Photovoltaic Performance Characteristics of Devices Based Nano-Particle Allura Red Dye Thin Films: A Comparison Study
Abstract
In the present work, nanocrystalline Allura Red dye thin films based devices were prepared by using electro-spin technique under best conditions. The nanostructure characteristics of the Allura Red dye surfaces were found to be nanosphere-like structure by using scanning electron microscopy (SEM). The dark and 100 mW/cm2 illuminated current density–voltage (J–V) characteristics of the prepared heterojunctions by using different substrates like Si, Al and Cu were discussed at a temperature of 300 K. A diode-like behavior was obtained for all the studied devices and the rectification characteristics were obtained and compared. Moreover, the series resistance and ideality factors showed high interface dependence. Exposure of light on the Allura Red dye / different substrate structures produces a photocurrent which suggests the production of free carriers in all cases with different values which elucidate that these structures are promising for optoelectronic device applications.
Keywords: Allura Red dye; Nano-sphere; electro-spin technique; Photovoltaic
Introduction
Recently, thin films of organic semiconductors have much consideration in the various fields like the technology of various optoelectronic equipment’s [1-4].The wide-spread applications of this type of materials owing to their advantages such as low cost of fabrication, light weight, and flexibility [4-7]. Aydin and Yakuphanoglu [8] have investigated the influence of both the surface and interface on the metal/semiconductor structure. They concluded that these interfaces are complex and can affect the mechanical, electrical and photovoltaic characteristics of the MS devices. Further, Gunduz et al. [9] have reported the influence of P3HT as an organic semiconductor on the electrical and photoconductivity performance of P3HT:MEH-PPV diodes. They concluded that the prepared diodes show non-ideal current–voltage characteristics due to the ideality factor being higher than unity due to the effect of series resistance and the occurrence of metal/ semiconductor interface. Otherwise, Orak et al.[7] have fabricated a junction device of Al/Azure C/p-Si by thermal evaporation and spin coating techniques and studied electrical characterizations of the prepared devices and concluded that the device can be used for some optoelectronic applications and solar cells. Moreover, El-Nahass et al. [10] have prepared heterojunction of organic (CoMTPP)/inorganic (p-type Si wafer) by thermal evaporation technique and obtained a reasonable characteristic parameters solar cell. Zeyrek et al. [11] have investigated the influence of interface states and series resistance on the admittance spectroscopy parameters of Al/perylene/p-Si diode. Trombach et al. [12] have discussed the impact of thin film of the PcVO onto substrate of Si and extracted some photovoltaic parameter in the visible light range.
Furthermore, Ali et al.[13] have provided a junction of Au/2-(2-furanylmethylene) propanedinitrile, prepared onto substrate of p-Si by thermal evaporation technique, and extracted some of its main factors. The characteristics of both electrical and photovoltaic of the type metal/semiconductor junction, obtained using a thin organic interfacial layer, were detailed studied by Oyama et al. [14]. They found that these type of devices have been attracting increasing interest from researchers in recent years.
From the accessible literature, a large number of organic based heterojunctions has been investigated on Silicon, but not much work has investigated the impact of the substrate on their performance. Some junction parameters have been affected the type of substrate. In addition, we have compared the electrical and phototranisent properties of the organic layer that have the same thickness based on different substrates.
In the present communication, we investigated the electrical and photoelectrical performance of Allura Red dye / different substrates like Al, Cu, and Si. The aim of this study is to prepare Allura Red dye / different substrates of Al, Cu, and Si and compare their electronic and photoconductivity properties. In addition, the electronic and optoelectronic characteristic parameters were extracted using dark and illuminated current–voltage characteristics.
3. Results and discussion
3.1. Crystalline characterization and morphology of Allura Red
The crystallinity of the sample is investigated by the diffraction of x-ray pattern. As can be seen, the pattern confirms a polycrystalline nature for the structure with the monoclinic system and lattice parameters: a = 21.41 Å, b = 11.26 Å, c = 10.33 Å. The indexing of lattice plane was performed by the CRYSFIRE computer program [16] and the Miller indices for each diffraction line were computed and the cell parameters were realized by using CHECKCELL program [17] and denoted on each peak as observed in the figure. Broadening of the recorded pattern has a high significant to get the mean crystallite size of the studied material. Accordingly, a best fit by using a Gaussian fitting process of the broadened peaks was taken place, as illustrated in Fig.2.
