Solar cells play an important role in the future low carbon energy production of the earth. Highly efficient and cheap devices are necessary. The aim of the project SUNSPOT (SUrface and iNterface Science on PhOtovoltaic maTerials) is to improve the understanding of the surfaces and interfaces of polycrystalline materials that are relevant for photovoltaics. The project focusses on Cu(In,Ga)Se2 and hybrid organic inorganic perovskites (HOIPs) which show exceptionally high power conversion efficiencies despite their polycrystalline nature. Scanning tunneling microscopy and spectroscopy in conjunction with Kelvin probe force microscopy in ultra-high vacuum will be used to analyse these materials on the nanometer scale. The goal is to develop models that predict how the nature of polycrystalline surfaces and grain boundaries need to be in order to achieve high quality solar cells. The influence of foreign atoms on grain boundaries and the effect of the polycrystalline surfaces on junction formation will be analysed in detail. Absorber surfaces and cross sections of solar cell devices will be studied. Phase transitions in the HOIPs will be investigated by temperature dependent measurements and the effect of H2O and O2 will be analysed in detail in order to study the stability of the absorbers on the nanometer scale. It is the aim to understand how the modifications of the surfaces and grain boundaries contributed to the mostly empirical improvements of these high performance solar cell technologies in the last years. Only then further improvements can be made to close the gap between polycrystalline thin film solar cells and single crystalline devices. Moreover the models developed within this project can be translated to other new emerging thin film solar cell technologies. The CIGSe absorbers and devices will be prepared in already existing groups of the host institution where polycrystalline and epitaxial samples can be fabricated. The perovskite absorbers will be grown via vacuum evaporation within the SUNSPOT group. Special care will be taken to analyse clean surfaces which is a prerequisite for a development of consistent models. The proposal is in line with the focus of the host institution, covers one of the priorities of the University of Luxembourg and of the FNR and strong collaborations within the university and with the Luxembourg Institute of Science and Technology are foreseen.