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Abstract
Flexible ultraviolet (UV) photodetectors are considered as potential building blocks for future-oriented photoelectric applications such as flexible optical communication, image sensors, wearable devices and so on. In this work, high-performance UV photodetector was fabricated via a facile combination of single ZnO microwire (MW) and p-type polyaniline. Due to the formation of effective organic/inorganic p-n junction, the as-prepared flexible UV photodetector based on ZnO MW/polyaniline hybrid heterojunction exhibits high performance (responsivity ∼ 60 mA/W and detectivity ∼ 2.0 ×1011 Jones) at the reverse bias of −1 V under the UV illumination. The ZnO MW/polyaniline photodetector displays short response/recovery times (∼ 0.44 s/∼ 0.42 s), which is less than that of most reported UV photodetectors based on ZnO/polymer heterojunction. The fast response speed and recovery speed can be attributed to the high crystallinity of ZnO MW, built-in electric field in space-charge region and the passivation of oxygen traps on the surface. Further, the photodetector using ZnO MW/polyaniline junctions shows excellent flexibility and stability under bent conditions. This work opens a new way to design next-generation high-performance, low-cost and flexible optoelectronic devices for lab-on-a-chip applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-performance photodetectors play an increasingly important role in novel optoelectronic applications, among which ultraviolet (UV) photodetectors are a key element for various devices, such as missile warning, health care, fire monitoring, communication and so on [1–5]. To meet the demand of next-generation miniaturized UV photodetectors, one-dimensional (1D) nano/micro-structures are potential candidates owing to their unique optoelectronic properties and high surface-to-volume ratio of the active area by comparing with the bulk materials [6–8]. ZnO micro/nanostructures have been applied to developing UV photodetectors widely because of its proper broad bandgap ($\sim$ 3.37 eV), large exciton binding energy ($\sim$ 60 meV) and quantum confinement effect [9–12]. Many attempts have been made to enhance the photodetecting behavior of 1D ZnO micro/nanostructures by designing various device structures including the photoconductive type [13–15], the Schottky junction type [5,14,16–18] and the p-n junction type [19,20]. Among these structures, the photodetectors based on p-n junction attract intensive research interest owing to its unique carrier transport mechanism [21–23]. When a junction-based device is operated under illumination, the photo-induced carrier can be separated by the inner electric field rapidly in the depletion region leading to high photocurrent and rapid response [24–26]. However, it is difficult to achieve high-performance photodetectors constructed from ZnO homojunction due to the lack of high quality p-type ZnO [27,28]. Thus, the as-prepared ZnO micro/nanostructures combined with other p-type materials have been regarded as an alternative way to realize ZnO heterojunction-based detectors [24,29–31].
Recently, flexible photoelectronic devices are thriving rapidly because of the urgent demand for foldable, portable and wearable applications [32–35]. Most of the attention has been attracted to the stability and flexibility of these devices with high efficiency and versatility. Combinations of ZnO micro/nanostructures with p-type polymers are promising candidates in flexible photoelectronic applications such as solar photovoltaic, light-emitting diodes and light sensors [6,36]. Furthermore, p-type conducting polymer, such as polyaniline, poly(9-vinylcarbazole) and poly(9,9-dihexylfluorene), has many advantages including solution processability, high mobility and tunable carrier concentration, which is suitable for utilization in the high-performance photoelectronic devices [8,37]. Inorganic/organic hybrid photodetectors based on ZnO nanostructures and polyaniline heterojunction have been reported widely [38–40]. Being similar to all-inorganic heterojunction, the carrier transport mechanism in inorganic/organic heterojunction can be an effective method to direct the movement of the photo-induced carriers and enhance high-performance photoresponse. However, the long response time (from seconds to dozens of minutes) restricts the practical applications of these flexible ZnO nanostructures/polymers devices because of O$_{2}$ adsorption/desorption in the photo response process [36,38]. Moreover, most of the reported heterostructures based on ZnO nanostructures/polymer are constructed via surface coating of the polymer on the ZnO nanorod arrays [27,30,41,42]. The relatively fixed structure leads to low flexibility of the prepared devices and the cumbersome fabrication process limits their practical applications [23,37,43].
In this work, individual ZnO microwires (MWs) via high crystal quality were synthesized by using chemical vapor deposition (CVD) method. A simple method of constructing flexible UV photodetectors based on single ZnO MW/polyaniline organic-inorganic hybrid heterostructures was demonstrated. The fabricated single MW-based photodetector has a high UV photoresponsivity and a rapid response speed because of the superior crystal quality of the as-synthesized samples, the efficiently formed p-n junction and the passivation of oxygen traps on the surface of ZnO MWs. Furthermore, the fabricated ZnO MW/polyaniline photodetectors show high flexibility and stability under bent conditions. The experimental findings will provide a workable scheme to design simple, low-cost and efficient functional UV photodetectors, having promising applications in fields including industrial control, space optical communication and fire monitoring.