The following Scherrer’s equation is used for determining the mean crystallite [18]:
, (1)
where D is the mean crystallite size; K is the shape factor (k0.9), is the wavelength of the X-rays, is Bragg diffraction angle and is the full width at half of its maximum intensity and measured in radians. The mean crystallite size D measured using Scherrer’s formula is calculated and found to be 30 nm. The obtained result confirms the nanostructural characteristic of the prepared Alura Red samples.
Fig.3 (a-c) illustrates the topography of Allura Red sample, scanned by SEM. These images showed the presence of high aggregations for the particles which composed from individual nanoparticles of a particle size of 25-40 nm, with a stick-like morphology ( see Fig.3(c)), in matching with those obtained by using the X-ray results. Large grain growth suppression may be due to the high contract of the particles in the studied material. Moreover, Fig. 3(d) cross-sectional image of SEM at the edge of the vertical boundaries at Allura Red/substrate edge was used to determine the film thickness which is found to be ≈100-140 nm. Furthermore, an adequate adhesion of the film on the substrate was also estimated as shown in Fig. 3(d).
3.2. Current-voltage characteristics
The schematic diagram of the prepared junction of Allura Red /Al, Allura Red /Cu and Allura Red/Si substrates is shown in Fig.4. Dark current density–voltage characteristics of Allura Red based junction using various types of Al, Cu, and Si substrates at room temperature (300 K) under steady state provision is represented in Fig.5(a) and can be expressed by the following equation[19]
J=J_0 (e^(q(V-JR_s )/nkT)-1)+((V-JR_s ) )/R_Sh (2)
where J0 is the reverse saturation current density, Rs is the series resistance, Rsh is the shunt resistance and n is the ideality factor. As consequence from Fig.5, for all the studied junctions, the current increases exponentially with applied potential, in the desired range, but a deviation from linearity has been observed under high bias region due to the impact of some factors such as RS. The Allura Red/Al, Allura Red /Cu, and Allura Red/Si-based junctions follow Schottky-diode behavior. In addition, there are two distinct linear regions depending on the voltage region are recorded as shown in Fig. 5(a). Kasis and Saad [20] have attributed this behavior to the presence of different predominant conduction mechanisms depending on the operating voltage range and the junction can be modeled as non-ideal characteristics.
To understand the conduction mechanism, Fig. 5(a) has been re-plotted in log-log scale as shown in Fig. 5(b) for the three junctions. The same behavior has been taken place with two distinct linear regions which confirming the presence of two predominant conduction mechanism. In the first region (region I), the J-V characteristics are close to linear with slope 1. But in the second region (region II), the behavior are found to obey the relation, J α V2 [21]. The transition from linear (ohmic behavior) to a super-linear dependence (quadratic) is dependent on the voltage region. Behavior in the region I can be explained by the presence of a single trap of type shallow under low forward bias which can be due to the obeying of ohm’s law. With increasing the applied voltage, Region II, the current in the three junctions obey the space charge limited conduction due to the presence of single trap levels [22].
There are a major role of some parameters like series, Rs and shunt, Rsh resistances that control the mechanism of the heterojunction. These resistances can be estimated based on the current-voltage characteristics at low and high bias, respectively. Figs.6 (a) and (b) show the substrate type dependence of both resistances. As observed, for the junction that of Si substrate has low Rs and high Rsh as compared with those other substrates of Cu and Al. The source of Rs is the summation of all the resistances such as resistances of the front and back metallic contacts as well as the resistances of the semiconductors. But the value of Rsh can be attributed to the leakage current through the junctions. The higher value of Rsh confirms that the shunting loss can be neglected [23].
In addition, the J-V characteristics of Allura Red/Al, Allura Red /Cu, and Allura Red/Si-based junctions show rectification characteristics as shown in Fig. 7(a). The rectification factor can be explored from the forward and reverse current at certain applied voltage, i.e. RF=⌊J_F/J_R ⌋_(V=constant). Fig.7 (a) confirms that the junction based Allura Red/Si shows high rectification characteristics as compared to other junctions which can be attributed to the higher barrier at the interface of Allura Red/Si that controls the flow of the forward and reverse biased carriers through the junction [24].
