Nanophotonic design and nanomaterial assembly for next-generation optoelectronics

Nanomaterials are widely deployed in many optoelectronic technologies, with applications in solar energy harvesting, light emission, bio-sensing, computing and communications. The unique advantages of colloidal nanomaterials include size-tunable optical properties and room-temperature solution-processability, which translates to low-cost materials growth and fabrication processes associated with nanomaterials-based technology. Moreover, their lightweight and thin-film nature enables optoelectronic devices made from nanomaterials to be flexibly coated on almost any surface, which is ideal for applications such as wearable electronics and building-integrated photovoltaics. This thesis focuses on combining optical modeling, nanomaterials synthesis, nanofabrication, and advanced optical and electrical characterization techniques to develop nanomaterial-based next-generation optoelectronic devices. The first section of this thesis focuses on applying nanophotonics design principles to optically engineer solar cell and photodetector device structures for specific applications. One of our studies demonstrated a high-performing visible-blind ultraviolet (UV) thin film photodetector by introducing nanoheterojunctions for enhanced absorption and carrier injection. In another study, we used optical simulations and an effective medium model to investigate and predict light-trapping enhancements by embedding plasmonic nano-inclusions in the absorbing layer of solution-processed solar cells. We also combined thin-film interference engineering and multi-objective optimization algorithms to control the color and transparency of colloidal quantum dot (CQD) solar cells for applications in building-integrated photovoltaics and multi-junction photovoltaics. In the final study of this section, we proposed and investigated engineering photonic bands in strongly absorbing materials to tune the spectral selectivity of optoelectronic films. We then focus on developing lead sulfide CQD-based light emitting diodes (QLEDs) and solar cells with novel functionality. We developed a room-temperature-processed silver-nanowire-based transparent electrode for flexible optoelectronics. With carefully-tuned nanomaterials synthesis conditions, we fabricated PbS QLEDs with near-infrared emission that can be easily detected by inexpensive silicon-based photodetectors, paving the way for our proposed flexible transparent light emitting membrane technology, which has many target applications including in next-generation virtual reality googles and motion-capture suits for the film industry. We also built a semi-automated spray-casting system to demonstrate an all-solution-processed CQD solar cell, as a scalable and portable method for manufacturing CQD solar cells, expanding the application areas of this technology