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Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes

Zhao Xi Chen Jing Peng Teng Liu Jun-Hong Wang Bo Chen Xiao-Li Xiong Zu-Hong

Zhao Xi, Chen Jing, Peng Teng, Liu Jun-Hong, Wang Bo, Chen Xiao-Li, Xiong Zu-Hong. Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes. Acta Phys. Sin., 2023, 72(16): 167201. doi: 10.7498/aps.72.20230765
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Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes

Zhao Xi, Chen Jing, Peng Teng, Liu Jun-Hong, Wang Bo, Chen Xiao-Li, Xiong Zu-Hong
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  • Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex state are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependence which weakens with the increase of bias-current. This is because the bias-current increases by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device is enhanced, which facilitates the electric-field-induced dissociation of polaron-pair and exciplex states and then reduces their lifetime. That is, less polaron-pair and exciplex states participate in the ISC process and RISC process, leading these two processes to weaken. Here, magneto-electroluminescence (MEL) is used as a fingerprint probing tool to observe various current-dependent ISC and RISC processes in EB-OLEDs with different charge balances via modifying the device hole-injection layer. Interestingly, current-dependent MEL traces of the unbalanced device display a conversion from normal ISC (1–25 μA) process to abnormal ISC (25–200 μA) process, whereas those of the balanced device show conversions from normal ISC (1–5 μA) into abnormal RISC (10–50 μA) and then into normal RISC (50–150 μA) and finally into abnormal ISC (200–300 μA) process. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC process and RISC process in these two devices first increase then decrease as the bias-current increases. These non-monotonic current-dependent ISC process and RISC process are attributed to the competition between the increased number and the reduced lifetime of polaron-pair state and exciplex state during improving the bias-current. Furthermore, the RISC process in the balanced device is stronger than that in the unbalanced device. This is because the balanced carrier injection can facilitate the formation of triplet exciplex states and weaken the triplet-charge annihilation (TQA) process between triplet exciplex states and excessive charge carriers, which leads the number of triplet exciplex states to increase. That is to say, more triplet exciplex states can be converted into singlet exciplex states through the RISC process, causing the external quantum efficiency of the balanced device to be higher than that of the unbalanced device. Obviously, this work not only deepens the understandings of current-dependent ISC and RISC processes in EB-OLEDs, but also provides an insight into the device physics for designing and fabricating high-efficiency EB-OLEDs.
      PACS:
      72.80.Le(Polymers; organic compounds (including organic semiconductors))
      71.35.Ji(Excitons in magnetic fields; magnetoexcitons)
      78.60.Fi(Electroluminescence)
      78.55.Kz(Solid organic materials)
      Corresponding author: Xiong Zu-Hong, zhxiong@swu.edu.cn
    • Funds: Project supported by the National Natural Science Foundation of China (Grant No. 11874305).

    在激基复合物有机发光二极管(exciplex-based organic light-emitting diodes, EB-OLEDs)中存在自旋对态(spin-pair states)的两种重要演化过程. 它们是单重态与三重态极化子对(singlet and triplet polaron pairs, PP1和PP3)和激基复合物(singlet and triplet exciplexes, EX1和EX3)之间的系间窜越(intersystem crossing, ISC) (PP1 → PP3, EX1 → EX3)和反向系间窜越(reverse ISC, RISC) (PP1 ← PP3, EX1 ← EX3)过程[1-3]. 显然, 这两种微观过程可以有效地调控这些单/三态的数量和比例. 为了通过增大单/三态比例来提高器件的量子效率, 需要很好地理解ISC和RISC过程的物理机制. 最近, 磁电致发光(magneto-electroluminescence, MEL)作为指纹式探测工具经常被用来研究EB-OLEDs中ISC和RISC过程的物理机制[4-6]. 这是因为ISC和RISC过程是高度自旋依赖的, 并且产生指纹式MEL曲线[4-6], 见补充材料图S1. 具体地, ISC和RISC过程的特征MEL曲线分别展示倒置和正置的洛伦兹线型. 2019年, Yuan等[7]利用MEL发现EB-OLEDs中EX态的RISC过程随着EX态中电子-空穴耦合距离的增大而增强. Chen等[8]利用MEL发现EB-OLEDs中PP态的ISC过程随着电子给体材料的最高占据分子轨道(highest occupied molecular orbital, HOMO)与电子受体材料的最低未占据分子轨道(lowest unoccupied molecular orbital, LUMO)之间能隙的减小而增强. 在我们之前的工作中, 通过测量EB-OLEDs在不同注入电流下的MEL曲线, 发现ISC和RISC过程通常展示正常的电流依赖关系, 即随着电流的增大而减弱[9-11]. 虽然对EB-OLEDs中的ISC和RISC过程的物理机制已有一定的理解, 但是仍然需要对它们进行更深入的探索. 这是因为ISC和RISC过程的物理机制与器件结构、注入电流、环境温度和有机材料的薄膜形貌密切相关.

