The electronic density of states (DOS) plays a central role in controlling the charge-carrier transport in amorphous organic semiconductors, while its accurate determination is still a challenging task. We apply the low-temperature fractional thermally stimulated luminescence (TSL) technique to determine the DOS of pristine amorphous films of organic light-emitting diode (OLED) host materials. The DOS width is determined for two series of hosts, namely, (i) carbazole-biphenyl derivatives, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), 3,3′-di(9$H$-carbazol-9-yl)-1,1′-biphenyl (mCBP), and 3′,5-di(9$H$-carbazol-9-yl)-[1,1′-biphenyl]-3-carbonitrile (mCBP-CN), and (ii) carbazole-phenyl (CP) derivatives, 1,3-bis(N-carbazolyl)benzene (mCP) and 9-[3-(9$H$-carbazol-9-yl)phenyl]-9$H$-carbazole-3-carbonitrile (mCP-CN). TSL originates from radiative recombination of charge carriers thermally released from the lower-energy part of the intrinsic DOS that causes charge trapping at very low temperatures. We find that the intrinsic DOS can be approximated by a Gaussian distribution, with a deep exponential tail accompanying this distribution in CBP and mCBP films. The DOS profile broadens with increasing molecular dipole moments, varying from 0 to 6 D, in a similar manner within each series, in line with the dipolar disorder model. The same molecular dipole moment, however, leads to a broader DOS of CP compared with CBP derivatives. Using computer simulations, we attribute the difference between the series to a smaller polarizability of cations in CP derivatives, leading to weaker screening of the electrostatic disorder by induction. These results demonstrate that the low-temperature TSL technique can be used as an efficient experimental tool for probing the DOS in small-molecule OLED materials.

中文翻译：

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热激发发光导致的OLED主体材料的状态密度

状态电子密度（DOS）在控制非晶有机半导体中的电荷载流子传输中起着核心作用，而准确确定其仍然是一项艰巨的任务。我们应用低温分数热激发发光（TSL）技术来确定有机发光二极管（OLED）主体材料的原始非晶膜的DOS。确定两个系列主体的DOS宽度，即（i）咔唑-联苯衍生物，4,4'-双（N-咔唑基）-1,1'-联苯（CBP），3,3'-di（ 9$H$-carbazol-9-yl）-1,1'-联苯（mCBP）和3'，5-di（9$H$-咔唑-9-基）-[1,1'-联苯] -3-腈（mCBP-CN）和（ii）咔唑-苯基（CP）衍生物，1,3-双（N-咔唑基）苯（ mCP）和9- [3-（9$H$-咔唑-9-基）苯基] -9$H$-咔唑-3-腈（mCP-CN）。TSL源自从本征DOS的低能部分热释放的载流子的辐射复合，这导致在非常低的温度下捕获电荷。我们发现本征DOS可以通过高斯分布来近似，在CBP和mCBP薄膜中伴随着这种分布的是深指数尾巴。DOS分布随着分子偶极矩的增加而加宽，分子偶极矩从0到6 D不等，在每个系列中以相似的方式，与偶极障碍模型一致。然而，与CBP衍生物相比，相同的分子偶极矩导致CP的DOS范围更广。使用计算机模拟，我们将系列之间的差异归因于CP衍生物中阳离子的极化率较小，从而导致通过感应对静电障碍的筛选较弱。