Anomalous Ambipolar Transport of Organic Semiconducting Crystals

Aug 2, 2017 - ... a deep highest occupied molecular orbital level compared to conventional “standing-up” crystals. The lattice-strained pentacene ...
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Anomalous Ambipolar Transport of Organic Semiconducting Crystals via Control of Molecular Packing Structures Beomjin Park,†,∇ Kyunghun Kim,†,∇ Jaesung Park,‡,∇ Heeseon Lim,‡ Phung Thi Lanh,‡,§ A-Rang Jang,∥,⊥ Chohee Hyun,∥,⊥ Chang Woo Myung,∥ Seungkyoo Park,† Jeong Won Kim,‡,§ Kwang S. Kim,∥ Hyeon Suk Shin,∥,⊥ Geunsik Lee,∥ Se Hyun Kim,*,# Chan Eon Park,*,† and Jin Kon Kim*,† †

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea Korea Research Institute of Standards and Science, Daejeon 305-340, Korea § Korea University of Science and Technology (UST), Daejeon 34113, Korea ∥ Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea ⊥ Center for Multidimensional Carbon Materials, Institute of Basic Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, Korea # School of Chemical Engineering, Yeungnam University, Gyeongsan, 712-749, Korea ‡

S Supporting Information *

ABSTRACT: Organic crystals deposited on 2-dimensional (2D) van der Waals substrates have been widely investigated due to their unprecedented crystal structures and electrical properties. van der Waals interaction between organic molecules and the substrate induces epitaxial growth of high quality organic crystals and their anomalous crystal morphologies. Here, we report on unique ambipolar charge transport of a “lying-down” pentacene crystal grown on a 2D hexagonal boron nitride van der Waals substrate. From in-depth analysis on crystal growth behavior and ultraviolet photoemission spectroscopy measurement, it is revealed that the pentacene crystal at the initial growth stage have a lattice-strained packing structure and unique energy band structure with a deep highest occupied molecular orbital level compared to conventional “standing-up” crystals. The lattice-strained pentacene few layers enable ambipolar charge transport in field-effect transistors with balanced hole and electron field-effect mobilities. Complementary logic circuits composed of the two identical transistors show clear inverting functionality with a high gain up to 15. The interesting crystal morphology of organic crystals on van der Waals substrates is expected to attract broad attentions on organic/2D interfaces for their electronic applications. KEYWORDS: van der Waals substrates, hexagonal boron nitride (h-BN), organic/h-BN heterostructures, ambipolar charge transport, organic field-effect transistors

1. INTRODUCTION

Recently, OSCs deposited on 2-dimensional (2D) layered materials, such as graphene and hexagonal boron nitride (h-BN, Figure 1a), have attracted strong attention due to their unprecedented thin film morphologies and electrical properties.6−10 For instance, atomically flat h-BN provides epitaxial growth of high quality rubrene single crystals, leading to high field-effect mobility (μFET) OFETs.6 Benzothienobenzothiophene-based OSCs grown on h-BN favored assembly with “lying-down” orientation where a π-conjugation plane of the OSC molecule is contacted with the underlying substrate, and the OFETs employing the single-crystalline monolayer showed high μFET.9 However, even though fast charge transport in OSCs is obtained on 2D surfaces, there have been no report for

Ambipolar organic semiconductors (OSCs) utilizing both holes and electrons are attractive as a key component in future electronic circuits because they bring simplification and stabilization to fabricating of complementary circuits.1,2 Even though ambipolar organic field-effect transistors (OFETs) based on either blending p-type and n-type OSCs or using their layered structures have been reported in the literature,3,4 high-cost and complicated fabrication steps limit the development of unit ambipolar OSCs. Ambipolar OSCs have also been synthesized from halogen substitution or donor−acceptor conjugated structures to induce their low band gap, but synthesizing high purity OSCs is very difficult and expensive.5 Thus, simple polarity tuning of existing OSCs by controlling their nanostructures could be an ideal way for cost-effective fabrication of ambipolar OSCs. © XXXX American Chemical Society

