Transient electroluminescence determination of carrier mobility and charge trapping effects in heavily doped phosphorescent organic light-emitting diodes
Introduction
Organic light-emitting diodes (OLEDs) have attracted significant research attention for potential applications in emissive displays [1] and solid state lighting [2], [3] due to their potential for full-color, high efficiency and low fabrication cost. In order to achieve efficient emission from OLEDs, good charge balance and exciton recombination in the center of the emissive layer are desirable [4], [5]. However, organic semiconductors are characterized by low carrier mobility compared to their inorganic counterparts, and large differences in electron and hole mobility have been reported [6]. Engineering good OLED performance requires an understanding of carrier mobility and transport phenomena, which can be achieved by time dependent and transient measurements. The use of transient electroluminescence methods to investigate OLED performance, the onset of light emission, and charge transport and trapping has been documented [6], [7], [8]. The delay time between the drive pulse and onset of electroluminescence (EL) is essentially the average time that is required for electrons and holes to be injected, meet and recombine in the emissive layer, and therefore provides information about the recombination zone. Delayed recombination phenomena in OLEDs are related to the electric field and charges remaining in the device after the drive pulse have been removed, and thus are related to the trapping and de-trapping processes in device layers and at interfaces [9].
Understanding and controlling carrier transport is a primary method of improving OLED performance. Often exciton blockers, electron blockers and hole blockers are included in device structures to confine excitons, electrons and holes to the emissive layer, and thus enhance radiative recombination and device performance [10], [11], [12]. Similarly, electron and hole injection and transport layers are frequently used. Thompson, Forrest and co-workers [11], were among the first to detail the effect of electron blocking layers in OLEDs. They obtained a peak power efficiency of 7.3 lm/W in OLEDs without an electron blocker [11], and an improvement to 12.2 lm/W with Ir(ppz)3 (Ir(ppz)3 = Tris(phenylpyrazole)iridium) as an electron blocker in the device. Lee and co-workers have reported that TAPC (TAPC = 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) is a better a hole transport material compared to NPB (NPB = N,N′-bis(1-naphthyl)-N-N′-biphenyl-1,1′-biphenyl-4, and 4′-diamine) in OLED applications, and the difference in performance has been ascribed to the higher mobility of TAPC (1.0 × 10−2 cm2/V s) compared to NPB (5.0 × 10−4 cm2/V s) [13], [14]. In addition, TAPC provides a larger barrier for blocking electrons and confining them to the emissive layer compared to the common electron blocker mCP (mCP = N,N′-dicarbazolyl-3,5-benzene).
In this work, the delay in the onset emission was used to determine the electron mobility of doped CBP films in OLEDs with the structure: ITO/NPB (40 nm)/mCP (10 nm)/65% Pt(ptp)2:CBP (25 nm)/TPBI (30 nm)/Mg:Ag (100 nm). The emissive layer consisted of the electrophosphorescent dopant, Pt(ptp)2 = bis[3,5-bis(2-pyridyl)-1,2,4-triazolato] platinum(ΙΙ) doped into the common OLED host 4,4′-bis(carbazol-9-yl)triphenylamine (CBP). Other layers in this structure include NPB as a hole transport layer, mCP as an electron blocking layer, and TPBI (TPBI = 1,3,5-tris(phenyl-2-benzimidazolyl)-benzene) as an electron transport layer. The first part of this study used the information from EL onset delay-time measurements to determine the electron mobility of the emissive layer. The second part of this study used transient EL measurements to understand the observed delayed radiative recombination, which was only observed in devices with the mCP, electron/exciton blocking layer.
Section snippets
Experimental setup and method for transient EL measurements
A schematic of the experimental setup for the transient EL response measurement is shown in Fig. 1. A Tektronix AFG 3102 pulse generator with a rise time of ∼1 ns was used to excite the sample, and the resulting EL signal was detected by a PMT (Oriel model 77348, response time 2.2 ns) attached to an automated Cornerstone 260 monochromator with appropriate gratings. The EL signal from the PMT was captured for analysis by a Tektronix DPO 4104 digital oscilloscope. The RC time constant of the OLEDs
Results and discussion
Fig. 4 shows the photoluminescence (PL) and photoluminescence excitation (PLE) spectra of CBP doped with 65% Pt(ptp)2, while Fig. 5 is the EL spectrum of devices featuring the CBP:65% Pt(ptp)2 emissive layer (see Fig. 3). The absence of CBP emission in the PL spectrum indicates complete energy transfer from the host to the dopant. CBP doped with 65% Pt(ptp)2 was used here because this dopant concentration was found to produce optimal electro-optical performance. Specifically, CBP films doped at
Conclusion
In summary, transient electroluminescence was used to measure the electron mobility of a heavily doped emissive layer consisting of 65% Pt(ptp)2 doped into CBP. The frequency and pulse length data suggests that trapping and de-trapping processes at the interface between the electron/exciton blocker layer (mCP) and emissive layer associated with disorder-induced carrier localization [19], [20] are responsible for the delayed recombination behavior, and that the recombination zone is close to the
Acknowledgement
This work is supported by the US Department of Energy under contract DE-FC26-06NT 42859.
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