SUPPLEMENTARY INFORMATION

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1 Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency Jaesang Lee 1, Hsiao-Fan Chen 2, Thilini Batagoda 2, Caleb Coburn 3, Peter I. Djurovich 2, Mark E. Thompson 2, Stephen R. Forrest 1,3,4 1 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, USA 2 Department of Chemistry, University of Southern California, Los Angeles, CA, USA 3 Department of Physics, University of Michigan, Ann Arbor, MI, USA 4 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, USA Contents S1. Transport mechanisms in the emission layers of * PHOLEDs S2. Optical simulation based on Green s function analysis for PHOLED optimization S3. Solid-state phosphorescence characteristics of * Ir(pmp) 3 S4. Density functional theory calculation results S5. Summary of reported performance of selected deep blue PHOLEDs * PHOLED: phosphorescent organic light-emitting diode * Ir(pmp) 3 : tris-(n-phenyl, N-methyl-pyridoimidazol-2-yl)iridium (III) NATURE MATERIALS 1

2 S1. Transport mechanisms in the emission layers of PHOLEDs Hole-only (HO) device structure: ITO (ultraviolet-ozone treated) / 10 nm CzSi:MoO 3 at 15 vol. % / 5 nm CzSi / 5 nm fac-ir(ppz) 3 / 40 nm TSPO1:mer-Ir(pmp) 3 at 4, 8 and 20 vol. % / 5nm TSPO1 / 5 nm MoO 3 / 100 nm Al Electron-only (EO) device structure: ITO (untreated) / 10 nm CzSi:MoO 3 at 15 vol. % / 5 nm CzSi / 5 nm fac-ir(ppz) 3 / 40 nm TSPO1:mer-Ir(pmp) 3 at 4, 8 and 20 vol. % / 5nm TSPO1 / 1.5 nm LiQ / 100 nm Al Current density (ma/cm 2 ) wt % 8 wt % 20 wt % Voltage (V) Current density (ma/cm 2 ) wt % 8 wt % wt % Voltage (V) Figure S1. Current density-voltage characteristics of hole-only (left) and electron-only (right) devices with different concentrations of 4, 8 and 20 vol. % (squares, circles, and triangles, respectively), of mer-ir(pmp) 3 in the emission layer. The current density of HO device increases with the doping concentration of mer-ir(pmp) 3 in the emission layer and that of EO device is nearly independent of the concentration. These measurements indicate that holes are predominantly transported via the dopant, whereas electrons are transported via the host, TSPO1. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION S2. Optical simulation based on Green s function analysis for PHOLED optimization Distributed fraction 1 Outcoupling Glass mode ITO Organics Cathode Distance from EBL / EML interface, t dipole (nm) Outcoupling efficiency (%) Position (nm) Figure S2a. (left) Calculated fractions of emission intensity as a function of dipole location 1, t dipole, within the EML from the electron blocking layer (EBL) / EML interface (t dipole = 0 nm) to EML / hole blocking layer (HBL) (t dipole = 40 nm). (right) Outcoupling efficiency extracted from the left figure. Electric field intensity (a.u.) ITO EML Position (nm) Al Reflectance (%) Reflectance Normalized intensity Wavelength (nm) Normalized intensity (a.u.) Figure S2b. (left) Calculated optical field intensity to optimize the position and thickness of the EML. (right) Reflectance and the measured EL spectrum of the PHOLEDs. The thicknesses of the PHOLED components were chosen to minimize microcavity effects, and thus to obtain the most saturated blue emission. 3 NATURE MATERIALS 3

