Thin films are essential in our research and enable us to study new materials, the influence of microstructure or defined model electrodes. Thin film preparation is either done by pulsed laser deposition, magnetron or RF sputtering. Moreover, in the last years we took a step forward to measuring thin film properties already at growth. We developed a technique to measure electrochemical impedance spectroscopy while a film grows. This unique combination allows us to get deep insights for example in the evolution of electrochemical elementary parameters with increasing film thickness and thus gives us an edge in our research. By this method, we could also show that several yet unknown processes exist that change thin film surfaces and their defect chemistry shortly after their growth. By combining our new in-situ technique with cutting-edge analytical methods such as high resolution transmission electron microscopy, positron annihilation lifetime spectroscopy or synchrotron-based measurements, done together with our collaboration partners, we get new insight into defect concentrations on the ppm-scale, reaction mechanisms, degradation processes, or the concentration and energy levels of electronic trap states.