High-performance amorphous organic semiconductor-based vertical field-effect transistors and light-emitting transistors†‡

Haikuo Gao ^ab, Jinyu Liu ^ab, Zhengsheng Qin ^ab, Tianyu Wang ^a, Can Gao ^a, Huanli Dong *^ab and Wenping Hu ^c
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: dhl522@iccas.ac.cn
^bUniversity of Chinese Academy of Sciences, Beijing 100049, China
^cDepartment of Chemistry, School of Science, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, Tianjin 300072, China

First published on 6th August 2020

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Introduction

Organic field-effect transistors (OFETs) have gained extensive attention from both academia and industry due to their attractive advantages of light-weight, low-cost, easy fabrication and good flexibility, which demonstrate their great potential with application in various fields.^1–3 Over the past decades, significant advances have been achieved in OFETs and their related functional devices.^4–19 In comparison with conventional planar OFETs, vertical OFETs (VOFETs) have an intrinsic short conducting channel without special high precision lithography, which could easily afford high current density, low operation voltage and high working frequency.^20–26 These unique characteristics of VOFETs make them highly promising for organic integrated optoelectronic devices, such as vertical organic light-emitting transistors (VOLETs). As possibly minimized integrated optoelectronic circuits embodying the functionalities of OFETs and organic light-emitting diodes (OLEDs) in a device, OLETs demonstrate great potential for the next-generation display technology with features of minimization, high integration density and simplified device fabrication process.^27–29 Due to the great efforts of scientists from the fields of chemistry, materials sciences and device physics, intriguing advances have been achieved in this field.^30–42 For instance, based on vertical OFETs, in 2011, McCarthy et al. developed a kind of carbon nanotube enabled vertical OLETs incorporating dinaphtho-[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT) as the channel layer and three phosphorescent materials as the emitting layer, which demonstrated a very low parasitic power dissipation of 6.2% and high aperture ratio of up to 98% in three primary RGB colors.³² More recently, by stacking an Ag nano-wire electrode based VOFET and a quantum-dot light-emitting diode (QLED) vertically, a novel vertical quantum-dot light-emitting transistor (VQLET) has been successfully fabricated with a high current efficiency of 37 cd A⁻¹.⁴¹

Despite these attractive advances, in general, the performances of most reported vertical devices are still poor, especially with a relatively low on/off ratio and complex device fabrication processes are usually required.^35,43,44 The main challenges existing in this field are as follows. In a typical vertical device, the different parts are vertically stacked together with the deposition of the top electrode to complete the whole device. One of the construction difficulties in these devices is the inevitable penetration of the top electrode, especially the penetration of metal atoms via a thermal evaporation process onto the active layer, resulting in large current leakage and poor device performances.⁴⁵ Another difficulty is to improve the film-forming characteristics with very smooth and pin hole-free film surfaces for each part vertically stacked together for good device stability and reproducibility. These cases usually occur in thin-film devices especially those incorporating a crystalline organic semiconductor as the active layer because they are prone to form crystalline films with large surface roughness and grain boundaries, which may promote the penetration of metal atoms and increase the complexity of device fabrication techniques. Although increasing the film thickness or using composite films could alleviate this problem to some extent, it is usually at the expense of decreased current density, increased operation voltage and complicated fabrication process.^32,46 More recently, a successful demonstration of organic single crystals with good surface and intact molecular packing in vertical devices has been reported.⁴⁷ However, the limited size and highly anisotropic transport in crystals prevent their further applications. In comparison, amorphous materials have good film-forming ability with a smooth surface, homogeneous structure, no grain boundaries, isotropic charge transport and less dependence on different experimental conditions, which is very beneficial for easy fabrication with good device stability and reproducibility and also it is very important for potential actual device applications and useful for fundamental studies of the device working mechanism.^48–51 In addition, due to the unique architecture of VOFETs and VOLETs with an inherent short conducting channel, high performances with high current density and good gate modulation via appropriate device engineering in these devices are relatively easy to achieve even with low-mobility amorphous organic semiconductors as the active layer. However, to the best of our knowledge, very few studies regarding amorphous organic semiconductor based-vertical devices have been reported in the literature,^43,44 and the current performances are still very low.

