR structure with many more layers. This new structure reduces the back reflector complexity and is hence, amenable to large scale fabrication processes.;In the latter sections of this thesis, I present a host of fabrication techniques that are used to realize micro and nano scale light trapping features. These techniques range from standard silicon wet etching processes, to customized and elaborate deep ultra-violet lithography, which is combined with inductively coupled plasma etching and used in order to realize sub-micron diffraction gratings. These textures are then applied to substrates on which thin film amorphous silicon solar cell structures are deposited, subsequent analyses on the effectiveness of these texturing processes is performed. Finally, this thesis concludes with the presentation of a blueprint for future explorations and applications of the developed light trapping techniques, to other thin film solar cell materials and technologies.