Tape-Based Photodetector: Transfer Process and Persistent

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Functional Inorganic Materials and Devices

Tape-Based Photodetector: Transfer Process and Persistent Photoconductivity Li Wang, Peng Chen, Yu-Cheng Wang, Gui-Shi Liu, Chuan Liu, Xi Xie, Jingzhou Li, and Bo-Ru Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02233 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Tape-Based Photodetector: Transfer Process and Persistent Photoconductivity Li Wang, Peng Chen, Yu-Cheng Wang, Gui-Shi Liu, Chuan Liu, Xi Xie, Jing-Zhou Li,* Bo-Ru Yang* School of Electronics and Information Technology, State Key Lab of Opto-Electronic Materials & Technologies, Guangdong Province Key Lab of Display Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China. * Email: [email protected], * Email: [email protected] KEY WORDS: Tape-based photodetector, Tape transfer printing, CdS nanowires, Persistent photoconductivity, Silver nanowires electrode

ABSTRACT: We report a facile transfer method to fabricate flexible photodetectors directly on tape, wherein the films formed by different processes were integrated together. The tape-based photodetectors with CdS nanowire (NWs) active layers exhibited good performances as those fabricated by conventional processes. The obvious persistent photocurrent (PPC) in our device was eliminated by introducing a conductive polymer poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) onto the CdS NWs layer. By adjusting the concentration of the PEDOT:PSS aqueous solution, a device with a fast response, ultrashort decay time, and relatively large photocurrent was obtained. The decay times were 11.59 ms and 6.64 ms for

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devices using electrodes of silver NWs and gold, respectively. These values are much shorter than the shortest decay times (on the order of hundreds of milliseconds) reported previously.

1. INTRODUCTION Flexible photodetectors based on low dimensional nanomaterials have attracted much attention due to their excellent flexibility, solution-processability, high photosensitivity, and possibility of application in wearable image sensors.1-4 Nanostructured cadmium sulfide (CdS) with a direct bandgap of 2.42 eV5-7 has a wide range of potential applications in visible light photodetectors due to its large surface to volume ratio, convenient high yield synthesis,8 good single-crystalline phase,9 high sensitivity to oxygen gas,10 and higher light sensitivity compared to bulk materials.10 Moreover, metal nanowires (NWs) have been considered as promising alternatives to conventional transparent indium tin oxide (ITO) electrodes. Therefore, the combination of a CdS NWs active layer and a metal NWs electrode would be a better design strategy for flexible photodetectors. Among the processes used to construct these nanomaterials on flexible substrates, transfer printing is an effective method suitable for large area electronics in flexible and stretchable formats. In this method, the components fabricated on the mother substrates are transferred onto the desired substrates using an elastomeric stamp (e.g., polydimethysiloxane (PDMS)), thus avoiding high temperature processing on plastic substrates. However, transfer printing dominated by PDMS is limited in its actual application by several requirements, such as the difference of surface energy, surface smoothness, and kinetic control of adhesion.11 To ensure a reliable adhesion, a bonding agent was introduced to ensure a clean and full transfer of all components.12-13 The disadvantages of using a bonding agent are the possibility that the mother substrate adheres strongly with the bonding agent and the cured adhesive loses the ability to pick up another functional layer.


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Many studies have aimed to develop a more sensitive, more stable, and more reproducible photodetector with faster photoresponse. Despite these efforts, there always exists a persistent photoconductivity (PPC) effect or PPC tail in the NWs network structure.14-16 The PPC tail refers to when there is a residual photocurrent after the illumination is terminated. This can result in slow recovery (in some cases taking hundreds of seconds)17-18 to the dark state, which is not a desired feature for applications in fast photoelectric switches, high frequency optical communications or other sensing applications. Despite some efforts to develop the photoresponse of photodetectors with network structure,19-21 the decay time of the PPC was still above the order of hundreds of milliseconds, and no effective means have been found to cut off the PPC tail. In this paper, CdS NWs photodetectors were fabricated using a facile and high fidelity tape transfer method.22 Fabricating all-NWs structured devices on tape not only satisfies the need for a flexible format but also facilitates putting devices on arbitrary surfaces, even those with low surface energies and modest roughnesses. This is especially important when the photodetectors are being freely integrated with wearable devices. To cut off the PPC tail existing in our devices, conductive polymer poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was introduced to eliminate the residual photocurrent. As a result, the photodetectors exhibit ultrashort decay times and relatively large photocurrents under illumination. This is the first reported method that effectively cuts off the PPC tail, leading to potential applications in fast optical switches for flexible devices. 2. EXPERIMENTAL SECTION The preparation of CdS NWs and silver NWs (AgNWs): CdS NWs were prepared according to Ref. 8-9 with a few modifications: 0.2665 g Cd(CH3COO)2·2H2O was first dissolved in 35 mL