Received: April 12, 2017 Accepted: August 2, 2017 Published: August 2, 2017 A

DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

its efficient lateral charge transport and also unique ambipolar behavior. We investigated the crystal growth behavior of pentacene on h-BN using various crystal analysis tools. Their charge transport properties, characterized by fabricating OFET structures, show clear ambipolar charge transport with balanced μFETs of 0.20 cm2/(V·s) (hole) and 0.26 cm2/(V·s) (electron). To understand the origin of ambipolar characteristics, we performed ultraviolet photoemission spectroscopy (UPS) to measure the highest occupied molecular orbital level (EHOMO) and to deduce the lowest unoccupied molecular orbital level (ELUMO) of “lying-down” pentacene crystals close to h-BN. It is shown that a deep EHOMO and thereby deep ELUMO of a lattice-strained “lying-down” pentacene crystal, compared to “bulk lying-down” or “standing-up” one, is a key factor for its ambipolar charge transport. With two identical ambipolar OFETs, we also fabricated complementary inverters showing voltage gains as high as 15. Figure 1. Pentacene growth on h-BN. (a) Schematic illustration of hBN on a SiO2/Si substrate, (b) “lying-down” pentacene layer formation on h-BN at the initial stage of the growth, (c) top view of “bulk lying-down” pentacene layer formation on h-BN, and (d) cross-sectional view of entire pentacene crystals growth on h-BN, showing the evolution of molecular orientation from “lying-down” to “bulk lying-down”, and final to “standing up”.

2. EXPERIMENTAL SECTION Preparation of h-BN/SiO2/Si Substrate. 300 nm-thick SiO2/Si substrates were (i) cleaned in piranha solution (6:4 mixture of H2SO4:H2O2) for 30 min at 250 οC, (ii) washed with distilled water, and (iii) cleaned under UV-ozone exposure for 20 min. Thin h-BN (20 nm), the crystals become “standing-up” orientation. Considering the sensitivity of charge transport properties in OFETs to the few layers of OSC molecules near the dielectric interface, large and epitaxially grown pentacene crystals with “lying-down” orientation near h-BN surface would provide unique charge transport properties. To determine the overall packing structure of pentacene on h-BN, we performed GIWAXS experiment with a synchrotron light source with varying T (1.5 nm, 20 nm, 50 nm) (Figure 2b). At T = 1.5 nm (left image of Figure 2b), intense (00l) reflections tilted by 18ο from the qxy axis were only observed, indicating that pentacene layer was wholly “lying-down”.7 However, as T increased to 20 nm (middle image of Figure 2b), while the “lying-down” orientation was still dominant because of intense (00l), (020), {1, ± 1} and {1, ± 2} reflections (blue brackets), standing-up orientation also appeared, supported by weak (00l), (020), {1, ± 1} and {1, ± 2} reflections (red brackets).31 It coincided with the AFM result where the “standing-up” pentacene was also observed at T = 20 nm. It is noted that the AFM image only represents the top surface of the pentacene film. When the T increases to 50 nm (right image of Figure 2b), the reflection intensity arising from “standing-up” orientation becomes more dominant. GIWAXS D

DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Table 1. Electrical Characteristics of “Lying-Down” Pentacene OFETs Depending on Device Geometry for holes

for electrons

device geometry

μFET (cm2/(V·s))

Vth (V)

SS (V/decade)

μFET (cm2/(V·s))

Vth (V)

SS (V/decade)

BGBC BGTC

0.20 ± 0.02 0.23 ± 0.03

−63.4 −20.1

17.1 11.0

0.26 ± 0.03 0.007 ± 0.002

63.4 64.2

17.4 43.3

Figure 4. UPS measurement on pentacene crystals. Valence electronic structure of pentacene in situ grown on (a) h-BN and (b) SiO2 substrates. Their resultant EHOMOs were shown in (c).

