Wojciech Mróz

Cyclometallated platinum(II) complexes of 1,3-di(2-pyridyl)benzenes for solution-processable WOLEDs exploiting monomer and excimer phosphorescence

Two cyclometallated platinum(II) complexes, N^C^N-5-fluoro-1,3-di(2-pyridyl)benzene platinum(II) chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2-pyridyl)benzene platinum(II) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.

Near‐IR Emitting Iridium(III) Complexes with Heteroaromatic β‐Diketonate Ancillary Ligands for Efficient Solution‐Processed OLEDs: Structure–Property Correlations

Three NIR-emitting neutral Ir(III) complexes [Ir(iqbt)2 (dpm)] (1), [Ir(iqbt)2 (tta)] (2), and [Ir(iqbt)2 (dtdk)] (3) based on the 1-(benzo[b]thiophen-2-yl)-isoquinolinate (iqtb) were synthesized and characterized (dpm=2,2,6,6-tetramethyl-3,5-heptanedionate; tta=2-thienoyltrifluoroacetonate; dtdk=1,3-di(thiophen-2-yl)propane-1,3-dionate). The compounds emit between λ=680 and 850 nm with high luminescence quantum yields (up to 16 %). By combining electrochemistry, photophysical measurements, and computational modelling, the relationship between the structure, energy levels, and properties were investigated. NIR-emitting, solution-processed phosphorescent organic light-emitting devices (PHOLEDs) were fabricated using the complexes. The devices show remarkable external quantum efficiencies (above 3 % with 1) with negligible efficiency roll-off values, exceeding the highest reported values for solution-processible NIR emitters.

Perylene diimide derivatives as red and deep red-emitters for fully solution processable OLEDs

We report on the photophysical characterization of two solution-processable red-emissive perylene diimide molecules and on their use in fully solution assembled OLEDs. The two emitters contain sterically hindered aromatic naphthalene or acenaphthene substituents on the perylene core. These fused rings limits the perylene diimide intermolecular π–π interactions in the solid state due to highly sterically hindered effects, while preserving an extended conjugation between the substituents and the perylene core. Indeed, these features generate a broad absorption for both perylene derivatives along with an efficient red and deep red emission in the solid state for the PDIs with naphthalene and acenaphthene, respectively. The performances of both emitters were tested on OLEDs, where the active layer is a film of bulk PDI, fabricated by simple solution processing. The devices with acenaphthene-substituted perylene diimide provide deep red electroluminescence with emission wavelength at 690 nm, CIE coordinates of (x = 0.69, y = 0.29) and show the best efficiency reported so far for OLED based on PDI fluorescent emitters.

CdTe nanocrystal assemblies guided by breath figure templates

We successfully utilized the breath figure (BF) technique to create thin polystyrene films exhibiting, on the surface, holes of a few micrometres of diameter, arranged in hexagonal packed array. Three-dimensional CdTe microporous structures have been obtained by dipping these BF films into an aqueous solution of CdTe nanocrystals, and stabilized by thermal treatment. The resultant hybrid film joins together the morphology feature of polymeric breath figures and the photoluminescence feature of the semiconducting nanoparticles. In addition, micrometric disk-shaped CdTe aggregates have been anchored in hexagonal fashion to a silicon substrate, which has previously been imprinted with a hydrophilic/hydrophobic pattern. These patterned surfaces made of assembled quantum dots are promising tools for a variety of applications including light-emitting devices and hybrid organic/inorganic solar cells.

Influence of electronic and steric effects of substituted ligands coordinated to Ir(III) complexes on the solution processed OLED properties

Orange and green phosphorescent heteroleptic iridium complexes 1 and 2 [(1): iridium(III)bis[2-(5′-benzylsulfonyl)phenylpyridinato-N,C2′](2,4-decanedionate) and (2): iridium(III)bis[2-(5′-benzylsulfonyl-3′,6′-difluoro)phenylpyridinato-N,C2′](2,4-decanedionate)] are used as dopant emitters in solution processed organic light-emitting diodes. These emitters bear one sterically hindered 2,4-decanedionate and two phenylpyridine (ppy) ligands functionalized with electron withdrawing benzylsulfonyl groups and fluorine atoms. The steric and electronic effects of such substituents on the photophysical properties and performance of devices of 1 and 2 are investigated by making comparison with the green emitting reference phosphor Ir(ppy)2(acac) [iridium(III)bis(2-phenylpyridinato-N,C2′)-acetylacetonate]. In particular, the functionalization of ppy ligands with the benzylsulfonyl group enhances the photoluminescence quantum yield (Φ) and red shifts the emission of complex 1 with respect to Ir(ppy)2(acac). Further substitution of the same ligands with two fluorine atoms in 2 restores the green emission of the reference complex, increasing its Φ. Hence, the combination of the two kinds of substituents represents a suitable functionalization pattern to increase the photoluminescence efficiency of 2vs. the unfunctionalized Ir(ppy)2(acac). Interestingly, a fully solution processed phosphorescent light-emitting device (PHOLED) made using complex 2 as an emitter and with properly tailored architecture, which includes an electron transporting layer of a PEG substituted polyfluorene (PEG: poly(ethylene glycol)), exhibits high external emission quantum efficiency (EQE up to 12%) and a high luminous efficiency (LE up to 24.2 cd A−1). We attribute such a high performance to the bulky effect of both benzylsulfonyl groups in ppy and of the alkyl chain in the β-diketonate ligand, as well as to the enhanced electron mobility induced by fluorine atoms. This device outperforms the control diode based on Ir(ppy)2(acac) (EQE 5.5% and LE 17 cd A−1) and approaches the best efficiencies achieved so far for green Ir(III) complex based PHOLEDs.

