Improved Air Processability of Organic Photovoltaics Using a Stabilizing Antioxidant to Prevent Thermal Oxidation

During the production of organic solar cells, the exposure of constituting materials and resulting devices to light, heat, oxygen and water is a potential source of degradation. We show that thermo-oxidation of the active materials occurs upon annealing the active layer or upon stirring the active materials solutions in air at elevated temperatures. By comparing various blends of donor polymers with fullerene or non-fullerene acceptors, the fullerene is identified to be the most susceptible component to thermal oxidation in both film and solution, while the polymer remains unaffected. In both cases, the effect on the photovoltaic device performance consists predominantly in a decreased short-circuit current (JSC), suggesting that the underlying degradation mechanism is the same in the solid state and in solution. In addition, we find that thermo-oxidation of the active layer during annealing of the device can be decelerated by blending nickel(II) dibutyldithiocarbamate (Ni(dtc)2) as antioxidant additive into the active layer. While small concentrations of additive stabilize the JSC and therefore the power conversion efficiency, higher contents negatively affect device performance. On the other hand, thermo-oxidation of the active materials in solution during stirring in air at elevated temperatures can be avoided for several hours by adding as little as 0.5% of Ni(dtc)2. The observation that the antioxidant is consumed during stabilization of the active materials suggests a sacrificial protection mechanism