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Needle-like and flower-like antimony sulfide nanostructures were synthesized and applied for both rigid and flexible photodetectors. Rigid photodetectors based on both nanostructures have the features of linear photocurrent characteristics, low linear dynamic range and good sensitivity to light intensity. Especially, the rigid Sb2S3 nanoflowers photodetector has high photoresponse characteristics and its response time and decay time were found to be relatively fast as 6 ms and 10 ms respectively. The flexible Sb2S3 nanoflowers photodetector has high flexible, light-weight and adequate bendability with a response time of about 0.09 s and recovery time of 0.27 s. Our results revealed that the rigid and flexible photodetectors based on Sb2S3 nanostructures have great potential in next generation optoelectronic devices.
©2013 Optical Society of America
1. Introduction
Next generation electronic and optoelectronic systems require more rigorous elements with low-cost, small size, high efficiency, and excellent flexibility. As one of important devices, photodetector may find important applications in binary switches [1], imaging techniques [2], optical communications [3, 4], memory storage and optoelectronic circuits [5–8]. To achieve high performances, some critical properties of devices including photosensitivities, photoresponses and repeat abilities have been widely investigated in previous work [9–17]. In recent years, flexible optoelectric devices have attracted extensive attention due to their potential applications and a number of different flexible electronics technologies have been developed [18–25]. For example, Chen et al. have successfully fabricated flexible asymmetric supercapacitors based on transition-metal-oxide nanowires/single-walled carbon nanotube hybrid thin film electrodes [18]. Flexible hybrid photodetectors constructed on Polyethylene terephthalate (PET) and printing paper show an extremely high on/off switching ration, a fast time response, and excellent wavelength-dependence [22]. Highly flexible batteries were fabricated to achieve high flexibility, excellent electrical stability and superior electrochemical performance [25].
Sb2S3 is an important semiconductor of the chalcogenide semiconductors family with orthorhombic crystal structure and a space group of D1620 [26, 27]. With a suitable band gap of 1.5~2.2 eV, it covers the range of the solar spectrum [28–30], which makes it a good candidate for high performance visible or infra-red photodetectors. In this work, by synthesizing Sb2S3 nanoneedles and nanoflowers via a facile polyol refluxing process under the open-air condition, we successfully fabricated high performance Sb2S3-based visible light photodetectors on rigid SnO2 transparent conductive glass doped fluoride (FTO) substrate as well as flexible PET substrate. Both the rigid and flexible photodetectors have the features of high stability, fast response and recovery time.
2. Experiment
Sb2S3 nanostructures were prepared via a facile polyol refluxing process. To get Sb2S3 nanoneedles, appropriate amount of Sb(DDTC)3 prepared according to previous report [31,32], was put into 50 mL ethylene glycol (EG) solution in a 250 mL three-neck flask with the addition of 0.1 g polyvinyl pyrrolidone (PVP), which was heated and kept at 190 °C for 1 h. The resulted dark-brown precipitates were filtered, washed with distilled water and absolute ethanol and then dried at 60 °C for 3 h in vacuum. As for Sb2S3 nanoflowers, 3 mmol antimony trichloride and 2.5 mmol L-cystine were mixed into 60 mL EG solution in the presence of 6 mL ethylenediamine. The refluxing system was kept at 180 °C for 1 h and the resulted precipitates were collected for the characterizations. To get thin films on FTO glass substrates, the pre-cleaned substrates were directly putting into the fluxing solution and then dried for device fabrication.
To fabricate the rigid photodetector device, parallel silver wires with an interval of 1 mm were fixed onto the as-synthesized Sb2S3 thin film with silver paste as the binder. Then the devices were annealed at 100 °C for 1h to solidify the silver paste. While to fabricate the flexible photodetector device, a suitable amount of the as-synthesized powders were first dispersed in ethanol solution containing small quantity of terpineol and ethylene cellulose to form uniform paste. Then the paste was coated onto the PET substrate with parallel Ag electrodes at room temperature. Finally, the PET device was dried at 80 °C for 10 h in a vacuum oven to improve the mechanical strength and electrical contact.
The Sb2S3 samples were studied by using scanning electron microscopy (SEM, JSM-6701F), Transmission electron microscopy (TEM, JEOL, JEM-2010 and JEM-3000F), X-ray power diffractometer (XRD, X’Pert PRO, PANalytical B.V., the Netherlands), and UV-vis spectrophotometer (Shimadzu UV-3150). Photoresponse properties were measured under the UV-filtering solar (AM 1.5G) conditions with an Autolab (model AUT84315).
