Degradation Mechanisms in Blue Organic Light-Emitting Diodes

Open AccessCCS ChemistryMINI REVIEW1 Aug 2020Degradation Mechanisms in Blue Organic Light-Emitting Diodes Dan Wang†, Cong Cheng†, Taiju Tsuboi and Qisheng Zhang Dan Wang† MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 , Cong Cheng† MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 , Taiju Tsuboi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 and Qisheng Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027 https://doi.org/10.31635/ccschem.020.202000271 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail An organic light-emitting diode (OLED) is required to exhibit long-time operation without degradation as an inorganic LED. Sufficiently long operation time has been demonstrated for green- and red-emitting OLEDs. However, a blue device that is important for full-color display and lighting exhibits a much shorter operational lifetime than the other color devices. The short lifetime is mainly attributed to the molecular dissociation and the defects and radical species formation through various unimolecular and bimolecular processes, including direct photolysis, exciton–exciton interaction, and exciton–polaron interaction, and so on. Different novel techniques of chemistry and physics have been employed for blue devices to suppress the degradation process induced by high-energy excitons. A deep understanding of the degradation mechanism is still needed for all three kinds of blue OLEDs employing fluorescence, phosphorescence, and thermally active delayed fluorescence. In this brief review, we introduce and discuss several degradation mechanisms for these three kinds of OLEDs. Download figure Download PowerPoint Introduction The realization of efficient and stable blue light-emitting diodes (LEDs) allows LED technology to be used in the solid-state light source and various electronic equipment. In contrast to the single-crystal-based inorganic LEDs, organic light-emitting diode (OLED) fabricated by rapid organic thin-film deposition is considered as an ideal technique for large-area high-resolution displays.1 In recent years, more than 10 active-matrix OLED (AMOLED) production lines have been built in East Asia, although the stability problem in blue OLEDs has not been entirely overcome (see Tables 1 and 2).2–25 The highly efficient blue OLEDs employing phosphorescence (PHOLEDs) have a problem of short operation lifetime. In commercial OLED displays, inefficient blue fluorescence OLEDs (FOLEDs) work together with efficient green and red PHOLEDs, pulling down the total efficiency and reliability of the displays. A deep understanding of the degradation mechanism in blue OLEDs will help us to find new approaches to break the bottleneck in the OLED industry and realize a leap-frog style development in display technology. In this brief review, we distinguish the blue OLEDs into three categories based on their luminescence mechanisms, that is, FOLEDs, PHOLEDs, and thermally activated delayed fluorescence (TADF) OLEDs, and provide an introduction to the current status on these devices, focusing on the operational lifetimes and degradation mechanisms. Table 1 | The International Commission on Illumination (CIE) Coordinates, Current Efficiency (ηc), and Half-Life (LT50) of Commercialized FOLEDs and PHOLEDs2 Devices CIE ηc (cd/A) LT50 (h) Red FOLED 0.67, 0.33 11 160,000 Green FOLED 0.29, 0.64 37 200,000 Blue FOLED 0.14, 0.12 9.9 11,000 Red PHOLED 0.67, 0.33 22 20,000 Green PHOLED 0.33, 0.63 64 200,000 Note: Lifetimes were measured with an initial luminance of 1000 cd/m2. Table 2 | Lists of Recent Device Performances of the Blue OLEDs Emitters CIE EQEmax (%) L0 (cd/m2) Lifetime (h) References FOLED DPAVBi — 3.7 1000 200 (LT67) 3 BITPI 0.15, 0.13 6.6 500 55 (LT50) 4 3Me-1Bu-TPPDA 0.13, 0.15 9.3 1000 1160 (LT50) 5 DFDPA 0.14, 0.12 — 500 300 (LT97) 6 BmPAC 0.13, 0.17 6.5 2100 83 (LT80) 7 DPFCz-TRZ 0.15, 0.10 15.5 500 78 (LT50) 8 PHOLED Ir(dmp)3 0.15, 0.29 18 1000 616 (LT80) 9 MS2 0.16, 0.38 25 327 2203 (LT50) 10 Ir(dbi)3 0.19, 0.41 12.7 1000 118 (LT70) 