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

Abstract

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.

What can the activation energy tell about the energetics at grain boundaries in polycrystalline organic films

“…Monolayer thin films of the organic semiconductor PDI1MPCN2 were fabricated using previously reported solution processes [20,23] from dimethyl phthalate (DMP) solution onto degenerately-doped Si wavers covered with 30 nm of Al2O3 formed using atomic layer deposition (ALD). As per reference [23], the process for monolayer formation implemented in this work typically yields energy valleys at the grain boundaries, which are distinguished by higher surface potentials, corresponding to energetically smaller lowest unoccupied molecular orbital (LUMO) levels at grain boundaries than within the surrounding grains. Source and drain contacts for the OFETs were defined using shadow masks with channel lengths of L = 20 μm or L = 200 μm and were applied after semiconductor film formation, and the doped Si substrate was used as the gate (e.g.…”

“…Instead, Kelvin Probe Force Microscopy (KPFM) has become an established method for the characterization of grain boundaries, since it allows concurrent local probing of the topography and the potential landscape [21,22]. We recently extended this to monolayer PDI1MPCN2 films containing a small density of grain boundaries [23]. Importantly, the use of the monolayer films (as opposed to multilayer films) guarantees that the probed layer is the same layer within which charge-carrier transport takes place.…”

The Electrical Behaviors of Grain Boundaries in Polycrystalline Optoelectronic Materials

“…[79] Even for the same film undergoes post-processing, both electron and hole barriers will appear simultaneously. [69,81,82] In addition to band bending at GBs, some GBs exhibit the same energy band structure as the GIs, meaning that no band bending occurs at the GBs. In the study of polycrystalline Cu(In,Ga)Se 2 , Nicoara et al [69] referred to such GBs as neutral (ΔCPD GB = 0).…”