(A) study on side chain effects in polymer light-emitting devices using polymer blends and white organic light-emitting devices

Abstract

Polymer light-emitting devices (PLED) have attracted strong attention due to their potential use in display technology. Until now, many researches have been focused on improving the efficiency of electroluminescence (EL) devices. For improving the efficiency of EL devices, energy transfer can be used by conjugated polymer blend. Especially this energy transfer is called Foster energy transfer which is dependent on the energetic overlap of the donor emission spectrum and the acceptor absorption spectrum. Although the energetic overlap is the primary condition for energy transfer, it is not enough to explain what factors affect the efficient energy transfer. To find other conditions in the efficient energy transfer, EL devices were made by blending polyfluorene derivatives (PFO) with poly (2-methoxy-5-(2``-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEHPPV). As changing side chain length and shape at the C-9 position of PFO, we observed not only the side chain length effects but also side chain shape effects in the energy transfer. PFOs, such as poly(9,9-dihexylfluorene) (PDHF), poly(9, 9-dioctylfluorene) (PDOF) and poly(9,9-dicyclohexylheptylfluorene) (PDHHF), were used as donor materials and MEHPPV was used as an acceptor material in this experiment . In photoluminescence (PL), photoluminescence excitation (PLE) and EL, the efficient energy transfer was observed in PDHF/MEHPPV and PDOF/MEHPPV blend, while the relatively inefficient energy transfer was observed in PDHHF/MEHPPV blend. The phenomena of that can be explained by the morphology of polymer films and the rate and efficiency of energy transfer calculated by the fluorescence decay life time in polymer blends. In AFM images, it is confirmed that MEHPPV components were dispersed within both PDHF and PDOF matrices. However, phase separation occurred between PDHHF and MEHPPV. That is, MEHPPV was completely separated from PDHHF. The latter case showed the inefficient energy transfer. In the calculation of rate ...