ethylenediamine, and then 0.0641 g of sulfur powder was added and mixed by vigorous stirring. This homogeneous mixture was transferred into a Teflon-lined autoclave, gradually heated to 200 °C, kept for 12 h, and then cooled to room temperature. The resulting yellow precipitate was collected, washed several times with deionized water and absolute ethanol, and then dried in vacuum at 70 °C overnight. AgNWs were synthesized by a polyol method (Supporting information). Device fabrication: The CdS NWs film was obtained by vacuum filtration. 0.01 mg CdS NWs dispersed in 30 mL absolute ethanol was poured into a vacuum filter with an anode aluminum oxide (AAO) membrane. The AAO membrane with 200 nm apertures was premodified by octadecyltrichlorosilane (OTS) to form an anti-adhesion surface. The AgNWs interdigitated electrode was formed by wettability contrast patterning in which the hydrophilic area was defined by a shadow mask followed by UV/O3 exposure.23 AgNWs (0.3 wt%) dispersed in the mixture of ethanol and deionized water with a mass ratio of 2:1 were dripped onto the UV/O3 treated PDMS at a spin speed of 2500 rpm. The detailed transfer process is depicted in Figure 1a, including the first and second tape transfer. The AgNWs electrode was adhered onto a 20 µm thick double-sided optically clear adhesive (OCA) pasted on a polyethylene terephthalate (PET) substrate. The obtained CdS NWs film on AAO was entirely picked up by a flat PDMS that served as a transfer stamp. Finally, the CdS layer on the PDMS was adhered onto the AgNWs layer. The PEDOT:PSS was introduced by spin-coating in which three drops (about 32 µL for each droplet) of aqueous solution of PEDOT:PSS (2.8 wt% dispersion in H2O) were dripped onto the CdS NWs layer that had been transferred on the PDMS at the spin speed of 3000 rpm. Due to the hydrophobicity of PDMS, the PEDOT:PSS was confined to the area of CdS NWs. Then, the


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solution was further diluted by deionized water. We use dilution ratio to describe the diluted degree of PEDOT:PSS solutions. It denotes the volume ratio of deionized water to the undiluted PEDOT:PSS. The dilution ratio of the undiluted PEDOT:PSS is 0:1. The other dilution ratios are 2:1, 4:1, 6:1, and 8:1, respectively. The coated polymer was adhered to the CdS NWs network in amorphous form after dried at 70 ℃ for 10 minutes. Characterization: All the opto-electrical response curves were measured by a Keithley 2400 source meter. Laser diodes with wavelengths of 450 nm and 520 nm were used as light sources. In most cases, unless specially stated, the light source used in this paper is 450 nm. The light power was measured by a laser power meter (VLP-2000-200 mW), and the measured value was taken as the incident light power since the light spot is far smaller than the photosensitive area. The frequency of the light source was modulated by a chopper. The morphology of the CdS NWs was observed using a scanning electron microscope (SEM, Carl Zeiss SUPRA 60). Photoluminescence (PL) spectra were obtained by a laser microRaman spectrometer (inVia Reflex) using a 405 nm laser with the power of 0.1 mW. The hydrophilic treatment of PDMS was conducted under UV/O3 exposure at an intensity of 18 mW/cm2 (SEN, PL17-110). The contact angles were measured by a contact angle meter (Dataphysics OCA 15EC), and water and diiodomethane were used as test liquids. According to the measured results, the surface energy of the hydrophilic PDMS was calculated by using the Owens, Wendt, Rabel and Kaelble method. 3. RESULTS AND DISCUSSION