11 Ir(dpIPY)3: A 0.14, 0.27 10.5 4000 79 (LT70) 12 Ir4 – 17.8 500 93 (LT70) 13 PtNON: TBPe 0.16, 0.25 15.2 1000 602 (LT70) 14 TADF OLED DDCzTrz 0.16, 0.22 5.5 500 52 (LT80) 15 DCzTrz 0.15,0.16 14.9 500 18 (LT80) 15 4CzBN 0.17, 0.20 10.6 500 62 (LT50) 16 4TCzBN 0.16, 0.22 16.2 500 167 (LT50) 16 5CzBN 0.17, 0.34 20 500 700 (LT50) 17 3Ph2CzCzBN 0.17, 0.36 17.8 1000 118 (LT80) 18 oBFCzTrz 0.18, 0.31 20.4 191 2.6 (LT95) 19 BDpyInCz 0.17, 0.27 8.7 1000 21 (LT80) 20 BCz-TRZ 0.20, 0.36 8.8 1000 454 (LT50) 21 DCz-TRZ 0.16, 0.38 15.5 500 80 (LT50) 22 ICz-TRZ 0.14, 0.19 11.8 1000 95 (LT50) 23 ICz-TRZ 0.15, 0.23 18.1 1000 174 (LT50) 23 Exciplex: v-DABNA —, 0.36 18 1260 300 (LT50) 24 TADF sensitizer: t-DABNA 0.13, 0.12 32.5 1000 60 (LT80) 25 Overview on the lifetime studies of blue OLEDs Unlike photoluminescence (PL), the molecule in OLED, which is responsible for electroluminescence (EL), captures an electron and a hole injected from electrodes simultaneously. It has a 1/4 probability to generate the lowest singlet (S1) state exciton and a 3/4 chance to create the lowest triplet (T1) state exciton with lower energy than the singlet exciton (Figure 1a). The earliest commercial passive-matrix OLEDs (PMOLEDs) were operated by doping high-efficiency fluorescent blue, green, or red dye in each emitting layer (EML) for a full-color display. Figure 1b shows some currently reported stable blue fluorophores having a short radiative lifetime (∼2 ns) and small structural deformation in their S1 state,3,4–25 which enable a high color purity and a reduced device efficiency roll-off at high luminance. These compounds are not emissive in their T1 state. Therefore, the electro-optical conversion efficiency of the fluorescent molecule-based OLED is only 25% at most. Although, several bright ideas currently realize high efficiency of FOLED by employing triplet–triplet annihilation (TTA),26,27 TADF,24,28,29 and hot exciton assisted singlet harvest,30,31 the operational lifetime of blue FOLEDs is still at least one order of magnitude shorter than that of the green and red devices (Table 1). Table 2 summarizes some laboratory works with repeatable experiment details.3–25 The LT97 of blue [International Commission on Illumination (CIEy) < 0.2] fluorescent devices with an initial luminescence (L0) of 500 cd/m2 is generally a few hundred hours. Here LT97 means a time of EL luminance to reach 97% of the L0 value. However, it is not fruitful to compare these lifetime data because the color coordinate CIE, initial luminescence, and efficiency are not the same in all the reports. Figure 1 | (a) Luminescence mechanism of OLEDs employing fluorescence, phosphorescence, and TADF. Flu = fluorescence; Ph = phosphorescence; PF = prompt fluorescence; DF = delayed fluorescence; ISC = intersystem crossing; RISC = reverse intersystem crossing; ΔEST = the energy difference between the S1 and T1; IC = internal conversion. (b) Chemical structures of some blue emitters reported in refs 3–25. Download figure Download PowerPoint In the late 1990s, luminescent complexes containing noble metals such as platinum and iridium were introduced in OLEDs.32–36 Due to the heavy atom effect, the S1 → T1 intersystem crossing (ISC) and T1 → S0 radiative transition become less forbidden (Figure 1a), shortening the phosphorescence lifetime to approximately 1 µs. The energy utilization ratio of OLED employing phosphorescence is up to 100%, achieved by fully harvesting singlet and triplet excitons. Red and green phosphorescent materials have been widely used in commercial AMOLEDs, but the stability of blue PHOLEDs needs to be further improved. A state-of-the-art sky blue PHOLED achieved an LT80 of 616 h with an L0 of 1000 cd/m2,9 although such a lifetime value is still incomparable with that of FOLED. Note that there are only a few reports available on the lifetime of deep blue phosphorescent devices with CIEy < 0.2. Since 2011, metal-free TADF emitters have been successfully applied to OLED,37–43 and achieved high efficiency, comparable to that of phosphorescent devices based on noble metallic