Improvement of electrical and photovoltaic properties of methyl red dye based photoelectrochemical cells in presence of single walled carbon nanotubes

Abstract

In this work, we investigated the effect of single walled carbon nanotubes (SWCNT) on the electrical and photovoltaic properties of methyl red (MR) dye based photoelectrochemical cell (PEC). MR dye based PEC with LiClO₄ as ion salt were fabricated with and without mixing SWCNT. The cells were characterized through electrical and optical measurements. The performance of the devices changed drastically in presence of SWCNT. The transition voltage and trap energy of the cells were estimated from the steady-state dark current voltage (I-V) analysis. The transition voltage and trap energy decreased for MR dye cell in presence of SWCNT. Open circuit voltage (V_oc), short circuit current (J_sc), fill factor (FF) and power conversion efficiency (η) increased due to the addition of SWCNT. Further measurement of the transient photocurrent showed that the growth and decay of photocurrent was quite faster in presence of SWCNT. The photocurrent decay with time was fitted for both the cells and found to follow a power law relation which indicates dispersive transport mechanism with exponential trap states distributed in between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels. Possible interpretation is done on the lowering of trap energy with the photocurrent. These results suggest that SWCNT lowers the trap energy of the cells by providing efficient percolation pathways for the conduction of charges. It is expected that due to lowering of trap energy the residing time of the free carriers within the traps decreases. In other words, it may also be said that the charge recombination decreases. These factors affect the overall conduction of charges and improve the electrical and photovoltaic properties.

Keywords

methyl red (MR) / single walled carbon nanotubes (SWCNT) / photoelectrochemical cell (PEC) / trap energy / percolation pathways

1 Introduction

Recently organic dye based solid state photoelectrochmical cells (PECs) have emerged as an immensely attracting field mainly due to their low cost, light weight and high flexibility [ 1– 4]. They are being widely used as photovoltaic devices to harvest solar energy. In any PEC, organic dye is mixed with a solid electrolyte. The ionic conductivity in these PECs plays a vital role to enhance the quantum efficiency of these devices. However, compared to their inorganic counterparts, these cells suffer from low efficiency and stability [ 5– 8]. The important factor is the presence of traps in these organic systems, which severely affect their performance [ 9– 11]. Due to the presence of traps, organic photovoltaic devices exhibit unusual electrical and optical behavior. This affects the dark current voltage (I-V)characteristics and also the photocurrent. Traps are modeled by different workers in different ways such as discrete trap, multi traps, transport hopping via localized states, etc. But in all these cases, the trap energy is a very important parameter. Trap energy, which may be considered as the potential depth of the traps, has a major impact on the charge transport process of these devices. During transport, the free carriers interact with the traps and in the process a majority of these carriers get entrapped. Due to it, the motion of the carriers get slowed down. In other words, it may be considered that the traps behave as potential wells and the carriers reside and become immobilized within the traps. The residing time may be considered proportional to the depth or the energy of the traps for a particular concentration of the trap charge density. Due to this, the recombination and mobility of charge carriers get affected. With an increase in trap charge density, the recombination of charges increase and also the mobility gets reduced. For the last few years, we have been studying the effects of traps in organic dye based photovoltaic devices [ 12– 17]. Efforts to reduce the trap energy of these devices are being carried out as it would lower the rate of recombination and thereby enhance the conduction of charges through these devices. Different approaches are being undertaken in this regard. By changing the device structure, incorporating different guest materials such as nanoparticles, carbon nanotubes (CNT), etc, researchers are trying to control the effect of traps and improve the performance of these organic dye based devices [ 18– 20]. Reports suggest that CNTs help to enhance the efficiency of organic photovoltaic (OPV) devices [ 21, 22]. In our previous work, we have observed that due to the addition of COOH- single walled carbon nanotubes (SWCNTs), the electrical and photovoltaic properties of Malachite Green dye based photovoltaic devices improved [ 23].

In the present work, we have investigated the changes in electrical and photovoltaic properties of methyl red (MR) dye based PEC in presence of SWCNT. Generally, in a PEC, an ionic salt is used in a solid matrix. In this work, we have used LiClO₄ as an ion source. Poly (ethylene oxide) (PEO) of molecular weight 100000 is used as the solid polymeric matrix. LiClO₄ is mixed with solid polymer matrix PEO to form the solid-state ionic conductor. The ionic conductivity of PEO:LiClO₄ complexes is very low. To enhance the ionic conductivity ethylene carbonate (EC) and propylene carbonate (PC) are used as plasticizers. Our earlier works [ 24] show that use of plasticizers EC and PC assist charge migration. MR dye is used as the light sensitizer. We fabricated cells with this blend by drop casting on a spin coater. For fabrication of these cells, we used indium tin oxide (ITO) coated glass as the front electrode and aluminum (Al) coated on mylar (M) sheet as the back electrode. Next, we added SWCNT to this blend and fabricated cells in a similar manner. Our interest is to observe the change in electrical and photovoltaic properties of the MR cells due to addition of SWCNT. Such a study is important to understand the role of CNT in dye based PEC. After fabrication is done, all the cells are kept in vacuum for around 12 h before characterization.