Figure 1. (a) Schematic of the fabrication process of a tape-based CdS NWs photodetector. (b) Photodetector arrays on tape. Inset: AgNWs circuit with line width of 300 µm. (c) and (d) CdS NWs photodetectors transferred onto PDMS and paper, showing that the devices can be transferred onto substrates with low surface energy and moderate roughness. 3.1 Tape-based photodetector by tape transfer method The tape transfer method is used to assemble various functional layers together on the target substrate, avoiding the non-compatibility of different film-forming processes. In the fabrication process for a tape-based photodetector, the PDMS not only serves as a transfer stamp but also as a substrate for AgNWs patterning23 due to the tunable surface energy of PDMS under UV/O3 exposure. Longer UV/O3 exposure time makes more methyl groups (-CH3) turn into hydroxyl groups (-OH). As shown in Figures 2a and 2b, the polar component of the PDMS surface energy dramatically increases after 12 minutes of UV/O3 treatment. The PDMS-hydrophobic surface


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becomes super hydrophilic after more than 25 minutes of UV/O3 treatment. The pattern of an interdigital electrode was spontaneously formed after the AgNWs ink was spin-coated onto the selectively hydrophilic-treated PDMS. The actual gap width of the electrode varied with the change in surficial hydrophilicity when the gap width of the shadow mask was fixed at 100 µm, (this can be seen from the optical micrographs in Figure 2c-f). Another variable dependent on the surface energy is the work of adhesion between the AgNWs and the PDMS. This can be written as: = 2 + 2 (1) where γ is the surface energy; subscripts h and f represent two different surfaces, and the superscripts P and D stand for the polar and dispersion components of γ, respectively. The increased polar component of the PDMS surface energy results in a high work of adhesion, and strong bonding between two surfaces can be achieved provided that there is full contact between these two surfaces. It is detrimental that using liquid adhesive in transfer printing forms strong adhesion between the adhesive and hydrophilic PDMS, leading to the failure of transfer printing. Fortunately, the limited fluidity of the polymer on tape makes it difficult to have full contact with the hydrophilic PDMS in atomic-scale. Thus the strong adhesion between the adhesive polymer and hydrophilic PDMS can be avoided, rather than our previous work by using uncured epoxy resin.23 Accordingly, the AgNWs can be directly transferred onto the tape without taking steps to restore the hydrophobicity of the hydrophilic area. With the increase of UV/O3 exposure time (especially beyond 25 minutes), strong adhesion was formed between AgNWs and the hydrophilic PDMS. This was evidenced by the fact that in the first tape transfer step some AgNWs remained on the PDMS after peeling off the tape, thus resulting in damage to the integrity of the AgNWs electrode (see Figure S3).



As the CdS NWs layer was transferred onto the tape with the AgNWs electrode, the tape-based photodetector was obtained. The two NWs networks were closely connected by the strong adhesion of tape without further thermal treatment (Figure 2g). Tape-based photodetectors demonstrate good performances as those fabricated on rigid substrates.5 Compared to Ref. 5, the electrode gaps here are much wider (fixed at 175 µm), resulting in very low dark currents on the order of 10~100 pA. Also, CdS NWs are densely distributed in the films obtained by filtration, and thus more CdS NWs contribute to photoconductivity. As a result, the photo-to-dark current ratio reaches 1309 (Table S1) in comparison with that of 741 reported in Ref. 5. In addition, statistics of different devices fabricated by the same process are shown in Figure S4. The fluctuation of photocurrents is in the acceptable range, and the photoresponse has a good reproducibility for the same device in repeated switching cycles as shown in Figure 2 h. Concerning the change of electrode gaps, the photocurrent increases with the decrease of the width of electrode gap (Figure 2 h), and obvious PPC tails appeared in the dark state of each switching cycle. As mentioned above, longer UV/O3 exposure time increases the work of adhesion between the AgNWs and hydrophilic PDMS, causing damage to the AgNWs electrode during the peeling process. Thus, a moderate exposure time should be adopted (approximately 22 minutes) to balance damage to the electrode and attain a larger photocurrent. In addition to the reliable and reproducible performance of tape-based photodetectors, this process is convenient and adaptable to any substrate, as shown in Figures 1b, 1c and 1d. Moreover, the strong tape adhesion is conducive to the high fidelity of transferring NWs as well as the adhesion of NWs to a substrate. The unused part of the tape can be used to bond another functional layer, showing the flexibility of this method compared to those of the transfer methods that use fluidic adhesive.


