Stable radical based conjugated electrolytes as a cathode interlayer for organic solar cells with thickness-insensitive fill factors

In this work, a perylene bisimide (PBI) based electrolyte PDIN-TEMPO pendent with a stable radical 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) has been synthesized and used as a cathode interlayer (CIL) in organic solar cells (OSCs). The aligned energy levels of PBI and electron-conducting nature of TEMPO enabled the new CIL to show excellent performance in OSCs, displaying not only high power conversion efficiencies (PCEs) of 18.48-15.86% and high fill factors (FFs) over 0.80 with the CIL thickness in the range of 5 nm-100 nm, but also high stability and the desired generality to be used in both regular and inverted OSCs. Two controlled conjugated electrolytes by removing TEMPO or PBI units were synthesized and used as a CIL in OSCs, but their FFs were reduced based on thick CILs. Therefore, the synergism of integrated PBI and TEMPO units into one material is critical for the superior performance of PDIN-TEMPO, which was confirmed by its high conductivity and thickness-insensitive electron mobilities in OSCs. This work demonstrates that the TEMPO radical is a promising unit to design semiconducting materials for high performance OSCs.