Low-temperature cross-linking of polyethyleneimine ethoxylated using silane coupling agents to obtain stable electron injection layers in solution-processed organic light-emitting devices

This study investigates low-temperature cross-linking of polyethyleneimine ethoxylated (PEIE) using four types of silane coupling agents, including trimethyl[3-(trimethoxysilyl)propyl]ammonium chloride (TTSPAC), trimethoxyphenylsilane (TMPS), trimethoxy[3-(phenylamino)propyl]silane (TPAPS) and 1,2-bis(trimethoxysilyl)ethane (BTMSE). The results of this study indicated that all the silane coupling agents reacted with PEIE at low temperatures ranging from 65 to 120 °C. The reacted PEIE exhibited solvent tolerance, indicating the cross-linking of PEIE. The cross-linked PEIE films were applied to electron injection layers (EILs) in solution-processed organic light-emitting devices. The device with PEIE: TTSPAC EIL showed a shorter device lifetime than that with only PEIE EIL. The shorter device lifetimes were attributed to the migration of chloride anions of TTSPAC. Conversely, the devices with PEIE: TMPS, TPAPS and BTMSE EILs, which did not contain mobile ions, had longer device lifetimes than that with only PEIE EIL. These results suggested that these improvements of device stability resulted from the cross-linking of PEIE.