In recent years, the flexible and integrated ultraviolet photodetector (UV PD) has become a crucial active component for photosensing, optical communications, wearable devices, foldable displays, etc. [1–5]. To satisfy the ever-increasing demands of realistic applications, the construction of flexible and integrated photodiodes should necessarily mandate advantages such as low energy consumption, low dark current, fast response time, high mechanical stability, etc. [6–8]. In previous studies, low-dimensional organic and inorganic materials, such 0D quantum dots, 1D micro-/nanostructures, and 2D layered materials have been extensively used to fabricate flexible and integrated devices [9–15]. However, the PDs based on 0D and 2D materials have several drawbacks owing to material intrinsic problems and complex fabrication techniques [16,17]. For example, the solution-synthesized 0D materials are prone to problems such as instability, low photogenerated charge carrier density, poor charge transport, and so on, which seriously hinder further applications of quantum dot-based flexible photovoltaic devices [18–21]. Additionally, although significant progress has been achieved in developing flexible optoelectronic devices based on the 2D layered materials. The presence of poorer light absorption, large-scale, complex preparation process, difficult transfer, poor ductility, etc., also makes them unable to carry out large-area practical applications [22–24]. In contrast, 1D micro-/nanostructure materials have been widely applied in flexible detectors because of their superior mechanical stability, high carrier mobility, high surface-to-volume ratio, and simple synthesis [17,25]. In particular, assembling these 1D micro-/nanostructures into highly ordered arrays as ideal building blocks can realize the low-power, flexibility, and integration practical applications for novel optoelectronic devices [26–28].

ZnO, an important third-generation semiconductor, has been widely considered as potential material for the construction of UV PDs owing to the low cost, abundance of raw materials, nontoxicity, large bandgap of 3.37 eV, and high exciton binding energy of 60 meV [29–31]. Low-dimensional ZnO, including nanofibers, microwires (MWs), nanowires, and so on, has attracted extensive scientific interest in flexible detectors owing to their large length/diameter ratio, miniaturization, and flexibility. Generally, there are three types of detectors: metal-semiconductor-metal (MSM); Schottky; and p-n junction [1,32–35]. The limitations of a typical MSM structure detector such as complex metal electrode contacts, large operating voltage, and low photoresponse would inevitably restrict the development toward low-power consumption, flexibility, and integration [1,32,36,37]. Compared with photovoltaic p–n junction detectors, the traditional Schottky devices usually suffer from numerous bottlenecks, such as serious absorption of metal electrodes, relatively thinner depletion layer, and photocarriers transporting barriers. These shortcomings severely hinder the further application of Schottky-based PDs [38–40]. The realization of p–n junction photodiodes has received widespread attention owing to its excellent photodetection properties. Because of the lattice mismatch, heterojunction interface discontinuity of energy levels, etc., p–n heterojunctions usually have numerous defects, such as trapping states, large leakage current, surface defects, and high strains, thus bringing about economic difficulties in constructing high-performance heterojunction photodiodes [41–44]. Currently, the achievement of ZnO-based homojunction photodiodes has been blocked by the presence of high-quality p-type ZnO. Developing ZnO homojunction UV detectors that meet the requirements of low power consumption, high-performance, flexibility, integrability, and practical applicability remains a long-term and arduous task [4,45,46].

In this work, the sample of Ga-doped ZnO (ZnO:Ga) MWs showing n-type conductivity was synthesized individually, while the product of p-type conductivity ZnO MWs, which were doped with Sb-dopant (ZnO:Sb), was also obtained. The conducting properties were subsequently further verified using single MW-based field-effect transistors (FETs). A vertically structured p-n homojunction containing ZnO:Sb MW crossed with ZnO:Ga MW was constructed, showing excellent UV photodetection capability. When operated under 360 nm light irradiation at −0.1V, the largest responsivity, detectivity, on/off ratio, noise equivalent power, and photoelectron conversion efficiency can reach ∼2.3A/W, ∼6.5×1013Jones, 105, 4.8×10−15WHz−1/2, and 7.8%, respectively. The device also exhibited a fast rise/decay speed (τr/τd∼0.48/79.41ms). Further, the PD could be integrated into large-scale flexible optoelectronic arrays, which shows small degradation in the photocurrent after 400 bending cycles. Interestingly, the array unit as a pixel sensor exhibits its huge potential for UV imaging. This work offers a novel approach to design and fabricate high-performance, low-power, flexible, and integrable optoelectronic devices.

