Electronic Properties of Conjugated Polyelectrolyte Thin Films

Abstract

The electronic properties of conjugated polyelectrolytes (CPEs) with poly(fluorene-co-phenylene) backbones and different counterions and charges have been investigated using absorption and ultraviolet photoelectron spectroscopy (UPS). The optical energy band gap of CPEs depends mainly on their conjugated backbone and are nearly insensitive to the charges or counterions. UPS measurements reveal that electron injection from Au to polymers with cationic groups is more efficient than for the neutral and anionic counterparts. The vacuum levels of CPEs were also shifted toward higher or lower binding energy, relative to that of Au, depending on the charge and counterion presence, and provide insight into the general alignment of dipoles at the metal/organic interface.

High-Efficiency Polymer LEDs with Fast Response Times Fabricated via Selection of Electron-Injecting Conjugated Polyelectrolyte Backbone Structure

“…In a series of earlier reports it has been shown that the transient response of mobile ion possessing CPE-based PLEDs is strongly dependent on the ion size and environment (including neighboring functional groups). 9,12,13,19,29,31 The striking difference here between F8im-Br and F8imBT-Br which possess the same mobile bromide ion and imidazolium side chain tethered ionic groups represents a significant deviation from typical behavior. The low electron injection barrier for F8imBT-Br in contact with the Al electrode might also contribute to the fast transient response observed by minimizing the field dropped across the EIL.…”

“…[4][5][6][7][8][9][10] It was reported that the charged nature of CPEs has the ability to enhance polymer light-emitting diode (PLED) efficiencies as a consequence of reduction in the energy barrier for electron injection from high work function metals, much in the same way that charge trapping can, but with greater control and no requirement for pre-stressing. 11 This is understood mainly due to the formation of permanent dipoles at the CPE/metal interface 12,13 as also achieved with dipolar self-assembled monolayers (SAMs), but without their need for specific interface chemistry. 14 CPEs can also be used to improve the device performance of organic solar cells.…”

Origin of Work Function Modification by Ionic and Amine‐Based Interface Layers

“…[ 1,5,6 ] A recent study on conjugated polymer zwitterions revealed a similar lowering in work function, attributed to electrostatic realignment of the dipolar zwitterionic side-chains near the metal interface. [ 18 ] The formation of interfacial dipoles and electric fi eld-induced reorientation of molecules has also been reported.…”

“…[ 20 ] This however does not explain the work function modifi cation of conductors in absence of fullerenes. [ 6 ] The anionic polyelectrolyte Nafi on, which is also an acid, is found to give the opposite effect compared to cationic polyelectrolytes. A thin fi lm of Nafi on on ITO is found to increase the work function by 0.5 eV, [21][22][23] which indicates that Nafi on can modify the metal work function.…”

“…A new generation of WMLs fi rst introduced by Cao et al is based on polyelectrolytes or tertiary aliphatic amines, such as poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt -2,7-(9,9-dioctylfluorene)] (PFN) and the corresponding ethyl ammonium bromide. [ 1 ] Although these materials improve carrier injection or extraction in different organic diode devices, [1][2][3][4][5][6][7] the mechanism behind this improvement is not completely clear. This ambiguity hinders design, selection and optimization of new WMLs.…”