Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical

Article pubs.acs.org/JPCC

Achilles Heels of Phosphine Oxide Materials for OLEDs: Chemical Stability and Degradation Mechanism of a Bipolar Phosphine Oxide/ Carbazole Hybrid Host Material Na Lin,† Juan Qiao,*,† Lian Duan,† Haifang Li,‡ Liduo Wang,† and Yong Qiu† †

Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education and ‡Beijing Key Laboratory for Analytical Methods and Instrumentation, Department of Chemistry, Tsinghua University, Beijing, 100084, China ABSTRACT: For long-living organic light-emitting diodes (OLEDs), the chemical stability of all employed materials is essential. In this work, we take a typical bipolar material, 9-(3,5-bis(diphenylphosphoryl)phenyl)-9H-carbazole (CzPO2), as an example for exploring the intrinsic chemical stability of the hot-spot phosphine oxide (PO) based materials for OLEDs. Compared to the carbazole-only counterparts, PO-based carbazole materials typified by CzPO2 have prominent advantages in terms of electrochemical stability and bipolar character, which are generally required for improving the device stability. However, we discovered that CzPO2 suffers a fatal chemical instability just originating from the PO moieties. Under UV irradiation or electrical stress, the identified degradation products of CzPO2 point to the dissociation of relatively weak C−P bonds as the initiating step. Quantum chemical calculations were carried out to gain further insight into the role of the C−P single bond in the intrinsic degradation mechanism associated with the aging process of OLEDs. The cleavage of vulnerable C−P single bonds may occur not only in excited states, but also more easily in charged states. These findings strongly suggested that the chemically unstable C−P bond of PO derivatives could undermine the stability of the corresponding OLEDs, regardless of the function that the PO materials played in devices. For improving the lifetimes of OLEDs, it is highly suggested to consider the relative bond strengths in charged states or excited states of OLED materials, in addition to the generally required thermal stability. driving voltage.18−29 In particular, by employing bipolar PO/ carbazole hybrid host materials, extraordinarily high external quantum efficiency approaching the theoretical limit (above 20%) has already been achieved for blue, even deep blue, PhOLEDs.17 It seems that these PO-based materials meet almost all the key requirements as an effective host for PhOLEDs. However, to the best of our knowledge, little is known about the degradation and lifetime of the above materials and devices. In our group, we incorporated carbazole and diphenylphosphine oxide units into the star-shaped molecule 9-(3,5bis(diphenylphosphoryl)phenyl)-9H-carbazole (CzPO2), which exhibits desirable bipolar character, high triplet energy, and good thermal stability and solution processability. With CzPO2 as host, the solution-processed PhOLEDs demonstrated excellent performance among the highest reported values for small-molecule-based solution-processed blue and white PhOLEDs.30 For further practical application, its chemical stability is crucial for long-lifetime OLEDs. Herein, we systematically investigated its chemical stability, including electrochemical stability and photochemical stability, and degradation behaviors in electrically aged devices. Quantum chemical calculations were also carried out to help understand

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have been extensively studied over the past decades because of their unique features and potential applications in flat-panel displays and solid-state lighting. The efficiency of OLEDs has been boosted significantly especially since phosphorescent OLEDs (PhOLEDs) were invented.1 However, the device lifetime is still a challenge, in particular for blue PhOLEDs, which has been recognized as the formidable bottleneck for a broad application of this promising technology.2,3 It is generally accepted that the device lifetime is dominated by the intrinsic degradation.4,5 Recent studies have well established that the intrinsic degradation has a chemical basis.2,4,6−14 Through bond cleavages and further radical addition reactions, the resulting degradation products act as deep charge traps, nonradiative recombination centers, and luminescence quenchers. However, up to now, the chemical degradation mechanisms of only a few commonly used organic materials in OLEDs have been known. For PhOLEDs, it is proved that the choice of host materials dramatically influences the device lifetime.11,15,16 To date, the host materials for PhOLEDs have been based mainly on holetransporting carbazole.3 Recently, aryl phosphine oxide (PO) derivatives have attracted great attention, due to characteristics such as high triplet energy and efficient electron transport, for use as host or electron-transport materials in PhOLEDs.17 Many PO-based host materials have demonstrated improved device performance in terms of high device efficiency and low © 2012 American Chemical Society

