Our future energy supply with considerably lower carbon emissions than today will have to make extensive use of photovoltaic (PV) solar energy). Thin film solar cells will play a particularly important role because of their lower energy consumption and carbon foot print. Thin film solar cells based on the compound semiconductor Cu(In,Ga)Se2 have reached the highest efficiency in the laboratory of any thin film technology: 22.6%. They are produced by several companies worldwide at a multi-GW/year scale.Recent progress in several PV technologies has been made possible by an improved understanding of the role of defects. In Cu(In,Ga)Se2 the electronic defects are controlled by native defects such as vacancies (a missing atom) or antisites (a wrong atom on a crystal site). The Se vacancy plays a particular role. It has been correlated with metastable behaviour of the solar cells and chalcopyrite thin films. Metastable behaviour is for example observed by light soaking, i.e. a slow increase of the open circuit voltage and the fill factor of the solar cell when exposing it to light, or by persistent photocurrent, i.e. the photoconductivity disappearing only slowly when the light is turned off. It is to be expected that a change in Se pressure during growth or an annealing in Se containing atmosphere changes the density of Se vacancies. Although it is difficult to directly access information about point defects like the Se vacancy, our molecular beam epitaxy growth method allows unique control over the Se pressure during growth and our luminescence methods allow to characterise the electronic structure in details. This gives us the tools needed to study and understand the influence of the Se vacancy on the metastable effects and the efficiency of chalcopyrite solar cells.We will grow Cu(InGa)Se2 films with special attention to the control of the Se pressure. Furthermore, we can influence the Se content by annealing in Se atmosphere. We will study the defect density of states by low temperature photoluminescence spectroscopy. Temperature dependent Hall measurements also allow conclusions on the defect levels. We can perform them in the dark and under illumination. By preparing solar cells from the same absorbers we can study any changes in the metastable behaviour and the efficiency of the solar cell. This combination of methods will allow us to answer questions like: is it the Se vacancy that it is responsible for metastabilities? Which defects are influenced by changing the Se pressure during growth or annealing and thus the Se content of the semiconductor? What is their influence on doping, transport and recombination behavior and thus on the efficiency of solar cells.