2. Experimental section
2.1 Device fabrication
For the synthesis of polyaniline, 20 mL H$_{ 2 }$SO$_{4}$ solution (1 M) was added to a 50 mL flask and cooled to 0 $^{\circ }$C for 1 h in an ice bath. The aniline (28 $\mu$L) was added dropwise under gentle stirring, subsequently. The dissolved aniline solution was injected quickly into an ice-cold $K_{2}$S$_{2}$O$_{8}$ (20 mL, 3.5 mg/mL solution in H$_{2}$SO$_{4}$ (1 M) under vigorous stirring for 40 s. The resultant solution was left undisturbed at 0 $^{\circ }$C for 12 h. Finally, the produced polyaniline was gathered through centrifuged and drying process. A polyaniline solution was prepared by dissolving polyaniline (6 mg) into formic acid (3 ml). Individual ZnO MWs employed in this study were synthesized via CVD method, as we reported previously [9]. For the synthesis of individual ZnO MWs, high purity powders of ZnO and graphite (C) with the weight ratio of 1:1 were mixed, and then used as the precursors [44]. A flexible polyethylene terephthalate (PET) substrate with 25 mm length, 1 cm width, and 0.3 mm thickness was treated by plasma. An individual high-quality ZnO MW was picked out and then moved onto the PET. An indium (In) particle was used to fix one end of the MW serving as the cathode. A portion of MW was deposited by the polyaniline solution subsequently. Finally, the In electrode was fixed on the polyaniline to construct a ZnO MW/polyaniline heterojunction photodetector. To avoid the movement of contact area in the testing process, the edge of electrode was fixed by thin epoxy film.
2.2 Device characterization
The morphology properties of ZnO MW were characterized by scanning electron microscope (SEM). X-ray diffraction (XRD) was employed to analyze the crystal structure of the ZnO MWs. Photoluminescence (PL) measurements of the as-synthesized ZnO MWs were performed by using a He-Cd laser (the wavelength of 325 nm) employing as the excitation source via LABRAM-UV Jobin Yvon spectrometer. Optical response of polyaniline films was investigated by a Raman spectroscopy and ultraviolet-visible (UV-vis) spectrophotometer. Electrical characteristics and photoelectric performance of the fabricated Zn MW/polyaniline heterojunctions were characterized by a photoelectric detection system consisting of a Xe lamp, monochromator, chopper and semiconductor analysis device (Keithley B1500A). All the measurements were measured in air at room temperature.
3. Results and discussions
As we previously reported, individual ZnO MWs were successfully synthesized by using CVD method [9,44]. The samples were collected around Si-substrate. Figure 1(a) illustrates the optical image of as-prepared samples. The length of wires is about 1 cm, and the longest samples can reach up to 2 cm. Figure 1(b) shows a typical SEM image of ZnO MW, illustrating hexagonal cross-section and straight sidewalls. It is found that the ZnO MW has smooth facets and its diameter is $\sim$ 10 $\mu$m. The XRD pattern demonstrates that individual ZnO MWs show a wurtzite-type ZnO structure (see Fig. 1(c)). The typical intense peak at (002) plane indicates that the samples are crystalline with a preferential growth in the [001] direction [7,10]. Figure 1(d) displays PL spectrum of a single ZnO MW, in which a typical near-band-edge recombination emission is at $\sim$ 380 nm [9,45]. Besides, a much weaker and negligible visible emission band is also observed in the sample, which can be originated from deep-level emission relative to intrinsic defects [11,46]. It is indicated that the ZnO MW has good crystal structure and relatively low concentration of surface defects, and is appropriate for the construction of high-performance photodetector to detect UV light signals [10,46]. Meanwhile, optical characterization of polyaniline film was tested. Figure 1(e) shows that the polyaniline film has transmittance of over 40$\%$ in the UV-vis region. The Raman spectrum, which can prove that the bond vibration modes of polyaniline film, is shown in Fig. 1(f). The characteristic peaks at 1188 cm$^{-1}$, 1340 cm$^{-1}$, 1482 cm$^{-1}$, and 1606 cm$^{-1}$ are originated from C-H bending of benzene rings, C-N$^{+}$ stretching vibration, C=C stretching of the quinoid ring, and C-C stretching of the benzene ring, respectively. It reveals that polyaniline film was successfully prepared [47,48].
Fig. 1. Morphology and structure of the as-synthesized ZnO MWs: (a) The photograph of the as-synthesized ZnO MWs. The scale bar is 2 cm. (b) SEM image of a ZnO MW. The scale bar is 10 $\mu$m. (c) XRD pattern of ZnO MWs. (d) PL spectrum of a ZnO MW. (e) Transmittance spectrum of the prepared polyaniline film. (f) Raman spectrum of polyaniline film.
The schematic diagram illustration in Fig. 2(a) shows a flexible UV photodetector device based on ZnO MW/polyaniline heterojunction. The fabrication procedure in details can be obtained in the experimental section. Figure 2(b) represents an optical image of a flexible ZnO MW/polyaniline photodetector. The SEM image of the ZnO MW/polyaniline junction boundary is also demonstrated in the inset of Fig. 2(b). Clearly, a segment of ZnO MW is covered by polyaniline film and the fabricated single ZnO MW based UV photodetector shows good flexibility. Electrical characterization of the ZnO MW/polyaniline photodetector was carried out. In the device configuration, ohmic contacts formed between the semiconductor and electrodes play a key role in constructing high-performance photodetectors. The contacting behavior between In electrodes and ZnO MW (polyaniline) were studied through electrical measurement. Both In electrodes were applied onto two ends of ZnO MW to form metal-semiconductor-metal (MSM) type structure. The current-voltage ($I$-$V$) characteristic curve of a single ZnO MW is shown in Fig. 2(c). The linear and symmetric $I$-$V$ curve in darkness reveals that ZnO MW and In electrodes are under ohmic contact. $I$-$V$ characteristic curve of the In-polyaniline-In plotted in Fig. 2(d) suggests that good ohmic contact between polyaniline and In electrodes was also formed. In addition, the $I$-$V$ curve remains unchanged under light illumination, indicating that the polyaniline film exhibits no photosensitivity to ultraviolet irradiation and visible light. The polyaniline polymer in ZnO MW-based ultraviolet photodetectors acts as a hole transporting channel [6,8].
