Enhancing Red QLED Efficiency with NiOx Surface Modification

By Samudrapom DamJun 17 2024Reviewed by Susha Cheriyedath, M.Sc.

In an article recently published in the journal Molecules, researchers proposed a novel approach to fabricating efficiency-improved nickel oxide (NiOx)–based red quantum-dot light-emitting diodes (QLEDs) with interfacial modification.

Challenges of Fabricating QLEDs

QLEDs have emerged as a suitable candidate for developing flexible display technologies owing to their tunable emission colors, narrow spectral linewidth, superior luminescence efficiency, and efficient fabrication using all-solution-based methods. However, the challenges of operating and fabricating QLEDs on flexible substrates remain due to the lack of low-temperature processable and stable charge-injection/transporting layers with aligned energy levels.

Although poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) is often used in hole-injection/transporting layers due to its superior conductivity and high work function, its acidic properties and susceptibility to moisture absorption can degrade the underlying indium tin oxide (ITO) electrode, hindering efficient and sustained operation of the device.

Role of NiO_x

Recently, NiO_x, a non-stoichiometric transition metal oxide, has gained significant attention as a suitable hole-injection/transporting layer in perovskite solar cells and QLEDs. The NiO_x layer preparation approaches primarily include solution-combustion and vacuum-based techniques.

However, both approaches have several drawbacks, including the need for high annealing temperatures and complex and costly equipment, which present challenges in the fabrication of flexible QLEDs. Alternatively, pre-crystallized NiO_x nanoparticles can be directly used for a uniform thin film formation at low annealing temperatures.

Additionally, these nanoparticles are also compatible with flexible substrates. However, QLEDs utilizing NiO_x nanoparticles display unsatisfactory electroluminescence performance when compared with PEDOT: PSS-based QLEDs owing to poor hole injection and transportation ability.

The Study

In this work, researchers used NiO_x nanoparticles compatible with flexible substrates as the hole-injection layer and a low-temperature/100 °C annealing method for film preparation. They introduced a self-assembled dipole modifier, referred to as 4-(trifluoromethyl)benzoic acid (4-CF3-BA), on the NiO_x nanoparticle surface to modify the surface.

During the NiO_x film preparation, the NiO_x nanoparticles' ethanol solution was initially diluted to 0.15 wt%. Then, the solution was spin-coated on the patterned ITO glass substrates for 30 seconds at 3000 rpm after filtration through a 0.45 µm polyvinylidene fluoride membrane. This was followed by annealing for 10 minutes at 100 °C. The NiO_x film was treated using UV/ozone for 30 minutes.

Subsequently, 5 mg of 4-CF3-BA was dissolved in 2 mL of anhydrous ethanol to prepare the precursor solution. 300 µL of the precursor solution was dropped on the NiO_x-coated substrate and immersed for 2 minutes after filtration of the solution through a 0.45 µm polyvinylidene fluoride membrane.

This was followed by spin-coating for 20 seconds at 5000 rpm and annealing for 5 minutes at 100 °C. Finally, after cooling, anhydrous ethanol was used to rinse the film during spin-coating for 20 seconds at 5000 rpm to remove molecules that were not bonded to NiO_x, which was followed by annealing for 5 minutes at 100 °C.

Researchers then fabricated the QLEDs and the hole–only device and characterized the thin film and device using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FTIR) spectroscopy.

Importance of this Work

Both FTIR and contact angle tests confirmed the modification of the NiO_x nanoparticle surface and passivation of the defects using the 4-CF3-BA, which validated the successful attachment of dipole molecules. The alteration of NiO_x nanoparticles' surface electronic states improved the carrier balance by decreasing the hole injection barrier and prevented exciton quenching by passivating defects in the film.

The introduction of dipole molecules through adsorption treatment significantly altered the electronic characteristics and wettability of NiO_x nanoparticles, resulting in the formation of NiO(OH) at the interface and a shift in vacuum level. Specifically, the interaction of dipoles shifted the vacuum level of NiO_x, reduced hole transport barriers, and promoted efficient light emission in the device. XPS characterization of NiO_x films before and after modification displayed a substantial increase in NiO(OH) and Ni³⁺ components, indicating improved film conductivity.

Overall, the NiO_x-based red QLEDs with interfacial modification /4-CF3-BA-modified NiO_x utilized as a hole injection layer in QLEDs demonstrated a maximum current efficiency of 16.1 cd/A and a peak external quantum efficiency of 10.3 %, which represented approximately a 200 % increase compared to control devices. Moreover, the efficiency roll-off with increasing luminance was effectively reduced.

To summarize, the low annealing temperatures and mild fabrication requirements in the proposed approach indicate the potential applications of dipole molecule-modified NiO_x nanoparticles in flexible optoelectronic devices. The adoption of cost-effective and mild fabrication conditions, coupled with high efficiency after optimization, can offer promising prospects for the extensive application of red QLEDs in emerging fields like augmented reality and virtual reality, and large-area flexible optoelectronic devices.

Journal Reference

Xu, S., et al. (2024). Flexible Substrate–Compatible and Efficiency–Improved Quantum–Dot Light–Emitting Diodes with Reduced Annealing Temperature of NiO_x Hole–Injecting Layer. Molecules, 29(12), 2828. https://doi.org/10.3390/molecules29122828, https://www.mdpi.com/1420-3049/29/12/2828

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