Study Of Realizing Highly Efficient Polymer Light-emitting Diodes By Balancing Charge Carrier Injection And Transport

Organic light-emitting diodes (OLEDs) have drawn great attention due to their broadapplication prospects in full-color flat-panel displays, back-lighting sources for liquid-crystaldisplays and solid-state lighting. Compared to vapor-deposited organic light-emitting devices,polymer light-emitting diodes (PLEDs) based on solution-processed technology attracts muchmore attention due to their some obvious advantages in low-cost manufacturing process, largearea fabrication, and so on. However, the efficiency and stability of PLEDs can not meet theactual applications and should be further improved. In addition, the color rendering index(CRI) is also an important parameter to measure the performance of white PLEDs. How toobtain a perfect white light with superior efficiency and excellent CRI simultaneously is animportant target. We provide an avenue to fabricate highly efficient PLEDs by studying theaggregation state of the polymer films, charge carrier injection and transport and differentdevice structures.Firstly, a systematic study on how the solvent with different polarity and boiling point(chlorobenzene, p-xylene and1-chloronaphthalene) affects the film morphology, chargetransport properties and device performance of blue light-emitting poly(9,9-dioctylfluorene)(PFO) was reported. The content of β phase in PFO can be controlled by the selection ofsolvent/mixed solvent with a range of polarities and boiling points. It was found that as muchas26.9%β phase in films was obtained through casting from a common solvent p-xylene anda lower solvent power and higher boiling points solvent1-chloronaphthalene mixture. Mostimportantly, the efficiency and color purity of the device can be improved simultaneously dueto many more β phase that possess higher photoluminescence quantum efficiency and makethe hole and electron transport balanced. A maximum luminous efficiencies (LEmax) of1.42cd A-1with CIE coordinates (0.16,0.10) was achieved by using mixed solvent, about200%and120%increase compared to the devices based on chlorobenzene and p-xylene-spun films,respectively. It may have an enormous potential in improving the device performance ofPFO-based copolymers. This mixed solvent effect on a PFO-based white-emitting copolymerPF-DTBTA0.1was further investigated based on the former study. The LEmaxcan beimproved from3.87cd A-1to5.10cd A-1。Considering the CRI of the white PLEDs was low, a red phosphorescent molecule Ir-G2was doped in PF-DTBTA0.1. The white emissionspectra was greatly broadened and the CRI was successfully improved from59to86.Secondly, a series of blue (B), green (G) and red (R) light-emitting,9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene (PPF) based polymers containing adibenzothiophene-S,S-dioxide (SO) unit (PPF-SO polymer), with an additionalbenzothiadiazole (BT) unit (PPF-SO-BT polymer) or a4,7-di(4-hexylthien-2-yl)-benzothiadiazole (DHTBT) unit (PPF-SO-DHTBT polymer) arecharacterized. These polymers exhibit high fluorescence yields and good thermal stability.Light-emitting diodes (LEDs) using PPF-SO25, PPF-SO15-BT1, and PPFSO15-DHTBT1asemission polymers have maximum efficiencies LEmax=7.0,17.6and6.1cd A1with CIEcoordinates (0.15,0.17),(0.37,0.56) and (0.62,0.36), respectively. The optical gainproperties of the blue light-emitting PPF-SO in amplified spontaneous emission (ASE)measurement and1D distributed feedback (DFB) lasers were studied. These polymersdisplayed an excellent ASE threshold stability.1D DFB lasers using PPF-SO30as the gainmedium are demonstrated, with a wavelength tuning range467to487nm and low pumpenergy thresholds (≥18nJ). Blending different ratios of B (PPF-SO), G (PPF-SO-BT) and R(PPF-SO-DHTBT) polymers allows highly efficient white PLEDs to be realized. With adiode configuration incorporating a PFN EIL and a B:G:R blend ratio100:8:7by weight, theLEmaxreached9.8cd A-1and the maximum power efficiency reached8.9lm W-1. Moreover,the optimized devices have an attractive color temperature close to4700K and an excellentcolor rendering index (CRI)≥90. They are relatively stable, with the emission colorremaining almost unchanged when the current densities increase from20to260mA cm2.The use of these polymers enables white PLEDs with a superior trade-off between deviceefficiency, CRI, and color stability.Thirdly, inverted PLEDs (IPLEDs) using solution-processed ZnO film with aconfiguration of ITO/ZnO/PEIE/PF-3,7FSO10/MoO3/Al were fabricated. By using the PEIEas a surface modifier layer and varying its thickness, the LEmaxreached3.16cd A-1and themaximum luminance (Lmax) reached8429cd m-2. The performance of optimized IPLEDs isequal to the conventional PLEDs (CPLEDs), which is attributed to the goodelectron-injecting and hole-blocking ability of PEIE and less quenching of photoluminescence at the interface of ITO/ZnO/PEIE. In addition, the different thickness ofthe PEIE affects on the roll-off of LE was studied, which is attributed to quenching at theinterface induced by the charge accumulation. At the current densities of300mA cm-2, theLE of IPLEDs decreased about18%from the maximum LE value, but52%for the CPLEDs,showing excellent stability. LE and LE roll off trade-off was preferably realized. In addition,the lifetime of CPLEDs and IPLEDs were tested. Compared to CPLEDs, the lifetime ofIPLEDs was increased greatly, but still to be further improved. It is available for fabricatinghighly efficient and stable white PLEDs.