Performance Degradation of Dye-Sensitized Solar Cells Induced by Electrolytes

Abstract

We investigated the change of the electric characteristics in dye-sensitized solar cell (DSSC) when the electrolyte has been injected and measured initially and lately for a period of time. It was found that the short-circuit current density decreased from 9.799 mA/cm² to 7.056 mA/cm² and the fill factor increased from 0.406 to 0.559 when the cell had stood for an hour, while the open-circuit photovoltage did not change due to fixed difference between the Fermi level of TiO₂ and the oxidation-reduction potential of electrolyte. The results can be explained by using the variation of the series resistance in the equivalent circuit of the DSSC.

1. Introduction

Dye-sensitized solar cells (DSSC) have attracted much attention ever since Grätzel et al. demonstrated the possibility to convert solar energy to electricity with a photoelectro-chemical dye-sensitized device [1]. The DSSC provides a technically and economically credible alternative concept to present day p-n junction photovoltaic devices. The development of these new types of solar cell is promoted by the increasing public awareness that the earth’s oil reserves will run out during this century. Because the energy needs of the planet will at least double within the next 50 years, the stage is set for a major shortage of energy supply unless renewable sources can cover the substantial deficit that fossil fuels can no longer furnish.

This accomplishes the optical absorption and charge separation processes by the association of a sensitizer as a light-absorbing material with a wide-band-gap semiconductor of mesoporous or nanocrystalline morphology [1–3]. It also offers the prospect of very low cost fabrication without expensive and energy-intensive high-temperature and high-vacuum processes, compatibility with flexible substrates, and variety of presentations and appearances to facilitate market entry, both for domestic devices and in architectural or decorative applications. Generally, the semiconductor, which accepts the electron from the sensitizer and transfers it to the electrode, is nanocrystalline anatase TiO₂ which is mesoporous because of high surface area. High surface area of TiO₂ thin film improves the adsorption of sensitizer and hence increases the light-harvesting capacity of the mesoporous film. However, the electrolyte may penetrate into the mesoporous film and contact the dye-free TiO₂ and the electrode surface, resulting in the increase of the reverse saturation current. On the other hand, it also increases the contact area between the electrolyte and the sensitizer so that facilitates the reduction of 𝐼₃₋ by the combination of the hole. It was found that the short-circuit photocurrent decreased and the fill factor of the dye-sensitized solar cell increased as time goes on.

In this study, the change of the electric characteristics in dye-sensitized solar cell (DSSC) when the electrolyte has been injected and measured initially and lately for a period of time was investigated. The short-circuit photocurrent density (𝐽_sc) and the fill factor (F.F.) were varied and the results can be explained by using the variation of the series resistance in the equivalent circuit of the DSSC.

2. Experimental Apparatus and Procedure

2.1. Preparing the Working Electrode

A TiO₂ thin-film electrode was designed and fabricated for use in a dye-sensitized solar cell. This TiO₂ thin-film was deposited on the top of an FTO-glass (Fluorine doped tin oxide, SnO₂:F) substrate, which has a size of 20 × 20 × 3 mm and a sheet resistance of 8 Ω/sq. The TiO₂ particles (P-25) are composed of 30% rutile and 70% anatase, with a particle size range (<100 nm).

Figure 1 shows the procedure of preparing the working electrode of DSSC with a layer of TiO₂ particles and the steps are described as follows. (1) Diluting 40 μL of acetylacetone in 1 mL of de-ionic (DI) water; (2) mixing 1 g of TiO₂ particles with the diluted acetylacetone, and stir well to make the colloid of TiO₂ particles; (3) adjusting the glutinosity of the colloid of TiO₂ particles by adding 1.5 mL of DI water and 40 μL of surfactant (Triton-100, C₈H₁₇C₆H₄(OCH₂CH₂)nOH), and then homogenize the colloid of TiO₂ particles for 30 minute in a mortar; (4) dropping the quantitative colloid of TiO₂ particles on the top of the F:SnO₂ (FTO) glass substrate, and then spin coat the working electrode with rotational speed of 3000 rpm; (5) drying the working electrode with a layer of TiO₂ particles at room temperature for 10 minute, and then sinter this working electrode at 500°C for 30 minutes in a high-temperature furnace; (6) immersing the FTO-glass substrate with a layer of TiO₂ particles into the (3 × 10⁻⁴mol/L) solution of N-719 dye (Ruthenium, RuL₂(NCS)₂) and ethyl alcohol (CH₃CH₂OH, 95%) for 24 hours at room temperature. A conventional working electrode with a layer of nanocrystalline TiO₂ on the FTO-glass substrate was fabricated using above procedures to demonstrate the feasibility and advantages of the working electrode with a layer of TiO₂ particles. An α-step (Dekeak 6 M) surface profiler was utilized to obtain the surface profile of the thin film and the average thickness of the thin film on the FTO-glass substrate of the working electrode. The thickness of the TiO₂ thin films was about 10 μm. Figure 2 shows the practical working electrode.

