Study On Semiconducting Polymers Light-emitting Diodes

Polymer /Organic light-emitting, as new research field, has been a hotspot in flat panel displays and attratted more and more people. Higher and higher technology for information display is required as a result of rapid development of information technology. Display panel with rich color, low cost,environmentally protection, lightweight, even flexibility, has become a goal of modern people. Polymer /Organic light-emitting diodes would be an ideal technology for information display of modern times due to its low driving voltage, rich color, fast response,wide viewing angle, portability,etc. but their shortcomings oflow efficiency short life and unstable properties need solving urgently. Nowadays researchers in this field throw themselves into exploiting high electroluminescence efficiency, stable luminescence matenals and carrier transport materials, improving device architecture and otherwise. In contrast to the rapid progresses made in the experimental front, systematic comprehensive understanding of the various processes and the physics involved Polymer /Organic light-emitting diodes is important.The purpose in this dissertation is improving of efficiency and stability for polymer/organic light-emitting diodes. Relatively in-depth study is processed at material synthesizing and device structure as well as fundamental theory. The main researchful method and conclusion in my dissertation are summarized as following:1) Basing on molecular and material designs, a series of the novel polymers with charge transporting property have been designed and syntgesized. Monomer N,N'-diphenyl-N,N'-bis(4-alkylphenyl)-benzidine(alkyl-TPD) prepared by Ullmann reaction of N,N'-diphenylbenzidine with 1-halogen-4-alkylbenzene using 18-crown-6 as phase transfer catalysts and o-dichlorobenzene as solvent was reacted with 1,4-bischloromethylbenzene (BCB) or 9,10-bischloromethylanthracene (BCA) by condensation polymerization through Friedel-Crafts reaction in chlorobenzene,using SnCl₄ or AlCl₃ as catalysts TPD units are introduced into the main chain of polymer. The energy band structure of poly-TPD (PTPD) have been studied by combing cyclic voltammetry (CV) and UV-Vis absorption. It is indicated that the energy band structure of poly-TPD have few change after TPD polymerize. The glass transition temperature (T_g) of poly-TPD was measured. It find that all polym-TPD had higher T_g than that of TPD itself(maximum of T_g was 245℃). Consequently, the polymers with charge transporting property have good themal stability. They have an advantage for the light-emitting diodes with high stability prepared.2) A novel structure of light-emitting diodes have been designed and fabricated. The method adopted is doping in polymer/small-molecules heterojunction. The basic structure of the heterostructure is PTPD/Alq3. The highly fluorescent rubrene and DCJTB were selected as dopant. The devices of efficiency and stability were research. When hole transport layer (HTL) and electron transport layer (ETL) are doped simultaneously with Rubrene, the quantum efficiency of the doped devices at 1.47% is about 2 times larger than that of the undoped device at 0.74%. When hole transport layer (HTL) are doped with Rubrene and electron transport layer (ETL) are doped with DCJTB, the EL quantum efficiencies are about 3 times greater than that of the undoped device. The stability of the doped devices is significantly higher than that of the undoped device and the stability of the double doped devices with different dopant is the highest. In addition, the durability of the four devices with PTPD as HTL is higher than that of the referenced device with TPD as HTL.3) The emission mechanism for polymer/small-molecules heterojunction doped device is proposed. It is found that there exists a phenomenon between the host and guest interconverted. Based on their EL spectra, the emission mechanisms for doped device are carrier trapping and Forster energy transfer processes working together. In addition, the mechanism of the enhanced stability for light-emtting diodes is proposed. We consider that the stability for light-emtting doped diodes is extremely relation with reactivity of singlet (S₁) and triplet (T₁) exciton of host and dopant materials, and the electrochemical stability of materials.4) The model of trap effect for Polymer doped small-molecules light-emitting diodes is founded. Polymer doped small-molecules light-emitting diodes have been fabricated using a novel PTPD (poly-TPD) as the hole transport material and the highly fluorescent Rubrene as the dopant. Electroluminescent characteristics are studied at different doped concentrations and different film thicknesses. The results indicate the presence of doping trap effect. It is assumed that the traps are limited at discrete levels. Based upon the double-carrier injection theory, an analytical model of J-V characteristics of doped device is derived by solving the Poisson' equation. Calculated data of this model are consistent with our experimental results. Based on the trap modle, there are conclusions as following: a) doping result in effective charge mobility changed in organic layer of device. The more doped concentration, the less effective charge mobility, the less electric current in the same voltage.b) discrete trap energy-level is more sensitive to model of J-V characteristics of doped device than trap density. In addition, influence of factor for light-emitting performance in polymer doped small-molecules device is investigated. It is found that doping in device result in light-emitting efficiency increase and can control light-emitting color. In low doped concentration, light-emitting efficiency increase as doped concentration increase. In high doped concentration, light-emitting efficiency decrease as doped concentration increase. Thus, there exists an optimization doped concentration.