    本文利用MEL特征线型, 从具有不同电荷平衡的1, 1-bis[(di-4-tolylamino)phenyl]-cyclohexane (TAPC)/2, 4, 6-tris[3-(diphenylphosphinyl) phen-yl]-1, 3, 5-triazine (PO-T2T)异质结EB-OLEDs中观察到多种电流依赖的ISC和RISC过程. 具体地, 非平衡器件中电流依赖的MEL曲线展示从正常ISC (1—25 μA)向反常ISC (25—200 μA)过程的转换. 但是, 通过修饰器件的空穴注入层来提高载流子注入平衡后, 平衡器件中电流依赖的MEL曲线则呈现从正常ISC (1—5 μA)→反常RISC (10—50 μA)→正常RISC (50—150 μA)→反常ISC (200—300 μA)过程的转换. 为了解释这些转换过程, 利用由洛伦兹函数和非洛伦兹函数组成的公式来拟合MEL曲线. 拟合结果显示非平衡和平衡器件中的ISC和RISC过程随着电流增大都先增强后减弱. 这是由增大电流时PP态和EX态增加的数量与它们减短的寿命之间的竞争所引起. 另外, 因为平衡的载流子注入可以促进EX3态的形成并减弱EX3与多余电荷载流子之间的三重态-电荷湮灭(triplet-charge annihilation, TQA)过程, 所以平衡器件中的RISC过程比非平衡器件中的更强. 这个增强的RISC过程可以增大EX1/EX3的比例, 从而导致平衡器件的外量子效率(external quantum efficiency, EQE)比非平衡器件的更高. 因此, 本文不但丰富了对EB-OLEDs中电流依赖的ISC和RISC过程的理解, 还为高效率EB-OLEDs的设计制作提供了理论指导.

    采用超高真空有机分子束沉积技术, 制备了具有不同空穴注入层的TAPC/PO-T2T异质结EB-OLEDs. 器件1 (Device 1, Dev. 1)的结构为ITO/poly(3, 4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (40 nm)/TAPC (80 nm)/PO-T2T (80 nm)/LiF(1 nm)/Al(120 nm). 器件2 (Device 2, Dev. 2)的结构为ITO/1, 4, 5, 8, 9, 11-hexaaza-triphenylene-hexacarbonitrile (HAT-CN) (20 nm)/TAPC (80 nm)/PO-T2T (80 nm)/LiF(1 nm)/Al(120 nm). 带有ITO阳极的玻璃衬底从商业公司购买. 在把衬底放入真空沉积系统进行有机分子束沉积之前, 首先采用去离子水和清洗液(Decon 90, 浓度为4%)对其正面和反面进行反复擦洗并在超声清洗仪中进行水浴超声(水浴温度为60 °C). 然后用丙酮和无水乙醇分别对衬底进行有机杂质的溶解和脱水处理. 随后, 采用旋涂法在衬底上制备PEDOT:PSS空穴注入层. 最后, 将衬底传入真空系统(10–6 Pa)进行有机功能层和金属电极的蒸镀. 在蒸镀有机功能层时, 采用INFICON公司的膜厚检测仪(XTM/2)对有机材料的生长速率和各功能层的厚度进行原位监测. 其中, 有机材料和金属电极的生长速率分别控制为0.5 Å/s和1.2 Å/s (1 Å = 10–10 m).

