Dynamics of Polarons in Guest-Host-System Polymer Light Emitting Devices
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ABSTRACT The conjugated ladder-type poly(paraphenylene) is an attractive material for blue polymer light emitting devices (PLED). Blending the active layer with small amounts of a red emitting guest polymer, the emission shifts from blue to red with increasing guest concentration due to efficient excitation energy transfer. The results of photoluminescence detected magnetic resonance, electroluminescence detected magnetic resonance measurements and current detected magnetic resonance measurements on PLEDs based on 0.05w%/o - 2w%/o red emitting poly(perylene-co-diethynylbenzene) (PPDB) in the active layer of the PLED are presented and discussed. INTRODUCTION One of the advantages of using conjugated polymers as electroluminescent emitters is the possibility to vary the degree of conjugation by chemical tailoring[1,2] which results in a change of the emission color[3,4]. Blending a certain amount of the organic red dye PPDB into films of the blue emitting methyl-substituted ladder-type poly(para-phenylene) (m-LPPP) results in white light emitting devices [5,6]. In order to improve the performance of these PLEDs there is considerable interest in the role of the non-emissive states like polarons in the emission processes. To probe these states we use electroluminescence (ELDMR) and photoluminescence detected magnetic resonance (PLDMR) to compare the dynamics of the involved polarons in both emission creating processes. Both methods are sensitive to states with lifetimes in the range of -10 microseconds to 30 milliseconds, while the decay time of electroluminescence (EL) and photoluminescence (PL) is in the nanosecond range [7]. EXPERIMENT The synthesis of the utilized polymers m-LPPP and PPDB is described in [8] and [9], respectively (see Figure 1 (a), (b)). The PLEDs were fabricated in a single layer configuration, containing both the host and the guest polymer, using Indium-Tin-Oxide (ITO) on glass as the hole injection electrode. The polymer layer (=100nm) was spin-coated from a cosolution of m-LPPP/PPDB onto the ITO coated glass substrate of dimensions 1 mm x 4.5 mm x 12 mm. As the electron injecting electrode Al was evaporated onto the polymer film so that an active device area of -6 mm 2 was obtained. For preparing the films for PLDMR measurements, the polymer powders were dissolved in toluene and filled in electron spin resonance quartz-tubes. The toluene was evaporated leaving thin polymer films on the inner tube walls, which were sealed under vacuum. The sealed sample tubes were placed in the quartz dewar of an Oxford Instruments He gas-flow cryostat, enabling temperature control from 4-300 K, inside an optically accessible X-Band microwave cavity. 99 Mat. Res. Soc. Symp. Proc. Vol. 488 © 1998 Materials Research Society
(b)
(a) C1 0H2 ,
I
00H1
/I RR
3 CI0
0C 6H 13
#
RR
13 C60
O 1RR
6 H13
/ R
Figure 1: Chemical structure of m-LPPP (wn25 ) (a) and PPDB (b) (R=phenoxy-t-butyl) The ,,/" in figure (b) denotes the random nature of the location of the single bond connecting the
perylene to the ethynyl
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