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Organic solar cells, characterized by a symmetrical regular layered structure, are very promising systems for developing green, low cost, and flexible solar energy conversion devices. Despite the efficiencies being appealing (over 17%), the technological transfer is still limited by the low durability. Several processes, in bulk and at interface, are responsible. The quick downgrading of the performance is due to a combination of physical and chemical degradations. These phenomena induce instability and a drop of performance in working conditions. Close monitoring of these processes is mandatory to understand the degradation pathways upon device operation. Here, an unconventional approach based on Energy Dispersive X-ray Reflectivity (ED-XRR) performed in-situ is used to address the role of Wells–Dawson polyoxometalate (K6-P2W18O62, hereafter K6-P2W18) as hole transporting layer in organic photovoltaics. The results demonstrate that K6-P2W18 thin films, showing ideal bulk and interface properties and superior optical/morphological stability upon prolonged illumination, are attractive candidates for the interface of durable OPV devices. [ABSTRACT FROM AUTHOR]