2 Experiments

2.1 Materials

The structures of MR dye (Finar Chemicals, Ahmedabad, India, 99.5% pure) and SWCNT (Sisco Research Laboratories, India) are shown in Figs. 1€(a) and 1€(b), respectively. Polyvinyl alcohol (PVA) has been purchased from S.D. Fine Chem. Ltd., Boisar, India and PEO from BDH, England, UK. We have procured LiClO₄, EC, PC from Fluka (99.5% pure). We have used ITO coated glass as the front electrode and Al coated with M sheet as the back electrode. It can be seen from our works [ 26] that Al-M has better optical reflectivity that Al. As a result of this back reflection, more optical energy can be confined in the organic dye.

Fig.1 Structure of (a) methyl red (MR) dye; (b) single walled carbon nanotubes [25]

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As mentioned, we have chosen MR for our study as it is one of the less reported dyes in photovoltaic mode and has a good optical response. It is a typical aromatic azo dye with the chemical formula NC₆H₄COOH (2-[4-(dimethylamino) phenylazo] benzoic acid). The reddish color of MR comes from its absorbance in the visible region of the spectrum due to the delocalization of electron in the benzene and azo groups forming a conjugated system [ 27, 28].

2.2 Sample preparation

For sample preparation, 1 mg of PVA is added in 30 mL of double distilled water and stirred well to form a transparent viscous solution of PVA. Here, PVA acts as an inert binder. In this solution, 1 mg of MR dye is added and stirred well in a magnetic stirrer. In a separate beaker, a gel like electrolyte is prepared by mixing LiClO₄, PEO, EC and PC in the percentage weight ratio of 30.60%–3.60%–19.60%–46.20% by weight. This electrolyte is heated at around 60°C and stirred well in a magnetic stirrer for about 6 h. Then, this electrolyte is mixed with the previously prepared dye solution to form the ionic blend. This ionic blend is then spin coated on a ITO coated glass and Al-M electrodes and sandwiched to form the device. This blend is then separated into two parts in two beakers and in one of them we add SWCNT in the ratio 1:1 of MR dye and SWCNT and again stirred well.

Next we have fabricated different cells with these two blends. To prepare the cells, the MR dye blend is spin coated on a pre cleaned ITO coated glass at a speed of 2500 r/min and the film is semi-dried in vacuum. The same blend is drop cast on the Al-M electrode, which was placed on the spin coater. When both the electrodes are in semi-dry state, they are sandwiched together to form the MR dye based photoelectrochemical cell. By using the same procedure cell are prepared with the SWCNT mixed MR dye blend. All the cells have an area of 1.5 cm × 1.5 cm. The cells are kept in vacuum for 12 h before characterization. To make the system cost effective, we have used all the components including the MR dye and the electrodes in commercial grade and also the measurements were carried out in the open atmosphere of the laboratory. So, due to the less costly chemicals, the overall efficiency of the device is quite poor. Figure 2 shows the diagram of a typical photoelectrochemical cell.

Fig.2 Schematic diagram of the photoelectrochemical cell. Indium tin oxide (ITO) coated glass is used as the front electrode and a thin layer of aluminum (Al) coated on a mylar (M) sheet is used as the back electrode. Here, methyl red (MR) dye is used as a light sensitizer in the ionic blend comprising of LiClO4, PEO, EC and PC

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3 Measurements

The absorption spectra of the cells are measured by using an Elico scanning mini spectrometer. Dark current voltage (I-V) characteristics of the cells have been measured with a Keithley 2400 source measure unit. During measurement, the bias voltage is varied from 0 to 6 V in steps of 0.5 V with 1500 ms delay. A solar simulator (Model 150 W Newport Corporation) has been used as the light source for the photovoltaic measurement. The photocurrent and photo voltage are measured with a digital current nanometer and Keithley 2000 multimeter, respectively. The light intensity is measured by a calibrated lux meter (Kyoritsu Electrical Instruments Works Ltd., Toyko, model 5200). Pulsed photocurrent measurements are done with an Agilent data-logger (Model 34970A) data acquisition system. The experiments have been done in the clean open atmosphere of the laboratory at a room temperature of 22°C.