In the present study, the sample of MWs was synthesized using a chemical vapor deposition (CVD) method [47,48]. The rational preparation of samples with tunable n-type conductivity is available using Ga2O3 as the doping source. In the synthesis process of individual ZnO:Ga MWs, the precursor mixture containing ZnO, Ga2O3, and graphite (C) powder with a weight ratio of 9:1:10 served as source materials. Using the CVD method, individual ZnO:Ga MWs were successfully obtained. Compared with the preparation of n-type ZnO:Ga MW, the Sb element is commonly used to substitute Zn to induce p-type conductivity. The experimental scheme has been illustrated in our previous literature [29,49]. In the synthesis process of individual ZnO:Sb MW, a powder mixture of ZnO, Sb2O3, and C high purity powder (99.99%) in a weight ratio of approximately 9:1:10 was used as the source.

To investigate the electronic transport properties of CVD-prepared samples, an MW-based FET was fabricated by combining SiO2/Si substrate [50]. First, a single MW was mechanically transferred to the cleaned SiO2/Si wafer. Second, the Ti/Au serving as source and drain electrodes were welded at both the ends of the wire, while the Si substrate was used as the global back gate electrode for the FETs. The corresponding schematic illustration of the single MW-based FET is shown in Fig. 7 in Appendix A.

A representative cross-stacked homojunction device is constructed, which consists of a single p-type ZnO:Sb MW crossed with another n-type ZnO:Ga MW in a cross-shape layout, with each MW having a matching electrode forming an ohmic contact with it, respectively. The detailed fabrication process is summarized as follows: (i) deposition of ∼70nm thick and ∼30μm wide Au channel electrode on sapphire using an electron-beam evaporation method (∼0.07Å/s); (ii) 5 μm thick MgO blocking layers were deposited on both sides of the Au channel electrode (∼0.5Å/s); (iii) a ZnO:Sb MW was placed on the Au channel electrode; (iv) the other ZnO:Ga MW was then transferred to cross over the ZnO:Sb MW; (v) deposition of ∼5μm blocking layer on the indium tin oxide (ITO), leaving ∼30μm of the ITO channel was utilized as the top electrode; and (vi) using ITO to apply pressure to the crossed areas of the MW, which in turn makes good physical contact between the crossed samples. The corresponding schematic diagram of the detailed construction process of the device is shown in Fig. 8 in Appendix B.

The morphology and structure of CVD-synthesized ZnO:Ga and ZnO:Sb MWs were characterized using a scanning electron microscope (SEM) (TESCAN LAYRA3 GM). The chemical compositions of samples were detected using an energy-dispersive spectrometer (EDS), which was attached to the SEM. The crystal structure of the samples was characterized using X-ray diffraction (XRD) (Malvern Panalytical, Empyrean). The optical photograph of samples was obtained by the microscope (Olympus, BX53M). Photoelectrical properties of the constructed devices were conducted using a semiconductor analyzer (Keysight, B1500A), a light source (EQ99X), and a monochromator (Zolix, Omni-λ200i). The temperature of the device was tested using a handheld thermal imaging camera (FOTRIC 321Q). The response speed of PD devices was evaluated by connecting the detector with a 10GΩ resistor. A 365 nm Nd:YAG pulsed laser (Ekspla, NT342) and a digital oscilloscope (Tektronix, DPO 2012B) were used as the light source and data collection, respectively.

Figure 1(a) presents the optical photograph of the CVD-prepared ZnO:Sb MW, and a single ZnO:Sb MW sample was successfully picked out by using a facile mechanical transfer method. The corresponding surface morphology of a single ZnO:Sb MW in Fig. 1(b) was obtained by the SEM, where the image displays that the sample exhibits a regular hexagonal prismatic structure, smooth and straight sidewall facets, and sharp edges. The chemical composition of the ZnO:Sb wire was characterized via EDS mapping. As shown in Fig. 1(c), the homogeneous distribution of the Sb element was witnessed, indicating that a high degree of homogeneity of Sb doping was successfully achieved in the synthesis of ZnO:Sb MWs. The phases and crystal structure of CVD-synthesized ZnO:Sb wires were further tested via XRD scanning at a two-diffraction angle ranging from 26° to 40°. The XRD result in Fig. 1(d) shows diffraction peaks positioning at 32.65°, 34.32°, and 36.15°, which are assigned to the (100), (002), and (101) planes, respectively. Thus, the CVD-grown ZnO:Sb MWs are belonging to crystalline hexagonal phase of ZnO (JCPDS No. 36–1451). Further, the diffraction peaks of the samples are relatively sharp without any additional phases, indicating that the synthesized samples have excellent crystalline quality. Further, the microscopic image of the sample [Figs. 9(a) and 9(b) in Appendix C] shows that a high aspect ratio ZnO:Sb MW has uniform morphology and good mechanical flexibility, which benefits its applications in flexible optoelectronic devices.