    器件制备完成后, 将其固定在一套高真空闭循环冷却系统(Janis: CCS-350S, 温度范围: 15—310 K)的冷头上. 然后将装有器件的冷却系统放入电磁铁(Lakeshore: EM647)之间, 并且使器件表面平行于外磁场. 该测量系统由计算机通过Labview软件对系统中的电磁铁、霍尔探头、Lakeshore 421高斯计、Keithley 2400数字源表、硅光电探头和Keithley 2000数字源表进行实时监控. 其中, Lakeshore 421用来测量外磁场的大小. Keithley 2400不但为器件提供恒定偏压, 还可同时测量流过器件的电流. 硅光电探头测量器件的电致发光(electroluminescence, EL)强度并通过Keithley 2000读取. 测量器件的EL谱时, 先利用Keithley 2400给器件提供偏置电压使其发光, 当器件发光经过凸透镜汇聚、斩波器提供参考信号频率、Spectra-2300i光栅光谱仪分光、光电倍增管将光信号转变为电信号后由锁相放大器将电信号放大, 最后通过SpectraSense光谱软件将电信号绘制成光谱. 另外, 测量薄膜的光致发光(photoluminescence, PL)谱时, 先利用由氙灯发射的波长为285 nm的光线作为激发光源来激发薄膜中的有机小分子使其发光, 然后用爱丁堡荧光光谱仪FLS 1000测量光谱, 最后通过Fluoracle软件绘制光谱.

    图1(a)图1(b)所示, Dev. 1和Dev. 2分别为空穴注入层为PEDOT:PSS和HAT-CN的TAPC/PO-T2T平面异质结OLEDs. TAPC和PO-T2T的化学分子结构见图1(c). 因为TAPC与PO-T2T之间具有大的LUMO能级差(0.8 eV)以及HOMO能级差(1.3 eV), 所以从阳极注入的空穴和从阴极注入的电子会被阻挡在TAPC/PO-T2T界面. 这些电子和空穴分别位于两个不同的分子上, 并且它们在库仑吸引的作用下复合形成EX态. 为了验证Dev. 1和Dev. 2中EX态的形成, 测量了这两个器件的EL谱以及TAPC纯膜, PO-T2T纯膜和TAPC:PO-T2T共混薄膜的PL谱, 如图1(d)所示. 可以看出, TAPC:PO-T2T共混薄膜的PL发射峰(570 nm)相对于TAPC纯膜和PO-T2T纯膜的PL发射峰(372 nm和375 nm)具有明显的红移和展宽. 这表示TAPC:PO-T2T薄膜的PL谱呈现TAPC:PO-T2T激基复合物发射. 与TAPC:PO-T2T薄膜的PL谱相似, Dev. 1和Dev. 2的EL谱显示TAPC/PO-T2T激基复合物发射.

    图 1 器件的能级结构和光电特性 (a), (b) Dev. 1和Dev. 2的能级结构; (c) TAPC和PO-T2T的化学分子结构; (d) TAPC纯膜, PO-T2T纯膜和TAPC:PO-T2T共混薄膜的PL谱以及Dev. 1和Dev. 2的EL谱; (e) Dev. 1和Dev. 2的电流-电压特性曲线; (f) Dev. 1和Dev. 2的EQE-电流密度特性曲线\r\nFig. 1. Energy-level structures and photoelectric properties of devices: (a), (b) Energy-level structures of Dev. 1 and Dev. 2; (c) chemical molecular structures of TAPC and PO-T2T; (d) PL spectra from pure TAPC and PO-T2T films and TAPC:PO-T2T co-deposited film and EL spectra from Dev. 1 and Dev. 2; (e) current-voltage characteristics of Dev. 1 and Dev. 2; (f) EQE-current density characteristics of Dev. 1 and Dev. 2.
    图 1  器件的能级结构和光电特性 (a), (b) Dev. 1和Dev. 2的能级结构; (c) TAPC和PO-T2T的化学分子结构; (d) TAPC纯膜, PO-T2T纯膜和TAPC:PO-T2T共混薄膜的PL谱以及Dev. 1和Dev. 2的EL谱; (e) Dev. 1和Dev. 2的电流-电压特性曲线; (f) Dev. 1和Dev. 2的EQE-电流密度特性曲线
    Fig. 1.  Energy-level structures and photoelectric properties of devices: (a), (b) Energy-level structures of Dev. 1 and Dev. 2; (c) chemical molecular structures of TAPC and PO-T2T; (d) PL spectra from pure TAPC and PO-T2T films and TAPC:PO-T2T co-deposited film and EL spectra from Dev. 1 and Dev. 2; (e) current-voltage characteristics of Dev. 1 and Dev. 2; (f) EQE-current density characteristics of Dev. 1 and Dev. 2.