Fig. 2. (a) Schematic illustration of UV photodetector, which made of a single ZnO MW and p-polyaniline polymer. (b) Optical image of ZnO MW/polyaniline photodetector structure. The inset is SEM image of ZnO MW/polyaniline junction. (c) $I$-$V$ curve of a single ZnO MW, with In particles serving as electrodes. (d) $I$-$V$ curves of the In-polyaniline film-In under the dark and light illumination via various incident wavelengths. (e) Logarithmic $I$-$V$ curves of the ZnO MW/polyaniline heterostructure device in darkness and under the UV irradiation (the pumping power of 10.0 mW/cm$^{2}$). The inset is $I$-$V$ curves of the ZnO MW/polyaniline heterostructure device in darkness and under the UV irradiation condition. (f) $I_{\mathrm {ph}}$/$I_{\mathrm {d}}$ ratio of the fabricated ZnO MW/polyaniline heterostructure.
Figure 2(e) displays the logarithmic $I$-$V$ curves of the fabricated ZnO MW/polyaniline heterojunction in darkness and under UV irradiation. It is clearly seen that the device displays a significant rectification characteristic with low dark current of 10$^{-10}$ A at −3 V. The rectification ratio is extracted to be $\sim$ 2 $\times$ 10$^{3}$ at $\pm$3 V. The results mentioned above confirm that the rectification characteristics originate from the junction between ZnO MW and polyaniline. According to thermionic emission model, the $I$-$V$ characteristic can be obtained from [49]:
where $I_{0}$, $q$, $V$, $k_{B}$ and $T$ is the reverse saturation current, the elementary charge, the forward voltage, the Boltzmann constant and the temperature, respectively. $n$ is the quality ideality factor, which is determined from the slope of the semi-logarithmic $I$-$V$ curves at forward bias. According to Eq. (2) and Fig. 2(e), the slope $d \ln I / d V$ at low voltage can be estimated and the ideality factor is calculated to be $\sim$ 2.17. For an ideal p-n junction, the calculated ideality factor should be one at a relatively lower voltage according to the Sah-Noyce-Shockley theory [49]. The deviation of ideality factor for the fabricated ZnO MW/polyaniline heterojunction may be derived from a small mount of surface defects, which mediates the recombination of electrons and holes in the space charge region [50]. Significantly, the ideality factor obtained from ZnO MW/polyaniline photodetector is closer to the ideal value compared with other ZnO-based heterojunction, indicating the high quality of as-prepared heterojunction [51–53]. When the photodetector was exposed in the UV illumination (the incident wavelength of 365 nm, 10.0 mW/cm$^{2}$), the photocurrent $I_{\mathrm {ph}}$ is rapidly enhanced to $\sim$ 10$^{-7}$ A at −3 V. At near zero bias, the $I_{\mathrm {ph}}$/$I_{\mathrm {d}}$ ratio of ZnO MW/polyaniline photodetector can reach up to $\sim$ 10$^{4}$ shown in Fig. 2(f), which indicates that the as-prepared device exhibits high photosensitivity to UV irradiation.To evaluate the device performance of the fabricated ZnO MW/polyaniline photodetector, $I$-$V$ tests were performed under 365 nm light irradiation by varying intensities from 0 to 10.0 mW/cm$^2$. The photocurrent highly depends on the light power intensity and enhances monotonously with the increase of light power intensity (see Fig. 3(a)). The dependence of photocurrent on light intensity at −1 V is further displayed in Fig. 3(b). The photocurrent ($I_{\mathrm {ph}}$) observed in devices is described by a power law [54]:
In the formula, $\alpha$ is power-law factor. The $I_{\mathrm {ph}}$ almost enhances linearly with the light power intensity and the $\alpha$ is calculated to be $\sim$ 0.89. It is indicated that fewer carrier trap states exist in the ZnO MW and the incident photons are almost absorbed in the ZnO MW to produce photocurrent under different light intensity [15,55]. The responsivity ($R$) of the ZnO MW/polyaniline photodetector is obtained from:
where $S$ is the effective cross-sectional area ($\sim$ 1.2 $\times$ 10$^{-4}$ cm$^{2}$). According to the Eq. (3), the responsivity $R$ was estimated to be over 200 mA/W under 365 nm UV irradiation (0.1 mW/cm$^{2}$) at −3 V bias. The three-dimensional responsivity mapping of the ZnO MW/polyaniline photodetector in Fig. 3(c) distinctly demonstrates that the responsivity can be modulated by the light intensity and different biases. Apart from the responsivity, other important parameters, such as the detectivity ($D^{*}$) and the linear dynamic range ($LDR$), can be defined as where $e$ is the elementary charge. The detectivity can reach up to $\sim$ 3.0 $\times$ 10 $^{11}$ Jones when the device is operated under the bias of −1 V, confirming that the fabricated ZnO MW/polyaniline heterojunction can capture the light signals of relatively weak UV irradiation (see Fig. 3(d)). The maximum $LDR$ of the ZnO MW/polyaniline photodetector is estimated to be $\sim$ 64 dB at 365 nm, which is close to the that of commercial InGaAs photodetectors ($\sim$ 66 dB) [1,33]. The high value of the $LDR$ reveals a large signal to noise ratio of the ZnO MW/polyaniline photodetectors.Fig. 3. (a) Logarithmic $I$-$V$ characteristics of the ZnO MW/polyaniline photodetector under different light intensities. (b) Light intensity-dependent photocurrents of the devices at the bias of −1 V. (c) Three-dimensional responsivity map of a ZnO MW/polyaniline photodetector. (d) The detectivity ($D^{*}$) and LDR of ZnO MW/polyaniline photodetector under different light intensities. (e) The spectral responsivity of ZnO MW/polyaniline photodetector devices at −1 V. (f) The spectral detectivity of ZnO MW/polyaniline photodetector devices at −1 V.
