Quantum dot polymer composite materials for light management in optoelectronic devices

This dissertation will highlight a path to achieve high conversion efficiency of optoelectronic devices, including photovoltaic concentrators and LED display modules. Semiconductor nanocrystals, also known as quantum dots (QDs), serve as the pivotal luminescent materials in these devices. A quantum dots encapsulation method was developed here to homogeneously disperse QDs in a transparent polymer matrix, enabling high optical quality devices and thorough investigation of light material interactions. A luminescent solar concentrator (LSC) typically consists of a luminophore embedded in a polymer sheet with a high-performance solar cell attached at the side. In such a device, sunlight is absorbed in a luminophore, emitted into the waveguide modes of the polymer sheet, and directed to a photovoltaic cell where it is absorbed and converted to electricity. Since the area of the polymer sheet is greater than the area of the photovoltaic cell, concentration of the solar photon flux is achieved. Approaching high concentration ratio will require a luminophore with large Stokes Shift, high quantum yield, minimal overlap between absorption and emission, and a narrow emission spectrum. We have examined the performance of LSCs utilizing CdSe/CdS core-shell QDs, with significantly reduced absorption-emission overlap and long propagation distances in the waveguide. Furthermore, a distributed Bragg reflector dramatically mitigates the negative impact of scattering in the waveguide, allowing efficient photon collection and concentration ratio. White-light LED is achieved by using a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light. However, tradition yellow phosphors suffer from low color rendering index due to the broad emission spectrum of the phosphors. QDs have been proposed as better candidate than traditional yellow phosphors due to their narrow and tunable emission spectrum, and wide absorption spectrum in the UV-blue spectrum range. We have fabricated QD-polymer thin films as color conversion layers on blue LED via different methods, including spin-coating, drop casting and electrohydrodynamic jet printing. The polymer surface has been incorporated with nano-sized features to create photonic crystal structure. Up to 8 times QD excitation and emission enhancement have been demonstrated. We have also designed and fabricated QD-polymer based concentrating cavity on blue LED that acts both as color conversion layer and light concentrator. Distributed Bragg reflector and sputtered silver have been used as reflectors surrounding QD-polymer thin film. The exterior of the concentrator cavity was coated with black absorber to suppress light reflection, and a small aperture in the center allows concentrated photons to exit. High power conversion efficiency and high ambient contrast have been achieved in module devices