Figure 1 

The procedure of preparing the working electrode.

Figure 2 

The photograph of the working electrode.

2.2. Assembling the DSSC

Figure 3 shows the schematic of the DSSC. The thin film on the FTO-glass substrate of the working electrode was fringed using the Scotch magic transparent tape (3 M). Then, the working electrode and the counter electrode that a thin film of platinum was deposited on the FTO-glass substrate using a ion sputter (Hitachi E-1010) were fitted together, such that the space between the two electrodes was adjusted to approximately 160 μm for embarking the liquid electrolyte. After sealing, the liquid electrolyte, which was composed of 0.5 M lithium iodine (LiI) and 0.05 M iodine (I₂) in 3-methoxypropionitrile, was injected into the cell through a hole on the cell that was prepared in advance.

Figure 3 

Schematic of the dye-sensitized solar cell.

2.3. Testing

A home-made I-V measurement system was adopted to measure the open-circuit photovoltage and short-circuit photocurrent of the DSSC. A solar simulator with a 500 W halogen lamp and a light intensity of 100 mW/cm², was employed to illuminate the DSSC. In addition, the power conversion efficiency η of the DSSC was determined by 𝜂(%)=𝑉^oc𝐽_scFF𝑃_in×100.(1)

In (1), 𝑉_oc, 𝐽_sc, and 𝑃_in represent the open-circuit voltage, the short-circuit current density, and the incident light power (100 mw/cm²), respectively. Moreover, the fill factor FF is given by FF=𝑉^max𝐽_max𝑉_oc𝐽_sc.(2)

In (2), 𝑉_max and 𝐼_max represent the voltage and the current at the maximum output power point, respectively. The I-V characteristics of the prepared DSSC were measured at initial assembling stage and at one hour lately.

3. Results and Discussion

Figure 4 shows I-V characteristics of the DSSC when the electrolyte is just injected into the space between the working electrode and the counter electrode. The measured 𝑉_oc, 𝐽_sc, and F.F. are 9.799 mA/cm², 0.658 V, and 0.406, respectively. Figure 5 shows I-V characteristics of the dye-sensitized solar cell after the electrolyte has been injected and stay for one hour. The measured 𝑉_oc, 𝐽_sc, and F.F. are 7.056 mA/cm², 0.668 V, and 0.559, respectively. as Comparing Figure 4 to Figure 5, it is found that the 𝐽_sc decreases from 9.799 mA/cm² to 7.056 mA/cm² and F.F increases from 0.406 to 0.559. The 𝑉_oc does not change due to the fixed difference between the Fermi level of TiO₂ and the oxidation-reduction potential of electrolyte. To understand the difference in their 𝐽_sc for the measurement at initial assembling stage and at one, two, three and so forth hours lately. We conduct the standard I-V curve of the solar cell and discuss the change of the series resistance and the shunt resistance.

Figure 4 

I-V curve of DSSC when the electrolyte is just injected.

Figure 5 

I-V curve of DSSC when the electrolyte has been injected and stay for one hour.

It has been known that the tangent slope of vertical part (near 𝑉_oc) is proportional to the reciprocal of the series resistance (1/𝑅_𝑠) and the tangent slope of transverse part (near 𝐽_sc) is proportional to the reciprocal of the shunt resistance (1/𝑅_sh). When 𝑅_sh increase or/and 𝑅_𝑠 decrease, F.F. will become better. The tangent slope of 𝑉_oc part in Figure 5 is larger than that in Figure 4, which means 𝑅_𝑠 decreases as time goes on. However, the tangent slope of 𝐼_sc does not change obviously so that 𝑅_sh is almost the same. We thus discuss the factor that affects the series resistance 𝑅_𝑠 and ignores the shunt resistance 𝑅_sh.

