Revealing and Controlling Energy Barriers and Valleys at Grain Boundaries in Ultrathin Organic Films

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2022-08, Vol.18 (34), p.e2200605-n/a
• Main Authors: Walter, Lisa S., Axt, Amelie, Borchert, James W., Kammerbauer, Theresa, Winterer, Felix, Lenz, Jakob, Weber, Stefan A. L., Weitz, R. Thomas
• Format: Article
• Language: English
• Subjects: 2D‐crystals; Carrier density; Charge transport; Crystal defects; Crystal structure; Crystallinity; Current carriers; energy transformation; Grain boundaries; grain boundary; Kelvin probe force microscopy; Knowledge management; Light emitting diodes; Nanotechnology; organic semiconductors; Photovoltaic cells; Solar cells; Thin films
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202200605

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. The energetic properties of grain boundaries in highly‐crystalline monolayers of an organic semiconductor are examined using high‐resolution Kelvin probe force microscopy. Analyses in the charge‐carrier density ranges typically present in organic light emitting diodes and organic solar cells reveal both energetic barriers and valleys that may have significant implications for charge‐transport efficiency in these devices.