4 Results and discussion

The absorption spectra of MR dye in doubled distilled water is shown in Fig. 3. As observed, MR shows a peak at 524 nm in distilled water. For comparison, the spectrum of SWCNT mixed with MR has also been shown. It is observed that due to addition of SWCNT there occur no significant changes in peak absorption wavelength of MR dye. From this observation, it can be interpreted that SWCNT has no significant role in the generation of charges carriers.

Fig.3 Absorption spectra of methyl red (MR) and single walled carbon nanotube (SWCNT) mixed with MR in distilled water

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The dark I-V characteristics of the cells are shown in Figs. 4(a) and 4(b). From the curves, it is observed that after a certain transition voltage (V_th) the conduction mechanism of the cells change drastically. It is expected that at sufficiently low voltages the current increases in accordance with Ohm’s law. As the applied voltage is increased, the injection of charges increase and the density of free carriers increase slowly. With the increase of free carriers, the charge trapping mechanism also increase and part of the free carriers become trapped. After a certain transition voltage, the density of trapped charges become higher and the current becomes trap limited. This process continues until another transition voltage is reached where the traps get filled up and the current becomes trap filled current. Here, in Figs. 4(a) and 4(b), it can be seen that even below the transition voltage, there is a certain considerable amount of current in both the cells. Also for the MR cell with SWCNT, the transition voltage shifts to a lower value from 2.52 to 1.99 V. But still a clear picture of the curves cannot be understood from the I-V curves and so we plotted the data on a logarithmic scale.

Fig.4 Dark I-V characteristics for (a) methyl red (MR) dye based cell and (b) single walled carbon nanotube (SWCNT) mixed with MR dye cell

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Figures 5 (a) and 5(b) show the dark I-V characteristics of the cells on a logarithmic scale. The curves show that there are three distinct regions of current conduction for both the cells. In these three regions, the mechanism of current conduction is different as mentioned above. In case of the MR cell without SWCNT, the first transition voltage (V_th1) is found to be at 2.52 V. Below this, the conduction mechanism is Ohmic. Above this voltage, the current is basically space charge limited in presence of exponentially distributed trap states. This mechanism continues until another transition voltage (V_th2) is reached when all the traps become filled up. After V_th2, again the current conduction mechanism changes and the current becomes trap filled current. For the MR cell without SWCNT, this second transition voltage is found to be at 4.2 V. The three regions mentioned above have been identified as region I, II and III in Figs. 5(a) and 5(b). In case of the MR cells with SWCNT, V_th1 occurs at 1.99 V and V_th2 at 3.55 V. Thereafter to estimate the slope of the curves and to estimate the trap energy, we recollect the I-V relation.

We already know that organic materials are disordered solids with no well-defined band structure. The molecules are bound together by weak van-der-Walls forces and the charge transport occurs through the loosely bound π- conjugated electrons. Here, instead of conduction and valence band, there are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels. These weak molecular bonds and structural disorders give rise to electronic traps in these systems, which are assumed to play a dominant role in the current conduction process.

Fig.5 lnI-lnV characteristics for (a) methyl red (MR) dye based cell and (b) single walled carbon nanotube (SWCNT) mixed with MR dye cell

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According to Yang and Shen [ 29], these traps introduce energy levels inside the energy gap between the HOMO and LUMO levels of the organic layers. The entrapping and de-trapping of charges in these trap levels has a significant effect on the operation and performance of these photoelectrochemical cells. Because of the presence of traps in these devices, a large part of the injected charges get immobilized and result in crowding of charges near the electrodes. This kind of conduction process is usually referred to as the trap limited conduction process. This is a special case of the general space charge limited (SCL) regime [ 30]. The process is referred to as trap-free SCL conduction when the space charges are dominantly free carriers. The distribution of trap energy level is generally described by one of the following ways: 1) an exponential distribution, 2) discrete levels, and 3) a Gaussian distribution. Assuming an exponential energy distribution, trap charge concentration (n_t) may be expressed as

(1) $$n_{\mathrm{t}}=H_n\exp \left({\displaystyle \frac{F_n}{k{T}_{\mathrm{c}}}}\right),$$

where H_n is the trap density, F_n is the electron Fermi energy, K is Boltzmann constant, and T_c is characteristic temperature of the exponential trap distribution (i.e., T_c = E_c/K, where E_c is the characteristic trap energy). Solving the Poisson equation with this form of trap distribution [ 31], the I-V characteristic is calculated and written in the following form:

