Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, 9747 AG, Groningen, The Netherlands.
Adv Mater. 2014 Jan;26(4):512-31. doi: 10.1002/adma.201303393.
This article reviews the basic physical processes of charge transport and recombination in organic semiconductors. As a workhorse, LEDs based on a single layer of poly(p-phenylene vinylene) (PPV) derivatives are used. The hole transport in these PPV derivatives is governed by trap-free space-charge-limited conduction, with the mobility depending on the electric field and charge-carrier density. These dependencies are generally described in the framework of hopping transport in a Gaussian density of states distribution. The electron transport on the other hand is orders of magnitude lower than the hole transport. The reason is that electron transport is hindered by the presence of a universal electron trap, located at 3.6 eV below vacuum with a typical density of ca. 3 × 10¹⁷ cm⁻³. The trapped electrons recombine with free holes via a non-radiative trap-assisted recombination process, which is a competing loss process with respect to the emissive bimolecular Langevin recombination. The trap-assisted recombination in disordered organic semiconductors is governed by the diffusion of the free carrier (hole) towards the trapped carrier (electron), similar to the Langevin recombination of free carriers where both carriers are mobile. As a result, with the charge-carrier mobilities and amount of trapping centers known from charge-transport measurements, the radiative recombination as well as loss processes in disordered organic semiconductors can be fully predicted. Evidently, future work should focus on the identification and removing of electron traps. This will not only eliminate the non-radiative trap-assisted recombination, but, in addition, will shift the recombination zone towards the center of the device, leading to an efficiency improvement of more than a factor of two in single-layer polymer LEDs.
本文综述了有机半导体中电荷输运和复合的基本物理过程。以单层聚对苯乙炔（PPV）衍生物为工作材料的发光二极管（LED）就是一个很好的例子。这些 PPV 衍生物中的空穴输运由无陷阱的空间电荷限制传导（SCLC）机制主导，迁移率取决于电场和载流子密度。这些依赖性通常可以用在高斯态密度分布中的跳跃输运框架来描述。而另一方面，电子输运则比空穴输运低好几个数量级。原因在于电子输运受到普遍存在的电子陷阱的阻碍，该电子陷阱位于真空中方 3.6 eV 处，典型密度约为 3×10¹⁷ cm⁻³。被捕获的电子通过非辐射的陷阱辅助复合过程与自由空穴复合，这是一个与发射双分子 Langevin 复合竞争的损耗过程。无序有机半导体中的陷阱辅助复合由自由载流子（空穴）向被捕获载流子（电子）的扩散控制，类似于自由载流子的 Langevin 复合，其中两个载流子都是移动的。因此，根据电荷输运测量得到的载流子迁移率和捕获中心数量，可以完全预测无序有机半导体中的辐射复合以及损耗过程。显然，未来的工作应侧重于识别和去除电子陷阱。这不仅可以消除非辐射的陷阱辅助复合，而且会使复合区向器件中心移动，从而使单层聚合物 LED 的效率提高一倍以上。