4 S3. Solid-state phosphorescence characteristics of Ir(pmp) 3 fac-ir(pmp) 3 80 mer-ir(pmp) PLQY (%) Doping concentration (vol %) Doping (vol %) PLQY of fac-ir(pmp) 3 (%) PLQY of mer-ir(pmp) 3 (%) 2-21 ± ± 2 41 ± ± 3 66 ± ± 4 73 ± ± 4 61 ± ± 7 47 ± 8 * Host matrix: TSPO1, excited at the wavelength of λ = 325 nm by HeCd laser. Figure S3a and Table S3a. Photoluminescence quantum yield (PLQY) of Ir(pmp) 3 at various doping concentrations. Absorbance (a.u.) Absorbance PL PL (a.u.) wavelength (nm) Figure S3b. PL and absorbance of the host, TSPO1. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Normalized photoluminescence (a.u.) Wavelength (nm) 2 vol% 8 vol% 14 vol% 20 vol% 30 vol% Normalized photoluminescence (a.u.) Wavelength (nm) 2 vol% 8 vol% 14 vol% 20 vol% 30 vol% Figure S3c. PL of (left) fac- and (right) mer-ir(pmp) 3 doped in TSPO1 at different doping concentrations. Solid-state PL of fac-ir(pmp) 3 Solid-state PL of mer-ir(pmp) 3 Normalized intensity (a.u.) UGH2 CzSi TSPO1 Solution PL Normalized intensity (a.u.) Wavelength (nm) Wavelength (nm) Figure S3d. PL of (left) fac- and (right) mer-ir(pmp) 3 doped into different host materials. Table S3b. Summary of solid-state vs. solution PL characteristics of fac- or mer-ir(pmp) 3. PLQY (%) FWHM (nm) CIE (x, y) Solid-state thin film host (11 vol%) Solution (1 vol%) Isomers UGH2 CzSi TSPO1 2-MeTHF fac 66 ± 3 23 ± 1 36 ± 2 76 ± 5 mer 85 ± 4 61 ± 3 66 ± 3 78 ± 5 fac mer fac (0.17, 0.16) (0.17, 0.15) (0.16, 9) (0.16, 4) mer (0.16, 0.18) (0.16, 0.19) (0.16, 0.14) (0.16, 9) * In other hosts not shown here (e.g., mcp, TPBi, and BCPO), the PLQYs of both isomers approach to zero. * Errors in thin film measurement is standard deviation of at least three samples. * Triplet energies (E T ) of UGH2 = 3.50 ev, CzSi = 3.02 ev, TSPO1 = 3.36 ev from the literatures (see the text). 5 NATURE MATERIALS 5

6 As shown in Figure S3a and Table S3a, two trends are observed in the photoluminescence quantum yield (PLQY) of Ir(pmp) 3 doped at different concentrations in TSPO1. The PLQY increases for doping concentrations of 2 to 14 vol%, decreasing at higher concentrations. A HeCd laser was used for an optical excitation of only the dopant molecules at λ = 325 nm 2. The low PLQY of the thin film at a low concentration significantly deviates from that in solution (76 78 %), due to the crystallite formation in TSPO1. This is substantiated by the PL and absorbance spectra of the TSPO1 host extended at > 400 nm as shown in Figure S3b (Note that the highest energy emission at λ 310 nm is from the TSPO1 monomer). The structured TSPO1 PL indicates that the emission originates from crystalline domains 3. The broad and redshifted host PL is superposed with that of 2 vol % fac-ir(pmp) 3 in Figure S3c. This feature is not observed in the mer-isomer due to its higher absorption at λ = 325 nm and hence its higher emission intensity overwhelming the host emission (see the text, Fig. 1b). As the doping concentration increases to 14 vol%, the PLQY increases due to the increased absorption by the dopants, and reduced quenching by the host crystallites. At >14 vol% doping concentration, the PLQY decreases due to the emitter aggregation. The lower PLQY values of fac-ir(pmp) 3 compared to the mer-isomer at the same concentration can be understood by (i) its lower absorptivity leading to the higher fraction of host quenching, and (ii) its higher dipole moment resulting in greater emitter aggregation (see text). Even though TSPO1 has much higher triplet energy 2 (E T = 3.36 ev) compared to the dopants [E T = 2.9 and 2.7 ev for fac and mer-ir(pmp) 3 ], the PLQY are lower than those in the non-polar UGH2 host (E T = 3.5 ev) at the same concentration of 11 vol%. Compared to other hosts, TSPO1 leads to a narrower and bathochromically shifted Ir(pmp) 3 emission, possibly due 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION to better solid-state solubility 4. Note that low PLQY of the fac-ir(pmp) 3 in CzSi host is due to its matched triplet energy with that of the dopant (3.02 vs. 2.9 ev). NATURE MATERIALS 7

