Donal Bradley

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:

Solution-processed organic field-effect transistors (OFETs) have attracted great interest due to their potential as logic devices for bendable and flexible electronics. In relation to n-channel structures, soluble fullerene semiconductors have been widely studied. However, they have not yet met the essential requirements for commercialization, primarily because of low charge carrier mobility, immature large-scale fabrication processes, and insufficient long-term operational stability. Interfacial engineering of the carrier-injecting source/drain (S/D) electrodes has been proposed as an effective approach to improve charge injection, leading also to overall improved device characteristics. Here, it is demonstrated that a non-conjugated neutral dipolar polymer, poly(2-ethyl-2-oxazoline) (PEOz), formed as a nanodot structure on the S/D electrodes, enhances electron mobility in n-channel OFETs using a range of soluble fullerenes. Overall performance is especially notable for (C60-Ih)[5,6]fullerene (C60) and (C70-D5h(6))[5,6]fullerene (C70) blend films, with an increase from 0.1 to 2.1 cm2 V−1 s−1. The high relative mobility and eighteen-fold improvement are attributed not only to the anticipated reduction in S/D electrode work function but also to the beneficial effects of PEOz on the formation of a face-centered-cubic C60:C70 co-crystal structure within the blend films.

Abstract:

The β-phase, in which the intermonomer torsion angle of a fraction of chain segments approaches ∼180°, is an intriguing conformational microstructure of the widely studied light-emitting polymer poly(9,9-dioctylfluorene) (PFO). Its generation can in turn be used to significantly improve the performance of PFO emission-layer-based light-emitting diodes (LEDs). Here, we report the generation of β-phase chain segments in a copolymer, 90F8:10BT, containing 90% 9,9-dioctylfluorene (F8) and 10% 2,1,3-benzothiadiazole (BT) units and show that significant improvements in performance also ensue for LEDs with β-phase 90F8:10BT emission layers, generalizing the earlier PFO results. The β-phase was induced by both solvent vapor annealing and dipping copolymer thin films into a solvent/nonsolvent mixture. Subsequent absorption spectra show the characteristic fluorene β-phase peak at ∼435 nm, but luminescence spectra (∼530 nm peak) and quantum yields barely change, with the emission arising following efficient energy transfer to the lowest-lying excited states localized in the vicinity of the BT units. For ∼5% β-phase chain segment fraction relative to 0% β-phase, the LED luminance at 10 V increased by ∼25% to 5940 cd m^-2, the maximum external quantum efficiency by ∼61 to 1.91%, and the operational stability from 64% luminance retention after 20 h of operation to 90%. Detailed studies addressing the underlying device physics identify a reduced hole injection barrier, higher hole mobility, correspondingly more balanced electron and hole charge transport, and decreased carrier trapping as the dominant factors. These results confirm the effectiveness of chain conformation control for fluorene-based homo- and copolymer device optimization.