Flexible ZnO Microwire Array UV Photodetector on PVA Substrate: Low‑Cost, High Responsivity, and Robust Flexibility

Abstract

Wearable electronics demand inorganic semiconductors that can be fabricated on flexible, inexpensive substrates. In this work, we present a novel ultraviolet (UV) photodetector (PD) that incorporates a dense array of ZnO microwires (MWs) embedded within a polyvinyl alcohol (PVA) film. The device delivers a remarkable photoresponsivity of 29.6 A W⁻¹ across the 350–380 nm band while maintaining low dark current (1.4 µA at 5 V) and a rapid response time of 4.27 ms. Importantly, the PD retains full functionality even when bent to 180° or to a radius approaching zero, underscoring its suitability for real‑time UV monitoring in wearable applications.

Background

UV detection is critical for astronomy, environmental safety, and biomedical diagnostics. Excessive UV exposure can mutate the p53 tumor‑suppressor gene, increasing skin‑cancer risk, making personal UV monitoring essential. The rise of wearable devices enables continuous, individualized UV exposure tracking, but requires photodetectors that operate reliably under mechanical strain. Zinc oxide (ZnO) is a wide‑bandgap II–IV semiconductor (3.37 eV at 300 K) with a large exciton binding energy (60 meV), making it an ideal material for UV photonics. While ZnO nanostructures—nanorods, nanowires, nanobelts, etc.—have been extensively studied, scalable production and flexible integration remain challenging. Conventional flexible substrates such as PET and PU often degrade under the high temperatures required for ZnO growth, and their rough surfaces can reduce device sensitivity.

Our study addresses these limitations by embedding ZnO MWs in a low‑temperature, flexible PVA matrix. PVA offers superior bendability, low cost, and compatibility with solution‑processed deposition, enabling the fabrication of a robust, flexible UV PD with high photoresponsivity.

Methods/Experimental

The ZnO MW array was grown by chemical vapor deposition (CVD) on a sapphire substrate. High‑purity Zn powder (99.99 %) was heated to 980 °C in N₂, followed by O₂ introduction, producing ZnO MWs with diameters of 40–50 µm and lengths of 3–5 mm (Figure 1b). After growth, the MWs were transferred to a glass slide, coated with 1 mL of liquid PVA glue, and dried at 60 °C for 1 h. The PVA‑embedded MWs were then peeled from the glass, and five pairs of Au interdigital electrodes (gap 100 µm, finger length 200 µm) were deposited to complete the device (Figure 2). Electrical and photoresponse measurements were performed at room temperature using an Agilent B2901A semiconductor characterization system.

Structural characterization involved SEM (ZEISS Gemini 500), optical microscopy, X‑ray diffraction (Bruker D8 ADVANCE), and absorption spectroscopy with a He–Cd laser (325 nm). Density Functional Theory (DFT) calculations (SIESTA, PBE‑GGA) assessed strain‑induced bandgap changes under axial tensile and compressive loads.

Results and Discussion

SEM images confirm uniform MWs with 40–50 µm diameter and multi‑millimeter length (Figure 1b). XRD reveals a pristine wurtzite phase (Figure 1c), while the absorption spectrum indicates low defect density (Figure 1d). The device exhibits ohmic I‑V behavior in the dark and a pronounced photocurrent under 380 nm illumination (Figure 4). The high responsivity (29.6 A W⁻¹) arises from excellent MW–PVA contact and efficient charge extraction via the Au electrodes.

Flexibility tests demonstrate stable operation at 0°, 90°, and 180° bending (Figure 3). Although bending introduces piezoelectric surface charges that slightly reduce photocurrent, the device maintains >0.9 A W⁻¹ responsivity even at 180° (Figure 7). Bandgap reduction from 3.37 eV (flat) to 3.29 eV (fully bent) correlates with a measurable red‑shift in the photoresponse peak (Figure 5). DFT confirms that axial strain modulates the band structure, supporting the experimental observations (Figure 6).

Photocurrent decay dynamics improve under bending: decay times shorten from 6.18 ms (flat) to 4.27 ms (180°) while rise time remains limited by the 30 ns laser pulse (Figure 8). The accelerated recombination is attributed to reduced surface recombination barriers caused by piezo‑induced electric fields.

Conclusions

We have demonstrated a facile, low‑cost fabrication route for a flexible ZnO MW array UV photodetector embedded in a PVA substrate. The device delivers high responsivity (29.6 A W⁻¹), low dark current, and sub‑5 ms response time while sustaining performance under extreme bending. These attributes make the PD a compelling candidate for wearable UV monitoring and suggest its applicability to other flexible optoelectronic components.

Abbreviations

CVD

Chemical vapor deposition

EBE

Electron beam evaporation

MOCVD

Metal organic chemical vapor deposition

MSM

Metal‑semiconductor‑metal

MWs

Microwires

PD

Photodetector

PET

Polyethylene terephthalate

PLD

Pulsed laser deposition

PVA

Polyvinyl alcohol

RFMS

Radio‑frequency magnetron sputtering

UV

Ultraviolet

Figure 1. (a) Schematic of the ZnO MW array UV PD. (b) SEM micrograph of synthesized MWs. (c) XRD pattern. (d) Absorption spectrum.

Figure 2. Schematic of the photodetector fabrication process.

Figure 3. Device appearance under 0°, 90°, and 180° bending.

Figure 4. I‑V characteristics in dark and under UV illumination at various bending angles.

Figure 5. Photoresponse wavelength shift with bending angle.

Figure 6. Bandgap variation of ZnO MWs under bending.

Figure 7. Spectral photoresponsivity at 0°, 90°, and 180° bending.

Figure 8. Decay time versus bending angle for 266 nm pulsed laser excitation.

Figure 9. (a) Band diagram of unbent MW; (b) band diagram of bent MW, illustrating piezo‑induced barrier modulation.