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Dye-sensitized solar cells have slightly lower photoelectric efficiency than silicon solar cells. Researchers have investigated various ways to address this problem. In this paper, we found that the optimized separation between the condenser lens and the cells was 8 mm. The cell efficiency increased from 2.5% to 8.3% compared to two isolated cells without a lens. If the efficiency of the basic cell can be increased sufficiently, it should be possible to commercialize the product.
©2011 Optical Society of America
1. Introduction
The dye-sensitized solar cell (DSSC) was developed in 1991 [1] by the Swiss scientist Gratzel, employing a photosensitive dye absorbed onto a thin film of TiO2. DSSCs, which operate using the principle of photosynthesis, have a lower efficiency (of around 10%) than that of silicon solar cells [2–5]. Efficiency is the ratio between the output energy of the DSSC and the input energy of solar, in energy terms. However, the unit cost is expected to be approximately 20% of that of silicon cells because of the low-cost manufacturing process technologies. Additionally, they have high permeability, and have been identified as the third generation cell to replace silicon solar cells. They can also be applied to glass substrates, making them suitable for applications including car windows.
The crucial weakness of DSSCs is their low conversion efficiency. In order to solve this problem, many studies have been carried out with the aim of increasing the efficiency. The most active research area has focused on increasing the surface area of the TiO2 photo-electrode. Since the dye polymer has a high efficiency when it is absorbed as a molecular monolayer on a semiconductor, the fraction of absorbed sunlight becomes larger as the surface area of the semiconductor with the dye polymer increases. Studies on reducing the size of the TiO2 particles using regular arrays of particles or nanorods, and generating core cell materials with relatively large specific surface areas have been carried out [6–9]. In addition, since the absorption spectrum of the dye does not cover the whole sunlight spectrum, the efficiency can be increased by developing new dyes and using several dyes with two or three layers [10].
Although the efficiency of DSSCs is extremely important, improvements in terms of safety and voltage are also important for commercialization. Designing of Z-type series connection, W-type series connection, and parallel-type series connection solar cells with large areas has the same effect as applying series or parallel connections of several individual cells [11,12]. Indeed, Míguez et al. reported remarkably improved light utilization using one-dimensional photonic crystals of a multilayer coupled inside a cell based on inorganic nanoparticles [13–15].
2. Purpose of the experiment
Most high-voltage DSSCs have a series or parallel design with a large area. A large area increases the output voltage; however, it also results in a decrease in the efficiency, and it cannot be readily applied to commercial devices. Here, we describe the development of a DSSC that has a high voltage and high efficiency, yet is compact enough to be utilized for real-life applications.
So we developed a vertical array DSSC for a high voltage and high efficiency per unit area by exploiting the biggest advantage of the DSSC: its transparency. And we estimated characteristics of a vertical array DSSC. The main problem of our vertical array DSSC is the permeability of the upper cell; it is necessary to supplement the resulting deficient light source. We solved this problem using a condenser lens.
3. Experimental method
3.1 Fabrication of DSSC
Transparent conducting oxide (TCO) was used for the electrodes, which were made from 2-mm-thick fluorine tin oxide (FTO)-coated glass, and had a sheet resistivity of 7 Ω. TiO2 sol-gel dye paste was used, with N719 dye and An50 electrolyte. 60µm-thick thermoplastic sealants sheet (Dye-sol, Surlyn®) was used to seal the device.
The manufacturing process was as follows. First, organic matter was removed by ultrasonic cleaning in acetone for 30 minutes. The acetone was removed by washing in ethanol for 30 minutes, and the ethanol was removed by washing with distilled water for 30 minutes. The TiO2 paste (Dye-sol, P50) was applied twice using a silk screen and baked in a furnace at 450–500°C for 3–4 hours. After soaking the device in N719 for 12 hours, it was sealed using the thermoplastic sealants sheet and assembled into cells by injecting An50 electrolyte.
We used polydimethylsiloxane (PDMS) blocks to stack individual cells. We could overlay the cells accurately using the blocks so that the active layer was not dispersed.
3.2 Structure of optical system
We constructed a system to improve the efficiency of stacked DSSCs by using a condenser lens. The configuration is shown in Fig. 1 (a) and (b) . The distance of the stacked devices was 8 mm, which resulted in the highest efficiency of the laminated system. After connecting the two stacked DSSCs in series and using the condenser lens, shown in Fig. 2 (a) , we measured the change in the efficiency compared with an isolated device. The specifications of the condenser lens are listed in Table 1 . We determined the specifications of the condenser lens using numerical simulation, as shown in Fig. 2 (b). The lens was separated from the DSSC stack by 8 mm. We used a spherical lens on one side only, and it was coated with MgF to reduce reflections. We used polydimethylsiloxane (PDMS) blocks to stack individual DSSCs. We could overlay the cells accurately using the blocks so that the active layer was not dispersed.
Table 1. Specifications of the condenser lens
4. Results and conclusions
The characteristics of the samples are listed in Table 2 . We used DSSCs with efficiencies of 3.1% and 2.9%, corresponding to samples (a) and (b), for the stacked condenser lens system. Sample (a) was stacked on top of sample (b). The configuration is shown in Fig. 3 .
Table 2. Specifications of the condenser lens
4.1 Efficiency as a function of distance from the lens
The results are summarized in Table 3 . Figure 4 shows the efficiency as a function of the distance between the condenser lens and the under DSSC. As the distance increased up to 8 mm, the efficiency increased. The efficiency decreased for larger distances because as the distance approached the focal length of the lens, the spot size was reduced so that the active layer could not be fully illuminated. The magnification reached approximately 4 times at a distance of 8 mm; here, the focal length of the lens contained both active layers.
Table 3. Specifications of the staked DSSC with a condenser lens placed at several distance
4.2 Cell improvement
We stacked DSSCs using the condenser lens and compared them to two isolated cells without a condenser lens. Figure 5 show the performance and efficiency of the devices. The results are summarized in Table 4 . Various factors characterize the cell performance, including Voc and Isc, which were enhanced. In particular, the efficiency in terms of Voc improved from 2.5% to 8.3%.
Table 4. Specifications of the condenser lens
5. Conclusion
We used a vertical stacked-cell configuration and a condenser lens to increase the efficiency of DSSCs. We found that the optimized separation between the condenser lens and the cells was 8 mm. The cell efficiency increased from 2.5% to 8.3% compared to two isolated cells without a lens. If the efficiency of the basic cell can be increased sufficiently, it should be possible to commercialize the product. Our highly efficient, high-voltage laminated DSSC is packaged in a module with a lens. Laminating allows a desired voltage to be designed into the device. These cells can be used as power sources in situations where it is difficult to install a generator, or where the solar cell cannot be driven without strong sunlight, as well as for sensors used to monitor high-pressure gases.
Acknowledgments
This work was supported by ERC (ENGINEERING RESEARCH CENTER FOR NET SHAPE & DIE MANUFACTURING) and "Development of Multi-Physics based Micro Manufacturing (MP-M2) Technologies for Biomedical Products" International Collaborative R&D Program project of ministry of knowledge economy.
References and links
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