1. The author developed a transparent perovskite light-emitting diode (PeLED) structure that has both low optical loss and excellent electrical injection characteristics;
2. The leading edge of the synchronized light pulse and the strong electrical pulse co-excited the PeLED and reduced the ASE threshold by 1.2 ± 0.2 μJ/cm², indicating that electrically injected carriers contribute to the optical gain;
3. Probing PeLED using 1 microsecond optical excitation, observing continuous wave ASE at a threshold of 3.8 kW/cm² to evaluate the feasibility of perovskite semiconductor optical amplifiers;
4. The electroluminescence brightness level produced by strong electric pulses is close to half of the irradiance produced by continuous wave optical pumping at the ASE threshold.
1. Challenges of electrically excited ASE in PeLEDs
Challenges of electrically excited amplified spontaneous emission (ASE) in perovskite light-emitting diodes (PeLEDs): This challenge arises from the complexity of electro-optical co-design of complete device stacks. Requirements for extreme current density and high internal quantum efficiency : PeLEDs need to operate at extreme current densities at the kiloamp level per square centimeter while maintaining high internal quantum efficiency (IQE). This imposes strict thermal management requirements on PeLEDs. Device stack design for high net mode gain : The device stack must be carefully designed to achieve high net mode gain. Highly conductive electrodes (such as aluminum or silver) with high extinction coefficients over a broad spectral range that overlap with the perovskite gain bandwidth. The electrodes cannot move far enough vertically or horizontally without sacrificing the conductivity of the device. Although the perovskite gain medium has a high refractive index (approximately 2.0-2.6), the guided modes leak into the contact layer, increasing the ASE threshold. The trade-off between device conductivity and optical losses is a critical factor in realizing thin-film semiconductor optical amplifiers and ultimately perovskite laser diodes.
2. Introduction to results
Metal halide perovskite materials serve as promising gain materials for thin film laser diodes. However, achieving electrically excited ASEs in PeLEDs is a prerequisite for realizing perovskite laser diodes, but this goal has been limited due to the conflicting requirements between high conductivity and high net mode gain of the device stack. The team of Robert Gehlhaar of the Belgian Microelectronics Research Center (IMEC) and Paul Heremans of the University of Leuven developed a transparent PeLED structure that combines low optical loss with excellent electrical injection properties. At 77K, using a 2.3 nanosecond light pulse, an ASE threshold of 9.1 μJ/cm² was achieved. At the same time, by submicrosecond electrical excitation of the same device at 77K, a current density greater than 3kA/cm² was achieved, and an irradiance value exceeding 40W/cm² was obtained. Notably, co-excitation of the PeLED using optical pulses, synchronized with the leading edge of strong electrical pulses, resulted in a reduction in the ASE threshold of 1.2 ± 0.2 μJ/cm², indicating that electrically injected carriers contribute to the optical gain. Furthermore, to evaluate the feasibility of perovskite semiconductor optical amplifiers, the authors used 1 microsecond optical excitation to probe PeLEDs and observed continuous wave ASE with a threshold of 3.8kW/cm². Finally, we demonstrate that such intense electrical pulses produce electroluminescence brightness levels close to half the irradiance produced by continuous wave optical pumping at the ASE threshold.
3. Results and Discussion
Point 1: Structure and optoelectronic properties of transparent PeLED
Figure 1. Structure and optoelectronic properties of transparent PeLED
1. Vertical transparent PeLED architecture : A vertically transparent PeLED architecture is proposed that can significantly reduce free carrier absorption losses while providing excellent electrical injection. A 20-nanometer-thick indium tin oxide (ITO) electrode is used and placed on both surfaces of the device. The active area is within the aluminum oxide (Al 2 O 3 ) insulation layer above the ITO anode. In addition, current-carrying metal traces were placed close to the thin ITO electrode but away from the recombination area to reduce the parasitic series resistance of the PeLED.
2. PeLED stack structure and materials : Cs 0.1 FA 0.9 PbI 2.855 Br 0.145 perovskite double cationic material is used as the basis of the PeLED stack, which includes a 3D structured perovskite layer, an electron transport double layer (PCBM and ZnMgO), and Solution-processed self-assembled monolayers. A precursor solution with the addition of methylamine hydrochloride (MACl) is used to regulate the growth of perovskite, producing grain sizes on the order of several hundred nanometers.
