Transparent Solar Cells
The next big step in the world of solar energy is transparent solar cells. These transparent solar cells can efficiently be used in windows or some sheet of glass to generate solar energy.
The photo-voltaic solar cells work by absorbing photons from the sunlight and converting them into electricity. Usually, the traditional solar cells consist of silicon that exhibits this property. As a majority of the light is absorbed, the panels are dark in color. But for a transparent solar panel the condition of transparency must be met, that is, all the visible light must pass through the glass.
Transparent Solar Window Technology:: Transparent luminescent solar concentrator (TLSC):
A team of researchers, headed by Richard Lunt, an assistant Prof, at Michigan State University, developed a fully transparent solar cell. The concept used by them is that of a Luminescent solar concentrator. This works by concentrating the radiation from a large surface to a smaller surface. This squeezing of light increases the power density thus improving the solar cell efficiency.
Working Principle:
The basic design is such that the sunlight is absorbed by chromophores. The absorption of energy causes the electron to get excited and jump to a higher level. When the electrons return to the ground state, radiation is emitted. Also, some fraction of this re-emitted light is trapped inside by total internal reflection. ( When the propagated wave strikes the boundary of the surface at an angle larger than the critical angle, total internal reflection occurs. The critical angle is the angle of incidence for which angle of refraction is 90°.)
At the edge, Photovoltaic cells are attached that absorbs this light.
TLSC consists of organic salts of a synthetic dye, also known as Cyanine. These allow the visible light to pass through it while it absorbs the ultraviolet and the infrared light of the spectrum, that is invisible to the human eye.
A lot of research is being carried out in this field with good results but still, it has not become fully functional. The main challenge that is faced is to improve its efficiency. The efficiency is only 5% as compared to the traditional solar panels having an efficiency of approx 15% to 18%.
Semi Transparent perovskite Solar cell:
A perovskite material uses organic-inorganic lead halides ( halogen atom bearing a negative charge), that act as the active layer, that is, light absorbing layer. Some properties of this material include photoelectric effect ( Electrons or other free carriers emitted when the light is incident on the material), possibility to make it transparent, broad absorption spectrum ( At a particular wavelength the absorption of ultraviolet rays of a molecule is maximum), etc. The efficiency of solar cells using this material have increased from 3.8% in 2009 to 22.7%in 2017.
This works on the concept of how efficiently a light or other signal is detected as a function of wavelength or frequency of the signal by human visual perception. The wavelength of green light is at which there is maximum visibility. Therefore this material absorbs the red and the blue light while allowing the green light to pass.
In particular, perovskite is good at absorbing the blue light. Therefore to increase light capturing efficiency, another layer of nano sized cubes of silver is added that focuses on harvesting the red wavelengths.
Though this solar cell is not fully transparent, this can be a revolutionary technology that can be improved and commercialized as a lot of research and development work is being put into it.