Light generation: principles of OLED

Transcription

1 Advanced Course on ORGANIC ELECTRONIC Principles, devices and applications Light generation: principles of OLED Marco ampietro Exciton Electron Hole ORGANIC LED EXCITON neutral excited molecule (presence of e - - h + couple) ITO h ITO Vext m e metal 1. Charge injection 2. Transport 3. Exciton formation 4. Radiative recombination Position depends on n and p C.W.Tang et al. LED on organic crystals APL 51, 1987, H.Burroughes et al. LED based on polymers Nature 347 (1990)

2 ORGANIC LED Reflecting metal nm Organic semiconductor Transparent conductor ubstrate - V + Collaborazione TEEO ( 2002) Light goes through one of the electrodes! Light First layered OLED Polymers 2

3 RADIATIVE RECOMBINATION ITO h ITO m e metallo - favour injection from contacts - minimize leakage current - confine carriers in space m ITO ITO h e Cathode Different & specialized materials Anode Hole Transport Layer - HTL Electron Transport Layer - ETL First layered OLED mall molecules 3

4 CHARACTERITIC CURVE m ITO V ext ELETTROLUMINECENCE EFFICIENCY Loss mechanisms (limiting devices efficiency): Charge un-balance (g) inglets vs Triplets (r st ) Intrinsic material efficiency ( pl ) Light extraction ( coupling ) These factors can be combined to define the external quantum efficiency as: ext g rst PL coupling n.of emittedphotons n.of injectedcharges 4

5 CHARGE UN-BALANCE (I) e metallo ITO h Holes in excess, give rise to current but not to light Equivalent barriers for holes and electrons are beneficial BARRIER MINIMIATION mall barriers increase power efficiency P eff h ext ev ame I ame light V 1 V 2 >> V 1 5

6 BARRIER MINIMIZATION (Anode) Hole injection (1) Anode contact ITO (transparent) ~4.2 ev (too low) ITO ITO h m e metallo O 2 plasma treatment (~ 4 min at 50mW/cm 2, ~4.7 ev) D.J.Milliron et al., JAP 87, 572, 2000 Poly(aniline) or poly(thiophene) - MEH-PPV Thin conductive layer between ITO and polymer (smooth surface, barrier for ion diffusion lifetime) A.J.Heeger et al., ynt.met. 67, 23, 1994 Y.cott et al., ynt.met., 85, 1197, 1997 PEDOT:P, best with PFO A.Elschner et al, ynt.met.111,139,2000 L.B.Groendaal et al., Adv.Mat. 12, 481, 2000 A.J. Campbell et al., JAP 89, 3343, 2001 BARRIER MINIMIZATION (Anode) Hole injection (2) ~ ITO (4.7eV) E PEDOT:P (5.2eV) E Graded barrier for hole injection (PEDOT:P complexed with poly-pxylylee-a-tetra-hydrothiophenum) Efficiency 50 times greater than standard PEDOT:P P.K.H.Ho et al.,nature,404,481,2000 6

7 BARRIER MINIMIZATION (Cathode) Electron injection Cathode contact ITO ITO h m e Ca(3), Ba, Mg, Yb(2.6), Li low work-function metals (important in blue LED) 1. Insertion of additional layers at the cathode side (Alkane-thiol in MEH-PPV) I. H. Campbell et al., Phys. Rev. B 54, , 1996 Local electric field increase Tunneling increase Importance of dipole alignment Barrier tuning over a 1 ev range 2.Use of fluorides, oxides, sulphides E.g. LiF, CsF capped with a thick Al T. M. Brown et al., APL 79, 174, 2001 T.M.Brown JAP 93, 6159 (2003) Luminance increase with fluorides insertion Back to : CHARGE UN-BALANCE (II) ext g r st PL coupling Probability that e - &h + meet and form excitons If barrier is small (no injection limit) current depends on transport layers resistance I n V R ETL R ETL 1 q n n L Un-matched mobilities produce different electrons and holes fluxes I p V R HTL R HTL 1 q p p L 7

