Low-Temperature ZrAlO_x-PVP Hybrid Dielectrics with Crosslinking-Regulated Leakage Suppression for Flexible IGZO TFTs

Abstract

Flexible oxide electronics require dielectric layers that combine low-temperature processability, low leakage current, high capacitance density, and mechanical reliability. In this work, we prepared ZrAlO_x-PVP hybrid dielectric films through a low-temperature self-combustion solution process at 180 °C and systematically investigated the effect of PVP doping (0–2 wt%). The results show that PVP promotes the formation of M-O-C related bonding environments, suggesting the construction of an organic–inorganic crosslinked structure. Moderate PVP incorporation effectively suppresses leakage pathways, whereas excessive PVP induces polymer aggregation and trap-assisted conduction. Among all samples, the film on flexible PI (polyimide) with a PVP doping concentration of 0.5 wt% exhibits the best overall performance, with a leakage current as low as 1.89 × 10⁻⁸ A/cm² at 1 MV/cm, a dielectric constant of 8.88. After static bending at a radius of 20 mm, the film maintains stable dielectric behavior, indicating improved stress tolerance. Flexible IGZO TFT fabricated with the optimized dielectric shows a mobility of 11.84 cm² V⁻¹ s⁻¹, a threshold voltage of 0.48 V, and a subthreshold swing of 0.24 V dec⁻¹ before bending. This work demonstrates that moderate PVP crosslinking provides an effective balance between defect suppression and stress relaxation, offering a practical interface-engineering strategy for low-temperature flexible high-k dielectrics.

1. Introduction

Flexible oxide electronics have attracted considerable attention for next-generation displays, wearable sensors, medical electronics, and low-power integrated systems [1,2,3,4,5]. In these devices, the gate dielectric is not merely an insulating layer but a key interface-regulating component in charge regulation. For flexible thin-film transistors (TFTs), developing high-performance dielectric materials that combine low-temperature processability, low leakage current, a high dielectric constant, and mechanical stability is of great significance for achieving low-voltage driving, low power consumption, and long-term stability in flexible devices [6]. However, there is a fundamental contradiction between the conventional fabrication routes of high-performance dielectric materials and flexible substrates. Generally, metal oxide dielectric films require high-temperature annealing to remove residual organic components of the precursors, promote the thorough progress of condensation reactions, and reduce defects such as oxygen vacancies [7], whereas flexible polymer substrates (PI, PEN, PET, etc.) have low glass transition temperatures that restrict processing to low temperatures, resulting in poorly densified films with high defect density and excessive leakage current [8]. Therefore, achieving low-temperature fabrication of dielectric layers that offer both superior electrical properties and mechanical flexibility remains a critical challenge in flexible electronics.

In terms of fabrication methods, the solution methods are promising for low-cost and large-area flexible electronics because of their compositional tunability, simple processing, and compatibility with scalable fabrication. Nevertheless, conventional solution method usually relies on high-temperature annealing (>400 °C) to achieve the hydrolysis and polycondensation of the precursors, which limits its application on flexible substrates [9]. To overcome this problem, many researchers have been working to reduce the processing temperature while maintaining the electrical performance of the devices [10]. Common low-temperature strategies often rely on external energy assistance to drive film formation, including deep ultraviolet (DUV) annealing, high-energy flash annealing, and high-pressure annealing [11,12,13,14]. Different from the above approaches, Kang et al. [15] introduced the self-combustion method, providing an effective route to reduce the external annealing temperature by utilizing internal chemical energy released from redox reactions between metal nitrates (oxidizers) and metal acetylacetonates (fuels) in the precursor system. For example, Bae et al. [16] used Al(NO₃)₃·9H₂O and Al(C₂H₅O₂)₃ to obtain an AlO_x gate dielectric layer via a self-combustion reaction at a low temperature of 250 °C. The leakage current density was 1.80 × 10⁻⁸ A/cm², and the dielectric constant reached above 8.7. Although self-combustion method can achieve low annealing temperature, it still suffers from limited defect passivation within the films and poor mechanical stability under flexible conditions.

