Multinary Semiconductor Quantum Dots: Photophysical Properties and Photovoltaic Applications

Multinary quantum dots such as AgInS₂ and alloyed AgInS₂-ZnS are an emerging class of semiconductor materials being explored in numerous applications such as bioimaging, LEDs, catalysis, and solar cells. The ternary chalcogenide AgInS₂ forms a solid solution with ZnS which affects the properties greatly. The emission of AgInS₂-ZnS quantum dots can be tuned across the visible spectrum and into the infrared, with high photoluminescence quantum yields. These quantum dots can absorb much of the visible spectrum, making them potentially suitable for photovoltaic applications.

In this dissertation, the photophysical properties of AgInS₂-ZnS with varying cation ratios are investigated through the use of steady state and transient spectroscopy. These multinary chalcogenides demonstrate radiative and non-radiative mechanisms atypical of many semiconductors. In addition to band edge absorption and emission, AgInS₂-ZnS demonstrate defect-driven absorption and emission pathways which greatly influences their overall optical properties. These AgInS₂-ZnS quantum dots display wavelength-dependent photoluminescence decays, large stokes shifts, and long photoluminescence lifetimes, strongly suggesting that donor-acceptor pair recombination arising from intrabandgap defects accounts for the vast majority of emission. Both surface defects and intrinsic crystallographic defects, which form easily in these compounds, play a role in the optical properties and will be discussed in detail. Excited-state interactions between AgInS₂-ZnS quantum dots and TiO₂ are probed through time-resolved absorption and emission spectroscopy to elucidate the photoinduced electron transfer mechanism. Photovoltaic devices utilizing these quantum dots as light harvesters are fabricated and tested in order to assess their feasibility in a practical application.