Towards High-Stability Organic Solar Cells Using Thermally Cleavable Side Chains in the Active Layer

The power conversion efficiency (PCE) of organic solar cells (OSC) has steadily improved over time and now exceeds commercially viable levels (>16%). However, the lack of stability in OSCs due to environmental factors such as oxygen, humidity, heat, light and mechanical stress is the primary challenge preventing commercial application. The OSCs can be protected from oxygen, humidity and mechanical stress through proper encapsulation techniques but the effects of thermal degradation are due to the intrinsic properties of the active layer material. After the active layer of an OSC is kinetically trapped in optimum morphology during processing, the thermal instability causes morphological drift as the system attempts to relieve thermodynamic stress. Thermal instability leads to a sharp drop in efficiency with the PCE decreasing by more than half during the first few hours of operations; this is referred to as the “burn-in” effect.
Previous work has shown that incorporation of thermally cleavable side chains (TCSs) into the active layer material help retains their desirable solution-processable properties prior to cleavage and improve thermal resistance post-cleavage. In this work, OSC devices were fabricated using a TCS-containing donor material, specifically a PffBT4T derivative. Accelerated thermal aging tests were performed at 100C for a one-month period, following partial cleavage of the side chains at 225C for 30 minutes to evaluate the effect of side-chain cleavage on device stability. The power conversion efficiency was correlated with morphology and domain purity changes using both solar simulation and atomic force microscopy coupled with infrared spectroscopy (AFM-IR).
The results demonstrate that employing thermally cleavable side chains leads to more stable active layer morphology, which improves the long-term efficiency and stability of OSCs. This research provides valuable insights into developing both solution-processable and thermally stable organic solar cells, offering the breakthrough needed for the commercialization of organic solar cells.