The combination of several non-destructive techniques with multi-scale sensitivity (from nanometers to millimeters) is essential for tracking the fundamental physicochemical processes involved in device optimization. Two of the specific objectives of our subproject (GraPerOS_MULTI) are linked to the multi-scale characterization: (i) Alloy ordering, molecular bonding, and aging of materials and devices by optical spectroscopies at the microscale. This covers the characterization of the different layers of the device (perovskites, oxides, graphene based materials), addressing their composition, molecular structure, and stability by means of optical techniques. (ii) Study of charge separation and recombination at interfaces with nanometric resolution.  Scanning probe microscopy can elucidate the electrical properties of grain boundaries and interfaces in terms of charge accumulation/depletion and energy barriers, which are crucial for the improvement of the device. This is complemented by a third specific objective: (iii) Surface functionalization with metal nanoparticles for plasmonic energy conversion. This explores the use of metal nanoparticles of the appropriate structure and size for boosting device performance without detrimentally increasing its complexity and cost.  The new scientific and technical knowledge acquired by the end of the project constitutes the best tool to unveil the fundamental phenomena underlying present limitations and improve device performance.