Huaping Li

Fullerene–Carbene Lewis Acid–Base Adducts

The reaction between a bulky N-heterocylic carbene (NHC) and C(60) leads to the formation of a thermally stable zwitterionic Lewis acid-base adduct that is connected via a C-C single bond. Low-energy absorption bands with weak oscillator strengths similar to those of n-doped fullerenes were observed for the product, consistent with a net transfer of electron density to the C(60) core. Corroborating information was obtained using UV photoelectron spectroscopy, which revealed that the adduct has an ionization potential ∼1.5 eV lower than that of C(60). Density functional theory calculations showed that the C-C bond is polarized, with a total charge of +0.84e located on the NHC framework and -0.84e delocalized on the C(60) cage. The combination of reactivity, characterization, and theoretical studies demonstrates that fullerenes can behave as Lewis acids that react with C-based Lewis bases and that the overall process describes n-doping via C-C bond formation.

Reconstruction of Conjugated Oligoelectrolyte Electron Injection Layers

Surface reconstruction of electron injection layers based on conjugated oligoelectrolytes atop an electroluminescent layer occurs in the presence of air. The proposed mechanism involves hydration and concomitant increase of the interfacial energy with the underlying hydrophobic surface followed by dewetting via a nucleation process. No such changes are observed in the case of a conjugated polyelectrolyte, presumably because the lower mobility of the polymer chains leads to a kinetically locked bilayer.

Electrolyte-Gated Red, Green, and Blue Organic Light-Emitting Diodes

We report vertical electrolyte-gated red, green, and blue phosphorescent small-molecule organic light-emitting diodes (OLED), in which light emission was modified by tuning the electron injection via electrochemical doping of the electron injection layer 4,4-bis(N-carbazolyl)-1,1-biphenyl (CBP) under the assistance of a polymer electrolyte. These devices comprise an electrolyte capacitor on the top of a conventional OLED, with the interfacial contact between the electrolyte and electron injection layer CBP of OLEDs achieved through a porous cathode. These phosphorescent OLEDs exhibit the tunable luminance between 0.1 and 10 000 cd m-2, controlled by an applied bias at the gate electrode. This simple device architecture with gate-modulated luminance provides an innovative way for full-color OLED displays.

Gate-Tunable Electron Injection Based Organic Light-Emitting Diodes for Low-Cost and Low-Voltage Active Matrix Displays

Low-cost and low-voltage active matrix displays were fabricated by simply patterning gate electrode arrays on a polymer electrolyte (PE)-coated polymer light-emitting diode (PLED). Structurally, a PE capacitor seamlessly stacked on a PLED by sharing a common Al:LiF composite electrode (PEC|PLED). This monolithic integrated organic optoelectronic device was characterized and interpreted as the tunable work function (surface potential) because of the perturbation of accumulated ions on Al:LiF composite electrode by PEC charging and discharging. The modulation of electron injection by the PEC resulted in increases in the electroluminescent brightness, from <100 cd m-2 to >8000 cd m-2, and the external quantum efficiency from <0.025% to 2.4%.

Electrolyte-gated light-emitting transistors: working principle and applications

Adding solid electrolytes into organic semiconductors broadens the scope of material properties and electronic applications. Successful examples include polymer light-emitting electrochemical cells, electrolyte-gated organic transistors, and electrolyte-gated organic light-emitting transistors (EGLETs). EGLETs combine an organic light-emitting device and a transistor with a high capacitance electrolytic dielectric. Here we summarize recent progress in the development of EGLETs in both planar and vertical device architectures. The former offers a lateral geometry and in-plane light-emission for scientific scaffolds in the fundamental study of organic semiconductor opto-physics. The latter features surface emission with a unity aperture ratio, and it can be used in matrix displays without the requirement of external thin-film transistor arrays as the switching circuitry. This strategy paves an easy avenue towards fabricating highly integrated organic optoelectronic devices, and it offers a new test bed for research in iontronics and organic electronics.

Composite electrode with gate-tunable work function for optoelectronic devices

The work function (WF) of Al:LiF composite electrode is first reported to be tunable by electrostatic field effect via the polarization of LiF on Al atom layer. This observation was demonstrated in Kelvin probe force microscope measurement. The optimized Al:LiF composite electrode was employed as organic light emitting diode (OLED) cathode to show the electroluminescent brightness increased from 0 cd m−2 to >8000 cd m−2 and sub second temporal response in an electrolyte gated OLEDs, superior to LiF/Al and Al electrodes. These results are plausibly ascribed to the augmented electron injection with the concurrence of the upward shift of cathode WF induced by gating potential, leading to the improved electron/hole injection balance for efficient electroluminescence.