Donal Bradley

Abstract:

The unique electronic structures of heteroatomic conjugated polymers (HCPs) offer an attractive platform to tune optoelectronic properties via a supramolecular coordination strategy. This study reports on an sp(2) nitrogen heteroatom containing fluorene-based copolymer namely poly(9,9-dioctylfluorene-co-9,9-dioctyldiazafluoren-2,7-yl) (PF8-co-DAF8), with ≈20% DAF8 units. Tuning the optoelectronic properties of PF8-co-DAF8 via supramolecular coordination with a Lewis acid (B(C6 F5 )3 or AlCl3 ) is explored. Formation of either the PF8-co-DAF8-B(C6 F5 )3 or PF8-co-DAF8-AlCl3 adducts reduces the optical gap and causes an attendant redshift of the photoluminescence spectra. Controlling the degree and strength of the coordination allows the emission color to be tuned from blue through to green and yellow. This strategy is successfully implemented for polymer light-emitting diodes, confirming the large degree of spectral tuning whilst maintaining good device performance. Maximum luminous efficiencies, η ≈ 1.55 cd A(-1) @ 2120 cd m(-2) , 1.32 cd A(-1) @ 1424 cd m(-2) , and 2.56 cd A(-1) @ 910 cd m(-2) are, respectively, recorded for the blue-emitting diodes with Commission Internationale de L'Eclairage (CIE) (x, y) coordinates = (0.16, 0.16), the white-emitting diodes with CIE (x, y) = (0.28, 0.38) and the green-emitting diodes with CIE (x, y) = (0.33, 0.52). The results highlight the versatility of the supramolecular coordination strategy in modifying the electronic structure of HCPs.

Abstract:

A novel supramolecular system comprising a complex of 9,9′-diphenyl-9H,9′H-2,2′-bifluorene-9,9′-diol (DPFOH) with poly(methyl methacrylate) (PMMA) is presented as an attractive system for optical gain in the ultraviolet. The analogue compound 9,9′-diphenyl-9H,9′H-2,2′-bifluorene (DPFO8) without an -OH substituent was synthesized alongside DPFOH to confirm the importance of its chemical structure to the thin-film microstructure. A hydrogen-bonding interaction allows the molecule such as DPFOH and a combination of DPFOH and PMMA to have an excellent solution-processed high quality coating film. In stark contrast to the DPFO8 system, we find that the addition of 1 wt % DPFOH to PMMA leads to spontaneous formation of a supramolecular complex via hydrogen bonding interactions, giving rise to a homogeneous film with relatively high photoluminescence quantum efficiency ∼38 (±5)%. The demonstration of ultraviolet laser action with peak wavelength emission at 385 nm provided further evidence of the high optical quality of the DPFOH/PMMA supramolecular complex films. The DPFOH/PMMA supramolecular complex has great potential for use in low-cost solution-processed optoelectronic devices.

Abstract:

The spin crossover (SCO) phenomena are a remarkable example of state spin switching at the molecular level. The low- and high-spin states can be reversibly selected through application of external stimulus - often simply a variation in temperature. Since the particular spin-state embodies optical, electronic and structural characteristics, the spin switching can be readily detected or probed using a variety of techniques. In this regard, SCO phenomena show great promise for a range of devices. The key to this uptake is the preparation of high-quality, thin-films capable of retaining SCO properties, and solution-based materials, in particular, provide further opportunities for integration or blending with other functional materials. The present work examines SCO behaviour from two iron(II)-triazole polymers, with short and long side chains, prepared and examined in a variety of formats – from bulk powders to thick and thin films. Magnetic, optical and electronic techniques all verify the SCO behaviour is faithfully maintained for all formats. The two materials serve to highlight the impact on key properties arising from the different density of Fe atoms. The results, all from solution-based materials, are extremely promising and clearly emphasise the growing capability of materials and processing advances associated with SCO materials.

Abstract:

A charge-carrier density dependent mobility has been predicted for amorphous, glassy energetically disordered semiconducting polymers, which would have considerable impact on their performance in devices. However, previous observations of a density dependent mobility are complicated by the polycrystalline materials studied. Here charge transport in field-effect transistors and diodes of two amorphous, glassy fluorene-triarylamine copolymers is investigated, and the results explored in terms of a charge-carrier density dependent mobility model. The nondispersive nature of the time-of-flight (TOF) transients and analysis of dark injection transient results and transistor transfer characteristics indicate a charge-carrier density independent mobility in both the low-density diode and the high-density transistor regimes. The mobility values for optimized transistors are in good agreement with the TOF values at the same field, and both have the same temperature dependency. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge-carrier density dependent model, and does not follow the expected power-law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge-carrier density independent mobility, and this is discussed in terms of polaron-dominated hopping and interchain correlated disorder.