The series-internal resistance of DSSCs was investigated by some researchers based on an equivalent circuit of DSSCs [4, 5]. It was found that series-internal resistance correlates with the charge-transfer at Pt counter electrode, TCO substrate resistance, and Nernstian diffusion in the electrolyte. According to these, the equivalent circuit of DSSCs is constructed as shown in Figure 6. The series resistance is the sum of the charge transfer resistance at Pt/electrode interface (𝑅₁), the diffusion resistance of 𝐼₃₋ ions in electrolyte (𝑅₂), and the sheet resistance of TCO (𝑅_ℎ). Because 𝑅₁ and 𝑅_ℎ relate to the property of the material, they cannot change as time goes on. We think the major factor that influences 𝑅_𝑠 must be 𝑅₂. The probable explanation is that the electrolyte only contacts the top surface of TiO₂ thin film when the electrolyte is just injected. However, the electrolyte may permeate into the mesoporous film after an hour and the contact area with dye-adsorbed TiO₂ will increase, so the charge-transfer from the electrolyte to the oxidized dye will be accelerated. The large contact area between the electrolyte and the dye-adsorbed TiO₂ film improve the rate of 𝐼₃₋ reduction and hence decrease 𝑅_𝑠.

Figure 6 

Equivalent circuit based on current-voltage characteristics of DSSCs. The sum of 𝑅1, 𝑅2, and 𝑅ℎ corresponds closely to the series resistance of DSSCs. 𝐶1 is capacitance elements.

According to the diode equation that correlates with Figure 6: 𝐼=𝐼_ph−𝐼_𝑜exp𝑞𝑉+𝐼𝑅^𝑠𝑛𝑘𝑇−1−𝑉+𝑅𝐼_sh𝑅^𝑠,(3) where 𝐼_ph is the ideal photocurrent, 𝐼₀ is the reverse saturation current, 𝑛 is the ideality factor whose value is between 1 and 2 for the DSSC, 𝑅_𝑠 is the series resistance which is the sum of 𝑅₁, 𝑅₂, and 𝑅_ℎ, and 𝑅_sh is the shunt resistance. The gain in open circuit voltage can be calculated from (3): 𝑉_oc=𝑛𝑘𝑇𝑞ln𝐼𝐼_𝑜−𝑉^oc𝐼_𝑜𝑅_sh+1.(4) As 𝑅_sh increase, 𝑉_oc will also increase. as comparing Figure 4 to Figure 5, it is found that 𝑉_oc does not change obviously. Since 1/𝑅_𝑠 is unchanged, it confirms that the shunt resistance in DSSCs is almost the same as time goes on.

The short circuit current can be obtained from (3) when 𝑉=0, and it can be found that 𝐼_sc will increase as 𝑅_𝑠 decreases. However, 𝐽_sc decreases from 9.799 mA/cm² in Figure 4 to 7.056 mA/cm² in Figure 5. It must be the influences of the dark current which correlates with the recombination of charge carriers by reduction of 𝐼₃₋ at dye-free TiO₂ surface and bare FTO surface exposed to electrolyte through porous electrode [6]. As we discussed before, the electrolyte will permeate into the mesoporous film when DSSC has been staid for an hour. It may apply the electrolyte to contact the dye-free TiO₂ surface and even the FTO surface, so that accelerates the recombination of the charge carriers from the conduct band of TiO₂ to the electrolyte and produces the reverse saturation current. The increase of the dark current lowers 𝐼_sc seriously so that it is important to modify the working electrode surface to decrease the reverse charge flow.

Based on the above discussion, it is known how to improve the performance of DSSCs by modifying the interface between the working electrode and the electrolyte, as investigated by some research workers [7–9]. Su et al. fabricated layer-by-layer Au nanoparicles, which were prepared using chemical reduction method, onto the working electrode as a Schottky barrier in a water-based DSSC [10]. It was found that an interfacial blocking layer could suppress the interfacial recombination between TiO₂ and excited dye molecules or I⁻/𝐼₃₋ couple, which led to an improvement on cell performance [7–9]. Moreover, in order to improve the performance of DSSCs, it is also necessary to suppress the dark current. The reverse charge flow, recapture of the electron by the electrolyte, could be impaired by judicious design of the sensitizer. It can also form a tightly packed insulating monolayer blocking the dark current. Consequently, decreasing the recombination of charges in the dye or electrolyte is one of the important issues to increase the conversion efficiency of DSSC and is worthy of ongoing study.

4. Conclusion

In this study, we discussed the change of the current-voltage characteristics in DSSCs as time goes on and found that fill factor (F.F) got better but short circuit current density 𝐽_sc decreased. The results were explained by using the variation of the series resistance in the equivalent circuit of the DSSC. The increase of F.F. may result from the decrease of series resistance 𝑅_𝑠 due to the electrolyte permeating into the mesoporous working electrode and hence increase the contact area with the dye-adsorbed TiO₂. However, it also applies the electrolyte to contact the dye-free TiO₂ surface and even the FTO surface, so that accelerates the recombination of the charge carriers from the conduct band of TiO₂ to the electrolyte and produces the reverse saturation current.