Solid-state light-emitting electrochemical cells (LECs) based on ionic transition metal complexes (iTMCs) have several advantages such as high efficiency, low operation voltage and simple device structure. To improve the device efficiency of iTMC-based LECs for practical applications, improving the carrier balance to achieve a centered recombination zone would be an important issue. In this work, incorporating a hole-transport material (HTM) into the emissive layer of iTMC-based LECs is shown to improve device performance. When mixed with an HTM (12%), the LECs based on a Ru complex exhibit 1.9× and 1.5× enhancement in peak light output and peak external quantum efficiency (EQE) as compared to neat-film devices. Furthermore, over 2× enhancement in stabilized EQE can be achieved in LECs mixed with an HTM. It is attributed to that a more centered recombination zone in LECs mixed with an HTM is beneficial in reducing exciton quenching in the recombination zone approaching extended doped layers. Estimating the temporal evolution of the recombination zone in the LECs mixed with an HTM by employing the microcavity effect is demonstrated to confirm the physical origin for improved device performance. These results reveal that incorporating of an HTM in the emissive layer of LECs based on an iTMC is a feasible way to improve carrier balance and thus enhance light output and device efficiency.

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High light-quality and low color temperature are crucial to justify a comfortable healthy illumination. Wet-process enables electronic devices cost-effective fabrication feasibility. We present herein low color temperature, blue-emission hazards free organic light emitting diodes (OLEDs) with very-high light-quality indices, that with a single emissive layer spin-coated with multiple blackbody-radiation complementary dyes, namely deep-red, yellow, green and sky-blue. Specifically, an OLED with a 1,854 K color temperature showed a color rendering index (CRI) of 90 and a spectrum resemblance index (SRI) of 88, whose melatonin suppression sensitivity is only 3% relative to a reference blue light of 480 nm. Its maximum retina permissible exposure limit is 3,454 seconds at 100 lx, 11, 10 and 6 times longer and safer than the counterparts of compact fluorescent lamp (5,920 K), light emitting diode (5,500 K) and OLED (5,000 K). By incorporating a co-host, tris(4-carbazoyl-9-ylphenyl)amine (TCTA), the resulting OLED showed a current efficiency of 24.9 cd/A and an external quantum efficiency of 24.5% at 100 cd/m2. It exhibited ultra-high light quality with a CRI of 93 and an SRI of 92. These prove blue-hazard free, high quality and healthy OLED to be fabrication feasible via the easy-to-apply wet-processed single emissive layer with multiple emitters.

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A wet- and dry-process feasible host material is crucial to realize, respectively, low cost roll-to-roll fabrication of large area and high performance organic light-emitting diodes (OLEDs) with precise deposition of organic layers.

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