Received: June 2, 2012 Revised: August 16, 2012 Published: August 22, 2012 19451

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Figure 1. Repeated cyclic voltammograms for CzPO2 and 3Cz in CH2Cl2 and DMF solutions.

stricted method was used for the geometry and frequency calculations of neutral radicals, cation radicals, and anion radicals. The bond dissociation energy (BDE) was calculated according to the enthalpy change in the corresponding reaction of homolytic cleavage of a single bond in the gas phase at 298 K and 1 atm.34 Time-dependent DFT (TD-DFT) at the B3LYP/ 6-31G(d) level was used to calculate the excited states energies of CzPO2 molecules.

the intrinsic degradation mechanism of CzPO2 associated with the aging process of OLEDs.

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. CzPO2 was synthesized as described in a previous report.30 Cyclic voltammetry (CV) was performed on a Princeton Applied Research potentiostat/ galvanostat Model 283 voltammetric analyzer. The laser desorption ionization (LDI) mass spectra were obtained using a Bruker Daltonics MALDI-TOF Auto flex instrument in both positive and negative detection modes with an applied high voltage of 19 kV between the target and the aperture of the time-of-flight analyzer. The samples were excited at a wavelength of 337 nm, where the maximum pulsed laser beam intensity can be set to 118 μJ/pulse (at 50 Hz), a spot size of 0.01 mm2, and a pulse duration of 3−5 ns. High performance liquid chromatography (HPLC) analysis and preparative separations of extracts of operationally degraded OLEDs were performed with an Agilent 1100 Series instrument equipped with an Agilent ZORBAX SB-C18 chromatographic column. The mass spectra of the electrical degradation products were performed using electrospray ionization mass spectroscopy (ESI-MS). 2.2. Device Fabrication and Aging. The electron-only device contained the following layers: ITO/CzPO2 (120 nm)/ CsCO3 (2 nm)/Al (100 nm). The hole-only device containeed the following layers: ITO/CzPO2 (120 nm)/HAT−CN (5 nm)/Ag (5 nm)/Mg:Ag (100 nm)/Ag (5 nm). To suppress degradation caused by water or oxygen from air or by deterioration of the encapsulation glue by UV radiation, the devices were aged in a nitrogen atmosphere. Immediately after preparation, the devices were transferred to a nitrogen-filled glovebox and aged under a constant current density of 100 mA/cm2 for 24 h. An undriven reference sample was stored for the same time under the same conditions. After dissolving the devices, we analyzed the solutions by HPLC/ESI-MS to identify the possible degradation products due to the electrical aging. Here ESI-MS was chosen to overcome the propensity of these products to fragment when ionized like LDI-TOF-MS. 2.3. Quantum Chemical Calculations. All our calculations were performed by using the Gaussian 03 software package.31 Considering the prohibitively large molecule size, the geometry optimization and frequency calculations were carried out at the level of B3LYP/6-31G(d)32,33 for each molecule, radical, and charged species. The standard unre-

3. RESULTS AND DISCUSSION 3.1. Electrochemical Stability. The charge-carrier transport in small molecules is regarded as a chain of redox processes between the neutral molecules and the corresponding radical ions.35 The irreversible anodic oxidation of the prototypical tris(8-hydroxyquinoline)aluminum (Alq3) and the instability of cationic Alq3 species were discovered to be responsible for the long-term degradation of Alq3-based OLEDs.36 Therefore, materials used in the emitting layer should desirably possess a bipolar character, that is, have both electron-transporting and hole-transporting properties, to permit the formation of both stable cation and stable anion radicals.37,38 In previous work, we found that CzPO2 has the desirable bipolar character. Here, we focused on its electrochemical behaviors under repeated CV scans. The PO-free 1,3,5-tri(N-carbazolyl)benzene (3Cz), which has a similar starshaped structure, was selected as the reference sample. As shown in Figure 1, CzPO2 not only undergoes fully reversible oxidization and reduction, but its CV profile and redox potentials remain unchanged, even after more than 10 repeated scans. This indicates that both the cations and anions of CzPO2 are stable entities in solutions, which agrees well with its bipolar character. In contrast, the PO-free counterpart exhibited irreversible reduction and oxidation. The oxidation potential gradually shifts to lower potential and the current increases during repeated CV scans. Similar characteristics were also found on the other carbazole-based host materials.39,40 Therefore, it is clear that PO materials are superior to the carbazole-only counterparts in terms of the electrochemical stability and bipolar character, which is generally anticipated to enhance the device stability. 3.2. Photochemical Behaviors. In addition to the electrochemical stability, the emitting materials should be robust toward excitons. We investigated the photochemical behaviors of CzPO2 using laser desorption ionization time-offlight mass spectrometry (LDI-TOF-MS). Leo’s group has 19452

