Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

Abstract

One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface.Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono-to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to band-like in subsequent layers.Such abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only ~3nm. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals. 3Organic field-effect transistors (OFETs) offer unique advantages of low cost, lightweight and flexibility and are widely used in electronics and display industry.While the mobility of bulk organic semiconductors has increased dramatically [1][2][3], an outstanding issue is to directly examine the structure-property relationship at the semiconductor-dielectric interface [4], where charge transport actually occurs [5][6][7].Ultrathin organic semiconductors down to few-nanometre thickness are often dominated by traps and disorders and far away from intrinsic transport regime [8][9][10].Another challenge in organic electronics is the development of layer-by-layer epitaxy with the precision similar to molecular beam epitaxy in their inorganic counterparts [11]. These challenges may be alleviated if molecular crystals are processed into large-area, highly crystalline monolayers. Such 2D form factor will also bring about new applications such as nanoporous membranes and insulating dielectrics [12,13].Several recent breakthroughs in various types of 2D organic materials such as polymers [14,15], oligomers [16] and covalent organic frameworks [17] have already shown great promises along this direction. However, one of the most fundamental questions regarding the nature of charge transport at the 2D limit has not been addressed. In this work, we study the benchmark molecule pentacene epitaxially crystallized on BN substrate because of its high mobility and simple structure to model. The highly clean system allows us to provide the first definitive scenario of how molecular packing and charge transport are modulated near the interface, without being dominated by extrinsic factors. Our results suggest the possibility of band-like transport...

Precise Layering of Organic Semiconductors

“…Our structure measurements were further supported by ab initio DFT calculations [18]. Figure S17 in the Supplemental Material [18] shows the optimized molecular packing for 1L and 2L, whose lattice constants are in good agreement with experimental values within 1.5%.…”

“…1(a), the molecular packing is very different near the dielectric interface. The average thickness of the wetting layer (WL, also referred to as interfacial layer in the literature), the first conducting layer (1L), and the second conducting layer (2L) is 0.5, 1.14, and 1.58 nm, respectively 18]), similar to that of pentacene on graphite [35] and on metal [36]. The thickness of 2L is consistent with the thinfilm phase of pentacene [37].…”

“…1(f), Supplemental Material, Figs. S2b, S2c] [18] indicated that the pentacene was crystalline, as later confirmed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As schematically illustrated in Fig.…”

“…Statistical analysis of the SAED patterns showed a sharp peak near 16°between pentacene (010) and BN (100) (Supplemental Material Fig. S4c [18]), indicating that pentacene had a quasiepitaxial relationship with BN [12]. The quasiepitaxy nature was due to weak vdW interactions and incommensurability between pentacene and BN.…”

“…Therefore, we also performed TEM characterization to crosscheck with AFM. Figure S4a in the Supplemental Material [18] shows a typical low-magnification TEM image of a fewlayer pentacene on BN. Selected-area electron diffraction (SAED) was typically performed over a ∼10 μm 2 area and exhibited a single set of diffraction patterns from pentacene [ Fig.…”