J. Dresner, RCA Review, 30, 223 (1969) Recombination statistics 1/4 3/4

Transcription

1 J. Dresner, RCA Review, 30, 223 (1969) Recombination statistics 1/4 3/4 Na-K / / Se-Te

2 Bi-layer: Recombination at organic / organic interface Cathode Electron current Hole current Anode

3 OLED Multi-layer structure: Optimized for efficiency & life Layer designation: Equivalent: HIL HTL BTL ETL EIL p + p i n n + Cathode Anode EML Strong D-A (conductor) Weak Donor-Acceptors (semi-insulating) Strong D-A (conductor) Recombination yield in BTL (light emitting layer) Leakage electron-hole recombination in transport layers Conductivities in HIL and EIL layer Interfacial energy level alignment

4 Anode and work func.on modifica.on: ITO (indium +n oxide) IZO (indium zinc oxide) ITO/UV Ozone ITO/O2 plasma ITO/CFx plasma ITO/PEDOT ITO/metal oxides (e.g. MoOx, WOx) ITO/strong acceptors with deep (~5 ev) LUMO (e.g. LG101) ITO/strong donor acceptor complexes (e.g. MTDATA F 4 TCNQ) Instability issues: Reduc+on of the oxidized anode surface Electrochemical reac+ons (e.g. acidic species in PEDOT) Mobile ions and electro migra+on

5 Cathode and work func.on modifica.on: Low work func+on Metal alloys (e.g Mg:Ag) Alkaline and alkaline earth metal / Al (e.g. Li / Al, Ca / Al) Alkaline metal halides / Al (e.g. LiF / Al) Alkaline metal doped electron transport layer (e.g. BCP / Li) Forma.on of cathode contacts: Reac+on with impinging metal vapor can cause surface damage in the organic layer Penetra+on of metal atoms/clusters into the so\ organic layer Metal ions diffusion in the organic layer Mobile ions and electro migra+on

6 Ag/Li/BCP/ITO/Glass Ag/Li/CuPc/ITO/Glass G. Parthasarathy et al, J. Appl. Phys. 89, 4986 (2001)

7 Li diffusion in BCP films: SIMS profiling data Li concentra+on profile due to 10A of Li vapor deposited on BCP on a) 800A; b)1600a Uniformly Li doped region G. Parthasarathy et al, J. Appl. Phys. 89, 4986 (2001)

8 Resis+vity of Alkaline metal (K) doped C60 films B.E. Koela et al, J. Appl. Phys (1999)

9 Dark Spots EL view Emissive area Dark spots with Perforated center Op+cal view Dark spot with center pimple 100 microns

10 Dark Spot Growth EL bright field Dark spots 100 microns Time

11 OLED materials

12 Hydroly.c instability of Alq Papadimitrakopoulos et al, Chem. Mater. 8, 1363 (1996)

13 From S.R.Forrest, IDMC

14 Watanabe et al. Proc. SPIE Vol. 4105, 175 (2001) Lo = 280 cd/m 2 Lo = 450 cd/m 2 14

15 OLED degrada.on Loss of PL and EL efficiency and rise of V T ITO / 75nm NPB / 40 nm CBP+8% Irppy3 /10 nm BAlq / 35 nm Alq / LiF/Al PL V T EL Kondakov et al, J. Appl. Phys. 101, (2007)

16 ITO / 75nm NPB / 40 nm CBP+8% Irppy3 /10 nm BAlq / 35 nm Alq / LiF/Al Loss of CBP is significant; other components are rela+vely minor CBP degrada+on is via its 1 st singlet excited states CBP+ and CBP radicals do not play a significant role in CBP degrada+on Kondakov et al, J. Appl. Phys. 101, (2007)

17 OLED Schema+cs: under various bias HTL ETL V < V bi Reverse bias V = V bi Flat band V > V bi Forward bias 17

18 Voltammogram scans for ITO NPB(750 Å) Alq 3 (750 Å) Mg:Ag at various +mes during electrical 40 ma/cm h Kondakov et al, J. Appl. Phys. 93, 1108 (2003) 18

19 Correla+on between luminance efficiency and transi+on voltage for ITO NPB(750 Å) Alq 3 (750 Å) Mg:Ag at various +mes during electrical 40 ma/cm 2 Immobile posi+ve charge accumulated mostly at HTL/ETL interface ~ per cm 2 Probable non radia+ve recombina+on centers 19

20 Metal diffusion in organics Why does the following structure work? ITO/CuPc/NPB/Alq/CuPc/Li:Al but not ITO/CuPc/NPB/Alq/CuPc/MgAg 20

21 Hung et al: J. Appl. Phys., 86, 4607 (1999) Spu]ered Al:Li cathodes 15nm MgAg 10nm 5nm b,c,d: CuPc / Al:Li 21

22 Hung et al: J. Appl. Phys., 86, 4607 (1999) Spu]ered Al:Li cathodes CuPc / Al:Li CuPc / MgAg MgAg CuPc / MgAg CuPc / Al:Li MgAg 22

23 Alq/CuPc/Li Energy Levels Alq CuPc Li injec+ng Li + CuPc Mg non injec+ng

24 LiF/Al Cathode: What is the role of LiF? Hung et al., Appl. Phys. Lett. 70, 152 (1997) Mason et al., J. Appl. Phys., 89, 2756 (2001)

25 UPS: Alq at cathode interfaces 1.6 ev New gap state Intensity (a.u.) K Na Li Mg Ca Al/CsF Al/LiF Courtesy of W. R. Salaneck

26 = Li + Alq Gap State LUMO 2.8 ev E f HOMO 1.6 ev 26

27 Calculated binding energies of Li and Cs doped Alq Ionic Radius 1.32A 2.48A Binding Energy -1.70eV -0.17eV Suggest: Cs + may be more mobile in Alq than Li + D. Giesen, DFT Optimized Geometry

28 Effect of Cs-doped Alq electron-injection contact Thermal Anneal Luminance loss Cs migra+on in L. Gerenser et.al. SID Digest, 2004 Current stress Luminance recovery Cs migra+on out

29 Tandem OLEDs V L V J Current Efficiency (cd/a): Single stack = L / J n-stacks ~ n (L / J) 3V V L L L V V J 3V V L L L V V J p-n degenerate and transparent contacts Kido, SID Digest 2003 Liao et al, Appl. Phys. Lett. 84, 167 (2004)

30 Fabrica.on of OPV panels: Interconnec.ng OPV subcells in series Subcell electrical interconnects Anode terminal Cathode Organics Anode Substrate Cathode terminal OPV Panel fabrica.on requires: Paverning anode Paverning cathode Interconnec+ng subcells Integrated shadow masks or laser/mechanical scribing Poten.al problems: Disrup+on of organic and electrode layers Moisture and oxygen ingression from edges Expensive encapsula+on schemes

31 OLED Passive Matrix Paverning Method Metal vapor stream Cathode Pillar Organic layers ITO Anode Cathode Isola+on Gap Base

32 SEM of Cathode Isola+on Pillers