Stabilization of high-performing non-fullerene acceptor based organic solar cells using naturally occurring antioxidants

In the age of climate change and the scarcity of fossil raw materials, a rapid transitionto the renewable and sustainable energy resources must be carried out in order to cope withthe increased world energy consumption associated with growing world population. The solar photovoltaics is the fastest growing renewable energy generation technology, offering many advantages over its competitors. The solar energy systems currently in use are mostly based on silicon and use modules with monocrystalline silicon or poly/multicrystallinesilicon as photoactive semiconductors. Although these classic inorganic solar cells haveproven themselves and the production costs are steadily being reduced, the research interest is increasingly directed towards alternative materials for converting light energy into electrical energy. Solar cells whose photoactive components are based on semiconducting organic compounds are particularly attractive. Compared to the inorganic variants,Organic Solar Cells (OSC) offer several advatages: The production costs of organicsemiconductor materials are significantly lower. Due to high absorption coefficients, onlyvery thin layers are needed, which combined with their flexible nature and low process temperatures allows production on foil or paper, which further lowers overall costs. The use of soluble organic semiconductors also allows the deposition of photoactive layers from the liquid phase. This means that the solar cells can be manufactures using energy-saving manufacturing processes such as mass printing or Roll-to-Roll (R2R) processes.

Despite the many benefits offered by OSCs, the stability and associated lifetime of these devices is a major drawback that needs to be addressed to succeed in themarket. Oxidative degradation reactions in the organic active layer of OSCs are the main destabilizing factor that remains to be the main challenge confronting the technology. Chemical structures of the target molecules, miscibility of the donor-acceptor components, interactions of the organic active layer with the neighboring layers and the morphology they adopt have significant influence on the severity of the degradative reactions. Combination of light, oxygen, humidity, heat and perhaps impurities inside the blend can create an excellent environment for oxidative degradation reactions to occur, which can result in drastic disturbance in the molecular structure and subsequent defunctionalization of theOSC devices.

To address the above-mentioned challenges and to improve the lifetime of the devices, this thesis discusses the fundamental principles governing OSCs, mechanisms related todegradation, and the approaches to prevent or slow down the reactions that occur. 

The antioxidative stabilization is a beneficial and a promising approach to resist the aggression of the destructive oxidative reactions. Therefore, the application of antioxidants in OSCs is systematically investigated in the thesis. Various classes of antioxidants, including hydrogen donors, hydroperoxide decomposers, and radical scavengers, are tested for their ability to mitigate oxidative degradation. Among others, Retinoic acid and curcumin are identified as promising candidates due to their ability to neutralize reactive oxygen species and stabilize charge transport pathways. Experimental results demonstrate that the incorporation of antioxidants leads to reduced photochemical degradation, as evidenced by slower reductions in power conversion efficiency (PCE) under continuous illumination. 

The effect of curcumin on the photovoltaic performance and the operational stabilityof the devices was investigated in the thesis in detail. The efficiency of the devices containing curcumin was compared to the reference samples without antioxidant based ondifferent donor-acceptor systems, such as PBDB-T:ITIC, PM6:ITIC-2F and PM6:Y7. Wehave demonstrated that curcumin synergistically improves the initial performance andthe stability of the devices. 

We have studied the degradation of the organic active layer films, among others,with UV-Vis spectrometry, Atomic Force Microscopy (AFM) and Photoluminescence (PL)measurements to understand the mechanism behind the degradation. The results of the PL measurements showed that, by employing DPBF and hydroethidine probes to identify reactive oxygen species, there is two main degradation pathways, namely singlet oxygendriven and superoxide anion driven. The measurements show that the dominance of that reactions depends on the donor-acceptor couples in the film.

In the final part of the thesis, we have discussed the effect of chemical structures onthe properties of OSCs. The synthetic part of this study took part at the University of Copenhagen, where we showed the synthetic procedures, purification and characterization of the subphthalocyanine (SubPc) based organic semiconductors. When used in combination with established OPV systems, and after optimization of fabrication conditions, the SubPcs improve photovoltaic performance and stability of the devices.