3. Device operation and performance : By exploring the photoelectric operation of 50µm diameter devices, PeLED shows high irradiance values (above 40 W/cm²) and belongs to Lambertian emission. At 77K, PeLED exhibits significantly superior performance, including an external quantum efficiency (EQE) of approximately 0.9% and an IQE of up to 23.2%.
4. Changes in the electroluminescence spectrum : Under different current densities, PeLED shows red shift and asymmetry characteristics of the emission spectrum, which may be related to the changes in the recombination area, the quantum confinement Stark effect and the occurrence of ASE.
5. Performance analysis : Despite a certain circuit delay, the performance of PeLED at 77K is significantly better than that of PeLED of the same size at room temperature. Under extreme conditions, PeLEDs exhibit spatially uniform luminescence that is maintained even at maximum current densities.
Point 2: Co-pump dynamics of submicrosecond voltage pulses and 2.3 nanosecond optical pulses
Figure 2. Co-pumping dynamics
In the context of electronic excitation gain, the ASE threshold carrier density (Nth) is approximated through optical excitation experiments to infer the threshold current density (Jth). Using the square root of the transient PL signal recorded below the ASE threshold at 77K to fit the carrier density rate equation, the Nth of the 50μm PeLED was estimated to be approximately 7.3×10^17cm−3 and the Jth to be approximately 3.0 kA cm−2 . Experimental results show that electrical injection can achieve a comparable number of carriers to the Nth value required for optical pumping to obtain ASE. However, under extreme electrical pumping, apart from slight spectral changes at higher current densities, electrical pumping alone has no obvious ASE characteristics.
The amplification dynamics of PeLEDs under combined photoelectric excitation were studied at comparable excitation levels with separate optical and electrical pumping. The experiments used fixed voltage pulses (VP) and synchronized 2.3 nanosecond light pulses (Iopt, ns) applied at different time points related to electrical biasing. Experimental results show that optical excitation at certain moments can significantly affect the luminescence spectrum of PeLED. Among them, optical excitation before VP induces a strong ASE signal. When optical excitation is superimposed recently after VP is fully established, a clear current injection contribution is observed. However, delaying optical excitation until the end of VP results in a significant reduction in peak signal intensity. With the optical excitation of VP ending approximately 250 ns later, a spectrum similar to the reference PLns is produced. Each spectrum can be decomposed into two (in the case of optical pumping only) or three (in the case of electrical pulse superposition) Gaussian distributions, representing the spontaneous emission of PL, ASE and EL respectively.
The spectral effects caused by the photoelectric co-excitation of PeLED were studied by subtracting the reference PLns spectrum from the three spectra after VP. When the PeLED is co-excited at timestamp '2', a narrow feature of the ASE bandwidth in the spectrum is observed, interpreted as electrically assisted ASE enhancement. As the optical pulse approaches the end of VP, there is a partial suppression of the ASE intensity. At this current density, PeLED continuously accumulates Joule heat, resulting in high-energy tail broadening and an obvious trough at the ASE wavelength. Finally, by maintaining the low voltage pulse frequency, the emission of the PeLED fully returns to the reference PLns on the next pulse period (i.e. at timestamp '1').
Under the condition of maintaining EL bias, PeLED was excited with gradually increasing light intensity of Iopt,ns. It was found that when Iopt,ns was less than 7.7 μJ cm −2 , PeLED did not show obvious ASE characteristics, while when Iopt,ns ≥ 7.7 At μJ cm −2 , as the input light intensity increases, the ASE peak is gradually enhanced. Further quantitative analysis of the photoelectric pumping effect shows that the impact of electrical injection on Ith is reduced by 1.2±0.2 μJ cm −2 or about 13%.
Point 3: Independent continuous wave optical and continuous wave photoelectric co-excitation
Figure 3. Independent continuous wave optical and continuous wave photoelectric co-excitation
The authors performed experiments on scaled PeLEDs pumped with CW light to achieve a better comparison with electrical excitation.
1. Experimental objectives and methods:
· Conduct experiments with CW light pumping to enable better comparison with electrical excitation.
· CW optical pumping has lower peak excitation intensity and increased Joule heating effect.
2. Experimental results:
· Conduct CW light pumping experiment on PeLED using 447 nm laser diode.
· The evolution of the emitted CW spectrum (PLCW) was observed under a 500 ns, 15 kW/cm² light pulse.
· In the absence of electrical excitation, the transient PLCW spectrum decomposes into SE(PL) and ASE Gaussian functions.
· The ASE Gaussian amplitude exceeds the SE(PL) Gaussian envelope after about 300 ns, accompanied by the narrowing of PLCW at different delay times.
