Experimental Studies and Numerical Simulations on Light-Harvesting Devices.

Abstract

Obtaining high solar energy conversion efficiencies with materials that require minimal processing or refining is critical to next generation light-harvesting systems. Organic dyes and inorganic nanostructured semiconductors are two material types that address this need and are studied herein.

Two sets of organic chromophore systems were characterized. First, triarylamine multi-chromophore dendrimers with purposely designed biphenyl-based trap sites were investigated using fluorescence upconversion spectroscopy. A rise in the fluorescence from the biphenyl site after the excitation pulse demonstrated that excitons were trapped with 99% efficiency. These data show that excitons can be directed to a specific site in a molecular chromophore. Separately, thiophene macrocycles were investigated to determine if molecular systems could show high energetic degeneracy. The chromophore coupling constants of two thiophene rings were quantified using time-resolved fluorescence anisotropy measurements. The calculated chromophore coupling constants for the cyclic system were an order of magnitude higher than linear chains. In addition, the cyclic system had a two photon absorption cross section of 1470 GM, which is over a thousand times greater than the linear chain and useful for applications in imaging and lithography.

Nanostructured inorganic semiconductors were also the subject of study. In one set of experiments, the first example of macroporous p-GaP(100) was reported and its ability to perform photosynthetic water splitting was demonstrated and assessed. Macroporous films were prepared using a two-electrode cell with a halogen acid electrolyte and pulsed anodic etching voltage waveform. Control over the macroporous film morphology was explored by varying halogen acid type, concentration, and etching voltage. Macroporous p-GaP has applications in photonic and light-harvesting systems. To this end, the relationship between optoelectronic properties and the obtainable solar energy conversion efficiency was determined in nanostructured semiconductors. The photocurrent-potential response of lightly and heavily doped silicon nanowires were quantified, with the heavily doped semiconductors demonstrating superior energy conversion. For low dopant density nanowires, the low energy conversion efficiencies were attributed to a lack of an internal electric field, which resulted in a high majority carrier recombination at the interface. These data provide design principles for efficient solar energy conversion systems based on nanostructured semiconductors.