Development of novel flexible photodetectors based on 0.5PVA/0.5PVP/Fe:NiO nanocomposite system with enhanced optoelectronic properties

Ordinary casting technique has been used to fabricate the intrinsic films of polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) polymer blend matrix incorporated with dissimilar weight percent contents of Fe:NiO nanocomposite (NC). X-ray diffraction (XRD) and infrared (IR) spectroscopy has been implemented to analyze NC structure of these films. Significant interaction and tuning of PVA/PVP blend matrix due to Fe:NiO NC is detected. XRD pattern reflects the structural modification and partial crystalline nature of the pristine blend matrix. The corresponding peaks of Fourier transform IR identifies the vibrational group of the synthesized samples. Atomic force microscope images indicate that a change in the Fe:NiO concentration in a pristine blend leads to an increase in the roughness and clusters. Numerous optical factors such as E g (transition band gap), refractive index (n), and E ed (absorption edge) of pure blend and blend films with different concentrations of Fe:NiO were assessed via UV–Vis optical absorption spectra. As we increased the doping concentration in pristine blends, the value of E g goes to decrease. The dispersion (E d) and oscillator (E o) energies were calculated from Wemple and Di. Domenico of a single oscillator. Although, optical susceptibilities and nonlinear refractive index were enhanced by doping with Fe:NiO NC. The change in doping content leads to modifications in the optical limiting. The photocurrent density–voltage properties of the present polymers were studied at different values of white light intensity. It was found that the photoconductivity of the PVA/PVP blend is 1.03275 × 10−8 (Ω cm)−1, while NC films varied in the range (1.03–10.6954) × 10−8 (Ω cm)−1 at 8600 lux. In addition, the photosensitivity increased from 13.82 to 24.08. The recombination process was found monomolecular process for pure and doped polymers. The present films assume the possibility of their uses in optical and photo-electric devices.


Introduction
Nowadays, nickel oxide (NiO) has been considered a promising functional material with a cubic lattice structure. The space group of NiO is Fm3m having lattice parameters a = b = c = 4.177 Å [1]. NiO has attracted the researcher due to its numerous potential applications such as chemical gas sensors, transparent conducting electrodes for display including spintronics, supercapacitors, and magnetic properties for optoelectronic devices [2][3][4][5]. NiO has shown remarkable chemical stability and durability along with good thermal stability, low cost, and is environmentally friendly [6][7][8][9]. Besides this, NiO has been used as a hole transport layer due to its varied wide bandgap (3.45-3.85 eV) tuning with a new class of hybrid perovskite solar cells [10].
It has been well known that the properties of a material undergo extensive alteration on doping with external elements. In such cases, host matrixes only act as a vehicle, but their properties are exclusively controlled by the dopants. For this purpose, diverse metals (such as Co, Mg, Mn, Sn, Fe, and In) with varied concentration has been used as a dopant in NiO matrix [11][12][13][14]. The distribution and size of the dopant material in the polymer matrix is a considerable interest because such type of nanocomposite (NC) can paved the way to control and adjust the optical parameters of the materials [15][16][17][18][19][20].
It is well known that NiO is a prototype p-type transparent conducting transition metal oxide [21,22] due to vacancy at Ni 2+ sites [23] exhibiting fcc structure. It is observed that the magnetic, electrical, optical, and structural properties can be tuned with the incorporation of transition metal elements, particularly at Ni site which facilitates the further exploration of this material. Due to these key and unique properties of NiO, it is necessary and justified to bring some modifications to its key properties by implementing the appropriate size metal dopant. This can provide a huge conducting charge and can be retained by doping concentration according to requirements. There are diverse methods to synthesize NiO nanoparticles and their NC; among them, combustion, thermal decomposition, microwave irradiation, chemical precipitation, sol-gel, microemulsion, and hydrothermal are most frequently and widely used [24][25][26][27][28][29][30].
The comparable ionic radii of dopants (Fe 2+ ) with the pristine Ni 2+ (0.70 Å) allows the effective substitution of Ni 2+ ions which results in significant alteration of structural electronic and optical properties of the pure film. The switching of magnetic properties of NiO that is from antiferro to ferro has been observed at room temperature with the aforementioned doping with Fe 2+ ions.
It has found from the comprehensive literature survey that different researchers around the globe have used different methods for the preparation of NiO thin films. Recently, Pepe et al used the spin coating method to fabricate binary NC films of NiO doped with Cr and Sb metal ions along with annealing treatment of films at 400 • C in air ambient [31]. Whereas, Shkir et al also used the spin coating technique to fabricate binary NC films of Fe:NiO and annealed them for 2 h at 450 • C [32]. Godbole et al prepared a binary composite of Fe:NiO films through simple spray pyrolysis and investigated the effects of the volumetric concentration of the precursor solution and the substrate temperature [33]. The inclusion of Fe-doped NiO in a polymer matrix enhances its applicability in the optoelectronic field. It is found that doping of polymers brings strong changes and permits a researcher to get advanced material with superior belongings [34,35]. Polyvinyl alcohol (PVA) attracted the scientific community towards itself because of its unique physical and chemical features [36], which make it an essential candidate to be used in material science. PVA is very useful and mostly used in medical devices due to its higher water solubility; low protein adsorption features and its biocompatible nature [37]. An environmental stability, smooth formation, miscible water-soluble properties in all concentrations, and intermediate electrical conductivity lead to the frequent usage of polyvinyl pyrrolidone (PVP) in electrochemical devices (batteries, displays) [38] and in combination shows the characteristics of both polymers [39].
Among available matrices, a blend of more than one polymer matrix is also appropriate for fabricating metal-doped polymer films. The correlation between polymers as blends is dependent on the H-bonding and their miscibility. Every so often, the blend has a marked effect on the thermal stability, mechanical, and optical properties of the parent polymers that cannot be talented by a single matrix. In the present work, we used PVA/PVP as a host polymeric blend.
The composites of metal oxides and PVA/PVP blend polymers have an enormous phenomenon that has excellent applications in recent electronics. The scheme of the paper is as follows: at first mechanism of synthesis of Fe:NiO NC and subsequently formation of polymer composition films with varying weight percent concentration, followed by the analysis of physical properties. This is carried out by common methods which include atomic force microscopy (AFM), x-ray diffraction (XRD), Fourier transform infrared (FTIR), linear and nonlinear optical, and photocurrent properties.

