Introduction
Organic-inorganic halide perovskites (OIHPs) have emerged as a revolutionary class of optoelectronic materials, demonstrating exceptional performance in solar cells, photodetectors, lasers, and notably, light-emitting diodes (PeLEDs). Their high color purity, tunable bandgap, and low-cost solution processability make them ideal candidates for next-generation displays and solid-state lighting. Recent breakthroughs have achieved power conversion efficiencies over 25% in perovskite solar cells, while PeLEDs continue to advance toward commercial viability.
Despite these advantages, the performance of PeLEDs is often limited by poor morphological quality of the perovskite thin films. During spin-coating, uncontrolled crystallization frequently results in non-uniform coverage, pinholes, and large grain boundaries—leading to device inefficiency and short operational lifetimes. To overcome this, researchers have explored various strategies including solvent engineering, additive engineering, and interfacial modification.
This study focuses on enhancing both charge injection and film morphology in green-emitting PeLEDs by introducing two key innovations:
- A PSS-modified PEDOT:PSS hole-injection layer (HIL) to improve energy-level alignment.
- A PEO-doped MAPbBr₃ composite emission layer to achieve dense, pinhole-free films with near-complete surface coverage.
Together, these modifications significantly enhance device efficiency, stability, and reproducibility.
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Optimizing Charge Injection with PSS-Modified HIL
The efficiency of any LED hinges on balanced carrier injection. In conventional PeLEDs, the mismatch between the work function (WF) of the standard PEDOT:PSS HIL (~4.99 eV) and the valence band of MAPbBr₃ (~–5.7 eV) creates a significant hole injection barrier. This imbalance suppresses hole transport and leads to inefficient radiative recombination.
To address this, we modified the PEDOT:PSS layer by incorporating additional poly(4-styrenesulfonate) (PSS) at varying volume ratios—1:100, 1:50, and 1:20 (PSS:PEDOT:PSS). UV photoelectron spectroscopy (UPS) revealed that PSS doping increased the WF of PEDOT:PSS from 4.99 eV to 5.18 eV, effectively reducing the energy barrier at the HIL/perovskite interface.
Device measurements confirmed this improvement:
- All PSS-modified devices showed higher current density than the pristine counterpart.
- The 1:20 ratio yielded optimal performance with a maximum current efficiency of 0.69 cd A⁻¹.
- Enhanced hole injection led to improved luminance and reduced turn-on voltage.
However, excessive PSS content did not further improve performance due to its inherently low electrical conductivity—a known trade-off in such systems.
Enhancing Film Morphology via PEO Doping
Even with efficient charge injection, poor perovskite film quality can severely limit device output. Undoped MAPbBr₃ films exhibited large crystallites (~200 nm) and low surface coverage (~70%), increasing the risk of current leakage and non-radiative recombination.
By blending poly(ethylene oxide) (PEO) into the perovskite precursor solution at different volume ratios (1:0.75, 1:1, 1:1.25 relative to MAPbBr₃), we achieved remarkable improvements in film uniformity:
- At 1:1 ratio, grain size reduced to ~100 nm with ~100% surface coverage.
- XRD analysis showed broader (100) diffraction peaks, confirming smaller crystallite size and suppressed crystal growth along that plane.
- SEM imaging verified homogeneous dispersion and dense packing without cracks or voids.
Interestingly, at a 1:1.25 ratio, larger cubic crystals reappeared due to Ostwald ripening—a thermodynamic process where smaller particles dissolve and redeposit onto larger ones. This underscores the importance of precise compositional control.
Photoluminescence (PL) spectra revealed that PEO doping had no adverse effect on emission characteristics—both pristine and composite films emitted at 539 nm, maintaining excellent color purity.
High-Performance PeLEDs: Efficiency and Stability Gains
Integrating both modifications—PSS-doped HIL and PEO-composite emissive layer—we fabricated high-efficiency green PeLEDs. Key results include:
- Maximum luminance: 2476 cd m⁻² (achieved at 1:1.25 PEO ratio)
- Peak current efficiency: 7.6 cd A⁻¹, representing an 18.5× improvement over undoped devices
- Turn-on voltage: As low as 3.2 V
- Electroluminescence peak: Stable at 524 nm, indicating no spectral shift from PEO incorporation
Furthermore, device stability improved significantly:
Half-lifetime (time for brightness to drop to 50% at 100 cd m⁻²) increased from 15 s (undoped) to:
- 125 s (1:0.75)
- 275 s (1:1)
- 203 s (1:1.25)
This enhancement is attributed to:
- Denser, more uniform films reducing defect-mediated degradation
- PEO passivating surface traps on perovskite grains
- Better charge balance minimizing Joule heating
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Frequently Asked Questions
What are perovskite light-emitting diodes (PeLEDs)?
PeLEDs are a type of light-emitting diode that uses organic-inorganic halide perovskites as the emissive layer. They offer high color purity, tunable emission wavelengths, and compatibility with low-cost solution-based fabrication methods like spin-coating.
Why is surface coverage important in PeLEDs?
Low surface coverage leads to pinholes and exposed substrate areas, which can cause local current concentration, short circuits, and non-radiative recombination. High coverage (~100%) ensures uniform electroluminescence and improves device yield and longevity.
How does PEO improve perovskite film quality?
PEO acts as a crystallization modulator during film formation. It restricts uncontrolled grain growth, promotes homogeneous nucleation, and enhances film continuity. Additionally, it passivates surface defects, improving both efficiency and stability.
What role does PSS play in the hole-injection layer?
PSS increases the work function of PEDOT:PSS through vertical phase segregation, creating a more favorable energy alignment with the perovskite valence band. This reduces the hole injection barrier and enhances overall charge balance in the device.
Can these modifications be applied to other optoelectronic devices?
Yes. The strategies of interfacial energy tuning and additive-induced morphology control are broadly applicable to perovskite solar cells, photodetectors, and lasers—where efficient charge transport and high-quality thin films are equally critical.
Are green PeLEDs suitable for commercial displays?
Green-emitting PeLEDs are among the most advanced in terms of efficiency and stability. With further improvements in operational lifetime and blue/red counterparts, they hold strong potential for use in future high-dynamic-range displays and micro-LED applications.
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Conclusion
We have demonstrated a synergistic approach to fabricating high-performance green PeLEDs by combining a PSS-modified PEDOT:PSS hole-injection layer and a PEO-doped MAPbBr₃ composite emissive layer. The former enhances hole injection by aligning energy levels, while the latter enables the formation of dense, uniform perovskite films with near-perfect surface coverage.
These modifications collectively resulted in a maximum current efficiency of 7.6 cd A⁻¹ and luminance of 2476 cd m⁻², with an 18.5-fold improvement over reference devices. Operational stability also saw dramatic gains, with half-lifetimes extending up to 275 seconds under constant driving conditions.
This work highlights the importance of simultaneous interfacial and bulk engineering in achieving high-efficiency PeLEDs. The methodologies presented here—particularly polymer-assisted crystallization control and work function tuning—are transferable to other perovskite-based optoelectronic devices, offering a scalable pathway toward commercially viable perovskite technologies.
Core Keywords: perovskite, light emitting diode, composite film, charge injection, film morphology, energy alignment, solution processing