Tuning the color emission of thin film molecular organic light emitting devices by the solid state solvation effect

122

This Full Paper investigates a series of strongly fluorescent donor–acceptor‐substituted spirobifluorene compounds, red 2‐diphenylamino‐7‐(2,2‐dicyanovinyl)‐9,9′‐spirobifluorene (DCV), green 2‐diphenylamino‐9,9′‐spirobifluorene7‐carxoxaldehyde (CHO), and blue 2‐diphenylamino‐7‐(2,2‐diphenylvinyl)‐9,9′‐spirobifluorene (DPV), together with their spiro‐linked “dimeric” analogs, 2DCV, 2,2′‐bis(diphenylamino)‐9,9′‐spirobifluorene‐7,7′‐dicarboxaldehyde (2CHO), and 2,2′‐bis(diphenylamino)‐7,7′‐bis(2,2‐diphenylvinyl)‐9,9′‐spirobifluorene (2DPV), respectively. The emission optical density and, hence, the intensity of photoluminescence (PL) or electroluminescence (EL) of the “dimeric” analogs is presumed to increase, which is beneficial for organic light‐emitting diode (OLED) applications. The physical properties, including the dipole moments obtained from quantum chemistry calculations, emission solvatochromism, fluorescence quantum yield (Φf) as well as the EL of these six spirobifluorene compounds have been examined in detail. We found that Φf as well as OLED performance (EL efficiency and intensity) of the strongly dipolar DCV decrease significantly in the “dimeric” analog 2DCV, but less so in the moderately dipolar CHO and 2CHO, and only slightly in the weakly dipolar DPV and 2DPV. This is parallel to the intramolecular dipole moment, which is large for 2DCV, medium for 2CHO, and very small for 2DPV. Here, we show for the first time systematically that the luminescence intensity is closely correlated with the local electric field induced by the molecular dipole. A strong electric field may facilitate radiationless decay channels with a charge‐transfer nature, leading to a high quenching rate. Consistent with this conclusion, which is derived from the red DCV/2DCV and green CHO/2CHO, our new blue fluorophore DPV with an essentially zero dipole moment has successfully achieved one of the best electrofluorescent blue OLEDs. At the same time, by doping the highly dipolar DCV into an isolated environment with the low‐polarity Alq3 as the host matrix, we obtained a very high performance of saturated yellow OLEDs as well, This is possibly due to the reduction of emission‐quenching dipoles from the neighboring molecules. Our results have provided an important insight in designing luminescent materials, as follows: molecular dipole moments should be kept at a low magnitude to avoid quenching induced by a strong local electric field in the chromophore.

Section: Photophysical Propertiessupporting

confidence: 69%

Influence of Molecular Dipoles on the Photoluminescence and Electroluminescence of Dipolar Spirobifluorenes

Chiang

Tseng

Chen

et al. 2008

122

“…Similar spectral changes have been observed with other DCM derivatives. [4,24±26] In analogy to the work by Bulovic et al, [20] we have attributed this effect to solid-state solvation. The polarity of the doped layer increases with doping concentration, leading to a pronounced red-shift.…”

Section: Electroluminescence Characteristicssupporting

confidence: 55%

“…In particular, the polarity of the matrix has a determining effect on the emission color in doped OLEDs. [20] In the case of DCM2 doped in Alq 3 , the red-shift of the electroluminescence with concentration has been attributed to a solid-state solvation effect induced by the doping molecules themselves. [4] Here, we are interested in trapping effects that result from the energy-level position of the frontier orbitals of the doping molecules with respect to the ones of the host material.…”

Section: Polarization Effectsmentioning

confidence: 99%

Doping‐Induced Charge Trapping in Organic Light‐Emitting Devices

Nüesch

Berner

Tutiš

et al. 2005

By using pyran‐containing donor–acceptor dyes as doping molecules in organic light‐emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro‐active host material. Laser dyes 4‐(dicyanomethylene)‐2‐methyl‐6‐[2‐(julolidin‐9‐yl)phenyl]ethenyl]‐4H‐pyran (DCM2) and the novel 4‐(dicyanomethylene)‐2‐methyl‐6‐{2‐[(4‐diphenylamino)phenyl]ethenyl}‐4H‐pyran (DCM‐TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8‐hydroxyquinoline)aluminum (Alq3) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM‐TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq3 region. To impede charge‐recombination processes taking place in the undoped host matrix, a charge‐blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.‐% we obtain a maximum luminous efficiency of 10.4 cd A–1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices.

“…6b), presumably due to the change in the polarity of the medium. [24] Further optimization can be achieved by the incorporation of a thin layer of poly(styrene sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT) to improve the surface smoothness and to serve as a hole injection layer. [25] The best results, with an external quantum efficiency (EQE) of 7.03 %, luminous efficiency of 8.02 cd A -1 , and power efficiency of 2.74 lm W -1 , have been obtained using this configuration at a current density of 20 mA cm -2 .…”

Section: Fabrication Of Oled Devicesmentioning

confidence: 99%

Orange and Red Organic Light‐Emitting Devices Employing Neutral Ru(II) Emitters: Rational Design and Prospects for Color Tuning

Tung

Chen

Chi

et al. 2006

132

A new series of charge‐neutral Ru(II) pyridyl and isoquinoline pyrazolate complexes, [Ru(bppz)2(PPh2Me)2] (bbpz: 3‐tert‐butyl‐5‐pyridyl pyrazolate) (1), [Ru(fppz)2(PPh2Me)2] (fppz: 3‐trifluoromethyl‐5‐pyridyl pyrazolate) (2), [Ru(ibpz)2(PPhMe2)2] (ibpz: 3‐tert‐butyl‐5‐(1‐isoquinolyl) pyrazolate) (3), [Ru(ibpz)2(PPh2Me)2] (4), [Ru(ifpz)2(PPh2Me)2] (ifpz: 3‐trifluoromethyl‐5‐(1‐isoquinolyl) pyrazolate) (5), [Ru(ibpz)2(dpp)] (dpp represents cis‐1,2‐bis‐(diphenylphosphino)ethene) (6), and [Ru(ifpz)2(dpp)] (7), have been synthesized, and their structural, electrochemical, and photophysical properties have been characterized. A comprehensive time‐dependant density functional theory (TDDFT) approach has been used to assign the observed electronic transitions to specific frontier orbital configurations. A multilayer organic light‐emitting device (OLED) using 24 wt % of 5 as the dopant emitter in a 4,4′‐N,N′‐dicarbazolyl‐1,1′‐biphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (NPB) as the hole‐transport layer exhibits saturated red emission with an external quantum efficiency (EQE) of 5.10 %, luminous efficiency of 5.74 cd A–1, and power efficiency of 2.62 lm W–1. The incorporation of a thin layer of poly(styrene sulfonate)‐doped poly(3,4‐ethylenedioxythiophene) (PEDOT) between indium tin oxide (ITO) and NPB gave anoptimized device with an EQE of 7.03 %, luminous efficiency of 8.02 cd A–1, and power efficiency of 2.74 lm W–1 at 20 mA cm–2. These values represent a breakthrough in the field of OLEDs using less expensive Ru(II) metal complexes. The nonionic nature of the complexes as well as their high emission quantum efficiencies and short radiative lifetimes are believed to be the key factors enabling this unprecedented achievement. The prospects for color tuning based on Ru(II) complexes are also discussed in light of some theoretical calculations.