    为进一步研究Dev. 1和Dev. 2的光电特性, 测量了这两个器件的电流-电压和EQE-电流密度特性曲线, 见图1(e)图1(f). 可以看出, Dev. 2的开启电压比Dev. 1的更低. 这是因为Dev. 1和Dev. 2具有相同的电子注入能力, 但Dev. 2的空穴注入能力比Dev. 1的更强. 具体地, Dev. 1和Dev. 2中从LiF/Al阴极到PO-T2T的LUMO的电子注入势垒是0.1 eV, 但Dev. 2中从HAT-CN的LUMO到TAPC的HOMO的空穴注入势垒(0.2 eV)小于Dev. 1中从ITO/PEDOT:PSS阳极到TAPC的HOMO的空穴注入势垒(0.4 eV). 显然, 相对于Dev. 1的空穴注入势垒(0.4 eV), Dev. 2的空穴注入势垒(0.2 eV)更接近其电子注入势垒(0.1 eV). 也就是说, Dev. 2中的载流子注入比Dev. 1中的更平衡, 从而引起Dev. 2具有比Dev. 1更高的EQE. 虽然平衡的载流子注入可以提高器件的EQE, 但其本质上的原因缺乏充分的分析. 这是因为从载流子的注入到器件的荧光发射, 器件中存在PP态和EX态的多种物理微观过程并且这些过程都会影响最后的荧光发射. 这些过程包括单重态与三重态之间的ISC和RISC[9-11], 三重态与多余电荷载流子之间的TQA[12,13], 以及三重态-三重态湮灭(triplet-triplet annihilation, TTA)[14-16]. 如文献[4-9]所报道, 这些过程可以被MEL这个指纹式探测工具灵敏识别. 因此, 为了探索EQE提高的物理起源, Dev. 1和Dev. 2的MEL曲线将在后文讨论.

    图2(a)图3(a)分别为Dev. 1和Dev. 2中电流依赖的MEL曲线. MEL被定义为有外加磁场和无外加磁场时器件发光强度的相对变化[4-6], 如(1)式所示. 其中, EL(B)和EL(0)分别表示有外加磁场和无外加磁场时器件的发光强度:

    图 2 (a) Dev. 1中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 1中ISC, RISC和TQA过程的强度因子\r\nFig. 2. (a) Current-dependent MEL traces of Dev. 1 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 1 at different bias-currents.
    图 2  (a) Dev. 1中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 1中ISC, RISC和TQA过程的强度因子
    Fig. 2.  (a) Current-dependent MEL traces of Dev. 1 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 1 at different bias-currents.
    图 3 (a) Dev. 2中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 2中ISC, RISC和TQA过程的强度因子\r\nFig. 3. (a) Current-dependent MEL traces of Dev. 2 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 2 at different bias-currents.
    图 3  (a) Dev. 2中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 2中ISC, RISC和TQA过程的强度因子
    Fig. 3.  (a) Current-dependent MEL traces of Dev. 2 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 2 at different bias-currents.
    $$ \text{MEL =}\text{}\frac{\text{EL(} {B}\text{)}-\text{EL(0)}}{\text{EL(0)}}\text{}\times \text{}100{\text{%}}. $$ (1)

    图2(a)所示, Dev. 1的MEL曲线是由快速上升的低场效应(low-field effect, LFE)(B ≤ 10 mT)和缓慢上升的高场效应(high-field effect, HFE)(10 < B ≤ 300 mT)组成. 根据文献[9-11]报道的特征MEL曲线, Dev. 1的LFE和HFE分别归因于被磁场抑制的PP态的ISC过程和被磁场抑制的EX3态的TQA过程. 此外, ISC过程通常展示正常的电流依赖关系, 即随电流的增大而减弱[9-11]. 这是因为激基复合物有机发光二极管中偏置电流的增大是通过提高器件的偏置电压来实现的. 当器件的偏置电压被提高, 器件内的电场增强. 因为增强的电场会促进PP态的电场致解离, 所以PP态的寿命减短, 从而减弱PP态的ISC过程. 有趣地, 虽然Dev. 1中的ISC过程在小电流(1—25 μA)下展示正常的电流依赖关系, 但是它在大电流(25—200 μA)下具有反常的电流依赖关系. 也就是说, 一个从正常ISC到反常ISC过程的转换发生. 这个有趣的转换很少有文献报道.

