The π-conjugated polymers with semi-conducting properties have recently attracted much attention due to their applicability in the field of optic and optoelectronic devices such as light-emitting diodes (LEDs) and lasers. The LEDs based on conjugated polymers have attracted much attention because of their potential application to flat, large area displays, which can be operated at low driving voltage. To make more efficient electroluminescent (EL) device, it is necessary to balance the injection rate of charge carriers. In this dissertation, several studies to improve the luminescent efficiency, to enhance the environmental stability and to tune the emitting colors have been accomplished.
One approach to improve the luminescent efficiency of the EL devices is to employ several different kinds of charge injecting materials such as ionomers, single-ion conductors, and organic-salt doped polymer blends.
First, ionomers have been employed as an electron injecting and hole blocking material. The effect of ion concentration, the neutralization level and the metal counter-ions in ionomers was systematically studied to obtain the optimal EL characteristics in the polymer light-emitting diodes using poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as an emissive layer and sulfonated polystyrene ionomers as an electron-injecting layer.
Second, single-ion conductors are employed as charge injection materials. A single-cationic conductor was used for the electron injection and a single-anionic conductor for the hole injection. The charge injection can be greatly promoted due to the ionic space charges near both electrodes. A light-emitting single-energy-well device was fabricated by employing both the single cationic- and anionic- conductor. It can be a further advanced polymer light-emitting device than the polymer light-emitting electrochemical cell (LEC) using bi-ionic conductors.
Third, to reform the LEC devices, we fabricated multi-layer EL devices employing ammonium salt-doped poly(ethylene oxide) blend as hole- or electron-injecting materials. The current-voltage-optical output characteristics of the triple-layer device using both hole- and electron-injecting layer are very similar to the well-known LEC devices.
Another approach to improve the luminescent efficiency of the EL device is thermal annealing and electrical annealing. Thermal annealing after Al deposition dramatically enhanced the EL efficiency about 230 times at 350 mA/cm$^2$. With the bias voltage annealing as an additional treatment, not only the onset voltage of the device can be reduced and but also the efficiency is enhanced.
The novel approach to improve the luminescent efficiency of the EL device and environmental stability of the materials is two-dimensional organic-inorganic nanocomposites. To simultaneously solve low quantum efficiency of the EL device and poor stability of emitting material against oxygen and moisture, light-emitting devices using the polymer/layered silicate nanocomposite with good gas-barrier property was fabricated. The nanocomposite materials were prepared by blending MEH-PPV with organo-clay. The two-dimensional nanocomposite film shows higher photoluminescence output and better photostability when compared with the pure MEH-PPV film of the same thickness. EL efficiency is also hugely enhanced.
F$\ddot{o}$rster energy transfer has been employed to obtain desired emission spectrum and enhance luminescent efficiency in EL devices. White light emission was obtained from a LED prepared from a ternary polymer blend (19:1:1 by wt.) consisting of poly(9-vinylcarbazole) (PVK), poly(9,9$\prime$-dihexlyfluorene-2,7-divinylene-m-phenylenevinylene-stat-p-phenylene vinylene) (CPDHFPV), and MEH-PPV. Poor miscibility among the components resulted in partial energy transfer causing the blend to emit two colors simultaneously, resulting in generation of a pure white color. On the other hand, white emission from a completely miscible ternary polymer blend of liquid-crystalline poly(2,7-bis(p-stiryl)-9,9’-di-n-hexylfluorene sebacate) (PBSDHFS) doped lightly with poly(9,9’-di-n-hexyl fluorenediylvinylene-alt-1,4-phenylenevinylene) (PDHFPPV) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was also obtained by controlling the excitation energy transfer at very low doping levels. When the dopant concentration in the host is enough for complete transfer from the donor to the guest, cascade energy transfer occurs in a ternary blend of PBSDHFS, PDHFPPV, and DCM, which can be utilized to tune the emission color and to enhance the emission efficiency.
The optical characteristics of a fullerene-containing material, $(PS)_xC_{60}(PMMA)_y$ has been investigated and it has been employed in a polymer EL device. In forward biased ITO/MEH-PPV/$(PS)_xC_{60}(PMMA)_y$/Al device, the luminescence was strongly quenched. In contrast, when the device was operated in reverse bias direction, the luminescence and the yield was enhanced. From this result, it was newly found that the $C_{60}$-containing polymer possesses a hole-injecting property.
Stimulated emission and laser action can be also observed from the conjugated polymers. A low-threshold blue amplified spontaneous emission (ASE) has been obtained in a statistical alternating copolymer, CPDHFPV and its blend with PVK. The threshold energy of the PVK/CPDHFPV (95:5 by wt.) blend device was as low as 20 nJ/㎠/pulse. On the other hand, liquid crystalline PBSDHFS/PDHFPPV (98:2 by wt.) blend film also possesses threshold energy of 20 nJ/㎠/pulse. It can be understood that F$\ddot{o}$rster energy transfer works well to improve the performance of lasing actions as well as to tune the color. We also fabricated microcavity-lasing devices of the blend of PBSDHFS and PDHFPPV. The microcavity device of PBSDHFS/PDHFPPV blend demonstrated a very low-threshold energy (∼3 nJ/㎠/pulse), which is lower than any other values previously reported on the organic or polymeric microcavity devices with metal or dielectric mirrors. This result implies that the liquid crystalline polymer blends could be a good candidate for gain material of photo- and electrically-pumped lasing devices.
