US 20160240821A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2016/0240821 A1 CARROLL et al. (43) Pub. Date: Aug. 18, 2016 (54) FREQUENCY DEPENDENT LIGHT HOIL 33/42 (2006.01) EMITTING DEVICES HOIL 33/30 (2006.01) (71) Applicant: WAKE FOREST UNIVERSITY, HOIL 33/00 (2006.01) Winston-Salem, NC (US) (52) U.S. Cl. CPC ........ HOIL 51/5296 (2013.01); HOIL 51/5206 (72) Inventors: David CARROLL, Winston-Salem, NC (2013.01); HOIL 51/5234 (2013.01); HOIL (US); Robert SUMMERS, Clemmons, 33/30 (2013.01); HOIL 33/004I (2013.01); NC(US); Yonghua CHEN, Cleveland HOIL 33/42 (2013.01); H05B33/08 (2013.01); Height, OH (US) HOIL 225 1/303 (2013.01); HOIL 5 1/5016 (21) Appl. No.: 15/024,624 (2013.01) (22) PCT Filed: Sep. 26, 2014 (57) ABSTRACT (86). PCT No.: PCT/US2014/057774 S371 (c)(1), (2) Date: Mar. 24, 2016 An electroluminescent device described herein, in one aspect, comprises a first electrode and second electrode and a light Related U.S. Application Data emitting layer positioned between the first and second elec trodes. A current injection gate is positioned between the first (60) Provisional application No. 61/883,710, filed on Sep. electrode and the light emitting layer or the second electrode 27, 2013. and the light emitting layer. In some embodiments, the cur Publication Classification rent injection gate comprises a semiconductor layer of elec tronic structure restricting injected current flow from the first (51) Int. Cl. or second electrode through the semiconductor layer as a HOIL 5/52 (2006.01) function of alternating current Voltage frequency applied to H05B33/08 (2006.01) the first and second electrodes. tes Patent Application Publication Aug. 18, 2016 Sheet 1 of 4 US 2016/0240821 A1 tes 18 WAC FIGURE 1 Patent Application Publication Aug. 18, 2016 Sheet 2 of 4 US 2016/0240821 A1 FIGURE 2 Patent Application Publication Aug. 18, 2016 Sheet 3 of 4 US 2016/0240821 A1 8000 - NOne O TiO, 1000 RPM 6000- a-TO,4000 RPM CN O E O Sl 9 4000 (S E - 2000 20 40 Frequency (kHz) FIGURE3 Patent Application Publication Aug. 18, 2016 Sheet 4 of 4 US 2016/0240821 A1 40 -- None 30 s E NY C (f Sl e 20 CD2 g 3 () 10 - g O--reer O 4 6 8 10 12 14 16 18 AC Voltage V is (V) 10 3.0 -HNOne e-e " ...; TiO, 1000 RPM is-É al-A-TO,4000RRig2.5 - 10 3 Et - // E i 20 S 10° 9. S. 3 1.5 S. 10 5 1.O O O.5 O 6 8 10 12 14 16 18 AC Voltage VRs (V) FIGURE 4 US 2016/0240821 A1 Aug. 18, 2016 FREQUENCY DEPENDENT LIGHT positioned on a first side of the organic light emitting layer EMITTING DEVICES and a hole dopant layer is positioned on the opposing side of the organic light emitting layer, wherein a nanoparticle phase RELATED APPLICATION DATA bridges an interface formed by the electron dopant layer and organic light emitting layer. Alternatively, the nanoparticle 0001. The present application claims priority pursuant to phase can bridge an interface formed by the hole dopant layer 35 U.S.C. S 119(e) to U.S. Provisional Patent Application and organic light emitting layer. Further, a nanoparticle phase 61/883,710 filed Sep. 27, 2013 which is incorporated herein can bridge an interface formed by the electron dopant layer by reference in its entirety. and organic light emitting layer and an interface formed by FIELD the hole dopant layer and organic light emitting layer. 0007 Methods of generating light are also described 0002 The present invention relates to light emitting herein. A method of generating light comprises providing an devices and, in particular, to light emitting devices demon electroluminescent device comprising first and second elec strating properties related to alternating current Voltage fre trodes, a light emitting layer positioned between the first and quencies. second electrodes and a current injection gate positioned BACKGROUND between the first electrode and the light emitting layer or between the second electrode and the light emitting layer. An 0003 Organic thin film electroluminescent (EL) devices, alternating current Voltage is applied to the first and second including organic light emitting devices (OLEDs), typically electrodes and current injected from the first or the second operate using constant Voltage or direct current (DC) power electrode is restricted from flowing into the light emitting Sources. The charge carriers, holes and electrons, are directly layer by the gate as a function of alternating current Voltage injected from high work function and low work function frequency, wherein holes and electrons are radiatively com metal electrodes, respectively. Several disadvantages exist bined in the light emitting layer. with direct current injection architectures. Direct current injection, for example, can precipitate charge accumulation 0008. These and other embodiments are further described in the recombination