Shengqian
Liu
,
Zongyu
Huang
*,
Hui
Qiao
,
Rong
Hu
,
Qian
Ma
,
Kai
Huang
,
Hongxing
Li
and
Xiang
Qi
*
Hunan Key Laboratory of Micro-Nano Energy Materials and Devices, Laboratory for Quantum Engineering and Micro-Nano Energy Technology, School of Physics and Optoelectronic, Xiangtan University, Hunan 411105, P. R. China. E-mail: zyhuang@xtu.edu.cn; xqi@xtu.edu.cn
First published on 4th January 2020
Flexible optoelectronic devices have been of great significance in recent years, owing to their extensive commercial and military applications. However, the manufacturing processes of most existing flexible photodetectors are particularly complicated and expensive. Employing a facile and low cost way for constructing a high performance flexible infrared photodetector is one of the effective strategies to facilitate its practical applications. Pencil-drawing is a popular method in novel electronic and optoelectronic devices, as it is a low cost and facile fabrication process. Herein, we report a novel flexible infrared photodetector using liquid-exfoliated Bi2Se3 nanosheets as a light sensitive material, pencil-drawn graphite as the electrodes, and paper as the substrate. The as-fabricated photodetector exhibits high photocurrent, excellent responsivity and long-term stability under 1064 nm infrared light irradiation. In addition, as the pencil-drawn photodetector is made of a flexible paper substrate, it also well exhibits stability and durability under bending conditions. This work is proposed to be a route to construct a novel flexible infrared photodetector with a facile manufacturing process and low cost.
Recently, two-dimensional (2D) materials have attracted widespread attention in advanced photodetectors due to their inherent atomic structure with excellent flexibility,9 superior photoelectric properties10–12 and high compatibility with flexible substrates. Among them, black phosphorus (BP) is a novel 2D material with an inherent direct bandgap,13 but it is well known that 2D BP is unstable when exposed to ambient conditions, which limits its practical applications.14 Transition metal dichalcogenides (TMDs) have extraordinary electrical and optical properties that evolve from quantum confinement and surface effects.15,16 TMDs can be used in infrared photodetectors by introducing appropriate defects, but the fabrication of devices is very expensive and complicated.17 Besides BP and TMDs, topological insulators (TIs) are considered as a novel class of 2D materials,18 revealing an exotic state of quantum matter with conductive surface states and insulating bulky states with an indirect band gap.19,20 In recent years, topological insulators have aroused the great interest of researchers, owing to their excellent bending ability, superior optoelectronic properties with a layer-dependent band-gap and polarization-sensitive photocurrent,21 enabling them to be widely applied in the new generation of optoelectronic devices.22,23 Bismuth selenide (Bi2Se3), which is one of the topological insulators, has attracted enormous attention from researchers because of its very exciting optoelectronic properties and 0.3 eV bandgap (can be tuned with the layer thickness) as well as high carrier mobility.24–26 Owing to these special properties, Bi2Se3 is a new promising star in the field of photodetection. For example, Wang et al. investigated 2D Bi2Se3 flakes, demonstrating a strong responsivity of 23.8 A W−1 and EQE of 2035% under 1465 nm laser irradiation.27 Zhang et al. fabricated a novel Bi2Se3/Si photodetector with superior responsivity at optical communication wavelength.28 According to the above reports, although great accomplishments in the field of optoelectronic devices have been achieved, it is particularly rigorous and complicated to manufacture Bi2Se3 photodetectors as they are expensive and lack mechanical flexibility, greatly restricting the practical applications of the device. Considering the commercial benefits and practical application issues, all components of the flexible photodetector should be optimized to address the requirements of malleability, portability, environment-friendliness, low-cost and durability. Writing is a state-of-the-art route and has been widely applied in the field of optoelectronics. Cao et al. fabricated a flexible photodetector with a high responsivity of 2.1 A W−1 using pencil-drawn electrodes on paper.29 Zhang et al. demonstrated that a humidity sensor based on pencil-tracing exhibited fast response speeds (1 s) and good long-term stability.30 Graphite is one kind of carbon allotrope, which is known to possess high carrier mobility and is commonly found in pencils. It can be used to easily draw on common paper with a slight force, which is the easiest way to construct graphite electrodes.31 In addition, for flexible devices, common paper is one of the most competitive substrates because of its extremely low-cost, eco-friendly nature and compatibility with other components of the devices. What's more, paper is pliable enough to support flexible devices.