The electrical transport properties of individual Sb-doped MW were researched using FETs [36]. Figure 1(e) shows a set of typical current and source-drain voltage (Ids–Vds) data under different gate voltage (Vg ranges from −4V to 4 V). The linear feature of the plotted Ids–Vds curves confirms that the metal electrodes make good ohmic contacts with the single wire. This is further confirmed by the local magnification curve of the Ids–Vds [Fig. 1(f)]. The Ids versus Vg curve for the FET recorded at Vds=1.2V is shown in Fig. 1(g). Electrical analysis shows that the conductance of the FET increases (decreases) with increasingly negative (positive) Vg. In consequence, a significant modulation of the typical p-type electrical transport properties of single MW was successfully realized via a simple Sb-doping.

Figure 2(a) shows a representative photograph of CVD-prepared ZnO:Ga MWs, which exhibits outwardly dispersing needle-like growth, indicating an excellent aspect ratio of the synthesized samples. According to the SEM result of a single ZnO:Ga MW in Fig. 2(b), the morphology of wire is a hexagonal prism with a diameter of ∼10μm. Moreover, the ZnO:Ga MW possesses a uniform morphology throughout its entire length with smooth and flat surfaces. The Ga element distribution is measured in Fig. 2(c) via EDS mapping analysis. It is apparent that the Ga component is uniformly distributed in the ZnO:Ga MWs, indicating that Ga elements can be well incorporated into the ZnO lattice. Figure 2(d) shows the XRD pattern of the samples, in which all the diffraction peaks can be well indexed to the hexagonal wurtzite phase of ZnO. Further, the value of full-width at half-maximum for the (002) plane is 0.071°, manifesting excellent crystal quality of CVD-synthesized samples.

Similarly, the electrical properties of a ZnO:Ga MW can be clarified on the basis of single-wire backgated FETs. Typical Ids versus Vds curves under varied Vg are shown in Fig. 2(e). The enlarged result of the output characteristic curve clearly exhibits a typical linear feature of the device [see Fig. 2(f)]. The transport characteristic curve Ids–Vg for the device recorded at Vds=1.2V is shown in Fig. 2(g). The result shows that the device exhibits a noticeable gating effect, and Ids increases with increasing Vg, which is a typical behavior of the n-type electronic transport properties of our synthesized ZnO:Ga wires [51].

Further, the carrier mobility (μ), conductivity (σ), and carrier density (nc) in CVD-synthesized ZnO:Sb and ZnO:Ga MWs were estimated by fitting the Ids–Vg curve, respectively. The carrier mobility can be calculated due to dIds/dVg=μ(C/L2)Vds, where C and L are the capacitance and the channel length of single wire, respectively [37]. The capacitance is given by C≅2πε0εSiO2L/ln(2h/d), in which ε0, εSiO2, h, and d are the vacuum dielectric constant, relative dielectric constant of SiO2, thickness of the SiO2, and diameter of the wire, respectively [36]. The conductivity can be calculated according to R=ρL/LScs=L/LσScs, where R, ρ, σ, and Scs are the resistance, resistivity, conductivity, and cross-sectional area of the MW, respectively. The carrier density can be determined by nc=σ/eμ, where e is the elementary charge [30]. Therefore, the carrier mobility, conductivity, and carrier density of ZnO:Sb MWs and ZnO:Ga MWs are as shown in Table 1. The result shows that the corresponding electrical parameters of the ZnO:Ga MW are evidently better than those of ZnO:Sb MW, suggesting that Sb doping can achieve the change of conduction behavior from n- to p-type [47].Table 1.

