1 Introduction

Metal/semiconductor structures (Schottky diode) have become an important tool in the identification of semiconductors. Schottky diodes and similar metal/semiconductor structures are widely used in various experiments to assess the properties of semiconductor materials. These structures provide information about the electrical properties, carrier lifetimes, surface conditions, and other characteristics of semiconductor materials. In particular, Schottky diodes are utilized to examine the electrical properties of semiconductor surfaces. The current–voltage (I–V) characteristics of Schottky diodes can be correlated with factors, such as the surface condition, surface area, and surface cleanliness of the semiconductor. Therefore, Schottky diodes are commonly employed as a tool to comprehend the structural and electrical properties of semiconductor materials, because these structures play a critical role in the production of important electronic devices, such as solar cells, photodetectors, photodiodes and field-effect transistors (FET) [1].

Schottky diodes have rectifying properties like p-n junctions and consist of a metal and semiconductor with a region where active charges occur [2,3,4]. “p”- or “n”-type semiconductor materials can be used in the production of Schottky diodes.

In Schottky junctions, the interface layer between the semiconductor and metal is an effective layer that influences the diode’s performance, stability, and dependability. Interface states are induced by the location of the electrical charges present in the interface as well as its composition, quality, and non-stoichiometric structure. The image force lowering effect is the main parameter influenced by Schottky barrier inhomogeneities. The metal–semiconductor barrier is lowered as a result of image force interaction with the field at their interface, a phenomenon known as image force lowering [3]. In addition to these junctions, many researchers are also interested in organic or inorganic interfaces that can change the electrical and optical properties of these devices. Performance parameters of metal/semiconductor contacts can be varied as a function of various external factors, such as illumination and temperature. For this reason, studies in this field have always been attractive.

It is very important to detect light with a device and use it in industrial applications. On the other hand, with the increasing energy need, studies continue to convert light into electrical energy in a low-cost and efficient way [5,6,7,8,9]. These devices are called photodiodes, and can convert light into current or electrical signals or detect it. On the other hand, metal/semiconductor structures with interface layers are very important in creating photodiodes due to their material diversity and applicability. In this field, devices consisting of p-type and n-type silicon ground conductors and thin-film connections are mostly used [10]. Although both silicon and germanium are generally used in semiconductor devices, the current trend is to use silicon. At room temperature, a silicon crystal has fewer free electrons than a germanium crystal. This means that silicon will have much smaller collector cut-off current than germanium. Silicone has a higher operating temperature. Therefore, silicone devices are not easily damaged by excessive heat. Peak reverse voltage values of silicon diodes are larger than germanium diodes. For this reason, Si is the most preferable semiconducting material in that field.

The interfacial layers are important factors that determine the usage areas of metal/semiconductor devices. Therefore, different inorganic and organic interface layers are used for material diversity and applicability. Some researchers have conducted research on visible and ultraviolet photodiodes with interfacial layers. Gozeh et al. have reported the Zn-doped CdO interlayers in Al/Zn–CdO/p-Si/Al photodiodes, with their electrical and optical attributes [11]. Çetinkaya et al. have reported Photovoltaic characteristics of Au/PVA (Bi-doped)/n-Si Schottky photodiodes [12]. In addition, metal oxides such as TiO2, Ga2O3, and WO3 have a wide band gap and are generally used to detect ultraviolet light [13,14,15]. Zinc oxide (ZnO), an inorganic compound with a wide and direct band gap, is also promising material for opto-electronic applications [16,17,18].

