Title

Engineering of Semiconductor Nanocomposites for Harvesting and Routing of Optical Energy

Date of Award

2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Mikhail Zamkov

Second Advisor

George Bullerjahn (Committee Member)

Third Advisor

Peter Lu (Committee Member)

Fourth Advisor

Ksenija Glusac (Committee Member)

Abstract

The increasing demand for renewable energy sources requires a significant effort to be invested in the development of inexpensive and efficient light-harvesting materials. It is estimated that the usable portion of the solar energy is an order of magnitude greater than projected energy needs of the entire world in upcoming decades and most of the solar power is delivered at photon energies that are capable of driving photocatalytic hydrogen production or the reduction of carbon dioxide. Our ability to harvest these resources comes down to a number of technological challenges that are largely synthetic in nature. Among those, the design of materials that efficiently absorb solar light and convert its energy into long-lived charge separated states is of the key importance.

We addressed two crucial technology issues concerning composite nanomaterials application for photovoltaics (solar cells) and photocatalytic systems (water splitting):

(i) engineering of active-layer materials for hydrogen production systems and (ii) development of a novel strategy for processing colloidal nanoparticles into highly photoconductive films for low-temperature production of high-efficiency solar cells.

We investigated the effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals. By tuning the energy alignment at the semiconductor-ligand interface, we demonstrate that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play a crucial role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation.

Assembly of thin film devices from semiconductor nanocrystal (NC) "inks" has emerged as a cost-effective approach for the development of next generation light-harvesting materials with the potential to create a considerable technological impact on the solar cell industry. We explored a novel strategy for low-temperature (<150 °C) assembly of colloidal semiconductor nanocrystals into solid films, which resulted in the development of solar cells, processable entirely from solution. Specifically, the main accomplishments of this work included:

(i) hot-injection synthesis and assembly of copper zinc tin sulfide (CZTS) semiconductor NCs into solid films through spin-coating with consecutive ligand exchange, (ii) optimization of the film morphology towards improving the carrier mobility, and (iii) fabrication, characterization, and evolution of prototype solar cells comprising the developed morphology.

Improving the efficiency of prototype solar cells was achieved through material optimization (Cu2ZnSnS4 synthesis modification), adjustment of the thickness of functional solar cell layers, and study of ligand exchange and exhalation process. Light harvesting performance of fabricated cells comprising conductive transparent electrodes, CZTS/CdS film, and gold/palladium top contacts was routinely tested by measuring power conversion efficiency (PCE). We have also analyzed the oxidation stability of fabricated devices.