Aiming at the above-mentioned problems existing in this field and considerations, herein we construct two kinds of amorphous organic semiconductor-based vertical devices, VOFETs and VOLETs, incorporating N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as the hole injection and transport layer, tris(8-hydroxy-quinolinato) aluminum (Alq₃) as the emitting layer and graphene as the source electrode. By modulating the hole injection barrier at the graphene–NPB junction, high device performances were achieved with a large on/off ratio of 6 × 10³, good gate modulation capacity, a fast device response of 5 ms at 20 Hz operating frequency and a high electroluminescence brightness of 126 cd m⁻², which are among the highest values previously reported for amorphous organic semiconductor-based optoelectronic devices. Moreover, due to the good film-forming characteristics of amorphous materials, both these devices demonstrate good stability and repeatability. These results suggest the potential of amorphous organic semiconductors in vertical devices for optoelectronic circuit applications.

Experimental section

Materials

Graphene on a Si-SiO₂ substrate was purchased from Hefei VIGON Material Technology Co., Ltd, China and was prepared by the traditional chemical vapor deposition (CVD) method and transferred onto the substrate. NPB and Alq₃ were purchased from Luminescence Technology Corp. All metals were purchased from Dongwan DINGXIN New Materials Corp. and used directly.

Preparation of the graphene source electrode patterns for VOFETs and VOLETs

The graphene-source electrode patterns were prepared through the following processes (Fig. S1‡): (i) 50 nm aluminum was deposited on a graphene film via a shadow mask in a vacuum chamber under a pressure of 8 × 10⁻⁴ Pa; (ii) then, the graphene film without Al capping was removed by exposing the samples to O₂ plasma at a power of 100 W for 5 minutes; (iii) finally, aluminum was etched away using dilute nitric acid (5% volume ratio) to finish the preparation of graphene-source electrodes. The characteristics of the resulting graphene electrodes were investigated by atomic force microscopy (AFM) and Raman spectroscopy (Fig. S2‡) which demonstrate the smooth surface and absence of defects in the constructed graphene electrodes. Before device fabrication, graphene electrodes were transferred into a quartz tube and heated to 240 °C, and kept for 2 hours in a hydrogen environment for the reduction process. The following experimental results demonstrate that this process is very essential for achieving high device performances. Finally, a 20 nm gold layer was deposited onto each side of the graphene electrode with a shadow mask, which is used for the following probe-contact during measurements.

Construction of VOFETs and VOLETs

As for the construction of VOFETs, an active layer of NPB semiconductor was thermally evaporated onto the prefabricated graphene source electrodes at a speed of 2 Å s⁻¹ under a pressure of 6 × 10⁻⁴ Pa in a glove box with the concentration of H₂O and O₂ below 0.1 ppm. A series of NPB films with contrast thicknesses of 300 nm, 450 nm and 600 nm were prepared for comparative studies. Finally, by thermally depositing a 20 nm gold layer on the NPB layer as the top drain electrode, the fabrication of NPB-based vertical OFETs was completed.

A similar process was adopted for fabricating vertical OLETs. After the deposition of an NPB layer, a layer of Alq₃ was then thermally evaporated on top as the emitting layer at an evaporation rate of 2 Å s⁻¹. Finally, the electron injection electrodes of 1 nm LiF, 2 nm Al and 47 nm Ag were subsequently deposited on the top.

Device performance measurement

The electrical and optical characteristics were measured using a semiconductor parameter analyzer Keithley 4200 coupled with a Photomultiplier tube PMT H10721-20. AFM images were obtained using a Digital Instruments Nanoscope III atomic force microscope in air. The electroluminescence spectra of VOLETs were obtained using a cryogenically cooled PyLoN: 400. All the above measurements were carried out in a glove box.