To further investigate the wavelength-dependent selectivity of the fabricated ZnO MW/polyaniline heterojunction, the responsivity of the photodetector in the wavelength varying from 300 to 600 nm is depicted in Fig. 3(e). The detector exhibits photoresponse to UV irradiation obviously and a extremely weak photoresponse in the visible region, revealing that the ZnO MW/polyaniline photodetector possesses a large sensitivity to the UV irradiation, while is blind to visible light. To better evaluate the photodetecting performance of ZnO MW/polyaniline heterojunction, external quantum efficiency ($EQE$) was calculated at −1 V bias. The $EQE$ is the ratio of photo-induced charge carriers and the excitation photons, which can be obtained from [55]
where $h$, $c$ and $\lambda$ is Planck’s constant, the speed of light and the incident light wavelength, respectively. $EQE$ being equal to 20%, 10%, and 1%, respectively, is plotted as a function of light wavelength (see Fig. 3(e)). It is clearly illustrated that $EQE$ of the fabricated ZnO MW/polyaniline photodetector is larger than 20% in ultraviolet region, whereas $EQE$ is smaller than 1% when the device is operated under visible light. The fabricated ZnO MW/polyaniline photodetector has high photoelectric conversion to ultraviolet light rather than visible light. $EQE$ obtained in this work is relatively lower than that of a conventional semiconductor photodetector in ultraviolet region, which can be attributed to the low transmittance of polyaniline and surface defect density of ZnO microwires [11]. Figure 3(f) displays the wavelength-dependent detectivity of ZnO MW/polyaniline UV photodetector at −1 V bias, and the maximum detectivity can arrive $\sim$ 2.0 $\times$ 10 $^{11}$ Jones, confirming the photodetector’s capability to detect weak signals.The reproducibility and stability are also the key performance of the fabricated UV photodetectors. The current-time ($I$-$t$) curve of the fabricated ZnO MW/polyaniline photodetector is obtained under various light intensities at −1 V bias in Fig. 4(a). The photocurrent increases to a stable value rapidly once UV illumination is switched on, and subsequently declines to initial value quickly in the darkness, indicating superior reproducibility and stability of as-prepared devices. Furthermore, the photocurrent increases step by step when the UV light intensity increases gradually, whereas the response speed remains almost invariant. The corresponding rise and decay time of the ZnO MW/polyaniline photodetector are calculated to be $\sim$ 0.44 s and $\sim$ 0.42 s, respectively, seen in Fig. 4(b). It should be noted that the response time is much shorter through considering the limit of time-resolution in testing process. The response speed of ZnO MW/polyaniline photodetector is faster than that of previously reported ZnO/polymer-based photodetectors, whose response times varying from seconds to minutes (see Table 1). In general, photo-induced electrons and holes are produced upon UV irradiation. Subsequently the holes migrate to the surfaces forced by the depletion field and then combine with $\mathrm {O}_{2}^{-}$ ($\mathrm {h}^{+}+\mathrm {O}^{2-} \rightarrow \mathrm {O}_{2}$) [19,27,46]. The corresponding procedure is displayed in Fig. 4(c). The unpaired electrons enhancing the carrier concentration in ZnO MW can contribute the photocurrent. As the UV light is switched off, oxygen-related intrinsic defect states re-adsorbs the carriers on the facets of ZnO MW, making the device return to its initial state [15,42,56]. The O$_{2}$ adsorption/desorption in the surfaces of ZnO MW will take a great deal of time, and contribute to poor photoresponse speed of ZnO-based photodetectors [57]. However, for ZnO MW/polyaniline heterojunction based UV photodetector, the photo-generated hole trapping process can be eliminated by the passivation effect owing to the cover of polyaniline [36,42]. Meanwhile, the carriers generated in the depletion layer will be separated by the inner electric field, leading to fast response speed, as shown in Fig. 4(d).
Fig. 4. (a) $I$-$t$ curve of a ZnO MW/polyaniline photodetector under different power light intensities. (b) Time response of the ZnO MW/polyaniline photodetector at −1 V bias. (c) The top drawing displays oxygen molecules adsorbed at the ZnO MW surfaces, that can capture the free electron forming $\mathrm {O}_{2}^{-}$. The bottom drawing displays photo-inducing holes are trapped by $\mathrm {O}_{2}^{-}$, leaving unpaired electrons in the ZnO MW. (d) The top drawing displays the transport process of photo-generated carriers in ZnO MW/polyaniline UV photodetector. The bottom drawing displays the corresponding cross section of the photodetector. (e) Energy band diagram of the ZnO MW/polyaniline heterojunction at equilibrium. (f) Energy band diagram of the ZnO MW/polyaniline heterojunction at negative bias.