Polyaniline (PANI), which has many advantages, is the material that attracts the most attention among various conductive polymers. Because it is cheap, simple to prepare, resistant to oxidation and has controllable properties. Electrical properties can also be changed by doping [19,20,21]. In recent years, two-dimensional materials such as grapheme [22], silicene [23,24,25], germanene [26], phosphorene [27], arsenene [28], and antimonene [29] have received increasing attention from researchers. Many two-dimensional materials have been synthesized by researchers or their theoretical predictions have been made [30]. In addition to their unique physical and chemical properties, two-dimensional materials have linear band structures close to the Fermi level and high electrical and thermal conductivity. Therefore, they have a wide potential for use in electronic devices and energy storage [30]. Borophene is an atomic monolayer of boron crystal and has a two-dimensional allotrope. Many experiments and theoretical studies have been reported on many properties of borophene, such as its electronic structure, superconductivity and mechanical features, optical properties, thermal conductivity of lattice, atomic adsorption, and reactivity of surface [31,32,33,34,35,36,37,38]. Borophene has shown wide application potential as an anode material in metal ion batteries, thanks to its high capacity and excellent electronic and ionic conductivity properties, as well as some unique physical and chemical properties [30].

As it is known, oceans are the largest water resource in the world and are extremely important for human life. Today, determining the salinity of oceans, which have rich minerals and nutrients necessary for human life, is used to monitor the ocean environment and climate [39]. Because of changing in seawater salinity cause the formation of water masses and stratification in the ocean and the formation of internal ocean waves [40,41,42]. For this reason, studies on measuring seawater salinity have gained momentum in recent years [43,44,45,46,47], and the salinity of seawater is measured according to electrical conductivity, which is based on the relationship between ion content and conductivity [48].

Many studies on metal/semiconductor photodiodes created using different interface layers have been presented in the literature [49,50,51,52]. To our knowledge, no study has yet been reported on the a silicon-based metal/semiconductor structure consisting of PANI:Borophene interfacial material. In this context, PANI:Borophene/p-Si and PANI:Borophene/n-Si structures have been prepared and their photodiode properties have been examined using various measurements. Also, the PANI:Borophene/Si Schottky diodes have tested for salt detection. We believe that the results obtained in this novel study will potentially benefit further research.

2 Experimental details

2.1 Synthesis of PANI:Borophene and structural properties

The preparation stages of the exfoliated borophene was previously reported in literature [53,54,55,56] and this process can be seen in Fig. 1.

Fig. 1
figure 1

Preparation stages of the exfoliated borophene [53,54,55,56]

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2.2 Device fabrication

To prepare PANI:Borophene nanocomposites, 1-mg borophene and 1-mg PANI were added to 2-ml N-methyl-2-pyrrolidone solvent. The ingredients were mixed onto a magnetic stirrer for 5 min. Subsequently, the prepared solution was coated on cleaned p-type and n-type Si wafers. The wafers were cleaned via RCA procedure [57]. To prevent SiO2 formation and impurities on the surface, they were dealt with HF:H2O(1:10) solution. Aluminum Ohmic contact was formed at 10−6 Torr using physical vapor deposition. Spin coater was used to create a uniform PANI:Borophene interlayer at 1500 rpm. 150-nm Al and Au was coated onto the nanocomposite layer to create metallic contacts via physical vapor deposition [58,59,60,61,62,63,64]. Figure 2 shows the schematic diagram of fabricated Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices.

Fig. 2
figure 2

a The schematic diagram of devices. b Spin-coating processes of PANI:Borophene films

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2.3 Characterization

Characterization of the borophene nanosheets was carried out using X-Ray Diffraction Spectroscopy (XRD, PAN analytical Xpert-Pro) and UV–Vis diffuse reflectance spectroscopy (Shimadzu UV-2501PC). Fytronix FY-7000 solar simulator measurement system was used to obtain I–V and I–t characteristics under different power intensities of light illumination. The surface characterization of prepared borophene and PANI:Borophene nanocomposite were fulfilled by High-Resolution Transmission Electron Microscopy (HRTEM) JEOL JEM-ARM200CFEG UHRTEM) and Scanning Electron Microscopy (SEM).

3 Results and discussion

The TEM images of the prepared borophene are depicted in Fig. 3. In the 50-nm scale image in Fig. 3a, exfoliated borophene nanosheets are seen free-standing in DMF. When a 20-nm scale image is taken by focusing on one of the borophene nanolayers, it is seen that the borophene nanolayer consists of hexagonal honeycomb structures with a regular morphology (Fig. 3b). When this borophene nanosheet was selected and FFT analysis was performed, it was proven that the crystal structure of borophene was hexagonal, as seen in Fig. 3c. When the selected borophene nanosheet was focused a little more with a microscope and 10-nm and 2-nm scale images were taken (Fig. 3d, e), it was proven that borophene had a hexagonal structure with a very smooth morphology.