Results and discussion

Fig. 1a shows the schematic of the constructed NBP-based VOFET and NPB-Alq₃-based VOLET. It should be noted that unlike conventional planar devices, the source electrode of VOFETs and VOLETs is sandwiched between the insulator and the active layer, which directly determines the effective penetration of the gate electric field and the modulation ability for charge transport. Thus, the selection and preparation of source electrodes possessing features of being ultrathin and porous with an appropriate energy barrier compared to that of active layers is crucial.²⁴ Among the various source electrodes currently developed for VOFETs and VOLETs, graphene was selected due to its intrinsic small thickness and good capacity for the penetration of the gate electric field and the easily tunable Fermi levels to modulate the energy barriers for gate modulation.^35,47,52Fig. 1b shows the energy levels of graphene, NPB, Alq₃ and Li/Al/Ag in NPB-Alq₃-based VOLETs. In principle, there is a high enough Schottky barrier (SB) value from the Fermi level of the graphene source (−4.5 eV) to NPB (HOMO: −5.4 eV) to enable good gate modulation capacity. However, in the experiment, it was found that based on the initially obtained graphene as source electrodes, the device demonstrated poor performances with a very low on/off ratio. The main reason is possibly due to the natural oxidation of graphene with a certain p-doping characteristic during the CVD-process and the subsequent transfer process.⁵³ The oxidation of graphene leads to a smaller Fermi surface for graphene and decreased injection barrier, resulting in an increased off-state current and poor gate modulation ability in device operation. Thus, a necessary reduction process for graphene was adopted under a H₂ atmosphere before it was used. Fig. 1c shows a typical image of the constructed VOLET in which the graphene electrodes and top electrodes are all bar-shaped and perpendicular to each other. The active area is 100 μm × 100 μm, which is highlighted with a red-dotted line. Fig. 1d and e show the typical AFM images of the thermally evaporated NPB single layer film and bilayer films of NPB-Alq₃, demonstrating smooth surface and high homogeneity. Because of the simplified device structure and good film-forming characteristics of the amorphous compounds incorporated, these two vertical devices can be easily fabricated with good repeatability.

Fig. 2a and b show the typical transfer and output characteristics of NPB-based VOFETs. Obviously, well-defined on- and off-states with good gate modulation capacity are observed under the gate voltage (V_GS) sweeping from 40 to −60 V and the source–drain voltage (V_DS) varying from −5 V to −25 V. The on/off ratio was up to 6 × 10³ with a low leakage current value of ∼10⁻¹¹ A in the whole operation window (Fig. S3‡). The working mechanism of this VOFET can be interpreted by the tunable Fermi level of graphene due to its low density of states (DOS). The main processes are as follows. (i) When a negative voltage is applied to the gate electrode (Si substrate), negative charges accumulate on the Si substrate, whose density depends on the value of the capacitor formed by the Si/SiO₂/graphene structure. Meanwhile, equivalent positive charges also accumulate on the graphene source electrode, the quantity of which is directly proportional to the voltage between the source and gate electrodes. (ii) Due to the low DOS characteristic of graphene, the increased positive charges can easily tune the Fermi level of graphene to a lower position, which further reduces the SB value between graphene and the adjacent organic layer. (iii) Depending on both the SB and V_DS values, the mechanism of hole-injection from graphene to the organic layer shifts between the tunnelling process and the thermionic process. This can be analyzed using the following relationship obtained from the Fowler–Nordheim (F–N) tunnelling model:^35,54,55

where d_DS is the distance between the drain and source electrodes, m* represents the effective mass of carriers, and Φ_B represents the SB value.