Table 1. A comparison between the single ZnO MW/polyaniline photodetector in this work and other previous works
The detailed operational mechanism of the ZnO MW/polyaniline photodetector is further illustrated through the energy band diagram. The electron affinities of ZnO and polyaniline are 4.5 and 3.2 eV, respectively. The bandgap of ZnO and polyaniline are 3.37 and 2.76 eV, respectively [8,16]. When the polyaniline and ZnO MW contact in the darkness, holes can diffuse from polyaniline to ZnO MW; while the transport of electrons moves in the opposite direction, giving rise to aligned Fermi levels and the formation of the band bending at the junction (see Fig. 4(e)). When the fabricated ZnO MW/polyaniline photodetector is irradiated by UV light, photo-created carriers will be formed in the junction. Due to the inner electric field, the photo-induced holes move from the valence band of ZnO to the highest occupied molecular orbital (HOMO) of polyaniline attached to the anode [8]. The electrons can not transfer from the conduction band to the lowest unoccupied molecular orbital (LUMO) due to the potential barrier, which will be directly collected by the cathode [32]. Furthermore, the enhanced barrier height will broaden the depletion layer and increase the strength of inner electric field when the photodetector is applied by negative bias. The photo-created carriers can be separated efficiently and the recombination in the junction can be reduced, which significantly increases the photocurrent (see Fig. 4(f)).
A bending test was performed on the ZnO MW/polyaniline photodetector by varying the bending angles and cycles, as shown in Fig. 5(a). In the bending test, the $I_{\mathrm {ph}}$ of flexible ZnO MW/polyaniline photodetector decreases slightly with the increase of the bending angle (Shown in Fig. 5(b)). In addition, the $I_{\mathrm {d}}$ exhibits a declining tendency and large fluctuation with the increase of bending angle, which may be originated from a tiny change in the contact area or the piezoelectric effect [1,33,51]. The $R$ and $D^{*}$ are calculated in Fig. 5(c). It is clearly seen that $R$ and $D^{*}$ remain a high level when the bending angle is below 75$^{\circ }$. The performance of photoresponse was measured after cycles of repeated bending test. As shown in Fig. 5(d), the $I_{\mathrm {ph}}$ and $I_{\mathrm {d}}$ show a substantial decrease in the first 50 bending cycles and subsequently level off until 300 bending cycles. After 400 bending cycles, the declined $I_{\mathrm {ph}}$ and increased $I_{\mathrm {d}}$ indicate the device failure. Accordingly, the $R$ declines rapidly in the first 50 bending cycles and then remains almost unchanged before a sharp drop after 400 bending cycles (See Fig. 5(e)). The $LDR$ of the ZnO MW/polyaniline photodetector has a high level until the device is broken down. Considering the fabricated photodetector is simple and unprotected, the device performance during bending tests is satisfactory and can further be enhanced in future study.
Fig. 5. Photodetecting performance of flexible photodetector based on ZnO MW/polyaniline heterojunction. (a) The optical image of a flexible ZnO MW/polyaniline photodetector on PET substrate. (b) $I$-$t$ curve of the flexible ZnO MW/polyaniline photodetector under 365 nm UV switching (3.0 mW/cm$^{2}$) at −1 V bias with various bending angles. (c) The responsivity ($R$) and detectivity ($D^{*}$) of ZnO MW/polyaniline photodetector under various bending angles. (d) $I$-$t$ curve of the flexible ZnO MW/polyaniline photodetector under 365 nm UV switching (3.0 mW/cm$^{2}$) at −1 V bias after bending cycles. (e) The responsivity ($R$) and LDR of ZnO MW/polyaniline photodetector with increasing bending cycles.
4. Conclusion
In summary, a flexible UV photodetector device consisting of a ZnO MW/polyaniline heterostructure was fabricated. The photodetector device exhibits high UV detecting performance (responsivity $\sim$ 60 mA/W, detectivity $\sim$ 2.0 $\times$ 10$^{11}$ Jones, the rise time $\sim$ 0.44 s and the decay time $\sim$ 0.42 s) under the condition of 365 nm radiation and −1 V bias. The superior photodetecting performance can be attributed to the high crystal quality of the as-synthesized ZnO MWs, the effective hybrid p-n junction in the photodetector and the passivation of oxygen traps on the surfaces of the wires, facilitating the separation and transport of photo-induced carriers. Further, the ZnO MW/polyaniline photodetectors show superior flexibility and stability under bent conditions. The successful fabrication of flexible ZnO MW/polyaniline photodetectors demonstrates the potential of developing organic/inorganic-based heterojunction on other flexible substrates by other methods, including spraying, spin coating and self-organization, which accelerates the blossom of flexible optoelectronic devices.
