Revealing the Microstructure of the Active Layer of Ternary Organic Solar Cells using Energy-Filtered TEM

Fullerene-free ternary organic solar cells have emerged as a promising class of devices with higher efficiencies when compared with solar cells that encompass binary active layers. Compared with traditional fullerene acceptors, non-fullerene acceptors have stronger absorption in the visible and near-infrared, which helps to generate high photocurrent for solar cells. Nevertheless, controlling phase separation and domain size in the photoactive layer morphology is crucial to enable efficient charge separation, transport and collection. Here, we demonstrate high-efficiency devices by combining the organic semiconductors with complementary light absorption, using PTB7-Th as donor, and IDIC, COi8DFIC, Eh-IDTBR, and O-IDTBR as acceptors. The combination of Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS), Energy-Filtered Transmission Electron Microscopy (EFTEM), Resonant Soft X-ray Scattering (RSoXS) reveals the impact of crystallization on the mesoscale morphology. We show that ternary systems composed of compatible nonfullerene acceptors exhibit fibril-like nanostructures in the active layer, leading to devices with efficiencies in excess of 10%. The morphology suggests that compatibility between two of the three components is important for maximizing performance in devices.