Molecular wire points to bio solar cell

Japanese researchers have taken a step towards hybrid biological-synthetic solar cells by building molecular wires to hook natural light-harnessing enzymes to artificial electrodes.

The efficiency of photovoltaic energy conversion is of critical significance for the practical application of solar installations. Theoretically, every photon absorbed should release one electron. Whereas modern solar cells are far from achieving high efficiency, natural photosynthetic systems achieve nearly 100 per cent quantum yield.

Plants, algae and cyanobacteria are able to almost completely transform captured sunlight into chemical energy. The electrons set free by the photons hitting the photosynthetic elements in these organisms are transported out of the light receptor and are used as the driving force for chemical reactions. The team from the National Institute of Advanced Industrial Science and Technology in Tosu has developed a process to capture light energy with nearly equal efficiency.

Reporting in the journal Angewandte Chemie, they plug a molecular wire directly into a biological photosynthetic system to efficiently conduct the free electrons to a gold electrode.

To improve the efficiency of synthetic systems, experiments were attempted in which biological light-capturing units were deposited onto electrodes as thin films. However, the transfer of electrons from the light-capturing layer into the circuit in this type of system is so inefficient that most of the electrons don't even make it to the target electrode.

The secret to the success of natural photosystems is the perfect fit of the individual components. The molecules fit precisely together like plugs and sockets and can pass electrons on directly and nearly without loss.

The approach taken by the Tosu team connects a Photosystem I enzyme taken from the cyanobacteria Thermosynechococcus elongatus to a synthetic apparatus. An important component of the electron transmission sequence of Photosystem I is vitamin K1. The researchers removed the vitamin K1 from the Photosystem I protein complex and replaced it with a synthetic analogue. This consists of two molecular plugs and a hydrocarbon chain built to be the same length as that of K1.

One plug connects to Photosystem I, the other is a molecular, viologen group that anchors the ensemble to a coated gold electrode. Electrons released by irradiation of Photosystem I and transmitted along the wire are relayed to the gold electrode by the viologen group.

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