(2) $$J=N_{\mathrm{c}}\mu q^{\mathrm{1}-m}{\left[{\displaystyle \frac{mϵ}{H_n(m+\mathrm{1})}}\right]}^m{\left({\displaystyle \frac{\mathrm{2}m+\mathrm{1}}{m+\mathrm{1}}}\right)}^{m+\mathrm{1}}{\displaystyle \frac{V^{m+\mathrm{1}}}{L^{\mathrm{2}m+\mathrm{1}}}},$$

where N_c is the effective density of states in LUMO or HOMO, μ is the mobility of majority carrier, L is the thickness of the layer, ϵ is equal to ϵ₀ϵ_r with ϵ₀ being the permittivity of vacuum and ϵ_r the dielectric constant, V is the applied voltage and m = T_c/T, T_c is a characteristic temperature that describe the trap distribution. The most notable feature in the above equation is the power law dependence of

(3) $$J\backslash vskip\; 3pt~V^{m+\mathrm{1}}.$$

In this work, the dark I-V curves have been analyzed to find the characteristic temperature T_c and from it the characteristic trap energy E_c. Figures 5(a) and 5(b) show the logarithmic plots of the dark I-V curves. From these figure, we can calculate the ‘m + 1’ values for the different cells using Eq. (3) and estimate the trap energy of the cells and analyze the change in the conduction behavior of the charges due to the addition of SWCNT. From those values, we have extracted the values of characteristic trap energy of the system. The values are shown in Table 1.

Tab.1 Extraction of values of ‘m’ and trap energy ‘Ec’ from lnI-lnV curves

cell type	Vth1/V	Vth2/V	m1 region I	m2 region II	m3 region III	Ec in region II/eV
MR cell without SWCNT	2.52	4.2	1.19	4.61	1.99	0.091
MR cell with SWCNT	1.99	3.55	1.13	3.42	0.33	0.061

The trap energy is a very important parameter controlling the conduction of charges in these devices. Depending on the energy values of traps, the immobilization time of trapped carriers also varies. So any changes in trap energy will affect the overall performance of the devices. From the values calculated in Table 1, it is observed that both V_th and E_c have decreased in presence of SWCNT. Decrease in trap energy indicates that the residing time or the immobilized time of the charges will decrease. This leads to fast collection of charges. It is expected that this process will reduce recombination and will enhance the device performance. To ensure this, we will also perform the photovoltaic measurements.

Figures 6(a) and 6(b) show the photovoltaic characteristics of the cells. The curves are obtained by varying the load resistance. Different photovoltaic parameters such as open circuit voltage (V_oc), short circuit current density (J_sc), fill factor (FF) and power conversion efficiency (η) of the cells are evaluated from the curves and are listed in Table 2.

Tab.2 Different photovoltaic parameters extracted from the light I-V curves

cell type	cell area/cm2	Jsc/(µA·cm-2)	Voc/mV	FF	η/10-2%
MR cell without SWCNT	1.5	296.92	79.35	0.38	1.12
MR cell with SWCNT	1.5	734.26	195	0.48	68.7

From the results obtained, it is seen that V_oc, J_sc, FF and η values have increased for the MR cell in presence of SWCNT. It is also observed that there is a significant change in the short circuit current density but the open circuit voltage of the cells does not change considerably. For the MR cells with SWCNT, the FF increased from 0.38 to 0.48. It can be expected that due to a decrease in trap energy, charge conduction process becomes faster and causes an increase in the output current and overall device performance.

Fig.6 Photovoltaic characteristics of (a) methyl red (MR) cell and (b) single walled carbon nanotube (SWCNT) mixed with MR cell. Different photovoltaic parameters such Voc, Jsc, FF and η have been extracted from the curves, the values of which are shown in Table 2

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To further ensure this fact that decrease in trap energy causes faster migration of charges, we performed the transient photocurrent measurements. The charge separation and relaxation dynamics of the cells can be understood from the pulsed photocurrent measurements. Figures 7(a) and 7(b) show the transient photo-response of the cells. We have performed the measurements with different light intensity. To perform the measurement, the cell is exposed to radiation for about 100 s and then radiation is turned off. The growth and decay rate for the two cells are observed to be quite different. From the curves shown in Fig. 8, it is observed that the photocurrent growth and decay rate for SWCNT added MR cells are quite faster. A sharp increase and decrease in photocurrent indicates that the charge separation rate is fast and charge recombination is reduced. Analysis of this data will give further information about the charge transport mechanism of the system.

