Friday 30 September
13:15 - 16:15
Silicon (Si) serves as the basic material of the system-on-a-chip industry and photovoltaic panels nowadays. This is mostly thanks to its high abundance in the earth’s crust, thereby low cost, virtually non-toxicity, and superior stability. Nano-silicon, especially silicon quantum dots (Si QDs), is endowed by the quantum confinement effect with the ability to emit light efficiently under photoexcitation, different from the bulk counterpart. The bright photoluminescence (PL), first found in the 1990s, has paved the way for this nanomaterial to be applied for light conversions in the last decades, such as for biosensing/biolabeling, light emitting diodes and luminescent solar concentrators (LSCs). The latter is used to concentrate sunlight in the slab on the edge-attached solar cells by means of PL. This thesis, on the one hand, deepens the comprehension on the optical properties of Si QDs by single-dot spectroscopy; on the other hand, a low-cost mass synthesis of high-quality Si QDs is developed here, which favors high QD loading applications, demonstrated as large-area “quantum dot glass”.
First, the photo-physics mechanism behind PL was studied by single-dot spectroscopy, excluding the QD size inhomogeneity in the ensemble measurements. A new method was developed to fabricate large-area (~mm2) isolated oxide-passivated Si QDs on a silicon-on-insulator wafer. Linearly polarized PLs were observed on those single dots. System-limited PL linewidths, ~250 μeV, were measured at 10 K on QDs here, indicating a good quality of oxide shell endowed by high temperature annealing. Based on this method, it is possible to modify the ambient optical environment of QDs without tenuous alignments. With Si QDs residing on a metal membrane with an oxide spacer, the PL yields of single dots were enhanced ~10 times in average compared to those residing outside the membrane. Next, we have achieved, for the first time, direct observation on the temperature-dependent radiative lifetimes on single ligand-passivated Si QDs. Most importantly, these single-dot PL decays can be well-fitted mono-exponentially, indicating trap-free dynamics, as opposite to oxide-passivated counterparts.
Secondly, a chemical synthesis method of ligand-passivated Si QDs by using triethoxysilane (TES) as precursors is introduced. The quantum yield of as-synthesized Si QDs is ~40% in solution and ~55% in Si QDs/polymer nanocomposites. Such QDs have near-unity internal quantum efficiency both in the liquid and solid phase. With a comparably good quality of Si QDs, the QD cost of this TES method is about an order of magnitude less expensive than that of the established HSQ method.
Finally, the application of Si QDs in photovoltaic devices was demonstrated. A 9 × 9 × 0.6 cm3 LSC device based on Si QDs was fabricated, delivering ~7.9% optical power conversion efficiency under one standard sun. This performance is very similar to the state of the art of direct-bandgap semiconductor QDs. To further expand the application area of this kind of transparent photovoltaic devices, a concept of transparent “quantum dot glass” (TQDG) is introduced, fulfilling requirements as both power-generating components and building construction materials. A 20 × 20 × 1 cm3 TQDG device was fabricated with the overall power conversion efficiency up to 1.57% and the average visible transmittance 84%. The light utilization efficiency (LUE) is 1.3%, which is among the top reported TPVs based on the LSC technology with a similar size. Moreover, to facilitate the characterization of large-area LSC-like light-harvesting devices a new concept of an “optical center” is introduced. A procedure of whole device PCE estimates from optical center excitation measurements with basic laboratory instruments was provided, with a negligible error to the measured one by the conventional method.