Design and analysis of photonic structures for spectra dependent energy harvesting of photovoltaic devices

Abstract

In this thesis, we present a detailed study to design spectra selective photonic structures for harvesting energy efficiently from ambient light sources employing photovoltaic (PV) energy conversion technology. To choose the appropriate absorber material of the photonic structure, efficiency limit of single-junction PV devices for different absorber materials have been evaluated and optimum absorber bandgaps have been identified for natural and different artificial light sources. Due to widespread use of Red-Green-Blue (RGB) white LEDs, performance characteristics of ideal and practical PV devices have been studied in detail under white LED sources having a wide range of correlated color temperatures (CCTs) and fraction of blue in their corresponding spectrum and a relation has been established between maximum efficiency of the PV device and photometric features of the sources. Depending on bandgap of the absorber material, both positive and negative correlations are observed between photon conversion efficiency of PV devices and CCT values of the white LED sources. For material bandgaps of ~1.5eV or lower, higher photon conversion efficiencies are obtained for warm glow white LEDs. On the contrary, white LEDs characterized to emit cool light are found to be more conducive for PV devices having absorber layer bandgaps of ~2eV or higher. Upon selecting the absorber material we have designed the photonic structure having periodically arranged dielectric filled circular holes in the window layer of a practical PV device and evaluated optical absorption along with conversion efficiency of the device under solar irradiation and different white LEDs for a wide range of diameters and filling factors of the circular arrays. Based on the obtained results, a relation between optimum conversion efficiency of the PV device, filling factor of dielectric holes and photometric features of LED sources has been established. For absorber material bandgap of ~1.5eV, warm glow white LEDs outperform the cool light white LEDs and peak efficiency is obtained at relatively low filling factor (~40%) of dielectric holes. On the contrary, photonic structures with relatively high filling factor of holes (~70%) exhibit optimum conversion efficiency under white LEDs emitting cool light. Conversion efficiency of the photonic structures with filling factor in between 50% -70% is found to be optimum for operation under solar irradiation. These results in effect provide the necessary guidelines for designing homojunction, heterojunction or patterned PV devices suitable for operation under different light sources.