First author : Hyeon-Dong Lee, Seung-Je Woo, Sungjin Kim.
Corresponding author: Tae-Woo Lee
Correspondence unit: Seoul National University, South Korea
Research highlights
1. Proposed a hybrid tandem structure that superimposes nanocrystalline perovskite light-emitting diodes (PeLED) and organic light-emitting diodes (OLED) to achieve vivid and efficient tandem display;
2. Through optical simulation, the key optical microcavity structure, namely the h-tandem valley, was discovered, which enables the hybrid tandem PeLED to achieve narrow-bandwidth light emission;
3. The central structure of the hybrid series structure valley (valley center series) has near-perfect charge balance and the best microcavity effect;
4. The performance of hybrid tandem PeLEDs, including external quantum efficiency up to 37.0%, narrow half-maximum width (27.3nm), higher color purity and fast switching response compared with organic light-emitting diodes;
1. The importance and difficulties of connecting LEDs in series
The series structure has wide application in optoelectronic devices, especially its advantages in LEDs. By using tandem LEDs, especially those using a charge generation layer (CGL), luminous efficiency can be improved, voltage stress can be reduced, and device life can be extended. By adjusting the microcavity structure of series-connected LEDs, the emission spectrum of stacked LEDs can be narrowed. In this case, tandem structures must be considered in the commercial development of PeLEDs, and the most promising strategy currently is to superimpose thin, optically transparent solution-processed colloidal nanocrystalline PeLED units with vacuum-deposited OLED units. However, simply combining narrowband PeLEDs and broadband OLEDs with different current density-voltage-brightness (J–V–L) and electroluminescence (EL) characteristics does not guarantee narrowband emission and high efficiency like traditional tandem OLEDs. This simple combination of two LEDs with different emission characteristics can lead to a broadening and shift of the emission spectrum, depending on the driving voltage, because of the charge imbalance present.
2. Introduction to results
The study identified PeLEDs as a promising new light source in the display field. In order to commercialize PeLEDs, Tae-Woo Lee's team at Seoul National University in South Korea proposed a development roadmap using a tandem device structure, specifically superimposing thin nanocrystalline PeLED units with organic light-emitting diode units to achieve vivid and efficient tandem displays. However, simply combining light-emitting diodes with different characteristics cannot guarantee narrow-band emission and high efficiency, and may lead to emission spectrum expansion and charge imbalance. Through optical simulations, the study discovered a key optical microcavity structure, called the hybrid tandem structural valley, which enables hybrid tandem PeLEDs to achieve narrow-bandwidth light emission. The central structure of the valley exhibits near-perfect charge balance and optimal microcavity effect. Finally, the hybrid tandem PeLED achieved an external quantum efficiency of up to 37.0%, a narrow half-peak width (27.3nm), compared with 64.5nm of organic light-emitting diodes, and a fast switching response. These findings provide new strategies to overcome the limitations of nanocrystal-based PeLEDs and provide valuable optical and electrical guidance for integrating different types of light-emitting devices in practical display applications.
3. Results and Discussion
Point 1: Key optical microcavity structure of hybrid tandem PeLEDs
Figure 1. Concept of hybrid tandem PeLED and key optical microcavity structure
1. Material selection: Using optically transparent, nanocrystalline PeLEDs as superposition units, compared with traditional bulk polycrystalline PeLEDs hundreds of nanometers thick, helps avoid strong absorption of the luminous intensity of the top unit and makes it easier to control series devices charge balance in;
2. Optical simulation: Through optical simulation, the researchers discovered the existence of h-tandem valley. By adjusting the matching of the EL emission spectra of PeLED and OLED units, they successfully achieved a narrow emission spectrum below 28 nm;
3. Device preparation: Hybrid tandem PeLEDs located inside and outside the h-tandem valley were prepared, in which colloidal PNC was used as the PeLED unit. In a valley-centre tandem device, an EL emission spectrum as narrow as 27.3 nm is achieved;
4. Performance improvement: The valley center tandem structure exhibits excellent device efficiency, including an external quantum efficiency (EQE) of up to 37.0% and a luminous efficiency (CE) of 151.8 cd A−1, which is higher than the efficiency of a single PeLED and OLED unit. and;
5. Energy efficiency improvement: The performance improvement of hybrid tandem PeLED in terms of power efficiency and other aspects, especially in the case of narrow emission spectrum (27.3 nm), makes it very suitable for high-efficiency and vivid next-generation display technology;
6. Stability and application: Hybrid tandem PeLED shows high operational stability, has an expected lifespan of up to 5,596 hours, and has been successfully applied to flexible large-area displays, demonstrating its potential for practical applications.