8 OLED with Doped Transport Layers : reduction of voltage Low device resistance Low voltage drop Better injection irrespective of contact metal (small depleted region) h Peff ext ev V close to thermodynamic limit (e.g. 2.5V for blue) INGLET - TRIPLET RATIO ext g r st r EL - coupling Exciton Elettrone Lacuna EL e - and h + have spin. Capture of e - and h + (Exciton formation) is spin-independent. inglets Excitons Triplets Excitons pin momentum should be conserved for radiative decay 25 % 75 % 8

9 FLUORECENCE 25% Excitons with INGLET character 75% Excitons with TRIPLET character s k R T k IC k = decay rate. lifetime =1/k 0 undergo radiative recombination FLUORECENCE NON radiative recombination In conventional fluorescent devices only singlet excitons (25%) recombine radiatively (75% goes into heat) at best EL max = 25% FLUORECENCE LO R R R R R R R R 25% Excitons with R= H, INGLET character T4bb R= Butyl, T4bbR4 Initial situation PhT2bPh R= H, T4bb R= Butyl, T4bbR4 T4 T4B s k R 0 Fluorescenza T4b k NR T4 EL (ns) T T4B K R = EL / Torsional relaxation introduces NON RADIATIVE recombination channels T4b K R K NR T K IC T.Benincori et al. Phys. Rev. B 58, 9092 (1998) LE than 25% of excitons recombine radiatively EL < 25% 9

10 Fluorescent emitters : examples INGLET to TRIPLET LO 25% Excitons with INGLET character s k R 0 Fluorescenza k IC small 75% Excitons with TRIPLET character k IC NON radiative T k rad Very small Initial situation Relaxation toward triplet (Inter ystem Crossing) Another reason for LE than 25% of excitons recombine radiatively EL < 25% 10

11 FROM CHARGED POLARON TO NEUTRAL EXCITON Oppositely charged polarons Lacuna decay into Elettrone Polaron pair (called charge-transfer exciton-ct ), already obey to spin statistics (25% singlet, 75% triplet ) singlet or triplet neutral exciton Lacuna Elettrone If decay is fast (faster than any other process affecting the intermediate CT) spin statistics is mantained : CT singlet Exc. inglet 25% CT triplet Exc. Triplet 75% J.L.Bredas et al., Chem. Rev. 2004, 104, This is the case of small molecules TRIPLET MAY BE MADE RADIATIVE! Lacuna Elettrone If decay is slow, triplets may have time to feel spin-orbit coupling and undergo radiative recombination decay Lacuna Elettrone s 0 k R Fluorescenza mall energy difference (of the order of the reorganisation energy) k IC Very big Fosforescenza T Radiative k rad 10-4 s Big 11

12 PHOPHORECENT (triplet) HARVETER Z 4 Molecules containing heavy atoms (organometallic complexes with Ir, Pt) show large spin-orbit coupling In principle, 100% of excitons can recombine radiatively EL max = 100% PROBLEM : diffusion range is much longer (because of long lifetimes): >140 nm compared to ~10-20 nm for singlets. Device structure must be more complicated : Exciton confinement layers needed to prevent excitons from crossing the whole active layer and decay non-radiatively at the electrodes - BAD charge transporter (low ) - tend to aggregate (diliution is needed) M.A. Baldo,et al. Nature 395, 151 (1998), APL 74, 442 (1999) Triplet harvesters : Iridium complexes Emitted colour changes by changing the legand : 12

13 EFFICIENCY IMPROVEMENT Improvement thanks to phosphorescent emitters aturated trend for fluorescent emitters Triplet emitters MATRIX of TRANPORTER and EMITTER A WAY TO IMPROVE EFFICIENCY Exciton transfer: Exciton, generated on one material (matrix, host, donor), transfers onto another material (guest, dye, acceptor) having better emissive properties First group to use green phosphorescent dye to increase efficiency has been M. A. Baldo et al., Nature 395, 151 (1998), then V. Cleave et al., Adv. Mater. 13, 44,

