Recent development in the field of small molecular materials has led to great advances in the performance of vacuum-evaporated organic light-emitting diodes. However, as a significant class of phosphorescent emitters, ionic transition metal complexes are seldom sublimable due to the inherent ionic nature and low vapor pressure, restricting their applications in state-of-the-art devices fabricated by vacuum evaporation deposition. Here a facile, feasible and versatile strategy is shown to tune the volatility of ionic transition metal complexes through counter-ion control. By introducing counter-ions with large steric hindrance and well-dispersed charges, a series of evaporable ionic iridium complexes are developed, and efficient vapor-processed devices with a high brightness, small efficiency roll-off, and polychromic emission ranging from deep-blue to red-orange are achieved. Our findings unlock the utilization of ionic functional materials in vacuum-evaporated devices, and may open new doors for modern electronic materials technology.

In real devices, organic semiconductors are largely amorphous. Because accurate molecular packing in them cannot be obtained, the relationship between the molecular structure and the material properties can be difficult to understand. Nevertheless, knowing the charge transport processes is essential to material and device engineering. In amorphous organic semiconductors, charge transport is often apprehended as a hopping process that can be described using the Marcus or MillerAbrahams equations. The intrinsic disorder and frequently present traps have a great influence on the charge mobility. Carrier density, which would affect the effective density of states and create spacecharge perturbations, is also one important factor in the charge transport process. Herein, recent advances in the charge transport mechanism in amorphous organic semiconductors are summarized. The influences of disorder, carrier density, traps, and scatters are discussed in detail.

Organic light-emitting diodes (OLEDs) have attracted great attention because of their potential applications in full-color displays and solid-state lights. In the continual effort to search for ideal materials for OLEDs, small molecules with bipolar transporting character are extremely attractive as they offer the possibility to achieve efficient and stable OLEDs even in a simple single-layer device. In this Research News, we review the two design strategies of bipolar materials for OLEDs: molecules with or without donor-acceptor structures. The correlation between the experimental results and theoretical calculations of some of the materials is also discussed.

Organic light-emitting diodes (OLEDs) have attracted great attention because of their potential applications in full-color displays and solid-state lights. In the continual effort to search for ideal materials for OLEDs, small molecules with bipolar transporting character are extremely attractive as they offer the possibility to achieve efficient and stable OLEDs even in a simple single-layer device. In this Research News, we review the two design strategies of bipolar materials for OLEDs: molecules with or without donor-acceptor structures. The correlation between the experimental results and theoretical calculations of some of the materials is also discussed.

A pyridine-containing anthracene derivative, 9,10-bis(3-(pyridin-3-yl)phenyl)anthracene (DPyPA), which comprehensively outperforms the widely used electron-transport material (ETM), tris(8-quinolinolato) aluminum (Alq₃), is synthesized. DPyPA exhibits ambipolar transport properties, with both electron and hole mobilities of around 10⁻³ cm⁻² V⁻¹ s⁻¹; about two orders of magnitude higher than that of Alq₃. The nitrogen atom in the pyridine ring of DPyPA coordinates to lithium cations, which leads to efficient electron injection when LiF/Al is used as the cathode. Electrochemical measurements demonstrate that both the cations and anions of DPyPA are stable, which may improve the stability of devices based on DPyPA. Red-emitting, green-emitting, and blue-emitting fluorescent organic light emitting diodes with DPyPA as the ETM display lower turn-on voltages, higher efficiencies, and stronger luminance than the devices with Alq₃ as the ETM. The power efficiencies of the devices based on DPyPA are greater by 80–140% relative to those of the Alq₃-based devices. The improved performance of these devices is attributed to the increased carrier balance. In addition, the device employing DPyPA as the ETM possesses excellent stability: the half-life of the DPyPA-based device is 67 000 h—seven times longer than that of the Alq₃-based device—for an initial luminance of 5000 cd m⁻².

Using imidazole-type ancillary ligands, a new class of cationic iridium complexes (1–6) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′-bipyridine)-type ancillary ligands, imidazole-type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue-green to red emitting cationic iridium complexes are obtained. Single-layer light-emitting electrochemical cells (LECs) based on cationic iridium complexes show blue-green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A⁻¹ are achieved for the blue-green-emitting, yellow-emitting, and orange-emitting devices, respectively. By doping the red-emitting complex into the blue-green LEC, white LECs are realized, which give warm-white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color-rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A⁻¹, and 10 lm W⁻¹, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.

Two blue-emitting cationic iridium complexes with 2-(1H-pyrazol-1-yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)₂(pzpy)]PF₆ and [Ir(dfppy)₂(pzpy)]PF₆ (ppy is 2-phenylpyridine, dfppy is 2-(2,4-difluorophenyl) pyridine, and PF₆⁻ is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH₃CN solutions, [Ir(ppy)₂(pzpy)]PF₆ emits blue-green light (475 nm), which is blue-shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)₂(dtb-bpy)]PF₆ (dtb-bpy is 4,4′-di-tert-butyl-2,2′-bipyridine); [Ir(dfppy)₂(pzpy)]PF₆ with fluorine-substituted cyclometalated ligands shows further blue-shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand-centered ³π–π* states of the cyclometalated ligands (ppy or dfppy). Light-emitting electrochemical cells (LECs) based on [Ir(ppy)₂(pzpy)]PF₆ gave green-blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A⁻¹ when an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was added into the light-emitting layer. LECs based on [Ir(dfppy)₂(pzpy)]PF₆ gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs-based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron-donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.