In terms of material selection, high-k dielectric materials possess a large dielectric constant, which can effectively increase device capacitance density and reduce operating voltage. ZrO₂ and Al₂O₃ are representative high-k oxide dielectrics with complementary properties. ZrO₂ offers a relatively high dielectric constant, while Al₂O₃ provides a wide bandgap, high insulating capability, and good chemical stability. As an amorphous composite oxide of ZrO₂ and Al₂O₃, ZrAlO_x composite dielectrics can therefore combine the high capacitance of Zr-based oxides with the leakage-suppressing characteristics of Al-based oxides. A number of composite high-k systems have already been reported and proven effective, such as hafnium zirconium oxide (HfZrO_x) [17,18], zirconium aluminum oxide (ZrAlO_x) [19], and lanthanum-doped zirconium oxide (LaZrO_x) [20,21]. However, purely inorganic oxide networks are intrinsically rigid and can suffer from interfacial stress when integrated onto flexible polymer substrates, leading to delamination, cracking, and enhanced interfacial defects. This stress may result in a surge in leakage current, degraded dielectric performance, and overall device deterioration.

The organic-inorganic crosslinking strategy provides an effective route to address the above issues [22]. Poly(4-vinylphenol) (PVP) is widely used for organic doping owing to its favorable intrinsic flexibility, chemical stability, and solubility. More importantly, PVP enables its phenolic hydroxyl groups to coordinate into the inorganic oxide network, forming M-O-C crosslinked bonds and constructing a stable organic-inorganic crosslinked network [23,24]. For instance, Yu et al. [23] doped In₂O₃ films with PVP. By adjusting the In₂O₃:PVP weight ratio, they obtained the In₂O₃:5%PVP-based transistors that exhibit mobilities approaching 11 cm² V⁻¹ s⁻¹ before, and retain up to about 90% performance after 100 bending/relaxing cycles at a radius of 10 mm. On the one hand, such organic-inorganic crosslinking may fill the voids between inorganic frameworks, which can passivate defect-related oxygen species, reducing the overall defect density on the film surface. On the other hand, the flexible organic polymer chains can buffer thermal and bending stresses through conformational motion, improving mechanical flexibility. However, the effect of PVP incorporation is not necessarily monotonic. A moderate amount of doping can create a uniform and dense organic-inorganic crosslinked network, achieving synergy between defect passivation and stress buffering, and thus reducing leakage current. Excessive doping may lead to overly dense crosslinking, making the film rigid and unable to buffer stress, while also causing polymer aggregation, interfacial disorder, and trap-assisted leakage conduction. Therefore, systematic research on the influence of PVP crosslinking density on defect states, conduction mechanisms and bending stability is important for designing reliable flexible oxide electronics.

In this work, ZrAlO_x-PVP hybrid dielectric films with different PVP concentrations were fabricated on flexible PI substrates using a low-temperature self-combustion solution process. The focus of this study is to reveal how PVP-regulated crosslinking density modulates defect states, leakage transport, and mechanical stability. Flexible IGZO TFTs based on the optimized dielectric were further fabricated to verify device-level applicability. The results reveal that moderate PVP crosslinking balances defect suppression and stress relaxation, providing a practical interface-engineering strategy for low-temperature high-k hybrid dielectrics in flexible electronics.

2. Results and Discussion

The samples prepared with different PVP doping concentrations are designated as P00, P02, P05, P10, and P20, as summarized in

Table 1. Sample names for different PVP doping concentrations.

Precursor System	ZrAlOx	ZrAlOx–0.2 wt% PVP	ZrAlOx–0.5 wt% PVP	ZrAlOx–1 wt% PVP	ZrAlOx–2 wt% PVP
Doping concentrations	0	0.2 wt%	0.5 wt%	1 wt%	2 wt%
Sample names	P00	P02	P05	P10	P20

.

