Extending the Potential of Thin-film Optoelectronics via Optical Engineering

Optoelectronics based on nanomaterials have become a research focus in recent years, and this technology bridges the fields of solid state physics, electrical engineering and materials science. The rapid development in optoelectronic devices in the last century has both benefited from and spurred advancements in the science and engineering of pho- ton detection and manipulation, image sensing, high-efficiency and high-power-density light emission, displays, communications and renewable energy harvesting. A particu- larly promising material class for optoelectronics is colloidal nanomaterials, due to their functionality, cost -efficiency and even new physics, thanks to their exotic properties in the areas of light-matter interaction, low-dimensionality, and solution-processability which dramatically reduces the time and cost required to fabricate thin film devices, and at the same time provides wide compatibility with existing materials interfaces and device structures. This thesis focuses on exploring and assessing the capabilities of lead sulfide quantum dot-based solar cells and photodetectors. The discussion involves advances in techniques such as implementing novel photonic structures, designing and building novel characterization systems and methods, and coupling to external optical structures and components. This thesis comprises three sections. The first section focuses on the design and adap- tion of photonic structures to tailor the function and response of photovoltaics and other absorption-based optoelectronics for specific applications. in the first part, we introduce consideration of complete multi-layer thin film interference effects into the design of so- ii lar cells. By numerical calculation and optimization of the film thicknesses as well as the precise fabrication control, devices with specific target colors or optical transparency levels were achieved. In the second part, we investigate the presence of 2D photonic crystal bands in absorbing materials that can be readily incorporated into nanomaterial thin films through nanostructuring of the material. We carried out simulations and the- oretical analyses and proposed a method to realize simultaneous selectivity in the device reflection, transmission and absorption spectra that are critical for optoelectronic appli- cations. The next section focuses on designing and building a multi-modal microscopy sys- tem for thin-film optoelectronic devices, accompanied with analyses and explanation of complex experimental data. The goal of the system was to provide simultaneous 2D spatial measurements of, including but not limited to, photoluminescence spectra, time- resolved photocurrent and photovoltage responses, and a rich variety of all the possible combinations of these measurements and their associated derived quantities, collected with micrometer resolution. The multi-dimensional data helped us understand the in- tercorrelation between local defective regions in films and the entire device behavior, as well as a more comprehensive profile of mutual relationships between solar cell figures of merit. In the last section, we discuss a new implementation of miniature solar concentrator arrays for lead sulfide quantum dot solar cells. First, we design and analyze the effects of a medium concentration ratio lens-type concentrator made from polydimethylsiloxane, a flexible organosilicon polymer. The concentrators were designed and optimized with the aid of ray-tracing simulation tools to achieve the best compatibility with colloidal nanomaterial-based solar cells. Experimentally, we produced an integrated concentrator system delivering 20-fold current and power enhancements close to the theoretical pre- dictions , and also used our concentrator measurements to explain the rarely-explored carrier dynamics critical to high-power operation of thin film solar cells. Next, we de- iii sign a wide-acceptance-angle dielectric solar concentrator that can be adapted to many types of high- efficiency small-area solar cells. The design was generated using rigorous optical models that define the behaviors of light rays, and was verified with ray-tracing optical simulations to yield results for the full annual 2D time-resolved collectible power for the resulting system. Finally, we discuss strategies for further extending the possi- bilities of nanomaterial-based optoelectronics for future challenges in energy production and related applications