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With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

International audience; Understanding the degradation mechanisms in organic photovoltaics is a major issue in order to develop stable organic semi-conductors and robust device architectures. We investigated the role of the architecture of organic solar cells (OSC) in the stability of the device under different aging conditions.The impact of the light, especially the UV part of the solar spectrum, on the evolution of the photovoltaic characteristics of the inverted OSC devices was investigated. The results show that UV light induces an important Voc loss for several active layers. More importantly, we show that the type of electron transport layer (ETL) induces different degradation mechanisms. For TiOx-based devices, the formation of an interface dipole was identified, resulting in a loss of the flat-band potential (Vfb). For ZnO-based devices, chemical modifications of the metal oxide and active layer at the interface were detected, resulting in a doping of the active layer. The stability of inverted OSC under thermal stress was also investigated and the role played by the hole transport layer (HTL) material and metal electrode was examined. A roughening phenomenon of the silver electrode was detected upon thermal ageing. For sufficiently thick silver layers, this has no impact of the device stability. However, for devices with thin silver electrode, this de-wetting phenomenon leads to a complete failure of the solar cell due to the formation of gaps in the electrode. This problem can be overcome with the addition of a thin metal-oxide layer on top of the electrode. Using XPS depth profile, we also demonstrated the presence of inter-layer diffusion when silver is used as an electrode material. This inter-diffusion is correlated to declines in the performance of the device when used in conjunction with MoO3 as an HTL.The stability under storage in air of OSC in direct architecture was also investigated and we developed a novel strategy to replace the PEDOT:PSS. A poly(3-hexylthiophene) bearing a triethoxysilane function at the end of the chain (P3HT-Si) was anchored to the indium-tin oxide (ITO) electrode. The use of this novel grafted HTL creates a hole selective membrane which drastically reduces the leakage current. As a result, the power conversion efficiency of OSC was improved compared to devices with bare ITO. By replacing PEDOT:PSS by the hydrophobic P3HT-Si, the water penetration in the device is impeded which significantly improved the shelf lifetime of devices.These different studies highlight the role of the architecture and, more specifically, of the ETL, HTL and electrode in the stability of OPV devices.

International audience

International audience

International audience; In recent years the performances of organic solar cells have improved considerably, reaching power conversion efficiencies higher than 10%. However, long-term stability of these devices is still to be achieved. In order to improve the lifetime of these devices, a good understanding of the different degradation mechanisms is needed. Degradation pathways are complex and diverse: they can be induced by numerous stimuli (heat, light, presence of O2 and H2O…) which can affect different regions of the device (active materials, interfaces, morphology, etc…). In this work, we attempt to isolate the contribution of each different degradation mechanism by applying different kinds of stresses to the devices (thermal ageing, photo-ageing) and, in particular, focusing on the interfaces in the devices.The robustness under thermal and photo-ageing of the top interface in inverted architectures was first investigated. In particular, the influence of the hole transport layers (MoO3, WO3 and PEDOT:PSS) and the top electrode (Ag and Al) on the stability of the device was studied. Under thermal stress, the combination of MoO3 as HTL and silver as electrode was shown to be especially unstable due to a rapid loss of open-circuit voltage (Voc) while WO3 and aluminum proved to be very robust. In depth analysis (XPS, RBS) showed that interdiffusion was occurring in MoO3/Ag devices. We suspect that this phenomenon affects the Fermi level of the MoO3 and/or increases the recombination rate at the interface with the organic active layer.1 The robustness of the bottom interface was also investigated, especially in terms of resistance to photo-degradation. Different kinds of electron transport layers (ZnO or TiOx) and active layers (P3HT:PC60BM and PCDTBT:PC70BM) were submitted to photo-ageing with or without UV. As expected, the UV component of the light (300-400 nm) is detrimental for the device for both ETL and both active layers. This loss of performance is mainly due to a loss of Voc, which can be attributed to an increase of the recombination rate at the interface. While the loss of Voc is always observed, the degradation kinetics are different in the four cases, and depend not only on the active layer used, but also on the ETL chosen. In particular, the use of ZnO as ETL seemed to slow down this degradation process. These findings indicate that TiOx reacts faster with the active layer to form recombination centers, even in the absence of oxygen.Thanks to these different studies, we have managed to identify degradation mechanisms specific to some organic solar cell architectures. Specifically, the combination of MoO3/Ag seems to be very unstable with thermal ageing. The UV component of the light, while always detrimental for the device, seems to react faster with the active layer when TiOx is used as ETL. Knowing this, specific solutions can be designed to improve the stability of organic solar cells against thermal and/or photo-ageing.(1) Greenbank, W.; Hirsch, L.; Wantz, G.; Chambon, S. Interfacial thermal degradation in inverted organic solar cells Appl. Phys. Lett. 2015, 107 (26), 263301.