Funding
National Natural Science Foundation of China (11774171, 11874220, 11974182, 21805137).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. S. Li, Y. Zhang, W. Yang, H. Liu, and X. Fang, “2D perovskite Sr2Nb3O10 for high-performance UV photodetectors,” Adv. Mater. 32(7), 1905443 (2020). [CrossRef]
2. C. Lin, Y. Lu, Y. Tian, C. Gao, M. Fan, X. Yang, L. Dong, and C. Shan, “Diamond based photodetectors for solar-blind communication,” Opt. Express 27(21), 29962–29971 (2019). [CrossRef]
3. C. H. Kang, I. Dursun, G. Liu, L. Sinatra, and B. S. Ooi, “High-speed colour-converting photodetector with all-inorganic CsPbBr3 perovskite nanocrystals for ultraviolet light communication,” Light: Sci. Appl. 8(1), 94 (2019). [CrossRef]
4. L. Chen, C. Yang, and C. Yan, “High-performance UV detectors based on 2D CVD bismuth oxybromide single-crystal nanosheets,” J. Mater. Sci. Technol. 48, 100–104 (2020). [CrossRef]
5. F. Liang, M. Feng, Y. Huang, X. Sun, X. Zhan, J. Liu, Q. Sun, R. Wang, X. Ge, J. Ning, and H. Yang, “AlGaN-based schottky barrier deep ultraviolet photodetector grown on Si substrate,” Opt. Express 28(12), 17188–17195 (2020). [CrossRef]
6. H. Wang, H. Chen, L. Li, Y. Wang, L. Su, W. Bian, B. Li, and X. Fang, “High responsivity and high rejection ratio of self-powered solar-blind ultraviolet photodetector based on PEDOT:PSS/β-Ga2O3 organic/inorganic p–n junction,” J. Phys. Chem. Lett. 10(21), 6850–6856 (2019). [CrossRef]
7. F. Khan, H. M. S. Ajmal, K. Nam, and S.-D. Kim, “Enhancement in the photonic response of ZnO nanorod-gated AlGaN/GaN HEMTs with N2O plasma treatment,” Opt. Express 28(19), 27688–27701 (2020). [CrossRef]
8. Y. Wang, L. Li, H. Wang, L. Su, H. Chen, W. Bian, J. Ma, B. Li, Z. Liu, and A. Shen, “An ultrahigh responsivity self-powered solar-blind photodetector based on a centimeter-sized β-Ga2O3/polyaniline heterojunction,” Nanoscale 12(3), 1406–1413 (2020). [CrossRef]
9. M. Jiang, G. He, H. Chen, Z. Zhang, L. Zheng, C. Shan, D. Shen, and X. Fang, “Wavelength-tunable electroluminescent light sources from individual Ga-Doped ZnO microwires,” Small 13(19), 1604034 (2017). [CrossRef]
10. G.-D. Yuan, W.-J. Zhang, J.-S. Jie, X. Fan, J.-X. Tang, I. Shafiq, Z.-Z. Ye, C.-S. Lee, and S.-T. Lee, “Tunable n-type conductivity and transport properties of Ga-doped ZnO nanowire arrays,” Adv. Mater. 20(1), 168–173 (2008). [CrossRef]
11. H. Zhou, L. Yang, P. Gui, C. R. Grice, Z. Song, H. Wang, and G. Fang, “Ga-doped ZnO nanorod scaffold for high-performance, hole-transport-layer-free, self-powered CH3NH3PbI3 perovskite photodetectors,” Sol. Energy Mater. Sol. Cells 193, 246–252 (2019). [CrossRef]
12. Z. Li, M. Jiang, Y. Sun, Z. Zhang, B. Li, H. Zhao, C. Shan, and D. Shen, “Electrically pumped fabry-perot microlasers from single Ga-doped zno microbelt based heterostructure diodes,” Nanoscale 10(39), 18774–18785 (2018). [CrossRef]
13. X. Chen, F. Ren, S. Gu, and J. Ye, “Review of gallium-oxide-based solar-blind ultraviolet photodetectors,” Photonics Res. 7(4), 381–415 (2019). [CrossRef]
14. R. Tang, G. Li, C. Li, J. Li, Y. Zhang, K. Huang, J. Ye, C. Li, J. Kang, R. Zhang, and Y. Zheng, “Localized surface plasmon enhanced Ga2O3 solar blind photodetectors,” Opt. Express 28(4), 5731–5740 (2020). [CrossRef]
15. J. Dai, C. Xu, X. Xu, J. Guo, J. Li, G. Zhu, and Y. Lin, “Single ZnO microrod ultraviolet photodetector with high photocurrent gain,” ACS Appl. Mater. Interfaces 5(19), 9344–9348 (2013). [CrossRef]
16. Y. Wang, L. Zhu, Y. Feng, Z. Wang, and Z. L. Wang, “Comprehensive pyro-phototronic effect enhanced ultraviolet detector with ZnO/Ag schottky junction,” Adv. Funct. Mater. 29(5), 1807111 (2018). [CrossRef]
17. W. Jiang, T. Zheng, B. Wu, H. Jiao, and J. Wang, “A versatile photodetector assisted by photovoltaic and bolometric effects,” Light: Sci. Appl. 9(1), 160 (2020). [CrossRef]
18. X. Zheng, Y. Sun, H. Qin, Z. Ji, and H. Chi, “Interface engineering on ZnO/Au based schottky junction for enhanced photoresponse of UV detector with TiO2 inserting layer,” J. Alloys Compd. 816, 152537 (2020). [CrossRef]
19. Z. Wang, R. Yu, C. Pan, Z. Li, J. Yang, F. Yi, and Z. L. Wang, “Light-induced pyroelectric effect as an effective approach for ultrafast ultraviolet nanosensing,” Nat. Commun. 6(1), 8401 (2015). [CrossRef]
20. D. Wang, A. E. L. Allcca, T.-F. Chung, A. V. Kildishev, Y. P. Chen, A. Boltasseva, and V. M. Shalaev, “Enhancing the graphene photocurrent using surface plasmons and a p-n junction,” Light: Sci. Appl. 9(1), 126 (2020). [CrossRef]
