Shalong Wang,
Yousheng Zou*,
Qingsong Shan,
Jie Xue,
Yuhui Dong,
Yu Gu and
Jizhong Song*
Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced Display Materials and Devices, School of Materials Science and Engineering, Nanjing University of Science & Technology, Nanjing 210094, China. E-mail: songjizhong@njust.edu.cn; yshzou75@njust.edu.cn
First published on 1st October 2018
Wearable photodetectors (PDs) have attracted extensive attention from both scientific and industrial areas due to intrinsic detection abilities as well as promising applications in flexible, intelligent, and portable fields. However, most of the existing PDs have rigid planar or bulky structures which cannot fully meet the demands of these unique occasions. Here, we present a highly flexible, omnidirectional PD based on ZnO nanowire (NW) networks. ZnO NW network-based PDs exhibit the imageable level performance with an on/off ratio of about 104. Importantly, a ZnO NW network can be assembled onto wire-shaped substrates to construct omnidirectional PDs. As a result, the wire-shaped PDs have excellent flexibility, a large light on/off ratio larger than 103, and 360° no blind angle detecting. Besides, they exhibit extraordinary stability against bending and irradiation. These results demonstrate a novel strategy for building wire-shaped optoelectronic devices through a NW network structure, which is highly promising for future smart and wearable applications.
However, so far, the performances of wire-shaped PDs are still lower than their corresponding planar devices, because the existing wire-shaped PDs are mostly core–shell structures with an electrode inside the device and another electrode outside. Such a vertical structure will contribute to a high level of current in dark conditions, which more often than not cause low performance of the low on/off ratio. For example, Ko17 et al. fabricated wire-shaped ultraviolet PDs based on a nanostructured zinc oxide (ZnO)/nickel oxide (NiO) coaxial p–n heterojunction with a light on/off ratio of about 10. Chen20 et al. reported a UV wire-PD with ZnO nanowires (NWs) grown on an oxidized Ni wire, and the light on/off ratio was about 4. A self-powered fiber-shaped wearable omnidirectional ultraviolet (UV) PD with a light on/off ratio of 2 under zero bias was constructed by Dong and her co-authors.6 Therefore, it is desirable to design an all-purpose structure with excellent photodetecting performances for future wearable devices.
Various NW networks have been considered as a class of promising optoelectronic materials for flexible, stretchable, and wearable devices, owing to their superior elasticity for bending, stretching, twisting, and deformation into complex, non-planar shapes.21–24 In general, the strain applied by a mechanical stretching stress is accommodated by rotations and distortions of the NW network.25–27 Thus, unconventional nanostructures, which are generated by spatial displacement between NWs with a high aspect ratio, can maintain their continuous network structures, which act as an electrical charge path, guaranteeing excellent carrier transporting properties under various deformations. Thus, there is a high potential to develop wire-shaped PDs based on a NWs network for wearable devices.
Here, we present omnidirectional PDs based on a ZnO NW network, which is promising for future wearable and smart optoelectronic devices. First, high quality ZnO NWs with large aspect ratios of >100 are readily dispersed in alcoholic solvents, e.g., isopropanol, serving as ink. The ink can be used to process highly uniform NWs network films for various optoelectronic devices. NW network-based PDs exhibit an imageable level performance with an on/off ratio of about 104. Importantly, a ZnO NW network can be transferred onto wire-shaped substrates to construct omnidirectional PDs. As a result, the wire-shaped PDs have excellent flexibility (>500 cycles), large light on/off ratio of 103, and 360° no blind angle detecting. These results demonstrate a novel strategy to build the wire-shaped optoelectronic devices suitable for future smart and wearable applications.
KEVLAR wire with diameter of 1 mm was dipped in PDMS gel and pre-cured at 60 °C for 1 h. The PDMS (Sylgard 184, Dow Corning) gel was prepared from a mixture of a base and a curer (weight ratio 10:1). The KEVLAR wire was rotated on a PVDF filter membrane with the ZnO NWs. The sample was further twisted with CNT fiber wire to construct the PDs and then the devices were packaged with a thin layer of PDMS for testing.