compounds such as popular Ir(ppy)3 (Figure 1a).34–36 Owing to the lack of relatively weak coordination bonds in TADF molecules, TADF materials are expected to overcome the technical bottleneck of blue OLEDs. In 2015, Kim et al.15 indeed reported two stable blue TADF molecules DDCzTrz and DCzTrz, whose device color coordinates were 0.16, 0.22 and 0.15, 0.16, and LT80 reached 52 and 18 h with an L0 of 500 cd/m2, respectively. In 2016, Zhang et al.16 reported two blue luminescence TADF molecules 4CzBN and 4TCzBN. The color coordinates of their devices were 0.17, 0.20 and 0.16, 0.22, respectively, and the LT50 reaches 62 and 167 h with an L0 of 500 cd/m2. Recently, using an indolocarbazole/triphenyltriazine derivative (ICz-TRZ) as a luminescent material, we fabricated a stable blue TADF device successfully, which has the color coordinate CIE of 0.14, 0.19 and an LT50 of up to 95 h with an L0 of 1000 cd/m2.23 Note that bluish-green TADF devices could be much more stable than the blue TADF OLEDs as mentioned above.44,45 Some preliminary studies based on controlled experiments indicated that blue TADF OLEDs could be much more stable than blue PHOLEDs.15,46 One might suspect that the device structures used in these experiments might be more favorable for TADF emitters. The early reports on blue TADF device lifetime were encouraging, but the subsequent development has been slow. A full understanding of new types of EL materials would take time, although it is essential. Degradation mechanisms in blue FOLEDs The lifetime of blue FOLEDs cannot match that of green ones (Table 1). One of the reasons is that a significantly higher power (including Joule heat) is needed for the blue device than for green one to obtain the same luminance intensity. In traditional FOLEDs, at least 3/4 of the energy could not be converted to light and eventually dissipates as heat via a T1 → S0 nonradiative transition. The heat generated by the blue fluorescent device is higher than that of the green fluorescent device, resulting in reduced stability of blue OLEDs by the morphology change of the organic layers.47 In addition to the device degradation caused by Joule heat, it is believed that high-energy blue fluorescence could cause direct decomposition of the organic molecule, resulting in reduced stability of the blue devices. Most blue fluorescent materials are the derivatives of aniline and carbazole (Cz).3,5–8 Wide-bandgap host materials generally contain electron-donating/electron-withdrawing fragments such as Cz, aromatic amine, phosphine oxide, triazine, and cyano.20,48–51 The S1 energy level of the blue fluorescent materials could be as high as 3.0 eV, which is equivalent to the bond energy of the C–N bond and higher than the C–P bond. Therefore, the homolysis of the C–X (X = N, P, S, etc.) might occur on the host and guest in the EML,51 and the generated free radicals quench the emitters effectively as exciton quenchers.52 In the devices containing a Cz-based fluorophore, the breaking of the C−N bond in phenyl-carbazole moiety has been demonstrated. Kondakov et al.53 found the degradation products in the devices with EML composed of rubrene-doped 4,4′-N,N′-dicarbazole-biphenyl (CBP) by high-performance liquid chromatography/mass spectrometry (HPLC/MS) and confirmed that the breaking of the C–N bond in the host CBP is responsible to the device degradation. Sandanayaka et al.54 investigated the photodegradation of a (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN)-doped 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) film and revealed that the wide-bandgap mCBP host is extremely unstable under UV light (Figure 2a) in comparison with green-emitting Ir(ppy)3 and 4CzIPN. This suggests that one of the keys to device degradation is on the host materials. Figure 2 | (a) Photostability of 4CzIPN in 15 wt %-doped mCBP film, 4CzIPN neat film and mCBP neat film (left image), and degradation products of 4CzIPN and mCBP (right image). Adapted with permission from ref 54. Copyright 2015 American Chemical Society. (b) Effect of BDE by the