inverters. The ID−D output characteristic in Figure 3c also showed typical ambipolar behavior; (i) a super linear current increase at low VGs due to the injection of the opposite carrier, and (ii) a clear transition from linear to saturation at high VGs.34 More pronounced S-shaped curves at positive VGs compared to negative VGs indicated that there is a higher injection barrier for electrons from Au electrodes into pentacene.42 Compared to BGBC OFETs, in BGTC OFETs (Figure 3d), electron transport was significantly suppressed, while maintaining the hole transport. As summarized in Table 1, an average electron μFET dropped to 0.007 cm2/(V·s), and there was negligible change in an average hole μFET with a value of 0.23 cm2/(V•s) (Figure 3e). In the output characteristic (Figure 3f), weak electron transport was barely observed in high positive VG, and low negative VG, respectively. As mentioned above, it is an abnormal behavior that the μFETs are comparable (hole) or higher (electron) at the BGBC geometry, which will be discussed in the next section. 3.3. Origin of Ambipolar Transport of “Lying-Down” Pentacene Crystals on h-BN. Before comparing BGBC and BGTC OFETs, the origin of ambipolar transport of “lyingdown” pentacene crystals needs to be revealed. We speculated that reduction of electron injection barrier from Au electrodes to “lying-down” pentacene crystals compared to “standing-up” ones could enable not only their hole transport but also electron transport (i.e., ambipolar transport). Therefore, we performed UPS measurement to investigate the EHOMO, and anticipate the ELUMO of “lying-down” pentacene crystals. As shown in Figure 4, UPS spectra as a function of T revealed that EHOMOs of “lying-down” pentacene crystals on h-BN were deeper than those of “standing-up” ones on SiO2 substrates at all Ts. Actually, from previous reports, a shift of EHOMO depending on molecular orientations/packing structures of pentacene has been experimentally observed.43−46 It was due to changes in the intermolecular dispersion and electronic polarization energy. At all cases, EHOMOs of “lying-down” crystals were much deeper than those of “standing-up” crystals, which agreed well with our results. Considering the shift of

300 nm-thick SiO2 dielectric and Au source/drain electrodes. The Au electrodes were thermally evaporated through a shadow mask where channel width (W) and length (L) were 50 and 25 μm, respectively. The capacitance of h-BN/300 nmthick SiO2 (Ci) was 10.0 nF/cm2. Device measurements were carried out in a N2-purged glovebox (H2O, O2 < 0.1 ppm), and the electrical characteristics were averaged over 10 devices. In BGBC OFETs (Figure 3a), drain current (ID) increased with increasing drain voltage (VD) for low gate voltages |VG|, which is a typical phenomenon in ambipolar transistors (Figure 3b).34 At negative VG, the OFETs operated in hole-enhancement mode with an average μFET of 0.20 cm2/(V·s) at VG = −100 V. Average electron μFET at electron-enhancement mode was 0.26 cm2/(V·s) at VG = 100 V. Average threshold voltage (Vth) and subthreshold swing (SS) values were −63.4 and 17.1 V/decade (for holes), and 63.4 and 17.4 V/decade (for electrons), respectively. These electrical characteristics are summarized in Table 1. It should be noted that high and well-balanced hole and electron μFETs were obtained in the BGBC geometry with Au source/drain electrodes. Even though there have been a few reports on the ambipolar transport of pentacene OFETs, they were obtained with only BGTC geometry and low work function source/drain electrodes (e.g., Ca and Al) which are vulnerable to oxidation.35−39 BGBC geometry has a strong advantage compared to BGTC geometry in that the semiconducting layers are deposited in the final step because they are sensitive to additional detrimental processing steps. However, it was not favored in pentacene OFETs due to the significantly low μFETs caused by morphological variation at an electrode/channel interface; pentacene formed a “lying-down” orientation on commonly used Au electrodes, whereas it formed “standing-up” orientation on the channel.21,40,41 In this study, pentacene formed the “lying-down” orientation on both the electrodes and the channel by using Au electrodes and hBN layer. The high and well-balanced hole/electron μFETs and low off current at low |VG| implied that the ambipolar OFETs could show an ideal switching function with low power consumption at off-state when utilized in complementary E

DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Application of the ambipolar OFETs. (a) Schematic illustration of a complementary inverter comprised of two identical pentacene OFETs. The inset shows its polarized OM image. (b) Typical voltage-transfer characteristics, and (c) corresponding gains of the inverter.

