光谱学与光谱分析, 2019, 39 (4): 1018, 网络出版: 2019-04-11  

深陷阱对有机磷光双掺杂体系电致发光器件效率衰退的影响

The Influence of Deep Trap on the Efficiency Decrease in PhOLEDs Based on Double Dopants Strategy
王浩 1,2赵谡玲 1,2徐征 1,2宋丹丹 1,2乔泊 1,2王鹏 1,2郑伟业 1,2魏鹏 1,2
作者单位
1 北京交通大学发光与光信息教育部重点实验室, 北京 100044
2 北京交通大学光电子技术研究所, 北京 100044
摘要
研究利用溶液法制备的有机磷光双重掺杂体系电致发光器件的光致发光特性与电致发光特性, 并研究了在这种体系中深能级陷阱导致的器件效率衰退现象。 首先利用紫外光谱仪和光致瞬态寿命测试系统对基于旋涂法制备的以宽带隙材料4,4’-bis(N-carbazolyl)-1,1’-biphenyl(CBP)为主体, 绿色磷光材料tris(2-phenylpyridine) iridium(Ⅲ)(Ir(ppy)3)和红色磷光材料tris(1-phenylisoquinolinato-C2,N)iridium(Ⅲ)(Ir(piq)3)为客体材料的薄膜进行了光致发射光谱测试和薄膜在Ir(ppy)3发光峰516 nm处的光致发光寿命测试, 实验发现在Ir(ppy)3掺杂比例保持定值时, 随着深能级掺杂材料Ir(piq)3的引入, 其光致发光光谱中Ir(ppy)3的相对发光强度减弱且发光寿命变短, 当Ir(piq)3掺杂浓度继续提高时, 薄膜光致发光光谱基本保持不变且Ir(ppy)3的发光寿命基本不变。 实验说明在低浓度掺杂下两者的三线态能级之间存在着能量传递, 但当掺杂浓度达到高浓度时, 能量传递主要来自于主客体之间的传递, 两者作为独立的发光中心发光。 然后利用溶液法制备了发光层分别为CBP∶Ir(ppy)3, CBP∶Ir(ppy)3∶Ir(piq)3和CBP∶Ir(ppy)3∶PTB7的三组器件, 器件结构为ITO/PEDOT∶PSS/Poly-TPD/EML/TPBi(15 nm)/Alq3(25 nm)/LiF(0.6 nm)/Al(80 nm)。 在Ir(ppy)3和Ir(piq)3共掺杂器件和Ir(ppy)3单掺杂器件的对比实验中发现, 加入一定比例的深能级材料后, 器件的电致发光光谱发生改变, Ir(piq)3的相对发光强度增强, 器件发光效率下降且效率滚降现象明显。 通过对器件进行J-V测试, 发现在Ir(ppy)3单掺杂器件中陷阱填充电流随着掺杂材料浓度的提高而提高, 但在加入等浓度深能级材料Ir(piq)3后, 陷阱填充电流基本保持一致。 瞬态电致发光测试表明, 随着Ir(ppy)3掺杂比例的提高, 器件内由于陷阱载流子释放而产生的瞬时发光强度降低, 这是由于Ir(ppy)3具有一定的传导电荷作用, 会减少器件中的陷阱载流子, 这进一步说明了具有较深能级的Ir(piq)3是限制载流子的主要能级陷阱。 同时发现随反向偏压的增大, 瞬态发光强度增大且发光衰减加速, 这是因为位于深能级陷阱的载流子在高电压下被释放, 重新复合发光, 说明深能级陷阱的确限制住了大量载流子, 而由于主体三线态激子具有较长的寿命, 激子间相互作用产生的单线态激子在高反压下解离, 从而引起三线态激子-极化子相互作用的加剧, 导致发光衰减加速。 在窄带隙聚合物材料PTB7与Ir(ppy)3共掺杂器件实验中发现, 随着PTB7掺杂浓度提高, 陷阱浓度变大且器件效率降低, 具有较深能级的PTB7成为了限制载流子的深能级陷阱。 因此说明在双掺杂有机磷光电致发光器件中, 深能级材料会成为限制载流子的能级陷阱, 引起载流子大量堆积, 从而导致三线态激子与极化子相互作用加剧, 使器件的发光效率衰退。
Abstract
In this paper, in order to study the effect of trapped carrierson the efficiency decrease in PhOLEDs based on double dopants strategy, threegroupsof devices, in whichthehostwas 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) andtheguestswere tris(2-phenylpyridine) iridium(Ⅲ) (Ir(ppy)3), tris(1-phenylisoquinolinato-C2, N)iridium(Ⅲ)(Ir(piq)3)andpoly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1, 2-b:4,5-b′]dithiophene-2, 6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3, 4-b]thiophenediyl]] (PTB7), with the emitting layer of CBP∶Ir(ppy)3, CBP∶Ir(ppy)3∶Ir(piq)3 or CBP∶Ir(ppy)3: PTB7 were prepared by spin-coating method utilizing the double dopants strategy to realize high performance PhOLEDs. Transient photoluminescence measurement was used to test the lifetime of the films with different doping ratios. Asthe concentration of Ir(piq)3 increased, the lifetime of green emission became shorter which indicated that internal energy transfer between the dopants existedwhen the doping ratioofthe Ir(piq)3 was at a low value. When the concentration of Ir(piq)3 increased to 100∶3 and 100∶5, the luminescence of the dopants became independent, in which the internal energy transfer could be negleted. The devices with PTB7 or Ir(piq)3 performed significantly lower power efficiency compared with Ir(ppy)3 only devices, in which the PTB7 and Ir(piq)3 had become the traps which would influence the perfomance of the devices. Transcient electroluminanscence was investigated to penetrate how the trapped charges work in the double doped devices. When the device was driven by a pulse power, a spike occured when the reverse bias turned on. This was because that the trapped charges were released and then recombined under a high reverse bias. The results showed that the device with more Ir(ppy)3 showed weaker spike which indicated that charges trapped by the Ir(piq)3 reduced, and as the concetration of the Ir(ppy)3 increased, the sipke became weaker. This was mainly because that the Ir(ppy)3 could transmit charges, which would reduce the trapped charges in Ir(piq)3. Through the transicient electrolu minescence mesurement, we also found that the spike would decay faster when suppllied a higher reverse bia which was caused by the deep trapped charges releasing and then excerbate the triplet-polaron quenching (TPQ) effect. Hence, the material with deep energy level would trap a large number of charges and then aggravate the interaction between the triplet exciton and polaron, causing the efficiency decrease and roll-off in the PhOLEDs based on double dopants strategy.

王浩, 赵谡玲, 徐征, 宋丹丹, 乔泊, 王鹏, 郑伟业, 魏鹏. 深陷阱对有机磷光双掺杂体系电致发光器件效率衰退的影响[J]. 光谱学与光谱分析, 2019, 39(4): 1018. WANG Hao, ZHAO Su-ling, XU Zheng, SONG Dan-dan, QIAO Bo, WANG Peng, ZHENG Wei-ye, WEI Peng. The Influence of Deep Trap on the Efficiency Decrease in PhOLEDs Based on Double Dopants Strategy[J]. Spectroscopy and Spectral Analysis, 2019, 39(4): 1018.

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