[1] [1] Tang C W, Vanslyke S A. Organic electroluminescent diodes[J]. Appl. Phys. Lett.,1987, 51(12): 913-915.

[2] [2] Hong G, Gan X, Leonhardt C, et al. A brief history of OLEDs-emitter development and industry milestones[J]. Adv. Mater., 2021, 33(9): 2005630.

[3] [3] Lüssem B, Keum C M, Kasemann D, et al. Doped organic transistors[J]. Chem. Rev., 2016, 116(22): 13714-13751.

[4] [4] Huang J, Pfeiffer M, Werner A, et al. Low-voltage organic electroluminescent devices using pin structures[J]. Appl. Phys. Lett., 2002, 80(1): 139-141.

[5] [5] Oyamada T, Sasabe H, Adachi C, et al. Extremely low-voltage driving of organic light-emitting diodes with a Cs-doped phenyldipyrenylphosphine oxide layer as an electron-injection layer[J]. Appl. Phys. Lett., 2005, 86(3): 033503.

[6] [6] Lee J H, Kim J J. Interfacial doping for efficient charge injection in organic semiconductors[J]. Phys. Status Solidi A, 2012, 209(8): 1399-1413.

[7] [7] Lüssem B, Riede M, Leo K. Doping of organic semiconductors[J]. Phys. Status Solidi A, 2013, 210(1): 9-43.

[8] [8] Bin Z Y, Duan L, Qiu Y. Air stable organic salt as an n-type dopant for efficient and stable organic light-emitting diodes[J]. ACS Appl. Mater. Interfaces, 2015, 7(12): 6444-6450.

[9] [9] Salzmann I, Heimel G, Oehzelt M, et al, Molecular electrical doping of organic semiconductors: fundamental mechanisms and emerging dopant design rules[J]. ACC. Chem. Res., 2016, 49(3): 370-378.

[10] [10] Bin Z Y, Liu Z Y, Qiu Y, et al. Efficient n-dopants and their roles in organic electronics[J]. Adv. Opt. Mater., 2018, 6(18): 1800536.

[11] [11] Parthasarathy G, Shen C, Kahn A, et al. Lithium doping of semiconducting organic charge transport materials[J]. J. Appl. Phys., 2001, 89(9): 4986-4992.

[12] [12] Lee J H, Wu M H, Chao C C, et al. High efficiency and long lifetime OLED based on a metal-doped electron transport layer[J]. Chem. Phys. Lett., 2005, 416(4/6): 234-237.

[13] [13] Choudhury K R, Yoon J, So F. LiF as an n-dopant in tris(8-hydroxyquinoline) aluminum thin films[J]. Adv. Mater., 2008, 20(8): 1456-1461.

[14] [14] Kao P C, Lin J H, Wang J Y, et al. Li2CO3 as an n-type dopant on Alq3-based organic light emitting devices[J]. J. Appl. Phys., 2011, 109(9): 094505.

[15] [15] Wei H X, Ou Q D, Zhang Z, et al. The role of cesium fluoride as an n-type dopant on electron transport layer in organic light-emitting diodes[J]. Org. Electron., 2013, 14(3): 839-844.

[16] [16] Deng Y H, Li Y Q, Ou Q D, et al. The doping effect of cesium-based compounds on carrier transport and operational stability in organic light-emitting diodes[J]. Org. Electron., 2014, 15(6): 1215-1221.

[17] [17] Chu X B, Guan M, Niu L T, et al. The utilization of low-temperature evaporable CsN3-doped NBphen as an alternative and efficient electron-injection layer in OLED[J]. Phys. Status Solidi A, 2014, 211(7): 1605-1609.

[18] [18] Tsai C T, Liu Y H, Kao P C, et al. 2-Methyl-9, 10-bis (naphthalen-2-yl) anthracene doped lithium carbonate as an effective electron injecting layer for both inverted and conventional organic light-emitting diode structures[J]. ECS J. Solid State Sci. Technol., 2020, 9(5): 056001.

[19] [19] Cho K, Cho S W, Jeon P E, et al. Energy level alignments at tris(8-hydroquinoline) aluminum/8-hydroquinolatolithium/aluminum interfaces[J]. Appl. Phys. Lett., 2008, 92(9): 093304.

[21] [21] Gao C H, Zhu X Z, Zhang L, et al. Comparative studies on the inorganic and organic p-type dopants in organic light emitting diodes with enhanced hole injection[J]. Appl. Phys. Lett., 2013, 102(15): 153301.

[22] [22] Lee S H, Huseynova G, Choi H K, et al. Analysis of charge transfer complex at the interface between organic and inorganic semiconductors[J]. Org. Electron., 2021, 88: 106001.

[23] [23] Juang F S, Chittawanij A, Hong L A, et al. The study of n-type doping and stamping transfer processes of electron transport layer for organic light-emitting diodes[J]. IEICE Trans. Electron., 2015, 98(2): 66-72.

[24] [24] Kumar A, Srivastava R, Tyagi P, et al. Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris(8-hydroxyquinolinato) aluminum[J]. J. Appl. Phys., 2011, 109(11): 114511.

[25] [25] Tyagi P, Srivastava R, Kumar A, et al. Low voltage organic light emitting diode using p-i-n structure[J]. Synth. Met., 2010, 160(9/10): 1126-1129.

[26] [26] Kim H M, Seo J H, Han W K, et al. Improvement of mixed electron transport structure red phosphorescent organic light-emitting diodes[J]. Mo. Cryst. Liq. Cryst., 2011, 538(1): 53-60.

[27] [27] Soman A, Unni K N N. Enhancement in electron transport and exciton confinement in OLEDs: role of n-type doping and electron blocking layers[J]. Eur. Phys. J.-Appl. Phys., 2019, 86(1): 10201.

[28] [28] Yuan Y, Grozea D, Han S, et al. Interaction between organic semiconductors and LiF dopant[J]. Appl. Phys. Lett., 2004, 85(21): 4959-4961.

[29] [29] Bulovic V, Shoustikov A, Baldo M A, et al. Bright, saturated, red-to-yellow organic light-emitting devices based on polarization-induced spectral shifts[J]. Chem. Phys. Lett., 1998, 287(3/4): 455-460.

[30] [30] Mahdiyar R, Fadavieslam M R. The effects of chemical treatment on ITO properties and performance of OLED devices[J]. Opt. Quant. Electron., 2020, 52(5): 262.