Oct 29 2010
Work on organic solar cells is being carried out for more than ten years. The solar cells have many advantages such as being environmentally friendly to manufacture and the ability of being applied on any material like plastic film.
Their main disadvantage is that they provide only a fifth of the electrical energy that could be provided by silicon solar cells, as most of the current is lost. To unravel this mystery as to where the current is lost Helmholtz-Zentrum Berlin (HZB) scientists have devised a technique using magnetic fingerprint of the particles that carry the charge. The results of the research which was carried out along with Scottish researchers were published in the Physical Review Letters.
Organic or plastic solar cells as they are being called have at its nucleus, a very thin layer of fullerenes (shaped like soccer balls) and polymers. When light falls on this layer the polymer constituent becomes excited and is called an exciton.When this exciton collides with a fullerene, an electron leaps into the fullerene molecule leaving behind a hole in the polymer. For the current to flow, the holes and the electrons must move to the opposite contacts. The holes journey through the polymer chain, while the electrons pass through the fullerenes. The holes, also called polarons can impede one another through the path reducing the solar cell efficiency and limiting the amount of electrical energy yielded from the given solar energy.
Jan Behrends from the HZB Institute during his doctorate thesis confirmed that Electrically detected magnetic resonance (EDMR) has been used to show that polarons always act as obstacles in the path of other polarons when their magnetic spin or moment is the same proving the formation of these bipolarons. This method manipulates the polaron spin by making use of a microwave pulse and an external magnetic field. The spin could be rotated like a compass needle using the resonance effect. When the miniature magnets were aligned in opposite directions the current flowed freely, but was the reverse when the magnets’ alignment was identical. According to Project Leader, Dr. Klaus Lips, this important discovery would help in advancing organic solar cell technology along with the creation of no spin blockade plastics.