Estimation and Modification of Electrical Parameters of Organic Device in the Presence of Single Walled Carbon Nanotubes

Abstract

Present work will mainly focus on one of the most important applications of single-walled carbon nanotubes (SWCNTs). In this work, the different electrical parameters that are associated with the charge injection process at the metal−organic contact of the organic device will be estimated and subsequently, the effect of SWCNTs on those parameters will be measured. As we all know that high charge carrier trapping and high Schottky barrier at the metal−organic contact significantly affect the charge flow at the junction of organic dye-based device. It is of paramount importance to reduce these parameters which hinder the charge flow in the organic device. SWCNTs are one of the most prominent materials which can improve this charge flow at the metal−organic contact. Our main aim will be to study the physics behind the improvement of these electrical parameters in the presence of SWCNTs which will allow the device to perform more efficiently.

1. Introduction

Nanotechnology and nanomaterials play a significant role in the present scenario of research works. In the emerging fields of research works, nanotechnology will form the base for other technologies to emerge at nanoscopic level [1]. One of the most significant innovations in the field of nanotechnology is the discovery of Carbon nanotubes (CNTs), which is an allotrope of carbon. CNTs are arranged in hexagon and pentagon with a diameter of 3–30 nm [2]. CNTs consist of single or multiple layers of graphene sheets. Depending on number of layers, CNTs can be of two types, single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). MWCNTs were discovered by Sumio lijima in 1991 by a simple arc evaporation method and consist of multiple graphene layer sheets [3]. SWCNTs comprise single-layer graphene sheets, and their outer diameter is basically in the range of 1−2 nm [4]. Inbuilt structural defects and undefined diameter make MWCNTs less stable than SWCNTs [5]. Researchers have shown great interest in SWCNTs due to their excellent thermal, electrical, and mechanical properties. These excellent properties can be attributed to large aspect ratio as the length and diameter of SWCNTs are in the range of micrometer and nanometers respectively [6]. The structure of SWCNTs is akin to cylindrical shape with hexagonal carbon atoms which are sp² hybridized [7]. It has a hollow inner structure. SWCNTs take the form of one – dimensional (1D) material and they are used in membranes, capacitors, polymers, metallic surfaces, ceramics, nanomedicine etc. [8].

In this work, we will study one of the major applications of SWCNTs in the organic device regarding charge injection process at metal–organic contact, when the organic dye is sandwiched between two metallic electrodes to form the organic device. These organic devices possess several advantages compared to inorganic devices, such as low cost, mechanical flexibility, copious availability, light weight, low-temperature processing, and easy tunability of their properties via molecular tailoring [9, 10, 11]. In spite of these advantages, there are indisputable impediments to these devices. One of the impediments of these organic devices is low charge injection from metallic electrodes to organic layer. The poor injection of charges can be ascribed to the high trap concentration at the metal–organic contact. Traps basically act as recombination centers or as defects in the amorphous organic materials, in which active charge carriers get adrift. High Schottky barriers at the metal–organic contact also allow less charge carriers to flow through the interfacial area and it causes the formation of space charge layer at the junction area. Researchers are striving for reducing the parameters which affect the charge injection process at metal–organic contact, such as high trap concentration, Schottky barrier, and space-charge layer width to improve the device performance. In this work, we will incorporate SWCNT in the organic device and subsequently study the effect of SWCNTs on these above-mentioned parameters in order to improve the charge injection at metal–organic contact. Phenosafranin dye has been chosen as organic dye and it has been sandwiched in between Indium Tin Oxide (ITO) and Aluminum (Al). The current flow in the prepared organic devices will be analyzed by using Richardson – Schottky thermionic emission model. Dark current–voltage (I-V) and capacitance-voltage (C-V) analysis of the prepared organic device will be performed in this present work.

2. Experimental details

2.1 Materials and sample preparation

Phenosafranin (PSF) dye (3, 7-Diamino-5-phenylphenazinium chloride) belongs to cationic phenazine group of dyes [12, 13]. It can be used as photosensitizer in energy and electron transfer reactions [14]. It has amine functionalities which make it a useful material for future polymer composite work [15]. The product we have used in this experiment has 80% dye content and its molecular weight is 322.79 g/mol. This dye has been procured from Sigma Aldrich, Germany. Figure 1a shows the structure of this dye. Figure 1b shows the schematic diagram of SWCNT, which is obtained from Sisco Research Laboratories (SRL), India. We have used SWCNT of 2 nm outer diameter and 30 μm length.