A pure tunnelling process strictly obeys the above equation and the observed k should be constant at a fixed value of Φ_B.⁵⁶ As shown in the F–N plot of VOFETs in Fig. 2c, the plot truly shows a linear relationship when |V_DS| is large enough, which suggests that a tunnelling process occurs; when the |V_DS| decreases, the curve begins to bend and finally shows an opposite relationship when |V_DS| is smaller than the threshold voltage |V_T|, indicating that the injection has changed from tunnelling to thermionic process. Fig. 2d shows the local enlarged image of Fig. 2c in the 1/V_DS range of −0.05 to −0.04 V⁻¹, where the tunnelling process occurs. It is obvious that the slope k decreases when V_GS becomes more negative, indicating a smaller SB value. The most dramatic change occurs when V_GS shifts from 20 V to 0 V, which is consistent with the transfer curve at large V_DS as shown in Fig. 2a. Another phenomenon that can be observed from Fig. 2c is the variation of V_T for different V_GS values. To be specific, a more positive V_GS leads to a larger |V_T|. This is because as V_GS increases, SB increases too, and a larger V_DS is needed to form a small enough width for the triangle barrier, which is essential for the tunnelling process (Fig. 2e).

Fig. 2f shows the correlation of on/off ratio versus current density with different thickness of NPB films. Due to the weak influence of amorphous organic semiconductor films on different conditions, they have been widely used as good candidates for investigating the influence of thickness on electronic device performances and other parameters through independent ways.^49–51 Clearly, a consistent change trend of on/off ratio versus current density is observed for the studied films in the thickness range of 300 to 600 nm that significantly increases at low current density, reaches the highest on/off ratio values at a current density of around ∼10 mA cm⁻², and then demonstrates a decreasing trend with further increase of the current density. Such a change trend can be explained as follows. (i) At lower current density far below ∼10 mA cm⁻², a lower |V_DS| is applied, generating a relatively planar HOMO level for the adjacent organic layer. In this situation, the thermionic process plays a major role, resulting in a relatively low on/off ratio. (ii) When |V_DS| increases, a triangle barrier begins to form, as shown in Fig. 2c. Along with the shift of the Fermi level of graphene, both the height and width of the triangle barrier change accordingly, leading to the co-existence of thermionic and tunnelling processes, which together contribute to the enhanced on/off ratio. (iii) While at higher current density values over ∼10 mA cm⁻², the large source–drain voltages overcome the major fraction of the SB.⁴³ The finite SB value becomes difficult to affect the current generated by large |V_DS|, thus leading to a decreased on/off ratio. In addition, at a fixed on-state current density, the on/off ratio increases with an increase in the film thickness due to the fact that devices with higher thickness have lower leakage current at higher on/off ratios. Of course, there is a balance for the increase of film thickness. Too thick films may induce a decrease of the on-state current and also be unfavorable for good device performances. In this study, we found that in the thickness range of 300 to 600 nm, high on/off ratios of over 10³ can be achieved for all the devices, which is 1–2 orders of magnitude higher than the previously reported values for amorphous-vertical devices^43,44 and important for display applications. Benefiting from the usage of amorphous materials, these devices exhibit excellent stability, which is indicated by a continuous switching operation test between the on- and off-state over a large time span of 1000 s. Obviously, the loop test demonstrates a very stable on-state current value except for a slight decrease in the beginning, which is probably caused by the increased resistance induced by the heat effect during device operation (Fig. 3a and b), which could be relieved to some extent at high working frequencies (Fig. 3c and d). The off-state value is maintained at a stable and low value of ∼0.01 mA cm⁻² during the whole measurement process, resulting in an on/off ratio of ∼10³. A fast rise/fall-time both having a small value of ∼5 ms was also demonstrated at an operation frequency of 20 Hz, which is a basic requirement for display applications (Fig. 3c and d). It should be stated that a flat part was always observed during the fall time probably due to the existence of parasitic capacitance and a resistor formed by the drain and source electrodes and the active layer(s) sandwiched between them to form a RC oscillating circuit.

