Kesterites in Thin Film Solar Cells - KITS

Coordinating Institution: Université du Luxembourg
Other Partner(s): Helmholtz Zentrum für Umweltforschung, Leipzig (D)
From: 01/05/2009
To: 30/04/2012
Budget: 322,000.00€
Contact(s): Siebentritt Susanne

Summary

Thin film solar cells are considered the next generation of photovoltaic technology because of their high cost reduction potential. Among the available thin film technologies, solar cells based on chalcopyrites (Cu(In,Ga)Se2) have demonstrated the highest efficiencies in the lab and in production. However, concerns exist because of the increasing indium price. An alternative has recently appeared: the material class of kesterites (Cu2ZnSnS(e)4) – materials with a crystal structure very similar to that of chalcopyrites, which are expected to show also analogous electronic properties.

Its applicability as absorber in thin film solar cells has been demonstrated, but very little information is available on its basic material properties. We aim at improving the efficiency of kesterite solar cells and at a detailed analysis of their optical properties. The first goal was to development a co-evaporation process for the deposition of the kesterite absorbers. Previous attempts have used various other methods to prepare kesterite films, very often with a focus on low-cost methods. However, the best chalcopyrite solar cells are made by a co-evaporation process.

Aiming at high efficiencies and fundamental material properties we chose this method. The project started shortly after the vacuum chamber which allows co-evaporation was delivered. So far we have focused on the development of the deposition process. We started depositing Cu2ZnSnSe4. Re-evaporation of tin in the form of tin selenide turned out to be a major issue. This could be fixed by optimising the substrate temperature. The films obtained show photoluminescence at the expected band gap energy, albeit weak. First solar cells were prepared, which showed an open circuit voltage of 100mV. Currently we are optimising the composition. As expected, secondary phases represent a major challenge. Cross section composition profiles indicate the presence of ZnSe near the back contact. We are in the process of analysing the role of this layer in the solar cells. It might be highly doped and could represent a suitable back contact to the solar cell.

Refereed Scientific Publications

One in Preparation

Other Publications: ---

Figure: The new PVD system for the co-evaporation of kesterite absorbers, which arrived at LPV in February 2009