Figure 1.

(a) Structure of Phenosafranin (PSF) dye and (b) schematic diagram of SWCNT.

A 75 mm × 25 mm x 1.1 mm ITO coated glass slide (surface resistivity ∼20 Ω/sq) is used as front electrode and Al is used as back electrode for preparing the organic device. ITO-coated glass and Al have also been purchased from Sigma Aldrich. Poly vinyl alcohol (PVA) has been used in this work, as it is an excellent transparent inert binder. PVA was obtained from S. D. Fine Chem. Ltd., Boisar, India.

At first, the PVA solution is prepared. In a cleaned beaker, 5 mg of PVA is added to 15 ml of distilled water and is stirred with a magnetic stirrer for 30 minutes at 60°C. PVA is of laboratory grade. PVA is utilized to enhance the mechanical properties of organic dye films because it has compatible structure and hydrophilic properties. Molecular weight of PVA is approximately 1,25,000. PVA is used to stick the dye solution on the electrodes [13].

1 mg of PSF dye is added in the PVA solution and stirred for 30 minutes. One part of this solution is kept aside in a pre-cleaned test tube and in other part of PSF dye solution, 1 mg SWCNT is added and well stirred. After that, PSF dye solution without SWCNT is spin coated at 2500 rpm speed and dried at 4000 rpm speed on a pre cleaned ITO coated glass substrate. Similarly, the same solution is deposited on the Al and then ITO coated glass and Al are intercalated together to form the PSF cell without SWCNT. Similarly, the PSF solution with SWCNT is also spin coated to prepare the PSF cell consisting of SWCNT. The prepared devices are kept in vacuum desiccators for 12 hours to dry before characterization. Figure 2 expresses schematic diagram of PSF dye based organic device.

Figure 2.

Schematic diagram of prepared PSF dye based organic device.

2.2 Measurements

Dark I−V characteristics and C-V characteristics of the cells have been measured with a Keithley 2400 source measure unit and by using LCR meter respectively. For dark I–V measurement, the front electrode ITO is connected to the positive terminal of the battery and the back electrode Al is connected to the negative terminal of the battery [16]. The applied voltage is varied from 0 to 5 V in steps of 0.25 V with a delay of 1000 ms. Room temperature was kept at 25°C during the experiment.

3. Results and discussion

As stated by Richardson-Schottky model, interfacial current at metal–organic layer is shown in the eq. (1)

$$\mathrm{I}=\mathrm{A}{\mathrm{A}}^{\ast }{\mathrm{T}}^{2}exp.\left(-\frac{\mathrm{q}{\mathrm{\varphi }}_{\mathrm{b}}}{\mathrm{kT}}\right)(exp\left(\frac{\mathrm{qV}}{\mathrm{nkT}}\right)\left[1-exp\left(\frac{-\mathrm{qV}}{\mathrm{kT}}\right)\right]$$E1

${\mathrm{I}}_0$ is the saturation current, which is expressed in eq. (2).

$${\mathrm{I}}_0=\mathrm{A}{\mathrm{A}}^{\ast }{\mathrm{T}}^{2}exp.\left(-\frac{\mathrm{q}{\mathrm{\varphi }}_{\mathrm{b}}}{\mathrm{kT}}\right)$$E2

The interfacial Schottky barrier at metal- organic junction can be determined from the eq. (2) which has been expressed in eq. (3)

$${\mathrm{\varphi }}_{\mathrm{b}}=\frac{\mathrm{kT}}{\mathrm{q}}ln\left(\frac{\mathrm{A}{\mathrm{A}}^{\ast }{\mathrm{T}}^2}{{\mathrm{I}}_0}\right)$$E3

Here, q is the charge of electron, V is the voltage that is applied to the device, A is the device area, k is the Boltzmann’s constant, T is absolute temperature, A* is the effective Richardson constant, n is the ideality factor and ${\mathrm{\varphi }}_{\mathrm{b}}$ is Schottky barrier [17, 18, 19, 20, 21, 22]. The saturation current is determined by the interpolation of exponential slope of I at V = 0 and ${\mathrm{\varphi }}_{\mathrm{b}}$ is obtained from the extrapolation of I₀ in the semi log forward bias I - V characteristics. The term $\frac{\mathrm{q}}{\mathrm{kT}}$ can be replaced by the term β.

