High-Efficiency and Stable Organic Solar Cells with Stacked LiF and Organic Electrolytes as Cathode Interface Layers

The cathode interfacial layer plays a key role in enhancing the efficiency and stability of organic solar cells. Constructing efficient two-layer interfaces is one of the important strategies to improve device performance. In this work, we deposited a thin layer of lithium fluoride (LiF) beneath the electron transport layer poly[(9,9-bis(3′-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br) and (N,N-dimethyl-ammonium N-oxide)propyl perylene diimide (PDINO), respectively, to prepare conventional organic solar cells featuring a double electron transport layer. The Voc and FF of the optimized devices with a double electron transport layer were improved to achieve a power conversion efficiency (PCE) of 16.4% for LiF/PFN-Br-based devices and 16.0% for LiF/PDINO-based devices. The LiF-containing double electron layers reduce the work function of the active layer, makes electron injection more efficient, and promotes charge extraction and collection. As an optical spacer layer, the thin layer of LiF also changes the internal light intensity distribution, enabling the active layer to absorb more photons and generate more excitons. In addition to PCE enhancement, the LiF-containing double layers still reduce photodegradation of the active layer and improve the stability of the device.