substituents on the D–π structure. Reprinted with permission from ref 56. Copyright 2018 American Chemical Society. (c) Chemical structures, BDE, and photostability of sulfonyl-, phosphine oxide-, and carbonyl-containing host materials. Reprinted with permission from ref 60. Copyright 2014 American Chemical Society. Download figure Download PowerPoint Freidzon et al.55 calculated the bond dissociation energy (BDE) and the dissociation activation energy of the C−N and C−C bonds in the charged and excited states for four host materials by the multireference complete active space self-consistent-field (CASSCF)/XMCQDPT2 method. The results showed that the Cz-based molecules degrade naturally in their neutral excited and negatively charged excited states; meanwhile, other states are relatively stable. Wang et al.56 systematically studied the effect of different substitution positions and groups on BDE based on some D(Cz)–π structure molecules (Figure 2b). On the π-side, introducing a large steric hindrance substituent in the ortho-position of 9-PhCz could reduce the BDE by over 0.3 eV. On the D-side, molecules with a 1,8-substituted Cz usually have low BDE values due to their steric hindrance, while molecules with a 3,6-substituted, 2,7-substituted, or 4-substituted Cz, introducing electron-withdrawing groups on these positions of Cz lead to increase the BDE of the corresponding material. In contrast, electron-donating groups would decrease the BDE. These results indicated that the C−N bond connected to the Cz moiety is unstable. By the modification of the Cz groups, the C−N bond with high BDE is obtained to meet the blue OLED requirements. From the devices which employ aniline-based hole transport materials 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TCTA) and N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,10-biphenyl-4,4′-diamine (α-NPD), Sivasubramaniam et al.57 found the degradation products of TCTA and α-NPD created during the device operation. They used a laser desorption ionization equipment, which is coupled with a time-of-flight mass spectrometry (LDI-TOF/MS), for the measurements. Vijaya Sundar et al. used a theoretical approach, the CASSCF method, to study the single-molecule dissociation process of triphenylamine (NPh3) and N,N′-diphenyl-2-naphthylamine (DPNA) in their excited state. They found a conical intersection between the potential energy surfaces of the ground state and the first excited state for these two molecules. Near this cross-point, the molecules were in anionic and free radical state.58 After crossing a small energy barrier to reach the conical intersection, the NPh3 and DNPA molecules could undergo a cyclization reaction to generate dissociation products. Besides, the energy barrier of the intermediate product of the cyclization reaction could be controlled by group modification, which suppressed the decomposition reactions effectively. The aromatic compounds with a ketone, phosphine oxide, and sulfone group are used widely in blue OLEDs as the host materials and exciton blocking layer materials. Lin et al.59 investigated the chemical stability of phosphine oxide-based materials using 9-(3,5-bis(diphenyl phosphoryl)phenyl)-9H-carbazole (CzPO2) as an example. The results of matrix-assisted laser desorption ionization equipment, which is coupled with a time-of-flight mass spectrometry (matrix-assisted laser desorption/ionization time-of-flight mass spectrometry [MALDI-TOF/MS]) and electrospray ionization mass spectroscopy (ESI-MS) methods, indicated that the C−P bond is broken more quickly than the C−N bond in CzPO2. The BDE and transition energy calculations showed that the S1 energy is comparable to the BDE of C−P and C−N (only 2 kcal/mol difference). Consequently, the unimolecular homolytic dissociations of C−P and C−N is suggested to occur in the S1 state of CzPO2. In the anionic state, the BDE of the C-P bond is reduced by half, and therefore the C−P single bond is more easily broken. Subsequently, this research group studied the chemical stability of a series of blue-emitting