Low voltage organic electroluminescent element

1. An OLED device comprising a cathode, an electron- transporting layer (ETL), a light-emitting layer (LEL) containing a fluorescent light-emitting material, a hole-transporting layer (HTL), and an anode, in that order, wherein the ETL comprises a mixture of compounds, including a first compound and at least one second compound, and wherein there is present a hole blocking layer (HBL) adjacent to the LEL on the cathode side, wherein the first compound is present in an amount of 50% to 90% by volume and is represented by Formula (2):

wherein:

R 1 and R 4 represent the same substituent and neither are hydrogen;

R 2 R 3 represent hydrogen;

Ar 1 and Ar 4 represent the same aromatic group; and

Ar 2 and Ar 3 represent the same aromatic group.

2. The OLED device of claim 1 wherein the second compound is represented by Formula (4):

wherein:

M represents a metal;

n is an integer of from 1 to 4; and

each Z a and Z b independently represents the atoms necessary to complete an aromatic ring group.

3. The device of claim 1 wherein the second compound comprises a monovalent metal.

4. The device of claim 1 wherein the second compound comprises a divalent metal.

5. The OLED device of claim 1 wherein the second compound is represented by Formula (4a):

(M a )p(Q)p  (4a)

wherein:

M a represents a monovalent metal;

Q represents a bidentate ligand; and

p is an integer.

6. The OLED device of claim 1 wherein the second compound is represented by Formula (5):

wherein:

M is a metal;

L represents an aromatic group; and

each Z c and Z d independently represents the atoms necessary to complete an aromatic ring group.

7. The OLED device of claim 1 wherein the second compound(s) is represented by Formula (6), Formula (7), Formula (8) or Formula (9):

wherein:

Z is O, NR 29 , or S;

R 17 , R 18 , R 19 , R 20 , R 21 , R 22 , R 23 and R 24 are hydrogen or substituents, provided that any of the indicated substituents may join to form further fused ring;

R 25 , R 26 , R 27 , and R 28 represent hydrogen or a substituent;

R 29 represents a substituent;

R 30 , R 31 , and R 32 represent hydrogen or substituents or are the atoms necessary to complete a fused carbocyclic or heterocyclic ring group;

R 33 represents hydrogen or a substituent;

Y is a linkage unit comprising an alkyl group or an aryl group that connects the multiple heterocycles together;

k is an integer of from 1 to 4; and

m is an integer of from 3 to 8.

8. The device of claim 1 wherein the hole-blocking layer includes a compound represented by Formula (3):

wherein:

W 1 -W 10 independently represent hydrogen or an independently selected substituent, provided that two adjacent substituents can combine to form rings.

9. The device of claim 1 wherein the hole-blocking layer includes a compound of Formula (4):

wherein:

M represents a metal;

n is an integer of from 1 to 4; and

each Z a and Z b independently represents the atoms necessary to complete an aromatic ring group.

10. The OLED device of claim 1 wherein the hole-blocking layer includes a compound represented by Formula (5):

wherein:

M is a metal;

L represents an aromatic group; and

each Z c and Z d independently represent the atoms necessary to complete an aromatic ring group.

11. The device of claim 1 wherein the hole-blocking layer includes a compound represented by Formula (7):

wherein:

m is an integer of from 3 to 8;

Z is O, NR 29 , or S;

R 25 , R 26 , R 27 , and R 28 represent hydrogen or a substituent;

R 29 represents a substituent; and

Y is a linkage unit comprising an alkyl group or an aryl group that connects the multiple heterocycles together.

12. The device of claim 1 wherein the fluorescent material comprises a perylene group, a distyrylbenzene group, a distyrylbiphenyl group, or a rubrene group.

13. The device of claim 1 wherein the ETL includes a first compound, a second compound, and a metallic material based on a metal having a work function less than 4.2 eV.