The key requirements of current technology, aimed at reducing the environmental impact of excessive fossil fuel consumption through the effective integration of clean renewable energy conversion devices, have recently sparked increased interest in the development of semi-transparent solar cells (STCs) for direct incorporation into energy-sustainable buildings. In this context, organic solar cells (OSCs) emerge as a promising and innovative technology for the fabrication of highly efficient STCs for building-integrated photovoltaics (BIPV) applications. A comprehensive understanding of the physical and chemical properties of the active materials at the micro/nanoscale is essential for the fabrication OSCs with the desired operational efficiency, which can be achieved through precise design and optimization of their active components and mutual interfaces.  Indeed, as typical OSC designs consist of multi-layered structures, where the absorber material—processed as a thin film—is sandwiched between charge-transporting layers and electrodes, a deep understanding of the interface properties is also crucial. Importantly, photovoltaic devices need to operate under external stress or harsh environmental conditions such as light, heat, humidity, and oxidative agents. Valuable insights into the relationship between microstructural characteristics and failure mechanisms can be gained by simulating the devices operational conditions while simultaneously monitoring in-situ the changes in their structural and morphological properties under various stress factors. In this context, we present here an alternative approach for the study of the morphological and structural properties of multi-layered ST-OSCs for BIPV. The OSCs under investigation consist of a PM6:Y6 photoactive layer, a ZnO electron-transporting layer, an organic HTL-X hole-transporting layer (poly(3,4-ethylenedioxythiophene) (PEDOT)-based ionomer) and a transparent electrode made of silver nanowires (AgNWs). The work addresses stability issues related to both bulk and interface properties under prolonged heating and illumination conditions. Our experimental methodology combines in-situ Energy Dispersive X-ray Reflectometry (EDXR) with complementary ex-situ techniques, such as Atomic Force Microscopy (AFM), X-ray Diffraction (XRD), and micro-Raman spectroscopy. The combination of these techniques provided valuable insights into the chemical, structural, and morphological degradation processes affecting the stability of the multi-layer devices [1]. Specifically, an increased roughness at the ZnO/PM6:Y6 interface was observed by EDXR, although evidence of substantial structural stability under illumination was found by XRD. In contrast, the system exhibited overall stability when subjected to prolonged heating in the dark, suggesting the photo-induced origin of the observed degradation phenomenon. Such effect is related to a photo-oxidation process of the active material occurring during continuous illumination of the device due to the use of an the hygroscopic organic HTL under ambient moisture conditions, as confirmed by micro-Raman measurements. This process may also be activated by a photocatalytic role of the ZnO layer. The results obtained were highly valuable in designing an alternative cell configuration, where the organic hygroscopic HTL-X was replaced by the inorganic MoOx compound. Our findings indicate that the device in this alternative configuration was stable under light stress, suggesting that the use of the inorganic HTL MoOx can limit the photo-oxidation of the PM6:Y6 active material, thereby preventing the cell from degradation [2].

[1] B. Paci et al. Adv. Funct. Mater., 2011, 21: 3573-3582, https://doi.org/10.1002/adfm.201101047

[2] B. Paci et al. Nanomaterials, 2024, 14(3), 269, https://doi.org/10.3390/nano14030269

Fundings: Funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement No 101007084 (CITYSOLAR).