2.1. PVP-Regulated Morphology and Thickness Evolution

In order to investigate the influence of different PVP doping concentrations on the roughness of ZrAlO_x-PVP hybrid dielectric films, AFM was used for testing. Figure 1 shows the summary of the 3D images of the films and the corresponding root mean square (RMS) values of the roughness under different doping concentrations, which are 0.68, 0.74, 0.61, 0.46 and 0.70 nm respectively. All ZrAlO_x-PVP hybrid dielectric films exhibit smooth and dense surfaces, and no obvious pinholes or cracks are observed. When the doping concentration is 1 wt%, the film has the lowest surface roughness, indicating that it is the smoothest at this concentration. This may be because at a doping concentration of 1 wt%, PVP molecules condense with metal oxides through phenolic hydroxyl groups during annealing, uniformly filling the voids between inorganic particles and forming the densest organic-inorganic crosslinked network. Below this concentration, the amount of PVP participating in crosslinking is insufficient, which is not enough to fully fill the voids in the inorganic phase. When the concentration increases to 2 wt%, the roughness increases significantly, due to the stacking of excessive polymer chains. It should be emphasized that the lowest roughness of P10 on glass substrates does not necessarily guarantee the best electrical performance and mechanical stability on flexible PI substrates.

Figure 2a shows the XRR measurement and fitting curves. The thicknesses of the ZrAlO_x-PVP hybrid dielectric films extracted from the fits are summarized in Figure 2b: 12.3 nm for P00, 19.7 nm for P02, 20.5 nm for P05, 24.8 nm for P10, and 43.3 nm for P20. A clear trend is observed: as the concentration of PVP increases, the thickness of the films gradually rises. This is because during the annealing process at 180 °C, the polymer PVP did not undergo significant thermal decomposition. Instead, it crosslinked with the inorganic network in the form of molecular chains during film formation, directly increasing the film thickness. Notably, when the PVP concentration increased from 1 wt% to 2 wt%, the thickness sharply increased from 24.8 nm to 43.3 nm, with an increase of about 75%. This indicates that the excess PVP, unable to fully bond into the network, aggregated and piled up together, leading to a substantial thickening of the film. This inference is consistent with the trend in film roughness discussed above, further supporting the picture of PVP aggregation and surface inhomogeneity. Therefore, the thickness evolution not only confirmed the effective incorporation of PVP, but also revealed the saturation of the crosslinking sites and harmful polymer aggregation at excessive doping levels, which was later associated with the decline in electrical properties of the P20 sample.

Figure 2c shows the XRD testing curves of ZrAlO_x-PVP hybrid dielectric films. For the dielectric films with different doping concentrations, a single broad peak appears around 2θ = 23°, and no obvious diffraction peaks are observed, indicating that the films are amorphous. An ideal material for dielectrics should be amorphous, because the amorphous structure lacks grain boundaries, which can effectively reduce leakage current and improve the uniformity of the film.

2.2. Optical Properties and Bandgap Stability

To investigate the optical properties of the ZrAlO_x-PVP hybrid dielectric films, the transmittance of the films was measured using a UV-visible spectrophotometer. As shown in Figure 2d, the ZrAlO_x-PVP hybrid dielectric films exhibit high transmittance. Except for the P20 component, the transmittance in the visible range remains above 95%, indicating the feasibility of their application in transparent flexible devices. Figure 2e shows the (αhν)² –hv curves for the films from P00 to P20. The corresponding band gap (E_g) of ZrAlO_x-PVP hybrid dielectric films was calculated using the Tauc formula [23,25], as shown in Equation (1):

$$\alpha hv=A{\left(hv- E_g\right)}^{{\displaystyle \frac{1}{n}}}$$

(1)

