Advanced Science(IF:20.7)Yu-Jung (Yuri) Lu & Chu-Chen Chueh Novel Strategy to Mitigate Efficiency Degradation in Quasi-2D Perovskite LEDs
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As the urgency of global energy transition continues to increase, solar energy has become an important alternative energy source.
Among various available technologies, perovskite photovoltaic devices, especially quasi-2D perovskite light-emitting diodes (PeLEDs), have gained significant attention in the scientific community.
Notably, quasi-2D perovskite materials, as a subclass of PeLEDs, exhibit excellent optical properties due to quantum confinement effects and efficient energy transfer between different n phases.
However, these promising materials often suffer from poor conductivity, inadequate charge carrier injection, and significant efficiency degradation at high current densities, limiting their potential in solar energy conversion.
Recently, a team composed of renowned researchers, including Associate Research Fellow Yu-Jung Lu from the Academia Sinica and Associate Professor Ju-Chun Chou from National Taiwan University’s Department of Chemical Engineering, published a study aiming to improve the performance of quasi-2D perovskite light-emitting diodes (PeLEDs). The team focused on enhancing brightness, reducing trap density, and mitigating efficiency degradation at high current densities.
The research team proposed an innovative approach to enhance the performance of these quasi-2D PeLEDs, primarily focusing on improving brightness, reducing trap density, and mitigating efficiency degradation.
Understanding PeLEDs and their limitations
The core of this technology lies in the unique characteristics of perovskite materials.
These materials typically consist of hybrid organic-inorganic lead or tin halides and exhibit enticing features such as excellent light absorption, charge carrier mobility, and emission properties for photovoltaic applications. However, when these materials are employed in the quasi-2D configuration of PeLEDs, their performance is constrained by a range of limiting factors. Quasi-2D perovskite materials, despite having good stability, tunable bandgaps, and high photoluminescence quantum yields, suffer from reduced conductivity and diminished charge carrier injection, leading to significant efficiency degradation at increased current densities, thus diminishing brightness and overall device performance.
Resolving efficiency degradation in quasi-2D PeLEDs
This study explored a novel approach to alleviate these drawbacks by introducing a thin layer of conductive quaternary ammonium oxide at the interface between the perovskite and the electron transport layer.
Surprisingly, this innovative approach did not enhance energy transfer between different quasi-2D phases within the perovskite film. Instead, it significantly improved the electronic properties at the perovskite interface, introducing this additional layer to address two critical challenges. Firstly, it deactivated surface defects in the perovskite film. Secondly, it facilitated electron injection and restricted hole leakage at the interface.
As a result, the optimized pure-Cs quasi-2D devices exhibited unprecedented brightness of over 70,000 cd m−2, maximum external quantum efficiency (EQE) above 10%, and significantly reduced efficiency degradation at high bias voltages. These data demonstrated substantial improvements compared to the control devices, highlighting the effectiveness of the proposed technique.
Experimental methods and materials
The study explored the potential advantages of introducing the conductive quaternary ammonium oxides, PPT and PPF, into quasi-2D perovskites to reduce efficiency degradation in optoelectronic devices. The focus was on applying an additional layer of PPT or PPF onto the perovskite film before depositing the electron transport layer (ETL). This process was believed to enhance charge carrier injection and deactivate surface defects, thereby suppressing non-radiative recombination.
Preliminary investigations of the modified perovskite films did not observe significant changes in crystallinity or phase distribution. X-ray diffraction (XRD) and UV-Vis absorption spectroscopy confirmed that the modifications had no impact
on phase distribution and film quality. Additionally, the application of PPT and PPF did not significantly alter the morphology of the films, as confirmed by scanning electron microscopy (SEM).
To understand the influence of these modifications on charge carrier dynamics, steady-state photoluminescence (PL) spectroscopy and time-resolved photoluminescence (TRPL) measurements were conducted. Noticeable PL quenching was observed in both modified films, indicating charge carrier transfer between the perovskite layer and the PPT/PPF layer. Moreover, the average charge carrier lifetime increased in both modified films, suggesting effective deactivation.