3. Results and discussion
Figure 1(a) shows the XRD patterns of the as-grown samples under different conditions. All the diffraction peaks can be readily indexed to pure orthorhombic Sb2S3 phase (JCPDS: 42-1393). No peaks of any other phases are detected in both patterns, indicating the high purities of the two samples. Energy dispersive spectroscopy (EDS) analyses reveal that both samples are composed of S and Sb elements, with a molar ratio of 57.42: 42.58 and 58.51: 41.49, respectively, confirming the high purity of the samples. The UV-vis absorption spectra of the as-synthesized Sb2S3 samples are shown in Fig. 1(b), which demonstrate continuous broad absorption from UV to visible regions, indicating their potential applications in solar cells and visible light photodetectors. The energy band gaps of the Sb2S3 nanoneedles and Sb2S3 nanoflowers can be estimated by the formula (αhν)2 = A(hν-Eg), in which α is the absorbance coefficient, h is the Planck constant, A is the characteristic constant of semiconductors, Eg is the optical band gap energy and ν is the frequency. According to the Kubelka-Munk Theory: F(R∞) = K/S = (1 - R∞)2/2 R∞), where R∞ is the extreme limit of the reflectance coefficient, K is the absorption coefficient, S is the scattering coefficient, we can get the band gap values of both samples are 1.71 eV and 1.68 eV, respectively, in good agreement with previous reports [33, 34].
The morphologies and microstructures of the as-synthesized Sb2S3 products were characterized by SEM and TEM. As shown in Fig. 2(a), uniform Sb2S3 nanoneedles were prepared on a large scale. The higher-magnification SEM image (Fig. 2(b)) shows that the as-synthesized nanoneedles have the diameter of 200-300 nm and length of about 10 μm, which were further confirmed by the TEM image in Fig. 2(c). Inset in Fig. 2(c) shows the selected-area electron diffraction (SAED) pattern taken from an individual nanoneedle, which confirms the nanoneedle is an excellent single-crystalline in nature. Flower-like Sb2S3 nanostructures were also obtained when L-cystine was used as the S source in the presence of ethylenediamine. Figure 2(d) displays the corresponding panoramic SEM image, indicating the formation of a large number of 3D flower-like architectures with the diameter of about 7-10 μm. Figure 2(e) clearly exhibits that Sb2S3 flowers are built from dozens of nanoflakes with the thickness of 50 nm. Figure 2(f) shows the TEM image of a single Sb2S3 flower, which is in accordance with the SEM result and further demonstrates the porous structure of flower. The corresponding SAED pattern (Fig. 2(f) inset) reflected the single-crystalline nature of the petals of the flowers.
Fig. 2 (a,b) SEM images and (c) TEM image of the Sb2S3 nanoneedles. (d,e) SEM images and (f) TEM image of the Sb2S3 nanoflowers.
To investigate the photoresponse of the as-synthesized Sb2S3 nanostructures, rigid photodetectors were first fabricated by using the nanostructured films as the active channels and two silver wires as the electrodes. The corresponding device structures were depicted in Fig. 3(a,c) insets. Figure 3(a) shows the typical I-V curves of the nanoneedles-based device exposed to visible light and under dark condition with the bias from −1 V to 1 V, respectively. From the curves, we can clearly see that the current intensity exhibited an increase from 3.9 mA at dark to 7.5 mA upon illumination with a power density of 24.5 mW/cm2. The on/off switching curve at the bias of 1 V was shown in Fig. 3(b). A slow time response obtained may limit the scope of application of the device. While on the other hand, the nanoflowers-based device showed excellent performance, as can be seen from Figs. 3(c) and 3(d). Figure 3(c) shows that the photocurrent of the nanoflowers-based photodetector exhibits a significant increase under light illumination at the bias of 1 V. It clearly demonstrates the extremely high sensitivity of the Sb2S3 nanoflowers to visible light irradiation. The excellent photoresponse behavior was further proved by the time-dependent photoresponse (I-T) of the device (Fig. 3(d)). A low dark current of 2.5 nA and high photocurrent of 95 nA was recorded at a low bias of 1.0 V, increased about 38 times. From the I-T curves shown in Fig. 3(d), the photocurrent increased and decreased sharply upon repetitive on/off switching of the light illumination. After many cycles, the photocurrent intensity can still be rapidly changed and kept at a relatively stable value, indicating the highly stable and reproducible of the device.
Fig. 3 (a) I-V and (b) I-T curves of the Sb2S3 nanoneedles based rigid photodetector, (c) I-V and (d) I-T curves of the Sb2S3 nanoflowers based rigid photodetector illuminated with light density of 24.5 mW/cm2.