Fig. 3
figure 3

The TEM analysis images for (a) 50 nm (b) 20 nm (d) 10 nm (e) 2nm and (c) the FFT pattern of the borophene [53,54,55,56]

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Figure 4a shows the XRD analysis result of precursor boron and prepared borophene. Peaks related to the (003), (104), (021), and (324) planes of borophene have observed. These peaks prove that the prepared borophene is in the centrosymmetric β-rhombohedral phase (PDF31-0207 card).

Fig. 4
figure 4

a XRD analysis of the precursor boron and prepared borophene. b Tauc plots of the Kubelka–Munk function for the direct and indirect transitions of the prepared borophene

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Ultraviolet–visible (UV–Vis) diffuse reflectance spectroscopy has been conducted to determine the band gap energies. The Kubelka–Munk function method is applied to transform the collected data:

$$F\left(R\right)=\frac{{\left(1-R\right)}^{2}}{2R},$$
(1)

where \(R\) represents the diffuse reflectance value acquired from the diffuse reflectance measurements. The Tauc plot of the Kubelka–Munk function has been employed to estimate both direct and indirect band gap transitions of the prepared borophene as shown in Fig. 4b. The calculated band gap energies for the synthesized borophene have 0.94 eV and 1.74 eV for indirect and direct transition band gaps, respectively.

The SEM images of PANI and PANI:Borophene are given in Fig. 5, respectively. It can be seen from Fig. 5a that PANI thin film forms a smooth surface. The figure shows that PANI particles are nano-sized and the film is generally in agglomerated and uniform packages. The result of these uniform morphology and homogeneity provide an advantage for interfacial polymerization [65]. In Fig. 5b, a randomly grown PANI:borophene nano-shaped structure has observed. This PANI:Borophene nanocomposite thin film has created with fringes.

Fig. 5
figure 5

SEM images of the a prepared PANI thin film and b prepared PANI:Borophene nanocomposite thin film

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Figure 6 shows the typical semi-logarithmic current voltage (I–V) curves of the prepared structures Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si under dark and different light intensities (from 20 to 100 mW/cm2). As can be seen from the figure, both structures exhibit a rectification attitude. It can be seen from Fig. 6 that the plots show a linear behavior in the middle parts of the forward bias region. However, as the forward voltage increases, it deviates from linearity due to the series resistance effect. On the other hand, the current value of Al/PANI:Borophene/p-Si at − 5 V is lower than the current values of Au/PANI:Borophene/n-Si structure and showed a weak voltage dependence under reverse bias. Whereas, a reverse bias has a negligible forward current, a forward bias has a significant forward current. A diode’s depletion layer is significantly thicker in reverse bias and much thinner in forward bias. A diode’s resistance increases with reverse bias and decreases with forward bias. Herein, we can easily observe the increase of the currents in forward bias and decrease of it in reverse current in both devices. For the light intensity of 100 mW/cm2 at − 5 V in the reverse voltage region, the reverse current have been found 5.56 × 10−5 and 7.27 × 10−2 A for the structures Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si, respectively. This can be explained by the decrease in image force of the Schottky barrier height [64, 66].

Fig. 6
figure 6

Semi-logarithmic I–V characteristics of devices under different illumination and dark environment: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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In an ideal diode, the depletion region affects the forward bias current and this creates an exponential current–voltage correlation (Thermionic Emission Mechanism). The Thermionic Emission Mechanism is a dominant model, and it ignores other mechanisms. In this case it is possible to calculate the performance parameters such as diode’s ideality factor (n), barrier height (ΦB), and series resistance (Rs) using the thermionic correlation between current and voltage as follows [66]:

$$I={I}_{o}\left[{\text{exp}}\left(\frac{q(V-{IR}_{s})}{nkT}\right)-1\right].$$
(2)

