CHEMISTRY & MATERIALS
Solar cells A clear choice
Dye-sensitized solar cells that use carbon nanotube thin films as transparent electrodes offer significant cost savings
Solar energy is one of the most promising forms of renewable energy, but the high cost of conventional solar cells has so far limited its popularity. To increase the competitiveness of solar energy, scientists have turned to the development of dye- sensitized solar cells — solar cells that use low-cost organic dyes and titanium dioxide (TiO2
) nanoparticles in place of expensive
semiconductor and rare earth elements to absorb sunlight. Zhaohong Huang at the A*STAR Institute of Materials Research and Engineering and co-workers1
have now reduced
the cost of dye-sensitized solar cells even further by replacing indium tin oxide (ITO) — the standard material for transparent electrodes — with carbon nanotubes. A typical dye-sensitized solar cell comprises a porous layer of nanoparticles immersed in an organic dye. The dye absorbs
TiO2
the sunlight and converts the energy into electricity, which flows into the TiO2
and out of the solar cell. nanoparticles. The sun-facing side of the solar cell
is usually covered with a transparent electrode that carries the charge carriers away from the TiO2
“Unfortunately, ITO electrodes are brittle and crack easily,” says
Huang. “They are also expensive and could incur up to 60% of the total cost of the dye-sensitized solar cell.” Huang and his team therefore replaced the ITO electrode
with a thin film of carbon nanotubes. Carbon nanotubes conduct electricity and are almost transparent, flexible and strong, which make them the ideal material for transparent electrodes. The only drawback is that photo-generated charge carriers in the nanotube may recombine with ions in the dye, which reduces the power conversion efficiency of the solar cell. To overcome this problem, Huang and his team placed a TiO2
thin film in between the carbon nanotube thin film and the porous layer. They found that the performance of dye-sensitized solar cells with TiO2
thin film was significantly better than
those without. However, they also found that the solar conversion efficiency of
their new dye-sensitized solar cells was only 1.8%, which is lower than that of conventional solar cells using ITO electrodes. This is due to the higher electrical resistances and reduced optical trans- parency of the carbon nanotube films, which limits the amount of
28
Carbon nanotube (CNT) electrodes. The use of carbon nanotubes has a significant cost advantage. However, in earlier designs (left), the car- bon nanotubes degraded through chemical processes (e-
I3 -: ions in the liquid). Using a thin protective layer of titanium oxide
now stabilizes the nanotubes (right), increasing the performance of these cells.
e- I3 TiO2 CNT CNT -
e-
I3
-
TiOx
: electrons,
sunlight entering the cell. “We are now studying different ways to enhance the conductivity and transparency of the films,” says Huang. “Furthermore, we are planning to replace the bottom platinum electrode with carbon nanotube thin film to reduce the cost of dye-sensitized solar cells further.” If successful, the results could have a great impact on the cost and stability of dye-sensitized solar cells.
■
1. Kyaw, A. K. K. et al. Dye-sensitized solar cell with a titanium- oxide-modified carbon nanotubes transparent electrode. Applied Physics Letters 99, 021107 (2011).
A*STAR RESEARCH OCTOBER 2011– MARCH 2012
© 2011 AIP
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96