where hν, n, A, and α correspond to the photon energy, optical transition exponent, a proportionality constant in the absorption process, and absorption coefficient, respectively. The band gap obtained by linear extrapolation are 5.20 eV, 5.17 eV, 5.17 eV, 5.16 eV, and 5.14 eV, respectively. With increasing PVP concentration, the band gap shows a decreasing trend, which is attributed to the intrinsically lower band gap of the organic component PVP. Nevertheless, the decrease is slight and the E_g of all dielectric films are greater than 5 eV, indicating that the wide bandgap characteristic of the ZrAlO_x inorganic component is well preserved after PVP incorporation, and PVP does not introduce deep-level defect states that could significantly narrow the bandgap.

2.3. Chemical Bonding and Crosslinked Network Formation

To investigate the degree of crosslinking in the organic-inorganic network, XPS was used to analyze the internal bonding configurations. Figure 3a shows the O 1s spectra, which can be deconvoluted into three main components [26]: ~530 eV is attributed to lattice oxygen (M-O-M), ~531 eV is attributed to oxygen vacancy-related oxygen (V_O), and ~532 eV is attributed to hydroxyl-related oxygen (M-O-R). Figure 3b shows the corresponding peak area ratios. As the PVP concentration increases from 0 to 2 wt%, the proportion of M-O-M decreases, while the proportion of M-O-R increases, indicating that PVP inhibits the formation of a continuous inorganic lattice network. The abundant hydroxyl groups (-OH) in the PVP molecules, together with the interfacial oxygen (M-O-C) generated by condensation with ZrAlO_x, contribute to the adsorbed oxygen signal, confirming that PVP bonds with the inorganic network to form an organic-inorganic crosslinked structure. Referring to Figure 3e, the binding energies of Al 2p and Zr 3d shift positively with increasing PVP doping concentration. This is because the highly electronegative C attracts electrons, reducing the density of the outer electron cloud of the metal, weakening the shielding effect, and enhancing the attraction of the inner electrons. Conversely, the binding energy of C 1s decreases with increasing PVP concentration (Figure 3g), as the carbon atoms gain electron density from the metal atoms via the M-O-C bonds. This further confirms the successful construction of the organic-inorganic crosslinked network. Notably, as the PVP doping concentration gradually increases, the magnitude of the binding energy shift decreases. This indicates that the crosslinking sites in ZrAlO_x are approaching saturation, excess PVP can no longer form effective chemical bonds, thus tending to aggregate and stack.

2.4. Leakage Current Behavior and Defect-Assisted Conduction Mechanism

To investigate the electrical stability of flexible ZrAlO_x-PVP hybrid dielectric films before and after static bending at a bending radius of 20 mm, the electrical properties of MIM devices were characterized using a semiconductor parameter analyzer. Figure 4a shows the relationship between leakage current density (J) and electric field strength (E) of the MIM devices with different PVP doping concentrations before bending, and Figure 4b summarizes the leakage current densities at an electric field of 1 MV/cm. The results indicate that the leakage current varies non-monotonically with the concentration of PVP: as the PVP doping concentration increases, the leakage current density of the ZrAlO_x-PVP hybrid dielectric films first decreases and then increases. At a doping concentration of 0.5 wt%, the leakage current density is the smallest at 1.89 × 10⁻⁸ A/cm²@1 MV/cm. Leakage current density reflects the insulating performance of the dielectric layer under an electric field and is primarily determined by the defect state density within the film and the interface quality. A low leakage current is essential for reducing static power consumption, improving the on/off current ratio. When a low concentration of PVP is doped, the crosslinked network is sparse and discontinuous. Consequently, the film not only suffers from poor defect passivation, but also exhibits insufficient mechanical flexibility. An appropriate amount of PVP results in a moderate degree of crosslinking, achieving the most effective defect passivation and stress relaxation, thus significantly reducing leakage current pathways. When a high concentration of PVP is doped, aggregated PVP introduces additional interface defects and space charge accumulation, leading to a marked increase in leakage current density. After bending, as shown in Figure 4c, the leakage current density of each composition increases by about 1–2 orders of magnitude. This trend is attributed to the increase in crack and defect density in stress-concentrated regions, which provide additional leakage current channels for carriers. Although the leakage current increased after bending, the P05 component with the lowest leakage current density still maintains a relatively low level, with a value of 4.12 × 10⁻⁷ A/cm²@1 MV/cm, meeting the basic requirements of the dielectric layer for low-power flexible TFTs.

