Abstract
Polymer light-emitting diode (PLED) has drawn great attention due to its potentiality for fabrication of large-area, light-weight and flexible displays by solution process. Compare to the advantages of those conventional cathode ray tube (CRT) with heavy-weight and electricity consuming and liquid crystal display (LCD) with narrow angle of view and low reacting time, PLED owns the characteristics of light-weight, high brightness and fast reacting time. Owing to high electrical conductivity and excellent visible light transmitting property, Indium Tin Oxide (ITO) is widely used as the anode in optical devices. However, its work function (~4.7 eV) is low relative to the highest occupied molecular orbitals (HOMOs) (>5.5 eV) of fluorene-and phenylene-based emitting polymer, which results in a large hole injection barrier. Thus, further surface treatment is required to lower the barrier, such as (i) introducing thin organic interlayer like poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) as hole injection layer (HIL), (ii) utilizing O2 plasma, ultraviolet (UV)-ozone or CFx plasma treatment to adjust surface atomic composition and meanwhile remove the contamination on ITO, or (iii) inserting self-assemble monolayer (SAM) to increase the work function by interfacial dipole effect. However, the multilayer device structure and further surface treatment not only complex the fabrication, but also raise the risk of failure. Recently, Lu and coworkers reported that the work function of ITO can be raised dramatically from 4.7 to 6.13 eV via exposing ITO to o-dichlorobenzene (ODCB) under UV irradiation at 254 nm. With the chlorination, the hole injection barrier between Cl-ITO (work function 6.13 eV) and the hole transport layer, 4,4’-N’,N’-dicarbazole-biphenyl (CBP) (HOMO 6.1 eV), was eliminated completely and the resulting green phosphorescence organic light-emitting diodes achieved the outstanding performance, 97 lm/W at 100 cd/m2. This thesis is separated into four parts. To begin with, we study on the fabrication of Cl-ITO, alternate steps of process and discuss different factors affecting the work function and atomic composition of chlorinated-ITO (Cl-ITO). Then, Cl-ITO is introduced into the poly(9,9-dioctylfluorene) (PFO)-based deep blue and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV)-based orange PLED system. Finally, caesium fluoride and 1, 2-dibromoebnzene are used to modify ITO, enhance the work function of ITO and applied to the PFO-based device. In Chapter. 4, Cl-ITO was fabricated in several ways and its characteristics was carefully studied. For the fabrication of Cl-ITO, the UV-ozone treated ITO was covered with ODCB exposing to UV-irradiation from a low-pressure mercury lamp (SEN Light PL16-110) and the Cl radicals are liberated from solvent, displaced oxygen on the ITO and formed Cl-In monolayer. Both the Cl-In dipole layer and atomic composition of the surface of ITO affecting the enhancement of work function of ITO, the removal of carbon contamination is the most important role, especially. The Ar-etching analysis, to understand the depth element composition profile, reveals that carbon contamination mainly distributed on the surface of ITO. Therefore, we try several ways to reduce the amount of carbon. In the ITO pre-treatment, UV-ozone (15 min) is the most effective and easily control one; in the ITO post-treatment, the longer the UV-ozone treating time, the fewer the carbon. When the treating time up to 10 min, carbon amount drops obviously and remains similar while increasing the treating time, and it also gives the largest hole injection in the device. When about to the ODCB treatment, the much the ODCB amount and the more concentrated of the ODCB solution, the larger amount of the Cl-In and the higher work function of the Cl-ITO. Briefly, the work function of Cl-ITO could be raised to high as 5.58 eV in the process of pre-UV-ozone (15 min), ODCB (5 min) and post-UV-ozone (10 min). In the Chapter. 5, we introduce the Cl-ITO into the poly(9,9-dioctylfluorene) (PFO)-based deep blue device. Poly(9,9-dioctylfluorene) (PFO) is widely used because of its quite high photoluminescence quantum efficiency (PLQE) 59 % as thin film, good thermal and color stability and excellent film-forming quality. Also, β-PFO got by solvent treated has the capability of electron trapping and enhancement of hole mobility. However, the device suffers from low hole current due to the high hole injection barrier since it possesses much higher HOMO level (~5.8 eV). In this work, we introduce the Cl-ITO into the β-PFO-based device to solve the problem of poor hole injection and improve the device performance. We found that residual chlorine free radicals on the surface of Cl-ITO will quench the exaction on β-PFO and the quenching effect could be reduced by treating Cl-ITO with aqueous ammonia solution to neutralize radicals and remove it with different concentration and treating time. The device Cl-ITO (NH4OH 1 %, 3 min)/β-PFO/CsF/Al achieve the maximum brightness 16773 cd/m2 and maximum luminance efficiency 2.4 cd/A, which are higher than those of the device with untreated Cl-ITO by factors of about 7 and 9, respectively. Also, we also fabricate the device with cathode replacing poly[9, 9’-bis96’-((1, 4, 7, 10, 13, 16)hexaoxacyclooctadecanyl) methoxy] hexyl) fluorenen] (PCn6) chelating to K+ as the electron-injection layer (EIL) with unstable environmentally CsF as the cathode. The device gives the high brightness 28550 cd/m2 and high luminance efficiency 2.60 cd/A. The enhancement is ascribed to the reduction of the electron barrier and facilitation of electron transport provided by PCn6:K+ forming electron –transport channel. This method is not only simple in device fabrication but also owns a dramatic enhancement in the hole injection. In Chapter. 6, caesium fluoride (CsF) solution and 1,2-dibromobenzene are used to surface modify ITO. On the one hand, after treating with CsF solution (2 mg/ml), the work function of ITO can be raised to 5.75 eV which is suitable for using as anode to lower the injection barrier and increase the hole injection. The device gives poor performance and we ascribe the result to the some particles, may be CsF salt on the ITO leading to leakage current. On the other hand, we replace ODCB with 1,2-dibromoebnzene to treat ITO and discuss the characteristic of other halogenated-ITO. The device with Br-ITO anode exhibited bad performance because of particles on the ITO. Also, the work function of Br-ITO isn’t as high as Br-ITO so that it couldn’t provide larger hole injection into the device. In Chapter. 7, we introduce Cl-ITO into MEH-PPV-based orange PLED system. Even though there is no problem for hole injection from ITO (5.0 eV) or ITO/PEDOT:PSS (5.2 eV) to MEH-PPV (HOMO 5.1 eV), we utilize the high work function and the comparable high conductivity of Cl-ITO to enhance hole injection and on the other hand, we use CsF/Al as device cathode to increase electron injection. In this way, we aim to increase the chance of recombination, lower the turn-on voltage and enhance the performance. The device Cl-ITO/MEH-PPV/LiF/Al gives the highest brightness 12486 cd/m2, and the highest efficiency 1.23 cd/A. In addition, the device Cl-ITO/MEH-PPV/CsF/Al gives the highest brightness 12246 cd/m2, and the highest efficiency 0.68 cd/A. The lower luminance efficiency with cathode CsF/Al is ascribed to the unbalance of carrier injection due to the surplus of electron injection providing by high work function CsF (2.2 eV).