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Figure 2. LDI-TOF-MS spectra of CzPO2 taken at low laser energy: (a) 21% (positive mode) and (b) 15% (negative mode) of the maximum output energy per pulse laser.

Figure 3. LDI-TOF-MS spectra of CzPO2 taken in (a) 26% (positive mode), (b) 17% (negative mode), and (c) 32% (positive mode) of maximum output energy per pulse laser.

μJ/pulse) in positive and negative modes, respectively. Both spectra show the expected molecule ion mass/charge (m/z) signals of 644.010 in positive mode and 642.619 in negative mode. Also, the spectra display some unexpected fragmentation signals even under such low laser energy. In positive mode, the signal at 443.019 amu corresponds to the positively charged fragment [CzPO2−Ph2PO]+. That points to heterolytic cleavage of the C−P single bond between the aryl cation and the diphenylphosphine oxide free radical. In negative mode, the signals at 565.322 and 581.430 amu correspond to [CzPO2− Ph]− and [CzPO2−Ph+O]−, respectively. That refers to homolytic cleavage of the C−P single bond between the aryl phosphine oxide anion and the phenyl free radical. To note, CzPO2 has the hybrid structure of PO and carbazole units.

successfully applied this powerful technique to discover the laser induced photochemical reaction mechanism of organic layers as well to investigate the possible reaction products in fully processed organic devices.2,6−10 In our case, the sample is only pure CzPO2. To avoid the formation of additional photochemical reactions during the analysis process, the laser intensity was controlled as low as possible. We set it to gradually increase to help track the possible degradation process. Unfortunately, we found CzPO2 is highly reactive under UV irradiation. Photoinduced dissociation reactions can readily occur at the weakest point of the C−P single bond in the excited charged molecules of CzPO2. Figure 2 shows the LDITOF-MS spectra of CzPO2 taken at low laser energy (18−25 19453

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Scheme 1. Proposed Three Possible Pathways of Photochemical Reactions by LDI Experiments at Positive Mode

laser intensity at 47 μJ; a series of new peaks occur at higher m/ z of 720.233, 843.917, 1085.023, 1120.011, 1209.006, and 1285.011 amu. The signals detected at 720.233, 843.917, 1085.023, and 1209.006 amu are tentatively attributed to [CzPO2+Ph]+, [CzPO2+Ph2PO]+, [2CzPO2−Ph2PO]+, and [2CzPO2−Ph]+. To note, the signal at 1120.001 amu corresponds to the fragment [2CzPO2−carbazole]+, which indicates the dissociation at the C−N bond at higher laser intensity. However, that signal intensity is much lower than the

Apparently, these results reveal that the weakest bond of the molecule is just the C−P bonds from aryl phosphine oxide moieties, not the C−N bonds. The high intensity of fragment signals leads to the assumption that CzPO2 undergoes a strong fragmentation in the excited state and may therefore dissociate even under a low intensity of UV irradiation. With the increase of the laser intensity, additional peaks came out at higher m/z ratios, whether positive or negative mode was used. Figure 3c shows the spectrum in positive mode with the 19454