To analyze the leakage current conduction mechanism, we performed piecewise fitting of the J–E data using different leakage current models [27,28]. Since bending introduces stress-induced cracks and defects, which makes it difficult to evaluate the leakage mechanism with a single model, this work focuses on discussing the leakage current behavior on flexible PI substrates before bending. The Schottky emission and Poole–Frenkel (P–F) emission are expressed by Equations (2) and (3), respectively:

$$J_{\mathrm{SC}}=A{T}^2 exp\left[-\left(q{\varnothing }_{\mathrm{S}\mathrm{C}}-{\beta }_{\mathrm{S}\mathrm{C}}{E}^{1/2}\right)/kT\right]$$

(2)

$$J_{\mathrm{PF}}=CE exp\left[-\left(q{\varnothing }_{\mathrm{P}\mathrm{F}}-{\beta }_{\mathrm{P}\mathrm{F}}{E}^{1/2}\right)/kT\right]$$

(3)

where A and C are constants, $q{\varnothing }_{\mathrm{S}\mathrm{C}}$ is the schottky barrier height, $q{\varnothing }_{\mathrm{P}\mathrm{F}}$ is the trap energy level, ${\beta }_{\mathrm{S}\mathrm{C}}={\left({\displaystyle \frac{q^3}{4\pi \epsilon {\epsilon }_0}}\right)}^{1/2}$, ${\beta }_{\mathrm{P}\mathrm{F}}={\left({\displaystyle \frac{q^3}{4\pi \epsilon {\epsilon }_0}}\right)}^{1/2}$, q is electronic charge, $\epsilon$ is the dynamic dielectric constant and ${\epsilon }_0$ is the permittivity of free space. In the low electric field region (E < 0.6 MV/cm), as shown in Figure 5a, the slopes extracted from the lnJ–lnE fitting curves gradually approach 1 as the PVP doping concentration increases from 0 to 0.5 wt%, indicating that an appropriate amount of PVP doping effectively reduces the interface barrier and promotes ohmic conduction. When the doping concentration further increases, the slope begins to decrease, which is due to the excessive crosslinking and polymer stacking that introduces defect states, deviating from the ohmic conduction mechanism. In the high electric field region (E > 0.6 MV/cm), the slope of the fitted curve of ln(J/E)–E^1/2 was extracted, as shown in Figure 5b. The linearity of ln(J/E) versus E^1/2 for P10 and P20 suggests that trap-assisted Poole-Frenkel type emission becomes more pronounced at high electric fields. This demonstrates that excessive PVP doping introduces a large number of traps, facilitating electron hopping and thereby creating leakage current channels. Combined with the XPS and J–E analyses, these traps originate mainly from defects within the system, including oxygen vacancies (V_O), M-OH groups, and interfacial defects induced by the accumulation of excess PVP. The non-monotonic leakage behavior indicates that PVP incorporation plays a dual role: defect passivation at moderate concentrations and trap generation at excessive concentrations.