As one of the key factors for photodetector performance, a fast time response is very important. Figures 4(a) and 4(b) show the enlarged view of a single on/off cycle of the Sb2S3 nanoflowers-based device to the visible light, in which the response and the recovery time were measured to be 6 ms and 10 ms, respectively, indicating fast photoresponse features. The time-dependent (I-T) curves measured at different bias voltages of the photodetector are shown in Fig. 4(c), which shows that with the increase of the bias voltage from 0.1 V to 1 V, the photocurrent only shows commensurate increase, confirming that the rigid photodetector has different response for different bias. Figure 4(d) shows the I-V curves of the photodetector irradiated with the visible light at varied intensities. It displays that the conductance increases with the increase of irradiation power. At the bias of 1 V, the current changes from 2.5 nA at dark to 730 nA under irradiation with the light intensity of 133.4 mW/cm2. The photocurrent as a function of illumination density can be fitted with a linear dynamic range (LDR, typically quoted in dB). LDR is an important photodetector characteristic of merit, which is given by LDR = 20 log (Iph/Idark), where Iph is the photocurrent, tested at the light intensity of 1 mW/cm2 (see in Fig. 4(e)). In view of the dark current is 2.5 nA at the bias of 1V, the LDR is calculated to be 27.6 dB, which is less than those obtained from InGaAs photodetector (66 dB) and GaSe photodetector (32.8 dB) [35, 36]. The modest LDR values are contacted by the ratio of photocurrent and dark current intensity, thus leaving room for future improvement.
Fig. 4 (a, b) Enlarged view of a single on/off cycle of the Sb2S3 nanoflowers-based device; (c) Photoresponse of the photodetector at 1 V, 0.5 V, and 0.1 V bias voltages; (d) I-V curves recorded at different light intensity illuminated; (e) Linear fit curves of photocurrent at different light intensity at a bias of 1 V and (f) I-V curves illuminated at different wavelength with an intensity of 2mW/cm2.
In addition to the good sensitivity to light intensity, the Sb2S3 nanoflowers-based photodetector also exhibits perfect wavelength selectivity. Figure 4(f) shows the wavelength-dependent I-V characteristics obtained when the device was exposed to lights with different wavelengths at a constant intensity of 2 mW/cm2. It is clearly revealed that the photoconductance [5] (G = I/V calculated in the linear range) is sensitive to the excitation wavelength. The photoconductance increased and then decreased with the increase of light wavelength. From the I-V curves, the photoconductance is calculated to be 35.8 nS at 600 nm, 41.2 nS at 650 nm, 49.6 nS at 700 nm, 34.9 nS at 750 nm, 23.4 nS at 800 nm and eventually 2.5 nS at dark state. The cut-off wavelength of about 700 nm is obtained, which is in agreement with the band gap of the Sb2S3 nanoflowers. The photocurrent intensity originated mainly from the electron-hole pairs excited by incident light with energy larger than the band gap of the Sb2S3. In other words, only illumination with enough energy can induce a crucial increase in photoconductance, indicating that the sub-bandgap states make little contribution to the photocurrent generation.
The stability of the Sb2S3 nanoflowers-based photodetector to the testing temperatures was also investigated and the corresponding results were demonstrated in Fig. 5. From the curves, ranged from 273K to 463 K, the current of the device showed little variation, indicating the excellent stability of the device to ambient temperatures.
Compared with previous reports on metal sulfide based photodetectors, for example, ZnS nanowires [37], CdS nanowires [38], Bi2S3 nanotubes [39], etc. The present Sb2S3 nanostructures based photodetectors showed higher on/off switching ratio, excellent stability and fast response and recovery time. In addition, little work on visible-light photodetectors based on Sb2S3 nanomaterials with high performance has been reported so far. Our work here indicates that they are good candidate for high performance photodetectors.
Fabrication of flexible devices has become one of the focuses of optoelectronic research because they can be fit for many unique applications in various new areas, such as portable device, aerospace science and civil engineering [40, 41]. Figure 6(a) shows the photograph of the flexible Sb2S3 nanoflowers-based photodetector on PET substrate, exhibiting good flexibility. Figure 6(b) is the I-V characteristics of the flexible device in dark and under visible light with the intensity of 133.4 mW/cm2. Upon light irradiation, the current steadily increased from 0.3 nA in dark to about 1.5 nA upon light illumination at a bias of 10 V, indicating good photoresponse of the flexible device. We also investigated the mechanical stability of the flexible device by bending it for 100 times. The I-V curve was nearly unchanged as shown in Fig. 6(b), indicating the good mechanical strength between Sb2S3 materials and the PET substrate. The reproducible on/off switching curves were shown in Fig. 6(c), further confirming the stability of the device upon light irradiation. Figures 6(d) and 6(e) are the response part and the recovery part of a single on/off cycle of the flexible device, where the response time is about 0.09 s and it has a low fall edge, giving a recovery time of 0.27 s. The photoresponses of the flexible device at 10 V, 5 V, and 3 V bias voltages are shown in Fig. 6(f). Great different response to different bias voltages further confirmed that the flexible photodetector has higher sensitivity to voltage.
