Photoresponse properties of Au/(CoFe2O4-PVP)/n-Si/Au (MPS) diode


 Photo-response properties of the Au/(CoFe2O4-PVP)/n-Si (MPS) diode were investigated using current-voltage (I-V) measurements achieved under dark and various illumination conditions. The experimental results showed that the MPS diode has a good response to the illumination. Especially, in reverse-bias region, photocurrent (Iph) increases with increasing illumination intensity (P) due to the formation of electron–hole pairs. The double-logarithmic Iph-P plot has a good relation with 1.27 slope and such high value of slope indicates a lower density of the unoccupied trap level. This indicates that the diode exhibits a good photoconductive and photovoltaic behavior. The photo-to-dark current ratio confirms the photo-sensitivity of the diode. Thermionic emission (TE) theory was used to determine the diode electronic parameters such as saturation current (I0), ideality factor (n) and barrier height (ΦB0) and their values were calculated from the measured I-V data. Moreover, the ΦB0 and series resistance (Rs) were extracted from an alternative method suggested by Norde. All these parameters (ΦB0, n, Rs, and I0) decrease with increasing illumination intensity and there is a good linear correlation between ΦB0 and n as ΦB0 (n) = 4.72x10− 2n + 0.5464 eV. As a results, the fabricated MPS diode due to the excellent photo-response can be used for photovoltaic applications.


Introduction
A metal-polymer-semiconductor (MPS) structures are formed by sandwiching organic polymers between M and S and they are similar to the metal-insulator-semiconductor (MIS) type Schottky-barrier diodes (SBDs) due to the dielectric property of the polymers. In recent years, organic polymers are often used in various electronic and optoelectronic device applications including photodiodes, solar cells, transistors, sensors, detectors, polymer integrated circuits [1][2][3][4][5]. When a diode is exposed to illumination, the electron-hole pairs are generated in near the depletion region of the diode. Then, these pairs are separated under applied electric field. On the other hand, this separation is more effective at in the reverse bias. The number of photo-generated charge carriers increases under illumination. Especially, in the reverse bias region, these carriers create an additional current to the dark current. The reverse bias current is called as photocurrent (Iph) and it determined by the amount of photo-generated charge carriers increases with illumination intensity [6,7]. Moreover, the increase number of charge carriers leads to an increase in photoconductivity. In addition, the responsivity is used for the evaluation of the performance of light sensitive devices such as photodiodes, phototransistors, and photovoltaic cells.
Today, the main technical and scientific problems are relevant to the increase in the of photo-current and decrease cost of the photodiode or solar cells and it is remaining a challenge problem to the researchers yet. When these devices are exposed to the illuminated, some of the electrons may be generated and they could be trapped or release from the trap. These trapping and releasing processes can be considered as charging and discharging respectively. Although sunlight contains a huge amount of energy, these devices are very inefficient to absorb these photons/energy and can only utilize a small portion of these photons because of many photons have not enough energy to form electron-hole pairs so they will simply pass straight through the device without affecting it. Therefore, in the last years, researchers have been focused on the developing new technologies that allows us to capture and convert this energy from the sun to provide electricity.
In our previous study, the electrical properties of MPS and MS diodes with and without the CoFe2O4-PVP interfacial layer have been compared [17]. In this study, photoresponse and electrical properties of the Au/(CoFe2O4-PVP)/n-Si (MPS) diode were investigated under dark and different illumination intensities using the forward and reverse bias I-V measurements at room temperature. Experimental results showed that the prepared Au/(CoFe2O4-PVP)/n-Si (MPS) structure has a good response to the illumination.

Experimental details
Au/(CoFe2O4-PVP)/n-Si (MPS) type structures were fabricated with deposited of (CoFe2O4-PVP) polymer interface layer on n-Si substrate. A detail information both on the chemical cleaning processes and formation on the grown of (CoFe2O4-PVP) polymer interlayer at Au/n-Si wafer can be found in our previous study [17]. The forward-reverse bias I-V measurements were carried out by using a voltage-current source (Keithley 2400) both in dark and under illumination range of 30-100 mW/cm 2 in the VPF-475 cryostat with four optical window. The fabricated MPS diode was illuminated with Newport/Oriel solar simulator and illumination level was determined by the ILT1700 research radiometer.

