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Highly efficient organic electroluminescent devices based on cyclometallated platinum complexes as new phosphorescent emitters
Synthetic Metals 147 (2004) 253–256
Highly efficient organic electroluminescent devices based on cyclometallated platinum complexes as new phosphorescent emitters M. Cocchia,∗ , D. Virgilia , C. Sabatinia , V. Fattoria , P. Di Marcoa , M. Maestrib , J. Kalinowskic a
Institute of Organic Synthesis and Photoreactivity, National Research Council of Italy, Via P. Gobetti 101, 40129 Bologna, Italy b Department of Chemistry “G. Ciamician”, University of Bologna, 40126 Bologna, Italy c Department of Molecular Physics, Technical University of Gda´ nsk, 80-952 Gda´nsk, Poland Received 28 April 2004; accepted 17 May 2004
Abstract High efficiency electroluminescence (EL) in organic light emitting diodes (OLED) was achieved using cyclometallated platinum(II) complexes as new phosphorescent emitters. OLEDs containing bis[2-(2-thienyl)pyridine]platinum(II) [Pt(thpy)2 ] or bis[2-(5-trimethylsilanyl-2thienyl)-pyridine]platinum(II) [Pt(thpy-SiMe3 )2 ] show a high external EL quantum yield reaching 5.4 and 11.5% photon/electron, respectively. Both electroluminescent spectra fall in the “amber” color region defined by the automotive lighting standards; specifically the CIE coordinates are (0.58; 0.42) for Pt(thpy)2 and (0.60; 0.39) for Pt(thpy-SiMe3 )2 . © 2004 Elsevier B.V. All rights reserved.
1. Introduction The light generation in OLED is the result of the formation of emissive state via recombination of charge carrier injected from electrodes. Due to spin statistics from electron–hole recombination three times more triplets than singlet excited states are created. In common electrofluorescent devices only the singlets contribute directly to the emission and triplets, non-radiatively relaxing, are lost for light generation. Using phosphorescent materials with radiatively active triplets, instead, provides a straight way to a significant increase in quantum efficiency of organic LEDs [1]. It was originally realized by Forrest and co-workers in organic LEDs based on organometallic complex phosphors, Ptbased dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3 ), acting as guest emitters in a layer co-evaporated with a host material of tris-(8-hydroxyquinoline) aluminium (Alq3 ) or 4,4 -N,N -dicarbazole-biphenyl (CBP) [2,3]. We ∗
Corresponding author. Tel.: +39 051 6399818; fax: +39 051 6399844. E-mail address: [email protected] (M. Cocchi).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.05.021
have proposed a more easy way, using the spin coating technique and casting an emitting layer from a solution containing both the host and guest molecules [4], with no need of any active polymers as a host matrix [5]. This method allows an easier and better control of the doping level, which is a very critical point for the uniformity of amorphous films, affected by the cooling conditions in the thermal co-evaporation technique. Moreover, the spin coating technique provides a possibility to employ several compounds (neutral or ionic species) that cannot be sublimed because of their chemical and physical instabilities. In the past 20 years, the photophysical properties of a large number of phosphorescent organometallic complexes have been investigated and in particular cyclometallated iridium and platinum complexes have been shown to exhibit interesting optical properties in solution at room temperature, originating from the radiative decay of the triplet metal-toligand charge-transfer (MLCT) excited state allowed by the strong spin–orbit coupling effect of the heavy metal [6]. In search of new LED emitters, we have synthesized two cyclometallated platinum complexes: Pt(thpy)2 and Pt(thpySiMe3 )2 , which show an intense room temperature lumi-
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M. Cocchi et al. / Synthetic Metals 147 (2004) 253–256
Co., and PC (mw 32,000–36,000) from Polysciences Inc. All materials were used as supplied. The 60 nm thick host–guest layers were obtained by spin casting at 2000 rpm a 10 mg/ml dichloromethane solutions of TPD (74 wt.%), PC (20 wt.%), and Pt-complex (6 wt.%) mixtures onto quartz substrates for optical measurements and onto ITO coated glass substrates (20 /cm2 ) for OLED construction. The ETL of PBD was deposited over the spin coated film by high vacuum deposition (0.1 mPa) to the final thickness of 60 nm. The sandwich devices were completed by a high vacuum deposited Ca (25 nm thick) covered with a protecting Ag layer 100 nm thick. Each layer thickness was measured with a Tencor Alpha Step 200 profilometer. Absorption, PL and EL spectra were measured using a Perkin Elmer Lambda 9UV/Vis/NIR spectrometer and a Spex Fluorolog spectrofluorimeter. The time-resolved PL measurements were made using a singlephoton IBH Model 5000 counter. The current–voltage characteristics were measured with a Keithley Source-Measure unit model 236 under continuous operation mode, while the output light power was measured with an EG&G power meter. All measurements were carried out at room temperature, and the devices and thin film samples were kept in argon atmosphere to prevent the negative effects of oxygen and humidity. All characteristics have been reproduced for many runs excluding irreversible chemical or morphological changes in the samples.