    为了解释这个转换, Dev. 1中电流依赖的MEL曲线被(2)式拟合[9,17-19]:

    $$ \text{MEL}={{C}}_{1}\frac{{{B}}^{2}}{{{B}}^{2}+{{B}}_{1}^{2}}-{{C}}_{2}\frac{{{B}}^{2}}{{{B}}^{2}+{{B}}_{2}^{2}}+{{C}}_{3}\frac{{{B}}^{2}}{{{(|}{B}|+{{B}}_{3}{)}}^{2}}. $$ (2)

    在(2)式中, 两个洛伦兹函数和一个非洛伦兹函数分别模拟ISC, RISC和TQA过程, B是外加磁场, B1(~10 mT), B2(~10 mT)和B3(~100 mT)分别是ISC, RISC和TQA过程的特征磁场大小, C1, C2C3分别用来描述ISC, RISC和TQA过程的强度因子. 如图2(a)所示, 白色的拟合曲线与实验结果很好的一致. 另外, Dev. 1中ISC, RISC和TQA过程在不同注入电流下的强度因子分别被总结在图2(b)图2(c). 下文将详细解释这些强度因子. 接下来讨论Dev. 2在不同注入电流下的MEL曲线. 如图3(a)所示, Dev. 2在1—5 μA的电流下的LFE和HFE也分别展示被磁场抑制的ISC和TQA过程, 并且ISC过程具有正常的电流依赖关系. 有趣的是随着电流从1 μA增大到50 μA, Dev. 2的LFE从快速上升转变为快速下降, 也就是从倒置的洛伦兹线型转变为正置的洛伦兹线型. 根据已在文献[9-11]中被报道的特征MEL曲线, 快速下降的LFE归因于被磁场抑制的EX态的RISC过程. 另外, RISC过程通常具有正常的电流依赖关系, 即随电流的增大而减弱[9-11]. 这是因为器件内增强的电场促进EX态的电场致解离, 从而减短EX态的寿命并减弱EX态的RISC过程. 但是器件2中的RISC过程在10—50 μA的电流下呈现反常的电流依赖关系. 也就是说, 发生了一个从正常ISC (1—5 μA)向反常RISC (10—50 μA)过程的转换. 更有趣的是, 随着电流从10 μA增大到150 μA, 这个反常RISC转变为正常RISC (50—150 μA)过程. 此外, 当电流从50 μA进一步增大到300 μA, 这个正常RISC转变为反常ISC (200—300 μA)过程. 总体来说, 随电流增大, 从正常ISC (1—5 μA)→反常RISC (10—50 μA)→正常RISC (50—150 μA)→反常ISC (200—300 μA)过程的转换发生. 这些丰富的转换很少被报道. 为了解释这些转换, Dev. 2中电流依赖的MEL曲线被(2)式拟合. 如图3(a)所示, 白色的拟合曲线很好地模拟了实验结果. 至于Dev. 2中ISC, RISC和TQA过程在不同注入电流下的强度因子, 可见图3(b)图3(c). 这些强度因子将会在下文详细解释.