Fig. 6 (a) Photograph of the flexible photodetector based on Sb2S3 nanoflowers; (b) Typical I-V curves measured in dark, upon visible light with intensity of 133.4 mW/cm2 and after 100 cycles of bending, respectively; (c) Time-related response of the photocurrent under visible light illumination; (d, e) Enlarged view of a single on/off cycle; (f) Photoresponse of the device at 10 V, 5 V, and 3 V bias voltages.
Flexible photodetectors were also fabricated by using the as-grown Sb2S3 nanoneedles as the active materials. Figure 7(a) shows the optical image of the device on PET substrate. Figure 7(b) gives the photoresponse switching behavior of the flexible photodetector. It can also be observed that the photocurrent can be reproducibly switched from the “ON” state to the “OFF” state with a time period of 25 s measured at a bias of 20 V. In Fig. 7(c), we examined the photocurrent intensity at different bending conditions, including lateral bending, longitudinal bending, and diagonal bending. The flexible detector was quite stable and the photocurrent had tiny changes even after 100 cycles of bending. Further test was recorded on the current flow through the flexible device at four different bending states, which are shown in inset of Fig. 6(d). And we also observed that the light current almost unchanged under visible light illumination at a constant voltage of 20 V (Fig. 7(d)), revealing that the device has good flexible and stability.
Fig. 7 (a) Photograph of the flexible photodetector based on Sb2S3 nanoneedles. (b) Current vs time for the flexible device with visible light repeatedly turned on and off. (c) Typical I-V characteristics of the device measured in dark and after 40, 80, 100 cycles of bending. (d) I-T curves of the photodetector bent with different curvatures at a voltage of 20 V upon visible light with intensity of 133.4 mW/cm2 condition. The upper insets show the photos of the four various bending states of the device.
The photoresponse mechanisms of the rigid and flexible photodetectors on both the Sb2S3 nanoneedles and nanoflowers can be expressed as the follows. Owing to different surface structure, trapping at surface states mainly affects the transport and photoconduction properties, O2 is the primarily responsible for the current intensity increased in air. In dark, oxygen molecules absorbed on the samples surface capture the free electrons: O2 (g) + e-→O2- (adsorption,) and the holes concentration increase. A low-conductivity depletion layer is thus formed near the samples surface. When illuminated by visible light with a photon energy above the energy gap of the two Sb2S3 samples, electron-hole pairs are generated. The holes migrate to the surface along the potential gradient and combine with oxygen, inducing desorption of oxygen from the surface of the nanostructure: h+ + O2- (ad)→O2 (g). The unpaired holes are either collected at the electrode or recombine with the electrons. With the increase of the density of the oxygen anions, the thickness of the low-conductivity depletion layer deceased, which leads to the increase of the carrier concentration and the apparent enhancement in photocurrent. When the light is turned off again, oxygen re-adsorbed on the nanostructure surfaces, resulted in the returning to its initial state.
Based on the above results, we can see that the photo-response performance of Sb2S3 nanoflowers is better than the nanoneedles. The reasons are as following. First, the nanoflowers are assembled by numerous porous sheets, while the nanoneedles are just disordered in stacking. This apparently leads to the different quality of the conductive thin film. Nanoflowers, with higher density, can confine the active conductive pathway of the charge carrier and hinder the diffusion process, which result in a shorter transit time and obviously a faster device speed. On the other hand, higher surface-to-volume ratio of the nanoflowers could bring an increase of sensitive area and then enhance the oxygen absorption/desorption process, leading an enhancement of carrier injection and transport and also producing a fast device speed.
4. Conclusions
In summary, high performance visible light photodetectors on both rigid FTO substrates and flexible PET substrates using Sb2S3 nanoneedles and nanoflowers were demonstrated. The two kinds of photodetectors exhibited higher photoconductive performance to visible light, such as high stability, excellent reproducible properties, and fast response and recovery speed. Expecially, the flexible photodetectors showed higher photoconductive behavior, high flexibility, light-weight and excellent reproducible properties. Our results demonstrated that the Sb2S3 nanoneedles and nanoflowers are good candidates for high performance photodetectors and the performance may be further improved by surface modification and device structure design.
Acknowledgments
This work was supported by the National Natural Science Foundation (51002059, 21001046, 91123008), the 973 Program of China (2011CB933300), the Program for New Century Excellent Talents of the University in China (grant no. NCET-11-0179) and the Natural Science Foundation of Hubei Province (2011CDB035). Special thanks to the Analytical and Testing Center of HUST and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for using their facilities.
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