Zone and large leakage current, resulting in the detailed description which follows. in significant exciton quenching. Exicton quenching pro duces low brightness and series efficiency roll-off Further, BRIEF DESCRIPTION OF THE DRAWINGS DC driven architectures require power converters and increase device sensitivities to dimensional variations that 0009 FIG. 1 illustrates a cross-sectional view of an elec lead to run away current imperfections. More importantly, in troluminescent device according to one embodiment order to achieve effective charge injection, high work func described herein. tion metals are required for anodes, and low work function 0010 FIG. 2 illustrates a cross-sectional view of an elec metals are required for cathodes. Such requirements severely troluminescent device according to one embodiment restrict suitable electrode materials for DC devices. Addition described herein. ally, low work function metals are unstable in air and water, 0011 FIG. 3 illustrates luminance versus AC voltage fre thereby increasing fabrication complexities for DC devices. quency for electroluminescent devices according to some embodiments described herein. SUMMARY 0012 FIG. 4 illustrates AC current and luminance versus 0004 Electroluminescent devices are described herein AC voltage at set Voltage frequencies for electroluminescent which, in some embodiments, offer advantages over prior devices according to Some embodiments described herein. devices. For example, electroluminescent devices described herein can be driven by alternating current (AC), alleviating charge accumulation by the frequent reversal of applied bias. DETAILED DESCRIPTION Further, electroluminescent devices described hereincan pro 0013 Embodiments described herein can be understood vide radiant recombination in the absence of direct current more readily by reference to the following detailed descrip injection, thereby breaking electrode dependency on high and tion, examples and drawings. Elements, apparatus, and meth low work function metals. ods described herein, however, are not limited to the specific 0005 Briefly, an electroluminescent device described embodiments presented in the detailed description, examples herein, in one aspect, comprises a first electrode and second and drawings. It should be recognized that these embodi electrode and a light emitting layer positioned between the ments are merely illustrative of the principles of the present first and second electrodes. A current injection gate is posi invention. Numerous modifications and adaptations will be tioned between the first electrode and the light emitting layer readily apparent to those of skill in the art without departing or between the second electrode and the light emitting layer. from the spirit and scope of the invention. In some embodiments, the current injection gate comprises a semiconductor layer of electronic structure restricting 0014. The term “alkyl as used herein, alone or in combi injected current flow from the first or second electrode nation, refers to a straight or branched chain Saturated hydro through the semiconductor layer as a function of alternating carbon radical having from 1-20 carbon atoms. In some current Voltage frequency applied to the first and second embodiments, for example, alkyl is C. alkyl. electrodes. 0015 The term “alkenyl as used herein, alone or in com 0006. In another aspect, an electroluminescent device bination, refers to a straight or branched chain hydrocarbon described herein comprises a first electrode and second elec radical containing from 2-20 carbon atoms and at least one trode and an organic light emitting layer positioned between carbon-carbon double bond. In some embodiments, for the first and second electrodes. An electron dopant layer is example, alkenyl comprises Cs alkenyl. US 2016/0240821 A1 Aug. 18, 2016 0016. The term “aryl as used herein, alone or in combi trode (11) and the light emitting layer (13). The current injec nation, refers to an aromatic ring system radical. Aryl is also tion gate (16) can comprise a layer (17) of semiconductor intended to include partially hydrogenated derivatives of car bocyclic systems. material of electronic structure restricting injected current 0017. The term "heteroaryl as used herein, alone or in flow from the first electrode (11) through the semiconductor combination, refers to an aromatic ring radical with for layer (17) as a function alternating current Voltage frequency instance 5 to 7 member atoms, or to an aromatic ring system (18) applied to the first (11) and second (12) electrodes. In radical with for instance from 7 to 18 member atoms, con alternative embodiment, the current injection gate (16) can be taining one or more heteroatoms selected from nitrogen,