21. Y. Xiang, Q. Zhou, Z. Li, Z. Cui, X. Liu, Y. Liang, S. Zhu, Y. Zheng, K. W. K. Yeung, and S. Wu, “A Z-scheme heterojunction of ZnO/CDots/C3N4 for strengthened photoresponsive bacteria-killing and acceleration of wound healing,” J. Mater. Sci. Technol. 57, 1–11 (2020). [CrossRef]
22. W. Hu, H. Cong, W. Huang, Y. Huang, L. Chen, A. Pan, and C. Xue, “Germanium/perovskite heterostructure for high-performance and broadband photodetector from visible to infrared telecommunication band,” Light: Sci. Appl. 8(1), 106 (2019). [CrossRef]
23. M. Kumar, M. Patel, T. T. Nguyen, J. Kim, and J. Yi, “High-performing ultrafast transparent photodetector governed by the pyro–phototronic effect,” Nanoscale 10(15), 6928–6935 (2018). [CrossRef]
24. A. K. Rana, M. Kumar, D.-K. Ban, C.-P. Wong, J. Yi, and J. Kim, “Enhancement in performance of transparent p-NiO/n-ZnO heterojunction ultrafast self-powered photodetector via pyro-phototronic effect,” Adv. Electron. Mater. 5(8), 1900438 (2019). [CrossRef]
25. C.-J. Wang, X. Yang, J.-H. Zang, Y.-C. Chen, C.-N. Lin, Z.-X. Liu, and C.-X. Shan, “Ultraviolet irradiation dosimeter based on persistent photoconductivity effect of ZnO,” Chin. Phys. B 29(5), 058504 (2020). [CrossRef]
26. D. Xiong, W. Deng, G. Tian, Y. Gao, X. Chu, C. Yan, L. Jin, Y. Su, W. Yan, and W. Yang, “A piezo-phototronic enhanced serrate-structured ZnO-based heterojunction photodetector for optical communication,” Nanoscale 11(6), 3021–3027 (2019). [CrossRef]
27. K. Liu, M. Sakurai, and M. Aono, “ZnO-based ultraviolet photodetectors,” Sensors 10(9), 8604–8634 (2010). [CrossRef]
28. H. Zhou, P. Gui, L. Yang, C. Ye, M. Xue, J. Mei, Z. Song, and H. Wang, “High performance, self-powered ultraviolet photodetector based on a ZnO nanoarrays/GaN structure with a CdS insert layer,” New J. Chem. 41(12), 4901–4907 (2017). [CrossRef]
29. S. K. Singh, P. Hazra, S. Tripathi, and P. Chakrabarti, “Performance analysis of RF-sputtered ZnO/Si heterojunction UV photodetectors with high photo-responsivity,” Superlattices Microstruct. 91, 62–69 (2016). [CrossRef]
30. R. Sinha, N. Roy, and T. K. Mandal, “Growth of carbon dot-decorated ZnO nanorods on a graphite-coated paper substrate to fabricate a flexible and self-powered schottky diode for UV detection,” ACS Appl. Mater. Interfaces 12(29), 33428–33438 (2020). [CrossRef]
31. L. Su, W. Ouyang, and X. Fang, “Facile fabrication of heterostructure with p-biocl nanoflakes and n-zno thin film for uv photodetectors,” J. Semicond. 42(5), 052301 (2021). [CrossRef]
32. X. Zhang, J. Li, W. Yang, B. Leng, P. Niu, X. Jiang, and B. Liu, “High-performance flexible ultraviolet photodetectors based on AZO/ZnO/PVK/PEDOT:PSS heterostructures integrated on human hair,” ACS Appl. Mater. Interfaces 11(27), 24459–24467 (2019). [CrossRef]
33. M. Chen, B. Zhao, G. Hu, X. Fang, H. Wang, L. Wang, J. Luo, X. Han, X. Wang, C. Pan, and Z. L. Wang, “Piezo-phototronic effect modulated deep UV photodetector based on ZnO-Ga2O3 heterojuction microwire,” Adv. Funct. Mater. 28(14), 1706379 (2018). [CrossRef]
34. S. Lim, D.-S. Um, M. Ha, Q. Zhang, Y. Lee, Y. Lin, Z. Fan, and H. Ko, “Broadband omnidirectional light detection in flexible and hierarchical ZnO/Si heterojunction photodiodes,” Nano Res. 10(1), 22–36 (2017). [CrossRef]
35. X. Xu, J. Chen, S. Cai, Z. Long, Y. Zhang, L. Su, S. He, C. Tang, P. Liu, H. Peng, and X. Fang, “A real-time wearable uv-radiation monitor based on a high-performance p-CuZnS/n-TiO2 photodetector,” Adv. Mater. 30(43), 1803165 (2018). [CrossRef]
36. K. Ranjith and R. R. Kumar, “Facile construction of vertically aligned ZnO nanorod/PEDOT:PSS hybrid heterojunction-based ultraviolet light sensors: efficient performance and mechanism,” Nanotechnology 27(9), 095304 (2016). [CrossRef]
37. H. Wang, G. Yi, X. Zu, P. Qin, M. Tan, and H. Luo, “Photoelectric characteristics of the p-n junction between ZnO nanorods and polyaniline nanowires and their application as a UV photodetector,” Mater. Lett. 162, 83–86 (2016). [CrossRef]
38. S. Mridha and D. Basak, “ZnO/polyaniline based inorganic/organic hybrid structure: Electrical and photoconductivity properties,” Appl. Phys. Lett. 92(14), 142111 (2008). [CrossRef]
39. M.-Y. Li, M. Yu, D. Su, J. Zhang, S. Jiang, J. Wu, Q. Wang, and S. Liu, “Ultrahigh responsivity uv photodetector based on Cu nanostructure/ZnO QD hybrid architectures,” Small 15(28), 1901606 (2019). [CrossRef]
