Using exotic particles called quantum dots as the basis for a photovoltaic cell is not a new idea, but attempts to make such devices have not yet achieved sufficiently high efficiency in converting sunlight to power.
Zinc Oxide nanowires are conductive enough to extract charges easily, but long enough to provide the depth needed for light absorption. Using a bottom-up growth process to grow these nanowires and infiltrating them with lead-sulfide quantum dots produces a 50 percent boost in the current generated by the solar cell, and a 35 percent increase in overall efficiency, Jean says. The process produces a vertical array of these nanowires, which are transparent to visible light, interspersed with quantum dots.
Already, the test devices have produced efficiencies of almost 5 percent, among the highest ever reported for a quantum-dot PV based on zinc oxide. With further development, it may be possible to improve the devices’ overall efficiency beyond 10 percent, which is widely accepted as the minimum efficiency for a commercially viable solar cell. Further research will, among other things, explore using longer nanowires to make thicker films, and also work on better controlling the spacing of the nanowires to improve the infiltration of quantum dots between them.
Scanning Electron Microscope images show an array of zinc-oxide nanowires (top) and a cross-section of a photovoltaic cell made from the nano wires, interspersed with quantum dots made of lead sulfide (dark areas). A layer of gold at the top (light band) and a layer of indium-tin-oxide at the bottom (lighter area) form the two electrodes of the solar cell.
Images courtesy of Jean, Advanced Materials
Vertical arrays of ZnO nanowires can decouple light absorption from carrier collection in PbS quantum dot solar cells and increase power conversion efficiencies by 35%. The resulting ordered bulk heterojunction devices achieve short-circuit current densities in excess of 20 mA cm−2 and efficiencies of up to 4.9%.