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Figure 2. (a) The surface energy of the PDMS versus UV/O3 exposure time. The total surface energy includes the polar component and dispersive component. (b) Water contact angles before and after hydrophilic treatment. (c)-(f) The variation of electrode gaps with increasing surface energies of the PDMS. (g) SEM image of the boundary between the CdS NWs and AgNWs on the tape. (h) Reproducibility of photoresponse and the relationship between photocurrent and electrode gap. 3.2 Discussion of the PPC effect PPC not only occurs in NW-type photodetectors but also in heterojunctions and polycrystalline films,

such

as

GaN

thin

films,24

polycrystalline

CdS,25-26

and

GaAs-AlxGa1-xAs

heterostructures.27 However, there is not a unified explanation of the PPC effect, although theories include the presence of trap states,28 surface band bending,17 adsorption and desorption of molecules,29 and potential fluctuations.30 The PPC effect may be caused by a combination of these factors or one dominant mechanism. According to the early reports on polycrystalline CdS films,25-26, 31 the chemisorption of oxygen (O2) and grain boundaries have a significant impact on PPC. The adsorption and desorption of O2 enlarges the photocurrent and suppresses the dark



current, simultaneously acting as additional recombination centers or producing certain recombination centers to make the PPC decay rapidly, which acts to weaken the PPC effect. The depletion region induced by charge trapping at grain boundaries separates photogenerated electron-hole pairs, prolonging the carrier lifetime and strengthening the PPC effect. Meanwhile, the photogenerated holes nearby are trapped at the grain boundaries, which decreases the barrier height and width and allows the electrons cross the grain boundaries easily through tunneling or thermal emission. For the nanostructured CdS, the PPC in the CdS nanorod networks is mainly induced by the spatial separation of electrons and holes at the NW-NW junctions where back-to-back Schottky barriers are formed due to band bending.15 By improving the crystallization, the PPC can be ignored, as in the cases of single CdS nanowires or nanobelts.32-34


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Figure 3. Photoresponse of CdS NWs photodetectors with a bias of 5 V. (a) Devices with different electrode gaps under the incident power of 60 mW. (b) The same device exposed to weak (2 mW) and strong (60 mW) light power when the electrode gap was 300 µm. (c) The same device with an electrode gap of 250 µm in air and after PDMS encapsulation, under the incident power of 60 mW. (d) The device with an electrode gap of 300 µm under the incident power of 60 mW applied for over 2 minutes. As for tape-based photodetectors, the larger the photocurrent, the more significant the PPC. As shown in Figures 3a, 3b, the PPC shows fast decay when the electrode gap is larger than 500 µm, and no obvious PPC is observed when strong illumination (60 mW) is replaced by weak illumination (2 mW). In contrast, narrow electrode gaps and strong illumination increase the photocurrent and create long PPC tails. To demonstrate the effect of O2 on PPC, the CdS NWs were encapsulated in PDMS to cut off the air. Figure 3c shows the same device before and after PDMS encapsulation. It is clearly seen that the PDMS-encapsulated device exhibits a longer PPC tail than that in air, which is consistent with the result found for ZnO NWs fully embedded in PDMS.18 In addition, long time illumination makes the PPC larger, indicating an accumulation effect (Figure 3d). Combining previous reports and the results in this work, it is reasonable to consider the frustrated recombination of photogenerated carriers as the main origin of PPC in CdS NWs network. This could be caused by trap states and the separation of photogenerated electron-hole pairs at the NW-NW junction barriers. Therefore, owing to the long percolating pathways and weak electric fields when an electrode gap is wide, a large number of photogenerated carriers recombine before they are collected by the electrode. While fewer electron-hole pairs are generated by weak illumination, resulting in a relatively fast recovery to the dark state. The increase of PPC and photocurrent in Figure 3d should be assigned to a



