Study of Electrochemical Properties of Compared Indigo for Metal–Semiconductor–Metal Diode

Abstract

Indigo blue was discovered as a semiconductor material because of its organic semiconductor properties. This paper shows a primary study of the electrochemical properties of Sakon Nakhon-indigo strain used in the metal–semiconductor–metal (MSM) diode. The fermentation and extraction of our local indigo plant are explained. Indian indigo in the MSM diode is compared in the same conditions of preparation. The electrochemical properties, including the current–voltage (I–V) characteristic, static resistance, and rectification ratio, are discussed. The results show that the electron and hole characteristics and band gap energy of the indigo blue affects the electrochemical properties of the device. Our local MSM diode has a suitable operation between −1 and +3 V_MSM with a knee voltage of 1.0 V_MSM. Especially, it can produce the highest forward-bias current of about 3.19 mA at linear operation between +2 and +3 V_MSM, whereas the review MSM diode is about 2–3 hundred times lower. This shows that this strain has more conductive properties because of its effective electron and hole characteristics obtained by an indigo yield concentration. Therefore, the MSM diode based on Sakon Nakhon-indigo strain is an important role in an electronic semiconductor device for low voltage consumption and high sensitivity. In the future, the molecular characteristics of local indigo may be deeply analyzed to be further developed into a thin-film form used as an organic semiconductor material in several electronic devices.

1. Introduction

Natural-derived materials achieved from some animal and plant sources have provided the remarkable properties for semiconductor device applications, being also nontoxic for humans and low in cost for utilization [1,2]. Recently, these materials have been popularly applied in such devices based on their electrochemical properties, i.e., red cabbage thin-films in dye-sensitized solar cells, carotenoids utilization in a field-effect transistor, sensors fabricated by using a dopamine–melanin thin-film, and indigo blue used in semiconductor diodes [3]. Particularly, indigo blue has very low solubility because it contains the strong molecular hydrogen-bonding structure [4,5].

The indigo plant, categorized into the Indigofera tinctoria family, has been extensively focused on for planting around Europe and Asia [6]. Its fresh leaves provide indigo blue pigment used as materials for fabric dyes, cosmetics, and printing inks [7]. The indigo blue pigment indicates many outstanding advantages, such as being easy to synthesize, most compatible with other materials, high flexibility of molecule forming, and biodegradability [8].

Many researchers have discovered that indigo blue pigment has organic semiconductor properties because of its strength of intermolecular interactions, best charging transport, and responsible ultrafast proton transfers [9]. Based on those remarkable properties, the indigo blue pigment has been widely used in many electronic semiconductor devices, such as indigo-derivative-organic field-effect transistors, FETs, organic light-emitting diodes, LEDs, and metal–semiconductor–metal diodes, MSM diodes [10,11,12,13,14].

Especially, the MSM diode is a simple semiconductor device used extensively as an electron tunnel diode and a rectifier. Due to its noncomplex structure, the MSM diode has been utilized as a tester to verify electrochemical properties [15]. Indian researchers have introduced experiments to investigate the electrochemical properties of the indigo blue pigment in an organic semiconductor layer for MSM diodes [16,17,18]. Its properties were studied to evaluate potential electric current sensitivity and conductivity [19].

In Thailand, the indigo plant has been planted around the country, and Sakon Nakhon city is one of the top three planting cities [20]. Moreover, it has been supported by a national research agency to utilize it for cloth fabrics, costumes, and handmade products [21]. However, there has been no demonstration of using it for electronic products.

The objective of this paper is to primarily study the possibility of utilizing the electrochemical properties of an MSM diode based on Sakon Nakhon-indigo strain as a semiconductor layer. The fermentation and extraction processes of our local indigo plant are intensively described. The India-indigo-strain-based MSM diode is compared for its particular electrochemical properties with this proposed diode under the same conditions of preparation, i.e., the fermentation, extraction, and experiment methodologies. The fabrication process and experimental setup of the proposed MSM diode test are explained. The electrochemical properties of local-indigo-made MSM diodes, i.e., the current–voltage (I–V) characteristic, static resistance, and rectification ratio, are investigated. The potential of the Sakon Nakhon-indigo strain is discussed in the last section.