40. M. Zheng, F. Yang, J. Guo, L. Zhao, X. Jiang, G. Gu, B. Zhang, P. Cui, G. Cheng, and Z. Du, “Cd(oh)2@zno nanowires thin-film transistor and uv photodetector with a floating ionic gate tuned by a triboelectric nanogenerator,” Nano Energy 73, 104808 (2020). [CrossRef]
41. C. Xie, P. You, Z. Liu, L. Li, and F. Yan, “Ultrasensitive broadband phototransistors based on perovskite/organic-semiconductor vertical heterojunctions,” Light: Sci. Appl. 6(8), e17023 (2017). [CrossRef]
42. C. S. Lao, M. C. Park, Q. Kuang, Y. Deng, A. K. Sood, D. L. Polla, and Z. L. Wang, “Giant enhancement in UV response of ZnO nanobelts by polymer surface-functionalization,” J. Am. Chem. Soc. 129(40), 12096–12097 (2007). [CrossRef]
43. R. S. Veerla, P. Sahatiya, and S. Badhulika, “Fabrication of a flexible UV photodetector and disposable photoresponsive uric acid sensor by direct writing of ZnO pencil on paper,” J. Mater. Chem. C 5(39), 10231–10240 (2017). [CrossRef]
44. Y. Liu, M. Jiang, G. He, S. Li, Z. Zhang, B. Li, H. Zhao, C. Shan, and D. Shen, “Wavelength-tunable ultraviolet electroluminescence from Ga-doped ZnO microwires,” ACS Appl. Mater. Interfaces 9(46), 40743–40751 (2017). [CrossRef]
45. C. Tian, D. Jiang, B. Li, J. Lin, Y. Zhao, W. Yuan, J. Zhao, Q. Liang, S. Gao, J. Hou, and J. Qin, “Performance enhancement of ZnO UV photodetectors by surface plasmons,” ACS Appl. Mater. Interfaces 6(3), 2162–2166 (2014). [CrossRef]
46. W. Ruane, K. Johansen, K. Leedy, D. C. Look, H. von Wenckstern, M. Grundmann, G. C. Farlow, and L. J. Brillson, “Defect segregation and optical emission in ZnO nano-and microwires,” Nanoscale 8(14), 7631–7637 (2016). [CrossRef]
47. M. Dhingra, L. Kumar, and S. Shrivastava, “Impact of interfacial interactions on optical and ammonia sensing in zinc oxide/polyaniline structures,” Bull. Mater. Sci. 36(4), 647–652 (2013). [CrossRef]
48. H. Chen, P. Yu, Z. Zhang, F. Teng, L. Zheng, K. Hu, and X. Fang, “Ultrasensitive self-powered solar-blind deep-ultraviolet photodetector based on all-solid-state Polyaniline/MgZnO bilayer,” Small 12(42), 5809–5816 (2016). [CrossRef]
49. C.-X. Wang, G.-W. Yang, H.-W. Liu, Y.-H. Han, J.-F. Luo, C.-X. Gao, and G.-T. Zou, “Experimental analysis and theoretical model for anomalously high ideality factors in ZnO/diamond p-n junction diode,” Appl. Phys. Lett. 84(13), 2427–2429 (2004). [CrossRef]
50. Z. Di, J. Xu, A. N. Noemaun, J. K. Kim, E. F. Schubert, M. H. Crawford, and D. D. Koleske, “The origin of the high diode-ideality factors in GaInN/GaN multiple quantum well light-emitting diodes,” Appl. Phys. Lett. 94(8), 081113 (2009). [CrossRef]
51. P. Lin, X. Yan, Z. Zhang, Y. Shen, Y. Zhao, Z. Bai, and Y. Zhang, “Self-powered UV photosensor based on PEDOT:PSS/ZnO micro/nanowire with strain-modulated photoresponse,” ACS Appl. Mater. Interfaces 5(9), 3671–3676 (2013). [CrossRef]
52. G. C. Park, S. M. Hwang, J. H. Lim, and J. Joo, “Growth behavior and electrical performance of Ga-doped ZnO nanorod/p-Si heterojunction diodes prepared using a hydrothermal method,” Nanoscale 6(3), 1840–1847 (2014). [CrossRef]
53. J. H. He and C. H. Ho, “The study of electrical characteristics of heterojunction based on ZnO nanowires using ultrahigh-vacuum conducting atomic force microscopy,” Appl. Phys. Lett. 91(23), 233105 (2007). [CrossRef]
54. H. Kind, H. Yan, B. Messer, M. Law, and P. Yang, “Nanowire ultraviolet photodetectors and optical switches,” Adv. Mater. 14(2), 158–160 (2002). [CrossRef]
55. M. Dai, H. Chen, R. Feng, W. Feng, Y. Hu, H. Yang, G. Liu, X. Chen, J. Zhang, C.-Y. Xu, and P. Hu, “A dual-band multilayer inse self-powered photodetector with high performance induced by surface plasmon resonance and asymmetric schottky junction,” ACS Nano 12(8), 8739–8747 (2018). [CrossRef]
56. C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. Aplin, J. Park, X. Bao, Y.-H. Lo, and D. Wang, “ZnO nanowire UV photodetectors with high internal gain,” Nano Lett. 7(4), 1003–1009 (2007). [CrossRef]
57. Y. Jin, J. Wang, B. Sun, J. C. Blakesley, and N. C. Greenham, “Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles,” Nano Lett. 8(6), 1649–1653 (2008). [CrossRef]
58. R. A. Talib, M. Abdullah, H. S. Al-Salman, S. M. Mohammad, and N. K. Allam, “ZnO nanorods/polyaniline heterojunctions for low-power flexible light sensors,” Mater. Chem. Phys. 181, 7–11 (2016). [CrossRef]
59. F. Teng, W. Ouyang, Y. Li, L. Zheng, and X. Fang, “Novel structure for high performance UV photodetector based on BiOCl/ZnO hybrid film,” Small 13(22), 1700156 (2017). [CrossRef]