This model is used at the V > 3kT/q voltage limit to remove contributions from reverse bias, where I0 is the saturation current, n is the ideality factor, q is the electronic charge, V is the applied voltage, Rs is the series resistance, T is the temperature in Kelvin, and k is the Boltzmann constant. The linear regions of the semi-logarithmic I–V curves represent the current generated by the majority charge carriers using diffusion. On the other hand, the saturation current (I0) created by minority charge carriers is given as follows [2, 67, 68]:

$${I}_{o}=A{A}^{*}{T}^{2}{\text{exp}}\left(-\frac{q}{kT}{\Phi }_{{\text{B}}}\right),$$
(3)

where A is the effective diode area, A* is the effective Richardson constant (112 A/cm2 K2 for n-Si and 32 A/cm2 K2 for p-Si), and ΦB is the barrier height. The I0 values depending on the light intensity and dark environment obtained from the zero voltage point of linear region of semi-log I–V curves are tabulated in Table 1 for both structures. It can be seen from Fig. 6, the I0 values of Au/PANI:Borophene/n-Si device is greater than the Al/PANI:Borophene/p-Si device.

Table 1 Illumination-dependent electrical parameters of (a) Al/PANI:Borophene/p-Si and (b) Au/PANI:Borophene/n-Si devices
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Figure 7 shows the rectification ratios (RR) of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si structures depending on light intensity. The ratio of forward current to the reverse current (at a specific voltage) is used to determine the RR value of diode. The RR values of the produced devices have been obtained at ± 5 V. As seen in Fig. 7b, the RR values for structure Au/PANI:Borophene/n-Si decrease with increasing light densities. This may be due to the possible contribution of charge carriers produced by photons to dark current and the increase of reverse voltage as a result of illumination [67, 68]. The RR has been found as 44.14 and 22.15 at ± 5 V under dark environment for devices Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si, respectively.

Fig. 7
figure 7

The RR plots of devices under different illumination: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Figure 8 depicts the variation of ideality factor (n) and barrier height (ΦB) as a function of illumination intensity. n values have been obtained using the tilt of linear region [Eq. (2)] of current–voltage plots. ΦB values have been obtained with the help of Eq. (3). As seen from the figures, the n values of both devices are much larger than unity. The high n values obtained can be attributed to the presence of barrier inhomogeneities and series resistance [69, 70]. One of the reasons why the ideality factor is greater than unity is the generation-recombination currents that contribute to the thermionic emission mechanism of current. The increase in interface states as a result of possible defects creates inhomogeneity and this creates deviation from ideality. The n and ΦB values for Al/PANI:Borophene/p-Si device have found to be 2.27 and 0.667 eV, respectively, in dark environment. Similarly, for device Au/PANI:Borophene/n-Si, these parameter values have been found to be 3.96 and 0.506 eV, respectively. The n and ΦB values for devices Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si are listed in Table 1 as a function of light density. As seen from both figures, while the n values of the Al/PANI:Borophene/p-Si and the Au/PANI:Borophene/n-Si device started to decrease after a certain light intensity. This may be due to excess charged carriers produced under illumination. Moreover, the ratios of holes and electrons in the depletion region change under illumination. This causes the forward current in device to increase and, as a result, the ideality factor to decrease [71]. As seen in Fig. 8a and b, ΦB increases with increasing light intensity. The increases in ΦB can be attributed to the density of states present at the interface and the chemical reactions that occur. The increase in light density increased the mobility of charged carriers and, accordingly, the barrier height increased. Similar results have been reported by researchers [72, 73].

Fig. 8
figure 8

n and ΦB curves of devices under illumination: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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The exponential behavior of current–voltage curves is affected by series (Rs) and shunt (Rsh) resistance, and these parameters divert the curves from linear position when applied bias increases toward the forward and reverse voltage regions [64, 66]. In order to high power and speed, the Rs value is expected to be low. However, the trap states and contact points within the device increase Rs value [74, 75]. Despite the series resistance value, the shunt resistance of the diode is expected to be quite high. However, due to the contacts within the device, current mechanisms affected by interface conditions and inhomogeneous surface, it may be associated with the leakage currents [76,77,78]. Rs and Rsh parameters are obtained from the junction resistance (Rj) of structure using the following expression [79].

$${R}_{j}=\frac{\partial V}{\partial I}.$$
(4)

Figure 9a and b presents the Rs and Rsh curves of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices at different light intensities. Rs and Rsh values have been found 2.7 Ω and 119.3 kΩ for Al/PANI:Borophene/p-Si at 20 mW/cm2, respectively. At 100 mW/cm2, these values have been obtained as 0.62 Ω and 88.63 kΩ, respectively.