2.5. Dielectric Properties and Frequency Stability

Figure 6 shows the capacitance–frequency (C–F) characteristics of the ZrAlO_x-PVP hybrid dielectric films before and after static bending at a radius of 20 mm, along with the dielectric constant measured at 1 kHz. According to Equation (4):

$$C_i={\displaystyle \frac{{\epsilon }_{0}k}{d}}$$

(4)

where ${\epsilon }_0$ is the vacuum permittivity (8.85 × 10⁻¹² F/m), d is the film thickness and k is the relative dielectric constant. As the PVP doping concentration increases, the dielectric constant before and after bending both show a trend of first increasing and then decreasing. This trend arises because an appropriate amount of PVP improves the dispersion of inorganic particles and enhances interfacial bonding, thereby increasing the effective dielectric constant. However, as the PVP concentration further increases, a dilution effect dominates, causing the dielectric constant to decline. The values before bending are 5.10 (P00), 6.86 (P02), 8.88 (P05), 8.04 (P10), and 7.56 (P20); after bending, they are 4.84 (P00), 6.68 (P02), 8.88 (P05), 7.87 (P10), and 6.41 (P20). The dielectric constant of the components with PVP doping concentrations of 0–1 wt% shows a slight decreasing trend (<5.1%) after bending, with weak frequency dispersion, suggesting that both interface traps and polarization losses are low. While the P20 component shows a significant drop (−15.2%). This is because the bending stress leads to a sharp increase in interfacial defects and severe damage to polarization, resulting in a substantial decrease in the dielectric constant. At a doping concentration of 0.5 wt%, the dielectric constant reaches its maximum value of 8.88. A higher k value means a physically thicker dielectric layer can be used to suppress leakage current while maintaining or even increasing the capacitance density, which is beneficial for achieving good gate control in subsequent TFT devices. The P05 component exhibits the highest capacitance density: 384 nF/cm²@1 kHz before bending and 383 nF/cm²@1 kHz after bending. The capacitance density is determined by both the dielectric constant and the thickness. Therefore, although the P05 film is thicker than the P00 and P02 films, its excellent dielectric constant gives it the highest capacitance density. The P00 component also shows a relatively high capacitance density, which results from its small film thickness offsetting the influence of its lower dielectric constant. However, due to the film thickness of this component that has not been optimized by PVP doping, P00 component suffers from a much higher leakage current.

2.6. Substrate-Dependent Optimal Crosslinking Under Flexible Conditions

From the above analysis, the P05 component exhibits the lowest leakage current density, the highest dielectric constant, the best bending stability, and good high-frequency response. It is therefore the component with the best overall electrical performance and mechanical stability on flexible PI substrates. In the previous film structure characterization, the P10 component based on rigid glass substrates showed the most excellent surface morphology characteristics. The optimal PVP concentration shifts from the structurally favorable P10 condition on rigid substrates to the electrically favorable P05 condition on flexible PI substrates. A substrate-dependent optimal crosslinking behavior is identified, originating from the competition between structural densification and stress relaxation under flexible conditions. This substrate-dependent optimization behavior indicates that the dielectric performance of hybrid films is governed not only by intrinsic structural densification, but also by thermo-mechanical coupling at the film/substrate interface. On glass substrates, the P10 crosslinked network is the densest, with the smallest free volume and the best structural performance; while the crosslinking density of P05 is slightly lower than that of P10, and it has a larger free volume. When transferred to flexible PI substrates, there is a significant mismatch in the coefficient of thermal expansion (CTE) between PI and inorganic metal oxide films (the CTE of rigid glass is usually in the range of 7–10 × 10⁻⁶/°C [29], and the CTE of PI is 30–60 × 10⁻⁶/°C [30]), and thermal stress will be generated during the cooling step of the annealing process. Excessive crosslinking improves network rigidity but reduces segmental mobility, making the film less capable of relaxing thermal stress generated by CTE mismatch during cooling. In contrast, moderate crosslinking at 0.5 wt% PVP provides sufficient defect suppression while preserving stress relaxation capability, leading to improved electrical stability under bending. The mechanism is schematically illustrated in Figure 7.

