Ternary electron injection layers for highly efficient polymer light emitting diodes†
The high efficiency of polymer light-emitting diodes (PLED) with ternary electron injection layers (EILs), including polyethylenimine (PEIE), poly(vinylpyrrolidone) (PVP), and tetraoctylammonium bromide (TOAB), has been achieved and well-studied via a Mixture design strategy. With optimization by using a Mixture design, the ternary device shows 1.12 and 1.09 times the current efficiency and the power efficiency, respectively, of a PEIE device (11.24 cd m−2 and 10.69 lm W−1), which are higher than the values of a PEIE : PVP (binary) device (11.96 cd m−2 and 10.14 lm W−1). The enhancement in power efficiency in the ternary device over the binary device results from an increase in film morphology. AFM images show that TOAB significantly reduces the roughness of the aggregated binary film. This is corroborated from XPS results that show the additional hydrogen bonding formed from the interaction between PEIE and PVP being reduced with the addition of TOAB into the thin film. The strategy of Mixture design not only provides a quick optimization of the device performance but also shows the compositional dependence on each parameter for future applications. This is the first time that ternary EIL has been demonstrated to possess the electron injection, hole blocking, and smooth film morphology needed to achieve high device performance.