In organic electronics, local crystalline order is of critical importance for the charge transport. Grain boundaries between molecularly ordered domains are generally known to hamper or completely suppress charge transfer and detailed knowledge of the local electronic nature is critical for future minimization of such malicious defects. However, grain boundaries are typically hidden within the bulk film and consequently escape observation or investigation. Here, a minimal model system in form of monolayer-thin films with sub-nm roughness of a prototypical n-type organic semiconductor is presented. Since these films consist of large crystalline areas, the detailed energy landscape at single grain boundaries can be studied using Kelvin probe force microscopy. By controlling the charge-carrier density in the films electrostatically, the impact of the grain boundaries on charge transport in organic devices is modeled. First, two distinct types of grain boundaries are identified, namely energetic barriers and valleys, which can coexist within the same thin film. Their absolute height is found to be especially pronounced at charge-carrier densities below 10¹² cm^–2—the regime at which organic solar cells and light emitting diodes typically operate. Finally, processing conditions by which the type or energetic height of grain boundaries can be controlled are identified.

中文翻译：

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揭示和控制超薄有机薄膜晶界处的能量屏障和山谷

在有机电子学中，局部晶体顺序对于电荷传输至关重要。通常已知分子有序域之间的晶界会阻碍或完全抑制电荷转移，而对局部电子性质的详细了解对于未来最大限度地减少此类恶意缺陷至关重要。然而，晶界通常隐藏在体膜内，因此无法观察或调查。在这里，提出了具有原型 n 型有机半导体的亚纳米粗糙度的单层薄膜形式的最小模型系统。由于这些薄膜由大的结晶区域组成，因此可以使用开尔文探针力显微镜研究单个晶界处的详细能量景观。通过静电控制薄膜中的电荷载流子密度，模拟了晶界对有机器件中电荷传输的影响。首先，确定了两种不同类型的晶界，即高能势垒和谷，它们可以在同一薄膜内共存。发现它们的绝对高度在电荷载流子密度低于 10 时特别明显¹² cm ^{– 2} — 有机太阳能电池和发光二极管通常工作的状态。最后，确定了可以控制晶界类型或能量高度的加工条件。