Two-dimensional Organic Materials and Their Electronic Applications

Abstract

Soluble organic semiconductors have been widely researched for a wide range of application scenarios, such as organic field-effect transistors (OFETs), organic light-emitting diodes and solar cells. Due to their unique characteristics and great potentials as a platform for exploiting charge transport, two-dimensional (2D) organic crystals are emerging to investigate the device physics in OFETs, which is essential for functional device applications. Here, 2D organic crystalline films are briefly introduced from the deposition to the applications in electronic devices.

Semiconductor/dielectric interface, Two-dimensional organic films, Organic field-effect transistor (OFET)

1. Introduction

In recent years, intensive research has emerged in the field of two-dimensional (2D) materials, such as graphene,^1–3 MoS₂^4–6 and boron-nitride,^7,8 owing to their unique physical properties and diverse potential applications. However, the applications of these materials are often limited by specific substrates, transfer techniques and their crystal sizes. Their organic counterparts, which are assembled by weak van der Waals (vdW) forces between conjugated molecules and inherently could be used in low-cost and flexible electronics, have become a new class of materials to directly investigate the charge transport at the semiconductor/dielectric interface.^9–19 Besides, 2D organic materials, consisting of crystalline thin films with several monolayers, have attracted increasing attention due to unique characteristics of organic materials.^20–22 Thus, 2D organic materials are promising candidates for probing physics and realization of novel devices.^23,24

The semiconductor/dielectric interface plays a crucial role in the device physics of OFETs,^25,26 which affects the charge carrier transport in the semiconducting layer and the device stability. For organic semiconducting films, charge carriers mainly transport at the ultrathin active layer (a thickness of ∼10 nm) near the semiconductor/dielectric interface in OFETs. 2D architectures mainly compose of molecular monolayer or a few monolayers (bilayers and trilayers) limited to the spatial dimension and ultrathin polymer films (<10 nm). The applications of 2D organic crystalline films in electronic devices mainly include the following three aspects: i) 2D semiconducting films can act as the active layer in the OFETs, due to its highly ordered molecular packing and low contact resistance;^27,28 ii) 2D organic crystalline films serve as templates for the subsequent film growth, due to the long-range ordered molecular packing and lattice match with upper layers.^29–31 iii) The ultrathin crystalline films can modify the dielectric surface, such as reducing the interfacial trap states, the broadening of density of states, the carrier localization and the polarization due to the dielectrics, thus facilitating the charge transport.^32–35 Therefore, 2D organic crystalline films have shown great potentials in the applications for novel electronics and optoelectronics.^36,37 Here, we review our group’s work on 2D crystalline films, including the solution-processed deposition, the effect of 2D films on film growth and charge transport, and applications of 2D films in OFETs and transistor memories.

2. Deposition of 2D Organic Thin Films

2D organic crystals have been intensively studied in recent years to explore the charge transport and structure-property relationships at the 2D limit. Long-range ordered and defect-free 2D crystals present as potential platforms for high-performance devices and integrated circuits. Solution-processed depositions of 2D organic molecular films have also been reported, such as spin-coating,^9,10 dip-coating^11–13 and drop-casting.^14,15 However, the obtained 2D films are often composed of polycrystalline phases with small crystal sizes, which does not facilitate the charge transport in these semiconducting layers. Therefore, many innovative techniques have been proposed and developed to realize high-quality 2D crystals with large area and high crystallinity.

Wang et al. demonstrated the deposition of large-area 2D molecular crystals with precise layer control via the floating-coffee-ring-driven assembly (Figure 1a).³⁸ Floating-coffee-ring-driven assembly is an air-assisted crystallization method with a high assembly rate.³⁹ Based on the energy attachment model for crystal growth, the molecular crystal growth can be considered as a repeating process that slices molecules with a thickness d attached onto the crystal face.^40,41 The growth rate R under a temperature T is expressed as:

$$\begin{equation*}R \propto \text{d}f(\alpha)\exp(- \alpha)\end{equation*}$$

where α is the Jackson factor, which determines a smaller value for the 2D crystal growth than that of the bulk crystal. It indicates that a high growth rate is intrinsically necessary to deposit uniform 2D crystalline films. At the solvent/antisolvent (anisole/p-anisaldehyde) interface formed spontaneously in the droplet, the C₈-BTBT molecules can migrate sufficiently to the droplet edge to form 2D crystalline films, due to the reduced liquid thickness and the high evaporation rate.