Synthesis of NCs/blend films
Commercially Ni(NO 3 ) 2 ·6H 2 O (Nickel nitrate hexahydrate), Fe(NO 3 ) 3 ·9H 2 O (Iron nitrate nonahydrate), and citric acid source from Alfa Aesar Pvt, LTD to synthesize the Fe:NiO NC via facile flash combustion synthesis as reported elsewhere [40]. PVA and PVP were purchased from Sigma-Aldrich to prepare the blend matrix through the casting approach. PVP and PVA have Molecular Weight (M.W) of 40 000 g mol −1 and 115 000 g mol −1 . First of all 45 g of PVA was dissolved in 1 l of deionized water (DW) under constant stirring (ω = 400 rpm) at 90 • C for 3 h till a homogeneous and transparent solution was obtained. In a separate bottle, the same conditions were applied using a magnetic heating stirrer for 45 g PVP/ 1 l DW at 50 • C/2 h. Then, both solutions are mixed for 10 min via an ultrasonic probe at room temperature. Different weights of Fe:NiO NCs were mixed with PVA/PVP blend by adjusting the ultrasonic amplitude to 100 for 2 min. Each mixture was cast in clean circular Petri dishes. The dried free films were removed from plates after leaving them in an oven at 35 • C for 3 d. Using the following equation the percent of Fe:NiO in a polymeric blend solution was calculated: As w blend and w Fe:NiO represent the weights of polymers and NCs. The weight percentages of Fe:NiO in the blend are 0.037, 0.37, 1.85, and 3.7 wt%. The final film's thickness is an important parameter for optical properties, as it was measured by a micrometer. The average thickness of films with 0, 0.037, 0.37, 1.85, and 3.7 wt% Fe:NiO doped blend are 0.024, 0.025, 0.023, 0.024, and 0.025 cm, respectively.