Electrical Transport Properties of As-Synthesized Individual ZnO:Sb and ZnO:Ga MWs

Figure 3(a) shows that a representative cross-shape device was constructed by using a vertical structure, which consists of two MWs in a cross-shape layout, with the ITO and Au films acting as contact electrodes for the ZnO:Ga and ZnO:Sb MWs, respectively. An I–V curve in the dark of the constructed device is displayed in Fig. 10(a) in Appendix D. The curve shows that a p-type ZnO:Sb MW crossed with n-type ZnO:Ga MW (p-ZnO:Sb⊗n-ZnO:Ga) exhibits clear current rectification characteristic with a turn-on voltage of ∼2.0V, after which the forward current increases rapidly. The inset in Fig. 10(a) in Appendix D shows an optical photograph of the constructed p-ZnO:Sb⊗n-ZnO:Ga device. Figure 10(b) (in Appendix D) exhibits that the I–V measurements made on the individual ZnO:Sb and ZnO:Ga MWs show a linear response characteristic of ohmic contacting features. It suggests that the rectification characteristic is mainly attributed to the fabricated crossed-shape MWs device, illustrating that a high-quality homojunction is formed in the physical contact region toward the p-ZnO:Sb⊗n-ZnO:Ga interface. To obtain clearer insight into the temperature dependence of the dark current behavior [dark current versus voltage measured at different temperatures as shown in Figs. 11(a)–11(c) in Appendix E], we represent the dark current and the resistance-area product (R0A) using an Arrhenius plot, as shown in Fig. 3(b). It is found that varying the temperature from 260 to 330 K, the dark current exhibits a considerably linear increase. This phenomenon may be derived from the fact that the low temperature would lead to a slower transport efficiency of the interfacial diffusion current, which in turn weakens the dark current. Also, the R0A product at 330 K is evaluated to about 1269Ωcm2. It also increases to 33,670Ωcm2 as the temperature reduces to 260 K. The higher value of R0A results from the tiny junction region interface. In this case, the change through diminishing the surface leakage current would lead to the reduced dark current of as-constructed devices [52].

The photoelectrical properties of the p-ZnO:Sb⊗n-ZnO:Ga MW homojunction device were systematically tested under light illumination with different wavelengths. As depicted in Fig. 3(c), the logarithmic I–V characteristic curves under several selected wavelengths (300, 320, 340, 360, 380, and 400 nm) were measured. It can be found that the device has a pronounced photoresponse behavior and shows the largest photocurrent at the wavelength of 360 nm. Figure 3(d) shows that the photo–dark current ratio of the homojunction device can reach as high as ∼1.68×105 at −0.1V bias. Additionally, room temperature absorption versus wavelength (λ) plots of ZnO:Ga and ZnO:Sb MWs and the logarithmic I–V characteristic curves of the device in the range of 200–600 nm are shown in Figs. 12(a) and (b) in Appendix F. The absorption edges of the samples appear at 380 nm without any shift for ZnO:Ga and ZnO:Sb samples, indicating that the modulation effect of doping via introducing electron donor (Ga) or acceptor (Sb) elements into the ZnO host crystal on absorption characteristics could be negligible. The optical bandgaps of CVD-grown ZnO:Ga MW and ZnO:Sb MW were calculated to be 3.19 and 3.21 eV, respectively. Compared with bulk ZnO, the deviations could be derived from the self-absorption of CVD-synthesized monocrystalline wirelike structures. Figure 12(c) in Appendix F shows a wavelength-dependent photoresponse at the ultraviolet wavelengths. Owing to the excellent light absorption properties of the samples, the constructed devices exhibit a pronounced photoresponse at the UV wavelengths, with the strongest response occurring at 360 nm.

To quantitatively evaluate the photodetection properties of p-ZnO:Sb⊗n-ZnO:Ga homojunction PD, several critical performance parameters, including photocurrent responsivity (R), specific detectivity (D*), external quantum efficiency (EQE), and linear dynamic range (LDR), were considered. These parameters are described below [40,53,54]: R(λ)=Iph−IdPS,(1)D*=R2eId/S,(2)EQE=hcR(λ)eλ,(3)LDR=20log(IphId),(4)where S is the effective irradiated area (1×10−6cm2) of device; P is the incident power intensity (∼0.1mW/cm2); Iph and Id are the photocurrent and dark current of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD, respectively; e is the electron charge constant (∼1.6×10−19C); h is the Planck constant (∼6.62×10−34J·s); c is the speed of light (∼3×108m/s), and λ is the wavelength of incident light.