Fig. 1. (a) Molecular structures of the materials used: (TPD) N,N diphenyl-N,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine; (PBD) 2(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4 oxadiazole; (PC) bisphenol-Apolycarbonate; (PtOEP) 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum (II); [Pt(thpy)2 ] bis-[2-(2-thienyl) pyridine] platinum(II); [Pt(thpySiMe3 )2 ] bis-[2-(5-trimethylsilanyl-thiophen-2-yl-pyridine] platinum (II). (b) Configuration of the electroluminescent device.
nescence in liquid solution (the photoluminescence quantum yield are 0.35 ± 0.05 for both compounds in deoxygenated solution). Both these complexes are not stable toward thermal deposition. We have realized double-layer (DL) devices with a spin-cast first layer of these compounds as guests in a host matrix that consists of N,N -diphenylN,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine (TPD) as hole transporting molecule and polycarbonate (PC) as a matrix preventing molecular movements, and a second layer of 2-(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4 oxadiazole (PBD) as electron transport layer (ETL) deposited by vacuum evaporation. Fig. 1 shows the chemical structure of the active molecules and the DL OLED configuration.
2. Experimental The cyclometallated Pt-complexes were synthesized as reported in Ref. [7] and purified by chromatography. TPD and PBD were purchased from Aldrich, PtOEP from H.W. Sands
3. Results and discussion Our discussion is focused on cyclometallated Ptcomplexes as emitters in OLEDs but to evidence their good performance, they are compared with the well known PtOEP emitter [2]. In cyclometallated Pt-complexes the emission originates from the lowest metal-to-ligand-charge-transfer (MLCT) triplet state, while in PtOEP the state responsible of radiative emission is the lowest ligand-centered (LC) triplet state. Fig. 2 compares the PL spectra of TPD:PC:Ptcomplex films with the PL spectrum of the undoped film. It shows that the TPD molecule is an excellent sensitizer for the emission of all Pt-complexes; the emission spectrum is, in fact, due to the phosphorescent decay of the 3 Ptcomplex* triplet excited states, this being confirmed also by their long lifetimes (τ ∼ = 80 s for PtOEP and τ ∼ = 8 s for both the cyclometallated complexes). Hence, the energy transfer process from 1 TPD* singlet excited state generated by light absorption to the Pt-complexes is very efficient. The strong emission at 650, 580 and 590 nm observed in PL spectra for PtOEP, Pt(thpy)2 and Pt(thpy-SiMe3 )2 complexes, respectively, is also observed in the electroluminescence (EL) spectra (Fig. 3a). The CIE (Commission Internationale de L’Eclairage) chromaticity coordinates for our devices are (0.71, 0.29) for PtOEP, (0.58, 0.42) for Pt(thpy)2 , and (0.6, 0.39) for Pt(thpy-SiMe3 )2 , as shown in Fig. 3b. All of them fall in the acceptable red emitters range, though the emission color changes from saturated red for
M. Cocchi et al. / Synthetic Metals 147 (2004) 253–256
255
Fig. 2. Photoluminescence (PL) spectra of the host–guest systems 74% TPD:20% PC:6% Pt-complex compared with the PL spectrum of a 75% TPD:25% PC undoped film.
PtOEP to reddish-orange (amber) for Pt(thpy)2 and Pt(thpySiMe3 )2 . Fig. 4 shows the dependencies of the electroluminescence (IEL ) and current density (j) on applied voltage for the Ptcomplexes based devices. The maximum emission (IELmax ) of 610 W/cm2 is achieved at about 21 V with a current den-
Fig. 4. Electroluminescence (IEL ) and current density (j) vs. bias voltage for devices based on PtOEP (a), Pt(thpy)2 (b) and Pt(thpy-SiMe3 )2 (c).