    为了理解Dev. 1和Dev. 2中电流依赖的MEL曲线, 这两个器件中PP态和EX态的形成和演变机制分别如图4(a)图4(b)所示. 可以看出, 从阳极注入的空穴和从阴极注入的电子首先在库仑吸引的作用下复合然后形成弱束缚的PP态. 因为PP1和PP3具有简并的能级并且自旋翻转可以在超精细相互作用下实现, 所以PP1和PP3可以通过ISC和RISC过程相互转换[20-22]. 接下来, PP1和PP3分别以速率常数kSkT进一步演变成弱束缚的EX1和EX3. 因为kT通常大于kS, 所以PP1与PP3之间的相互转换是由ISC过程主导[21]. 当一个外加磁场存在, 简并的PP3(PP3, 0, PP3, +, PP3, –)发生塞曼分裂[22]. 由于PP1与PP3, 0之间的能级差仍然很小, PP1依然可以转变成PP3, 0. 但PP1与PP3, +和PP3, –之间的能级差较大, 从而导致PP1不能转变为PP3, +和PP3, –. 这引起被磁场抑制的ISC过程和PP1增加的数量. 因为PP1的数量增加, 所以更多的PP1演变成EX1, 从而增加EX1的数量. 这会在几个mT的外磁场范围内快速增强器件的发光强度, 也就是MEL曲线快速上升的LFE[21,22], 详见补充材料中的图S1. 与PP态相似, 因为EX1和EX3在能级上几乎是简并的, 所以EX态也会在超精细相互作用下通过RISC和ISC过程相互转换[23]. 如文献[9, 10, 12]所报道, EX态的相互转换通常是由RISC过程主导. 这是因为EX3的数量是EX1的3倍[23], 并且EX3的寿命比EX1长3个数量级[24]. 另外由于简并的EX3(EX3, 0, EX3, +, EX3, –)的塞曼分裂, RISC过程也会被外加磁场抑制[9,10,12]. 当RISC过程被抑制, EX1的数量减少. 这会在几个mT的外磁场范围内快速减弱器件的发光强度, 也就是MEL曲线快速下降的LFE[9,10,12] (补充材料图S1). 基于以上对被磁场抑制的ISC和RISC过程的描述, 得出MEL曲线的LFE是被磁场抑制的ISC的正LFE和被磁场抑制的RISC的负LFE的叠加. 至于被磁场抑制的TQA过程引起的HFE, 因为它的形成机制已经在文献[9, 12, 13]中被详细解释, 所以不再赘述.

    图 4 不同注入电流下器件中PP态和EX态的形成和演变机制 (a) Dev. 1; (b) Dev. 2\r\nFig. 4. Formation and evolution mechanisms of PP and EX states in devices at different bias-currents: (a) Dev. 1; (b) Dev. 2.
    图 4  不同注入电流下器件中PP态和EX态的形成和演变机制 (a) Dev. 1; (b) Dev. 2
    Fig. 4.  Formation and evolution mechanisms of PP and EX states in devices at different bias-currents: (a) Dev. 1; (b) Dev. 2.

    图2(a)所示, Dev. 1的LFE是由ISC过程主导, 也就是说ISC过程强于RISC过程. 这是因为TQA过程($ {{\vec {\rm{e}}}} + {\text{E}}{{\text{X}}_3} \to {\text{e}} \downarrow + {{\text{S}}_0} $)发生在Dev. 1中, 并且TQA过程会通过减少EX3的数量来减弱RISC过程. 为了解释Dev. 1中从正常ISC向反常ISC过程的转换, 其MEL曲线被拟合. 拟合结果见图2(b)图2(c). 可以看出, 随着注入电流增大, Dev. 1的LFE值(C1 + C2)先减小后增大, 这貌似表示ISC过程先减弱后增强. 事实上, Dev. 1中的ISC和RISC过程(C1C2)都先增强后减弱, 如图2(b)图4(a)所示. 这是因为增大电流时PP态和EX态增加的数量与它们减短的寿命相互竞争. 具体地, PP态和EX态的绝对数量随着电流的增大而增加. 但是它们的寿命随着电流的增大而减短. 这是因为电流的增大是通过提高器件的偏压来实现的. 当器件偏压增大, 器件内部的电场增强. 根据Onsager理论, 增强的电场会促进PP态和EX态的场致解离, 从而导致它们减短的寿命[25-27]. 当PP态和EX态的寿命短于它们的自旋演变时间(10–9 s), 它们则不能经历ISC和RISC过程[25]. 因此, 虽然PP态和EX态的绝对数量随着电流的增大而增加, 但是可以参与ISC和RISC过程的PP态和EX态的相对数量先增加后减少, 也就是说ISC和RISC过程先增强后减弱.