The Schottky barrier can also be expressed as shown in the eq. (4)

$${\mathrm{\varphi }}_{\mathrm{b}}=\frac{1}{\mathrm{\beta }}ln\left(\frac{\mathrm{A}{\mathrm{A}}^{\ast }{\mathrm{T}}^2}{{\mathrm{I}}_0}\right)$$E4

Figure 3 shows the dark I-V plots of device without and with SWCNTs have been shown in. Figure 3 depicts that with SWCNT, the current flow in the PSF dye based organic device increases about two times. The threshold voltage has also been estimated from Figure 3. The threshold or turn-on voltage of the device is 2.5 V without SWCNT and the value of the threshold voltage lowers to 1 V with SWCNT.

Figure 3.

Current–voltage (I-V) plot of organic device without and with SWCNT.

Figure 4 shows the semilogarithmic current – voltage (lnI –V) plots of Figure 3 to calculate the Schottky barrier at the metal – organic junction of the PSF dye based organic device in absence and in presence of SWCNT. Schottky barrier is obtained from the extrapolation of I₀ in the semi log forward bias I - V characteristics, which are shown in Figure 4. The estimated value of I₀ is found to be 0.74 μA and 1.45 μA in absence and in presence of SWCNT. Putting the values of I₀ in eq. (4), the Schottky barrier of PSF dye based organic device has been calculated without and with SWCNT.

Figure 4.

Semi-logarithmic (ln I-V) plot of organic device without and with SWCNT.

The space - charge layer width can be estimated from following eq. (5)

$${\mathrm{W}}_{\mathrm{d}}=\sqrt{\frac{2{\mathrm{\epsilon }}_0{\mathrm{\epsilon }}_{\mathrm{s}}{\mathrm{V}}_{\mathrm{d}}}{\mathrm{q}{\mathrm{N}}_{\mathrm{D}}}}$$E5

${\mathrm{W}}_{\mathrm{d}}$= space - charge layer width, ${\mathrm{\epsilon }}_0$= vacuum permittivity, ${\mathrm{\epsilon }}_{\mathrm{s}}$= semiconductor permittivity, ${\mathrm{V}}_{\mathrm{d}}$= diffusion potential, q = charge of an electron, ${\mathrm{N}}_{\mathrm{D}}$= donor atom concentration.

In this work, we have considered the value of ${\mathrm{\epsilon }}_{\mathrm{s}}$ = 11.9 ${\mathrm{\epsilon }}_0$ and ${\mathrm{N}}_{\mathrm{D}}$ is in the order of 10¹⁷ cm⁻³ respectively.

C–V measurement is one of the most important methods to obtain the particulars of rectifying contact interfaces [23]. Diffusion capacitance dominates in the forward bias region and for reverse biased region, the junction capacitances [24].

Figure 5 shows C-V curves of PSF dye based organic device without and with SWCNT respectively at a fixed frequency of 10 kHz. It has been observed from the Figure 5, that with SWCNT, under forward bias region, the diffusion capacitance increases considerably.

Figure 5.

Capacitance – Voltage (C-V) plot of organic device without and with SWCNT.

Figure 6 shows C⁻²-V curves of PSF dye based organic device without and with SWCNT respectively. The linear pattern of C⁻²-V curves is an indication of forming the Schottky contact at metal – organic junction. The values of diffusion potential and the Schottky barrier have been estimated by using C⁻²-V characteristics. Voltage axis intercept of C⁻²-V curves gives the estimated value of diffusion potential which is about to be 3.3 V without SWCNT and about 2.7 V with SWCNT. Putting the values of diffusion potential in eq. (5), the values of space – charge layer width have been calculated without SWCNT and with SWCNT.

Figure 6.

C⁻²-V plot of organic device without and with SWCNT.

Trap energy has been measured by using double logarithmic current – voltage (ln I- ln V) plot which is shown in Figure 7 without and with SWCNT respectively.

Figure 7.

Double -logarithmic current – Voltage (ln I-ln V) plot of organic device without and with SWCNT.

The trap energy can be written as expressed in the following eq. (6)

$${\mathrm{E}}_{\mathrm{t}}=\mathrm{mkT}$$E6

Where, E_t = trap energy, m = T_c/T, where, T_c denotes the effective temperature of trap distribution and T denotes the room temperature in Kelvin scale, k is the Boltzmann’s constant [25] and m is calculated from ln I- ln V plot of Figure 7.

