Donal Bradley

Abstract:

Control is exerted over the exciton-polariton physics in metal-poly(9,9-dioctylfluorene)-metal microcavities via conformational changes to the polymer backbone. Using thin-film samples containing increasing fractions of β -phase chain segments, a systematic study is reported for the mode characteristics and resulting light emission properties of cavities containing two distinct exciton subpopulations within the same semiconductor. Ultrastrong coupling for disordered glassy-phase excitons is observed from angle-resolved reflectivity measurements, with Rabi splitting energies in excess of 1.05 eV (more than 30 % of the exciton transition energy) for both TE- and TM-polarized light. A splitting of the lower polariton branch is then induced via introduction of β -phase excitons and increases with their growing fraction. In all cases, the photoluminescence emanates from the lowermost polariton branch, allowing conformational control to be exerted over the emission energy and its angular variation. Dispersion-free cavities with highly saturated blue-violet emission are thus enabled. Experimental results are discussed in terms of the full Hopfield Hamiltonian generalized to the case of two exciton oscillators. The importance of taking account of the molecular characteristics of the semiconductor for an accurate description of its strong coupling behavior is directly considered, in specific relation to the role of the vibronic structure.

Abstract:

Controlling chain behavior through smart molecular design provides the potential to develop ultrastable and efficient deep-blue light-emitting conjugated polymers (LCPs). Herein, a novel supramolecular self-encapsulation strategy is proposed to construct a robust ultrastable conjugated polydiarylfluorene (PHDPF-Cz) via precisely preventing excitons from interchain cross-transfer/coupling and contamination from external trace H2 O/O2 . PHDPF-Cz consists of a mainchain backbone where the diphenyl groups localize at the 9-position as steric bulk moieties, and carbazole (Cz) units localize at the 4-position as supramolecular π-stacked synthon with the dual functionalities of self-assembly capability and hole-transport facility. The synergistic effect of the steric bulk groups and π-stacked carbazoles affords PHDPF-Cz as an ultrastable property, including spectral, morphological stability, and storage stability. In addition, PHDPF-Cz spin-coated gelation films also show thickness-insensitive deep-blue emission with respect to the reference polymers, which are suitable to construct solution-processed large-scale optoelectronic devices with higher reproducibility. High-quality and uniform deep-blue emission is observed in large-area solution-processed films. The electroluminescence shows high-quality deep-blue intrachain emission with a CIE (0.16, 0.12) and a very narrow full width at half-maximum of 32 nm. Finally, large-area and flexible polymer light-emitting devices with a single-molecular excitonic behavior are also fabricated. The supramolecular self-encapsulation design provides an effective strategy to construct ultrastable LCPs for optoelectronic applications.

Abstract:

We report the development of low operating voltages in inorganic–organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrOx gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer “Super Yellow” acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10–2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

Abstract:

Three triple bond-conjugated naphthalene diimide (NDI) copolymers, poly{[ N, N'-bis(2-R1)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-[(2,5-bis(2-R2)-1,4-phenylene)bis(ethyn-2,1-diyl)]} (PNDIR1-R2), were synthesized via Sonogashira coupling polymerization with varying alkyl side chains at the nitrogen atoms of the imide ring and 2,5-positions of the 1,4-diethynylbenzene moiety. Considering their identical polymer backbone structures, the side chains were found to have a strong influence on the surface morphology/nanostructure, thus playing a critical role in charge-transporting properties of the three NDI-based copolymers. Among the polymers, the one with an octyldodecyl (OD) chain at the nitrogen atoms of imide ring and a hexadecyloxy (HO) chain at the 2,5-positions of 1,4-diethynylbenzene, P(NDIOD-HO), exhibited the highest electron mobility of 0.016 cm2 V-1 s-1, as compared to NDI-based copolymers with an ethylhexyl chain at the 2,5-positions of 1,4-diethynylbenzene. The enhanced charge mobility in the P(NDIOD-HO) layers is attributed to the well-aligned nano-fiber-like surface morphology and highly ordered packing structure with a dominant edge-on orientation, thus enabling efficient in-plane charge transport. Our results on the molecular structure-charge transport property relationship in these materials may provide an insight into novel design of n-type conjugated polymers for applications in the organic electronics of the future.

Abstract:

This paper reports state‐of‐the‐art fluorene‐based yellow‐green conjugated polymer blend gain media using Förster resonant‐energy‐transfer from novel blue‐emitting hosts to yield low threshold (≤7 kW cm−2) lasers operating between 540 and 590 nm. For poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) (15 wt%) blended with the newly synthesized 3,6‐bis(2,7‐di([1,1′‐biphenyl]‐4‐yl)‐9‐phenyl‐9H‐fluoren‐9‐yl)‐9‐octyl‐9H–carbazole (DBPhFCz) a highly desirable more than four times increase (relative to F8BT) in net optical gain to 90 cm−1 and 34 times reduction in amplified spontaneous emission threshold to 3 µJ cm−2 is achieved. Detailed transient absorption studies confirm effective exciton confinement with consequent diffusion‐limited polaron‐pair generation for DBPhFCz. This delays formation of host photoinduced absorption long enough to enable build‐up of the spectrally overlapped, guest optical gain, and resolves a longstanding issue for conjugated polymer photonics. The comprehensive study further establishes that limiting host conjugation length is a key factor therein, with 9,9‐dialkylfluorene trimers also suitable hosts for F8BT but not pentamers, heptamers, or polymers. It is additionally demonstrated that the host highest occupied and lowest unoccupied molecular orbitals can be tuned independently from the guest gain properties. This provides the tantalizing prospect of enhanced electron and hole injection and transport without endangering efficient optical gain; a scenario of great interest for electrically pumped amplifiers and lasers.