3. CW ASE threshold:
· Quantitative analysis of CW ASE thresholds over the Iopt,CW range from 0.3 to 15 kW/cm².
· Monitoring the steady-state PLCW spectrum using a sustained excitation time of 1 µs established a reasonable photoexcited CW ASE threshold of 3.8 kW/cm².
· By fitting the Gaussian function to the PLCW spectrum, an obvious inflection point on the input-output curve of the ASE Gaussian amplitude was observed, marking the beginning of the ASE.
4. Co-excitation experiment:
· Co-excited PeLED (J=3.5 kA/cm²) by synchronizing 1 µs light pulse with 300 ns VP.
· The occurrence of ASE is observed under high Iopt,CW, but its intensity is significantly weakened compared with pure optical excitation at the same power density.
· When the optical power increases, both sets of spectra become systematically narrower, but the FWHM value of the co-excitation spectrum is still greater than that of only light excitation.
· At the end of VP, there is a drop in PLCW, but the signal quickly recovers within hundreds of nanoseconds after VP termination.
· At a maximum Iopt,CW of 15 kW/cm² and gradually increasing the current injection level, a sharper decrease in PLCW is observed, consistent with the severe but reversible Joule heating effect that PeLEDs undergo under strong electrical excitation.
Point 4: Irradiance levels of transparent PeLEDs under electrical pulses and CW optical bias
The obtained CW light excitation threshold is 3.8 kW/cm², which provides a more direct measurement of the equivalent Jth due to the similar width of the optical and electrical pulses, which is calculated to be approximately 4.6 kA/cm². Furthermore, by comparing the irradiance emitted by the PeLED at Iopt,CW = 3.8 kW/cm² with the EL emitted by the same PeLED at 3.5 kA/cm² (Figure 4), the authors observed that the EL reached the PLCW level at the ASE threshold. Nearly half. Based on these observations, it can be estimated that PeLED achieves approximately 50% Nth. Compared with the 13% lower ASE threshold under 2.3 ns photoexcitation obtained previously using similar electrical pulse conditions, this suggests that electrically exciting ASE requires a higher excitation density, mainly due to the more significant Joule heating effect.
Figure 4. Comparison of irradiance levels of transparent PeLED under 200 ns electrical pulse and 500 ns CW optical bias
4. Summary
Overall, the authors demonstrate a downsized transparent PeLED that combines the properties of reduced optical pump ASE threshold and high IQE at 77 K. Due to the presence of dual ITO electrodes, an ASE threshold as low as 9.1 µJ cm −2 for a functional 50 µm diameter PeLED was achieved under 2.3 ns optical pumping . In the target direction of realizing current-driven perovskite semiconductor optical amplifiers and laser diodes, some important achievements are reported: (1) Under simultaneous 2.3 ns optical and submicrosecond (3.5 kA cm −2 ) electrical co-excitation, Electrically enhanced optically pumped ASE is achieved in perovskite diodes, where electrical driving reduces the ASE threshold by an equivalent energy flux of 1.2 ± 0.2 µJ cm −2 or 13%; (2) In fully-contacted perovskite diodes , a CW ASE was achieved under 1 μs optical excitation with a threshold of 3.8 kW cm −2 ; and (3) the PeLED electrical injection produced an irradiance level that was approximately 50% of the optically pumped CW ASE threshold. These achievements were achieved in diodes with a three-dimensional morphology of the perovskite gain layer.
The authors believe that the optimized transparent PeLED, with faster modulation speed, can achieve CW electrically pumped ASE at 77 K. Specifically, the electrical rise time should be reduced below the time constant of Joule heating, demonstrating that Joule heating suppresses the ASE intensity after about 100 ns. A perovskite laser diode equipped with a high-quality factor optical cavity may support lower threshold lasing. However, a challenge lies in integrating this cavity correctly without degrading electrical performance. Finally, a perovskite current-driven laser would benefit from low operating voltages and reduced EQE attenuation at high current densities, while also requiring material innovations such as low-dimensional high-gain perovskite compositions that do not affect device conductivity.
5. References
Elkhouly, K., Goldberg, I., Zhang, X. et al. Electrically assisted amplified spontaneous emission in perovskite light-emitting diodes. Nat. Photon. (2024).
DOI:10.1038/s41566-023-01341-7
E-mail: info@chemborun.com
Tel: +86-574-87178138 
No. 1558, Jiangnan Road,, Ningbo, Zhejiang, China (Mainland)/31