The effect of light irradiation at different wavelengths on the performance of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD was researched. The R and D* at −0.1V were calculated using Eqs. (1) and (2), respectively. Figure 3(e) demonstrates that the largest Rmax can reach 2.32 A/W at a wavelength of 360 nm; thereby, the UV/visible rejection ratio R360/R400∼430 was achieved, which indicates that the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD has outstanding detection ability for UV lights, whereas the higher spectral response can be attributed to the absorption characteristics of CVD-synthesized samples. In addition, we found that the D* reaches the maximum value of ∼6.5×1013Jones at 360 nm and maintains the same order of 1013 in a broad UV wavelength range, which is superior to most previously reported ZnO-based homojunction detectors [41]. The EQE and LDR at different wavelengths obtained from Eqs. (3) and (4) are shown in Fig. 3(f). The EQE data shows that the PD exhibits high photoelectric conversion efficiencies in the range of 300–400 nm, and the highest EQE value is achieved at the wavelength of 360 nm. The calculated LDR values are averagely evaluated to ∼60dB at UV wavelengths of 300–400 nm, which is comparable with those of commercial photodetectors [55]. In particular, the largest LDR can raise up to 80 dB at the wavelength of 360 nm, which indicates that the fabricated detector has excellent signal-to-noise ratio.

To evaluate the photodetecting performance of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD under 360 nm light irradiation at −0.1V, we performed I–V characteristics tests of the PD under different light intensities ranging from 0 to 5mW/cm2, as shown in Fig. 4(a). It clearly illustrates that the photocurrent exhibits an observable enhancement with increasing light intensities. The photo–dark current ratio of the PD increases from ∼103 (∼0.1mW/cm2) to ∼105 at −0.1V bias [Fig. 12(d) in Appendix F]. The robust increase of the photo–dark current ratio can be attributed to the fact that the higher intensity of the incoming lights induces more photocarriers, in turn, enhancing the photocurrent response. Figure 4(b) presents the changes of the obtained photocurrents versus the light illumination intensity of the PD at −0.1V bias. The change can be well fitted by the power law [40] Iph∝APα,(5)where A is a constant at a specific wavelength, and α is a power-law factor. The α value is estimated to be 0.90 (ideal value is 1), suggesting that interfacial recombination loss in the homojunction could be neglected upon UV light irradiation. We also noted the linear character of the photocurrent via light illumination intensity, which is different from low-dimensional ZnO heterojunction photodiodes. Such an experimental phenomenon is mainly caused by the existence of fewer interface defects acting as the charge recombination centers in homojunction. Thus, the trapping centers are rapidly saturated upon UV light illumination, which effectively reduces recombination loss resulting from the growing concentration of photogenerated carriers in homojunction interface [39].

The illumination density-dependent photoresponse performances of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD were studied at the wavelength of 360 nm. Figures 4(c) and 4(d) demonstrate that the R and D* show a decreasing trend when increasing the light intensity of the incident light. That is, the largest R (∼2.3A/W) and D* (∼6.5×1013Jones) can be achieved under weak light density of UV irradiation, indicating the outstanding photosensitivities of the PD to weak UV light signals. The reduced visibility of the obtained photosensitive properties with increasing light intensity could be probably attributed to the surface and interfacial defects of ZnO:Sb and ZnO:Ga MWs as well as the constructed devices. In that case, a small part of the photocarriers may be recombined before being collected by electrodes, decreasing the average lifetime with the increasing carrier recombination rate [56].

The NEP and IPCE of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD were analyzed according to the following equations [56,57]: NEP=2eIdR,(6)IPCE=1240JλP,(7)where e and Id are the electron charge constant and dark current, respectively. J and P are the measured photocurrent density and light intensity at a fixed wavelength (λ=360nm). The variation of NEP as a function of different light intensities is plotted in Fig. 4(e) at −0.1V bias. As the UV light intensity increases, the NEP increases gradually, indicating that the PD shows lower NEP (∼4.8×10−15WHz−1/2) at the weak light signals. This phenomenon can be attributed to the tiny contact area between the ZnO:Sb and ZnO:Ga MWs, which effectively reduces the background noise caused by the much lower interfacial defect states. Figure 4(f) depicts the changes of IPCE versus optical densities of the incoming light, which were calculated by using Eq. (7). The observed negative relationship could be speculated to the fact that the internal gain effect of the PD diminishes with increasing UV light irradiance density [32,58]. Therefore, the PD gives a maximum IPCE of 7.8% upon the 360 nm light irradiation.

