For the development of thin film solar cells we follow

- a high efficiency path by physical vapour deposition

- a low cost path by electrochemical deposition

We investigate the electronic structure and transport properties of our new semiconductors by opto-electrical defect spectroscopy.


Physical vapour deposition

We use co-evaporation for the deposition of our polycrystalline absorber films. Thin film solar cells have reached 20% efficiency [I. Repins, M. A. Contreras, et al.,

Prog. Photovolt. Res. Appl. 16, 235 (2008)]. These solar cells are made from Cu(In,Ga)Se2  and are prepared by co-evaporation.

A year after we installed our machine we have reached for our Cu(In,Ga)Se2  solar cells an efficiency of 15% using the three-stage process.

Currently we are working on a new process from which we expect higher efficiencies and maybe even a simpler process.

We are also developing a co-evaporation process for kesterites Cu2 ZnSnSe4 , a new solar cell material, which consists only of abundant and cheap raw materials.The record for this material is almost 10% [T. Todorov, K. B. Reuter, and D. B. Mitzi, Adv. Mater. 22, in press (2010)], which was prepared in a sequential process. However, we believe, that the efficiency could be higher when using absorbers prepared by co-evaporation.

Opto-electrical defect spectroscopy

The semiconductors we use as absorbers in our solar cells are not as well understood as more conventional semiconductors like silicon, which is used for example in computer chips, or gallium arsenide, which is used for example in mobile phones.

There is a need to answer questions like:

  • how is light absorbed?
  • how is current transported?
  • how is light made into charge carriers?

To help answer these questions we investigate material that does not contain grain boundaries, so we can study the properties of the material itself.

We use epitaxy to grow single crystalline films. These epitaxial films as well as polycrystalline films and solar cells are characterised by

photoluminescence,  Hall measurements and  capacitance measurements.