Figure 1.

(a) Schematic of the growth of 2D molecular crystalline semiconductors by the floating-coffee-ring-driven assembly. The good solvent “floats” on the antisolvent layer near the solution edge, where the semiconducting molecules are assembled and crystallized into large-area 2D films. The ellipsoids represent the C₈-BTBT molecules. The inset shows the molecular structure of a 2D crystalline C₈-BTBT, which typically has a bilayer structure. (b) Optical microscopy image of a 2D crystalline C₈-BTBT with a step-and-terrace structure on the SiO₂/Si substrate. Monolayer, bilayer, and trilayer are observed. It indicates that the C₈-BTBT molecules are assembled via a layer-by-layer manner. (c) AFM morphology images of bilayer C₈-BTBT. (d), High-resolution AFM images of bilayer C₈-BTBT. (e) Transfer characteristics at a drain voltage of −20 V. The inset shows the optical microscopy image of the measured device. The bilayer C₈-BTBT bridging the source and drain electrodes is outlined by the white dotted line, and the scale bar is 50 µm.³⁸ Copyright 2016, Wiley-VCH.

Different molecular layers could be observed in the obtained films with a size of 220 µm (Figure 1b). Based on the atomic force microscopy (AFM) characterizations (Figure 1c), the second and the third layers exhibited high uniformity with an atomic smoothness of ∼0.14 nm and a thickness of ∼3.0 nm, while the first monolayers consisted of microstructures, and the monolayer thickness was 2.3 nm. The high-resolution AFM (HRAFM) characterizations with corresponding fast Fourier transform (FFT) (Figure 1d) showed nearly identical patterns with an oblique molecule packing of the first layer and more vertically oriented packing of the herringbone type of the second and the third layers. These results indicated that both the bilayer and trilayer C₈-BTBT crystals consisted of a single-crystalline phase with coincident lattice constants.

Then, bottom-gate top-contact (BGTC) OFETs based on the achieved 2D C₈-BTBT crystals were fabricated and exhibited the average and the maximum carrier mobility of up to 5.2 cm² V⁻¹ s⁻¹ and 13.0 cm² V⁻¹ s⁻¹, which were among the highest reported values for few-layer molecular semiconductors (Figure 1e). Attributing to the ultrathin semiconducting channel with highly ordered molecular packing and ultrathin thickness, which could dramatically facilitate the charge transport and decrease the access resistance, the contact resistance estimated in this OFET was low to 400 Ω cm. The value in bulk OFETs is around 10⁴–10⁵ Ω cm.^42–44

Apart from the high device performance of OFETs, power consumption remains a main barrier hindering the development of OFETs. Besides, there are growing demands of wearable and portable electronic devices. Thus, it is essential to develop low-voltage and high-performance devices. One effective strategy is to employ high-k dielectrics and 2D semiconductors. Recently, AlO_x (ε = 9.0)^45–48 was employed as the dielectric in the OFETs by Wang et al.

2D C₈-BTBT films with high morphologic uniformity were achieved on the AlO_x substrates (Figure 2a).⁴⁹ The HRAFM image and the corresponding FFT patterns (Figure 2b) suggested that the 2D C₈-BTBT films consisted of highly ordered molecular packing and exhibited a single-crystalline feature. The electrical performance of OFETs based on 2D C₈-BTBT films and AlO_x was measured, yielding the average and the maximum carrier mobility of up to 4.9 cm² V⁻¹ s⁻¹ and 9.8 cm² V⁻¹ s⁻¹, which were the highest reported values for low-voltage OFETs (Figure 2c). Besides, the device was stored in ambient condition for up to one month, and no obvious degradation of device performance was observed (Figure 2d), indicating a good device stability. To understand the charge transport of the 2D C₈-BTBT-based OFETs, the activation energy (E_A) was calculated (Figure 2e). The obtained E_A of OFETs utilizing AlO_x presented the lowest value, suggesting a low energetic disorder and a narrow width for the density of trap states (DOS).^50–55

Figure 2.