4. CONCLUSIONS In summary, we have demonstrated anomalous ambipolar charge transport of “lying-down” pentacene crystals on h-BN. van der Waals interaction between pentacene molecules and hBN substrate induced the molecules to have a “lying-down” and lattice-strained crystal structure at the interface. As the thickness of pentacene molecules increased, they gradually had a “standing-up” orientation due to weak van der Waals interaction. BGBC OFETs with the “lying-down” pentacene showed ambipolar charge transport with balanced μFETs of 0.20 (hole) and 0.26 cm2/(V·s) (electron), respectively. In BGTC OFETs, however, electron μFET dropped significantly to 0.007 cm2/(V·s). UPS measurement revealed that it was due to molecular orientation-dependent EHOMO and ELUMO values determining the charge injection barrier from Au to pentacene. Complementary logic circuits composed of two BGBC OFETs showed clear inverting functionality with a high voltage gain above 10. The results shown in this study are promising to broaden applications of organic crystals through the combination of 2D van der Waals substrates.

EHOMO and band gap of pentacene crystals, it is expected that ELUMO is also deeper in “lying-down” crystals. Deep ELUMO could lower the electron injection barrier from Au to pentacene because work function of Au is around 5.0 eV (Figure S1a,b in the Supporting Information, SI). Therefore, it could be concluded that ambipolar transport of “lying-down” pentacene OFETs with Au electrodes resulted from reduced electron injection barrier from Au to “lying-down” pentacene crystals. From morphological analysis and UPS measurement on “lying-down” pentacene crystals on h-BN, it is possible to explain the different charge transport behaviors depending on OFET geometries in Figure 3. In UPS measurement, EHOMO increased with increasing T and became close to that of conventional “standing-up” crystals on SiO2 (Figure 4). It is related to the changes in molecular orientations confirmed in AFM and GIWAXS experiments; pentacene orientation gradually transformed from “lying-down” to “standing-up” with increasing T (Figure 2a,b). Therefore, with 50 nm pentacene crystals on h-BN, in the BGTC OFET geometry, electron injection was severely restricted when the electrons were injected from top-contact Au electrodes into the channel due to the shallow ELUMO of “standing-up” pentacene crystals on upper part (Figure S1c,d). By contrast, in the BGBC OFET geometry, electron injection was easily allowed because bottom-contact Au electrodes were contacted with the h-BN/ pentacene interface and electrons were directly injected to the “lying-down” pentacene channel with deep ELUMO (Figure S1e,f). 3.4. Expended Applications: Complementary Inverters. The unprecedented ambipolar charge transport properties of pentacene OFETs enabled the fabrication of complementary inverters using two identical ones (Figure 5a), and their voltage transfer characteristics were measured with the DC drain supply voltages (VDDs) from 100 to 140 V. Z-shaped voltage transfer characteristics were observed due to the connected channels (Figure 5b), where the leakage current hindered each OFETs to be completely switched-off.47 However, by virtue of the promising initial OFET characteristics, the inverter showed clear inverting functionality with a maximum voltage gain of 15 for all VDDs (Figure 5c). Especially, the switching voltages of the inverter are almost close to VDD/2, indicating well-balanced electrical characteristics between two OFETs (i.e., wellbalanced hole/electron charge transporting properties in ambipolar OFETs).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b05129. Energy band structures and charge transport mechanism depending on OFET geometries are provided in this section (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.H.K.). *E-mail: [email protected] (C.E.P.). *E-mail: [email protected] (J.K.K.). ORCID

Chang Woo Myung: 0000-0002-9480-6982 Jeong Won Kim: 0000-0002-5881-9911 Kwang S. Kim: 0000-0002-6929-5359 Hyeon Suk Shin: 0000-0003-0495-7443 Geunsik Lee: 0000-0002-2477-9990 Se Hyun Kim: 0000-0001-7818-1903 Chan Eon Park: 0000-0002-3100-0623 Jin Kon Kim: 0000-0002-3872-2004 Author Contributions ∇

F

These authors contributed equally to this work. DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B. P., K. K., and J. P. contributed equally to this work. This work was supported by National Creative Research Initiative Program (No. 2013R1A3A2042196) supported by the National Research Foundation of Korea (NRF) funded by Korean government (MEST). This work was also supported by grants from the Basic Science Research Program through NRF and Realization of Quantum Metrology Triangle, funded by MSIP (2014R1A2A1A05004993) and Korea research Institute of Standards and Science (KRISS-2017-GP2017-0034), respectively.

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DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.7b05129 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX