Design,synthesis And Optoelectronic Application Of Arylamine-based Organic Molecular Hole-injection/transport Materials

Organic light-emitting diodes(OLEDs)find wide applications in the fields of flat panel displays,car taillight,solid-state lighting and health care.Perovskite solar cells(PSCs)have attracted extensive attention in recent years due to the advantages of high optical absorption coefficient,wide absorption spectrum range and easy processing.Organic hole-injection/transport materials(HI/HT materials)have been widely used in organic electronics such as OLEDs and PSCs.As an anode buffer layer,HI/HT materials would be conducive to facilitate hole injection into the emitting layer from the anode of the OLEDs or extraction of the photo-generated hole charge carriers out from the active layer of the PSCs.HI/HT materials with suitable HOMO energy level,high hole mobility(?_h)and high glass transition temperature(T_g)are favorable for high-performance optoelectronic devices.It is commonly accepted that the potential residual halogen impurities arising from the reactants or reaction by-products have a large adverse impact on the OLED lifetime.Generally,there is trade-off in obtaining both high T_g and charge mobility.Consequently,it is of high importance to develop hole-transport materials that combine enhanced purity,glass-transition temperature and hole mobility for optoelectronic devices such as OLEDs and PSCs.Arylamine units possess advantages such as strong electron-donating ability and easy functionalization.In this context,a series of high-performance hole-injection/transport materials were designed and synthesized based on an easily available 6-bromo-2-naphthylamine.The main contents are as follows:1.Following the general introduction of HI/HT materials in Chapter 1,Chapert 2 presents a hole-injection 2,2'-binaphthalene-6,6'-diamine derivative XL1 with methoxyphenyls as the end groups.XL1 was prepared via successive Ullmann and Suzuki reactions starting from 6-bromo-2-naphthylamine.The halogen impurities which affect the lifetime of OLEDs could be removed by column chromatography.XL1 has a high HOMO level of-5.05 e V,which is beneficial for promoting hole injection in OLEDs.It shows an enhanced T_g and?_h in contrast with the conventional 1,1'-biphenyl-4,4'-diamine hole-injection material Me O-TPD.Upon p-doping with tetrafluorotetracyanoquinodimethane(F4-TCNQ),the conductivity of XL1 is 2.64×10^-4 S m^-1.The hole mobility of XL1:F4-TCNQ reaches(4.09-2.34)×10^-2 cm² V^-1s^-1@E=(2-5)×10⁵ V cm^-1.In the red phosphorescent OLEDs,XL1 provides better performances due to improved conducting properties and thermal morphological stability.For instance,at a luminance of ca.1000 cd m^-2,the driving voltage(V),luminous efficiency(LE)and power efficiency(PE)of XL1 OLED are 5.0 V,9.9 cd A^–1 and 6.2 lm W^–1,respectively,while V=5.8 V,LE=9.5 cd A^–1,PE=5.2 lm W^–1 of Me O-TPD OLED;XL1 exhibits a t₉₅ of ca.230 h at an initial luminance of 1000 cd m^-2,vs.only 65 h for the analogue Me O-TPD OLED.In the high-efficiency green phosphorescent OLEDs,the LE of the OLEDs based on XL1 and the commercial material OMET-P008 are 57.1,56.5 cd A^–1and PE are 64.0,68.2 lm W^–1@1000cd m^-2,respectively,while the lifetime of both the OLEDs is very close with t₉₀ of ca.80 h@10000 cd m^-2.In tandem OLEDs,XL1 exhibits a LE of 80.8 cd A^–1,a PE of 24.9 lm W^–1@1000 cd m^-2;L=33060 cd m^-2 and LE=66.12 cd A^–1@50 m A cm^–2.The lifetime t₉₀ is about56.5 h driven under a high constant current of 50 m A cm^–2.2.In the 3^rd chapter,on the basis of XL1,we further synthesized hole-injection material XL2 through partly replacing the methoxyphenyl endgroups with 9-methyl-3-carbazole unit in order to improve the glass transition temperature.The HOMO level of XL2 is–5.01 e V.Compared with XL1,it shows a higher glass transition temperature of 163 ^oC.At an electric field of(2-5)×10⁵ V cm^-1,the hole mobility of XL2 is 1.51×10^-3–1.05×10^-2cm²V^-1s^-1.XL2 is characterized in the green phosphorescent OLED as a doped hole-injection layer.The resulting PE,LE and external quantum efficiency(EQE)are 45.9 lm W^–1,73.1 cd A^–1 and 20.09%@1000 cd m^-2,respectively.The CIE of the green phosphorescent OLED is(0.31,0.63).In the red phosphorescent OLED,LE=20.3 cd A^–1,PE=9.1 lm W^–1 and EQE=23.3%@1000cd m^-2,and the LE is 16.82 cd A^–1@50 m A cm^–2.The CIE of the red phosphorescent OLED is(0.68,0.32).The lifetime t₉₅ is 11.6 h driven under a high constant current of 50 m A cm^–2.It should be noted that reacting 2,6-dibromonaphthalene with N,N-bis(4-methoxyphenyl)-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)naphthalen-2-amine leads to XL3,which has a T_g of 117 ^oC and a HOMO level of–5.08 e V.3.In the 4^th chapter,hole-transport 2,2'-binaphthalenediamine derivatives XL4 and XL5with decreased HOMO levels are obtained.Due to the lack of methoxy group in the molecular structure,the HOMO levels of XL4 and XL5 are reduced to–5.14 and–5.28 e V,respectively.Compared with the common hole-transport material NPB,the glass transition temperatures of the new compounds are greatly improved to 160 and 145 ^oC,respectively,due to introducing rigid and bulk carbazole or fluorene groups.The hole mobility of both compounds exceeds 10^-3cm²V^-1s^-1,which is one or two orders of magnitude higher than that of NPB(ca.10^-5–10^-4cm² V^-1 s^-1).XL4 and XL5 exhibit more outstanding performances than NPB in deep red phosphorescent OLEDs as an undoped and thick(200 nm)hole-transport layer.For instance,at a luminance of ca.1000 cd m^-2,V,PE and LE are 3.57/3.45/3.59 V,19.34/20.41/19.16 lm W^–1 and 21.98/22.42/21.90 cd A^–1 for OLEDs based on XL4,XL5 and NPB,respectively.In addition,the lifetime t₉₅ is 17.4 h for XL5 OLED,17.0 h for XL4 OLED and 19.74 h for NPB OLED driven under a high constant current of 50 m A cm^-2.It's worth pointing out that the initial luminance of XL4 and XL5 OLEDs is 1.12 and 1.23 times that of the NPB OLED at 50m A cm^-2,respectively.The deduced t₉₅ is ca.20.61 h for the XL4 OLED and 24.74 h for the XL5 OLED at the same initial luminescence as the NPB OLED,thus higher than that of NPB OLED(19.74 h).4.In the last chapter,we further synthesized aromatic triamine compounds XL6,XL7 and XL8 involving the Suzuki coupling of dibromotriphenylamines with 6-N,N-diphenylamino-2-naphthylboron esters.All the compounds are easy to synthesize with good solubility in organic solvents such as dichloromethane and chlorobenzene,and hence the high purity materials can be easily obtained by column chromatography.The glass-transition temperatures of the compounds are between 130 and 140 ^oC.At an electric field of(1-4)×10⁵ V cm^-1,the hole mobility of XL6,XL7 and XL8 is(1.27-5.54)×10^-3,(1.16-5.96)×10^-3and(1.49-8.43)×10^-3 cm² V^-1s^-1,respectively.Due to the weak electron donating ability of methoxy group,the HOMO levels of the compounds increase from–5.36,–5.07 to–5.02 e V for XL6,XL7 and XL8.In the inverted PSCs,power conversion efficiencies(PCEs)of 18.44%for XL6-based device,17.89%for XL7-based device and 17.48%for XL8-based device can be achieved,and the corresponding V_oc/J_sc/FF are 1.06 V/22.15 m A cm^-2/78.23%,1.04 V/22.14 m A cm^-2/78.01%and 1.03 V/21.18 m A cm^-2/78.18%,respectively,while the PCE of the common hole-transport material PEDOT:PSS in the analogue PSCs is only 15.18%with V_oc=1.01 V,J_sc=20.3 m A cm^-2 and FF=74.04%.All compounds show higher quenching efficiency of steady-state photoluminescence(PL)and shorter average lifetime(?)of time-resolved photoluminescence(TRPL)than PEDOT:PSS.The quenching efficiency of the steady-state PL for the PSCs devices is in the order of XL6>XL7>XL8>PEDOT:PSS and the average lifetime(?)of TRPL is8.64,10.32,10.71 and 36.32 ns for the devices based on XL6,XL7,XL8 and PEDOT:PSS,respectively.In summary,starting from facilely available 6-bromo-2-naphthylamine and 6-bromonaphthalen-2-ol,a series of arylamines combining high T_g,high?_h and attractive hole injection/transport properties were developed.Among them,XL5 exhibits very inspiring properties,including a HOMO energy level of-5.28 e V,a T_gof 145 ^oC and hole mobility of7.86×10^-3-6.09×10^-3 cm² V^-1s^-1@(1.23-2.46)×10⁵ V cm^-1,as well as ease of facile synthesis and purification.It showed improved luminescent efficiency and lifetime in the deep red phosphorescent OLEDs,in contrast with a conventional hole-transport material NPB.We are currently collaborating with industrial partners to explore the commercial potentials of these hole-injection/transport materials.