14 EXCITON TRANFER : FÖTER type Energy levels should be resonant (dipole-dipole). It is a long range interaction (up to 100 Å). Mainly singlet excitons. Föster transfer equal equal Host (Donor) Guest (Acceptor) Host (Donor) Guest (Acceptor) pectral superposition between host emission and dye absorption The best transfer efficiency is obtained on molecoles with transition moments parallel and colinears Difficult to find a matrix where levels 1 and T 1 emit in the blue EXCITON TRANFER : DEXTER type Charge transfer from D to A. hort range (up to 10 Å). Possibile both for singlet and Triplet excitons. Dexter transfer Host (Donor) Guest (Accettor) emission shifts to the red auto absorption is minimum triplet emission can be exploited The long triplet exciton life times ( 100 s) favor T-T annihilisation at high current densities 14

15 Typ. derivative of Triarylamines in summary. Doped region (1-2% wt) with organic dye (tuned color) Typ. derivatives of metal chelates such as Tris(8-hydroxyquinolate)Alluminates (Alq3) Layered OLED mall molecules (1990) Brevetto Kodak CuP c NPB Alq 3 : colorante LiF/A l Δ e ITO Δ h Anodo HIL HTL EML ETL Catodo DiMetil QuinAcridone Naftalene Phenil Benzidina Thickness of single layers : from 20 to 80 nm 15

16 Layered OLED Polymers (2004) 2 breakthrough : - doping of transport layers - triplet emission 100 lm/w at 100 cd/m 2 (@2.7V drive voltage) 70 lm/w at 4000 cd/m 2 Iridium green triplet emitter 19 % external quantum efficiency Karl Leo et al., APL 85, 17 (2004) Layered OLED Polymers (2006) 16

17 mall molecules (M-OLED) and polymers (PLED) - Vacuum deposited (dry) - Easier layer engineering olution deposited (wet) Optical output coupling ext g r st coupling Is related to the extraction of the light toward the external environment PL Mismatch between refractive index of EL layer and air Total reflection for J>J c (critical angle) Light is trapped E.g.: n EL = 1.7 J c = 36 imple expressions (from nell s law) If one side is a prefect reflector and the emission is isotropic (molecules): 0 2 coupling.5/ n implified model, neglecting: 1. Optical losses 2. Interferences 3. Absorption at the metal contact Typical values : = 17% for isotropic emission!! 17

18 How to increase coupling? Lifetime : cd/m 2 Brightness cd/m 2 (400 cd/m 2 ) Lifetime > h Low voltage < 10 V (Backlight!) Time response < s (16 ms) ummary of OLED characteristics All colors calable emissive area from µm to cm Efficiency > 40 lm/w (2 lm/w) Colors covers almost 90% of NTC color spectrum standard Viewing angle >160 ( ) Contrast Thin & lightweight Deposition Temperature -80 C / 80 C (0 C / 40 C) (AMLCD- Langfelder) 18

19 AREA source of light - ILLUMINATION OLED (area source) competes with fluorescent bulbs (line source) and LED (point source) Large active area (wallpaper, light panels, signalling - diffused light, minimized shadows) Mechanical flexibility (curved lamps, adapting to other forms, ) Large choice of colors (ambient light, Color Rendition Index 100) Philips Lumiblade OLED Panel GL350 (125x125mm 2, 120 lm) In perspective : great opportunities for designers and architects Problems : non-uniformity over large surfaces, lifetimes, a little electrical short can kill a large area OLED!!! AREA source of light - ILLUMINATION Philips Lumiblade OLED Panel GL350 19

20 Multi PIXEL - CREEN INGLE MOLECOLE R G B pettri dei tre colori primari RGB e del bianco 5,0E-07 4,5E-07 Blu Verde Rosso Bianco Amplificazione dell'nvi Pixel 84 m x 151 m Radianza [W/(sr cm^2 nm)] 4,0E-07 3,5E-07 3,0E-07 2,5E-07 2,0E-07 1,5E-07 1,0E-07 Friday afternoon Marco ampietro 5,0E-08 0,0E Lunghezze d'onda [nm] Politecnico di Milano e Logic pa 20

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