Polyfluorene devices, performance

Keywords Device performance Electron-transport moiety Hole-transport moiety Polyfluorenes Polymer light-emitting diode... 

The stracture formed by donor acceptor BHJ blends is dependent on the solvent used. Studies of polyfluorene PCBM blends show that choice of solvent controls the segregation of materials within the blend film so that a vertical multilayer structure forms spontaneously in some cases (Bjorstrom and Moons, 2007). Similarly, the choice of solvent in a polymer-polymer blend controls the vertical arrangement of materials and the device performance (Arias et al, 2002). Exploitation of these effects may open new avenues for materials-processing procedures in the organic photovoltaic field. 

Fig. 3.5. (a) The general structure of polyfluorenes, and (b) the variant F8T2. Polyfluorenes and many of their derivatives are liquid crystals which can be crystallized after deposition to improve the pi-pi overlap and device performance. The alkyl chains tethered to the fluorene group solubilize the material. 

Koch, N., Elschner, A., and Johnson, R.L. (2006) Green polyfluorene-conducting polymer interfaces energy level alignment and device performance. /. Appl. Phys., 100,024512. 

Polyfluorenes have been developed as promising candidates in organic electronics applications such as polymer light-emitting diodes (PLEDs) and polymer transistors. In the early stages of development, the relationships between polymer structure and device performance were investigated. This is still an important... 

Edman, L., Liu, B., Vehse, M., Swensen, J., Bazan, G.C., Heeger, A.J., (2005), Singlecomponent Light-emitting Electrochemical Cell Fabricated from Cationic Polyfluorene Effect of Film Morphology on Device Performance. /. Appl. Phys. Vol.98, 044502. 

These compounds provided the source material for the first blue OLEDs.72 However, these devices were short-lived. Yet devices fabricated with improved blue-emitting amino oxadiazole fluorene did exhibit greater efficiency and stability,9 although their performance was still inferior to that of polyfluorene-based PLEDs (see below). 

Abstract Poly(dibenzosilole)s are an emerging class of polymers with similar optoelectronic properties to polyfluorenes. With increased stability towards oxidation, several poly(dibenzosilole)-based devices, such as light emitting diodes, have shown improved performance over their polyfluorene counterparts. As a consequence of reduced conjugation in the polymer chain, some poly(dibenzosilole)s have high triplet excited state energies, which make them suitable hosts for blue triplet emitters in electrophosphores-cent devices. 

Poly(dibenzosilole)s are a relatively new class of compounds for the area of organic electronic materials. Poly(dibenzosilole)s are not easily oxidised and have advantages of solubility and processibility over other polyfluorene analogues, such as carbazoles, dibenzophosphole oxides and dibenzothiophene dioxides. Several groups have already incorporated poly(dibenzosilole)s in the latest organic electronic devices such as OLEDs, OFETs and OSCs and have found improved performance over similar polyfluorene-based devices. With continual advancement in their synthesis, dibenzosilole-based polymers are set to match the popularity of polyfluorenes in organic electronic materials. 

The performance achieved by these devices was record-setting, with each of their characteristics independently superior to reported values from any other polarized OLEDs, including those using polyfluorenes [9-13]. Recently, an integrated polarization ratio of 31 has been reported but with an efficiency of 0.3 cd A-1 for a deep blue OLED comprising poly(9,9-dioctylfluorene) [198]. Two other papers have reported polarization ratios at their emission maxima of 25.7 [199] and 29 [200], which are still slightly lower than 31.2 reported for the blue dodecafluorene device [196]. 

Finally, white-emitting fluorescent/phosphorescent diodes using a polyfluorene host have been reported by Gong et al. Details about the composition and the performance of these devices are contained in Chapter 4. 

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