Balancing charge transport/injection

Likewise, copolymers 90 have higher EL efficiency than homopolymers DHF-PPV 57 or MEH-PPV 13, due to more balanced charge injection and transport properties. Copolymer 90 with 7.5 wt% loading of dialkoxyphenylene comonomer gave the highest QE. The device with an ITO/PEDOT/90/Ca configuration showed remarkably higher efficiency (2.4cd/A) than devices fabricated with other copolymers in the series (0.65-1.0cd/A) [141],... 

Enhancement in the performance of OLEDs can be achieved by balanced charge injection and charge transport. The charge transport is related to the drift mobility of charge carriers. Liu et al. [166] reported blue emission from OLED based on mixed host structure. A mixed host structure consists of two different hosts NPB and 9,10-bis(2 -naphthyl)anthracene (BNA) and one dopant 4,4 -bis(2,2-diphenylvinyl)-l,l -biphenyl (ethylhexyloxy)-l,4-phenylene vinylene (DPVBi) material. They reported significant improvement in device lifetime compared to single host OLEDs. The improvement in the lifetime was attributed to the elimination of heterojunction interface and prevention to formation of fluorescence quenchers. Luminance of 80,370 cd/m2 at 10 V and luminous efficiency of 1.8 cd/A were reported. 

In all charge transport processes, several factors govern overall efficiency [25]. First, the generation of free carriers within the CGL is field dependent. Second, at the interface of the CGL and CTL, injection processes can be rate limiting. Finally, the mobility of the carriers in the CTL can be limiting. These factors are balanced delicately in functional electrophotographic systems to optimize performance-response characteristics. A full discussion is beyond the scope of the present chapter. 

There are several possibilities for constructing bilayer OLEDs with a more balanced charge injection. These include an electron-transport layer (ETL) and a combined hole-transport (HTL) and emission layer. Conversely a hole-transport layer and a combined electron-transport and emission layer is also effective. 

In this chapter, we showed that OLED performance is clearly defined based on the simplified working mechanism of carrier injection and successive SCLC. The balance of holes and electrons injected within an emitter layer is a major factor contributing to overall EL quantum efficiency. The unipolar-charged-transport layers, HTL and ETL, contribute to the increase of quantum efficiency. 

An even better balance in the injection of the opposite charges was found when another layer more suitable for hole transport, such as unsubstituted PPV, is used [70]. A doublelayer LED, formed by ITO/PPV/cyano-PPV/Al has an internal quantum efficiency as high as 4% with emission at 610 nm [70]. The cyano-PPV approach was extremely useful and other withdrawing substituents were introduced in the PPV backbone with the aim of increasing the electron affinity other electron-deficient nitrogen containing groups such as oxadiazole [102,103], triazole [94], pyridine [104,105] and quinoxaline [106] were also introduced in the polymer structure. 

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