Article pubs.acs.org/JPCC
Discontinuous pn-Heterojunction for Organic Thin Film Transistors Boeun Cho,†,○ Seong Hun Yu,‡,○ Minwoo Kim,§,∥ Moo Hyung Lee,† Wansoo Huh,† Jiyoul Lee,# Jungwook Kim,∇ Jeong Ho Cho,‡,∥,⊥ Jun Young Lee,‡ Young Jae Song,*,§,∥ and Moon Sung Kang*,† †
Department of Chemical Engineering, Soongsil University, Seoul, 156-743, Korea School of Chemical Engineering, §Department of Physics, ∥SKKU Advanced Institute of Nanotechnology (SAINT), and ⊥Center for Human Interface Nano Technology (HINT), Sungkyunkwan University, Suwon, 440-746, Korea # Holst Centre/TNO, High Tech Campus 31, 5656 AE Eindhoven, Netherlands ∇ Department of Chemical and Biomolecular Engineering, Sogang University, Seoul, 121-742, Korea ‡
S Supporting Information *
ABSTRACT: Utilization of discontinuous pn-oragnic heterojunction is introduced as a versatile method to improve charge transport in organic thin film transistors (OTFTs). The method is demonstrated by depositing n-type dioctyl perylene tetracarboxylic diimide (PTCDI-C8) discontinuously onto base p-type pentacene OTFTs. A more pronounced impact of the discontinuous upper layer is obtained on the transistor performances when thinner base layers are employed; a >100-fold enhancement in hole mobility and a >20 V shift in threshold voltage are achieved after applying PTCDI-C8 discontinuously onto 2 nm thick pentacene thin films. Local surface potential measurements (Kelvin-probe force microscopy) and temperature-dependent transport measurements (77−300 K) reveal that the interfacial dipole formed at the pnheterostructures effectively dopes the base pentacene films p-type and leads to a reduction in transport activation energy.
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INTRODUCTION Research in organic thin-film transistors (OTFTs) for realizing low-cost, large-area, flexible electronics has made significant advances during the last few decades.1−6 In particular, carrier mobilities comparable to or even superior to those for amorphous silicon TFTs (>1 cm2/(V s)) are now achievable from OTFTs.7−9 This progress relied on multilateral efforts that include (i) designing advanced organic semiconductors,10−12 (ii) employing new gate dielectric materials,13−17 (iii) devising various thin-film processing techniques that are involved before, after, or upon depositing the organic semiconductor layers to improve film morphology,18,19 and (iv) developing novel device architectures.20,21 While the first two approaches inevitably involve complicated chemical syntheses, the latter two approaches, in principle, can be developed by utilizing readily available material systems and then can be extended to a wider selection of materials. This universalness is important for boosting the efforts of diverse research communities that can potentially contribute to the development of organic electronics. Among various strategies available, we take particular notice of the methods based on the organic heterojunction effect. Improvement in carrier mobility has been observed at the interface between p- and n-type organic semiconductors (pnorganic heterojunction) when the energy levels of the materials are appropriately aligned.22 For example, if the lowest unoccupied molecular orbital (LUMO) of an n-type layer lies near the highest occupied molecular orbital (HOMO) of a ptype layer, holes can be transferred from the n-type layer to the © 2014 American Chemical Society
p-type layer, which would lead to enhanced charge carrier mobility. Because formation of heterojunction is combinatory and complementary, such approaches allow employing diverse sets of material candidates for optimizing device performance.23 Recently, Kim et al.24 demonstrated the utilization of a discontinuous top n-type layer (called nanopatches) for improving carrier transport through the p-type layer underneath. As compared to the typical continuous pn-heterojunction, such an approach is advantageous for making unipolar transistors yielding a high ON/OFF current ratio. This is because the top layer in a discontinuous approach only induces charge transfer to the bottom layer but does not serve as a transport channel by itself and thereby does not contribute to the leakage current or OFF current of the devices. In contrast, the continuous pn-heterojunction typically contains conductive pathways for both electrons and holes that prevent complete device turn-off; ambipolar transistors, in general, suffer from poor power consumption problems due to high OFF current level.22,25,26 In this vein, we investigate the discontinuous heterojunction effect of p-type pentacene films partially covered with n-type dioctyl perylene tetracarboxylic diimide (PTCDI-C8). The PTCDI-C8 was selected as the top layer primarily because PTCDI-C8/pentacene exhibits a favorable energy alignment for charge transfer at the heterojunction interface (Figure 1a).27−30 Received: April 28, 2014 Revised: July 2, 2014 Published: July 9, 2014 18146
dx.doi.org/10.1021/jp504114f | J. Phys. Chem. C 2014, 118, 18146−18152
The Journal of Physical Chemistry C
Article
Figure 1. (a) Energy level diagram for pentacene/PTCDI-C8. (b) Schematic cross-section of a pentacene OTFT with discontinuous PTCDI-C8 layer. (c) Topography of 2 nm pentacene films partially covered with PTCDI-C8. (d) Topography of pentacene films with different thicknesses partially covered with 7.5 Å PTCDI-C8. For (c) and (d), the white scale bar in the images indicates 1 μm. For (d), the vertical color-gradient bar indicates the span in height variation of 20 nm for the first three images on the left and of 50 nm for the last image on the right.
at 1 mm, while the channel length varied between 100 and 50 μm. OTFT Measurements. Current−voltage relations of the asprepared OTFTs were obtained under vacuum (