To access the stabilities of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD, the I–t curves of the device at different reverse voltages are displayed in Fig. 5(a). The current has a strong dependence on the reverse bias. Further, the fabricated PD shows a superior time-dependent photoresponse even at low bias along with excellent reproducibility and stability, which is considered to be a critical factor for practical application in low-power devices. The dark current and photoresponse sensitivity as a function of different reverse bias are as plotted in Fig. 5(b). The results demonstrate that the photoresponse sensitivity decreases with increasing dark currents under reverse bias condition, where the sharply increased dark current may be attributed to the avalanche multiplication of carriers in the depletion layer caused by the larger reverse bias [58]. The environmental stability of the unencapsulated p-ZnO:Sb⊗n-ZnO:Ga homojunction PD was tested. In the measurement, the photocurrent intensity was recorded under continuous irradiation of 360 nm light (0.1mW/cm2) at room temperature (25°C) and ∼45% humidity. As shown in Fig. 5(c), the device has only ∼10% fluctuation of photocurrent intensity after prolonged UV irradiation. Meanwhile, the inset of Fig. 5(c) shows that the environmental stability of the PD was discussed as a function of storage time (15, 30, and 60 days later). Although the photocurrent intensity fluctuated after 60 days of storage, the device also maintained over 90% of the pristine photocurrent, indicating that the device can ensure long-term stable operation even in an unencapsulated state.

The response speed of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD was examined by measuring its rise τr (10%–90%) and decay τd (90%–10%) upon switching the UV irradiation on and off. As shown in Fig. 12(e) in Appendix F, the τr and τd are estimated to be 0.073 and 0.392 s, respectively. The much lower response speed could result from the test limits of the semiconductor parametric analysis instrument. To actually approach the response speed, the transient photoresponse of our as-fabricated p-ZnO:Sb⊗n-ZnO:Ga homojunction PD was conducted under a 360 nm pulse laser illumination at −0.1V. As shown in Fig. 12(f) in Appendix F, the captured pulse signals with steady, rapid, and repeatable photoresponse are clearly obtained. Figure 5(d) exhibits a single pulse of the time-resolved photoresponse; thereby the τr and τd can be derived to about 0.48 and 79.41 ms, respectively. The fast response speed of the PD was superior to most of previously reported ZnO-based homojunction detectors. It is mainly attributed to the cooperative effect of composite (external and built-in) electric fields at the junction interface and the formation of high-quality homojunction with fewer trap states; thus, it is conducive to the quick separation and transport of photoinduced carriers [35,59,60].

The photosensitive mechanism and photocarrier transport paths of our as-fabricated p-ZnO:Sb⊗n-ZnO:Ga homojunction device can be further understood by the energy band diagram, as shown in Fig. 5(e). Shown in the photograph (left), EC, EV, and EF are the conduction band, valence band, and Fermi energy level of CVD-synthesized ZnO:Sb and ZnO:Ga samples, respectively. Eg and χ are their bandgap energies and electron affinity, respectively. It has been reported that the bandgap energy and electron affinity values of ZnO are 3.37 and 4.35 eV, respectively [Fig. 5(e)-I] [59]. As both MWs are in contact with each other, the difference in Fermi levels would lead to the majority carriers interdiffusion between the ZnO:Ga MW (for the electrons) and ZnO:Sb MW (for the holes). When the device is in an equilibrium state, built-in potential E0 is established [see Fig. 5(e)-II]. The depletion layer thickness (w) created in ZnO:Sb and ZnO:Ga samples at 0 V bias can be estimated using the formula [54] w=2εnεpε0nZnO:SbE0enX(εnnZnO:Ga+εpnZnO:Sb),(8)where εp and εn are relative dielectric constant of ZnO:Sb and ZnO:Ga MW, respectively. nX (X=ZnO:Sb, ZnO:Ga) is carrier densities of ZnO:Sb and ZnO:Ga MW, respectively. E0 is the built-in field intensity of the PD, e is the elementary charge, and ε0 is vacuum dielectric constant. The w of ZnO:Sb and ZnO:Ga MW are estimated to be ∼8.16 and ∼2.63nm, respectively. The depletion layer thickness of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD is about 10.79 nm at 0 V bias. However, once the PD is operated upon UV light illumination at −0.1V bias, photoinduced generation of electron-hole pairs will be formed in the depletion layer and then could be separated by combining the external electric field and the built-in electric field [Fig. 5(e)-III]. In this case, the external electric-field associated with the preformed built-in potential is more favorable for photocarriers transport. As summarized in Table 2, the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD in this research exhibits much better photosensitive performance than other previously reported ZnO-based UV PDs.Table 2.