Fig. 3. (a) Electroluminescence (EL) spectra of the ITO/74% TPD:20% PC:6% Pt-complex (60 nm)/PBD (60 nm)/Ca + Ag devices: (1) Pt(thpy)2 , (2) Pt(thpy-SiMe3 )2 and (3) PtOEP. (b) CIE diagram with the coordinates of the emissions from the Pt-complex-based devices and RGB video display standards.
sity of 36 mA/cm2 for the device based on PtOEP, while IELmax = 1000 W/cm2 at 18 V with a current density of 12 mA/cm2 and IELmax = 3650 W/cm2 at 19 V with a current density of 30 mA/cm2 are achieved for devices based on Pt(thpy)2 and Pt(thpy-SiMe3 )2 respectively. The better performances of the cyclometallated complexes are also more evident in Fig. 5 where the external EL quantum efficiencies (ϕEL ) versus bias voltage are compared. OLEDs based on Pt(thpy)2 and Pt(thpy-SiMe3 )2 show an excellent ϕEL reaching 5.4 and 11.5% photon/electron, respectively. The two cyclometallated complexes, while having same ϕPL in solution and same energy transfer efficiency in the TPD:PC layer, show a strong ϕEL difference in the corresponding OLEDs. A relevant portion of EL emission, differently from PL emission, is due to energy transfer from exciplexes or electroplexex formed upon electron–hole recombination at the interface between the TPD and PBD layers [8,9]. The ϕEL decrease (roll-off) is as a rule observed at high voltages (large current density regimes) in all devices [10,11], the effect ascribed to either triplet–triplet annihilation [10] and/or electric field-
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cent devices, in fact both maximum luminance and external quantum efficiency are similar or higher than PtOEP based OLEDs. Besides, the OLED emission colors for both compounds falls in a spectral region of great interest for “amber” automotive lighting standards.
Acknowledgment This work was supported in part by MIUR project “Nanotechnologies” (legge 95/95), FIRB project RBNE019H9K entitled “Nanometric machines through molecular manipulation” and EUREKA project entitled “Flexible organic illuminators for automotive market”. Fig. 5. External quantum efficiency (ϕEL ) vs. bias voltage for devices with the three Pt-complexes.
enhanced dissociation of emissive states and their precursors [11]. The higher ϕEL in the device with Pt(thpy-SiMe3 )2 as the emitter can be attributed to a lower dissociation efficiency of excited states in high electric fields [12]. Anyway, OLEDs based on cyclometallated Pt-complexes show a lower effect compared with PtOEP based OLEDs. Further studies are necessary to understand which of the following quenching mechanisms more strongly contributes to the observed unwanted effect on ϕEL : triplet–triplet exciton annihilation, triplet exciton–charge carrier interaction, electric field-assisted dissociation of excitons and temperature effect on phosphorescence.
4. Conclusions We have used cyclometallated Pt-complexes as phosphors in simple DL devices and have shown that they are excellent candidates as emitting materials in electrophosphores-
References [1] J. Kalinowski, J. Phys. D 32 (1999) R179. [2] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151. [3] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [4] M. Cocchi, P. Di Marco, V. Fattori, G. Giro, J. Kalinowski, W. Stampor, D. Virgili, IT Patent No. BO2002A000165 (2002). [5] S. Lamansky, R.C. Kwong, M. Nugent, P.I. Djurovich, M.E. Thompson, Org. Electron. 2 (2001) 53. [6] M. Maestri, V. Balzani, C. Deuschel-Cornioley, A. von Zelewsky, in: D. Volman, G. Hammond, D. Neckers (Eds.), Advances in Photochemistry, vol. 17, Wiley, New York, 1992, p. 1. [7] L. Chassot, A. von Zelevsky, Inorg. Chem. 26 (1987) 2814. [8] G. Giro, M. Cocchi, J. Kalinowski, P. Di Marco, V. Fattori, Chem. Phys. Lett. 318 (2000) 137. [9] J. Kalinowski, M. Cocchi, P. Di Marco, W. Stampor, G. Giro, V. Fattori, J. Phys. D 33 (2000) 2379. [10] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. [11] J. Kalinowski, W. Stampor, J. M˛ez˙ yk, M. Cocchi, D. Virgili, V. Fattori, P. Di Marco, Phys. Rev. B 66 (2002) 235321. [12] M. Cocchi, V. Fattori, D. Virgili, C. Sabatini, P. Di Marco, M. Maestri, J. Kalinowski, Appl. Phys. Lett. 84 (2004) 1052.
