Degradation through Directional Self-Doping and Homogeneous Density of Recombination Centers Hindered by 1,8-Diiodooctane Additive in Non-Fullerene Organic Solar Cells

Non-fullerene-based organic solar cells (OSCs) have recently proven to perform with efficiencies above 18%. This is an important milestone for one of the most promising technologies in the fields of flexible and transparent/semitransparent photovoltaics. However, the stability of OSCs is still a challenging issue to meet the industry requirements. Herein, several devices with IT-4F:PM6 as the active layer with and without 1,8-Diiodooctane (DIO) additive are characterized before and after a 1400 h degradation test under 1 sun white light-emitting diode (LED) illumination intensity. The optoelectronic study via impedance spectroscopy under illumination at quasi-open-circuit correlates the use of DIO as an additive with a retarded degradation behavior and an overall improved device performance. In dark conditions, the Mott–Schottky analysis suggests that samples without DIO develop self-doping during degradation, changing the p-i-n doping profile into a p–n type, most likely related to the evolution of the blend demixing. These mechanisms are further confirmed by drift-diffusion simulations. Space-oriented redistribution of shallow trap levels (self-doping) and homogeneous increase in deep-trap levels (nonradiative recombination) are shown to be hindered by the use of the DIO additive.