As shown in Fig. 1a, upon sonication for 1 min, the ZnO NWs formed a dispersion in isopropanol. The NW inks were filtered with a polyvinylidene fluoride (PVDF) membrane to form a thin and uniform film, which then could be easily transferred onto various substrates. Fig. 2a shows the large-area ZnO NW network transferred onto glass substrates characterized by SEM images. The ZnO NWs lay on the substrate due to their unique 1D geometries, and can be assembled into dense, crack-free, and smooth films observed by the photograph (inset in Fig. 2a). The high magnification SEM image shown in Fig. 2b demonstrates that ZnO NWs are intertwined together to form a perfect network structure. As described in the introduction, such network structure, which is very favorable for adapting to the deformations under applied stress, could maintain high flexibility. Accordingly, the flexible ZnO NWs network transferred onto flexible polyethylene terephthalate (PET) and stretchable polydimethylsiloxane (PDMS) substrates are shown in Fig. 2c and d. Especially, after bending and recovering for about 500 times the NWs on the PDMS substrates still retained their integrity. This is ascribed to the NWs being partially embedded in PDMS polymer, which anchored the NW network and further preserved the integrity of junctions among the NWs.
The photoelectric response characteristics of a ZnO NW network are presented in Fig. 3. The device configuration of a ZnO NW network PD is schematically demonstrated in Fig. S2;† Au interdigital electrodes were deposited on ZnO network substrates. Fig. 3a and b show the I–V and I–V logarithm characteristics of a ZnO NW network PD in the dark and under 365 nm (0.35 mW cm−2) illumination, respectively. The linear I–V relationship with forward and reverse bias measured in the dark clearly shows an approximate ohmic contact between the ZnO NW network and electrodes.30,31 The NW network exhibits high photoresponse. For example, the dark current and photocurrent are 7.73 nA, and 30 μA at 5 V applied bias, respectively whereby the resulting on/off current ratio was calculated to be 3.9 × 103 at 5 V.
To illustrate the light-switching characteristic, the photocurrent–time response measured in the dark and under illumination was controlled by a light shutter. As shown in Fig. S3,† when the UV light was turned on, the photocurrent sharply increased under the applied bias voltage. Meanwhile, the device immediately decayed to a dark current as soon as the UV light was turned off. The device exhibited stable and reproducible responses to on–off cycles. The current–time (I–t) curves at 2 V bias under 365 nm laser (1 mW cm−2) with different light irradiation intensity are presented in Fig. 3c. As shown in this picture, under illumination the device produced a photocurrent which steadily increased with increasing light power from 10 μW cm−2 to 200 μW cm−2. Especially, the current measured at 2 V reached a maximum value of 64 μA with a light intensity of 50 μW cm−2, which is about 3 orders of magnitude compared with that under dark conditions.
Furthermore, the spectral responsivity of the device considered as a function of excitation wavelengths was measured. The responsivity Rλ was calculated according to the following equation:32
(1) |
The detectivity (D*), demonstrated the ability of the photodetector for detecting a weak signal, also a critical parameter, which is calculated by the following equation:
(2) |
To further demonstrate the high photoresponse properties, a distinct imaging picture with letters of NUST (Nanjing University of Science and Technology) is demonstrated in Fig. 3d–f. The photocurrent microscopy system was constructed as shown in Fig. 3d (complete imaging setup is shown in Fig. S5†). Through a current pre-amplifier and lock-in amplifier, signal photocurrents are extracted from the NW network-based device to form images.33–35 Fig. 3e shows a metallic object (NUST) with a line width of 5 μm patterned. The metallic objects can be any other patterns with any shape or size. The corresponding images were acquired with the NW network-based detector by applying a focused light spot (365 nm) scanning across the patterns. Simultaneously, signal photocurrents from a pattern edge cross section in the images were extracted (see Fig. 3f). The location of photocurrent and background appeared in sharp contrast with the clear imaging letters on the picture. Meanwhile, the green color of the letter indirectly reflects the highly uniform feature of a NW network.