8 S4. Density functional theory calculation results fac-ir(pmp) 3 (dipole moment = 17.2 D) HOMO LUMO LUMO+1 LUMO ev ev ev ev mer-ir(pmp) 3 (dipole moment = 1 D) HOMO LUMO LUMO+1 LUMO ev ev ev ev Triplet spin density fac-ir(pmp) 3 mer-ir(pmp) 3 S 0 T 1 = 410 nm S 0 T 1 = 424 nm Figure S4. Molecular orbitals of fac and mer-ir(pmp) 3 based on time dependent density functional theory (TD-DFT) / DFT calculations using a B3LYP/LACVP** functional in a CH 2 Cl 2 solvent continuum dielectric model. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Table S4. S 0 T 1 and S 0 S 1 transitions, energies (λ cal and E cal ), oscillator strengths (f), orbital contributions (>10%), and assignments of fac and mer-ir(pmp) 3 from TD-DFT calculations State λ cal (E cal ) f Orbital contribution Assignment T nm (3.18 ev) 0 HOMO LUMO (78%) MLCT, ILCT fac- S nm (3.40 ev) 427 HOMO LUMO+1 (82%) HOMO LUMO (14%) MLCT, ILCT Ir(pmp) 3 S nm (3.40 ev) 436 HOMO LUMO+2 (98%) MLCT, ILCT S nm (3.43 ev) 016 T nm (3.13 ev) 0 HOMO LUMO (80%) HOMO LUMO (14%) HOMO LUMO (71%) HOMO LUMO+2 (17%) MLCT, ILCT MLCT, LLCT mer- Ir(pmp) 3 S nm (3.23 ev) 035 HOMO LUMO (98%) MLCT, LLCT S nm (3.28 ev) 007 HOMO LUMO+1 (99%) MLCT, LLCT S nm (3.33 ev) 067 HOMO LUMO+2 (98%) MLCT, LLCT Calculations were performed using the Jaguar 8.4 (release 17) software package on the Schrödinger Material Science Suite (v2014-2). The geometries were calculated using a B3LYP functional with the LACVP** basis set in a CH 2 Cl 2 solvent continuum. Figure S4 illustrates the calculated HOMO, LUMO and the spin density surfaces from the optimized triplet states for facand mer-ir(pmp) 3, which were obtained from density functional theory (DFT) and timedependent DFT calculations, respectively. Commonly, the HOMOs of both isomers are disposed on the phenyl-π and Ir-d orbitals, while their LUMOs are preferentially formed in the methylpyridoimidazole ligands. 5 The HOMO and LUMO of fac-ir(pmp) 3 are equally distributed among the three ligands due to its C 3 symmetry (nearly identical LUMO, LUMO+1, LUMO+2), NATURE MATERIALS 9

10 whereas for C 1 -symmetric mer-ir(pmp) 3, the phenyl π-orbitals in the two mutually trans pyridoimidazole ligands (both denoted as L) form the HOMO, and its lowest LUMO is localized in π*-orbitals in the third ligand (denoted as L and LUMO < LUMO+1, LUMO+2). This electron configuration of the mer-isomer elongates its transoid Ir C bonds that leads to the destabilized HOMO and the slightly affected LUMO 5. The calculated HOMOs are at 5.21 ev and 5.10 ev and LUMOs of 1.27 ev and 1.20 ev from the vacuum level for fac and mer- Ir(pmp) 3, respectively. The spin density distribution of triplet states of fac and mer-ir(pmp) 3 show that both isomers have pronounced metal-ligand charge-transfer ( 3 MLCT) character; however, the greatest difference between two isomers is that triplets in a fac-isomer are localized within a single ligand represented by intraligand-charge-transfer (ILCT) admixed with 3 MLCT states, whereas those in the mer-isomer are delocalized across the ligands represented by combined ligand-to-ligand charge-transfer ( 3 LL CT) and 3 MLCT states (see Table S4). The dispersed electron distribution in the mer-isomer as opposed to the ligand-localized condition in the fac configuration results in its higher transition dipole moment. 6 This partly explains the larger discrepancy in mer-ir(pmp) 3 between its calculated S 0 T 1 value (Fig. S4) and the peak energy at room temperature PL (Fig. 1 in the text) of ΔE mer = hc 24 hc 64 5eV, compared to those in fac-ir(pmp) 3 where ΔE fac = hc 10 hc 18 6eV, since the PL of the isomers was measured in the polar solvent (2-methyltetrahydrofuran, or 2-MeTHF). Here, h is the Planck constant and c is the speed of light in vacuum. 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION S5. Summary of reported performance of selected deep blue PHOLEDs. Dopants EQE (%) a Luminance J 1/2 (ma cm -2 ) b L (cd m -2 ) at J 1/2 CIE c τ (µs) d Ref. mer-ir(pmp) ± / 13.3 ± / 1000 nits 210 ± 10 22,000 ± , 0.15 ± 0.1 This work fac-ir(pmp) ± / 9.0 ± / 1000 nits 160 ± 10 7,800 ± , ± 0.1 This work (fpmi) 2 Ir(dmpypz) 17.1 / / 1000 nits ,000 16, , (TF) 2 Ir(fptz) 8.4 / 7.4 max / 100 nits ,000 2, , PtON / / 1000 nits , Pt / 8.3 max / 1000 nits , PtON7-dtb 24.8 / 1 max / 1000 nits , Ir(dbfmi) 18.6 / 6.2 max / 1000 nits , Ir(fbppz) 2 (dfbdp) 11.7 / 8.2 max / 100 nits , (HF) 2 Ir(fptz) 8.4 / 7.1 max / 100 nits , mer-ir(cn-pmic) cd/a < , PtOO7 4.1 / / 1000 nits , (dfbmb) 2 Ir(fptz) 6.0 / / 100 nits , Ir(fpmi) 2 (tfpypz) 7.6 < 50 nits N / A 3446 (max) 0.14, a External quantum efficiencies (EQE) of PHOLEDs measured at the specified luminance levels in the next column. b Current density at half maximum EQE (J 1/2 ) is obtained from the reported maximum values, otherwise EQEs at the specified conditions were used. c Color chromaticity coordinate (CIE) in this work was measured at 10 ma cm -2. Values are rounded off to two decimal digits. d Observed triplet lifetimes (τ) of the phosphors except (TF) 2 Ir(fptz) and (HF) 2 Ir(fptz) were measured either in solution or the polymer matrix. Pyrazole and carbonyl ligand-based phosphors, and the rest are carbene ligand-based. * Errors for EQE and J 1/2 are standard deviation from at least three devices and error for τ is the 95% confidence interval. NATURE MATERIALS 11

