Solution Processable STAR-Shaped BipoLar Conjugated Host MaterIals for HiGHly Efficient ElectrophosphorescenT Devices (STARLIGHT)

Project leaders

Umberto Giovanella, Irina Khotina

Agreement

RUSSIA - RFBR-suspended - Russian Foundation for Basic Research

Call

CNR/RFBR 2015-2017

Department

Chemical sciences and materials technology

Thematic area

Chemical sciences and materials technology

Status of the project

New

Research proposal

The host-guest strategy with triplet emitters (guests) as a dopant into suitable organic matrix (the host) is currently the most effective way to achieve organic light-emitting devices (OLEDs) with 100% internal quantum efficiency. While very efficient phosphors are commercially available, the design and synthesis of efficient host is still challenging.
A good host materials should possess: i) high chemical compatibility with the guest, ii) triplet energies higher than those of the guest molecules, iii) suitable energy levels for efficient charge carrier injection and decent charge carrier transport properties.
As an example, hole transporting carbazole-based derivatives, with high triplet energy, has been largely applied as a universal host for Iridium(III) complexes giving extremely high efficient devices [Chou et al. Adv. Mater. 2010, 22, 2468]. However, the utilization of these conventional "unipolar" hole-transport host materials creates the risk of a narrow recombination zone, close to the interface with the electrodes, and the use of electron-transport layer (ETL) or hole-transport layer (HTL) is imperative to balance the charge carrier transport. More recently, a rational molecular design led to the synthesis of bipolar-transport host materials. In this framework, the introduction of perfluorinated phenyl rings within the conjugated backbone can represent a valuable approach [Giovanella et al. J. Mater. Chem. C 2013, 1, 5322].
Thermally evaporated phosphorescent OLEDs based on small organic molecules have shown great improvements and surpassed their competitive counterpart [Reineke et al. Nature 2009, 459, 234]. Alternatively, in spite of a lower device performance achieved to date, solution-processed OLEDs are more attractive in the scientific and industrial communities because of their unique advantages such as simple device structures, low-cost solution-processing manufacturing, and the compatibility with effective large-area production by ink-jet printing or roll-to-roll coating, etc. [Wu et al. Chem. Soc. Rev. 2009, 38, 3391].
In this proposal, with the aim to achieve competitive solution processed OLEDs, we intend to synthesize novel and soluble bipolar host small molecules and test them in combination with commercially available efficient phosphorescent Ir(III) complexes.
For the synthesis of host molecules, we will apply the basic strategy: the introduction of carbazole groups on bromine-containing dendrimers and oligophenylenes. The reaction of carbazole with the aromatic bromine will be carried out in the presence of CuI, K2CO3, and crown ether 18-crown-6. So, based on 1,3,5-tris (p-bromophenyl) benzene molecule is obtained tris(4-carbazol-9yl)-1,3,5-triphenylbenzene (I). Previously synthesized oligophenylenes by Ni-catalyzed condensation of tribromides (1,3,5-tris (p-bromophenyl) benzene or tris(p-bromophenyl)amine) had a large number of end groups of the unreacted bromine. These groups are substituted on the N-carbazolyl residue(II). The replacement of carbazolyl groups for two bromine atoms in the 1,3,5-tris (p-bromophenyl) benzene, tris(p-bromophenyl)amine, and 1.3.5-tribromobenzene will lead to the formation of monobromides. They will interact following Suzuki reaction with the bis(pinacolato)-diboron. These compounds are converted into the corresponding pinacolborates that upon interaction with the 1,4-dibromo-tetrafluorobenzene will be converted to the corresponding symmetrical dendrimers (also called butterfly-shaped molecules) (III-V). Pseudo branched oligophenylenes will be prepared starting from 1,3,5-tris (p-bromophenyl) benzene by replacing only one bromine atom on carbazolyl group. Other two bromine atoms will be substituted with pinacolborate groups and Suzuki reaction of the difunctional monomer 1,4-dibromo-tetrafluorobenzene is expected to give a high molecular soluble polymer with carbazole residues as end groups and tetrafluorobenzene as a core of the molecule (VI).
Phosphorescent OLEDs based on the new molecular hosts embedding reference Ir(III) compounds such as Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)Ir(III) (FIrpic), Tris[2-phenylpyridinato-C2,N]Ir(III) (Ir(ppy)3) and Bis(2-(9,9-dibutylfluorenyl)-1-isoquinoline (acetylacetonate) (BTIr) emitting blue, green and red respectively, as guest emitters will be investigated and compared to tris(4-carbazo-9-ylphenyl)amine (TCTA)-based devices prepared in the same conditions. Initially, devices will be fabricated with a "basic" architecture ITO/PEDOT:PSS/emitting layer/cathode and then optimization will be explored by introducing solution processed HTL and ETL to give the "optimized" architecture ITO/PEDOT:PSS/HTL/emitting layer/ETL/cathode. The effect of the different electrodes on the OLEDs performance will be explored. Recent studies indicated that stable metal oxides such as MgO, ZnO and MoO3, can be successfully used for coating electrodes in the most advanced optoelectronic devices [Williams et al. Adv. Funct. Mater. 2013, 23, 2239], called "hybrid" OLEDs. Performance (i.e. current density-voltage characteristics, external quantum efficiency, luminous efficiency, power efficiency, luminance, electroluminescence spectrum and chromaticity coordinates) will be recorded. Phosphorescent OLEDs with performance exceeding the current state-of-the art devices fabricated with small molecules by following wet deposition methods are expected. [Chen et al. J. Pol. Sci., Part A: Pol. Chem. 2014, 52, 1037].
CNR-ISMAC/RAS-INEOS team is highly qualified in the field of the proposal and a long-term cooperation has produced achievements related to the synthesis and the study of the electronic properties of conjugated compounds and OLED fabrication, with joint publications. Laboratories are available for chemical synthetic work and physical-chemistry characterisation (RAS-INEOS) as well as complete facilities for device fabrication and characterization (CNR-ISMAC).

Research goals

o Synthesis of new unipolar carbazole-based star-shaped small molecules. Firstly, tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as reference compound, and successively new 1,3,5-triphenylbenzene derivative of carbazole. An oligophenylene with carbazole groups will be synthesized as well.
o Synthesis of new bipolar carbazole/fluorinated phenyl-based small molecules. The molecules of bis-carbazolyl derivatives of 1,3,5-triphenylbenzene, triphenylamine and benzene in 1,3 position, connected symmetrically through tetrafluorobenzene bridge will be obtained.
o Synthesis of new bipolar pseudo branched oligophenylene embedding carbazole/fluorinated moieties within the chain.
o Manufacturing phosphorescent OLEDs based on host/guest approach and wet deposition methods, and testing their performance.
o Development of optimized multilayer device architectures with proper charge regulating layers and electrodes.
o Performance exceeding the current state-of-the-art devices fabricated by following wet deposition methods (external quantum efficiency above 10 %, and luminous efficiency above 50 cd/A) are expected.
o Contribute to the field by publication of the results and conference presentations.

Last update: 08/07/2025