Silicon solar cells are dominating the solar market with a market share of over 90%, due to their high efficiency and low-cost. As silicon is nearing its maximum efficiency potential, thin film technology emerges as an opportunity to make Silicon-based tandem solar cells, where thin-film solar cells can be grown on top of silicon.  Tandem solar cells is a proven route to achieve efficiencies exceeding what is possible in traditional singe-junction devices. Unfortunately, current tandem solar cells are expensive or rely on materials that are scarce or toxic.  A candidate material, which in theory meets the requirements for silicon-based tandem solar cells is cuprous oxide. It is an abundant, non-toxic semiconductor with a suitable bandgap of 2.1 eV.

At the University of Oslo, researchers are investigating the properties of cuprous oxide and other materials for possible applications in thin-film tandem solar cells. At the Micro and Nanotechnology Laboratory (MiNaLab) cuprous oxide films have been deposited by reactive DC magnetron sputtering. Sputtering is a flexible and scalable deposition method, which can be used for a variety of material systems. In sputtered cuprous oxide there is a high concentration of unwanted defects, believed to be copper vacancies, “missing” copper atoms in the crystal lattice. This limits its applicability in solar cells and poses a challenge to realize a cuprous oxide – silicon tandem solar cell.

In the recent paper published in Thin Solid Films, Martin Nyborg and co-authors investigated if lithium could passivate defects by introducing lithium in the deposition process. Theoretical calculation predicted that Lithium is favorable by occupying the missing copper sites in the lattice and thereby passivate the electrically active defects.  They used lithium doped copper targets in the sputtering process and successfully deposited cuprous oxide films with varying lithium concentrations in the range of 0.01-1 %. The group report phase pure and stoichiometric Cu2O thin films of about 500 nm (0,00005 cm).  However, at the concentrations investigated, no decrease in defect concentration was detected. On the contrary, Hall effect measurements show an increase in concentration at high lithium doping concentrations. The study concludes that lithium can be used to alter the electrical properties of cuprous oxide, however, not to decrease, but rather increase the apparent acceptor concentration. The origin of the acceptor is not revealed, but the authors speculate that it could be related to changes in the relative concentration of acceptors and donors. Lithium is not expected to form acceptor defects in cuprous oxide, therefore it could be speculated that instead, lithium passivates a compensating donor, increasing the acceptor concentration reported by Hall effect measurements.

The full paper can be found here.