Photo-degradation, thermal degradation and storage: the impact of the organic solar cell architecture on the degradation mechanisms
Résumé
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.