Fig. 9
figure 9

Rs and Rsh curves of devices at different light intensities: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Series resistance values of structures can also be determined by different methods such as the Cheung method as defined below [80,81,82,83].

$$\frac{dV}{d{\text{ln}}I}=I{R}_{s}+n\left(\frac{kT}{q}\right)$$
(5)

and

$$H\left(I\right)=n{\Phi }_{{\text{B}}}+I{R}_{s}.$$
(6)

With this effective method, it is possible to determine n and ΦB as well as calculating Rs. Figure 10a and b presents the dV/dlnI plots depending on I for devices Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si at different light intensities. As can be seen from both figures, Eq. (5) gives a straight line in the area where there is bending in the high forward bias region of the I–V plots. The slope of this straight line gives Rs and the point where it intersects the y-axis gives nkT/q.

Fig. 10
figure 10

The dV/dlnI vs. I plots at different illumination intensities: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Figures 11a and b depicts the H(I) plots depending on I of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices at distinct light intensity. Each plot gives a straight line and nΦB is obtained from the point where it intersects the y-axis. The Rs values of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si that obtained from the plots at different light densities according to Cheung method are listed in Table 2. It is seen from Table 2, the Rs values of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices decreased with increasing light intensity. This may be caused by increased charge carriers at the interface as a result of increased illumination [83]. Moreover, Rs values calculated with the help of Cheung functions are close to each other and decrease with increasing light intensity.

Fig. 11
figure 11

The H(I) vs. I plots at different illumination intensities: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Table 2 Illumination dependent series resistance values of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices according to Cheung method
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Figure 12 shows the light intensity dependence of Rs values and used Cheung functions. As seen in the calculations, the second Rs value obtained from the slope of the H(I) function is also in agreement with the previous calculated value. The plots in Fig. 12 have been used to verify the consistency of the Cheung approach.

Fig. 12
figure 12

The calculated Rs (a) and Cheung functions (b) plots depending on illumination intensities

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Photocurrent structures of diodes can be analyzed with the following expression depending on light intensity and photocurrent [84, 85].

$${I}_{{\text{ph}}}=A{P}^{m},$$
(7)

where Iph is the photocurrent caused by illumination intensity, P is the applied illumination density, m is the illumination constant, and A is the constant of proportionality between photocurrent and light intensity. In Fig. 13, the plots of log(Iph) − log(P) of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices are presented. As can be seen from Fig. 13a and b, a linear behavior has obtained for both devices. This shows that the diodes have a linear photoconductivity property. Accordingly, it is possible to determine the photoconductivity mechanisms in devices with the m value [86]. This process depends on the lifetime of the photo-carriers and is carried out in three different ways. When m = 1, the photoconductivity mechanism of device is under the effect of monomolecular recombination. As m values increase, it is possible to talk about low density or unoccupied trap levels. The m values obtained from the linear curves in Fig. 13a and b are 0.44 and 0.33 for structures Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si, respectively. These values are in the 0 < m < 1 range and indicate the existence of permanent localized states inside device [68, 78, 85, 86]. For device Al/PANI:Borophene/p-Si (in Fig. 13a), it can be seen that the photocurrent increases from 1.40 × 10−5 to 2.85 × 10−5 A with increasing light intensity. Similarly, for device Au/PANI:Borophene/n-Si (in Fig. 13b), it is seen that the photocurrent increases from 2.85 × 10−2 to 5.54 × 10−2 A with increasing light intensity. This situation shows that the electrons in valence band jump toward the conduction band with light energy they gained [87,88,89,90]. The photocurrent values at different light densities are listed in Table 3 for both devices.