Characterization methods
XRD-6000 Shimadzu LabX diffractometer of Cu-Kα monochromatic radiation (λ = 1.5418 Å) was operated to measure the crystal structure of Fe:NiO NCs and polymer-based materials from the XRD patterns. The XRD system was worked at 30 mA and 40 kV, over 5 • -70 • at 2 • m −1 . For functional vibration group's analysis, a Thermo Nicolet 6700FT-IR was used, with a wavelength range of 400-4000 cm −1 and resolution of 5 cm −1 .
AFM is used to measure the 3D morphological surface of the films. Semi-contact mode with a tip of 10 nm radius was used for measuring. The cantilever of spring constant 25 N m −1 was operated at 265 kHz.
The linear and nonlinear optical parameters were calculated by measuring the transmittance and optical absorbance of the films by utilizing JASCO model V-570 spectrophotometer in UV-Vis-NIR ranges.
A sensitive power meter (Master Ultima Lab, COHERENT, USA) and photodetector (1916R model; Newport) were connected to detect the power of the output laser that passed through the films. In this measurement, we used a He-Ne source of 635 nm and a green beam of 532 nm.
The photocurrent density-voltage at various light intensities were measured to specify the photosensitivity and the recombination process mechanism in all films. This measurement was carried out via a simple photoelectric circuit in which the sample was joined with a DC-power supply, specific electric resistance, and a Keithley electrometer (Model: 4200-SCS). For good connection, the film was deposited by a silver paste electrode and secured on a sample holder through Canada balsam.

Results and discussion
3.1. XRD analysis XRD spectra of the pure PVA/PVP blend matrix and blends doped with Fe:NiO NC with varied concentrations has displayed in figure 1. For the spectrum of the PVA/PVP matrix, there is a single and broader peak corresponding to (110) plane at 2θ = 19.46 • that only exists. The good compatibility among the blend components may be indicated by the appearance of this single peak for the blend matrix. It indicates the semicrystalline nature of the blend. After doping with Fe:NiO NC the broadness and the intensity of this peak decreased more. Besides this, it is perceived clearly that the peak position is marginal changes towards the higher 2θ. However, the intensity of the peak decreases with a gradient increase in Fe:NiO NC content. The interactions between the PVA/PVP polymer matrices with dopant NCs are verified from the shifting and decrease in the intensity of the peak. Consequently, this leads to a decline in intermolecular interactions.
XRD pattern of Fe doped NiO (Fe:NiO) NC shows intense peaks at 2θ = 37.30, 43.32, 62.94 • which indexed for the (111), (200), and (220) reflections as reported by Choudhary and Sengwa [41]. It is found that at low doping concentrations of Fe:NiO (1.85 and 3.7) wt% in pure blend these reflection peaks have not been found, which may be due to the presence of Fe:NiO in insignificant fillings. But, as we increase the doping concentration of Fe:NiO NC in the pure blend the intensity of these peaks goes on increasing according to Rietveld refinement. XRD

Atomic force microscopy (AFM)
AFM study gives an idea of surface modifications in material and roughness of films in numerous scales along with a good resolution [42]. In current research work 3D pictures of PVA/PVP blend matrix and Fe:NiO NC doped films are presented in figures 3(a)-(e). AFM images clearly identified bright spots on the surface of films, aforementioned the dopant ions alter the surface morphology [43].
Diverse surface morphology is observed at each dopant level as compared to pristine blend film. It also confirms the interaction between PVA/PVP blend and Fe:NiO like other characterization techniques. The

FTIR absorption spectra
IR spectroscopy is a dynamic technique that is very widely used to recognize and designate the interface interaction between the host and dopant [45]. FT-IR spectra for pure blend matrix and Fe:NiO doped blend over a wide range of wavenumber (4000-500 cm −1 ) are displayed in figure 4. From PVA and PVP spectrum all the vital bands are clearly perceivable. Table 2 summarized the details of the different typical absorption band positions and their assignments for chemical groups. However, in the case of doped with Fe:NiO, there is a slight shift in bands, and their respective band intensities are perceived with respect to the pristine blend one which is a clear indication of the significant interaction between the blend and doped materials synchronizes to the XRD spectra. The peak of adsorption spectra at 1654 cm −1 corresponds to ν(C=C) stretching vibration while the peak at 1080 cm −1 corresponds to the carbonyl group ν(C=O). The sensitivity of intensities of Fe:NiO NC can be altered with the effect of doping [46]. Figures 5(a) and (b) displayed the UV-Vis-Near Infrared (NIR) data points of pure PVA/PVP blend and blend/Fe:NiO films. The incorporation of Fe:NiO NC with varied concentrations led to a decrease in the transmittance (T r %) from 92% to 5% at 1000 nm ( figure 5(b)). The low peak PVA/PVP of the pristine blend is identified in the visible region of absorption spectra ( figure 5(a)). However, optical absorption increased for blend/Fe:NiO doped films in the visible spectrum may be due to some agglomeration of particles. On the other hand, doping of Fe:NiO in the blend affected the absorption edge. With varying concentrations of Fe:NiO NC in the pristine blend, a redshift has been detected in the absorption edge. The adsorption edge energy, Urbach, and band gap values for fabricated films are calibrated through the absorption coefficient (α). Equation (2) is used to estimate α [47], here Abs is absorbance and t is the thickness of the corresponding film:

