Study on the Overshoot Effect of Doped PhOLED with Transient Electroluminescence
HONG Xiao-xia1, 2, XU Zheng1, 2*, ZHAO Su-ling1, 2, QIAO Bo1, 2, ZHANG Cheng-wen1, 2, WANG Peng1, 2
1. Key Laboratory of Luminescence and Optical Information (Beijing Jiaotong University), Ministry of Education，Beijing 100044，China 2. Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044，China
Abstract：The accumulation carriers and the trapped carriers are found in many organic light-emitting diodes (OLEDs) more or less, which can lead to a great loss of carriers and weaken the performance of devices. We have investigated a host-guest-system containing the green phosphorescent emitter tris[2-phenylpyridinato-C2,N]iridium(Ⅲ) [Ir(ppy)3] and one host material with transient electroluminescence (EL). The charge recombination, accumulation and light emission mechanisms of the phosphorescent organic light-emitting diodes (PhOLEDs) with different host materials were analyzed. The structure was fabricated as ITO/NPB(30 nm)/host: Ir(ppy)3/BCP(10 nm)/Alq3(20 nm)/LiF(0.7 nm)/Al(100 nm),the hosts were CBP, PVK and TAZ respectively. These results showed the transient EL was strongly dependent on host materials. Compared to devices of host material CBP and PVK, only those with the host material TAZ as the emitting layer exhibited strong electroluminescence overshoots between 1 and 3 μs after turning off the voltage pulse at room temperatures. To further elucidate the generality of the overshoots, we monitored their dependence on the dopant concentration. The transient EL results in host-guest-system devices demonstrated a direct link between the strong overshoot effect and charge trapping in the emitting guest molecules. The excessive electrons in the guest sites could be a major factor inducing significant strong overshoot phenomenon in the TAZ: Ir(ppy)3 layer. We attributed these overshoot effect to the electrons accumulated on Ir(ppy)3 sites and accumulated holes in the vicinity of the HBL/EML interface. As a result, we obtained a better understanding of carriers’ dynamics and recombination process of PhOLEDs after turning off the voltage pules. The new understanding of the charge carriers and exciton dynamics of PhOLEDs is instrumental in directing the efforts of developing stable and high-efficiency PhOLEDs.
[1] Tang C W, VanSlyke S A. Applied Physics Letters, 1987, 51(12): 913. [2] Luo Y, Aziz H. Applied Physics Letters, 2009, 95(7): 073304. [3] Luo Y, Aziz H, Xu G, et al. Chemistry of Materials, 2007, 19(8): 2079. [4] Klenkler R A, Xu G, Aziz H, et al. Applied Physics Letters, 2006, 88(24): 242101. [5] Popovic Z D, Aziz H.Journal of Applied Physics, 2005, 98(1): 013510. [6] Gan Z, Liu R, Shinar R, et al. Applied Physics Letters, 2010, 97(11): 113301. [7] Luo Y, Aziz H. Advanced Functional Materials, 2010, 20(8): 1285. [8] Mu H, Reddy I, Hunt J, et al. Journal of Physics D: Applied Physics, 2010, 43(19): 195103. [9] Youn Jun Ho, II Lee Yeon, Moon Hie Tae, et al. Journal of the Korean Physical Society, 2009, 55(3): 1111. [10] Liu R, Gan Z, Shinar R, et al. Physical Review B, 2011, 83(24): 245302. [11] Weichsel C, Burtone L, Reineke S, et al. Physical Review B, 2012, 86(7): 075204. [12] Xiao L, Chen Z, Qu B, et al. Adv. Mater., 2011, 23(8): 926. [13] Minaev B, Baryshnikov G, Agren H. Physical Chemistry Chemical Physcics, 2014, 16(5): 1719.