In this work, Bi2Se3 nanosheets were obtained via a simple liquid exfoliation method.32 As-exfoliated materials are utilized as the light absorption layer of the flexible infrared photodetector due to their narrow band gap and high optical absorption coefficient. The novel flexible infrared photodetectors exhibited excellent flexibility, bendability and extendable durability under 1064 nm infrared illumination. It can be mass-produced on common paper using pencils and brushes. It is found that the photocurrent of the flexible IR photodetector can reach 0.82 μA under a low operating voltage of 5 V; the bending stability test demonstrates that the pencil-drawn device shows no significant photocurrent attenuation after 1000 bending cycles and retains 60% of the pristine photocurrent under various bending angles. In addition, the comparison of the Bi2Se3-based photodetector in this work with photodetectors of other reported related materials is shown in Table 1. Some of them have exhibited better performances in terms of their responsivity or response time. However, it is clear that the as-prepared pencil-drawn device based on our Bi2Se3 nanosheets not only involves an even simpler manufacturing process than the previously reported photodetectors, but also exhibits good photoresponse in the near-infrared region, and the pencil-drawn device has superior flexibility and durability owing to its unique features. Herein, we believe that this work has a positive reference value for the design and applications of future wearable infrared devices.
Device structure | Materials | Wavelength (nm) | Responsivity (μA W−1) | τ r/τf (s) | Flexible | Ref. |
---|---|---|---|---|---|---|
a Photoconductive type. b Photoelectrochemical type. c Phototransistors. | ||||||
ITO/Teb | Te nanosheets | 400 | 2.79 | — | No | 38 |
ITO/BPb | BP nanosheets | Simulated light | 2.2 | 0.5/1.1 | No | 13 |
ITO/Bib | Bi quantum dots | 365 | 285.7 | 0.2/0.2 | No | 39 |
Ag/Bi2Se3/Aga | Bi2S3 nanoflowers | 980 | — | 0.8/3 | Yes | 11 |
ITO/Bi2S3b | Bi2S3 nanosheets | Simulated light | 210 | 0.1/0.1 | No | 40 |
Au/Bi2Se3/Si/Auc | Bi2Se3/Si | 808 | 24.28 × 106 | 2.5/5.5 | No | 28 |
ITO/Bi2Se3b | Bi2Se3 nanosheets | 532 | 20.48 | 0.7/1.48 | No | 41 |
Graphite/Bi2Se3/graphitea | Bi2Se3 nanosheets | 1064 | 26.69 | — | Yes | This work |
Fig. 1 (a) Schematic diagram of the device of the fabrication procedure for direct writing using a pencil and Chinese brush on paper. (b) Schematic diagram of the flexible device structure. |
Fig. 3 demonstrates the optoelectronic performances of a photoconductive photodetector under 1064 nm laser illumination. It is obvious that the as-prepared device presents excellent photoresponse characteristics. The linear current–voltage (I–V) curves, as shown in Fig. 3(a), imply that there is good ohmic contact between 2D Bi2Se3 and the graphite electrode, which is considered to benefit the extraction of photogenerated charge carriers.29 The inset of Fig. 3(a) shows that the electrons of Bi2Se3 move from the valence band to the conduction band via the infrared light excitation. Subsequently, with the help of the applied electric field, the photogenerated electron–hole pairs are quickly separated and transported through graphite electrodes.37 Thus, the pencil-drawn device can work effectively. The photocurrent switching characteristics of the pencil-drawn photodetector were further evaluated. As shown in Fig. 3(b), the temporal photoresponse of the photodetector was recorded by manually keeping out the light source every 20 seconds at 5 V bias voltage, and it was found that the electrical conductivity of the photodetector was affected by the light illumination. Under light irradiation conditions, the photocurrent rises immediately, and drops sharply when it is sheltered. Consistently, it can be seen that the photocurrent is closely related to the light intensity and monotonously increases with the change of light intensity from 3.16 mW mm−2 to 10.65 mW mm−2, which results from the increase in the number of non-equilibrium carriers with the increase of light intensity. Meanwhile, the