Thin film solar cells are considered the next generation of photovoltaic technology because of their highcost 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 materialclass of kesterites (Cu2ZnSnS(e)4) – materials with a crystal structure very similar to that ofchalcopyrites, which are expected to show also analogous electronic properties. Its applicability asabsorber in thin film solar cells has been demonstrated, but very little information is available on its basicmaterial properties.The main goal of this project is to improve the efficiency of kesterite solar cells from the current 7% to12%. This will be achieved by a systematic approach to the absorber design and by using co-evaporation,the deposition method which has provided the highest efficiency solar cells with chalcopyrite absorbers.This method has hardly been used for the deposition of kesterite solar cells so far. We will design gradedabsorbers, which have a much higher potential than the homogeneous absorbers used so far.A prerequisite for the development of useful kesterite absorbers is the definition of a co-evaporationprocess that results in single phase kesterite films without detrimental secondary phases. These films willthen be used to establish the optical properties of these materials, particularly the dependence of the bandgap on the composition. This information is the basis to define a composition gradient within theabsorber that allows a favourable band gap profile, with increased band gap towards the front and backcontact. This profile has been proven extremely important in the case of chalcopyrite solar cells. Asimilar approach is active in Si and in CdTe solar cells. This band gap gradient is expected to reducerecombination at the interfaces, thereby increasing the open circuit voltage, and simultaneously becauseof the reduced central band gap, to increase the short circuit current. This is the first time that such asystematic approach is taken towards the design of kesterite absorbers and justifies the ambitiousefficiency goal.