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High-performance CsPb1-xSnxBr3 perovskite quantum dots for highly efficient light-emitting diodes

High-performance CsPb1-xSnxBr3 perovskite quantum dots for  highly efficient light-emitting diodes
   All inorganic CsPbBr3 perovskite quantum dots (QDs) have attracted considerable interest because of their high photoluminescence, tunable and narrow emission wavelength, and facile synthesis. Given these advantages, all inorganic CsPbBr3 perovskite QDs can be used in solar cells, lasing, light-emitting diodes (LEDs) and bioimaging. However, the toxicity of Pb2+ has a negative impact on the environment. Hence, the total or partial replacement of Pb2+ is necessary. At present, the most suitable substitute elements are the less toxic Sn(II), Sn(IV), Bi(III) and Mn(II) ions. Thus, partially Sn-substituted perovskite QDs were prepared in a colloidal solution. After a long reaction time, the Sn(IV) ions reached the maximum substitution ratio, and the valence state of the Sn ions was confirmed by X-ray absorption near-edge spectroscopy (XANES). The Sn K-edge XANES spectra of the CsPb1-xSnBr3 QDs indicated that the absorption edge energies of all the analyzed samples were the same and fit the Sn(IV) ion standard curve. These results are attributed to the easy oxidation of Sn(II) ions during the synthesis of QDs at high temperature. According to the inert pair effect, the stable valence state of Sn is Sn(IV). The absolute photoluminescence quantum yield (PLQY) of CsPb1-xSnxBr3 QDs substituted with Sn(IV) increased from 45% to 83%, where the best substitution ratio was x = 0.33. Based on femtosecond transient-absorption (TA), time-resolved photoluminescence (TRPL), and single-dot spectroscopies, we concluded that the observed PLQY enhancement was due to the reduction of trion formation in the perovskite QDs. These highly luminescent CsPb0.67Sn0.33Br3 QDs that exhibit an emission wavelength of 517 nm have potential for application as emitters for electroluminescent displays. Thus, CsPb1-xSnxBr3-based quantum dot LEDS (QLEDs) with the structure ITO/PEDOT:PSS/TFB/PQDs/TPBi/LiF/Al were fabricated.

   The optimized device exhibited a luminescence of 12,500 cd/m2, a current efficiency (CE) of 11.63 cd/A, an external quantum efficiency (EQE) of 4.13%, a power efficiency (PE) of 6.76 lm/w, and a low turn-on voltage of 3.6 V.

   
In summary, we demonstrate the hot-injection synthesis of CsPb1-xSnxBr3 perovskite QDs with Sn(IV) substitution. Sn(IV) doping effectively suppresses the formation of trions, as revealed by single-dot, TRPL, and TA spectroscopies. The best perovskite QLED device displays a luminescence of 12,500 cd/m2, a CE of 11.63 cd/A, an EQE of 4.13%, a PE 6.76 lm/w, and a low turn-on voltage of 3.6 V, which are the highest values reported among all Sn-based perovskite QLEDs.


Figure 1. Schematic of trion formation suppression in CsPb1-xSnxBr3 perovskite QLEDs used for a backlight display.

Reference
Hung-Chia Wang, Weigao Wang, An-Cih Tang, Hsin-Yu Tsai, Zhen Bao, Toshiyuki Ihara, Naoki Yarita, Hirokazu Tahara, Yoshihiko Kanemitsu, Shuming Chen, and Ru-Shi Liu (2017). High-Performance Novel CsPb1-xSnxBr3 Perovskite Quantum Dots for Highly-Efficient Light-Emitting Diodes. Angewandte Chemie International Edition, 129(44), 13650–13654. DOI:10.1002/ange.201706860.

Glossaries
Exciton: the combination of an electron and a positive hole, which is free to move through a nonmetallic crystal as a unit.
Trion: a localized excitation which consists of three charged quasiparticles. A negative trion consists of two electrons and one hole and a positive trion consist of two holes and one electron.
PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate): a polymer mixture of two ionomers that functions as a hole injection layer in QLEDs.
TFB (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)diphenylamine)]): a polymer that functions as a hole transport layer in QLEDs.
TPBi (2,2′,2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)): a kind of electron transport layer in QLEDs.

Professor Ru-Shi Liu
Department of Chemistry
rsliu@ntu.edu.tw