Highly efficient organic electroluminescent devices based on cyclometallated platinum complexes as new phosphorescent emitters M. Cocchia,∗ , D. Virgilia , C. Sabatinia , V. Fattoria , P. Di Marcoa , M. Maestrib , J. Kalinowskic a
Institute of Organic Synthesis and Photoreactivity, National Research Council of Italy, Via P. Gobetti 101, 40129 Bologna, Italy b Department of Chemistry “G. Ciamician”, University of Bologna, 40126 Bologna, Italy c Department of Molecular Physics, Technical University of Gda´ nsk, 80-952 Gda´nsk, Poland Received 28 April 2004; accepted 17 May 2004
Abstract High efficiency electroluminescence (EL) in organic light emitting diodes (OLED) was achieved using cyclometallated platinum(II) complexes as new phosphorescent emitters. OLEDs containing bis[2-(2-thienyl)pyridine]platinum(II) [Pt(thpy)2 ] or bis[2-(5-trimethylsilanyl-2thienyl)-pyridine]platinum(II) [Pt(thpy-SiMe3 )2 ] show a high external EL quantum yield reaching 5.4 and 11.5% photon/electron, respectively. Both electroluminescent spectra fall in the “amber” color region defined by the automotive lighting standards; specifically the CIE coordinates are (0.58; 0.42) for Pt(thpy)2 and (0.60; 0.39) for Pt(thpy-SiMe3 )2 . © 2004 Elsevier B.V. All rights reserved.
1. Introduction The light generation in OLED is the result of the formation of emissive state via recombination of charge carrier injected from electrodes. Due to spin statistics from electron–hole recombination three times more triplets than singlet excited states are created. In common electrofluorescent devices only the singlets contribute directly to the emission and triplets, non-radiatively relaxing, are lost for light generation. Using phosphorescent materials with radiatively active triplets, instead, provides a straight way to a significant increase in quantum efficiency of organic LEDs [1]. It was originally realized by Forrest and co-workers in organic LEDs based on organometallic complex phosphors, Ptbased dye 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3 ), acting as guest emitters in a layer co-evaporated with a host material of tris-(8-hydroxyquinoline) aluminium (Alq3 ) or 4,4 -N,N -dicarbazole-biphenyl (CBP) [2,3]. We ∗
Corresponding author. Tel.: +39 051 6399818; fax: +39 051 6399844. E-mail address: [email protected] (M. Cocchi).
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.05.021
have proposed a more easy way, using the spin coating technique and casting an emitting layer from a solution containing both the host and guest molecules [4], with no need of any active polymers as a host matrix [5]. This method allows an easier and better control of the doping level, which is a very critical point for the uniformity of amorphous films, affected by the cooling conditions in the thermal co-evaporation technique. Moreover, the spin coating technique provides a possibility to employ several compounds (neutral or ionic species) that cannot be sublimed because of their chemical and physical instabilities. In the past 20 years, the photophysical properties of a large number of phosphorescent organometallic complexes have been investigated and in particular cyclometallated iridium and platinum complexes have been shown to exhibit interesting optical properties in solution at room temperature, originating from the radiative decay of the triplet metal-toligand charge-transfer (MLCT) excited state allowed by the strong spin–orbit coupling effect of the heavy metal [6]. In search of new LED emitters, we have synthesized two cyclometallated platinum complexes: Pt(thpy)2 and Pt(thpySiMe3 )2 , which show an intense room temperature lumi-
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M. Cocchi et al. / Synthetic Metals 147 (2004) 253–256
Co., and PC (mw 32,000–36,000) from Polysciences Inc. All materials were used as supplied. The 60 nm thick host–guest layers were obtained by spin casting at 2000 rpm a 10 mg/ml dichloromethane solutions of TPD (74 wt.%), PC (20 wt.%), and Pt-complex (6 wt.%) mixtures onto quartz substrates for optical measurements and onto ITO coated glass substrates (20 /cm2 ) for OLED construction. The ETL of PBD was deposited over the spin coated film by high vacuum deposition (0.1 mPa) to the final thickness of 60 nm. The sandwich devices were completed by a high vacuum deposited Ca (25 nm thick) covered with a protecting Ag layer 100 nm thick. Each layer thickness was measured with a Tencor Alpha Step 200 profilometer. Absorption, PL and EL spectra were measured using a Perkin Elmer Lambda 9UV/Vis/NIR spectrometer and a Spex Fluorolog spectrofluorimeter. The time-resolved PL measurements were made using a singlephoton IBH Model 5000 counter. The current–voltage characteristics were measured with a Keithley Source-Measure unit model 236 under continuous operation mode, while the output light power was measured with an EG&G power meter. All measurements were carried out at room temperature, and the devices and thin film samples were kept in argon atmosphere to prevent the negative effects of oxygen and humidity. All characteristics have been reproduced for many runs excluding irreversible chemical or morphological changes in the samples.
