Opitz Andreas
Humboldt-Universität zu Berlin, Berlin, Germany.
J Phys Condens Matter. 2017 Apr 5;29(13):133001. doi: 10.1088/1361-648X/aa5a6c. Epub 2017 Feb 14.
Planar organic heterojunctions are widely used in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. The energy level alignment in the devices plays an important role in obtaining the aspired gap arrangement. Additionally, the π-orbital overlap between the involved molecules defines e.g. the charge-separation efficiency in solar cells due to charge-transfer effects. To account for both aspects, direct/inverse photoemission spectroscopy and near edge x-ray absorption fine structure spectroscopy were used to determine the energy level landscape and the molecular orientation at prototypical planar organic heterojunctions. The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated molecular semiconductors. Following an introduction on heterojunctions used in devices and on energy levels of organic materials, the energy level alignment of planar organic heterojunctions will be discussed. The observed energy landscape is always determined by the individual arrangement between the energy levels of the molecules and the work function of the electrode. This might result in contact doping due to Fermi level pinning at the electrode for donor/acceptor heterojunctions, which also improves the solar cell efficiency. This pinning behaviour can be observed across an unpinned interlayer and results in charge accumulation at the donor/acceptor interface, depending on the transport levels of the respective organic semiconductors. Moreover, molecular orientation will affect the energy levels because of the anisotropy in ionisation energy and electron affinity and is influenced by the structural compatibility of the involved molecules at the heterojunction. High structural compatibility leads to π-orbital stacking between different molecules at a heterojunction, which is of additional interest for photovoltaic active interfaces and for ground-state charge-transfer.
平面有机异质结广泛应用于光伏电池、发光二极管和双层场效应晶体管中。器件中的能级排列在获得理想的能隙排列方面起着重要作用。此外，所涉及分子之间的π轨道重叠例如由于电荷转移效应决定了太阳能电池中的电荷分离效率。为了兼顾这两个方面，采用直接/逆光电子能谱和近边X射线吸收精细结构光谱来确定典型平面有机异质结处的能级分布和分子取向。这种综合实验方法得出了一个关于两种被研究分子半导体之间界面的电子和形态特征的综合模型。在介绍了器件中使用的异质结和有机材料的能级之后，将讨论平面有机异质结的能级排列。观察到的能级分布总是由分子能级与电极功函数之间的个体排列决定的。这可能会由于施主/受主异质结在电极处的费米能级钉扎而导致接触掺杂，这也提高了太阳能电池的效率。这种钉扎行为可以在一个未钉扎的中间层上观察到，并导致施主/受主界面处的电荷积累，这取决于各自有机半导体的传输能级。此外，由于电离能和电子亲和性的各向异性，分子取向会影响能级，并且受到异质结处所涉及分子的结构相容性的影响。高结构相容性会导致异质结处不同分子之间的π轨道堆叠，这对于光伏活性界面和基态电荷转移来说具有额外的研究意义。
