Interface Stability Evaluation and Optimization for Organic Photovoltaic Devices – IN-STEP

In Organic Photovoltaic field, numerous laboratories are focuses on the first limiting point of the organic PV devices: the low power conversion efficiencies. To solve this issue, lots of efforts are put into the development of low-band gap polymers and optimization of the active layer morphology.
However, the second limiting factor, the instability of the devices, is not as investigated. But in the context of building long-lifetime efficient organic solar cells, the understanding of the degradation mechanisms in OPV is as important as the improvement of the devices efficiencies and should go in step with the development of new efficient materials.

In this overall context, the aim of the IN-STEP project is to evaluate the robustness of the different device architectures when submitted to different stress, understand the failure of the devices and investigate solutions to improve the robustness of the devices. However, the failure of the device can come from various modifications in the device: morphological changes in the active layer, intrinsic degradation of the materials and interfacial degradation. The IN-STEP project focuses on the degradation mechanism at the interfaces. The project will be divided in two stages.

The first stage of the project will be focused on the evaluation of the robustness of different kind of interfaces against various stresses (heat, light, presence of oxygen or water…). The stability of various interfacial layers for hole or electron extraction as well as the type of electrode will be studied under different conditions (light, heat). The aim of this research part is to elucidate the degradation mechanism of the commonly proposed architecture. Indeed, understanding the weakest points of the organic device is the first step of the improvement of their lifetime. To achieve this goal, we plan to build an aging chamber, with in-situ device characterization, in which several atmosphere parameters can be controlled (temperature, light, oxygen level, hygrometry).

After having understood the interfaces failure mechanism, the project will aim to proposing solutions to improve the performance stability of the organic solar devices. A way to improve the robustness of the bottom interface is to create of an intimate contact between the active layer and the holes or electron selective layer. The latter can serve multiple purposes, including alignment of energy levels at interfaces for better charge selectivity and robustness of the interface. The idea of this part is to investigate the potentiality in terms of robustness of covalently grafted layer on metal oxides (ITO, electron selective layers or hole selective layers). Such kind of covalently bonded layers should promote the adhesion between the metal oxide and the organic layer and might, in terms, improve the lifetime of the devices.