This has been achieved by doping the cells with copper, the scientists from Empa, the Swiss Federal Laboratories for Materials Science and Technology, say.

In order to make solar energy widely affordable scientists and engineers all over the world are looking for low-cost production technologies. Flexible thin film solar cells have a huge potential in this regard because they require only a minimum amount of materials and can be manufactured in large quantities by roll-to-roll processing. One such technology relies on cadmium telluride (CdTe) to convert sunlight into electricity. With a current market share that is second only to silicon-based solar cells CdTe cells already today are cheapest in terms of production costs. Grown mainly on rigid glass plates, these so-called superstrate cells have, however, one drawback: they require a transparent supporting material that lets sunlight pass through to reach the light-harvesting CdTe layer, thus limiting the choice of carriers to transparent materials.

The inversion of the solar cell’s multi-layer structure – the so-called substrate configuration – would allow further cost-cuttings by using flexible foils made of, say, metal as supporting material. Sunlight now enters the cell from the other side, without having to pass through the supporting substrate. The problem, however, is that CdTe cells in substrate configuration on metal foil has thus far exhibited low efficiencies of below eight per cent – compared to the record efficiency of 19.6 per cent for a lab-scale superstrate CdTe cell on glass. 

One way to increase the low energy conversion efficiency of substrate CdTe cells is p-type doping of the semiconductor layer with minute amounts of metals such as copper (Cu). This would lead to an increase in the density of “holes” (positive charge carriers) as well as their lifetimes, and thus result in a high photovoltaic power, the amount of sunlight that is turned into electrical energy. However, CdTe is very hard to dope. 

A team led by Ayodhya Nath Tiwari, head of Empa’s laboratory for thin films and photovoltaics, used using high-vacuum Cu evaporation onto the CdTe layer with a subsequent heat treatment to allow the Cu atoms to penetrate into the CdTe. The amount of Cu has to be carefully controlled. The electronic properties improved significantly, however, when Lukas Kranz, a PhD student, fine-tuned the amount of Cu evaporation so that a mono-atomic layer of Cu would be deposited on the CdTe. “Efficiencies increased dramatically, from just under one per cent to above 12,” says Kranz. Their best value was 13.6 per cent for a CdTe cell grown on glass; on metal foils Tiwari’s team reached efficiencies up to 11.5 per cent.

“Our results indicate that the substrate configuration technology has a great potential for improving the efficiency even further in the future,” said paper co-author Stephan Buecheler, a group leader in the lab. “I’m convinced that the material has the potential for efficiencies exceeding 20 per cent.” The next steps will focus on decreasing the thickness of the window layer above the CdTe, including the electrical front contact. This would reduce light absorption and allow more sunlight to be harvested by the CdTe layer. 

The study was supported by the Swiss National Science Foundation (SNSF) and the Competence Center Energy and Mobility of the ETH Domain (CCEM-Dursol).It is published in the current issue of Nature Communications.