Fig. 13
figure 13

log (Iph) − log(P) plots: a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Table 3 Various detector parameters of the Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si photodiode for various power
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The responsivity (R) and sensitivity (K) are characteristic parameters of a photodiode and are given by the following expressions [88,89,90,91,92].

$$R=\frac{{I}_{{\text{ph}}}-{I}_{{\text{d}}}}{{P}_{{\text{in}}}},$$
(8)
$$K=\frac{{I}_{{\text{ph}}}-{I}_{{\text{d}}}}{{I}_{{\text{d}}}},$$
(9)

where IPh is the photocurrent under light intensity, Pin is the incident light density, and Id is the dark current. In Fig. 14a and b, the R and K plot of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices are illustrated. For device Au/PANI:Borophene/n-Si, the R value has been found to be 70.79 A/W under the light density of 100 mW/cm2 at − 5 V spot voltage. Similarly, for device Au/PANI:Borophene/p-Si, the R value has been found to be 3.61 × 10−2 A/W. The responsivity for Au/PANI:Borophene/n-Si device at 100 mW/cm2 is 2.98 times higher than that of Co/p-Si/Al photodiode with Congo Red interfacial layer [91] and 18.05 times higher than the photodiode with the copper complex interfacial layer [92]. The K values of Al/PANI:Borophene/p-Si device have found to be 2.68 A/cm2 under 100-mW/cm2 illumination. The sensitivity of Al/PANI:Borophene/p-Si device is 2.31 times higher than that of photodiode with Congo Red interfacial layer [91]. These results show that the fabricated device has a good photo-sensing property. From these results, it can be seen that the devices produced have good photo-sensing ability. The R and K values of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si that are obtained at various light densities are tabulated in Table 3.

Fig. 14
figure 14

a R and b K plot with light intensity

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One of the characteristic parameters of photodiodes is detectivity (D). This parameter determines the diode’s ability to detect low optical signals and is expressed with the following formulation [93]:

$$D=\frac{R\sqrt{A}}{\sqrt{2q{I}_{{\text{d}}}}},$$
(10)

where R and Id are the photoresponsivity and dark current, respectively, A is the effective area where incidental light is absorbed, and q is the electronic charge. Light intensity-dependent detectivity for devices Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si is given in Fig. 15. As can be seen from the figure, the detectivity of both devices decreases with increasing light intensity. This decrease in detectivity may be attributed to traps present at the heterojunction interface [94]. The maximum detectivity under 5 V bias and at 20 mW/cm2 light intensity has been found to be 1.99 × 1011 Jones for the Au/PANI:Borophene/n-Si sample. As can be seen, a high level of detectivity has been obtained under low intensity illumination. This shows that the manufactured devices are very sensitive to low optical power.

Fig. 15
figure 15

D plots depending on illumination intensities

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For a more detailed photoconductivity examination, the time-dependent transient measurement method has been used. These measurements are a frequently used technique to understand the photoconductivity mechanism of a device for different optical power densities and to see the photoresponse properties during on–off states [95, 96]. Figure 16a and b displays the transient plots of Al/PANI:Borophene/p-Si and Au/PANI:Borophene/n-Si devices at − 5 V bias depending on the light intensity. When illumination is turned on, the number of free charge carriers increases and contributes to the flowing current. Thus, the photocurrent level quickly reaches saturation. When illumination is turned off, charge carriers drop to low levels and the current photocurrent quickly drops to the initial level [97]. Thus, a reverse switching behavior begins. As can be seen from the figures, the photoresponds of both produced devices are highly repeatable. As seen in Fig. 16a, b, the produced devices showed a fast photoresponse and good stability at different light intensities and reached the maximum photocurrent level in approximately 1 s. For sample Al/PANI:Borophene/p-Si, it increased from 5.78 × 10−4 to 2.26 × 10−3 A at an illumination intensity of 100 mW/cm2 and decreased to 6 × 10−5 A when the illumination has turned off. Similarly, for sample Au/PANI:Borophene/n-Si, these values increase from 1.8 × 10−4 to 7.54 × 10−4 A and decrease 2.48 × 10−5 A. These results clearly show that the produced devices exhibit photoconductive behaviors. This initial rise in photocurrent when illumination begins provides information about the production mechanisms of free charge carriers. The decrease in current when the lighting is turned off is a symptom of the trapping mechanisms of charge carriers at deep levels. This occurs as a result of the layered surface properties of the device [98].

