New under the sun

"My dream, regarding alternative energy, is that one day the roofs of all the houses will be painted in 'solar paint', which will provide them with a significant part of their electricity consumption," says Prof. David Kahn from the Department of Materials and Surfaces in the Faculty of Chemistry at the Weizmann Institute of Science

On the one hand, as we know, the sun is a tremendous source of energy. On the other hand, the use of this resource is not common. why? The answer to this question is related to the well-known fact that nothing in life is perfect, and there are no free meals. The technologies associated with utilizing solar energy are still expensive, and are unable to convert solar energy into electricity with sufficient efficiency to make it economically viable for most uses. Based on the laws of physics, the efficiency of the most successful single solar cell cannot exceed 31% (in converting solar energy into electrical energy), and the efficiency achieved in practice is even smaller. Why then is it worth investing in solar cells whose efficiency is so low? The simple answer to this is that the investment pays off if the cells are cheap enough. "The idea is to understand the limitations, as well as the capabilities of each type of solar cell, and to identify a unique niche suitable for each of them," says Prof. Kahan.

Half a century ago, William Shockley and Hans Queiser identified three basic limitations, which apply to every solar cell and result in maximum efficiency: their light absorption is limited to a small range of the full spectrum of sunlight; A large part of the light absorbed by the cell is wasted in the form of heat; And in addition, some of the electric current generated in the cell is lost before it can even be used. These three limitations, which received the name SQ after the initials of their two definers, were considered to date to be the only factors limiting the efficiency of solar cells. But what about the newer generation of solar cells, which are made of molecular materials - such as organic polymers - instead of inorganic crystalline material, such as silicon? In laboratory experiments, the efficiency of these cells barely compares to that of the less efficient cells currently in commercial use, and often even falls short of them. Is the reason for this due to difficulties in development, or maybe there are other limiting factors, besides those defined by Kweiser and Shukley, that should be taken into account?

To try to find out the answer to this question, Prof. David Kahn, together with the post-doctoral researcher Fabrita Naik (from the Tata Institute of Basic Research in India), and with the help of Prof. Juan Biscart from Spain, compared and analyzed different types of solar cells, referring to a wide range of criteria . Their findings, which were recently published in the scientific journal Advanced Materials, show that there are indeed additional limiting factors beyond those defined by Queiser and Shukley - at least for solar cells made of organic materials - which may explain the large energy losses.

A typical old generation solar cell is made of two layers of an inorganic semiconductor - in most cases silicon is used for this. One of the two layers is rich in electrons, while the other has a lack of electrons. When the layers are placed on top of each other an electric field is created. When light rays, which carry enough energy to free the electrons from their bonds, strike the semiconductor, an electric current is created. The electric field now acts as a one-way gate, and the free electrons move through a wire connecting the two layers - thus creating an electric current.

The problem is, not all the light that hits the cell is actually utilized: the light separates according to the energy levels of the different wavelengths that make it up (as you can see - regarding the visible wavelengths - in the colors of the rainbow). Only a certain part of this energy - the exact amount of which is dictated by the properties of the material - is necessary for the release of the electrons. If the light rays striking the material do not carry a sufficient amount of energy, they will pass through the cell and not be used. Conversely, if the light carries a larger amount of energy than required, the excess energy will be wasted in the form of heat. In addition to this, potential energy may also be lost when a freed electron returns to its previous, bound state, before it is sufficient to "escape" through the wire.

When talking about the new generation of collectors, made of molecular organic materials, additional factors must be taken into account, dictated by the chemical and physical properties of these materials. Prof. Kahn believes that this fact may explain their low efficiency. The structure of organic substances, for example, is less ordered. A greater amount of energy is required to release the electrons to create an electric current. When an electron is released, following the interaction of light with the organic material, some of the energy is lost in the form of fluctuations in the chemical bond. In addition, the weak chemical bond between organic molecules (compared to the bond between atoms in non-molecular inorganic materials) leads to a reduced movement of electrons through the semiconductor, which causes some of the freed electrons to lose their energy. Although these two mechanisms, in which energy is wasted, are common to all solar cells, they are small and even negligible in inorganic solar cells.

Still, Prof. Kahn is in no hurry to give up the dream of cheap and efficient solar cells, organic and inorganic. "Instead of investing time and effort in trying to achieve unrealistic efficiency, we must understand the limitations of each and every cell, develop more reasonable expectations about their capabilities, and make more appropriate use of them," he says. "For example, molecular cells could be perfect as solar dyes, despite their low efficiency, and would be much cheaper to produce solar cells made of silicon, which are not suitable for this use. They can also be suitable for utilizing certain parts of the spectrum - that is, for specific wavelengths that are difficult to utilize by other means."

The research findings may also be useful for the conversion of solar energy into chemical energy, so Prof. Kahn believes that the research may be relevant to artificial photosynthetic systems. "Solar cells are a kind of 'beta version' that allows learning about an important part of artificial photosynthesis. Obtaining insights that will allow progress towards the development of artificial photosynthetic systems and solar colors will advance us to assemble the assembly of our future energy sources."