2.7. Flexible TFT Device Demonstration with P05 Dielectric

To verify the feasibility of the P05 component for practical applications, flexible multilayer ZrAlO_x-PVP/IGZO TFT devices were fabricated. The transfer and output characteristic curves were measured, and the test results are shown in Figure 8. The mobility (μ_sat) and threshold voltage (Vth) were extracted from Equation (5). The Subthreshold swing (SS) was calculated by fitting Equation (6):

$$I_D= \mu {\displaystyle \frac{W}{2L}}{C}_i{ \left(V_G- V_{th}\right)}^2$$

(5)

$$\mathrm{S}\mathrm{S}={\left({\displaystyle \frac{d\log \left(I_D\right)}{d{V}_G}}\right)}^{-1}$$

(6)

where $C_i$ represents the capacitance density of multilayer ZrAlO_x-PVP hybrid dielectric, W and L are the width and length of the channel, V_G represents the gate voltage and I_D represents drain current. The TFT parameters extracted from the fitting are summarized in

Table 2. Electrical parameters of ZrAlO_x-PVP hybrid dielectric films with P05.

P05	Mobility (cm2 /V·s)	Ion (A)	Ioff (A)	Ion/Ioff	Vth (V)	SS (V/dec)
Before Bending	11.84	6.31 × 10−5	5.51 × 10−9	1.15 × 104	0.48	0.24
After Bending	5.91	4.45 × 10−5	1.72 × 10−8	2.59 × 103	0.15	0.36

. The data in the table show that the flexible TFT device with the P05 dielectric exhibits excellent performance before bending. Its electron mobility reaches 11.84 cm²/V·s, with a low Vth of 0.48 V, a subthreshold swing of 0.24 V/dec and I_on/I_off of 1.15 × 10⁴. This indicates that the device with P05 component enables good gate control and can be turned on without requiring a high driving voltage. After static bending with a radius of 20 mm, although the device performance degrades to some extent, it still maintains stable switching behavior, demonstrating that the P05 dielectric layer possesses structural integrity against mechanical stress and holds application value in flexible electronic devices.

3. Materials and Methods

3.1. Preparation of ZrAlO_x-PVP Precursor Solutions

The total metal ion concentration in the self-combustion precursor solution was fixed at 0.2 M. In our previous work, the optimal molar ratio of zirconium nitrate [Zr(NO₃)₄·5H₂O, 99.99%] to aluminum acetylacetonate [Al(C₂H₅O₂)₃, 98%] for the preparation of ZrAlO_x dielectric films was determined to be 1:1 [31]. PVP ([CH₂CH(C₆H₄OH)]_n, Mw ca. 25,000) was introduced as the organic component. Five groups of precursor solutions were prepared by adding PVP at mass percentages of 0 wt%, 0.2 wt%, 0.5 wt%, 1 wt%, and 2 wt%, as shown in Table 1. The solvent was mixed in a volume ratio of DMF:DI = 4:1, and 3 mL of solvent was taken for each component to ensure complete dissolution of the inorganic and organic phases. All reagents were purchased from Macklin Biochemical Co., Ltd. (Shanghai, China), and all are manufactured in China. The mixture was stirred at 50 °C for 4 h to ensure complete dissolution of the mixed precursors until it became clear and transparent. After that, the precursor solution was filtered through a 0.22 µm organic syringe filter to remove any undissolved particles or impurities. Finally, the filtered solution was aged for 24 h to obtain the spin-coating solution.

