The laboratory for Thin Films and Photovoltaics attached flexible CdTe thin-film solar cells on a lightweight polyimide film by using low temperatures (<450°C) vacuum evaporation process to grow cadmium sulphide/cadmium telluride (CdS/CdTe) layers and a subsequent annealing step in air where the materials are heated up and cooled down to harden them.

The researchers used zinc oxide doped on aluminium (ZnO:Al) as a transparent electric contact instead of the expensive indium tin oxide (ITO) layer used in earlier 11.4% solar cells.

In addition to being cheaper, the ZnO/ZnO:Al bi-layer improved process yield and reproducibility of high efficiency solar cells, EMPA says.

The 12.4% efficiency of the flexible thin-film solar cells was measured under standard AMI.5 illumination condition. The parameters were Voc = 823 mV, Jsc = 19.6 mA.cm-2, FF = 76.5%.

EMPA says all the process steps of the flexible thin-film solar cells are compatible with continuous in-line processing and can be transferred to roll-to-roll manufacturing of large area solar modules with high deposition speed.

“Such high efficiency flexible CdTe solar cells can become a low cost option for cost-effective solar electricity generation in near future,” EMPA concludes.