Significance of ions with an ordered arrangement for enhancing the electron injection/extraction in polymer optoelectronic devices

We demonstrate the significance of ions with an ordered arrangement for enhancing the electron injection in polymer light-emitting diodes (PLEDs) and the electron extraction in polymer solar cells (PSCs) by using tetraoctylammonium bromide (TOAB) as a model compound. The spin-coated films of TOAB are prepared on the active layers of both PLEDs and PSCs without any annealing (TOAB-No) as well as with annealing at 60 degrees C (TOAB-60) and 100 degrees C (TOAB-100). Subsequently, they are investigated by atomic force microscopy (AFM) and synchrotron X-ray diffraction (XRD) as well as simultaneously employed as the interfacial layers in PLEDs and PSCs. The AFM results show that the active layers are well covered with TOAB-No and TOAB-100, but poorly covered with TOAB-60 due to the formation of large islands. The XRD results show that TOAB-No and TOAB-60 are polycrystalline with the (002) lattice planes parallel to the surface of the active layers while TOAB-100 is amorphous. Of note, the electron injection in PLEDs and the electron extraction in PSCs are significantly enhanced using TOAB-No and TOAB-60, but both are worsened using TOAB-100. Evidently the enhanced electron injection/extraction is mainly attributed to the ordered arrangement of ions in the TOAB crystals. This study indicates that with an ordered arrangement of ions, all ionic compounds should be excellent for enhancing the electron injection/extraction in polymer optoelectronic devices.