thermal accumulation effect brought by the long time irradiation. This was discussed in the earlier work of CdS nanorods thin films15 wherein the PPC and photocurrent increase with the increase of temperature. This effect can be totally avoided when upon weak illumination (Figure S5). In summary, the PPC tail disappears temporarily but is still very likely to reappear again when the electrode gap and illumination intensity are changed. 3.3 The way to cut off PPC tails Based on the discussion above, for the CdS NWs network, we conclude that suppressing carrier recombination facilitates an increase in photocurrent while causing an unfavorable impact on the dynamical response. Finding a way to cut off the PPC tail should increase the recombination of electrons and holes that are trapped or separated by fluctuant potentials, thus effectively restraining the accumulation of photogenerated carriers. Similar to the case of O2, introducing extra recombination mechanisms is a plausible way to cut off the PPC tail. Here, PEDOT:PSS was spin-coated onto the CdS NWs networks by using its undiluted and diluted aqueous solution. The p-type conductive polymer can not only form a uniform film with CdS NWs but can also easily form defect states because of disorder in its solid-state nanostructures, which is beneficial to electron-hole recombination.35-36 As shown in Figures 4a and 4b, the devices with PEDOT:PSS (dilution ratio of 6:1) exhibit fast responses to light and ultrashort decay times in comparison with those without PEDOT:PSS. This fast response ability was further studied by using high time-resolved circuits in which an oscilloscope is placed in series with a photodetector at a frequency of 1000 Hz (Figure 4c and Figure S6). For clear comparison, the rise time is defined as the time between 10% and 90% of the maximum value when turning on the illumination. The decay time is defined as the time between 90% and 10% of the maximum value when turning off the illumination. Here, the rise time and decay time are 11.5


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ms and 11.59 ms (Figure S7a, Figure S7b), respectively. Additionally, the PDMS-encapsulated device with PEDOT:PSS (at the dilution ratio of 6:1) also shows a fast photocurrent decay, which is evidence of its ability to cut off PPC without O2 (Figure 4d). Meanwhile, note the trend of the photocurrent before the removal of photo excitation. Under equally strong illumination, the device without PEDOT:PSS showed an abrupt increase followed by a slow climb. In contrast, the device with PEDOT:PSS has an abrupt increase followed by a slow decay. This difference suggests that the suppression effect on the photocurrent is caused by the PEODT:PSS. To further clarify the influence of the PEDOT:PSS, the effects of different concentrations of PEDOT:PSS on the photocurrent and PPC were systematically studied. Figure 5a reveals the trend of the photocurrent and dark current for different dilutions of PEDOT:PSS. The devices with undiluted PEDOT:PSS show low photocurrents and high dark currents, implying the occurrence of strong recombination for photogenerated carriers. As the PEDOT:PSS was gradually diluted by deionized water, the undesired high dark current decreases and the photocurrent increases to the value close to its original one. Furthermore, all the devices exhibited ultrashort decay times until the PPC tail reappeared again at the dilution ratio of 8:1 (Figure 5b). It was also found that the PL intensity of the undiluted PEDOT:PSS/CdS NWs hybrid film shows a much weaker peak than those of the diluted cases (Figure 5c). This can be attributed to the effect brought by PEDOT:PSS, which could be interpreted as the formation of a recombination mechanism similar to the effect of O2. Since a Schottky barrier is supposed to be formed at the PEDOT:PSS/CdS NWs contact interface due to the energy level alignment (illustrated in Figure 5d),35 the depletion region with low conductivity in CdS NWs also accounts for the drastic decrease of photocurrents in devices with undiluted PEDOT:PSS. The micromorphology of the hybrid film can be seen from the SEM image (Figure 5e). For the



undiluted PEDOT:PSS, the polymer uniformly filled in the network voids and covered on the CdS NWs. With regard to the diluted PEDOT:PSS, although it was not clearly seen in the SEM image, we can judge its existence from the second tape transfer step where the PEDOT:PSS functioned as a binder to stick CdS NWs together (Figure S8a). In summary, the PEDOT:PSS weakens the photocurrent while cutting off the PPC tail. This effect could be caused by certain recombination centers provided by the PEDOT:PSS.

Figure 4. Photoresponse of CdS NWs/PEDOT:PSS hybrid photodetectors at the bias of 5 V and under the incident power of 60 mW. (a)-(b) Comparison between devices with electrode gaps of 175 µm with and without PEDOT:PSS (at the dilution ratio of 6:1). (c) Transient responses with higher time resolutions recorded by an oscilloscope in series with a photodetector at a frequency


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of 1000 Hz. (d) The same device with an electrode gap of 250 µm in air and after PDMS encapsulation.