    注意到, RISC(C2)变化得比ISC(C1)更快. 这是因为EX3的数量变化得比PP1更快由于EX3来自PP3的演变并且PP3的数量远大于PP1[23]. 也就是说, 电流依赖的LFE值是由被磁场抑制的RISC过程主导. 因为被磁场抑制的RISC过程展示负的LFE值并且其随着电流的增大先增强后减弱, 所以Dev. 1的LFE值先减小后增大. 这表面上呈现ISC过程先减弱后增强, 即从正常ISC (1—25 μA)向反常ISC (25—200 μA)过程的转换发生, 如图2(a)所示. 与Dev. 1相似, 因为增大电流时PP态和EX态增加的数量与它们减短的寿命之间的竞争也发生在Dev. 2中, 所以Dev. 2中的ISC和RISC过程(C4C5)随着电流增大也都先增强后减弱, 如图3(b)图4(b)所示. 但是Dev. 2中的RISC过程比Dev. 1中的更强. 这是因为Dev. 2中的载流子注入比Dev. 1中的更平衡并且平衡的载流子注入可以促进EX3态的形成并减弱EX3的TQA过程[28]. 由于这个增强的RISC过程和被磁场抑制的RISC过程的负LFE值, Dev. 2在10—150 μA的电流下展示负的LFE值. 因此, Dev. 2的LFE值(C4 + C5)先从正减小到负, 然后从负增大到正. 这表面上呈现从正常ISC (1—5 μA)→反常RISC (10—50 μA)→正常RISC (50—150 μA)→反常ISC (200—300 μA)过程的转换, 如图3(a)所示.

    本文利用MEL在具有不同电荷平衡的TAPC/PO-T2T异质结EB-OLEDs中观察到多种电流依赖的ISC和RISC过程. 具体地, 非平衡器件中电流依赖的MEL曲线显示从正常ISC (1—25 μA)向反常ISC (25—200 μA)过程的转换, 但是平衡器件中电流依赖的MEL曲线则呈现从正常ISC (1—5 μA)→反常RISC (10—50 μA)→正常RISC (50—150 μA)→反常ISC (200—300 μA)过程的转换. 这些转换被合理地解释通过拟合并解析MEL曲线. 拟合结果反映非平衡和平衡器件中的ISC和RISC过程随着注入电流增大都先增强后减弱. 这是由增大电流时PP态和EX态增加的数量与它们减短的寿命之间的竞争所引起. 另外, 因为平衡的载流子注入可以促进EX3态的形成并减弱EX3的TQA过程, 所以平衡器件中的RISC过程比非平衡器件中的更强. 这个增强的RISC过程会增大EX1/EX3的比例, 从而引起平衡器件的EQE比非平衡器件的更高. 显然, 本工作不但进一步理解了EB-OLEDs中电流依赖的ISC和RISC过程, 还为高效率EB-OLEDs的设计制作提供帮助.

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  • 图 1  器件的能级结构和光电特性 (a), (b) Dev. 1和Dev. 2的能级结构; (c) TAPC和PO-T2T的化学分子结构; (d) TAPC纯膜, PO-T2T纯膜和TAPC:PO-T2T共混薄膜的PL谱以及Dev. 1和Dev. 2的EL谱; (e) Dev. 1和Dev. 2的电流-电压特性曲线; (f) Dev. 1和Dev. 2的EQE-电流密度特性曲线

    Figure 1.  Energy-level structures and photoelectric properties of devices: (a), (b) Energy-level structures of Dev. 1 and Dev. 2; (c) chemical molecular structures of TAPC and PO-T2T; (d) PL spectra from pure TAPC and PO-T2T films and TAPC:PO-T2T co-deposited film and EL spectra from Dev. 1 and Dev. 2; (e) current-voltage characteristics of Dev. 1 and Dev. 2; (f) EQE-current density characteristics of Dev. 1 and Dev. 2.

    图 2  (a) Dev. 1中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 1中ISC, RISC和TQA过程的强度因子

    Figure 2.  (a) Current-dependent MEL traces of Dev. 1 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 1 at different bias-currents.

    图 3  (a) Dev. 2中电流依赖的MEL曲线和它们的拟合曲线(白色实线); (b), (c) 不同注入电流下Dev. 2中ISC, RISC和TQA过程的强度因子

    Figure 3.  (a) Current-dependent MEL traces of Dev. 2 and their fitted curves (white solid lines); (b), (c) intensity factors of ISC, RISC, and TQA processes in Dev. 2 at different bias-currents.

    图 4  不同注入电流下器件中PP态和EX态的形成和演变机制 (a) Dev. 1; (b) Dev. 2

    Figure 4.  Formation and evolution mechanisms of PP and EX states in devices at different bias-currents: (a) Dev. 1; (b) Dev. 2.

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Metrics
  • Abstract views:  2834
  • PDF Downloads:  47
Publishing process
  • Received Date:  11 May 2023
  • Accepted Date:  08 June 2023
  • Available Online:  26 June 2023
  • Published Online:  20 August 2023

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