Perovskite enhanced efficiency of quantum dots LEDs and blue emission

Light-emitting devices based on semiconductors have a huge impact on modern technologies. The brightness and durability of light-emitting diodes make them ideal for displays, while semiconductor lasers have been used in everything from optical communications systems to compact disc players. But these applications have been limited by the lack of materials that can emit blue light efficiently.

Image: Creating blue LEDs is challenging, especially when they are blue perovskite LEDs.

A new study could simplify the process and increase efficiency. Credit: Pixabay

Towards the industrialization of perovskite LEDs Perovskite light-emitting diodes, is an important question around high quality and their stability remain.

PeLEDs were playing a vital role in the quantum dot as well as the blue colored region [1]. Perovskite quantum-dot-based light-emitting diodes (QLEDs) possess the features of high-quality lighting and displays1. Thus red and green QD-LEDs with a maximum external quantum efficiency (ηEQE) of 20 %, while the efficiency of blue emission is still lag because of the difficulties in synthesizing stable materials and high quantum efficiency in the films [2].

The scientist’s (L. Xu et al., [1]) were proposed a bilateral passivation strategy through the passivation of both top and bottom interfaces of QD film with various applied organic molecules (Figure 1), which has enhanced efficiency and stability of perovskite QLEDs.

Figure 1. (a) Diagram of the interaction between perovskite and diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1), (b) Calculate bond order of surface Pb atom with TSPO1, and others ligands. (c) PLQY of QD films with unilateral and bilateral passivation [1].

The interesting concepts behind the QLEDs shown in Figure 2:

The decreased defects were further verified by transient absorption (TA) spectra analysis and space charge-limited-current (SCLC) method.
The improved exciton recombination efficiency is reflected in the increased PLQY of QD film (an increase from 43 to 79%) and increased electro-optic conversion efficiency.
The passivated device achieves a maximum external quantum efficiency (EQE) of 18.7% and current efficiency of 75 cd A−1. Moreover, the operational lifetime of QLEDs is enhanced by 20-fold, reaching 15.8 h.

These findings highlight the importance of interface passivation for efficient and stable QD-based optoelectronic devices.

To reduce the interfacial defects of perovskite QD film, a layer of organic molecules between QD films and carrier transport layer (CTL) was evaporated. The typical passivation molecule used was phosphine oxide molecule of diphenylphosphine oxide-4-(triphenylsily)phenyl (TSPO1). The density functional theory (DFT) calculations were used to reveal the decreased defect traps and non-radiative recombination.

Figure 2. Energy level diagram for unilateral passivation [(a) on the top and (b) bottom side] and (c) bilateral passivation, (d) current efficiency distribution histogram, and (e) EQE statistics of devices passivated Quantum dots (QDs) with various organic molecules, (e) Insets are illustration of carrier-transporting process of pristine and passivated QD films. Unsteady current dynamic in before passivation and stable current after passivation [1].

Moreover, profiting from the strong interaction with perovskite and blocking between perovskite and CTL, bilateral-passivated molecules endow the films and LEDs with enhanced stability.

Updated effective attempts to obtain blue PeLEDs were generating the quantum-well structure via reducing-dimensional (quasi-2D and 0D) perovskites with different large cations [2]. Sargent et al. have used shorter iso-propylammonium (IPA) molecular to replace long ligands phenyl ethyl ammonium (PEA) and tune the quasi-2D PEA₂A_n−1Pb_nX_3n+1 perovskite composition with the desired n, and showed 1.5% EQE in sky blue [3].

Z. Chu et al., have reported the efficiency of blue light-emitting diodes (LEDs) based on quantum-confined bromide perovskite via phenylbutylammonium bromide as quasi-2D phases combined with an anti-solvent film deposition method, a peak EQE of up to 9.5% was achieved [1,4].

Figure 3. (a) Schematics showing the EA cation doping in the perovskite lattice to replace Cs+ in quasi-2D perovskite and (b) Commission Internationale de l’Eclairage (CIE) values of the EL spectra of perovskite LEDs [2].

They have constructed LEDs using PEA₂(Cs_1−xEA_xPbBr₃)₂PbBr₄ as active layer for increase the work function forming good alignment for hole injection and electron blocking.

Here, a large cation CH₃CH₂NH₂⁺ is added in PEA₂(Cs_1−xEA_xPbBr₃)₂PbBr₄ perovskite to decrease the Pb–Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has successfully replaced Cs⁺ cations to form PEA₂(Cs_1−xEA_xPbBr₃)₂PbBr₄. This method modulates the photoluminescence from the green region (508 nm) into the blue region(466 nm), and over 70% of photoluminescence quantum yield in blue was obtained.

The scientists have introduced large cation CH₃CH₂NH₂⁺ (EA) into the Cs⁺ site in PEA₂(Cs_1−xEA_xPbBr₃)₂PbBr₄incorporation into three-dimensional perovskite lattice, and tuned the emission from green (508 nm) into blue (466 nm).

A submission of the collection of exciting perovskites results from nature publications (August 2020).

The role of QLED, the study of device demonstrates that the defects on the interface between the QD films and charge transporting layers are detrimental for devices by bilateral passivation.
The blue emission of perovskite LEDs with EA cation incorporation into three-dimensional perovskite lattice. The emission has been turned from green into blue, and showed high PLQY (>70%). Also, the emission spectra are stable under the photo and thermal stress. With tailoring the EA composition, an efficient of PeLEDs with 12.1% EQE in 488 nm emission was achieved [5].

Hence, this concept can be expected to be an open avenue for QLEDS, QD-based optoelectronic devices, including solar cells, photodetectors, and full-color display using PeLEDs.

References

L. Xu et al., A bilateral interfacial passivation strategy promoting efficiency and stability of perovskite quantum dot light-emitting diodes. Nature communications 11, 3902 (2020). https://doi.org/10.1038/s41467-020-17633-3.
Z. Chu et al., Large cation ethylammonium incorporated perovskite for efficient and spectra stable blue light-emitting diodes, Nature communications 11, 4165 (2020). https://doi.org/10.1038/s41467-020-17943-6.
S. Kumar et al., Efficient blue electroluminescence using quantum-confined two-dimensional perovskites. ACS Nano 10, 9720–9729 (2016).
J. Xing et al., Color-stable highly luminescent sky-blue perovskite lightemitting diodes. Nat. Commun. 9, 3541 (2018).
Y. Liu, et al., Efficient blue light-emitting diodes based on quantum-confined bromide perovskite nanostructures. Nat. Photonics 13, 760 (2019).

Blog Written By

Dr. K. RAJKUMAR

Central University of Tamilnadu

Thiruvarur, Tamilnadu, India

Editors

Dr. A. S. Ganeshraja

Dr. S. Chandrasekar

Reviewer

Dr. Y. Sasikumar

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