Figure 5. (a) Box-plot of the dark current and photocurrent for devices with various dilutions of PEDOT:PSS. The electrode gap is fixed at 175 µm, and the incident power is 40 mW at a bias of 5 V. The data for each box-plot was gathered from an on/off switching cycle. (b) Photoresponse (plotted on a logarithmic scale) of the devices with various dilutions of PEDOT:PSS. (c) Photoluminescence spectra of the CdS NWs/PEDOT:PSS hybrid layer with different dilution ratios. (d) Schematic of band diagram at the interface of CdS NWs and PEDOT:PSS. (e) Micromorphology of the CdS NWs network coated with undiluted (left) and diluted PEDOT:PSS (at the dilution ratio of 6:1, right). The scale bar in the SEM image is 1 µm. 3.4 Better performance brought by Schottky contact



Besides fast dynamic response, a high photo-to-dark current ratio (on/off ratio) is another essential parameter in the application for fast optical switches. The selection of electrode materials has a significant impact on the on/off ratio. Schottky contact-based devices (SCD) usually exhibit high on/off ratios and short response and decay times due to the strong built-in field at the electrode-semiconductor interface.37-38 To improve the performance of our device, we replaced the AgNWs electrode with a gold (Au) electrode formed by thermal evaporation. Furthermore, the difference between transferring NWs electrode and thermal-evaporated electrode in the second transfer can also be demonstrated. Tape-based photodetectors using Au electrodes show much better performances than those using AgNWs electrodes after introducing PEDOT:PSS (at a dilution ratio of 6:1). As seen from the current versus voltage curves in Figures 6a and 6b, the device using the Au electrode has a more pronounced Schottky rectifying characteristic. Under the same intensity of blue light (450 nm, with the power of 40 mW), the device with the Au electrode has a large photocurrent that is nearly two times that of the device using the AgNWs electrode when the bias voltage is greater than 2 V. Compared with blue light, green light (wavelength = 520 nm, with the power of 40 mW) generates a much weaker photocurrent in both devices. This limited visible light response range can be broadened by element doping.34 Especially, in the devices without PEDOT:PSS, as shown in Figure 6c, the device using an Au electrode shows a relatively shorter decay time than that of the device using the AgNWs electrode. After introducing PEDOT:PSS with the dilution ratio of 6:1, the rise time and decay time of the device using the Au electrode were 4.59 ms and 6.64 ms (Figure S7c), respectively. The advantage of SCD is to suppress the high dark current to improve the on/off ratio. The relationship between the on/off ratio and the applied voltage is given in Figure 6d. For the AgNWs electrode, the highest on/off ratio, (639), appears at the bias


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of 5 V. For the Au electrode, the highest on/off ratio increases dramatically to 6129 due to the low dark current of 11 nA when the bias is 2 V (see Table S2, S3). To clearly demonstrate the photoresponse

performance

of

the

tap-based

photodetector,

a

comparison

between

photodetectors with different structures of NWs arrays and networks is shown in Table 1. Thus, in addition to the intrinsic photoelectric property of the NWs, the recipe-optimized PEDOT:PSS and SCD can significantly improve the performances of our tape-based photodetectors, which may also be effective in other material systems. Despite the excellent performance brought by thermal-evaporated Au electrode, limitations still exist in the device using Au electrode for the potential application in flexible electronics. The Au electrode has a more compact structure, leaving no spare adhesive area. As shown in Figure S8a, b, the PEDOT:PSS/CdS NWs hybrid layer was completely transferred onto the tape with an AgNWs electrode. In contrast, for the tape with an Au electrode, the hybrid layer was confined to the electrode gap, leading to some local disconnections between CdS NWs and the Au electrode (Figure S9). The remained adhesion of the tape covered with AgNWs can be attributed to the network nanostructure where the adhesive can be exposed from the network voids (Figure S10c). For the application in flexible devices, bending tests were conducted (Figure S11-S13). The photocurrents of the device using Au electrode decay faster than those of the device using AgNWs electrode after hundreds of bending. This might be assigned to the appearance of cracks in Au electrodes during the bending cycles (Figure S12d-f) as well as the local disconnections mentioned above. Therefore, using Au NWs electrode would be a better strategy for flexible photodetectors, and we will continue this study, including synthesis of Au NWs and developing a novel patterning method like AgNWs, in our future work.



Figure 6. Comparison between the devices using AgNWs and Au electrodes. The AgNWs electrode gap was fixed at 175 µm, and the same gap was used for the Au electrode. (a)-(b) I-V curves of devices using AgNWs (left) and Au electrode (right) under the incident power of 40 mW. The dilution ratio of the added PEDOT:PSS aqueous solution was 6:1. (c) The comparison of photoresponse between devices (without PEDOT:PSS) using AgNWs and Au electrodes under the incident power of 60 mW and at a bias of 5 V. (d) The on/off ratio for devices using AgNWs and Au electrodes in the range of 0.5 V to 6 V under the incident power of 60 mW. The dilution ratio of the added PEDOT:PSS aqueous solution was 6:1.