Fig. 1. (a) Molecular structures of the materials used: (TPD) N,N diphenyl-N,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine; (PBD) 2(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4 oxadiazole; (PC) bisphenol-Apolycarbonate; (PtOEP) 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum (II); [Pt(thpy)2 ] bis-[2-(2-thienyl) pyridine] platinum(II); [Pt(thpySiMe3 )2 ] bis-[2-(5-trimethylsilanyl-thiophen-2-yl-pyridine] platinum (II). (b) Configuration of the electroluminescent device.
nescence in liquid solution (the photoluminescence quantum yield are 0.35 ± 0.05 for both compounds in deoxygenated solution). Both these complexes are not stable toward thermal deposition. We have realized double-layer (DL) devices with a spin-cast first layer of these compounds as guests in a host matrix that consists of N,N -diphenylN,N -bis(3-methylphenyl)-1,1 -biphenyl-4,4 diamine (TPD) as hole transporting molecule and polycarbonate (PC) as a matrix preventing molecular movements, and a second layer of 2-(4-biphenyl)-5-(4-tert-butylphenyl)1,3,4 oxadiazole (PBD) as electron transport layer (ETL) deposited by vacuum evaporation. Fig. 1 shows the chemical structure of the active molecules and the DL OLED configuration.
2. Experimental The cyclometallated Pt-complexes were synthesized as reported in Ref. [7] and purified by chromatography. TPD and PBD were purchased from Aldrich, PtOEP from H.W. Sands
3. Results and discussion Our discussion is focused on cyclometallated Ptcomplexes as emitters in OLEDs but to evidence their good performance, they are compared with the well known PtOEP emitter [2]. In cyclometallated Pt-complexes the emission originates from the lowest metal-to-ligand-charge-transfer (MLCT) triplet state, while in PtOEP the state responsible of radiative emission is the lowest ligand-centered (LC) triplet state. Fig. 2 compares the PL spectra of TPD:PC:Ptcomplex films with the PL spectrum of the undoped film. It shows that the TPD molecule is an excellent sensitizer for the emission of all Pt-complexes; the emission spectrum is, in fact, due to the phosphorescent decay of the 3 Ptcomplex* triplet excited states, this being confirmed also by their long lifetimes (τ ∼ = 80 s for PtOEP and τ ∼ = 8 s for both the cyclometallated complexes). Hence, the energy transfer process from 1 TPD* singlet excited state generated by light absorption to the Pt-complexes is very efficient. The strong emission at 650, 580 and 590 nm observed in PL spectra for PtOEP, Pt(thpy)2 and Pt(thpy-SiMe3 )2 complexes, respectively, is also observed in the electroluminescence (EL) spectra (Fig. 3a). The CIE (Commission Internationale de L’Eclairage) chromaticity coordinates for our devices are (0.71, 0.29) for PtOEP, (0.58, 0.42) for Pt(thpy)2 , and (0.6, 0.39) for Pt(thpy-SiMe3 )2 , as shown in Fig. 3b. All of them fall in the acceptable red emitters range, though the emission color changes from saturated red for
M. Cocchi et al. / Synthetic Metals 147 (2004) 253–256
255
Fig. 2. Photoluminescence (PL) spectra of the host–guest systems 74% TPD:20% PC:6% Pt-complex compared with the PL spectrum of a 75% TPD:25% PC undoped film.
PtOEP to reddish-orange (amber) for Pt(thpy)2 and Pt(thpySiMe3 )2 . Fig. 4 shows the dependencies of the electroluminescence (IEL ) and current density (j) on applied voltage for the Ptcomplexes based devices. The maximum emission (IELmax ) of 610 W/cm2 is achieved at about 21 V with a current den-
Fig. 4. Electroluminescence (IEL ) and current density (j) vs. bias voltage for devices based on PtOEP (a), Pt(thpy)2 (b) and Pt(thpy-SiMe3 )2 (c).
