Page 62 - Alternative Energy Systems in Building Design

DYE-SENSITIZED SOLAR CELLS 39

being produced. Also, since the DSC bandgap is slightly larger than it is in silicon,
fewer photons in sunlight become usable for generating solar power.
Furthermore, the electrolyte in DSCs limits the speed at which the dye molecules can
regain their electrons and become available for a renewed cycle of photo-excitation. As
2
a result of these factors, the maximum current output limit for a DSC 20 mA/cm , com-
2
pared with 35 mA/cm generated by silicon-based solar cells. The preceding, in turn, is
translated into a conversion efficiency of about 11 percent. Semiconductor-based solar
cells, on the other hand, operate at between 12 and 15 percent efficiency. In comparison,
flexible thin-film cells operate at a maximum of 8 percent efficiency.
Another approach used to enhance the efficiency of DSCs involves the process of
injecting an electron directly into the TiO , where the electron is boosted within the
2
original crystal. In comparison, the injection process used in the DSC does not intro-
duce a hole into the TiO , only an extra electron. Although, in doing so, electrons have
2
a better possibility of recombining when back in the dye, the probability that this will
occur is so low that the rate of electron hole recombination efficiency becomes
insignificant.
In view of low loss characteristics, DSCs can perform more efficiently under cloudy
skies, whereas traditional designs would cut out power production at lower limits of
illumination. This operational characteristic of DSCs renders them ideal for indoor
applications.
A significant disadvantage of DSC technology is the use of a liquid electrolyte, which
has stability problems relating to temperature. At low temperatures, the electrolyte can
freeze, ending power production and potentially leading to physical damage. Higher
temperatures cause the liquid to expand, making sealing of the panels very difficult.

FUTURE ADVANCEMENTS IN DSC TECHNOLOGY
The early, experimental versions of DSCs had a narrow bandwidth that functioned in the
high-frequency ultraviolet (UV) and blue end of the solar spectrum. Subsequently,
owing to the use of an improved dye electrolyte, a wider response to the low-frequency
red and infrared range was achieved.
At present, with the use of a special dye with a deep brown-black color, referred to as
black dye, the conversion rate of photons into electrons has improved significantly to
almost 90 percent, with only a 10 percent loss, attributed to optics and the top electrode.
A critically significant characteristic of the black dye is that over millions of cycles
of simulated exposure to solar irradiance, the output efficiency of DSCs is outstanding,
with no discernible decrease. In recent tests using an improved electrolyte, the thermal
performance of the solar cell has been pushed to 60°C with remarkable conversion effi-
ciency. An experiment conducted recently in New Zealand used a wide variety of
organic dyes, such as porphyrin, a natural hemoprotein present in hemoglobin, and
chlorophyll, to achieve an efficiency of 7.1 percent.
At present, DSC technology is still in its infancy. It is expected that in the near
future, with the use of newer dye electrolytes and quantum dots, there will be signifi-
cant improvement in efficiency gains.