Self-assembled tetraoctylammonium bromide as an electron-injection layer for cathode-independent high-efficiency polymer light-emitting diodes

Tetraoctylammonium bromide (TOAB), a general kind of quaternary ammonium salt, was spin-coated onto the surface of a green-emissive poly(9,9-dialkylfluorene) derivative (G-PF) to fabricate cathode-independent polymer light-emitting diodes (PLEDs). The electroluminescence efficiencies were 15.4, 11.4, and 9.1 cd A(-1) for TOAB with Al, Ag, and Au as the cathode, respectively, which are better than that of the device with Ca/Al as the cathode (6.1 cd A(-1)). The molecular nanomorphologies of TOAB deposited on G-PF were investigated using synchrotron X-ray diffraction. Results show that TOAB molecules nucleated on the hydrophobic G-PF surface and self-assembled into a highly ordered lamellar structure during the spin-coating process. This unique structure produces suitable molecular dipoles between N(+) and Br(-), significantly improving the electron-injection ability from stable metals to G-PF. The direction of the molecular dipole between N(+) and Br(-) can be reversed by using a hydrophilic ZnO for producing an efficient electron injection layer in an inverted device. The self-assembled molecules of TOAB create an anisotropic dipole on hydrophilic or hydrophobic surfaces, making them a potentially efficient electron-injection layer.