Photophysical and Electro-Optical Properties of Copolymers Bearing Blue and Red Chromophores for Single-Layer White OLEDs

Abstract

In this study, novel copolymers consisting of blue and red chromophores are presented to induce emission tuning, enabling the definition of white light emission in a single polymeric layer. These aromatic polyether sulfones exhibit high molecular weights, excellent solubility and processability via solution deposition techniques. In addition, by carefully controlling the molar ratios of chromophores composition, the energy transfer mechanism, from blue to red chromophores, takes place enabling us to define properly the emission covering the entire range of the visible spectrum. The optical and photophysical properties of the monomers and copolymers were thoroughly investigated via NIR-Vis-far UV Spectroscopic Ellipsometry (SE), Absorbance and Photoluminescence (PL). These copolymers are used as an emissive layer and applied in solution-processed WOLED devices. The fabricated WOLED devices have been subsequently studied and characterized in terms of their electroluminescence properties. Finally, the WOLED devices possess high color stability and demonstrate CIE Coordinates (0.33, 0.38), which approach closely the pure white light CIE coordinates.

A Comparative Study between Blended Polymers and Copolymers as Emitting Layers for Single-Layer White Organic Light-Emitting Diodes

“…More details on the synthetic processes are given in refs. [ 4 , 14 , 26 ]. The molecular weights for terpolymers with ratios 98.5:1.0:0.5 and 99:0.5:0.5 were estimated at 9540 and 8800.…”

“…The chemical structures of copolymers and terpolymers are shown in Scheme 1 and Scheme 2 , respectively. The copolymers were dissolved in N, N-dimethylformamide, DMF, with a resulting concentration of 1% wt, and the terpolymers were dissolved in o-DCB:CHCl 3 (9:1) with a resulting concentration of 1% wt [ 4 , 26 ]. The ratios for the copolymers and terpolymers are summarized in Table 2 , where the codes of studied samples and the copolymer type are included.…”

“…White organic light-emitting diodes (WOLEDs), produced through a solution deposition process, offer a formidable opportunity for low-cost, environmentally friendly, and flexible light sources capable of meeting the spectral, functional, and budgetary needs of emerging application fields. These fields include next-generation full-color displays and solid-state lighting [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Until now, the reported high-efficiency WOLEDs with multilayer device structures have been mostly fabricated through vacuum evaporation [ 8 ].…”

“…Among these strategies, multilayer polymer LED fabrication is more laborious, as their fabrication is more difficult and there is a risk that the solvent may destroy the underlayers. On the other hand, the alternative strategy based on single-emissive layer WOLEDs has shown to be a more reliable option in scientific and industrial communities because of the low fabrication costs, simple structure, and easy solution processing [ 4 , 13 , 14 ].…”

“…There have been several proposed luminescent material systems, which have been devised resulting in the exciton regulation to emit white light in a balanced manner. Regarding this issue, two of the proposed strategies involve either the blend of two or more light-emitting polymers [ 15 , 16 ] or the use of multicolor-emitting copolymers bearing different chromophores [ 4 , 14 ]. It is worth noting that for producing environmentally friendly and practical WOLEDs, fluorescent materials are superior to phosphorescent materials, as the former do not contain any heavy metals [ 17 ].…”