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others due to the dissociation of C−P bonds. The CzPO2 dimer at 1285.011 amu is also observed, albeit in much lower intensity. According to the above LDI-TOF mass spectra, we attempted to propose the following three possible pathways of photochemical reactions starting from the excited positively charged CzPO2 molecule (Scheme 1). In pathway 1, the excited positively charged CzPO2 molecule dissociates heterolytically at the C−P single bond between aryl and diphenylphosphine oxide with the formation of the fragment [CzPO2−Ph2PO]+ ion and diphenylphosphine oxide neutral radical. In pathway 2, a similar dissociation can also occur at another C−P single bond between one phenyl and PO moiety, by the loss of a phenyl radical and accompanied by the formation of the [CzPO2−Ph]+ ion. In the further reactions, the resulted positively charged ions and the highly reactive free radicals will quickly form complexes with another CzPO2 molecule nearby. In these cases, the reactive sites are varied, so the exact structures are too complex to figure out. Under high laser intensity, as described in pathway 3, a relatively weak dissociation at the C−N single bond will also be detected, yielding two complicated species of [CzPO2+carbazole−H]+ and [2CzPO2−carbazole−H]+. According to the spectra in negative mode, the excited negatively charged CzPO2 molecule and the related fragmentation and complexation anions are also obtained and recognized. Therefore, the excited negatively charged CzPO2 molecule suffers similar dissociation and further complexation reactions also starting at the scission of C−P single bonds. For clarity, the corresponding possible photophysical reactions are omitted. 3.3. Electrical Degradation in Stressed Devices. In order to investigate the intrinsic chemical behaviors of CzPO2 in electrically aged OLED devices, we studied the degradation products of its single-carrier-charge devices at 100 mA/cm2 for 24 h. The electron-only and hole-only devices have the structures of ITO/CzPO2 (120 nm)/CsCO3 (2 nm)/Al (100 nm) and ITO/CzPO2 (120 nm)/HAT−CN (5 nm)/Ag (5 nm)/Mg:Ag (100 nm)/Ag (5 nm), where HAT−CN is 1,4,5,8,9,11-hexaazatriphenylene−hexacarbonitrile and is used as a hole injection layer between CzPO2 and Ag. Not surprisingly, we found that CzPO2 undergoes some similar chemical processes in the electrically aged single-charge-carrier devices as under UV irradiation. Figure 4 shows the difference of the chromatograms for the solutions of electron-only devices operated at 100 mA/cm2 for 0 and 24 h. Besides visible negative peaks corresponding to losses of CzPO2, several positive peaks labeled with arrows arise as the result of degradation. The highest peak accounts for 38.5% of the degradation products, whose ESI-MS spectra gave a strong peak at m/z = 568.5, tentatively identified as [CzPO2−Ph+H] by the molecular weight. That species is a direct degradation product during device aging, and it is the cleavage product at the point of C−P bonds. Traces of other degradation products remain unidentified. Similarly, detectable degradation products were also discovered from the aged hole-only devices, but these products are too complex to identify. Although the underlying chemical mechanisms are too complex to be completely elucidated, this identified degradation product corresponds to the dissociation of relatively weak C−P bonds as the initiating step. 3.4. Quantum Chemical Calculations. With the aid of quantum chemical calculations, we obtained further insight into the role of the C−P single bond in the chemical degradation of

Figure 4. Difference chromatogram of the solutions extracted from electron-only devices after 0 and 24 h of operation at 100 mA/cm2. The inset shows the possible chemical structure of the identified degradation products (CzPO2−Ph+H).

