Unan Yusmaniar Oktiawati1, Norani Muti Mohamed2, and Zainal Arif Burhanudin1

1Electrical and Electronic Engineering Dept, Universiti Teknologi Petronas, Perak, Malaysia 2Centre of Innovative Nanostructures & Nanodevices (COINN), Universiti Teknologi Petronas, Perak, Malaysia

Email: oktiawati_g01979@utp.edu.my, noranimuti_mohamed@petronas.com.my, zainabh@petronas.com.my

Abstract—Solar energy being one of the renewable energy sources can now be harnessed by the third generation solar technology in the form of Dye Solar Cell (DSC). Although DSC can offer flexibility, low in production cost and easy to fabricate, the problem still remains with its low efficiency. Various aspects of the components of the DSC can be studied in order to improve the efficiency. As one of DSC elements, electrolyte concentration may affect the efficiency as well as the performance of DSC. Higher electrolyte concentration may increase the electron mobility as well as the efficiency of DSC. However, it may also enhance the possibility of recombination that may reduce the efficiency of DSC. In this paper, simulation of different electrolyte concentrations is conducted in order to optimize the performance of DSC. Parameters obtained from other experimental work were used as the input for the simulation program. The result is then compared and validated with other experimental work which has the closest experimental condition. Results showed that the optimum electrolyte concentration is 1.5M of iodide with efficiency of 7.5504% as the compromise for higher electron mobility and less recombination rate.

Index Terms— Dye Solar Cell, Electrolyte, Concentration,

Efficiency.

I. INTRODUCTION

With the future prediction of limited fossil fuel, solar

energy has been increasingly utilized for various applications. Harnessing solar energy can be done simply by using solar cell technology. Although the first generation Si-based solar cell is well-established in terms of its performance and production but the utilization is still low because of the high cost of production. Thin film solar cell as the second generation came next to offer flexibility in application but accompanied with issues of complexity in the fabrication and the use of rare and toxic materials. Then dye solar cell (DSC) was introduced as the third generation solar technology that offers more flexibility, low cost and easy to fabricate [1].

DSC consists of sandwich like dye-sensitized TiO2 layer, electrolyte and counter electrode between transparent conducting oxide (TCO) glasses. DSC may offer more flexibility since DSC is formed by flexible material that can

be used in variety of application. This type of solar cell can be produced on low production cost because most of its material used is low cost material. Moreover, its simple production makes it to fabricate solar cell.

Behind its advantages, there still remains an issue of low efficiency in DSC. Compared to the previous generation of silicon-based solar cell, its efficiency is still lower [2]. Its efficiency is described by equation (i)

where Jsc is short circuit density in ampere per m2, Voc is open circuit voltage in volt, FF is fill factor and Pin is input power of the solar cell in watt per m2. For efficiency calculation, ideal Pin used is 1kW/m2. Efficiency for the solar cell is determined as the capability of the solar cell to convert the solar energy into electricity. To get better performance of DSC, higher efficiency is important to be reached.

As one of DSC elements, electrolyte concentration may affect the performance as well as efficiency of DSC [3]. Different electrolyte concentration may cause different performance of DSC. When the electrolyte concentration is increased, electron mobility will also increase. Increasing electron mobility will increase Jsc. But on the other hand, higher electrolyte concentration will enhance the possibility of recombination on electrolyte cathode interface that lead to a decrease in the efficiency. By optimizing electrolyte concentration, it is expected to improve the efficiency of the DSC.

In this paper, simulation of different electrolyte concentration used in DSC is conducted. Simulation is carried out by varying the electrolyte concentration while maintaining other parameters in DSC configuration. Simulation result is then analyzed and compared with other experimental work. Simulation result can provide close prediction on the performance of DSC before implementation, although in every step of procedure in experimental work, a number of practical factors need to be considered.

Simulation of the Effects of Electrolyte Concentration on Dye Solar Cell Performance

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978-1-4799-4653-2/14/$31.00 © 2014 IEEE

II. RELATED WORKS

Research in DSC is still ongoing with some focusing on the dependency of DSC performance on its electrolyte concentration. Electrolyte concentration is directly proportional to the electrolyte conductivity which can lead to an enhancement of the electron mobility. This is supported by Desilvestro [4] who stated that low electrolyte concentration slowed down the electron transfer reaction. Experimental work by Zhang et al. [3] also showed that electron diffusion, D was higher for higher electrolyte concentration as shown in equation (ii)

where kex is the exchange-reaction rate constant, and c are the center-to-center distance between the redox species participating in the exchange reaction and electrolyte concentration. Higher D will make electron mobility faster as shown in equation (iii) where k is Boltzman constant, T is temperature, q is charge and µ is electron mobility [5]. This means that higher electrolyte concentration will results in higher electron mobility. Electron transfer occurred when the electron move from an atom to another. The electron mobility will affect the short circuit current density, Jsc of DSC. Thus, higher electrolyte concentration can improve DSC performance because higher Jsc can be achieved as a result of higher electron mobility. In line with that condition, other work by Lan et al [6] mentioned that iodine content in electrolyte influences the performance of DSC. Meanwhile, Wang et al [7], Han et al [8], and Lee et. al [9] reported that higher Jsc can be obtained for higher electrolyte concentration.

