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Photovoltaic conversion: We see more clearly!

In an organic photovoltaic cell, the electrical charges are created by the absorption of light near an interface existing between two materials. For the first time, we have observed the photogeneration charge at the nanoscale using our near-field microscopy technique that directly measures the electrical surface potential.

Published on 3 February 2010
Photovoltaic conversion in organic solar cells is a miracle! As a matter of fact, we have to mix (at the scale of tens of nanometers) an electron donor material with an electron acceptor material. To produce a lot of charges, the mixture should contain a maximum of donor / acceptor interfaces, while forming two percolating networks to carry these + and - charges to their corresponding electrodes.

Chemists, physicists and technologists are confidently working with this structural model in mind. Yet, virtually no studies have directly observed these percolating networks and their electronic properties. But, seeing is better than believing!

We improved the resolution of our UHV atomic force microscope in Kelvin probe mode. It allows for local measurement of electrical potentials at the nanometer scale, ten times better than what had been previously done. XLIM laboratory, in Limoges, has manufactured the P3HT (polymer, donor) / PCBM (fullerene, acceptor) blend. We observed it successively in the dark, and under illumination, at 532 nm, a wavelength adsorbed by the polymer. Under illumination, the potential image shows halos at the interfaces, denoting the presence of charges. Our work confirms the generally accepted scenario for charge behavior in P3HT/PCBM cells.

Superimposed images of topography and surface potential of P3HT (light) / PCBM (dark) blend under illumination at 532 nm. Donnor/acceptor interfaces (in blue) exhibit a surface potential very different from its value "in the dark" over a width of ca. 3 nm.

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