Fig.7 Pulsed photocurrent measurements for (a) only (a) methyl red (MR) cell and (b) single walled carbon nanotube (SWCNT) mixed with MR cell

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To investigate more critically, we consider the curves for particular incident intensity. Figures 8(a) and 8(b) shows the pulsed photocurrent measurements of the cells for an incident radiation of 100 mW/cm². From the curves, it can be seen that the growth and decay rate of photocurrent for the two cells are quite different. For the MR cell without SWCNT, the photocurrent decays for a long time and takes around 175 s to attain the initial value. While in presence of SWCNT, the decay is quite faster, and the photocurrent takes around 100 s to decay. Also the peak value of photocurrent is higher in presence of SWCNT.

Fig.8 Pulsed photocurrent measurements of (a) (a) methyl red (MR) cell and (b) single walled carbon nanotube (SWCNT) mixed with MR for an incident radiation of 100 mW/cm2

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To study further this decay process, we fitted the decay current with time in a logarithmic scale, as shown in Figs. 9(a) and 9(b). It is interesting to note that for both the cells the decay current is fitted and found to follow a power law relation with time, which can be expressed as I_ph~t^α, where I_ph is the photocurrent and α is the decay constant. Here, the value of this decay constant is found to be 0.96 and 0.055 for the MR cells with and without SWCNT, respectively.

Fig.9 Determination of decay constant from lnI-lnt curves for (a) methyl red (MR) cell without single walled carbon nanotube (SWCNT) and (b) MR cell with SWCNT

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From this power law relation of photocurrent with time, we also get some idea about the charge transport mechanism. Power law relation indicates a dispersive transport via hopping in between localized trap states. This may be presented in a schematic diagram. Figures 10(a) and 10(b) show the charging and discharging process of the charge carriers in between the traps during the growth and decay of the photocurrent respectively. During growth of photocurrent, the traps get charged and the trapped charges go to the higher energy levels. While during decay the carriers hop from higher energy states to lower energy states. In this process, majority of the carriers get released from the traps while a number of carriers are lost due to recombination.

Fig.10 (a) Charging and (b) discharging of trapped carriers during photocurrent growth and decay

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It is expected that addition of SWCNT lowers the trap energy by providing efficient percolation pathways for conduction of charges. To get an overall view of the different parameters affected due to addition of SWCNT, the comparison has been shown in Table 3. From this table, we get a clear idea that the electrical and photovoltaic properties of the MR cells change in presence of SWCNT.

Tab.3 Comparison of the different parameters extracted from the electrical and optical measurement of the MR cells with and without SWCNT

Parameter	without SWCNT	with SWCNT
trap energy (Ec)	0.172 eV	0.062 eV
transition voltage (Vth1)	2.52 V	1.99 V
transition voltage (Vth2)	3.55 V	4.2 V
short circuit current (Jsc)	296.92 µA/cm2	734.26 µA/cm2
open circuit voltage (Voc)	79.35 mV	195 mV
fill factor (FF)	0.38	0.48
decay constant	0.055	0.096

5 Conclusions

In this work, we report on the changes in electrical and photovoltaic properties of MR dye based photoelectrochemical cells in presence of SWCNT. Dark I-V analysis shows that three different types of current conduction mechanisms occur with the applied bias voltage. Our results show that due to addition of SWCNT the trap energy of the cells decrease from 0.172 to 0.062 eV. Consequently for the same devices, there is about 79% increase in FF. The V_oc, J_sc and FF are 195 mV, 734.26 µA/cm² and 0.48 respectively for the SWCNT added MR cell. To investigate further the role of SWCNT, transient photocurrent measurement performed and the photocurrent growth and decay rate is found to be faster in presence of SWCNT. By fitting the data of a logarithmic scale, it is found that the photocurrent follow a power law relation with time. From this, the decay constant is estimated and found to be 0.96 and 0.055 for the MR cells with and without SWCNT respectively. It is expected that SWCNT provide effective percolation pathways for conduction of charges. Due to presence of SWCNT, the trap energy of the cells get reduced. Due to decrease in trap energy, the residing time of the charge carriers in the traps also decrease since it is expected that the residing time of the trapped carriers is proportional to the depth or the energy value of the traps. This leads to better current conduction and improves the photovoltaic action of the cells. Further work is going on to understand the role of carbon nanotubes on the charge recombination process of organic dye based photoelectrochemical cells.

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Acknowledgements

The authors thank University Grant Commission (UGC), India and Council of Scientific and Industrial Research (CSIR), India for providing financial support for carrying out the research work.

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