Besides electron transfer reaction, another process that occurs in semiconductor material is the recombination of free electrons and holes. Recombination through defects which is introduced by Shockley-Read-Hall or SRH recombination does not occur in perfectly pure material [10]. In DSC, SRH recombination is the dominant recombination mechanism. SRH recombination started when an electron is trapped by an energy state in the forbidden region. When more electron moves up to the same energy state and recombines with the hole, more recombination occurred. The recombination processes in DSC may affect electron lifetime through equation (iv)

where τ is electron lifetime, Δn is the electron concentration and R is recombination rate [11]. Higher electrolyte concentration will cause lower electron lifetime which indicated high possibility of recombination. This electron lifetime also affected the diffusion, D by equation (v)

where W is the width [11]. It can also be explained that there is a proportional relation between D and R.

Since D has proportional relation with electrolyte concentration as shown in equation (ii) and also may cause more recombination as shown in equation (v), it can be concluded that higher electrolyte concentration may cause more recombination. Therefore, when the recombination happens, the amount of electron will decrease leading to a reduction in the short circuit current density, Jsc as well as the efficiency.

It can be hypothesized that increasing the electrolyte concentration can improve the efficiency of DSC but at the same time, increasing the recombination. For this reason, electrolyte concentration needs to be optimized in order to maximize the performance of DSC.

III. SIMULATION MODEL

This paper is focused on the simulation of the performance of DSC based on different concentration of electrolyte. Softwares used are ATHENA and ATLAS from Silvaco. ATHENA can build semiconductor fabrication design, device and process in automation technologies. ATLAS can facilitate researchers to simulate electrical behavior of semiconductor devices including solar cell [12]. Simulation work of solar cell by Michael et al. also used ATLAS as its modeling software [13]. It has several models that can be selected. Shockley-Read-Hall (SRH) model is the closest option to model DSC. SRH model considers mechanism of electron transport that is suitable for wide band gap semiconductor oxide as representation of TiO2. It also describes kinetics of generation-recombination process within semiconductor, not only on interface. Research conducted by Goudon et al. also used SRH model [10].

Simulation is conducted to reduce costly experimental works. Estimation from simulation can also be based on the physical design of the fabrication and may even include other factors that may influence practically. As the input for simulation, properties namely affinity, energy gap, and electron mobility are extracted from literatures. Data properties used in simulation are shown in Table I. DSC model was constructed using 10 µm thick TiO2 film adsorbed with N719 dye and covered with a layer of electrolyte in between FTO coated glass substrates. DSC built in ATHENA was evaluated in ATLAS under full sunlight condition. Since the investigation is on the effects of electrolyte concentration on DSC performance, the electrolyte concentration was varied. Iodine iodide was selected as the electrolyte based on the wide usage by the researchers [14]. The effect of electrolyte concentration on DSC parameters was analyzed to get the optimum performance of DSC. Simulation results of the performance of DSC from ATLAS will then be compared with the experimental work by Zhang et al [3].

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vD

W

TABLE I. DATA PROPERTIES

Parameter Properties

Light condition 1.5AM

Working temperature 300 K

Dye N719

TiO2 nanoparticle thickness 10 µm

Affinity of iodine 3.059 eV [15]

Energy gap of iodine 2.498 eV [16]

Electron mobility of iodine 65 cm2/Vs [17]

IV. RESULTS AND DISCUSSION

Performance of DSC for different concentrations of

electrolyte is shown in Table II with comparison to the experimental work by Zhang et al [3].

Results of Jsc from simulated DSC for different electrolyte concentration of 0.5M, 0.8M, 1M, and 1.5M are 12.7532 mA/cm2, 13.1485 mA/cm2, 14.2634 mA/cm2 and 14.4982 mA/cm2

respectively. It is observed that Jsc shown in Table II increases with higher concentration believed to be due to increasing electron mobility described in equation (ii). This increasing Jsc also agreed with the increasing simulated diffusion, D shown in Figure 1. Higher D is the representation of higher electron mobility. However, for electrolyte concentration beyond 2M, the value of Jsc decreases. Higher D may also means that more recombination can happen in DSC which can also limit the Jsc. In Table II, it can be seen that optimum value of Jsc of 14.4982 mA/cm2 is obtained for electrolyte concentration of 1.5M.

