Abstract
The object of this research is to develop convenient physical processes can be used to (1) enhance the device performance and color purity of blue poly(fluorene)s (PFs), and (2) simultaneously tune the emitting color and improve the performance of PFs-based device. We first study the effect of the emitting species formed by the interaction between PFs main chains on electroluminescence (EL) properties of PFs. A simple dipping process which can be applied to PFO for the enhancement of its EL color purity and device performance is developed. Moreover, this dipping process is innovated to be capable of tuning the emitting color and improving device performance by dipping TazPFO film into organic liquids with dye or functional molecules (i.e., cesium carbonate), and named as “wet-deposition process”. This wet-deposition process can be successfully applied to the fabrication of dual-color polymer light-emitting displays. In order to investigate the reason for the growth of long-wavelength part (470~650 nm, termed as green component) in EL spectrum, IR (infrared) and EL spectra for PFO-based device before and after operation are analyzed and find that the amount of keto defect after operation is 80% larger than that before use. However, the intensity of green component only increases 10% after use, indicating that the growth of green component does not totally result from the keto defect. This is further supported by the different wavelength of EL peak for this green component (at 485 and 520 nm) and keto-defect-based green emission (at 535 nm). From the PL and PLE spectra for PFO and POBOHF device before and after use, it is reasonable to infer that this green component results from the fluorene-based excimers. Furthermore, the excimers (hereafter, designated as FI-excimers) are formed due to the approach of main chains by the induction of electric field accompanied by the device operation and the motion of side chains as evidenced by the measurements of thermal annealing, EL spectra at various temperatures (from 78 to 330 K), and field-induction thermally stimulated current (FI-TSC) for POBOHF. By the comparison of intensity ratio of green component relative to blue component for POBOHF- and PFO-based devices, the higher polarity of side chain in the PFs can cause more pronounced FI effect. In addition, the motion of main chains can facilitate the formation of FI-excimers for PFO but lower the content of FI-excimers for POBOHF due to the different polarity of side chains for these two polymers. Lowering the content of cross-linkable commoner in the copolymers and cross-linking the POBOHF only moderately suppresses the formation of FI-excimers. Consequently, in view of obtaining high-purity blue color, the incorporation of high-polarity moiety at side chain should be avoided. By dipping spin-coated PFO films in mixed organic liquids (THF/MeOH with volume ratio = 1/2 or 1/1) for various time periods, the content of β phase can be finely tuned from a negligible level to 1.32%, which is the highest β-phase content can be formed by this dipping process. Due to its special conformation, β phase can improve EL color purity and stability by suppressing the formation of FI-excimers. In addition, energy transfer and charge trapping mechanism also play important roles on this suppression of green emission. The weaker intensity at long-wavelength part (480~650 nm) results in smaller CIE x+y value for β-PFOs, especially for β-PFO (1.32 %), (0.168, 0.115) at 4 V. Moreover, β phase can enhance maximum brightness and efficiency, especially at high β-phase content. For pristine PFO, its maximum brightness as well as current efficiency are 12,573 cd/m2 (9 V) and 1.26 cd/A (3.8 V), respectively. However, the device based on PFO with 1.32% β phase exhibits a current efficiency of 3.85 cd/A at 3.8 V and its maximum brightness reaches 34326 cd/m2 at 9 V, which is the highest performance among deep-blue PLEDs reported in literature. The enhancement of efficiency results from the better balance of hole and electron fluxes due to the increase of hole flux and the decrease of electron flux with increasing β-phase content as revealed from the current density measured from hole- and electron-only devices. The decrease of electron flux for β-PFOs is due to that β phase possesses the property of electron trapping (electron-trap depth is 0.07~0.11 eV). This is demonstrated by the result of TSC measurements and the analysis of ultraviolet-visible (UV) absorption spectra and cyclic voltammetry (CV) curves. The increase of hole flux is not due to the lowering of hole injection barrier height (ΦB), since ΦB values for PFO with or without β phase are very close as determined from ultraviolet photoelectron spectroscopy (UPS) measurements. From the result of time-of-flight measurements for the thin films prepared by spin-coating process, the presence of β-phase chains can promote the hole mobility. Hence, the larger hole flux for β-PFOs actually results from the higher hole mobility of β-phase chains. After TazPFO films are separately dipped in TBPe, C545T, and DCJTB organic liquids, PL spectra exhibit the emission colors of organic dyes, indicating that dye molecules can diffuse into TazPFO film during this wet-deposition process. From X-ray photoelectron spectroscopy (XPS) measurements assisted with argon-ion etching, C545T can diffuse into a TazPFO film after this dipping process (C545T concentration is 5 mg/ml and dipping time is 1 min) by a depth of about 6 nm with a maximum concentration of 3.22 wt% at 0.6 nm below the film surface. In addition, the maximum DCJTB concentration in TazPFO after the dipping process (DCJTB concentration is 5 mg/ml and dipping time is 1 min) is lower than 5 wt%. On the other hand, EL spectra of TazPFO-based devices can be shifted from blue color of β phase to sky-blue (TBPe), green (C545T) and orange-yellow (DCJTB) colors due to the charge-trapping mechanism, demonstrating the ability of color tuning for this process. Due to the decrease of hole and electron current densities resulting from the presence of organic dyes, device performance can be improved by treating an ITO glass with CFx-plasma (increase the hole injection numbers) and introducing Cs2CO3 material (increase electron injection numbers and serve as hole-blocking material) between emitting polymer and cathode by this wet deposition process. After these modifications, turn-on voltages are lowered to 4.2, 5.8, and 6.2 V for TazPFO dipped in TBPe, C545T and DCJTB organic liquids (5 mg/ml for 1 min), respectively. In addition, the maximum brightness is improved to 4931, 14966, and 5175 cd/m2 for TazPFO dipped in TBPe, C545T and DCJTB solutions, respectively. This proves that this process also can be applicable to improve device performance. Most importantly, this wet deposition process can be successfully applied to the fabrication of dual-color device by separately dipping two parts of one spin-coated TazPFO film into two dye organic liquids, demonstrating the potential of this process in the fabrication of dual-color, even full-color, displays.