Chinese Optics Letters

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Stable and high performance all-inorganic perovskite light-emitting diodes with anti-solvent treatment

Organic and inorganic halide perovskites have attracted a lot of attention due to their extensive use in various optoelectronic applications given their unique properties like broad band wavelength absorption, high photoluminance (PL), narrow full width at half-maximum (FWHM), band gap tunability, and intrinsic photophysical stability. Perovskite material is widely applied in light- emitting diodes (LEDs), lasers, and photodetectors. Particularly, perovskite LED (Pe-LED) has been studied, and its efficiency has been significantly improved by enhancing the crystallinity and morphology of perovskite film. Despite the advantages of the fast free exciton emission attenuation described above, it may help transfer non-radiant energy to trapped states.

All-inorganic perovskites have attracted extraordinary attention due to their high heat resistance compared to organic perovskites. For example, cesium lead bromide (CsPbBr3) polycrystalline film-based Pe-LEDs offer an auspicious substitute technique increasing the device performance taking account of their enhanced thermal stability. However, sensitivity to humidity, light, and heat are still the main challenges Pe-LEDs facing. Nonetheless, various polycrystalline CsPbBr3-based Pe-LEDs suffer from low brightness (L) and inefficiency owing to their low film surface coverage and PL quantum yield (PLQY). Through post-treatment of organic compounds such as methyl acetate (MA), chlorobenzene (CB), ethanol (EtOH), and isopropanol (IPA) plays an important role for achieving uniform thin films and prevent leakage current, due to completely covered CsPbBr3 polycrystalline film with high PLQYs have been verified as a significant prerequisite to improve both efficiency and luminance of Pe-LEDs.

The device introduced by Dr. Hussain from a research group in Southeast University China published in Chinese Optics Letter (COL), Volume 3, Issue 01, 2021 (S. Hussain, et. al., Stable and high performance all-inorganic perovskite light-emitting diodes with anti-solvent treatment). This work was supervised by Prof. Qasim Khan and Prof. Lei Wei from Southeast University China who believe that these characteristics are of great importance for optoelectronic devices. The performance of CsPbBr3-based device is studied. The lowest current density (J) of final device is an indication of the high crystalline perovskite film blocking the short-circuit current paths in the device, as revealed by the SEM images. The better surface coverage of the perovskite film in turn results in higher device performance due to better charge injection in the emissive layer (EML). The improvement in luminance of device with anti-solvent-treated perovskite film, indicate that the minimized short circuit as well as an incredible enhancement in current efficiency (CE) is found, and higher external quantum efficiency (EQE) is dedicated to the improved surface morphology and affects the performance of the device.

(a) Schematic diagram of Perovskite Light-emitting diode device and (b) Luminance-Voltage characteristics.

The patterned fluorine-doped tin oxide (FTO) substrates were cleaned with detergent, deionized (DI) water, acetone, EtOH, and IPA for 15 min each using the ultrasonication cleaner and treated with UV ozone for 30 min before use. The ZnO nanoparticles (NPs) (25 mg ml −1 in butanol, filtered through a 0.22 μm N66 filter) was spin-coated on the substrate at 2000 rpm for 40 s and annealed at 140°C for 10 min. The active layer perovskite precursor solution CsBr and PbBr2 with a molar ratio of 1.2:1 (0.3 M) was mixed in anhydrous dimethyl sulfoxide (DMSO, 80°C, 4 h, 800 rpm), spun onto the ZnO NPs at 2000 rpm for 50 s along the dropped MA delay of 5 s after starting the spin coater, and finally treated with an anti-solvent CB perovskite layer. Then, poly-TPD (10 mg ml −1 in CF) as an HTL was spin-coated onto the perovskite film at 2500 rpm for 40 s. Finally, Au was deposited by thermal deposition technique through a shadow mask. The active area of our fabricated devices was 0.04 cm−2. This device presents a simple structure and the fabrication was carried out in ambient conditions.

We have presented a simple and facile method for obtaining high quality CsPbBr3 perovskite film for efficient Pe-LEDs. Dropping anti-solvent CB onto the treated MA CsPbBr3 film can obtain high coverage of the active layer with better crystallinity. As a result, the devices showed a low turn on voltage (VT) of 2.0 V and maximum luminance of 17,866 cd m−2 with current efficiency of 45.8 cd A−1. This initiative suggests a simple way of controlling the carriers' injection along with reducing surface defects in optoelectronic devices and lighting technologies based on perovskites for their utilization in high resolution displays and cost-effective technologies. Most significantly, the process treated with solvent enabled the fabrication of all-inorganic CsPbBr3-based LEDs with high efficiency, taking advantages of the longevity of the device and stability against environmental moisture and oxygen.