Comparison of Detection Performance Parameters of the ZnO-Based UV PDs

The potential application of the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD in flexible integrated optoelectronic devices was further researched. The schematic architecture of the flexible integrated homojunction array based on a single unit of our preprepared p-ZnO:Sb⊗n-ZnO:Ga homojunction device is illustrated in Fig. 6(a). We fabricated an array unit of the PD in a 500μm×700μm area. First, Au electrodes were deposited on the flexible PET substrate, then ZnO:Sb MW and ZnO:Ga MW were welded on the Au electrodes by indium, and finally the device was simply covered with polymethyl methacrylate (PMMA). Figure 6(b) represents the optical photograph of a flexible integrated PD array unit on PET substrate; the corresponding microscopic photograph is shown in Fig. 6(c). The microscopic photograph of the device shows that all the ZnO:Ga MWs form good contacts with ZnO:Sb MWs. Meanwhile, to study the stability of the PD array unit under harsh conditions, the photocurrents of the device were measured after it was subjected to various bending angles and bending cycles, as shown in Figs. 13(a)–13(e) in Appendix G. It demonstrates that the photocurrent of the fabricated PD array unit remained fairly stable up to 30° bending angles and 50 bending cycles, keeping more than 80% of its initial photocurrent value even after 90° bending angles and 400 bending cycles. This reveals the potential of the PD array unit for versatile applications in flexible and integrated optoelectronic devices.

Finally, the proposed p-ZnO:Sb⊗n-ZnO:Ga homojunction PD has great potential for single pixel photoimaging, which is important functional components for information communication, image sensors, and flexible optoelectronic devices. However, high-performance flexible p-n homojunction PD with high resolution and stable photoresponse still need to be developed. Figure 6(d) shows the schematic diagram of the MW imaging measurement platform, which consists of the PD array unit, light source, lens assembly, 2D translation stage, and so on. Figure 6(e) shows an efficiently detected image of the “MW” letters collected by the array unit under 360 nm light source illumination at 0.1mW/cm2. Obviously, the device exhibits high contrast and resolution with sharp edges. As shown in Fig. 6(f), the current waveform of the twelfth pixel [position of the red line in Fig. 6(e)] is acquired by the array unit, which suggests that the device has the ability to track rapidly varied light signals. The average photocurrent and SNR of the device were obtained under different scanned pixels, as demonstrated in Fig. 6(g). The imaging results clearly indicate the excellent imaging capability and superior signal current uniformity of the device as a single sensing pixel. It is notable that the SNR of the array unit is all more than 50 dB, and the high SNR values reveal the outstanding low-light detection capability of the device, suggesting that the homojunction array unit can efficiently convert UV light into photocurrents. Therefore, the p-ZnO:Sb⊗n-ZnO:Ga homojunction PD as a UV signal receiving terminal can provide new ideas for the design and fabrication of miniaturized, low-power, and high-performance flexible integrated UV imaging sensor devices.

In summary, we have successfully constructed a high-performance flexible UV PD based on a p-ZnO:Sb⊗n-ZnO:Ga MWs homojunction. The device can work at a relatively low operation voltage, suggesting its low-power consumption detection. The PD exhibits an impressive overall photosensitive property, including a largest responsivity of 2.3 A/W, a large specific detectivity of 6.5×1013Jones, a superior NEP of 4.8×10−15WHz−1/2, an excellent IPCE of 7.8%, and fast response speed (0.48 ms/79.41 ms) under 360 nm light illumination at −0.1V. These comparable figure-of-merit parameters show that the homojunction device demonstrates its remarkable potential for UV imaging, optical communication, and biological detection. As a unit of p-ZnO:Sb⊗n-ZnO:Ga MWs homojunction was integrated into a flexible array, the device exhibits less than 20% degradation in photodetection performance after 90° angles and 400 times bending-releasing test, indicating its good mechanical flexibility and stability. Moreover, the fabricated PD array unit was also integrated into a string array pixel, giving rise to high-resolution imaging capabilities. Given their UV photodetection properties, excellent mechanical flexibility and stability, and photosensitive imaging, we believe that the as-constructed p-ZnO:Sb⊗n-ZnO:Ga MWs homojunction PD will boost the progress toward next-generation wearable technologies and integrated UV imaging sensor.