Abstract
AbstractEfficient charge‐carrier injection is a critical requirement for high‐performance organic electronic devices, such as light‐emitting diodes, solar cells, and field‐effect transistors. In this work, a significantly improved charge‐carrier injection from high work‐function metal‐oxide electrodes in organic field effect transistors (OFETs) is demonstrated for amorphous organic semiconductors (OSCs) by using organic interlayers with a high ionization energy (IE). Molybdenum oxide (MoO3) exhibits limited injection into amorphous 2,2′,7,7′‐tetrakis(N,N‐diphenylamino)‐9,9‐spirobifluorene (Spiro‐TAD) and tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA) active layers, resulting in high contact resistance and threshold voltage. By inserting an interlayer of a few nanometers thick with high IE between the MoO3 electrode and the amorphous OSC films, the Spiro‐TAD and TCTA OFETs show a substantial enhancement in hole current, subthreshold swing, and effective charge carrier mobility due to the decreased contact resistance. However, for discontinuous interlayers formed on distinct grain domains as in the case of polycrystalline 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) films, the effect of the interlayer reduces. These results demonstrate that the utilization of smooth interlayers with IE higher than the semiconductor is a general approach to elevate the hole injection into amorphous OSCs with high IE in OFETs.
Highlights
These results demonstrate that the utilization of smooth interlayers with ionization energy (IE) higher than the semiconductor is a general approach to elevate the hole injection into amorphous organic semiconductors (OSCs) with high IE in organic field effect transistors (OFETs)
(TIPS-TAP) show hole and electron mobilities above 10 cm2 V−1 s−1.[3,4] in contrast to the low contact resistance (Rc) in inorganic thin film transistors, OFETs typically suffer from high Rc since the electrode–OSC junctions consist of a heterogeneous material interface between metal electrode and OSC active film.[5]
The Fermi level is pinned at deeper states in the broadened interface density of states (DOS), from which charges have to escape to the narrower bulk DOS, leading to an injection barrier
Summary
The schematic energy-band diagram in the top graph of Figure 1a illustrates the injection barrier at the interface between OSC and MoO3. For Au contacts, a nonlinear Ids1/2 behavior and a pronounced hysteresis in forward and backward scans (Figure S3a, Supporting Information) are observed, which are basically eliminated in the IL/MoO3/Al configuration These results demonstrate that the insertion of a TCTA IL improves the hole injection from MoO3 into the active SpiroTAD film. The lower μeff for MoO3/Al and Au contact devices directly results from a high Rc. To prove the generality of the IL concept on improvement of the hole injection in OFETs, a 50 nm thick homogeneous TCTA film was vacuum evaporated as active layer on bare silicon/silicone dioxide substrate instead of OTS-modified substrate (Figure S5, Supporting Information). A homogenous and continuous morphology of IL is a prerequisite for an effective reduction of the hole injection barrier in p-type OFETs
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