Comparing eq. (3) and eq. (6), it can be inferred that Schottky barrier and trap energy are proportional to each other, which means that ${\mathrm{\varphi }}_{\mathrm{b}}\propto$ E_t, considering other parameters remain unchanged. So, when the concentration of traps reduces, the Schottky barrier also decreases. Hence, the interconnection between Schottky barrier and trap energy can be established analytically.

The space – charge layer width can also be related to the Schottky barrier by using the following eq. (7)

$${\mathrm{W}}_{\mathrm{d}}=\frac{{\mathrm{\varphi }}_{\mathrm{b}}}{\mathrm{F}}$$E7

Where, F = applied electric field.

From eq. (7), it can be said that when the applied electric field remains constant, the space – charge layer width is directly proportional to the Schottky barrier.

Hence, the interrelationship among these three parameters, namely, trap energy, Schottky barrier and space – charge layer width can be analytically established. When the SWCNT is incorporated within the organic device, reduction of one of these three parameters will lead to lowering of other two parameters as they are directly proportional to each other.

The values of threshold voltage, trap energy, Schottky barrier and space – charge layer width of organic devices without and with SWCNT are shown in Table 1.

PSF Dye Based Organic Device	Threshold Voltage (V)	Value of “m”	Trap Energy (eV)	Schottky Barrier (eV) using I-V characteristics	Schottky Barrier (eV) using C-V characteristics	Space - Charge Layer Width (Wd) (cm) × 10^−6
Without SWCNT	2.50	2.05	0.053	0.81	0.76	8.15
with SWCNT	1.00	1.63	0.042	0.70	0.65	7.45

Table 1.

Calculation of threshold voltage, trap energy, Schottky barrier, space – charge layer width of organic devices in absence and in presence of SWCNT.

Table 1 shows that, in presence of SWCNT, the trap energy has been reduced from 0.053 eV to 0.042 eV, resulting in 20.75% decrease in the concentration of traps. Estimation of Schottky barrier from both I – V characteristics and C – V characteristics has also shown a reduction of 13.58% and 14.47% respectively in the presence of SWCNT. There is a small discrepancy in measuring the Schottky barrier using both the C-V and I-V techniques. C-V method averages over the whole area and estimates the Schottky barrier [26]. As organic devices are prone to traps, the difference between the barrier height values can also be ascribed to interfacial trap states in organic device. With SWCNT, space – charge layer width has also been lowered by 8.59%. All these parameters, which are associated with the charge injection process at the metal – organic contact, such as trap energy, Schottky barrier and space – charge layer width have been decreased considerably in the presence of SWCNT. Lowering of these parameters will result in improvement of charge injection at the interfacial contact and will also enhance the device performance by enhancing the conductivity in the device.

4. Conclusions

In this work, we have studied the effect of SWCNT on the different electrical parameters, affecting significantly the charge flow at metal – organic interface of PSF dye based organic device. We all know that, SWCNT has wide range of applications. Present work shows one of the applications of SWCNT in which lowering of the trap energy, Schottky barrier and space – charge layer width happen at metal – organic contact due to the incorporation of SWCNT in the organic device. With SWCNT, the device will be turned on at lower voltages, as the threshold voltages gets reduced and this can also be attributed to the increasing flow of mobile charge carriers in the organic device. Schottky barrier has been measured by both I-V and C-V methods in presence and in absence of SWCNT. Both the methods are congruous to each other, indicating notable lowering of Schottky barrier in presence of SWCNT. In terms of device physics, it can be inferred that SWCNT acts as conductive fillers to the traps which permit more active charge carriers to flow, resulting in reduction of Schottky barrier and space – charge layer width. SWCNT basically provides more conductive pathways in the organic device by filling out the traps. Due to reduction of trap energy, space – charge layer width and barrier lowering, the current flow in the devices also increases considerably. The notable findings of the work is to study one of the major applications of SWCNT in the organic device in terms of lowering of parameters that affect the charge flow at metal – organic contact which will improve the device performance.

Acknowledgments

One of the authors, Sudipta Sen is thankful to UGC for awarding a research fellowship (Grant No.3482/ (NET-JULY2016)).

Conflict of interest

On behalf of all authors, the corresponding author states that there are no competing financial interests or personal relationships that could have appeared to influence the present work.