(a) Optical microscopy image of the 2D C₈-BTBT crystalline films on the AlO_X/Si substrate. The bilayer and trilayer films are clearly observed. Scale bar, 20 µm. (b) High-resolution AFM image of the bilayer C₈-BTBT molecules. Scale bar, 1 nm. The inset shows the corresponding fast Fourier transforms of the AFM image with lattice indices. (c) Transfer characteristics at a drain voltage of −4 V. The inset shows the optical microscope image of the measured device. Scale bar, 20 µm. (d) Stability characteristics of the C₈-BTBT-based OFET. (e) Temperature dependence of the field-effect mobility of the C₈-BTBT-based OFETs using AlO_X, HfO₂ and SiO₂ as the dielectrics. Copyright 2014, Nature Publishing Group.

In summary, the obtained 2D C₈-BTBT films exhibited a single-crystalline feature, with atomic smoothness and high uniformity over a large area. This method could precisely control the thickness of the organic thin films within a large area, demonstrating great potentials for 2D solution-processed crystals for low-cost, large-area and high-performance electronic device applications. The strategy of air-assisted floating-coffee-ring-driven assembly should be further developed for the deposition of other organic semiconductors and the fabrication of more complicated structures, such as heterojunctions.

3. Templating Effect of the Monolayer

The charge transport at the semiconducting channels, especially at the first few monolayers near the dielectric, has been widely investigated.^56–58 Generally, many external factors, including gate insulator roughness, interfacial trap states and electrostatic interactions between carriers and dielectrics, would impede the charge transport. Thus, a high-quality insulator/semiconductor interface is very necessary for high-performance OFETs. One effective strategy is to insert a crystalline film at the insulator/semiconductor interface, serving as a template for the film growth in the “bottom-up” structure. Besides, this template could also reduce the interfacial trap states and weaken the polarization and localization effect of the dielectrics on charge carriers, which will be discussed in the following section.

Wang et al. demonstrated an antisolvent-assisted spin-coating method (Figure 3a) to fabricate large-area molecular monolayers with highly uniform morphology (Figure 3b).⁵⁹ The C₈-BTBT solution and the antisolvent N,N-dimethylformamide (DMF) were drop-cast onto the substrate sequentially and the C₈-BTBT molecules could migrate at this anisole/DMF interface, aggregating through strong π-π interactions, and forming ultrathin nanosheets.⁶⁰ As the spin-coating procedure went on, the nanosheets evolved into large-area monolayers with a size of centimeters. Based on the AFM and transmission electron microscope (TEM) characterizations, the C₈-BTBT monolayer exhibited atomic smoothness and highly ordered molecular packing. Thus, this molecular monolayer could serve as a high-quality insulator/semiconductor interface to improve charge transport.

Figure 3.

(a) Schematic diagram of the antisolvent-assisted spin-coating deposition of C₈-BTBT thin films. (b) Optical microscopy image of the C₈-BTBT monolayer and its corresponding SAED patterns. (c) Detail of the multilayered Si/SiO₂/C₈-BTBT model where C₈-BTBT molecule has been artificially layered in two portions, one consisting of thiophenes (BTBT, pale orange shadow) and aliphatic chains portion (C₈, blue shadow). And experimental XRR and its fitting based on samples using spin-coating at 4000 rpm. (d) Transfer characteristics of OFETs based on 30 nm thick C₈-BTBT films with monolayers or multilayers spin-coated at speeds of 4000 rpm. (e) OFET mobility along of the transistors based on C₈-BTBT films spin-coated at different speeds.⁵⁹ Copyright 2018, American Chemical Society.

The C₈-BTBT monolayer also exhibited an excellent templating effect as expected for the molecular packing of upper films. The evaporated C₈-BTBT films were characterized by X-ray reflectivity (XRR) and analyzed by the multilayered stack model (Figure 3c). The almost perfect fit between the experimental data and the theoretical model demonstrated a highly ordered molecular packing, suggesting the templating effect of single-crystalline monolayer for crystal growth. The electrical performance was measured using BGTC OFETs based on the obtained hybrid-deposited C₈-BTBT films (Figure 3d). The average and the maximum carrier mobility values are 9.2 cm² V⁻¹ s⁻¹ and 11.3 cm² V⁻¹ s⁻¹, which were nearly one-order of magnitude higher than that without this scalable template layer (Figure 3e).

