Stretchable electronics, which can be stretched, deformed and wrapped onto nonplanar curved surfaces, have attracted much attraction due to their potential applications in wearable electronics, implantable biomedical devices, and artificial electronic skin. So far, many types of stretchable electronic devices have been developed including stretchable transistors, light-emitting diodes (LEDs), supercapacitors, photodetectors and sensors. Ultraviolet (UV) photodetectors have wide applications in crime investigation, biological analysis, fire monitoring, UV irradiation detection and many other applications as well.
With wide bandgap, abundant reserves, large specific surface area, high aspect ratio, and excellent stability, metal oxide nanowires (NWs) have been extensively studied as the active materials for high-performance UV photodetectors with much higher photo response compared with their bulk or thin-film counterparts. Recently, researchers also tried to fabricate stretchable photodetectors with metal oxide NWs. However, due to the presence of a large number of surface defects related to trapping centers, most of the reported UV photodetectors based on pure binary metal oxide NWs, stretchable or not, have displayed a very low response speed, which greatly limits their practical applications.
Several attempts have been carried out to improve the response speed of binary metal oxide-based photodetectors. Considering the real-time response of photodetectors is very important for their applications in wearable electronics and implantable biomedical devices. It is highly desirable to find simple and effective ways to fabricate stretchable UV photodetectors with high response speed.
This paper presents an interesting SnO-CdS NW interlaced structure to fabricate stretchable UV photodetectors with fast response speed. Considering the high spectral responsivity of SnO (direct E ~ 3.6 eV) NW photodetectors for UV light and the fast response speed of CdS (direct E ~ 2.4 eV) NW photodetectors, the SnO and CdS NWs were chosen to construct a nanowire interlaced structure by a multiple lithographic filtration method to combine the merits of both materials. Systematic investigations on the photo response properties of the SnO-CdS interlaced NW photodetectors showed that they had lower dark current and much faster response speed compared with SnO NW photodetectors, and higher spectral responsivity compared with the CdS NW photodetectors in the UV region. Waved wrinkles on the surface of the NW/PDMS layer offered the device excellent electrical stability and stretching cyclability within 50% strain. This strategy can also be applied to other interlaced nanowire pairs like ZnO-CdS NWs. These results indicate that nanowire interlaced structures may have broad applications in future stretchable and wearable optoelectronic devices.
Stretchable SnO-CdS interlaced NW photodetectors were fabricated with a multiple lithographic filtration method. Two pieces of ~1-mm thick PDMS masks were first made with a 3D printed mold. To make it easier to peel off the masks from the two molds, a 50-nm thick Au layer was deposited on their surfaces by thermal evaporation to reduce the surface roughness.
Then, PDMS mask I was removed and PDMS mask II for SnO2 NWs was placed on the top of the patterned Ag NW electrodes. Each vacancy in PDMS mask II only occupied one Ag NW electrode and 2/3 of the electrode gap. The SnO2 NW ethanol solution was dripped and filtered to form the SnO2 NW pattern. The size of a SnO2 NW film is about 2 mm×2.5 mm. Following a similar process, the CdS NW pattern was filtered on the top of Ag and SnO2 NW patterns. Every CdS NW film occupied another Ag NW electrode and 2/3 of the electrode gap. After removing PDMS mask II, the PC filter membrane with Ag, SnO2 and CdS NW patterns was placed in a plastic culture dish (diameter ~5.5 mm). Then, the PDMS liquid (about 1.3 mL) was injected on the top of the filter membrane, which can efficiently penetrate the porous NW films. After the liquid surface levelled and air bubbles disappeared, the PDMS liquid was thermally cured at 60°C for 4 h. Finally, the cured PDMS substrate was peeled off of the PC filter membrane with Ag, SnO2, and CdS NW patterns transferred onto the PDMS matrix.
The photo response characteristics of the as-fabricated SnO2-CdS interlaced NWs photodetectors were investigated systematically. For comparison, photodetectors with pure SnO2 NWs and pure CdS NWs were also fabricated. The spectral responsivity of the SnO2 NWs device increased from 510 to 310 nm, while it decreased for the CdS NWs device. As a result, from 510 to 310 nm, for the SnO2-CdS interlaced NW device, its spectral responsivity first increases, reaches a peak at 370 nm, and then decreases. The the light-to-dark current ratios (Ilight/Idark) of the corresponding three devices were found to be 900, 2578, and 7417, respectively.
We also studied the response speed of these three kinds of photodetectors. Then the corresponding I-t curves were measured during on-off switching under 370-nm UV illumination with a light intensity of 11.64 μW cm-2 at a bias voltage of 5 V, respectively. For the SnO2 NW photodetector, the rise time and decay times are about 158 and 89 s, respectively, defined as the time needed for current transition from 10% to 90% (or 90% to 10%) of the steady-state photocurrent. The extremely slow response of the SnO2 device made it unsuitable for practical applications. For the CdS NW photodetector, both the rise time and the decay times are about 0.8 s, much faster than those of the SnO2 device. The combined effect of the SnO2-CdS NW device shows fast rise and decay times of 1.5 and 0.6 s, respectively. Our results show that the SnO2-CdS interlaced NW photodetectors have lower dark current and much faster response speed compared with pure SnO2 devices, and higher spectral responsivity compared with pure CdS devices in the UV region.
For a stretchable device, the electrical stability under the stretching condition is a very important parameter to evaluate its potential for practical applications. We also measured the corresponding performance of the stretchable SnO2-CdS interlaced NWs photodetectors. To measure the stretching performance, the device was fixed on a homebuilt stretching stage. The required strain can be applied by adjusting the distance between two ends of the stretching stage. Our tests confirmed that the stretchable NWs photodetectors can obtain superior electrical stability and stretching cyclability after a prestretching process.
The performance of the stretchable SnO2-CdS NWs photodetectors can be easily tuned by design of the NW interlaced structure. Our results demonstrate that the interlaced devices display lower dark current and much faster response speed compared with the SnO2 device, and higher spectral responsivity compared with the CdS device. In fact, the NW interlaced structure can be easily expanded to many other oxide NW photodetectors to achieve similar effects. Due to the different bandgaps of different oxide NWs, the peak response wavelength of the interlaced NW photodetectors can be adjusted easily by changing oxide NWs.
In conclusion, a SnO2-CdS NW interlaced structure was used to fabricate stretchable UV photodetectors with fast response speed by a multiple lithographic filtration method. Systematic investigations were carried out to study the photo response characteristics of the as-fabricated stretchable devices. The results reveal that the SnO2-CdS interlaced NW device has lower dark current (as low as 19.2 fA) and much faster response speed (more than 100 times) compared with a pure SnO2 NWs device, and higher spectral responsivity compared with a pure CdS NWs device. In addition, the SnO2-CdS interlaced NWs photodetectors exhibit excellent electrical stability and stretching cyclability within 50% strain after five pre-stretching cycles, which can be attributed to the formation of waved wrinkles on the surface of the NW/PDMS layer during the prestretching cycles. As a simple and effective strategy to fabricate stretchable UV photodetectors with fast response speed, the NW interlaced structure can also be applied to other NW pairs like ZnO-CdS NWs. The researchers believe that this strategy may promote the development of future stretchable and wearable optoelectronic devices.
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