2. Indigo Blue Pigment Preparation from Local Indigo Plant

The indigo plant (Indigofera tinctoria) is planted and is grown at Ban Klang, Phanna Nikhom District, Sakon Nakhon, Thailand, as shown in Figure 1a.

A total of 1 kg of harvested fresh leaves was prepared for fermentation. Firstly, fresh indigo leaves were sorted and then fermented with 10 liters of water at 40 °C for 24 h to hydrolyze indican by using the enzyme. Appearance of colorless organic compound indican (indoxyl β-D-glucoside) in indigo leaves indicates the yield of indigo blue pigment. This hydrolysis resulted in the indican conversion to the yellow-green indoxyl. Then, the fermented indigo leaves were beaten by mechanical paddles for the oxidative process and dewatering. The oxidized indigo water provides the insoluble suspended blue solution. The molecular structure of the indigo blue in this fermentation process is shown in Figure 1b.

Indigo blue was evaporated, dried, and stored as an indigo blue pigment at room temperature, as shown in Figure 1c, and then well prepared for the MSM diode fabrication.

The orbital energy band of the indigo blue pigment is indicated in Figure 1d. It is explained that the energized electron flows from HOMO to LUMO through the band gap energy to result in the conductive state of indigo blue pigment. Its band gap energy, E_g, is reported at about 1.89 eV for the fundamental HOMO–LUMO transition, exhibiting the property of an organic semiconductor material [22].

3. Metal–Semiconductor–Metal Diode Fabrication

The back-to-back MSM diode is studied because of its noncomplex structure and equipment availability. The equipment of the MSM diode fabrication consists of an Al plate, a Cu sheet, and an invented mold.

The Al and Cu were initially set to be metallic electrodes due to their good conductivity, less oxidation, inexpensive material, and availability. The work functions of Al and Cu are about 4.30 and 4.70 eV, respectively, which explains that the minimum quality of energy is required to release the free electron from the surface of the solid material as introduced by [23].

Firstly, these electrodes were cleaned with distilled water and then dried at room temperature. The mold, consisting of a lid and a base, was built by a 3D printer to contain a 10 mg indigo blue pigment, as shown in the parametric design and fabricated equipment in Figure 2a,b.

The Cu electrode was firstly laid in the base pothole, filled with 10 mg of local indigo blue pigment. The Al electrode was then topped, and the lid part was consequently closed. These fabricated layer stacks are now an MSM diode, as illustrated in Figure 2c,d.

A measurement circuit invented to verify the room temperature electrochemical properties of the fabricated MSM diode consists of a −5 to 5 V DC power supply, V_dc, and a 1 kΩ load resistance, R_L. The Al and Cu electrodes in the MSM diode were anode and cathode, respectively. Figure 3a,b demonstrate the circuit of the forward-bias and reverse-bias, respectively. The behavior and electrochemical characteristics of this proposed MSM diode will be described in the next section of results and discussion.

4. Results and Discussion

4.1. I–V Characteristic and Static Resistance Analysis

4.1.1. I–V Characteristic

Hundreds of proposed MSM (Al/local indigo/Cu) diodes were examined for repeatability and they exhibited similar results. The two sets of them were chosen due to their best presence. The I–V characteristic, presented in Figure 4, shows the forward-bias of the MSM diode with the local indigo, showing in the positive right-hand side. It is seen that an increase of MSM diode voltage, V_MSM, between 0 and 1 V_MSM results in the forward current of about 0 mA because of the insufficient potential electron energy in the diode that overcomes the band gap energy for conductive state access of the indigo blue layer. The forward current at the knee voltage is increased to a maximum of 2.60 mA or 3.19 mA in the first and second MSM diodes, respectively, when V_MSM increases. Then, it tends to decrease because the indigo blue layer becomes burnt. The local indigo blue obtained from this extraction has a low boiling temperature because it was contaminated, such as with chlorophyll, while in the reverse-biased region, the negative left-hand side, it shows a steady ultra-low reverse current due to the large band gap energy of the indigo blue layer. However, it has a bit of error at large negative V_MSM because of the burnt indigo layer. It is observed that this MSM diode may be operated between about −3 and +3 V_MSM. In addition, it is noticed that the reverse current of about 0 mA is found at −1 V_MSM.

