The Influence of Solid Additives in the Active Layer on the Thermal Stability of Slot-Die-Coated Organic Photovoltaics

As organic photovoltaics (OPVs) have seen significant advances in performance at both the laboratory and large scale, there is an increased need to develop strategies for improving the overall lifespan under operating conditions. One strategy that has gained significant attraction is the use of solid additives in the active layer to enhance device performance, in addition to photo-/thermal stability of OPV devices. While there are examples of how these additives can influence the physical properties of the bulk-heterojunction (BHJ) morphology, the wide variety of additives used makes it difficult to reach a consensus on their specific functions. Another challenge is that the majority of studies involving solid additives have been conducted on devices fabricated via small-scale spin-coating, with the assumption that these procedures are directly scalable. In this work, the performance of PPDT2FBT:PC₆₁BM devices fabricated via slot-die coating was examined by using four different additives. These included previously reported additives, piperazine (PP), polyacenaphthylene (PAN), and neat C₇₀, as well as one new additive, 4,4'-bipiperidine (BP). The impact of these additives on the thermal stability of the devices was also investigated at two different temperatures, 85 and 120 °C. Devices containing any of the mentioned additives exhibited slightly improved thermal stability at 120 °C, while only the addition of neat C₇₀ improved the device's lifetime at 85 °C. This work demonstrates that, despite solid additives being able to have a positive influence on the thermal stability of OPVs at significantly elevated temperatures, it is essential to investigate thermal stability at temperatures close to the maximum usage temperature.