Results of Voc for simulated DSC shown in Table II for electrolyte concentration of 0.8 M, 1 M, 1.5 M, 2 M, 2.5 M, and 3 M are 0.7496 V, 0.7486 V, 0.7449 V, 0.7446 V, 0.74371 V and 0.7431 V respectively. It is observed that Voc decreases as the electrolyte concentration increases beyond 1.5M. As shown in Figure 1, although the diffusion is still increasing, but the recombination rate starts to dominate at the concentration of 1.5M, illustrated by the reducing electron lifetime. This then reduces the Voc for concentrations beyond 1.5M.

As can be seen in Table II and Figure 2, efficiency of simulated DSC for electrolyte concentration of 0.5M, 0.8M, 1M, 1.5M are 6.3719%, 6.7464%, 7.3970%, and 7.5504% respectively. Efficiency, together with Jsc was found to increase with increasing electrolyte concentration, peaking at 1.5M and then starts to decrease.

For simulated DSC with electrolyte concentration of 2M, 2.5M and 3M, the efficiencies recorded are 7.5244%, 7.4960% and 7.3160%, respectively. This decreasing trend is the result of higher recombination rate.

TABLE II. PERFORMANCE OF DSC WITH DIFFERENT ELECTROLYTE

CONCENTRATION

As shown in equation (v), D is inversely proportional to the

electron lifetime. This behavior is depicted by the simulation result of electron lifetime shown in Figure 1. While D value is increasing, the electron lifetime is decreasing. It can be concluded that more recombination made electron lifetime shorter for higher electrolyte concentration in DSC. In order to have optimum performance of DSC with highest efficiency, D and electron lifetime need to be maximized.

Simulation of DSC performance showed that the electrolyte concentration of 1.5M iodine reached the highest efficiency on 7.5504% as shown in Table II and also Figure 2. In Figure 2, simulation result is shown with the continuous line (-) while experimental work by Zhang et al is represented by the dot (o). It can be seen that the values are close with each other and have the same trend. This is attributed to the

FIGURE 1. SIMULATED D AND ELECTRON LIFETIME OF DSC WITH DIFFERENT ELECTROLYTE CONCENTRATION

Conc (M) Ref

Jsc (mA/cm2) Voc (V) FF

Efficiency (%)

0.5 simulation 12.7532 0.7297 0.6847 6.3719

[3] 12.3 0.73 0.69 6.2

0.8 simulation 13.1485 0.7496 0.6844 6.7464

[3] 13.6 0.755 0.67 6.8

1 simulation 14.2634 0.7486 0.6928 7.3970

[3] 14.2 0.75 0.7 7.2

1.5 simulation 14.4982 0.7449 0.6992 7.5504

[3] 14.7 0.745 0.7 7.7

2 simulation 14.4981 0.7446 0.6970 7.5244

[3] 14.6 0.745 0.71 7.6

2.5 simulation 14.4842 0.74371 0.6959 7.4960

[3] 14.6 0.75 0.71 7.6

3 simulation 14.0272 0.7431 0.70191 7.3160

FIGURE 2. EFFICIENCY OF DSC WITH DIFFERENT ELECTROLYTE

CONCENTRATION

dominant role played by the diffusion, D and electron lifetime with increasing electrolyte concentration where the maximum value is at the intersection point, which lies in between 1M and 1.5M as illustrated in Figure 1, making it the optimum value of electrolyte concentration for better performed DSC.

V. CONCLUSION AND FUTURE WORKS

Concentration of electrolyte can affect the performance of DSC. Different concentration of electrolyte has been simulated and validated with other experimental work. Result showed that optimum electrolyte concentration is 1.5M iodide with efficiency of 7.5504% as the result of optimized electron mobility with less recombination.

For future work in improving the efficiency of DSC, this optimum electrolyte concentration value may be combined with other improvement in DSC parameters, such as using optimum thickness of TiO2 layer [18] and adding another TiO2 layer as the passivation layer on the working electrode [19].

ACKNOWLEDGMENT

Supports from Centre of Innovative Nanostructures & Nanodevices (COINN), and Graduate Assistant Scheme by Universiti Teknologi Petronas, Perak, Malaysia are gratefully acknowledged.

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