3.2. Film Fabrication and Characterization

The glass substrates and the PI substrates with dimensions of 10 mm × 10 mm were ultrasonically cleaned in the sequence of deionized water—isopropanol—deionized water—isopropanol. Each ultrasonic treatment lasted for 15 min. Then, they were placed in an oven and dried at 80 °C for 12 h. The spin-coating deposition of the ZrAlO_x-PVP hybrid dielectric film is schematically illustrated in Figure 9. After UV treatment on the substrate surface for 20 min to improve hydrophilicity, 40 μL of the precursor solution was dropped onto the substrate. The spin-coating process was carried out with a low spin speed of 500 rpm for 5 s, and a high spin speed of 5000 rpm for 30 s, resulting in a uniform dielectric film. The spin-coated films were dried at 80 °C for 10 min and then annealed in air at 180 °C for 1 h. The atomic force microscopy (AFM, CSPM5500, Beijing, China) was used to measure the surface roughness of the films. Film thickness was measured by the X-ray reflectivity (XRR, PANalytical Empyrean, Almelo, The Netherlands). The transmittance and optical bandgap of the films were characterized using a UV-visible spectrophotometer (UV-Vis, UV-3600 Shimadzu, Kyoto, Japan). The X-ray photoelectron spectroscopy (XPS, Nexsa, Waltham, MA, USA) was used to analyze the chemical states of elements on the film surface.

3.3. MIM and TFT Devices Fabrication and Electrical Measurements

Metal–insulator–metal (MIM) capacitors were fabricated on flexible PI substrates. With a 300 nm thick Al as the bottom electrode, after spin-coating the dielectric film, a 101 nm thick array of aluminum top electrodes was evaporated onto the ZrAlO_x-PVP hybrid dielectric films. Each top Al electrode has an area of 1.256 × 10⁻³ cm². The MIM devices were obtained, as shown in Figure 10a. The current–voltage (I–V) and capacitance–frequency (C–F) characteristics of the MIM devices were measured using a semiconductor parameter analyzer (Primarius FS-Pro, Beijing, China). The devices were fixed onto a cylinder with a bending radius of 20 mm for static bending, and their I–V and C–F characteristics were measured while the devices remained in the bent state.

Flexible multilayer ZrAlO_x-PVP/IGZO TFT devices were fabricated using an interlayer annealing process. The multilayer hybrid dielectric structure improves the surface morphology and interface quality, enhancing the electrical performance of the TFT devices. A bottom-gate top-contact structure was adopted, as shown in Figure 10b. The spin-coating and annealing parameters were exactly the same as those used for the single-layer films in MIM devices. Additionally, each dielectric layer after annealing needed to undergo UV treatment for 20 min before proceeding to the next layer fabrication. A total of three ZrAlO_x-PVP dielectric layers were spin-coated. IGZO was then deposited by DC magnetron sputtering, followed by UV treatment for 30 min and annealing at 100 °C for 1 h. Finally, 100 nm thick Al electrodes were evaporated onto the IGZO layer. The transfer and output curves of the TFT devices were tested using the semiconductor parameter analyzer (Primarius FS-Pro, China). The devices were fixed onto a cylinder with a bending radius of 20 mm for static bending, and their transfer and output curves were measured while the devices remained in the bent state.

4. Conclusions

This work fabricated ZrAlO_x-PVP hybrid dielectric films on flexible PI substrates via a self-combustion method at 180 °C. The sample doped with 0.5 wt% PVP achieved the lowest leakage current of 1.89 × 10⁻⁸ A/cm²@1 MV/cm and the highest dielectric constant of 8.88 before bending, and maintained stable performance after bending. For the first time we reveal that the optimal doping concentration is substrate-dependent: 1 wt% gives the densest structure on rigid glass, while on flexible PI, due to thermo-mechanical coupling effects, the thermal expansion mismatch of film/substrate interface generates thermal stress. Consequently, the moderately crosslinked 0.5 wt% PVP composition is more effective at relaxing stress and maintaining fewer defect states, demonstrating superior electrical and mechanical stability. The shift indicates that the design of flexible dielectric layers must take into account both structural densification and stress relaxation capability in order to balance defect suppression and mechanical stability. Flexible ZrAlO_x-PVP/IGZO TFTs based on 0.5 wt% dielectric show good switching and remain functional after bending. In summary, the ZrAlO_x-PVP hybrid dielectric films have a good application prospect in flexible displays and wearable devices. This result provides insights into crosslinking regulation and thermo-mechanical coupling optimization for the design of low-temperature, flexible high-k hybrid dielectric materials.