Results and Discussion
Both in dark and various illumination intensities, the I-V characteristics of the prepared Au/(CoFe2O4-PVP)/n-Si (MPS) diode was analyzed with the help of the standard thermionic emission (TE) theory. To extract important diode parameters such as such as reverse saturation current (I0), ideality factor (n) and barrier height (ΦB0), and series-resistance (Rs) both TE and Norde-function were used. In the based on TE theory (V≥3kT/q), the current in the forward bias is given by [18,19], For a diode with series resistance (Rs), the V-IRs term describes the voltage drop across Rs. The where, A is the diode area, A * is the Richardson constant (=112 A/cm 2 .K 2 for p-Si), and ΦB0 is the zero-bias barrier height and it can be calculated by using he experimentally obtained value of I0 and Schottky/rectifier contact are (A). But, the main electrical parameters obtained from the TE theory are usually deviated from the ideal case due to the existence of interfacial layer, Rs, barrier-inhomogeneity, and surface-states/traps.  When a diode is illuminated, the current carriers (electrons and holes) are generated in depletion region where the carrier concentration is lower than its equilibrium. The current generated by illumination adds to the dark current. Thus, the current known as photocurrent is larger than the dark current. The photocurrent is proportional to illumination intensity [20][21][22][23][24].
Since electrons absorb enough energy by photons rather than the forbidden bandgap (Eg) of the semiconductor, then many electrons in the valence band (Ev) can be jumped into the conduction band (Ec) or trap to trap.
The diode electronic parameters including I0, n and ΦB0 obtained from the forward-bias I-V characteristics under dark and different illumination conditions, are shown in Table 1. It is seen that these parameters depend on the intensity of illumination. The n and ΦB0 value decrease with increasing illumination intensity. Besides, the n value was found to be much greater than 1. This result results from the presence of interfacial layer and surface states, image-force lowering, and series resistance [25][26][27][28][29].  As can be seen in Table 1 and Fig. 2, there is a good linear correlation between ΦB0 and n as ΦB0 (n)=4.72x10 -2 n+0.5464 eV and hence the value of ΦB0 was found as 0.5936 eV for n=1 (ideal case). The obtained higher values of n can be attributed to the existence of (CoFe2O4-PVP) organic interlayer, its thickness, barrier inhomogeneity at Au/n-Si surface, surface states/traps, and dislocations. In addition, Norde proposed an alternative method to extract barrier height (ΦB) and series resistance (Rs) [30]. According to this method, F(V) is defined as Norde function and is given by, where γ is an integer (dimensionless) greater than the obtained n value for the MPS diode. The ΦB value is calculated from the value corresponding to the minimum of the F(V)-V plot and is given as follows, the Rs value is calculated as follows: where Imin is the current value corresponding to the Vmin value. Fig. 3 Table 1. It is seen that the Rsh and Rs values decrease with increasing illumination intensity. The relation between the photocurrent (Iph) and illumination intensity (P) is defined by the power law given as follows [31,32], where m is an exponent extracted from the slope of Log(Iph) versus Log(P) curve. A is a constant.   The responsivity (R) is a measure of the sensitivity to light and is calculated using the equation, where P is the illumination power and A (=7.85x10 -3 cm 2 ) is the diode area [35][36][37][38][39]. Similar results were also found in recently by some researchers [40][41][42]. Fig. 7 shows plot of responsivity versus P at the reverse voltage (-2.5 V). As seen in Fig. 7, the responsivity is increased with increasing incident illumination. This increase in the responsivity may be due to the excitation of electron-hole pairs from incident light.

Conclusions
In this study, photoresponse and electrical properties of the fabricated Au/(CoFe2O4-PVP)/n-Si (MPS) diode were investigated both in dark and under illumination density range of 30-100 mW.cm -2 by using the forward and reverse bias I-V measurements at room temperature.
The diode electrical parameters including I0, n, ΦB0, and Rs were obtained from these data by using the standard TE theory and Norde method. All these electronic parameters were found strong function of illumination intensity and voltage and decrease with increasing illumination intensity level. Experimental results show a good linear correlation between ΦB0 and n as ΦB0 (n)=4.72x10 -2 n+0.5464 eV and hence the value of ΦB0 was found as 0.5936 ev for n=1 (ideal case). The illumination enhances the reverse current when compared to the forward current and so the diode has exhibited good photoconductivity or photovoltaic behavior. The results showed that the illumination has a significant effect on the current. In addition, the calculated sensitivity and responsivity of the MPS device showed a high photoresponse under various illumination intensities. As result, the prepared MPS diode may be used as photodiode, photodetector and photovoltaic cell in various optoelectronic applications.