Fig. 3. (a) Electroluminescence (EL) spectra of the ITO/74% TPD:20% PC:6% Pt-complex (60 nm)/PBD (60 nm)/Ca + Ag devices: (1) Pt(thpy)2 , (2) Pt(thpy-SiMe3 )2 and (3) PtOEP. (b) CIE diagram with the coordinates of the emissions from the Pt-complex-based devices and RGB video display standards.
sity of 36 mA/cm2 for the device based on PtOEP, while IELmax = 1000 W/cm2 at 18 V with a current density of 12 mA/cm2 and IELmax = 3650 W/cm2 at 19 V with a current density of 30 mA/cm2 are achieved for devices based on Pt(thpy)2 and Pt(thpy-SiMe3 )2 respectively. The better performances of the cyclometallated complexes are also more evident in Fig. 5 where the external EL quantum efficiencies (ϕEL ) versus bias voltage are compared. OLEDs based on Pt(thpy)2 and Pt(thpy-SiMe3 )2 show an excellent ϕEL reaching 5.4 and 11.5% photon/electron, respectively. The two cyclometallated complexes, while having same ϕPL in solution and same energy transfer efficiency in the TPD:PC layer, show a strong ϕEL difference in the corresponding OLEDs. A relevant portion of EL emission, differently from PL emission, is due to energy transfer from exciplexes or electroplexex formed upon electron–hole recombination at the interface between the TPD and PBD layers [8,9]. The ϕEL decrease (roll-off) is as a rule observed at high voltages (large current density regimes) in all devices [10,11], the effect ascribed to either triplet–triplet annihilation [10] and/or electric field-
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M. Cocchi et al. / Synthetic Metals 147 (2004) 253–256
cent devices, in fact both maximum luminance and external quantum efficiency are similar or higher than PtOEP based OLEDs. Besides, the OLED emission colors for both compounds falls in a spectral region of great interest for “amber” automotive lighting standards.
Acknowledgment This work was supported in part by MIUR project “Nanotechnologies” (legge 95/95), FIRB project RBNE019H9K entitled “Nanometric machines through molecular manipulation” and EUREKA project entitled “Flexible organic illuminators for automotive market”. Fig. 5. External quantum efficiency (ϕEL ) vs. bias voltage for devices with the three Pt-complexes.
enhanced dissociation of emissive states and their precursors [11]. The higher ϕEL in the device with Pt(thpy-SiMe3 )2 as the emitter can be attributed to a lower dissociation efficiency of excited states in high electric fields [12]. Anyway, OLEDs based on cyclometallated Pt-complexes show a lower effect compared with PtOEP based OLEDs. Further studies are necessary to understand which of the following quenching mechanisms more strongly contributes to the observed unwanted effect on ϕEL : triplet–triplet exciton annihilation, triplet exciton–charge carrier interaction, electric field-assisted dissociation of excitons and temperature effect on phosphorescence.
4. Conclusions We have used cyclometallated Pt-complexes as phosphors in simple DL devices and have shown that they are excellent candidates as emitting materials in electrophosphores-
References [1] J. Kalinowski, J. Phys. D 32 (1999) R179. [2] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson, S.R. Forrest, Nature (London) 395 (1998) 151. [3] M.A. Baldo, S. Lamansky, P.E. Burrows, M.E. Thompson, S.R. Forrest, Appl. Phys. Lett. 75 (1999) 4. [4] M. Cocchi, P. Di Marco, V. Fattori, G. Giro, J. Kalinowski, W. Stampor, D. Virgili, IT Patent No. BO2002A000165 (2002). [5] S. Lamansky, R.C. Kwong, M. Nugent, P.I. Djurovich, M.E. Thompson, Org. Electron. 2 (2001) 53. [6] M. Maestri, V. Balzani, C. Deuschel-Cornioley, A. von Zelewsky, in: D. Volman, G. Hammond, D. Neckers (Eds.), Advances in Photochemistry, vol. 17, Wiley, New York, 1992, p. 1. [7] L. Chassot, A. von Zelevsky, Inorg. Chem. 26 (1987) 2814. [8] G. Giro, M. Cocchi, J. Kalinowski, P. Di Marco, V. Fattori, Chem. Phys. Lett. 318 (2000) 137. [9] J. Kalinowski, M. Cocchi, P. Di Marco, W. Stampor, G. Giro, V. Fattori, J. Phys. D 33 (2000) 2379. [10] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967. [11] J. Kalinowski, W. Stampor, J. M˛ez˙ yk, M. Cocchi, D. Virgili, V. Fattori, P. Di Marco, Phys. Rev. B 66 (2002) 235321. [12] M. Cocchi, V. Fattori, D. Virgili, C. Sabatini, P. Di Marco, M. Maestri, J. Kalinowski, Appl. Phys. Lett. 84 (2004) 1052.