Fabrication and Study of White‐Light OLEDs Based on Novel Copolymers with Blue, Yellow, and Red Chromophores

Abstract

White Organic Light‐Emitting Diodes (WOLEDs) have attracted an enormous interest because they can be implemented in numerous lighting applications as next‐generation light sources. Several device setups such as multilayer, blends, or single layers of polymer nanomaterials have been proposed to achieve white light emission. In this work, novel copolymers bearing blue, yellow, and red chromophores, are used for the fabrication of WOLED systems including single emissive layer. These terpolymers provide easy tuning of white color emission by selecting the appropriate ratios between the chromophores while being solution processable from typical organic solvents, like chlorobenzene (CB), o‐dichlorobenzene (o‐DCB), etc. The chromophores can be directly excited by capturing charge carriers or through energy transfer from the other chromophores. Incomplete energy transfer between the chromophores is necessary to get white light as combined emission from all of them. The opto‐electronic and electro‐optical characterization is carried out by NIR–Vis–far UV spectroscopic ellipsometry, photoluminescence, and electroluminescence, to provide valuable information toward the optimization and functionalization of these WOLED devices.

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

“…Monomers 2,6-dihydroxyphenylpyridine (Py) [19], 2,7-diacetoxystyryl-9-(2-ethylhexyl)-9H-carbazole (Cz) and 4,7-diacetoxystyryl-2,1,3-benzothiadiazole (BTZ) [10,20] were synthesized according to the literature procedures. Dimethylacetamide (DMA) and toluene were purchased from Honeywell and were used as received.…”

“…Most of the WOLED devices reported so far have relied on the use of combinations of several organic components in blended systems or heterolayer architectures that emit different colors to fully span the entire visible spectrum [5]. However, the preparation of such blended or heterolayer devices is more complicated; in the case of blended systems, there is a probability of phase separation within the films, while regarding the heterolayer structure, the intermixing of the different layers may harm the previously deposited layer [5,10,11].…”

“…Compared to the multi-emitting-component WOLEDs, a promising alternative strategy has been proposed with the fabrication of a single-layer WOLED, based on the inclusion of copolymers that bear different emitting chromophores [10][11][12]. This approach has many advantages, such as a simple and cost-effective fabrication process, and utilization in spin-coating and printing methods over a large-scale range due to the easy processability of copolymers in common solvents [11,12].…”

“…More specifically, in Fluorescence Energy Transfer (FET) process, an excited fluorophore (host) transfers its energy to an adjacent molecule (guest) by dipole-dipole interaction. Guest-host systems are also attractive materials for investigating energy-migration processes [10][11][12][13][14][15][16][17][18]. There are two types of FET processes, which are dependent on the copolymer chain architecture, host/guest strength and composition.…”

“…The fruitful combination of the chromophores along with the appreciable high molecular weights of the copolymers, without sacrificing their solubility, and their processability in the form of solution inks, allows for optimum deposition and study of the photoactive layers. The ease of the monomers' preparation [10,19,20], the versatility and tolerance of the polymerization procedure [21,22] and the efficient and straightforward purification of the final semiconductive polyethers demonstrate the economic viability of the herein presented materials compared to analogous commercial semiconducting fully conjugated polymers. In order to identify their emission characteristics, we have compared white emitting copolymers with blue emitting polymers bearing either only carbazole or both carbazole and pyridine moieties in the polymeric chain.…”