Optical properties of the films
(2)   Figure 6 displayed the relationship between photon energy versus absorption coefficients (α) for all prepared thin films. By extending the linear part of this plot to Abs = 0 determined the E ed (absorption edge). The value of α depreciated in Fe:NiO/blend with respect to the pristine blend clearly indicates the band edges of nano-composites and hence it is directly related to the variations in the electron and the hole's number in the V (valence) and the C (conduction) bands [44]. The deviation of the value of α also points toward the geometrical alterations in blend matrices and the Fe:NiO NC which is in coherence with the XRD and FT-IR spectra. From the analysis, the value of α decreased from 5.11 eV to 0.62 eV with doping of Fe:NiO. This redshift indicates a rising of Urbach energy or localized bandwidth. The Urbach energy (E u ) value is calculated by following the formula (equation (3)): Figure 6(b) shows the relationship between the log α and photon energy for each polymeric NC film. The reciprocal of the slope of each line indicates the width of localized states improved and the band tail shifted inside the forbidden band. Its value is 0.745, 0.842, 1.369, 3.254, and 3.515 eV for blend polymer films doped with 0, 0.037, 0.37, 1.85, and 3.7 wt% Fe:NiO. This shows the impact of doping concentration of double metal on the band gap of PVA/PVP blend film. A similar effect was observed for PVA doped with Ni 2+ -ions [48].
The study of optical energy band gap for films for pristine blend and after doping of Fe:NiO NC is to be inspected from optoelectronics usage's point of view ( figure 7). The investigation of energy band gap for films was done by Tauc's equation [49,50] by applying equation (4):  To illustrate the electronic polarization of material via the ions in optical physics, the study of the refractive (n) index is one of the important and fundamental parameters for the development optical devices [52]. Herein, the refractive (n) index was calculated to study the impact of Fe:NiO interaction on the optical properties of the blend matrix. The change of (n) with the wavelength (λ) is shown in figure 8. From k (extinction) and R (reflectance) values, the refractive index was calculated using the following equation [53]: Figure 8 shows the decrease value of the refractive index (n) at a small wavelength for all films. However, at a longer wavelength, a saturated value of n is observed which is referred to as a normal dispersion of light. It is observed that the refractive index (n) is enhanced with the incorporation of the Fe:NiO NC at varied Consequently, with an effective value of n as mentioned above, the dispersion and oscillator energies were calculated Wemple and Di Domenico (WDD) of a single oscillator has given a relation [54]. These are the imperative parameters for optoelectronic devices: Herein, E d and E 0 have given the variations associated with the structure order and the intensity of electronic transitions in the materials. By plotting a graph between (n 2 − 1) −1 vs (hυ) 2 relations for the as-prepared films of pure blend and Fe:NiO doped blend at varying concentrations ( figure 9). The E d value significantly increased from 5.22 eV to 22.038×10 3 eV, whereas the E o decreased with an addition of Fe:NiO from 4.98 to 2.77 eV for Fe:NiO doped blend films. The present E d value is higher than that reported for PVA doped with 18.5 wt% Bi-nanoparticles (2.508 × 10 3 eV) and for PVA/Polyethylene glycol (PEG) doped with NiO (12.88 eV) [55]. E d signifies the strength of the interband optical transitions and it is correlated to the number of free valence electrons available for a specific transition. Thus, the significant increase in E d points to the rise of the charge transfer between the Fe:NiO NC and the blend's macromolecules, as well as proves the effect of doping to induce new structure modifications within the bandgap [44].
For a comprehensive understanding of the light-polarization property of pure blend matrix and Fe:NiO doped blend optical characteristics in terms of the nonlinear optical susceptibility and refractive index were calculated in the current research work. The relation between the applied electric field and the induced polarization in the material is provided by the linear and higher-order susceptibilities: χ (1) , χ (2) and χ (3) are optical susceptibility parameters: where A is given by 1.7 × 10 −10 e.s.u [56]. Thus, n (2) can be obtained from the below equation [57]:  The first and third-orders susceptibility tends to increase with the Fe:NiO doping percentage. As n (2) is calculated as a direct function of third-order susceptibility, it has a similar manner. From table 3, it is noticeable that the present Fe:NiO NC is displaying a considerable effect on the host polymeric blend and will gain a substantial impact for photonic devices.