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Table 1. Summary of on/off ratio, rise time, and decay time in this work and various previous studies. Photodetectors

I light/I dark

Rise time

Decay time

Aligned CdS NWs network

200

0.8 ms

240 ms

21

CdS nanotubes network

4016

820 ms

630 ms

5

CdS NWs network

741

400 ms

700 ms

5

Zn2GeO4 NWs network

3.8

300 ms

200 ms

19

Single CdS nanowire

183

320 ms

37

Aligned InP NWs array

19.4

80 ms

2080 ms

39

60 ms

2100 ms

39 40

Aligned GaP NWs array

Reference

TiO2 nanotube array

2.5×105

500 ms

700 ms

CdS NWs network

639

11.5 ms

11.59 ms

This work (AgNWs electrode)

CdS NWs network

6129

4.59 ms

6.64 ms

This work (Au electrode)

4. CONCLUSION We have developed a facile transferring process to fabricate tape-based CdS NWs photodetectors. By utilizing the strong adhesion of tape, the functional layers were integrated into devices. This method demonstrated more design flexibility for building multilayer structures as well as the ability to integrate thin films formed by non-compatible processes. In addition, to eliminate the undesired PPC tail, the conductive polymer PEDOT:PSS was introduced to form a CdS NWs/PEDOT:PSS hybrid structure, resulting in the suppression of photocurrents and the disappearance of the PPC tail. By optimizing the concentration of PEDOT:PSS solution, a large



loss of photocurrent was avoided, and the device showed an ultrashort decay time. Moreover, higher on/off ratios were achieved when the Schottky contact was introduced by an Au electrode. ASSOCIATED CONTENT Supporting Information. Preparation of AgNWs, SEM images and XRD pattern of CdS NWs, statistics of photocurrents for ten devices, the rise time and decay time of devices using AgNWs and Au electrodes, photographs of the second tape transfer, sheet resistance of AgNWs electrode before and after tape transfer, device bending flexibility tests, long time illumination tests, comparison of the photoresponse before and after introducing PEDOT:PSS, the effect of the CdS thickness on the performance of the device, tables of the dark currents and photocurrents. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *(J. -Z. Li) Email:[email protected] *(B. -R. Yang) Email:[email protected] Author Contributions All the experimental design, material synthesis, measurements, and data analysis were conducted by Li Wang. 3D stereogram drawing was done by Peng Chen. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes


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The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by the grant from National Natural Science Foundation of China (61307027), the National High Technology Research and Development Program of China “863 Programs” (2015AA033408), the Economic and Information Industry Commission of Guangdong Province, P. R. China (20140401), and the Science and Technology Program of Guangdong Province (2014B090914001, 2015B090915003). ABBREVIATIONS NWs, nanowires; CdS, cadmium sulfide; PEDOT:PSS, poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate; AAO, anode aluminum oxide; PL, Photoluminescence; PPC, persistent photoconductivity; PDMS, polydimethylsiloxane. REFERENCE (1) Li, L. D.; Gu, L. L.; Lou, Z.; Fan, Z. Y.; Shen, G. Z., ZnO Quantum Dot Decorated Zn2SnO4 Nanowire Heterojunction Photodetectors with Drastic Performance Enhancement and Flexible Ultraviolet Image Sensors. ACS Nano 2017, 11, 4067-4076. (2) Wang, X. F.; Song, W. F.; Liu, B.; Chen, G.; Chen, D.; Zhou, C. W.; Shen, G. Z., High-Performance Organic-Inorganic Hybrid Photodetectors Based on P3HT:CdSe Nanowire Heterojunctions on Rigid and Flexible Substrates. Adv. Funct. Mater. 2013, 23, 1202-1209.



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TOC

We report a full-nanowire photodetector fabricated on tape. The innovation of this work is utilizing a facile tape transfer method to integrate films formed different processes into one device. Additionally, the undesired persistent photoconductivity, a common phenomenon in photodetectors, was eliminated by introducing PEDOT:PSS onto the CdS NWs film. Finally, a device with a fast response, ultrashort decay time, and relatively large photocurrent was obtained. The decay times were 11.59 ms and 6.64 ms for devices using electrodes of silver NWs and gold, respectively.


Tape-Based Photodetector: Transfer Process and Persistent

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