The construction of high-performance NPB-based VOFETs further enables their applications in VOLETs, where an emitting layer of Alq₃ was introduced on top of NPB and the top drain electrode was replaced by a semi-transparent metal electrode of 1 nm LiF/2 nm Al/47 nm Ag for electron injection and light emission (a typical image of such a device is shown in Fig. 1d). Fig. 4a and b show the luminance transfer and output characteristics of a typical NPB (300 nm)–Alq₃ (300 nm)-based VOLET, respectively with V_GS sweeping from 40 to −60 V and a fixed constant V_DS at −15, −20 and −25 V in transfer curves as well as a V_DS sweeping from 0 to −25 V and a fixed V_GS in the range from 40 to −60 at a step of 20 V in the output curves. With the increase of applied voltages, the luminance is significantly enhanced with the highest value approaching 126 cd m⁻². The working mechanism for VOLETs can be explained as follows. When the gate electrode is applied with a positive voltage (V_GS > 0), the holes can hardly be injected from graphene to NPB due to the high SB value, making the device change to the off-state, so that light emission can hardly be observed; when a negative voltage is applied to the gate electrode (V_GS < 0), the holes can overcome the low SB easily, and combine with electrons in the organic layer, which eventually leads to light emission. Fig. 4c shows the CCD image of a VOLET working in the on-state with V_DS = −20 V and V_GS = −60 V. Apparently, strong and uniform surface emission can be observed over the whole active area of the device. Fig. 4d shows the electroluminescence spectra of a NPB-Alq₃-based VOLET for different V_GS values, indicating the consistent emission spectra with the main emission in the range of 500 to 600 nm and a color coordinate of (0.48, 0.47) in the CIE 1931 system. Interestingly, compared to the electroluminescence spectra of VOFETs based on unreduced graphene electrodes (Fig. S4‡), a long-wavelength electroluminescence emission is observed, which may be due to the shift of the emission position toward the NBP–Alq₃ interface induced by enlarged SB barriers at the reduced-graphene–NBP junction, which suggests the possibility of constructing colorful OLETs by well-controlling the injection barriers at the junction. Further investigations are underway in the lab. Similar to NPB-based VOFETs, NPB-Alq₃-based VOLETs also demonstrate good operational stability under continuous working conditions thanks to the good film formation characteristics and interface contact of amorphous organic semiconductors in the device (Fig. 4e). Moreover, the device performances of reduced graphene-based VOLETs are significantly improved compared to that of devices based on graphene without reduction, suggesting the importance of the source electrode with appropriate energy alignment in vertical devices (Fig. S5‡). When compared to poly-crystalline material-based devices,^46,57,58 amorphous organic semiconductor-based devices are easy to fabricate with good device-to-device consistency and stability, however, the current external quantum efficiency (EQE) values for these devices are still very low (<0.02%). The reasons are mainly due to (i) the inefficient charge transport in the amorphous semiconductor used, (ii) low light outcoupling efficiency induced by the semitransparent top electrode (with an efficiency of only 6%), and (iii) low exciton utilization of the fluorescent active layer (25%). It is believed that much higher device performances could be expected by using higher performance organic semiconductors, improving the exciton utilization and optimizing the device structures with more efficient light outcoupling.

Conclusions

In summary, we present two kinds of high-performance amorphous organic semiconductor-based vertical devices, NBP-based VOFETs and NBP-Alq₃-based VOLETs. Due to the intrinsically short conducting channel in such vertical devices, high current density could be easily achieved even with a low-mobility amorphous organic semiconductor as the active layer. By rationally modulating the injection barriers at the interface junction, significantly improved performance could be achieved with enhanced gate modulation ability as indicated by the large on/off ratio of over 10³. More importantly, the good film-forming characteristics of amorphous organic semiconductors simplify the device fabrication process and the resulting devices demonstrate good stability and reproducibility with fast response. These results suggest the great potential of amorphous organic semiconductors in vertical optoelectronic devices and circuit applications and will also stimulate the development of more versatile and high performance amorphous organic semiconductors for much higher-performance devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge financial support from the Ministry of Science and Technology of China (2017YFA0204503, 2018YFA0703200, and 2016YFB0401100), the National Natural Science Foundation of China (61890943, 91833306, 51725304, 51733004 and 21661132006), Beijing National Laboratory for Molecular Sciences (BNLMS-CXXM-202012), the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the National Program for Support of Top-notch Young Professionals.

Notes and references

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Footnotes

† Dedicated to Celebrate 60 years of the Fujian Institute of Research on the Structure of Matter.

‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nr03569f

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