Study On Mechanism Of Organic Light Emitting Devices Based On Nitrile Fluorescent Dyes

Organic light emitting devices (OLEDs) have been considered as one of the mostpotential flat-panel display and solid-state lighting due to their significant advantages,e.g. self-emission, low-energy cost, fast response, high contrast, super-thin thickness,temperature enduring, simple production process, widely sources of materials,flexibility, as well as large-scale fabrication. Although OLEDs have stepped to thecommercialization stage, there are some shortcomings to prevent their rapiddevelopment such as high processing cost, low external quantum efficiency and shortstability. Therefore, the basic research on the synthesis of novel materials, optimizationof device architecture and understanding of device mechanism are still playing animportant role to promote the device performance. Aiming at these problems, this paperpresent a systematic works on the complementary-colors white OLEDs, three-primarycolors white OLEDs (WOLEDs) and red phosphor-sensitized-fluorescent OLEDs byusing two novel nitrile fluorescent dyes. The balance of charge carriers trasporting,stability of chromaticity coordinate, enhancement of color rendering index and lowerenergy loss are especially focused on.1. A yellow nitrile fluorescent dye (2Z,2’Z)-3,3’-bis(2-phenylacrylonitrile)(1,4-phenylene)(BPhAN) as dopant and poly(N-vinylcarbazole)(PVK) as host wereblending together to fabricate complementary-colors white OLEDs via spin-castingprocess. The theoretical calculation of tristimulus values for BPhAN and PVK indicatesthat it is possible to obtain white light by using the two materials. In experimentally,balanced white emission with Commission Internationale d’Eclairage (CIE) coordinatesat (0.32,0.33) was observed. The effects of different doping ratio and driving voltage onthe device performance were discussed. The incomplete energy transfer from PVK toBPhAN and the charge trapping effect of BPhAN are the main factors to promoterecombination. Moreover, to solve the problem of low efficiency, a charge carriertransporting balanced experiment was designed based on a composite hole transportinglayer (CHTL) comprised of NPB and PVK. The results showed that dopingconcentration of NPB not only enhanced the competence of hole transporting ability, but also modified the recombination region of charge as well as the surface morphologyof doped film. The efficiency of the CHTL devices exhibited a power efficiency of164%higher than that of the single HTL devices.2. A red nitrile fluorescent dye3-(dicyanomethylene)-5,5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene (DCDDC) was employed to fabricate three-primarycolors WOLEDs by using doped structure and ultrathin structure via thermalevaporation process, respectively. The doped WOLEDs of dual emitting layers (EMLs)showed a turn on voltage of5V. The maximum power efficiency and brightness in thefront viewing direction is1.4lm/W and12580cd/m2, respectively. The CIE coordinatesnear (0.33,0.33) with CRI of85were obtained as the bias voltage over9V. This wasattributed to the shift of recombination zone between the EMLs. The ultrathin WOLEDsof three EMLs showed a turn on voltage of4V. The maximum power efficiency andbrightness in the front viewing direction is2.4lm/W and16690cd/m2. Pure whiteemission with a good color rendering index (CRI) of80was achieved as low as5V.The CIE coordinates near (0.33.0.30) shows slight variation of (-0.020,+0.002) from5-13V. The achievement of white emission at low-operation voltages and high-colorstability were attributed to the confined emission zone by the thin EML and enhancedenergy utilization by direct carrier trapping within ultrathin layer.3. The effect of energy level matched system incorporated of DCDDC and phosphorsensitizers on the performance of red phosphor-sensitized-fluorescent OLEDs werestudied. Two phosphorescent materials fac tris(2-phenylpyridine)iridium (Ir(ppy)3) andbis[2-(4-tertbutylphenyl)benzothiazolato-N,C2’]iridium(acetylacetonate)[(t-bt)2Ir(acac)]were used as phosphor sensitizers to dope with DCDDC and CBP as the emitting layer,respectively. By optimizing the doping ratio and layer thickness, two redphosphor-sensitized-fluorescent OLEDs were constructed. The Ir(ppy)3based deviceand (t-bt)2Ir(acac) based device illustrated a turn on voltage of3.6V, a maximumbrightness of11307cd/m2and15871cd/m2, and a power efficiency of4.7lm/W and8.5lm/W, respectively, which were two (or three) times higher than the correspondingfluorescent devices. The electroluminescence (EL) spectra of Ir(ppy)3based deviceshowed a main red emission peaked at625nm, with obvious green and blue emissiveshoulders, resulting in the CIE coordinates located at (0.54±0.02,0.41±0.02). In contrast,the EL spectra of (t-bt)2Ir(acac) based device exhibited a main red emission peaked at 635nm, with a weak yellow emissive shoulder, resulting in the CIE coordinates locatedat (0.62±0.01,0.36±0.01). The improved efficiency and red emission purity wereascribed to a lower energy barrier between (t-bt)2Ir(acac) and DCDDC, whichinherently suppresses the strong charge trapping on DCDDC molecules as well asreduce the energy loss.4. The effect of triplet energy level and other intrinsic parameters of electrontransporting materials (ETMs), hole transporting materials (HTMs) and host materialson the performance of red fluorescent-phosphor-sensitized OLEDs were investigated.By combining the codoped system of (t-bt)2Ir(acac):DCDDC:CBP as EML and differentfunctional materials as the host or charge transporting layers, the devices werefabricated and analyzed. The results showed that there is no apparent correlationbetween the triplet energy level of ETMs and luminous efficiency. Instead, a highelectron mobility of ETM can effectively balance the charge carriers transporting andenhance the efficiency. Interesting, the device performance was sensitive to the tripletenergy level of HTMs. A HTM with a higher triplet energy level could confine thetriplet excitons and reduce the energy loss. Moreover, a host possessed a bipolar chargetransporting characteristic can greatly enhance the carrier recombination efficiency.Therefore, by selecting the proper functional materials, the performance of redfluorescent-phosphor-sensitized OLEDs is significantly improved. A maximum currentefficiency and power efficiency were20.5cd/A and16.5lm/W, respectively. and amaximum brightness of25530cd/m2at the front viewing direction was achieved. TheEL spectrum showed a saturated red emission peaked at635nm, with CIE coordinatesat (0.61,0.37).