The Crucial Role of Confined Residual Additives on the Photostability of P3HT:PCBM Active Layers

We have investigated the impact of residual additives such as diiodooctane (DIO) and octanedithiol (ODT) on the photostability of state of the art P3HT:PCBM active layers. A series of active layers prepared with and without additives as well as neat additives were submitted to light irradiation in ambient air and analyzed by UV-vis and IR spectroscopy. We show not only that residues are sensitive to the combined action of light and oxygen but also that their presence can dramatically impact the polymer blend stability. DIO molecules are highly sensitive to light and can directly saturate the polymer conjugated backbone or be trapped by the fullerene moieties. ODT molecules can be photooxidized and may accelerate the intrinsic photooxidation of the active layer. Another important result is that the additives impact is directly linked to the presence of a top layer above the active layer. The confinement makes that additives react within the active layer, and thus accelerate its photodegradation, rather than decomposing in the gas phase (irradiation without top layer). Thus, a light-soaking step before top layer deposition could allow a clean removing of additives without affecting the optimized morphology and polymer blend stability. This process would be easily adaptable to industrial scale production.