The other important parameter is the ideality factor of the junction. This parameter supplies confirmation of the divergence of the junction from the ideal one, i.e. from the ideal Schottky diode. The ideality factor was extracted from the slope of the line fit using Fig.7 (b) and equation (2). As observed for all the studied junctions n value is larger than unity. According to the literature [25-28], a probability of the occurrence of interface states and/or inhomogenities osf the barrier height are the main reason for this deviation.
Both the saturation current density (J0 ) and barrier height (b) of all the studied junctions were obtained from the current axis intercept of the linear fit of the relationship of the semilog J-V characteristics (not shown here) in the lower bias region (V<3kT ). The determined values of J0 and b were examined as a function of substrate type and shown in Fig.8 (a) and (b). A lower J0 and higher b were obtained for Allura Red/Si-based junctions as compared to the other junctions. The differences in the barrier height of the studied junctions can be attributed to the occurrence of an interfacial layer through the manufacture of the junctions by the conventional techniques [29, 30, 31].
Current density-voltage measurements of the studied junctions, under illumination 100 mW/cm2 illumination condition, at 300 K are represented in Fig. 9(a). The obtained photocurrent can be generated due to the separation of bounded electron-hole (exciton) at the junction interface a result of light. It is well known that the excitons in organic compounds, like Allura Red, have a high binding energy, i.e. larger than 0.5 eV [32]. The current was generated under dissociation of excitons which means a separation of electrons and holes. These charge carriers are moving away from one another and then producing a current under the effect of electric field at junction interface [33]. The relationship of the output power vs. voltage is shown in Fig.9 (b). As observed, the maximum output power values are recorded and showed substrate type dependence. In addition, the photovoltaic parameters such as open-circuit voltage (Voc), short-circuit current density (Jsc), fill-factor (FF) and the power of maximum output (Pmax) were calculated and show substrate dependence for the studied junction. Figs.9 (a) and (b) show the substrate reliance of the junction for Jsc and Voc, respectively. All the studied junctions display photovoltaic characteristics with lower values of Jsc and Voc as respectively represented in Figs.10 (a) and (b). These results can be referred to some restrictions like the probability of free charge carrier recombination and the great value of the series resistance and/or a little value of Rsh [34]. Values of both Jsc and Voc, along with maximum output current, Jmax and maximum output voltage, Vmax, are used to calculate the main photovoltaic parameters [35]. A high value of both Jsc and Voc are obtained for Allura Red/Si-based junctions as compared to the other junctions. In addition, Fig.11 (a) and (b) show the measured substrate dependence of both FF and Pmax. For a comparison in such a figure, the decrease in FF and Pmax are observed as a transition from the junction of Allura Red/ Si to Allura / Al which may be attributed to the barrier characteristic of each type of the studied junctions. Fyroz et al.[35] and Phang et al.[36] have attributed this property to the higher value of ideality factor due to the influence of surface recombination and high leakage current that affect the junction performance for photovoltaic characteristics.
4. Conclusions
In conclusion, Allura Red thin films based junction on various substrates of Si, Cu, and Al were successfully manufactured by using the spin-coating technique. Results of both XRD and SEM results confirm the crystalline and morphology structure characteristics. The extracted value of the mean crystallite size from the Gaussian fitting of the XRD is found to be 30 nm and supported by the SEM results. The results of the current-voltage characteristics of the junctions (Si, Cu, and Al) /Allura Red showed substrate type dependence. It is observed that the main parameters of the junctions such as the ideality factors, barrier heights, series and shunt resistances have remarkable functions of substrate type. The results present that the interface states, ideality factor, and series resistances are almost effect on the electrical characteristics of the studied junctions. The predominant conduction mechanism of all the studied junctions can be explained by ohm’s law and quadratic space charge limited conduction, respectively under low and high voltage. The photovoltaic parameters such as Jsc , Voc, FF and Pmax offer substrate dependence for the studied junction. The acquired results confirm that Allura Red / Si junction, compared to Allura Red / Cu and Allura Red/ Al, can be employed for optoelectronic applications.