12 References 1. Celebi, K., Heidel, T. D. & Baldo, M. A. Simplified calculation of dipole energy transport in a multilayer stack using dyadic Green's functions. Opt. Express 15, (2007). 2. Jeon, S. O., Jang, S. E., Son, H. S. & Lee, J. Y. External Quantum Efficiency Above 20% in Deep Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 23, (2011). 3. Padmaperuma, A. B., Sapochak, L. S. & Burrows, P. E. New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence: 2,7-Bis(diphenylphosphine oxide)-9,9-dimethylfluorene. Chem. Mater. 18, (2006). 4. Erk, P. et al. 11.2: Efficient Deep Blue Triplet Emitters for OLEDs. SID Symp. Dig. Tech. Pap. 37, (2006). 5. Tamayo, A. B. et al. Synthesis and Characterization of Facial and Meridional Triscyclometalated Iridium(III) Complexes. J. Am. Chem. Soc. 125, (2003). 6. Kober, E. M., Sullivan, B. P. & Meyer, T. J. Solvent dependence of metal-to-ligand chargetransfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2 - bipyridine complexes of ruthenium(ii) and osmium(ii). Inorg. Chem. 23, (1984). 7. Lu, K.-Y. et al. Wide-Range Color Tuning of Iridium Biscarbene Complexes from Blue to Red by Different N^N Ligands: an Alternative Route for Adjusting the Emission Colors. Adv. Mater. 23, (2011). 8. Lee, S. et al. Deep-Blue Phosphorescence from Perfluoro Carbonyl-Substituted Iridium Complexes. J. Am. Chem. Soc. 135, (2013). 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION 9. Hang, X.-C., Fleetham, T., Turner, E., Brooks, J. & Li, J. Highly Efficient Blue-Emitting Cyclometalated Platinum(II) Complexes by Judicious Molecular Design. Angew. Chem. Int. Ed. 52, (2013). 10. Fleetham, T., Wang, Z. & Li, J. Efficient deep blue electrophosphorescent devices based on platinum(ii) bis(n-methyl-imidazolyl)benzene chloride. Org. Electron. 13, (2012). 11. Fleetham, T., Li, G., Wen, L. & Li, J. Efficient Pure Blue OLEDs Employing Tetradentate Pt Complexes with a Narrow Spectral Bandwidth. Adv. Mater. 26, (2014). 12. Sasabe, H. et al. High-Efficiency Blue and White Organic Light-Emitting Devices Incorporating a Blue Iridium Carbene Complex. Adv. Mater. 22, (2010). 13. Chiu, Y.-C. et al. En Route to High External Quantum Efficiency (~12%), Organic True- Blue-Light-Emitting Diodes Employing Novel Design of Iridium (III) Phosphors. Adv. Mater. 21, (2009). 14. Chang, C.-F. et al. Highly Efficient Blue-Emitting Iridium(III) Carbene Complexes and Phosphorescent OLEDs. Angew. Chem. Int. Ed. 47, (2008). 15. Hsieh, C.-H. et al. Design and Synthesis of Iridium Bis(carbene) Complexes for Efficient Blue Electrophosphorescence. Chem. Eur. J. 17, (2011). NATURE MATERIALS 13

Supplementary Figure 1. Potential energy, volume, and molecular distribution of the

1 2 3 4 5 6 7 8 Supplementary Figure 1. Potential energy, volume, and molecular distribution of the organic substrates prepared by MD simulation. (a) Change of the density and total potential energy of