Optical limiting properties
Optical limiting materials are most commonly used for various applications including laser protection and optical sensors [58]. Herein, the output power of pure blend PVA/PVP and blends doped with Fe:NiO NC with varied concentrations were examined via two sources of laser (He-Ne and green light). Also, the normalized (output/input) power was calculated as presented in figure 10. The output power from each film indicates the high transmission of the pure and Fe:NiO doped blend films with low concentration (0, 0.037 wt%). Nevertheless, the power of the incident light was reduced significantly using blend films with 0.37, 1.85, and 3.7 wt% of Fe:NiO. This is matching with the optical transmittance at the same wavelength. Thus, the varied doping percentage of Fe:NiO NC plays an essential factor in the optical limiting of the polymeric films. However, the variation in the normalized power between the different laser sources is related to the film's sensitivity to light. The optical limiting of blend NC films is associated with the ability of the material to absorb and/or scatter light via particles on the film's surface.

Photo-current properties
Studying the photoelectric properties of pure and doped polymers is important for optoelectronic applications. Thus photocurrent density (J)-Voltage (V) characteristics have been measured. In figures 11(a)-(e) the photocurrent density has been plotted as a function of the applied bias voltage for the studied polymers in the dark and the presence of different levels of illumination (0, 1500, 2600, 3600, 5800, and 8600 lux). For blended polymer ( figure 11(a)) and the blend containing different percentages of Fe:NiO NC (figures 11(b)-(e)), it was viewed that both the dark current and photocurrent density increase linearly with increasing the applied voltage indicating Ohmic behavior. Also, it was observed that the straight lines of higher illumination levels have higher slopes since the generation rate increases with light intensity. From the slope of all lines, the photoconductivity was calculated. Table 4 shows the results of the photoconductivity at the different values of the light intensity.
Moreover, the effect of Fe:NiO NC on PVP/PVA blend on dark and photo conductivities at different values of white light intensity (0, 1500, 3600, and 8600 lux) is shown in figure 12(a). By adding Fe:NiO NC and increasing its percentage to 0.37 led to a significant increase in dark and photo-conductivity, this means    accumulation of many nanoparticles and the formation of more bonds between the polymer and Fe:NiO [58]. The values of photosensitivity (S), for each illumination level, can be determined by the relation [59]: where σ ph and σ d are the photoconductivity and the dark conductivity respectively. The effect of Fe:NiO NC concentration on the photosensitivity of blend polymer is displayed in figure 12(b). The results show an increase in the photosensitivity with increasing the Fe:NiO NC content and the light intensity due to the increase in the generation rate of photo carriers. The photosensitivity of pure and doped polymers for all white light intensity values is recorded in table 5. The photocurrent has been plotted as a function of light intensity for the classification of the pure and doped polymers with respect to the types of the recombination process as shown in figure 12(c). These figures show the behaviors are straight lines indicating that the photocurrent and light intensity are related together by the power law [58]: The results showed that the gamma values ranged between 0.6 and 0.8, and this means that the recombination process is of the type a monomolecular process for pure and doped polymer. The monomolecular process means that there are traps distributed within the forbidden gap for all films [60].

Conclusion
In summary, FTIR and XRD characteristics have shown the successful formation of the intermolecular interaction between blend/Fe:NiO NCe films. The investigation of the refractive index illustrated a normal dispersion in the wavelength range of 1200-2000 nm. There is a decrease in the energy band gap as the concentration of Fe:NiO NC content in the polymeric blend increases. Also, the optical absorption coefficient confirmed the redshift of the band gap. The dispersion (E d ) and oscillator (E o ) energies that are calculated using WDD model of a single oscillator show a significant change in the Fe:NiO/ blend NC films. The static (n o ) and nonlinear (n (2) ) refractive indices were enhanced with the Fe:NiO concentration in the blend. In addition, there was also quite an improvement in the linear (χ (1) ) and third-order (χ (3) ) susceptibilities. The photosensitivity of pure PVA/PVP was improved with the light intensity and Fe:NiO content in the films which boosted the possibility of utilizing them as key applicant solar cells. The findings of this work significantly agree that the present films are encouraging flexible NCs for numerous photo-electric applications including optical limiting.

Data availability statement
The data generated and/or analysed during the current study are not publicly available for legal/ethical reasons but are available from the corresponding author on reasonable request.