Synthesis and characterization of charge-transporting π-stacked polybenzofulvene derivatives

Two new benzofulvene derivatives bearing two or three methoxy substituents on the benzene ring were synthesized and induced to polymerize spontaneously in order to investigate the photophysical and electronic properties of the corresponding polymers. The photophysical features of the newly synthesized polymers suggested a high degree of π-stacking both in the solid state and in diluted solutions, and the large Stokes shift was interpreted in terms of an efficient energy transfer within the excimer. Absorption and emission features in the solid state were found to be similar to those in diluted solutions and the stable PL quantum yield was considered a promising feature with regard to the potential applications of the polymers in light emitting devices. Finally, the remarkable hole mobility shown by poly-4,5,6-MO-BF3k along with the enhancing effect of the methoxy substituents in the charge mobility opens up new routes to the development of materials potentially useful in optoelectronics.

Tailorable perylene-loaded fluorescent nanostructures: a multifaceted approach enabling their application in white hybrid LEDs

Organizing materials into well-defined nanostructures by self-assembly is of great interest for manufacturing various photonic devices at limited costs. In this study, we report on a versatile strategy to mold a perylene-containing building block into different fluorescent nanometric objects from zero- to two-dimensional shapes: particles, fibers and honeycomb films. As a practical example of application of these nanostructures into a functional device, we make use of fluorescent electrospun fibers to develop an efficient and stable white hybrid LED. The straightforward approach consists of the encapsulation of the nanofibers, used as downconversion materials, in flexible elastomeric semitransparent discs, which are then applied on a blue GaN LED. The main advantages include the ease of nanofiber manipulation and assembling of the final device, flexibility/stretchability of the downconversion filter, and practical tuning of the color temperature and rendering by simply adjusting the composition and number of discs, allowing us to obtain white light with chromaticity coordinates (0.36, 0.35), a correlated color temperature of 4500 K and a power efficacy as high as 70 lm W−1. In addition to the benefits above, the promising stability values shown by the device make our nanofiber approach a good option for the fabrication of downconversion filters for solid state lighting purposes.

Bithiophene-based polybenzofulvene derivatives with high stacking and hole mobility

Four new benzofulvene derivatives bearing bithiophene chromophores at two different key positions of the phenylindene scaffold were prepared in order to evaluate the role of different chromophores in the optoelectronic features of polybenzofulvene derivatives. The results of the photophysical studies showed that the optical properties of the newly-synthesized bithiophene-functionalized polymers were affected by both the polymer enchainment and the substitution topology of the monomeric units. On the other hand, the hole-mobility appeared to be affected to a lesser extent, but the best performances were obtained in poly-6-HBT-BF3k showing the strongest bithiophene side chain packing. This work demonstrates that the optoelectronic properties of polybenzofulvene derivatives can be optimized by a targeted chemical design such as side chain engineering.

Multi-Colour Electroluminescence of Dendronic Antennae Containing Pyrenes as Light Harvesters

Dendronic antennae systems containing pyrene units as energy donors and a styrylpyridinium derivative as energy acceptor show efficient energy transfer from the green-emitting pyrene excimer to the red-emitting acceptor. For the third dendron generation the effective screening of the pyrene units on the acceptor provides thin films showing bright red emission. Single-layer light-emitting diodes prepared by properly balancing the dendrons and donor units concentration in polyvinylcarbazole show electroluminescence from the blue, green and red components of the monomeric donor, the donor excimer and the acceptor when excitons are generated in the polymer and subsequently transferred to the molecules by resonant energy transfer.

Low-Cost and Green Fabrication of Polymer Electronic Devices by Push-Coating of the Polymer Active Layers

Because of both its easy processability and compatibility with roll-to-roll processes, polymer electronics is considered to be the most promising technology for the future generation of low-cost electronic devices such as light-emitting diodes and solar cells. However, the state-of-the-art deposition technique for polymer electronics (spin-coating) generates a high volume of chlorinated solution wastes during the active layer fabrication. Here, we demonstrate that devices with similar or higher performances can be manufactured using the push-coating technique in which a poly(dimethylsiloxane) (PDMS) layer is simply laid over a very small amount of solution (less than 1μL/covered cm2), which is then left for drying. Using mm thick PDMS provides a means to control the solvent diffusion kinetics (sorption/retention) and removes the necessity for additional applied pressure to generate the desired active layer thickness. Unlike spin-coating, push-coating is a slow drying process that induces a higher degree of crystallinity in the polymer thin film without the necessity for a post-annealing step. The polymer light-emitting diodes and solar cells prepared by push-coating exhibit slightly higher performances with respect to the reference spin-coated devices, whereas at the same time reduce the amounts of active layer materials and chlorinated solvents by 50 and 20 times, respectively. These increased performances can be correlated to the higher polymer crystallinities obtained without applying a post-annealing treatment. As push-coating is a roll-to-roll compatible method, the results presented here open the path to low-cost and eco-friendly fabrication of a wide range of emerging devices based on conjugated polymer materials.