4.1.2. Static Resistance

The static resistance of the proposed MSM diode was analyzed with varied V_MSM, shown in Figure 5. A nearly zero static resistance, found in the forward-biased region at greater than 2 V_MSM, means that it is exhibited to be a good conductor. When decreasing V_MSM below the knee voltage, the highest static resistance of 97 kΩ and 107 kΩ at the first and second MSM diodes are found at −1 V_MSM. However, if V_MSM was further reduced, the static resistance become unstable due to its burnt indigo layers.

4.1.3. Summary of I–V Characteristic and Static Resistance

The commercial semiconductor diode, no. 4007IN, was chosen to verify the MSM diode in this work [24]. It is seen from the results that the I–V characteristic and static resistance obtained by testing the MSM diode with the local indigo show similar results to those of the commercial diode, although values are not as large as the commercial one. However, the I–V characteristic and static resistance of this proposed device suffer from a low-temperature burning point of the indigo layer when applying a high V_MSM. It may be said that the MSM diode with local indigo blue performs a suitable operation between −1 and +3 V_MSM. In addition, in the forward bias, there is an obvious linear operation at +2 to +3 V_MSM. This operating range is thought to be because this indigo blue layer has a suitable number of electrons and holes with low energy of band gap, and when it is burnt, this will be no longer suitable [15]. Thus, it is summarized that the electrons and holes characteristics and band gap energy of the indigo blue layer have an effect on the behavior of the I–V characteristic and static resistance. The improvement of these factors depends on the preparation of the indigo blue pigment, including planting, harvest, fermentation, and extraction.

4.2. Rectification Ratio

The rectification ratio is an important physical factor used to verify the current sensitivity and conductivity. It can be acquired from the I–V characteristic experiment. The calculation of the rectification ratio demonstrates the ratio between the forward-bias and the reverse-bias currents at various V_MSM, written as

$$RR=\frac{I_f \left(V_{MSM}\right)}{I_r\left(-V_{MSM}\right)}$$

(1)

where RR is the rectification ratio, I_f is the forward-bias diode current, and I_r is the reverse-bias diode current [25,26].

Figure 6 illustrates the rectification ratio at varied V_MSM. It is seen that the rectification ratio increases as V_MSM increases. The highest rectification ratios of about 32.50 and 19.12 are obtained in the first and the second MSM diodes, respectively. A high RR represents a high sensitivity of the current transfer and conductivity. The highest RRs of silicon-based MSM devices for photodetectors are normally between 105 and 108 at 2 V_MSM [27]. Although this proposed MSM diode shows similar characteristics of RRs to those of the silicon-based MSM devices, its RR, about three times lower than those of the silicon-based MSM devices, is found to be between 1 and 3 V_MSM.

4.3. Comparison of Electrochemical Properties of MSM Diode

Indian indigo was reviewed in this work because of its similarities in terms of environment and weather. Its yield extraction was introduced by Dutta [28]. The MSM diode was fabricated and its electrochemical properties investigated [18]. A comparison of six electrochemical properties of MSM diodes between the proposed and reviewed ones was investigated and compared with a commercial one, as shown in

Table 1. Electrochemical properties of local and review indigo of MSM diodes.

Parameter (25 °C)	Commercial Diode [24]	MSM Diode (Al/Indigo/Cu)
		Review Indigo Diode [18,28]	Local Indigo Diode
			1st Diode	2nd Diode
Indigo yield (mg g−1)	-	0.6	1.03
Knee voltage (VMSM)	0.7	2.0	1.0	1.01
Highest forward-bias current (mA)	1000	0.014	2.60	3.19
Average reverse-bias current (mA)	0.00239	0.001	0.1	0.15
Highest static resistance of reverse-bias (kOhm)	2090	2000	97	107
Highest rectification ratio	108	8.30	32.50	19.12

[18,28].