Figure 2. Device characteristics of PeLED, OLED and hybrid tandem PeLED
Point 2: Charge balancing of hybrid tandem PeLEDs
Figure 3. Changing the ETLs of the bottom and top units to achieve electrical optimization of series devices
1. By simply stacking the same type of light-emitting devices, conventional tandem OLEDs with similar J–V–L characteristics can almost double the device efficiency without significantly modifying the device structure.
2. In hybrid tandem PeLEDs, since the component devices have different resistances and J–V–L characteristics, the current flow from the CGL to each light-emitting unit may be unbalanced, which may result in loss of injected charge, drift of the emission peak, or Broadening of the emission spectrum caused by unbalanced brightness of each unit.
3. In order to achieve high device efficiency, various devices with different ETLs were used, and single PeLEDs and single OLEDs were simultaneously fabricated to compare the current density of the bottom and top cells in the series devices.
4. Introducing BCP:Li as CGL and by replacing the TPBi-BCP:Li interface with ZADN-BCP:Li, the charge generation and electron injection efficiency were successfully improved because the electron injection barrier was lowered.
5. In the in-valley series structure, the ETLs of the bottom and top cells are replaced, resulting in a perfect match of the current density–voltage curves of the bottom and top cells at a driving voltage of 2.6–3.2 V, achieving maximum EQEs.
6. Mid-valley tandem PeLED adopts ZADN as all ETLs, which exhibits stable EL peak position during operation, compared to unbalanced tandem 1–3, which exhibits a shift in the EL spectrum due to the charge imbalance of each unit. Shift and widen.
7. The narrowest FWHM of the EL emission of the series in the valley is 27.3 nm, while the values of the unbalanced series 1–3 are 31.8, 28.4 and 42.2 nm respectively, indicating that the EL emission of the series in the valley is more stable and concentrated.
8. The response time of the mid-valley series connection is 48 µs, which is significantly reduced compared to the 169 µs of the unbalanced series connection 1, indicating that the introduction of the mid-valley structure and its optimized thickness improves charge carrier injection and charge balance, achieving fast response and stable brightness. drive.
9. By optical simulation of the tandem device, assuming perfect charge balance, the calculated EQEs are 35.2%, 36.4%, 35.8% and 37.1% in the unbalanced tandem 1–3 and the valley-in-trough tandem, respectively, and the differences between them Not big.
10. The ratio of the measured EQE to the simulated value of the valley-series connection is 99.7%, indicating that the valley-series connection achieves almost perfect double-unit charge balance.
11. In contrast, the ratios for unbalanced series 1–3 are 73%, 73%, and 59%, respectively, indicating that charge imbalance results in substantial efficiency losses.
12. The EQE of the tandem device can be derived from the formula, where ξCGE is the number of electrons (holes) generated at CGL divided by the number of electrons (holes) injected from the electrode; γCB is the charge balance factor, that is, the number of excitons generated divided Take the number of injected electrons (holes); χS/T is the radiative exciton ratio, which is 1 for both phosphorescent OLED and PeLED; ΦPLQY is the photoluminescence quantum yield of EML; etaout is the output efficiency.
13. By performing EL spectrum simulations on the valley-series PeLED and OLED units assuming perfect charge balance, it was confirmed that the valley-series connection achieves almost perfect charge balance.
