Coating and Doping of Ge QDs
Oliva-Chatelain, Brittany Lynn
Barron, Andrew R
Doctor of Philosophy
The ability to incorporate a dopant element into nanocrystals (NCs) and quantum dots (QDs) is one of the key technical challenges for the use of these materials in a number of optoelectronic applications, particularly solar applications. Unlike doping of traditional bulk semiconductors materials, the location of the doping element can be either within the crystal lattice (c-doping), on the surface (s-doping), or within the surrounding matrix (m-doping). A range of attempts to dope Ge QDs both during and post-synthesis are reported here. The QDs have been characterized by TEM, XPS, and I/V measurements of SiO2 coated QD thin films in test cells using doped Si substrates. The solution synthesis of Ge QDs by the reduction of GeCl4 with LiAlH4 results in Ge QDs with a low level of chlorine atoms on the surface; however, during the H2PtCl6 catalyzed alkylation of the surface with allylamine, chlorine functionalization of the surface occurs resulting in p-type doping of the QD. A similar location of the dopant is proposed for phosphorus when incorporated be the addition of PCl3 during QD synthesis; however, the electronic doping effect is greater. The detected dopants are all present on the surface of the QD (s-type), suggesting a self-purification process is operative. Attempts to incorporate boron or gallium during synthesis were unsuccessful. The silica coating of these particles was successful using a modified Stöber method. Monodispersed silica nanoparticles 20 nm in diameter were synthesized with Ge QDs as seeds. The resulting structures comprise of Ge QD core within a silica sphere. Films of these particles result in an average QD…QD distance of 9.6 nm, which is less than the maximum distance required for good electron transfer (10 nm). Film thickness and annealing tests were done to optimize the cells. These cells were tested for efficiency, and it was found that the phosphorus doped quantum dots and the undoped quantum dots both produced the highest photo induced current on n-type silicon wafers at ¼ of the maximum concentration of these particles with the phosphorus doped quantum dots producing a higher efficiency overall. Thermal annealing the films prior to deposition of the front and back contacts enabled a doubling in the cell efficiency, but did not show any marked increase in the density or crystallinity of the films.
quantum dots; solar cells; silica coating; thin film; doping