In hopes of eventually generating power from photovoltaic materials that are both environmentally stable and low cost, researchers at Lawrence Berkeley National Laboratory and the University of California at Berkeley have combined desirable characteristics of organic and inorganic solar cells using inorganic nanocrystals in colloidal solution.
Nanometer-size crystals of cadmium selenide (CdSe) and cadmium telluride (CdTe) were synthesized and spin cast in separate layers from filtered pyridine solutions to create ultrathin flexible films of the semiconductor nanocrystals on a substrate of indium tin oxide (ITO) glass coated with 2 Å of alumina. Donor-acceptor, type-II heterojunctions, typical of organic materials (as opposed to conventional p-n junctions of inorganic materials) were created by alternately juxtaposing CdTe and CdSe layers on the substrate. Annealing the CdTe layer for 15 minutes at 200°C prior to deposition of the CdSe layer, minimized mixing at the interface.
Both nanocrystal layers (CdSe and CdTe) are electrical insulators in the absence of light, whereas full-sun illumination decreased resistance in each layer by at least an order of magnitude and increased conductivity in the bilayered device by more than three orders of magnitude. The researchers achieved photovoltaic conversion efficiency in the prototype device of almost 3%, which is good for organic materials but an order of magnitude below efficiencies achievable with conventional inorganic photovoltaic materials. While hoping to eventually improve efficiency, however, the researchers view the current demonstration as an important first step.
“Our colloidal inorganic nanocrystals share all of the primary advantages of organics-scalable and controlled synthesis, an ability to be processed in solution, and a decreased sensitivity to substitutional doping-while retaining the broadband absorption and superior transport properties of traditional photovoltaic semiconductors,” said Ilan Gur, lead author of a paper in Science on the work and a doctoral candidate in the research group of Paul Alivisatos, who directs the Materials Science Division at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA).1
REFERENCE
1. I. Gur, N.A. Fromer, M.L. Geier, A.P. Alivisatos, Science310, 462 (Oct. 21, 2005).