Dye-sensitized solar cells have slightly lower photoelectric efficiency than silicon solar cells. Researchers have investigated various ways to address this problem. This study improved the efficiency of a dye-sensitized solar cell by re-driving it with a reflector, reusing discarded light after it was absorbed. The reflector increased efficiency by about 50%, by increasing the size of the pattern shape and increasing the distance of the reflector.
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The Swiss researcher Gratzel was the first to develop a dye-sensitized solar cell, and many researchers have continued his research. A dye-sensitized solar cell has slightly lower photoelectric efficiency than a silicon solar cell. Nevertheless, it can be produced at almost one-fifth the price of a silicon cell due to the low cost of manufacturing facilities and process technologies. It also has high permeability and has excellent applicability, for example, in window glass and windshield glass; as a result, it has attracted attention as a third-generation technology that may replace the silicon solar cell. Since the first report on dye-sensitized solar cell (DSSC) in 1991 , progresses in both fundamentals and applications have been substantially made. As a result, solar-to-electricity conversion efficiency of above 11% was demonstrated [2–5].
The greatest disadvantage of the dye-sensitized solar cell is its low conversion efficiency. Many researchers have focused on solving this problem, typically by increasing the surface area of TiO2 photoelectrodes used in the dye-sensitized solar cell. Other studies have focused on making TiO2 particles smaller, keeping the original TiO2 array, or replacing it with relatively larger specific surface area. Because these studies have focused on material characteristics, they require development of new materials and are therefore difficult to conduct [6–9]. 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 [10–12].
DSSC is cheaper than silicon based solar cell. So it is easy to change. Figure 1 shows concept of compact DSSC. It will use in tough environment like dessert or the polar regions. Target of our research is compact DSSC that it can change and assemble.
In this study, we attempted to improve the efficiency of the solar cell by re-driving light with a reflector, reusing discarded light after it is absorbed.
2. Analysis of dye-sensitized solar cell with an applied reflector
Figure 2(a) presents the predicted path of sunlight with an applied reflector. Without a dye reaction, incident sunlight permeates the dye-sensitized solar cell and is reused by the reflector. In this case, the same area as the incident area is reused. However, no material can reflects incident light 100% perpendicularly, as shown in Fig. 2(b). Most sunlight is reflected in various directions, according to the shape of the surface. In this case, decreasing the surface roughness will result in greater reflectivity of incident light. However, it is not possible to machine surface roughness to 0, so in this study we tried to increase the reusability of the light by applying a pattern. Figure 1(c) shows a reflector with an applied pattern. Ambient sunlight can be reused by applying a pattern on the reflector.
Figure 3 shows reflector efficiency depending on distance. In Fig. 3(a), the reflection path is not uniform due to the random pattern. Most light is scattered into a diffused reflection pattern; this will reduce sunlight reactivity as the solar cell is farther away from the reflector. Figures 3(b) and 3(c) show that efficiency can also be improved due to the duplication area of sunlight on the reflector as the solar cell is farther away from the reflector. However, while this kind of reflector will increase reusability, it is problematic because it will increase the thickness of the solar cell system, and the system will therefore be difficult to fabricate over a certain distance.
Figure 4 shows an optical analysis of the size of the reflector pattern. Figures 4(a) and 4(b) show the various shapes of reflectors used. All reflectors had the same curvature ratio to eliminate any change in light collection due curvature ratios. Figure 4(c) presents the analysis. Figures 4(d) displays the detecting ratio of light rays by pattern shape and distance. In the graph, an increase in detecting ratio due to reflector application is expressed as a percentage (%); all light rays reflected by a reflector is 100%.
3. Evaluation of efficiency for dye-sensitized solar cell with an applied reflector
3.1 Fabrication of dye-sensitized solar cell and configuration of test apparatus
The dye-sensitized solar cell used in this test involved a TCO with an electrode made from 2-mm FTO glass with 7 Ω resistance. For TiO2, a dye-sol paste was used with N719 dye and An50 electrolyte. A sealing sheet was fabricated using a sulyn sheet with a thickness of 60 µm.
Figure 5 shows the manufacturing processes of the dye-sensitized solar cell used for this test. TiO2 paste (dye-sol, p50) was coated twice with a silk screen and baked in a furnace at 450-500°C for 3-4 h. It was then soaked in N719 dye for 12 h, sealed with the suryn film, and injected with An50 electrolyte.
Figure 6(a) shows the test apparatus used to measure characteristics of the dye-sensitized solar cell. Tests were conducted under a standard light source AM (airmass) 1.5 (1 sun, 100 mW/cm2) using a solar simulator, Isc, Voc, and current-voltage curves of the dye-sensitized solar cell were scanned using a 1 sun light source, a xenon (Xe) lamp using a Keithley 2400 sourcemeter at 10 point/s. Figure 6(b) shows the fabricated dye-sensitized solar cell and test apparatus.
3.2 Efficiency of dye-sensitized solar cell upon reflector application
First, the dye-sensitized solar cell was tested to assess whether reflector application increased efficiency. Figure 7 shows a graph indicating the IV-curves of a dye-sensitized solar cell without a reflector and one with an applied reflector. The results revealed that the dye-sensitized solar cell was about 50% more efficient after the reflector was applied (from 3.8% to 5.7%), as shown in Fig. 8 . These test results indicated that lost light could be reused by applying a reflector.
3.3 Efficiency of dye-sensitized solar cell upon shape and distance of the reflector
Figure 9 shows the pattern shapes used to evaluate the efficiency of the dye-sensitized solar cell depending on the shape of the reflector. Patterning involved a method for molding different pattern shapes on aluminum reflective sheets with a surface roughness (Ra) of 8. Table 1 shows the shapes and sizes of the patterns used in tests. Figure 10 displays the efficiency increase by pattern shape. In the graph, an increase in efficiency due to reflector application is expressed as a percentage (%); the efficiency of a cell without a reflector is 100%. Pattern (a) reduced the increase in efficiency due to diffused reflection as the distance increased. A comparison of Patterns (b), (c), and (d) reveals that efficiency increased more as the pattern size increased. This result is similar to the analysis findings. However, in one section, efficiency was not improved but was simply maintained as distance increased. This is probably due to a change in incident area of the solar cell. All patterns except the random pattern yielded increased efficiency, depending on distance, but the available distance for increase would be limited considering the thickness of the solar cell. Target of our research is compact DSSC. So we tested from 2mm to 12mm.
This study applied a reflector to increase efficiency of dye-sensitized solar cells by using the lost light that is discarded after it permeates the solar cell. Application of the reflector increased the efficiency of the dye-sensitized solar cell by 50%. A comparison of various reflector patterns revealed the following results:
- 1. When pattern shapes were not uniform, efficiency was reduced due to diffused reflection as distance increased.
- 2. As pattern shape increased, it can reflect sunlight on a larger area and efficiency increased.
- 3. As distance between the cell and the reflector increased, efficiency increased, but one section maintained constant efficiency, which increased as pattern size increased.
This work was supported by NCRC (National Core Research Center) program of the Ministry of Education, Science and Technology (2010-0008-277) and “Development of hybrid Multi-Axis Machine Tool on the Basis of Micro EDM” project of Ministry of Knowledge Economy.
References and links
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