14. Through bias-dependent device time-resolved PL experiments and EL spectral analysis in the voltage range of 5.6–7.7 V, it was confirmed that the charge balance of the series device in the valley and the contribution of the PeLED and OLED units to the total EL emission are almost at this remains unchanged within the voltage range.
15. Since the charge balance of each unit in the valley-series device is almost the same, the product of the charge generation efficiency and the charge balance factor can be calculated, and the results show that there is almost no loss of injected charge in the valley-series device.
Point 3: Comprehensive electrical and optical analysis of hybrid tandem PeLEDs
Figure 4. Optical simulation and mid-valley tandem and thick tandem device characteristics
1. The optically achievable efficiency is determined by the microcavity structure of the device.
2. Changing the thickness of the hole transport layer and ETL will affect the charge balance, so the maximum output coupling device structure obtained from optical simulations does not always lead to the highest device efficiency.
3. Optical simulations were performed on the thickness of ZADN and TAPC in the top unit, and the simulation results were compared with the experimental EL characteristics of a tandem in the valley and a thick tandem device with maximum output coupling efficiency.
4. The theoretical EQE of the mid-valley series connection is derived from optical simulations to be 37.1%.
5. The theoretical EQE of a thick series connection derived from optical simulations only, without considering charge balance, is 50.8%. However, the experimental EQE of the actually fabricated thick tandem device is only 29.4%, which is much lower than the optical simulation value of thick tandem and the experimental EQE of mid-valley tandem.
6. Thick series shows charge imbalance leading to large electrical losses, while mid-valley series shows almost no electrical losses due to almost perfect charge balance (nearly 100%).
7. The emission spectrum of the thick tandem is broader, with a FWHM of 50.5 nm, while the FWHM of the valley tandem is 27.3 nm.
8. Due to the spectrum shaping effect of the microcavity, the simulated EL spectrum of the thick tandem shows a wider FWHM (38.3 nm).
9. The thickness of the transmission layer of the bottom unit and the thickness of ZADN also affect the microcavity structure of the tandem device. The thickness of the ZADN ETL is critical to the charge generation efficiency and charge balance factor.
10. Optical simulation was performed on the thickness of ZADN in the bottom unit, and it was found that when the ZADN thickness is 20 nm, the microcavity effect of the series connection in the valley is optimal, with the highest optical calculation EQE and the highest charge balance factor.
11. The actual fabricated mid-valley tandem device achieved the highest EQE when the ZADN thickness was 20 nm.
12. The increase in ZADN thickness from 5 to 20 nm leads to an increase in the charge balance factor and output coupling efficiency, but an excessive ZADN thickness (40 nm) leads to a decrease in EQE.
13. Comprehensive electrical and optical analysis shows that the mid-valley series PeLED structure is optimal for achieving the highest EQE and narrow-band emission.
Figure 5. Optical simulation and mid-valley series and thick series device characteristics
4. Summary
Overall, the authors developed an optical and electrical design strategy to achieve vivid and efficient hybrid tandem PeLEDs by combining thin and transparent colloidal nanocrystalline PeLED units with OLED units that provide a more favorable display manufacturing process. . By designing optical valleys for optimal microcavity effects and charge balance, current levels are precisely matched, achieving near-perfect charge balance in the series device. Finally, a hybrid tandem PeLED with high efficiency (CE = 151.8 cd A−1; EQE = 37.0%) and narrowband emission with FWHM of 27.3 nm was achieved. Furthermore, optical simulations reveal how the device structure affects the microcavity structure and charge balance in tandem devices, resulting in different EL spectra and EQE, and ultimately verify that hybrid tandem PeLEDs have well-designed electrical and optical devices in hybrid tandem valleys structure. The experimental results and comprehensive electro-optical studies provide guidance and insights for further development of hybrid tandem displays containing different types of light-emitting units, leading to next-generation commercial displays with high efficiency and high color reproducibility.
5. References
Lee, HD., Woo, SJ., Kim, S. et al. Valley-centre tandem perovskite light-emitting diodes. Nat. Nanotechnol. (2024).
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