4. Charge Transport at the Semiconductor/Dielectric Interface

Recently, fruitful progress in the understanding of the charge transport at the semiconductor/dielectric interface of OFET devices has been achieved. Charge transport in the active layer is limited by the hopping process due to carrier localization and dipole polarization in gate insulators, and occurs within a few Å-thick ultrathin layers adjacent to the dielectrics.^61,62 Richards et al. developed a quantitative model to calculate the broadening of DOS in the active layer, resulting from the interaction between the charge carriers and the randomly oriented dipoles.⁶³ This interaction would be weakened exponentially by inserting an ultrathin film. Thus, 2D organic crystalline films have potential for acting as the inserting layer to weaken the carrier localization and dipole polarization.

Wang et al. successfully deposited a flat-lying molecular monolayer between the semiconductor and the dielectric via slow thermal evaporation (Figure 4a).⁶⁴ The AFM and ultraviolet photoelectron spectroscopy (UPS) both verified the existence of this dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) interfacial monolayer with a thickness of ∼0.3 nm.⁶⁵ Besides, this 2D film also exhibited a templating effect on the upper films with highly ordered molecular packing (Figure 4b). As expected, the carrier mobility was markedly enhanced due to the existing flat-lying monolayer (Figure 4c), which was consistent with the result obtained from Richards et al. And the carrier mobility would decrease at high gate voltages since the charge carriers were then located closer to the semiconductor/dielectric interfaces and suffered from stronger disorder broadening (Figure 4d). Besides, this effect was particularly pronounced for high-k dielectrics, such as Al₂O₃.

Figure 4.

(a) Schematic illustration of the molecular packing of slow-evaporated DNTT films. (b) XRD patterns of slow-evaporated 20 nm-thick DNTT films. The peak at 5.39° corresponds to the (001) peak. (c) Transfer characteristics at a drain voltage of V_D = −100 V of OFETs based on the slow- (red) and fast (blue)-evaporated DNTT films. The channel width and length are 1000 and 100 µm. (d) Mobility variation (μ_FET/μ_{–100 V}) with the gate voltage of OFETs based on slow-evaporated DNTT films. The turning points are selected when ∂(μ_FET/μ_{–100 V})/∂V_G= 0. (e) Activation energy (E_A) for the carrier mobility in slow- and fast-evaporated DNTT transistors as a function of the gate voltage (V_G). The lines are the fitting. And density of trap states [D(E)] obtained from the FET data using a trapping depth of 10 nm. The broadening of DOS in devices based on slow-evaporated DNTT films is much narrower than that based on fast-evaporated samples.⁶⁴ Copyright 2018, American Chemical Society.

To deeply understand the effect of this interfacial monolayer on the charge transport, temperature-dependent measurements were performed. The OFETs with a flat-lying monolayer exhibited a lower E_A in both analysis models. Furthermore, from the study of DOS, the closer DOS distribution to the HOMO level of slow-evaporated samples indicated that most trap states were located near the semiconductor/dielectric interface (Figure 4e). Thus, it was proved that the flat-lying DNTT monolayer could act as a buffering layer which effectively alleviated the polarization interaction induced by the dielectric and significantly reduce the energetic disorder at the semiconductor/dielectric interface.

5. Applications of 2D Organic Crystalline Films

By virtue of the templating effect and buffering effect of the interfacial layers at the semiconductor/dielectric interface, novel applications are further realized in transistor memory devices. By inserting an ultrathin film at the semiconductor/dielectric interface, the polarization fluctuation can be dramatically suppressed, which is beneficial to the enhancement of the device performance, such as Fe-OFETs (Figure 5b). Traditional non-volatile memory devices are facing some problems, such as serious leakage currents at nanoscale and limitations of fast operation and retention. For Fe-OFET memory devices, especially the 2D molecular films-based devices, the device performance are markedly enhanced, such as the retention of the memory state by high-quality ferroelectric films, faster operation speed, lower static power dissipation, better interference immunity and higher radio-resistance.

Figure 5.

(a) Schematic illustration of a Fe-OFET with a bottom-gate top-contact structure. An ultrathin (PMMA) film acts as a buffering layer between the ferroelectric insulator of P(VDF-TrFE) and organic semiconductor layers, buffering the polarization fluctuation at the semiconductor/insulator interface. (b) (left) Illustrative representation of the polarization fluctuation at the semiconductor/insulator interface, affecting the charge carrier transport. (right) Such a polarization fluctuation can be well suppressed by depositing an ultrathin PMMA film between the ferroelectric and semiconductor layers, and the charge carrier transport is enhanced. (c) Typical transfer curves of the devices without (red line) and with (blue line) PMMA buffering layer. (d) presents the pulse responses of the Fe-OFETs with PMMA buffering.⁶⁵ Copyright 2014, Nature Publishing Group. (e) Typical transfer curve of the 2D C₈-BTBT-based Fe-OFET with PMMA. Inset shows the optical microscopy image of a fabricated Fe-OFET (scale bar is 400 µm). (f) Switching behavior between the binary states of the Fe-OFET memory from on- to off-state.⁶⁹ Copyright 2018, American Chemical Society.

