Light Emitting Devices Modified with Ionic Polymer Electrolyte

• Main Authors: Hsing-Lung Wang, 王興龍
• Other Authors: Ten-Chin Wen
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/34993051627858215274

博士 === 國立成功大學 === 化學工程學系碩博士班 === 91 === Abstract Light emitting devices based on organic materials are of considerable interest due to their attractive characteristics and potential applications to flat panel displays. After a brief overview of the different device construction and operating principles, two types of light emitting devices, modification of polymer light emitting diode (PLED) by inserting additional functional layer and light-emitting electrochemical cell (LEC) admixing luminescent conjugated polymers with single ion polymer electrolytes were investigated. In the study on modification of PLED by inserting additional functional layer, at first the sulfonated polydiphenylamine (SPDPA) was employed as hole injecting layer for poly[l-methoxy-4-(2-ethylhexyloxy-2,5-phenylene vinylene)] (MEH-PPV) based PLED. The results on SPDPA as a hole injection layer could reduce the operating voltage to 3 V. SPDPA also makes the surface of ITO as smooth and has high transparency. Additionally, a waterborne polyurethane (WPU) ionomer having two different pendant groups, sulfonate and carboxylate group, was synthesized and used as an electron injecting layer in polymer light emitting diode, PLED. MEH-PPV was used as an emitting material. For comparison, a polyurethane ionomer (PUI) with carboxylate ion alone in the pendant part was used in the device. The current (I)-voltage (V)-luminance (L) characteristics of the devices, ITO/MEH-PPV/WPU/Al, ITO/MEH-PPV/PUI/Al and ITO/MEH-PPV/Al were measured and compared. The low turn-on voltage for current and emission with WPU in the device configuration originates from the better polarization capability of sulfonate ion for lithium ion. WPU also provides improved emission performance for the device. The lowering of barrier to electron injection and higher density of electron injection cause an improved device performance with WPU. AC impedance measurements were used to monitor the frequency dependent dielectric constant and dielectric loss. The observed changes in dielectric properties corroborate with the results from I-V-L measurements. LECs were fabricated based on luminescent conjugated polymers, poly(p-phenylenevinylene) (PPV) and MEH-PPV, by blending with PUI and WPU. The differences in device characteristics were critically compared with traditional biionic LECs. The feasibility of a thin layer prepared by PUI with PPV for the fabrication of LEC was investigated. The observed electroluminescence (EL) at a much lower turn on voltage favors the use of PPV + PUI composite film as a light-emitting layer for LEC. Further, the utility of this blend owns superior response time and stability. LEC was fabricated with PPV as light emitting material and lithium ion conducting WPU as solid electrolyte. We report the current-voltage-light output characteristics, morphology and ion transport behavior of the device. Cyclic voltammetry has been performed to reveal the ionic and electronic contributions of current. The threshold voltage for electrochemical doping of PPV in ITO/WPU+PPV/Al device was evaluated. Scanning force micrograph of blend of WPU with PPV is presented. The alternating current (ac) impedance analysis was made in the frequency range 1 MHz to 1 Hz to bring out the impedance changes with applied (bias) dc potentials. Results demonstrate that WPU can be used as a single component electrolyte in LEC fabrication in contrast to two components (polymer and lithium salt) used earlier. The frequency dependent conductivity of the blend containing two different charge carrying material, a conjugated polymer, PPV and ionomer, WPU, was investigated in the frequency range of 0.1 kHz to 1000 kHz by impedance spectroscopy and compared with the individual material, PPV and WPU, respectively. Clear differences could be seen in the dependence on both real and imaginary parts of impedance between WPU, PPV and the blend film. At high frequencies (> 200 kHz), all the three materials, exhibit power law dependence. Simulation of s value through theoretical fitting reveals that s value of the blend (1.108) is in between PPV (1.289) and WPU (1.035) and signifies the mixed contribution of carrier transport of PPV and WPU in the blend. The morphology of blend as observed from AFM picture informs that WPU connects the islanded parts of PPV and provides path for carrier conduction. Results observed indicate that carrier transport operates through barrier hopping in these materials and the difference in the dependence of on in the blend is attributed from the influence of SO3- group in WPU on the carrier transport of PPV. A blend of lithium ion conducting WPU and MEH-PPV was used in fabricating a single layer LEC, ITO/MEH-PPV+WPU/Al. Cyclic voltammetry, ac impedance spectroscopy and atomic force microscopy were employed to characterize the current density (I)-voltage (V)-light intensity (L), ion transport and morphology of the device. This device behaves differently than conventional LEC having a biionic electrolyte with regards to direction of applied potentials. The role of ions in WPU towards electrochemical doping of MEH-PPV is explained in the background of experimental results.