The indigo extraction yield of the proposed and reviewed diodes was acquired in the same fermentation conditions of 40 °C for 24 h. The local indigo shows a yield of concentration twice higher than that of the reviewed indigo because the suitable pH in our fermentation provides a large indican quantity [28].

The I–V characteristics provide the knee voltage, forward- and reverse-bias currents, and calculated static resistance.

The knee voltage is a voltage level at which the potential energy of the electron is higher than the band gap energy of the indigo layer, and it also overcomes barrier potential at the MSM junction. It causes the MSM diode to approach the current conduction state at the forward bias. The value of this voltage is dependent on the orbital energy of the indigo layer and the work function of metal electrodes in the device [17]. The 1.0 V_MSM of knee voltage is found in our local indigo diode, which is close to that of the commercial diode, 0.7 V_MSM, whereas that of the review indigo diode is large, at about 2.0 V_MSM. These results mean that our local indigo diode can easily be improved to operate at 0.7 V_MSM. The advantage of the local indigo diode is thought to be due to its greater number of electrons and holes obtained from a high yield concentration during the preparation [29].

It is seen that both forward- and reverse-bias currents of either review or local indigo diodes are much too low to compare with those of a commercial diode.

However, a forward-bias current of a local indigo diode is found to be about 2–3 hundred times higher than that of a review one. These results mean that for utilization in a gradual linear amplifier in a positive current, the local indigo diode is more usable than the review one.

A nearly zero reverse-bias current is normally required to ensure that a current must flow in only one direction of a positive bias. The reverse-bias current of a review indigo diode is about 100 times smaller than that of our indigo diode. This is possibly because of the difference in an indigo derivative between our local MSM diode and the review one. It may be attributed to that the electron and hole characteristics of both number and mobility in the local indigo are better than those of the review indigo, as it exhibits a lower energy band. Therefore, the local MSM diode is more conductive [22]; this means that both diodes may be used in some particular applications.

The static resistance of reverse-bias must have a high value in order to prevent the reverse-bias current from flowing in a negative direction. It is seen that the static resistance obtained by the local indigo diode is 20 times lower than both commercial and review diodes. This also supports the assumption that a local indigo diode tends to be more conductive than review indigo, as explained previously.

A high rectification ratio of about 100 at a particular operation is preferred to allow more precise control of the current. It is seen that both diodes are lower than that requirement, but it is noticed that our local diode is about 2–4 times higher than that of a reviewed diode, since it has more sensitivity of current transfer characteristics and conductivity in this device. This means that our local indigo diode is more effective than a review one, as per the same reason explained above.

5. Conclusions

This study shows the electrochemical properties of a Sakon Nakhon-indigo-based MSM diode and compares them with those of an India-indigo-based MSM diode under the same conditions of preparation. The electrochemical properties, including I–V characteristics, static resistance, and rectification ratio, were examined. It was found that the electrochemical properties of the device are based on electron and hole characteristics and the band gap energy in the prepared local indigo pigment. Our local MSM diode performed a suitable operation between −1 and +3 V_MSM with a knee voltage of about 1.0 V_MSM. The highest forward-bias current of about 3.19 mA was found in a linear operation between +2 and +3 V_MSM, whereas that of about 0.014 mA was found between +4 and +12.5 V_MSM of the review MSM diode. This advantage of the local indigo was discovered as having better conduction than the review one, because its greater electron and hole characteristics of a better yield of indigo concentration obtained during the preparation are thought to be the main reason for this advancement. Therefore, it is concluded that the Sakon Nakhon-indigo-strain-based MSM diode is suitable for utilization in an electronic device with preferred conductivity and high sensitivity at low voltage consumption. Local indigo may be deeply analyzed by XRD technique to investigate the crystallinity and structure of solid samples and further develop it into a thin-film form for use as an organic semiconductor material in several electronic devices, i.e., organic field-effect transistors and light-emitting diodes, in the future work.