steady photocurrent is about 0.35 μA when the light intensity is 3.76 mW mm−2, demonstrating that our device possesses high infrared response characteristics. In addition, we examined the photocurrent of the pencil-drawn device by applying different bias voltages varied from 1 to 5 V as shown in Fig. 3(c). Notably, the photocurrent of the device is related to the applied bias, where the photocurrent reached by the device is about 0.15 μA at a bias of 1 V and gradually ascends to 0.82 μA when the bias increased to 5 V. These phenomena indicate that we can adjust the photoresponse of the device by finding an optimal bias voltage. It is well known that responsivity is a critical factor to evaluate the performance of photodetectors, which can be acquired through the following formula: R = I/Jlight, in which I is the photocurrent and Jlight is the irradiance intensity. The dependence of responsivity on irradiance power intensity is demonstrated in Fig. 3(d). Noticeably, the responsivity basically does not vary as the light intensity changes from 2 mW mm−2 to 10.65 mW mm−2 and the responsivity undergoes a very slight decrease owing to the recombination of electron–hole pairs when light intensity rises over 5.46 mW mm−2. The outstanding photocurrent density and high responsivity of Bi2Se3 nanosheets suggest a good prospect for their practical application in infrared photodetectors. Apart from the photoelectric responsivity, the stability of the photodetector is another critical factor that should be evaluated before its practical application. Herein, cycle stability measurement has been conducted and is shown in Fig. 3(e) to evaluate the robustness of the device. It is apparent that the photocurrent displays negligible decay after successive switching operation for 800 s, revealing that the pencil-drawn device is highly stable and can maintain good reproducibility.
Another merit of the device is its flexibility. As described above, all the components of the as-fabricated device (active material, electrode, and substrate) are flexible enough to support the mechanically foldable device. The photocurrents of the device with various bending curvatures under 1064 nm light irradiation are shown in Fig. 4(a). The insets showing real photographs of the device in Fig. 4(a) correspond to the device being bent at 60°, 100°, 130°, 140° and 160°, respectively, indicating that the device possesses high flexibility and conductivity. In addition, in order to more intuitively understand the relationship between the bending angle and the photocurrent of the device, fitting curves of the photocurrent of the flexible photodetector under different bending angles were obtained and are depicted in Fig. 4(b). Notably, the photocurrent gradually decreases as the bending angle of the device increases and returns to the pristine level when the device was restored. The slight photocurrent decrease during bending may be related to the less active area under the deflected conditions, because the radius of the light spot is only about 1 mm, the area of illumination changes under bending conditions, resulting in changes of the photocurrent. Moreover, the photocurrent of the device is recorded in Fig. 4(c) after multiple bending cycles. Encouragingly, the photocurrent barely changed after being bent for 0, 100, 200, 400 and 1000 cycles except for a little fluctuation noise, powerfully demonstrating that the device has excellent electrical stability and long-term flexibility. Fig. 4(d) shows the time-dependent photocurrent (I–t) curves of the pencil-drawn device after the 1st and 1000th bending–recovery cycle. The photocurrent of the device is about 42 nA after bending 1000 times and the pristine photocurrent before bending is 44 nA, suggesting that the device has good stability and fluctuations are less than 5%. Noticeably, our device still maintains excellent switching performance after bending for 1000 cycles. These results indicate that the pencil-drawn device has high flexibility, stability and durability in the bending state.
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