Paul Stavrinou

Abstract:

Non-metallic mirror, such as semiconductor distributed Bragg reflectors (DBRs), has been widely integrated in the structure of optoelectronic devices. However, constructing conductive DBR in organic optoelectronic device is still scarce, because of the incompatibility of high-temperature processes in the preparation of inorganic DBR. Herein, it is confirmed that organic-oxide hybrid DBR can achieve high conductivity and light manipulation. When thermal evaporated material MoO3 is doped into organic material (1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane, TAPC), the conductivity of TAPC can be increased by ten thousand times with very small refractive index change. It is shown that 8.5 pairs DBR at 460 nm has a reflectivity of about 95%, and the driving voltage is 8.2 V at the current density of 100 mA cm-2. Then, a transparent organic light-emitting diode with integrated bottom conductive DBR are fabricated to confirm the functionality of light regulation. Our results confirm that integrated optoelectronic devices with DBR as reflector can be achieved with low operating voltage.

Abstract:

Exciton behavior is crucial for improving the optoelectronic property of a light-emitting conjugated polymer. Herein, the photoexcitation dynamics of exciton migration and energy transfer in a self-doped β-conformation film of the polydiarylfluorenes (poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9- diphenylfluoren-2,7-diyl], PODPF) are demonstrated. Compared to the first generation of the β-conformation polyfluorene, poly(9,9-dioctylfluorene) (PFO), energy transfer occurs in PODPF β-conformation films in a time period of ≈150 ps, much longer than those of the PFO ones (<5 ps), associated with the effective intrachain energy transfer (few hundred picoseconds), rather than interchain Förster energy transfer (a few picoseconds). Similar to PFO, the PODPF β-conformation also displays well-resolved vibronic emission peaks at 20 K, attributed to the planar and rigid conformation. Interestingly, a residual 0-0 band emission of nonplanar conformation chain segments (435 nm, 2.85 eV) at 20 K also further confirms the exciton migration from the amorphous state to the β-conformation domain in PODPF films. Therefore, the stable dual amplified spontaneous emission (ASE) behavior of the PODPF self-doped films at 461 nm (2.69 eV) and 483 nm (2.57 eV), originates from the individual amorphous and β-conformation domains.

Abstract:

We demonstrate proof-of-concept refractive-index structures with large refractive-index-gradient profiles, using a micro-contact photothermal annealing (μCPA) process to pattern organic/inorganic hybrid materials comprising titanium oxide hydrate within a poly(vinyl alcohol) binder. A significant refractive index modulation of up to Δ<i>n</i> ≈ +0.05 can be achieved with μCPA within less than a second of pulsed lamp exposure, which promises the potential for a high throughput fabrication process of photonic structures with a polymer-based system.

Abstract:

Solution-processed metal halide perovskite light-emitting diodes (Pe-LEDs) show great promise in a range of optoelectronic applications. However, these devices can be limited by poor interfaces to the perovskite films due to poor crystallization control during film processing caused by de-wetting from the bottom layer. The deep (Highest Occupied Molecular Orbital) HOMO levels of the emitters also lead to large charge injection barriers for standard electrodes. To improve this, we develop and report on a small molecule, BPS2, based on phenothiazine-benzimidazole with Lewis base sites. This promising interfacial material is then applied to blue PeLEDs where the energy band alignment of BPS2 to the blue perovskite emitter helps to reduce the hole-injection barrier while blocking electrons. BPS2 can be solution-processed with non-chlorinated organic solvents and provides improved wettability towards perovskite precursor solutions compared to conventional PEDOT:PSS hole transport films. A thin interlayer of BPS2 introduced between PEDOT:PSS and a perovskite emission layer is shown to improve both the device external quantum efficiency and luminance in comparison to the reference device without the interlayer.

Abstract:

The vivid iridescent response from particular butterflies is as an excellent example of how micro-engineered hierarchical architectures that combine physical structures and pigmentary inclusions create unique colouration. To date, however, detailed knowledge is missing to replicate such sophisticated structures in a robust, reliable manner. Here, we deliver spheres-in-grating assemblies with colouration effects as found in nature, exploiting embossed polymer gratings and self-assembled light-absorbing micro-spheres.