Synthesis, Surface Functionalization of Germanium Nanoparticles, and Their Application in Polymer Hybrid Solar Cells

Abstract

Germanium nanoparticles (GeNPs) have immense potential in various applications, such as opto-electronics, batteries, bioimaging, etc. Germanium has a small band gap (0.67 eV in bulk), large exciton Bohr radius (~24 nm), high absorption coefficient (ca. 2.0 × 105 cm 1 at 2 eV), and high carrier transport (µe = 3900 cm2V-1S-1, µh = 1900 cm2V-1S-1). Despite the favourable opto-electronic properties, their potential cannot be utilized fully due to the lack of a robust synthetic method, complex surface chemistry, and non-uniform shape and size distribution. Significant research has been done over the past years; however, unlike silicon nanoparticles, the chemistries are often unpredictable. Till now, synthetic strategies could not achieve spherical GeNPs. There are examples of cubic and hexagonal shapes, but their distributions are not uniform. In addition to size and shape, surface chemistries of GeNPs also are not explored much. Hydrogermylation has been done on the surface of GeNPs, but there are other surface reactions that need to be explored, such as dehydrocoupling. In this thesis, dehydrocoupling surface reactions were studied. GeNPs also were applied as a dopant to enhance the efficiency of the polymer solar cells.The thesis starts with an introduction to germanium nanoparticles, their common synthetic procedures, surface passivation strategies, properties, and applications. Chapter 2 focuses on the synthesis and surface functionalization of GeNPs via dehydrocoupling. A series of reactions were performed with hydride-terminated GeNPs (H-GeNPs) and octadecylsilane to determine the optimum reaction conditions. Then, dimethyloctadecylsilane and hydride-terminated polydimethylsiloxane were reacted with H-GeNPs to passivate the surface of H-GeNPs. Hydrogermylation with 1-octadecene also was carried out in order to compare their reactivity. In Chapter 3, GeNPs were used as a dopant or efficiency enhancer in the active layer of polymer solar cells. Upon addition of small amounts of GeNPs, the efficiency of solar cells was improved. Chapter 4 focuses on the synthesis of GeNPs via induction heating, which allows for repeated fast heating and cooling cycles in order to obtain a better shape and size distribution of GeNPs. Finally, Chapter 5 summarizes the outcome of the experimental results and describes future directions.