Molecular doping of organic semiconductors and beyond: resolving fundamental processes and increasing doping efficiency

通过引入杂质来掺杂无机半导体（如晶体硅）是所有晶体管技术的基础，这使得当今的电子器件具有功能性。在过去的几十年里，有机半导体成为无机物的迷人替代品，有望在柔性衬底上实现大面积加工和巨大的多功能性，因为它们的光电特性，特别是它们的带隙，可以通过它们的化学结构来调节。掺杂的概念直到最近才通过采用分子掺杂剂而适用于有机物，其中今天基于有机发光二极管（OLED）的所有新型显示技术都使用掺杂的共轭材料。然而，尽管在应用方面取得了巨大的成功，但对有机半导体掺杂的基本过程仍然知之甚少，材料设计在很大程度上仍然是经验性的。在以前的工作中，我们确定了半导体和掺杂剂之间的化学相互作用是其低掺杂效率的关键因素，因为，而不是离子对，电荷转移复合物倾向于形成并接管掺杂剂的作用，然而，强度显着降低。在目前的多学科研究计划中，我们的目标是，首先，利用这种新的理解，通过修改分子间耦合的机制来恢复掺杂剂的强度。我们将采用能够与一种以上的掺杂剂相互作用的半导体，这将使有效的受体能级在能量上更容易接近。或者，由于电荷转移复合物的形成是由于耦合，我们将通过采用空间屏蔽功能核的种类来抑制耦合。其次，由于我们发现复合物的形成只发生在分子半导体中，而共轭聚合物在掺杂时确实会形成离子对，因此我们的目标是推导出一个对两者都有效的统一图像，从中自然出现它们不同的掺杂行为。为此，我们将增加从寡聚体到聚合物极限的重复单元的数量，从而评估相互作用局部性的作用，并观察看似不协调的掺杂现象之间的过渡。第三，我们的目标是通过使用刘易斯酸作为p型掺杂剂，通过化学选择性来增加分子掺杂，其中电荷转移被限制为能够充当刘易斯碱的物质。最后，我们将超越传统的掺杂，以获得空间控制的电荷密度在埋层。代替掺杂剂混合物，定制的能级对准将通过与衬底的能级钉扎而使层p-/n-掺杂。* 总的来说，拟议的计划将建立有机半导体掺杂的完整图景。它将使基于知识的化学设计能够控制电荷密度，就像我们今天通常对无机半导体所做的那样，最终达到类似的功能。

Doping inorganic semiconductors such as crystalline silicon by introducing impurities is the basis of all transistor technology, which enables the functionality of today's electronic devices. In the past decades, organic semiconductors emerged as fascinating alternative to inorganics promising large-area processability on flexible-substrates and enormous versatility, as their opto-electronic properties, their bandgap in particular, can be tuned by their chemical structure. The concept of doping has only recently been adapted for organics by employing molecular dopants, where all novel display technology based on organic light emitting diodes (OLEDs) today uses doped conjugated materials. Despite enormous success in applications, however, the fundamental processes of doping are still poorly understood for organic semiconductors and material design remains largely empirical. In previous work, we identified the chemical interaction between semiconductor and dopant as key factor for their low doping efficiency, as, rather than ion pairs, charge transfer complexes tend to form and take over the role of the dopants, however, of significantly reduced strength.******In the present multidisciplinary research program, we aim to, first, exploit this novel understanding for regaining dopant strength by modifying the mechanism of intermolecular coupling. We will employ semiconductors capable of interacting with more than one dopant, which will make effective acceptor levels energetically more accessible. Alternatively, as charge-transfer complex formation is due to the coupling, we will inhibit the coupling by employing species of sterically shielded functional cores. Second, as we found that complex formation only occurs for molecular semiconductors, while conjugated polymers indeed form ion pairs upon doping, we aim to derive a unifying picture valid for both, from which their different doping behavior emerges naturally. To this end, we will increase the number of repeat units from the oligo- to the polymer limit, thereby assess the role of interaction locality, and observe the transition between the seemingly incongruous doping phenomenologies. Third, we aim to augment molecular doping by chemical selectivity through using Lewis acids as p-dopants, where charge transfer is restricted to species capable of acting as Lewis base. Finally, we will go beyond traditional doping to gain spatial control over the charge density in buried layers. Instead of dopant admixture, tailored energy-level alignment will render layers p-/n-doped via energy level pinning with the substrate. ******Overall, the proposed program will establish a complete picture of doping in organic semiconductors. It will enable knowledge-based chemical design for controlling charge density just as precisely as we customarily do today with inorganic semiconductors, for ultimately approaching similar functionality.