PO materials. Since charge recombination is one of the main pathways to generate excitons in OLEDs, the most relevant excited states of molecules in the devices would be either S1 or T1 states. Thus, it is essential to find out whether CzPO2 molecules in these excited states undergo C−P bond cleavage. In principle, it depends on the relative C−P bond dissociation energy (BDE) and the excited state energies.12 Although the experimental bond dissociation energies for CzPO2 are not available, the density functional calculations provided the average BDE of a C−P single bond as about 81.2 kcal/mol (listed in Table 1), which is almost the same as that of a C−N bond (80.7 kcal/mol) in the CzPO2 molecule. In view of the B3LYP functional with the relatively large average absolute deviation from experimental heats of formation,34 we also calculated the excited state energies of S1 (3.58 eV, 82.6 kcal/ mol) and T1 (3.18 eV, 73.2 kcal/mol) through the TD-DFT calculation with the same B3LYP model. It is clear that the BDEs of both C−P and C−N single bonds are within 2 kcal/ mol less than the S1 energy of CzPO2, which suggests that unimolecular homolytic dissociations of C−P and C−N single bonds may occur in the singlet excited states. Considering the transport of electrons or holes into CzPO2, we also calculated the dissociation energies of C−P and C−N bonds in these charged species. Surprisingly, the dissociation energy of the C−P bond in charged species is significantly reduced. In particular, the anion radical of CzPO2 gives the dissociation energy of the C−P bond between the phenyl radical and the aryl PO anion reduced nearly by half (42.9 kcal/ mol). This is in good accordance with the above identified degradation products due to the cleavage of the C−P bond in the LDI-TOF-MS measurements and the aged electron-only devices. In contrast, the dissociation energy of the C−N bond in cation radical does not decrease, but markedly increases over 12 kcal/mol. Therefore, it is reasonable to conclude that the cleavage of a vulnerable C−P single bond may occur not only in excited states, but also more easily in charged states. These results quantitatively confirm that the C−P single bond in aryl PO materials is more vulnerable than the C−N bond in carbazole-based materials. As for the carbazole-based materials such as 4,4′-bis(N-carbazolyl)biphenyl (CBP), the homolytic cleavage of the exocyclic C−N bonds in excited states has been revealed as the key step in operational degradation of OLEDs.11 19455

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Table 1. Bond Dissociation Energies (BDEs) for the C−P and C−N Bonds in the CzPO2 Molecule bond

initial state

bond dissociation reaction

C−P

neutral neutral cation anion cation anion neutral cation anion

CzPO2 → [CzPO2−Ph]• + Ph• CzPO2 → [CzPO2−Ph2PO]• + Ph2PO• CzPO2+ → [CzPO2−Ph]+ + Ph• CzPO2− → [CzPO2−Ph]− + Ph• CzPO2+ → [CzPO2−Ph2PO]+ + Ph2PO• CzPO2− → [CzPO2−Ph2PO]− + Ph2PO• CzPO2 → [CzPO2−Cz]• + Cz• CzPO2+ → [CzPO2−Cz]+ + Cz• CzPO2− → [CzPO2−Cz]− + Cz•

C−N

4. CONCLUSIONS In conclusion, CzPO2, a typical bipolar host material for PhOLEDs, was taken as an example to explore the intrinsic chemical stability of the hot-spot PO-based materials and possible chemical mechanism associated with the operational degradation of OLEDs. Thanks to the unique hybrid structure of the PO and carbazole units, we have simultaneously got reasonable information about the key differences between the two important building blocks of PO and carbazole for host materials of PhOLEDs. PO-based materials typified by CzPO2 prominently outperform the carbazole-only counterparts in terms of electrochemical stability and bipolar character. However, it suffers a fatal chemical instability just at the weakest point of C−P bonds rather than C−N bonds. The cleavage of the C−P single bond appears to occur not only in excited states, but also more easily in charged states. These results reveal a potential problem of device lifetime for the ongoing development of PO-based materials in practical OLEDs, albeit they have outstanding device efficiency. AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-62773109. Fax: +86-10-62795137. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China under Grant 51073089 and the National Key Basic Research and Development Program of China under Grant 2011CB808403. J.Q. is grateful to Prof. Xi Zhang and Prof. Bo-Qing Xu for their valuable suggestions on the preparation and submission of the manuscript.

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81.5, 81.3, 81.4 80.8 68.9, 72.0, 72.0 44.3, 41.3, 41.6 88.8 59.9

av BDE(kcal/mol) 81.2 70.5 42.9 88.5 59.8 80.7 93.4 61.3

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Therefore, it is envisioned that the CzPO2 molecule could undergo C−P bond cleavage in electrical and photoexcited devices. These results strongly suggested that the chemically unstable C−P bond of PO derivatives could undermine the stability of the corresponding OLEDs, regardless of the function that the PO materials played in devices.

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BDE(kcal/mol) 81.2, 80.9, 68.9, 44.5, 88.2, 59.7, 80.7 93.4 61.3

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