As a complementary investigation of the interactions between these modifications and perovskite, nuclear magnetic resonance (NMR), electrostatic potential (ESP) maps, and X-ray photoelectron spectroscopy (XPS) were employed. The test data confirmed the successful spin-coating of the PPT/PPF layer onto the perovskite film during the post-processing. The results indicated that the P=O groups in the quaternary ammonium oxides successfully interacted synergistically with surface defects and vacancies, leading to significant deactivation effects.
Following the exciting findings, PeLEDs were fabricated based on the modified perovskite films and compared with control devices. Both modifications with PPT and PPF significantly improved performance, preventing hole leakage from the perovskite layer to the ETL and promoting electron transport. The modified devices exhibited over twice the brightness of the control devices and significantly reduced efficiency degradation at high voltage. These results underscore the potential of using PPT and PPF quaternary ammonium oxides in pure-Cs quasi-2D perovskite PeLEDs.
In conclusion, the introduction of conductive quaternary ammonium oxides for deactivating quasi-2D perovskite materials offers a promising strategy for enhancing optoelectronic device performance. Further research in optimizing the application of these materials in future device structures will be valuable.
In this study, the research team utilized the Enlitech LQ100X-PL photoluminescence and photoluminescence quantum yield measurement system from Enlitech, which features compact design and NIST-traceable advantages. With dimensions of only 502.4 mm (L) x 322.5 mm (W) x 352 mm (H), this equipment provides a space-saving solution and can be seamlessly integrated into a glovebox, overcoming integration challenges. This glovebox integration capability is particularly important for in situ experiments, enabling precise measurements without the influence of hydrolysis or oxidation, ensuring accurate efficiency measurements of tested samples.
The advanced instrument control software of LQ-100X-PL enables in situ time-resolved photoluminescence spectroscopy analysis and simultaneous generation of 2D and 3D plots. This capability expedites the material characterization process, rapidly providing insights into the samples. Additionally, the optical design of LQ-100X-PL extends the spectral wavelength range from 1000 nm to 1700 nm and is compatible with various sample types, including powders, solutions, and thin films. These features highlight the versatility of the system and played a crucial role in the successful completion of this research.
In conclusion, this study conclusively demonstrates that strategic interface engineering can significantly enhance the performance of quasi-2D PeLEDs. By introducing a thin layer of conductive quaternary ammonium oxide at the perovskite/electron transport layer interface, surface defects can be reduced, and charge carrier dynamics can be improved. This enhanced electron injection and improved hole blocking effect result in increased device brightness and reduced efficiency degradation at high current densities. This research reveals the critical role of interface properties in PeLEDs’ performance, paving the way for
new avenues in research and development in this field.
a) Chemical structures of PPT and PPF and schematic representation of the post-treatment process and illustration of the interfacial engineering. b) PL emission spectra, c) PLQYs, and d) TRPL curves of pristine, PPT-treated and PPF-treated perovskite films, where the PLQYs were measured by a 368 nm laser.
31P NMR spectra of a) PPT and b) PPF and their mixtures with different perovskite precursor components. c) ESP map of PPT molecule. d) XPS spectra of Pb 4f signal for the pristine, PPT-modified, and PPF-modified perovskite films. e) Schematic illustration of the passivation function of PPT on a perovskite surface.
a) Structure and b) the energy-level diagram of the fabricated PeLEDs. c) J−V−L characteristics, d) normalized EQE-voltage curves, e) normalized EQE-current density curves and f) EQE-luminance curves of the fabricated devices.
Performance of the fabricated PeLEDs
Color plot for transient absorption (TA) in the visible regions for a) pristine, b) PPT-modified, and c) PPF-modified perovskite films. Ultrafast time-resolved TA spectra of d) pristine, e) PPT-modified, and f) PPF-modified perovskite films. Power-dependent carrier dynamics at the probe wavelength of 505 nm for g) pristine, h) PPT-modified, and i) PPF-modified perovskite films.
Recombination decay constants of the control, PPT-modified and PPF-modified perovskite films
a) EIS analyses and b) capacitance-voltage curves of control, PPT-modified, and PPF-modified devices. c) Energy levels of the pristine, PPT-modified, PPF-modified perovskite films, and TPBi. d) Schematic representation of the better carrier dynamics in the modified device.