Sun et al. demonstrated a novel interfacial buffering method, in which an ultrathin polymer of poly(methylmethacrylate) (PMMA) was deposited onto the poly(vinyledene fluoride-trifluoroethylene) [P(VDF-TrFE)] layer in Fe-OFETs (Figure 5a).^66,67 The electrical results of Fe-OFETs showed the average field-effect mobility of 3.4 cm² V⁻¹ s⁻¹ (Figure 5c), which was much higher than that of the devices without PMMA buffering layers (0.32 cm² V⁻¹ s⁻¹). Besides, the frequency response and the pulse response, representing the “reading” process and the “programming” process respectively, both showed much improvement (Figure 5d). Especially, in the device with this buffering layer, the short delay time of about 30 ms was close to the switching time between two polarization states of P(VDF-TrFE) under the same voltage bias.⁶⁸

Not only the charge accumulation and depletion at the semiconductor channel have a significant impact on the switching between binary states of Fe-OFETs, the contact resistance, indicating charge injection, is also a key factor for the operation speed of Fe-OFETs. 2D molecular crystal structures showed a great potential in the realization of high-speed memory devices. Song et al. proposed a strategy for the fabrication of high-speed Fe-OFETs by employing 2D C₈-BTBT crystalline films as the semiconducting layers instead of bulk films.⁶⁹ The Fe-OFETs showed a high mobility of 5.6 cm² V⁻¹ s⁻¹ (Figure 5e) and a quick switching time with only several milliseconds between the binary states (Figure 5f). The improvement of the operation speed of Fe-OFETs was mainly attributed to the application of 2D molecular crystals, which inherently possessed low access resistance. Moreover, since P(VDF-TrFE) was the most commonly used ferroelectric polymer for Fe-OFET memories, the results provided a promising strategy for future developments in the field of Fe-OFETs.

6. Summary and Outlook

In conclusion, since solution-processed organic semiconductors show potential for low-cost and high-throughput electronic device fabrication and applications, they have attracted numerous research interests. In this review, we presented the deposition of 2D organic crystalline films by our unique solution-processed methods and their applications in electronic devices. High-quality 2D molecular crystals with atomic uniformity and ordered molecular packing served as the ideal platform to study the charge transport in OFETs, while the carrier mobility obtained in these transistors exceeded 10 cm² V⁻¹ s⁻¹.

In order to deposit high-quality organic semiconducting films, the template-assisted method has been employed. Single-crystalline monolayer served as the template layer and benefited the molecular packing in the upper films. Besides, the template or the interfacial monolayer at the semiconductor/dielectric interface could also suppress the carrier localization and polarization effect resulting from the dielectrics. It could also modify the DOS broadening to promote the charge transport and overall device performance.

2D organic crystalline films not only demonstrate great potentials in the applications of high-performance OFETs and Fe-OFETs, but also provide an ideal platform to exploit the charge transport and device physics in electronic devices. 2D organic crystalline films will be further employed in novel functional device structures and in industry manufacturing.

Acknowledgment

This work was supported by NSFC under Grant Nos. 61774080 and 61574047; NSFJS under Grant No. BK20170075.

Qijing Wang is a post-doctor as part of Post-doctoral Innovation Program at School of Electronic Science and Engineering, Nanjing University. He received his Bachelor’s degree in Physics (2012) and PhD degree in Electronic Science and Technology (2018) from Nanjing University. His current research focuses on organic electronics including molecular organic semiconductors, device engineering, and circuit applications.

Yun Li is a professor in the School of Electronic Science and Engineering, Nanjing University. He has undergraduate and Ph.D. degrees in physics from Nanjing University. From 2010–2012, he worked as a postdoctoral researcher in National Institute for Materials Science (NIMS), Japan. His current research interests include solution-processed 2D organic materials for electronic applications and the charge-transport physics of molecular crystals. In 2018, he was awarded the “Distinguished Lectureship Award” by the Chemical Society of Japan.