Fig. 16
figure 16

The time-dependent transient plots under illumination intensity. a Al/PANI:Borophene/p-Si and b Au/PANI:Borophene/n-Si

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Finally, the PANI:Borophene/Si Schottky diodes have been tested for salt (NaCl) detection. This salt detection experiment has carried out with different concentrations of salt solution (0–50 mM) deposited on the Al/PANI:Borophene/p-Si and the Au/PANI:Borophene/n-Si of the Schottky diode sensor and only, the Al/PANI:Borophene/p-Si diode response to salt solution concentration. The current–voltage plot under salt solution concentration at a range between + 5 and − 5 V for Al/PANI:Borophene/p-Si device is shown in Fig. 17. As seen in this figure, the average knee and the average breakdown voltage have obtained 3 V and − 4.6 V, respectively. Also, the current has increased with increasing salt solution concentration after the knee voltage of this diode. These increases in current are a result of the diode’s series resistance (Rs) to current under salt concentrations (0–50 mM). The salt solution concentration-dependent current and the salt solution concentration-dependent series resistance plots for Al/PANI:Borophene/p-Si diode are given in Fig. 18. As seen in Fig. 18, firstly, a minimal decrease in the current has observed with 10-mM salt solution concentration applied to the diode and then an increase in the current has observed according to the increasing salt solution concentration (Fig. 18a).

Fig. 17
figure 17

The current–voltage plot under salt solution concentration for Al/PANI:Borophene/p-Si diode

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Fig. 18
figure 18

a The salt solution concentration-dependent current and b the salt solution concentration-dependent series resistance plots for Al/PANI:Borophene/p-Si diode

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On the other hand, as can be seen Fig. 18b, firstly, a minimal increase in the Rs of Al/PANI:Borophene/p-Si diode has observed with 10-mM salt solution concentration applied to the diode and then an decrease in the Rs of Al/PANI:Borophene/p-Si diode has observed according to the increasing salt solution concentration. Then, we investigated the time-dependent transient plots under salt solution concentration for Al/PANI:Borophene/p-Si diode (Fig. 19). This device has showed a fast salt concentration detection and good stability at different salt concentration changes. As a result, the Al/PANI:Borophene/p-Si diode is more sensitive salt concentration detection after knee voltage. From these results, it can be concluded that this diode has the characteristics of salt concentration detection sensor and it successfully detected salt concentration changes with respect to current flow [99,100,101].

Fig. 19
figure 19

The time-dependent transient plots under salt solution concentration for Al/PANI:Borophene/p-Si diode

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4 Conclusion

In this study, a Schottky structure consisting of a PANI:Borophene/Si interface, which has not been reported before in the literature, has been fabricated and its photodiode properties have been investigated under different light intensities. It has been observed that the structures exhibit photoconductive behavior from I–V characteristics depending on illumination. The devices showed rectification features at all illumination intensities. Time-dependent photoconductivity measurements have shown that the photocurrent amounts of the devices depend on the light intensity, and the maximum photocurrent has reached in approximately one second. When the characteristic photodiode parameters have been examined, it has been seen that the structures had good photo-detection ability. It has been observed that it exhibits a very good response and quite high detectivity behavior to measurements at low light intensity. This shows that the produced devices may be candidates for opto-electronic applications such as a photodiode and photosensor. Finally, the PANI:Borophene/Si Schottky diodes have been tested for NaCl detection. Only, the Al/PANI:Borophene/p-Si diode response to salt solution concentration and this diode has the characteristics of salt concentration detection sensor and it successfully detected salt concentration changes with respect to current flow. This work will continue with trials of organic compounds/metabolite detection for possible biosensing applications for cheaper and volume fabrication in low-income countries.