Light generation: principles of OLED

Transcription

1 Advanced Course on ORGANIC ELECTRONIC Principles, devices and applications Light generation: principles of OLED Marco ampietro ORGANIC LED Exciton Electron EXCITON neutral excited entity (molecule) (presence of e - - h + couple) Hole Coulomb interaction Organic materials small (2.5) tightly bound excitons (molecular excitations) ilicon big (11) lousely bound excitons 1

2 ORGANIC LED ITO ITO Vext m e metal 1. Charge injection 2. Transport 3. Exciton formation 4. Radiative recombination h 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) ORGANIC LED Reflecting metal nm Organic semiconductor Transparent conductor ubstrate - V + Collaborazione TEEO ( 2002) Light goes through one of the electrodes! Light 2

3 First layered OLED Polymers a TEP forward of 25 YEAR 3

4 RADIATIVE RECOMBINATION ITO ITO h Exciton formation zone m e metallo m - favour injection from contacts - minimize leakage current - confine carriers in space to promote exciton formation ITO ITO h e Cathode Different & specialized materials Anode Hole Transport Layer - HTL Electron Transport Layer - ETL First layered OLED mall molecules 4

5 CHARACTERITIC CURVE m ITO V ext ELETTROLUMINECENCE EFFICIENCY Loss mechanisms (limiting devices efficiency): Charge un-balance (g) Fraction of excitons that decay radiatively (r st ) quantum efficiency of the emissive molecule ( pl ) Light extraction ( coupling ) These factors can be combined to define the external quantum efficiency, EQE, as: ext g rst PL coupling n.of emittedphotons n.of injectedcharges 5

6 CHARGE UN-BALANCE (I) metal e ITO h Holes in excess, give rise to current but not to light Equivalent barriers for holes and electrons are beneficial 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,

7 BARRIER MINIMIZATION (Anode) Thin conductive layer between ITO and polymer (smooth surface, barrier for ion diffusion lifetime) ITO (4.7eV) E ~ PEDOT:P (5.2eV) Poly(aniline) or poly(thiophene) - MEH-PPV 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 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 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 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 7

8 BARRIER MINIMIATION mall barriers increase power efficiency P eff h ext ev ame I ame light V 1 V 2 >> V 1 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 It is important to choose suitable HTL and ETL R HTL 1 q p p L 8

9 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 Hole Electron EL e - and h + have spin (spin number ½). Capture of e - and h + (Exciton formation) is spin-independent. pins can be up or down, and can precess in or out of phase (4 possible permutations) inglets Excitons (Total spin angular mom. = 0) Triplets Excitons (Total spin angular mom.= 1) pin momentum should be conserved for radiative decay 25 % 75 % 9

10 FLUORECENCE 25% Excitons with INGLET character 75% Excitons with TRIPLET character 1 Very different lifetime T k = decay rate. k R 10-9 s 10-3 s k IC 0 undergo radiative recombination (FLUORECENCE) Radiative Triplet to ground involves pin flip (very slow)! 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 k NR 0 T4b T4 EL (ns) T T4B K R = EL / T4b K R K NR T K IC T.Benincori et al. Phys. Rev. B 58, 9092 (1998) Torsional relaxation introduces NON RADIATIVE recombination channels LE than 25% of excitons recombine radiatively EL < 25% 10

11 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% 11

12 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! T Radiative k rad 10-4 s Big Phosphorescence If decay is slow, triplets may have time to feel spin-orbit coupling (electromagnetic interaction between the electron's spin and the magnetic field generated by the electron's orbit around the nucleus) spin flip radiative recombination 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% 12

13 Triplet harvesters : Iridium complexes Emitted colour changes by changing the legand Long exciton diffusion range (because of long lifetimes) >140 nm compared to ~10-20 nm for singlets. Exciton confinement layers needed to prevent excitons from crossing the whole active layer and decay non-radiatively at the electrodes. EFFICIENCY IMPROVEMENT Improvement thanks to phosphorescent emitters aturated trend for fluorescent emitters Triplet emitters 13

14 MATRIX of TRANPORTER and EMITTER A WAY TO IMPROVE EFFICIENCY Exciton formation zone ITO ITO h Anode HTL Exciton formation zone ETL Exciton may better be formed on a material different from the Emitter m e Cathode Different & specialized materials 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, 2001 EXCITON TRANFER : FÖTER type Dipole-dipole coupling of the donor and acceptor molecules. Energy levels should be resonant. pectral superposition between host emission and dye absorption equal equal Host (Donor) Guest (Acceptor) Host (Donor) Guest (Acceptor) It is a long range interaction (up to 100 Å). Mainly singlet excitons and triplet spin-orbit mediated. The best transfer efficiency is obtained on molecoles with transition moments parallel and colinears 14

15 EXCITON TRANFER : DEXTER type imultaneous exchange of electrons btwn D and A molecules. Host (Donor) Guest (Acceptor) emission shifts to the red auto absorption is minimum hort range ( < 10 Å). Possibile both for singlet and Triplet excitons. 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) 15

16 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 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) 16

17 Layered OLED Polymers (2006) mall molecules (M-OLED) and polymers (PLED) - Vacuum deposited (dry) - Easier layer engineering olution deposited (wet) 17

18 Power efficiency (lm/w) 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 > c (critical angle) Light is trapped E.g.: n EL = 1.7 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 Typical values : = 17% for isotropic emission!! (The remaining 83% of the photons are trapped in organic and substrate modes) How to increase coupling? Microlens arrays Textured surface H.Y. Lin, et al. Opt. Express 16 (2008) Half sphere Pattern Flat Luminance (cd/m 2 ). Reineke et al. Nature 459 (2009), 234-U

19 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 85% of NTC color spectrum standard Viewing angle >160 ( ) Contrast Thin & lightweight Deposition Temperature -80 C / 80 C (0 C / 40 C) (AMLCD- Langfelder) Multi PIXEL - CREEN INGLE MOLECOLE B G R 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 Thursday afternoon Marco ampietro 5,0E-08 0,0E Lunghezze d'onda [nm] Politecnico di Milano e Logic pa

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