High Efficiency Hybrid Solar Cells

Normally, a modern silicon solar cell exhibits a maximum theoretical efficiency of about 33.7 percent. A majority of the sunlight falling on the solar cell – more than 66 percent – is not converted to electricity and is simply wasted in heating up the cell. Now, a new type of solar cells may be able to boost this efficiency to 95 percent or more.

The University of Cambridge Cavendish Laboratories is researching on a new type of high-efficiency hybrid solar cell. The UK researchers are using an organic formulation to put in as a layer on top of a standard silicon solar cell. This layer will help the solar cell to reach its target of the hard-to-believe 100 percent efficiency.

The top layer of special organic formulation coating on the solar cell helps to absorb high-energy light and produce pairs of triplets. Inorganic solar cells underneath can efficiently absorb these triplets. Generally, the cells cannot convert the high-energy radiation into electricity and these radiations only serve to heat up the solar cells. The organic film on top of the solar cells converts the wasted energy into a form that the underlying solar cell can turn into useful electricity.

With an increase in efficiency brought about by the Cavendish Laboratory hybrid approach, solar energy harvesting farms can be reduced in size significantly, while still producing the same amount of electricity.

According to Maxim Tabachnyk, Scholar, and Akshay Rao, research fellow at Gates Cambridge, and other members of the Cavendish Laboratory at the University, they have developed a film to convert wasted energy into useful form. The traditional solar cell is unable to convert high-energy light and wastes it as heat because of the fundamental limit of the solar cell’s power conversion efficiency.

The researchers coated the silicon solar cells with a special organic layer. This layer functions to distribute the energy of the incoming high-energy photons into two triplet excitons that in turn transfer their electrons on to the silicon cells.

The researchers had to first characterize the ultra-fast processes occurring at the organic/inorganic interface. For this, they directed ultra-short laser pulses into organic pentacene and studied the effect with laser spectroscopy. By following the transfer of energy taking place within a femtosecond (a billionth of a billionth of a second), they confirmed the presence of two electrons for each high-energy photon. Normally, only one electron is generated per photon.

After proving the concept that each high-energy photon can generate two electrons, the researchers had to find an alternative candidate to replace pentacene, which is not a suitable candidate to produce electrons suitable for silicon to absorb. They have now found a suitable organic material that can produce electrons with excitation higher than the band gap or the minimum absorption energy of silicon. The organic material is cheap and can be printed or even sprayed on as ink on top of traditional silicon solar cells.

According to Tabachnyk, normal solar cells harvest only the bright single-spin excitation electrons produced by the photons. The organic layer extends the ability of the cells by allowing them to harvest additional electrons from high-energy photons producing dark spin-triplet excitations.