By coating conjugated polyelectrolytes (CPEs) on metals, the work function (WF) of metals can be tuned by electric dipoles formed through the self-assembly of cation–anion pairs on the side chains of CPEs. Recently, it has been reported that a pertinent oxidative doping of anionic CPEs results in a reversed direction in the net electric dipoles, thereby yielding opposite WF tunability compared with pristine CPEs. However, the fundamental mechanism of this reversed WF tunability is not clearly understood. Here, through a systematic study of the relationship between the chemical structure of anionic CPEs (n-type) and the corresponding oxidation-processed CPEs (p-type), it is clearly demonstrated that radical cations are generated on the π-conjugated backbone of the n-type CPEs and strongly paired with anions on the side chains of the n-type CPEs, thereby reversing the direction of the total net electric dipoles. Furthermore, it is found that the degree of radical cation–anion coupling in the CPEs is a key factor in determining their WF tunability. It is envisioned that the results will provide a fundamental understanding of CPEs for versatile interface engineering in organic electronics.

Radical Cation–Anion Coupling‐Induced Work Function Tunability in Anionic Conjugated Polyelectrolytes

This study investigated the effects of fluorine (F) diffusion from a CYTOP passivation layer into amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs). The F contained in the CYTOP passivation layer was diffused into a-IGZO through 350℃ annealing. The similar ionic radii of F and oxygen (O) allowed the passivation of oxygen vacancy (Vo) and weakly bonded oxygen by F. As a result, the a-IGZO TFTs with CYTOP passivation were highly stable under various stresses. The threshold voltage (Vth) shifts of a-IGZO TFTs without CYTOP passivation and with CYTOP passivation under a negative bias stress (NBS) test for 10,000s were -6.7 V and -2.5 V, respectively. In addition, the Vth shifts of each device under a negative bias illumination stress (NBIS) test for 4,000s were -10.9 V and -5.3 V, respectively. This improvement was caused by a reduction of Vo and a widened band gap of a-IGZO through the F diffusion effect. In addition, the CYTOP passivation layer maintained excellent properties as a barrier against moisture after 350℃ annealing.

https://iopscience.iop.org/article/10.1088/1361-6463/ab8e7d/pdf

Stability Improvement of Solution-Processed IGZO TFTs by Fluorine Diffusion from a CYTOP Passivation Layer

The effect of carbon contamination and argon ion sputtering on the work function of chlorinated indium tin oxide

The determination of the Gaussian density of states (DOS) width and its influence on charge injection/extraction across an interface are of critical importance for the accurate physical understanding of the optoelectronic properties of organic electronics. A new technique is developed here, whereby the DOS width is extracted by numerical fitting of experimental band-bending observed across an interface. This is done by exploiting the fact that the band-bending can be influenced by the disordered nature of energetic sites that are involved in charge transfer across an interface. This technique is applied on a single component and blended mixture of materials. This approach is successfully tested on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), poly(N-vinylcarbazole) (PVK), 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13) and poly(3-hexylthiophene-2,5-diyl) (P3HT) in a number of interfaces. The DOS width of PEDOT:PSS, PVK, SPPO13 and P3HT were calculated to be 105 meV, 144 meV, 178 meV and 80 meV respectively. The impact of DOS width on the interface between two materials is investigated. The Fermi level pinning model used to describe the hole barrier between an inorganic electrode and an organic semiconductor fails when applied to the interface between two organic semiconductors. Their energetic barriers are very highly dependent on the DOS of the materials.

http://iopscience.iop.org/article/10.1088/0022-3727/49/32/325106/pdf

Investigation into the Gaussian density of states widths of organic semiconductors

Hybrid blue polymer light emitting diodes (PLEDs) with high efficiencies, luminance >20000 cd m−2 and low operating voltages are obtained using processing temperatures ≤150 °C. By briefly applying an electric field across the device prior to measuring (pre-biasing), the PLEDs with unannealed Zn1−xMgxO/Cs2CO3 injectors have maximum luminances three times higher and operating voltages 26% lower than the previous state-of-the-art, which used ZnO cathodes processed at 400 °C. The high performance of our PLEDs is shown to be linked to the filling of trap states in the unannealed oxide cathode. Further reductions in the operating voltage are obtained through reductions in the electron-injection barrier by incorporating Mg into the ZnO cathode, as revealed by electroabsorption spectroscopy. Device characterization also shows that achieving efficient PLEDs requires the use of an interlayer (in our case Cs2CO3) to prevent non-radiative recombination at the cathode. The architecture and device processing methods we develop allow us to produce PLEDs with 80 nm thick emitters that have a turn-on voltage of only 3.7 V. This work takes a major step towards cheap, efficient flexible PLEDs for displays and lighting.

/pdf/bright-and-efficient-blue-polymer-light-emitting-diodes-with-141rn3v7b8.pdf

Bright and efficient blue polymer light emitting diodes with reduced operating voltages processed entirely at low-temperature

The work function of indium tin oxide (ITO) was modified using caesium fluoride (CsF). Various concentrations of CsF was spin-coated on top of ITO and baked while the residual CsF was washed away with DI water. The work function of all the ITO samples was measured using ultraviolet photoelectron spectroscopy and it was found that the work function of ITO reaches as high as 5.75 eV. The work function rapidly increases with small concentrations of CsF solution and then decreases for higher concentrations. Using atomic force microscopy and x-ray photoelectron spectroscopy, the cause was determined to be the change in surface roughness and the oxygen concentration, with the former having a much greater influence on the work function than the latter. The current density of ITO/poly(vinylcarbazole)/Al hole-only devices using the modified ITO increases by more than seven orders of magnitude compared with the control device.

https://iopscience.iop.org/article/10.1088/0022-3727/46/47/475102/pdf

Enhancement of the work function of indium tin oxide by surface modification using caesium fluoride

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Enhancement of the work function of indium tin oxide by surface modification using caesium fluoride