Such high performances (low dark current, high photocurrent, high responsivity, etc.) are attributed to a special carrier transport mechanism. Consequently, the generation of carriers and their transport in a NW network photodetector are depicted in Fig. 4. Usually, higher dark current is presented in a ZnO film photodetector, where the carriers have a smoother channel than in a NW network-based photodetector. In general, on a NW surface, oxygen molecules capture free electrons through [O2(g) + e− → O2−(ad)], leading to a low conductivity surface depletion layer under dark conditions. Carrier density in the NWs increased considerably under UV light (hυ > Eg ZnO), by electrons directly transported from the valence band to the conduction band. Adsorbed oxygen ions were discharged by the photo-generated holes through [O2−(ad) + h+ → O2(g)], reducing the thickness of the depletion barrier.36,37 Thus, the increase in carrier density and oxygen desorption contributes to the conductance increase in the NWs. Besides, the interaction between NWs also needs to be considered in NW network-based devices because the resistance of two crossed NWs was dominated by the NW–NW junction barrier and the resistances of the NWs themselves. In our ZnO NW network PDs, the electrons have to overcome the NWs–NWs junction barrier when tunneling from one NWs to another.38,39 The electron-transfer barrier originates from the surface depletion layers. As discussed in single NWs, the depletion layer can be narrowed by UV illumination due to increased carrier density, which is equivalent to a lowering of the effective barrier height.39,40 It is thus easier for electrons to go through the network upon UV illumination, therefore resulting in the increase of current.
Importantly, the NW network films can be integrated with wire-shaped substrates suitable for future wearable and smart devices, which can release part of bending stress and are suitable for wearable optoelectronic fields. A photograph of a ZnO NW network coated on KEVLAR exhibiting high flexibility is shown in Fig. 5a. The wire-shaped PD was fabricated by rolling a tensioning KEVLAR wire on the NW network substrates, which is illustrated in Fig. S6.† The film microstructure on the KEVLAR wire surface was characterized by SEM images (Fig. 5b and c). Similar to NW network films on planner substrates, the networks on wire-shaped substrates also exhibit high quality, crack-free, and smooth surfaces.
Fig. 5 (a) Photograph of a ZnO NW network coated on wire-shaped KEVLAR. SEM image (b) and high magnification SEM image (c) of the ZnO NW network film coated on KEVLAR substrates. |
The NW networks on KEVLAR can be assembled into wire-shaped PDs by twining the CNT electrodes. The structure of a wire-shaped PD is demonstrated in Fig. 6a. The KEVLAR wire substrate is coated by ZnO NWs with CNTs as the electrode. The device's structure was confirmed by the SEM images shown in Fig. 6b and c. The contact between CNT and the ZnO NW network is good due to the excellent flexibility of CNT, which enables charge collecting from the NWs active layers.
The wire-shaped PDs also exhibit high photoresponse features compared with those of a corresponding planner structure. As shown in Fig. 6d, the dark current and photocurrent for wire-shaped PDs are 4.7 nA and 5.71 μA at 2 V applied bias, respectively. The on/off current ratio was calculated to be 1.2 × 103 at 2 V. Meanwhile, the wire-shaped PDs exhibited high flexibility. The photoresponse behavior of the device under different bending situations was characterized by measuring the current as a function of time when a 365 nm UV light was periodically turned on and off (Fig. S7†). The current–time (I–t) curves at 2 V bias under 365 nm laser (1 mW cm−2) under a flat conformation and bending 180° are respectively presented in Fig. 6e, with almost no change in the photocurrent value, which illustrates extraordinarily flexibility of our wire-shaped PDs. Additionally, after 500 cycles of bending tests, the I–t curves still remained at the same level (Fig. 6f). These good mechanical properties probably resulted from the network structure of ZnO NWs. SEM images of wire-shaped ZnO PD after bending for 500 times are illustrated in Fig. S8,† showing compactness and interconnecting NWs in the PD after 500 cycles without severe damage.
Importantly, the wire-shaped structure possesses excellent features such as less limitation of incident light angle, wider range of the substrate materials and wearability, and especially it can absorb light in incident directions and greatly diversified applications of PDs. The relationship between normalized responsivity and detecting angle is shown in Fig. 7. Obviously, the responsiveness of planar PDs decreases under side irradiation while the wire-shaped 360° PDs are capable of detecting incident directions and show no blind spots when compared with rigid planar structure PDs (Fig. 7b). Due to their unique structure, the performances (e.g., on/off ratio) of the 360° PDs remained almost unchanged under different bending angles with the same light intensity (Fig. 7c). Therefore, they can be efficiently used for wearable optoelectronic fields. Besides, a wire-shaped PD is also capable of long-time operating as the stability of Ilight vs. time demonstrated in Fig. S9† in which the photocurrent showed no decrease after 12 hours irradiation.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra06555a |
This journal is © The Royal Society of Chemistry 2018 |