UCLA

According to recent research reports, the global market for display and lighting products is expected to grow to 169.17 billion USD by the year 2022. Display technologies include the light emitting diode (LED), liquid crystal display (LCD), organic light-emitting diode (OLED), quantum dot light emitting diode (QDLED), and electronic paper display (EPD), whereas LEDs have been the dominant lighting solution in the past two decades due to their long lifetime and use of non-hazardous materials. However, the growth in revenue for LED technology is predicted to slow down due to the advancement in OLED technology. OLEDs are known to be more energy efficient, have higher picture quality, and are more lightweight than LEDs. The bottleneck for OLEDs is their high production cost, which restrains it from the mass market. Fortunately, the development of conjugated polymer emitters shifts the manufacturing process from vacuum thermal evaporation to a solution processed methodology (i.e. ink-jet printing, roll-to-roll printing, screen printing, etc.) which is much more cost-effective. The resulting OLED, also referred to as a polymer light emitting diode (PLED), shows enormous potential in the future of display technologies. Alongside the PLED, a new class of solution-processable emitters, the organometal halide perovskite, has also shown promising performance in lighting applications.

Despite their advantages, solution-processed emitters have also shown lower device performance when compared to thermal vapor evaporated OLEDs. Due to the limitation of solution-processed materials to withstand the solvent attack from advancing layers, solution-processed light emitting devices usually have overly simple structures; this results in poor device performance, which can be attributed to the imbalance of charge injection. This dissertation focuses on improving the efficiency and stability of light emitting devices based on conjugated polymer emitters and organometal halide perovskite emitters by employing novel device architectures. In addition, based on the beneficial characteristics of the perovskite and polymer blend film, multifunctional devices including light emitting touch-responsive devices and stretchable perovskite light emitting diodes are also demonstrated here.

To begin, a white conjugated polymer emitter was applied on a flexible silver nanowire (AgNW)-nanoparticle composite electrode, which resulted in a high out-coupling efficiency with a 95% enhancement. The flexible nanocomposite electrode was made with silver nanowire electrode and barium strontium titanate (BST) nanoparticles. The high out-coupling efficiency is achieved by replacing the ITO electrode with AgNW and by incorporating BST nanoparticles. Replacement of the ITO avoids light trapping inside the ITO caused by the mismatch of the refractive index, and the BST nanoparticles serve as the light scattering center that changed the path of the trapped light inside the substrate. The use of white conjugated polymer emitters on the light extraction substrate demonstrates a means to fabricate high-efficiency flexible PLEDs with low fabrication cost. The white PLED achieved an external quantum efficiency of 27.3%.

The white conjugated polymer emitter was also used in a new type of optoelectronic device, the organic light emitting transistor (OLET). The white OLET combines both phosphorescent PLED and a transistor with a highly capacitive electrolyte dielectric material. By varying the gate potential, the luminance of the white OLET could be modified from 0.1 to 10,000 cd/m2. This work reveals the possibility of achieving a cost-effective active matrix display based on solution processed materials and a simple device structure.

Next, the organometal halide perovskite emitter is introduced. In one application, it was used to demonstrate a light emitting touch-responsive device (LETD) which could provide instantaneous visualization of pressure mapping. The new LETD incorporates perovskite-polymer composite film as the emitter and a flexible AgNW-polyurethane as the top electrode. The perovskite LETD emitted an intense green luminescence when a local pressure was applied on the AgNW-polyurethane due to the formation of a Schottky contact between the AgNW and the perovskite emissive layer. The fabrication of the perovskite LETD was simple and based on an all-solution processed procedure.

I also used the perovskite emitter to achieve a stretchable perovskite light emitting diode (PeLED). The PeLED integrates an intrinsically stretchable perovskite emissive layer and a stretchable AgNW-polyurethane composite electrode. The stretchable perovskite emissive layer incorporates polyethylene glycol diacrylate (PEG), siliconized acrylate (PDMS) and methylammonium lead bromide. The PEG polymer helps to control the perovskite grain size and achieve a uniform perovskite grain film, while the PDMS polymer serves as the elastic connection that holds the film together and provides stretchability to the film. The composite electrode, AgNW-polyurethane, provides high transparency, high conductivity, and high stretchability. With these elements combined, the PeLED could be stretched up to 60% strain while displaying a green electroluminescence.

Both conjugated polymer emitters and organometal halide perovskite emitters have been successfully demonstrated in various light applications as a proof of concept. A highly efficient PLED was achieved by employing nanoparticles in the substrate to scatter the trapped light into the air. In addition, by adjusting the three-terminal system to a suitable bias condition, the device luminance increased drastically due to the balance of electrons and holes in the OLET device. A robust and stable perovskite emissive layer was achieved by incorporating a PEO polymer, and an LETD was created that enables instantaneous pressure mapping. By adding an elastic polymer into the perovskite, a stretchable PeLED was also demonstrated. The results verify the potential of both conjugated polymer emitters and perovskite emitters for large-scale, low-cost lighting applications and for commercial use.