materials containing different electron acceptors such as diphenylsulfone (DPSO), diphenylthiophene sulfone (DBTO), and ketone (CO) in the excited state and the positive and negative ion radical states by the same method (Figure 2c).60 The analysis results indicate a molecular stability order of CO > DBTO > PO > DPSO. Very recently, Li et al.61 demonstrated minimal activation energy of ∼0.25 eV for the C−S bond breaking in DPSO in the T1 state. The activation energy of C−P bond breaking in the T1 of triphenylphosphine oxide (TPPO) is higher than that of the C−S bond. Also, it was found that the dissociation of C−P and C−S bonds were endothermic and exothermic reactions, respectively, and the stability of TPPO was better than DPSO. A proportional relationship was found between the BDE and the adiabatic T1 transition energy, indicated that the extension of the π-conjugation could significantly stabilize a molecule in T1 state. Such a conclusion based on BDE calculation also contributed to the design of the molecules for long-lifetime OLEDs. The up-conversion energy transfer that occurred through dipolar–dipolar interaction might also be responsible for the degradation in blue FOLEDs. The energy superposition of two blue-emissive singlet excitons through a Förster resonance energy transfer (FRET) pathway could break any chemical bond.46,62 The probability of singlet–singlet annihilation (SSA) in FOLEDs is believed to be very low because the radiative lifetime of a fluorescent emitter is as short as a few nanoseconds.63–65 However, taking into account the facts that small amounts of decomposition products can effectively quench the luminescence and the degradation measurement is undertaken in a time of thousands of hours,62,66 it cannot be ruled out that the molecule dissociation is caused by SSA in blue FOLEDs.67 It has been demonstrated that the accumulation of hole on the interface between HTL and EML causes device degradation due to either physical or chemical causations, especially in blue OLEDs containing a wide-bandgap host in EML. A graded heterojunction between the hole transport layer (HTL) and an electron transport layer (ETL) has been proposed to eliminate the discrete HTL/ETL interface by Bai et al.68 They fabricated the graded junction layer by insertion of a layer composed of NPB and TBADN between the HTL composed of NPB and the EML composed of is used as blue and TBADN is It was suggested that the graded junction the molecular due to a decrease of Joule heat in the resulting in of the device lifetime. Besides, the graded junction design of the device et the stability of the blue FOLEDs by using EML by and materials to eliminate the barrier and the two blue-emitting devices with and without a host was used as blue is and was In the device, two kinds of were One was the and the other was EML. The EML structure was employed to hole into the electron layer and to better of in the EML and to reduce the interface by the of the a a large increase of LT50 from to h at the L0 of cd/m2 was the of of the blue FOLED an was reported by Zhang et They fabricated the two blue FOLEDs with the structures of One OLED has the composed of and the other has the composed of has exciton and high electron which is more than two of magnitude higher than that of The of OLED with is much better than OLED with for the operation the OLED exhibits LT50 than the The better of the is considered to be caused by a higher of T1 state in the which the chance of and in the the OLED the LT50 is to be the same for the two OLEDs, although the OLED with is a than the OLED with a between EL and stability was not Such a new suggests degradation mechanism for blue FOLEDs. Degradation mechanisms in blue PHOLEDs It is that the operational stability of blue PHOLEDs is not which is usually to unstable coordination bonds and extremely long lifetimes of the phosphorescent materials. complexes with heavy and other transition have a luminescent transfer state and a due to electronic is as a the molecule the potential energy barrier from the state to the state, the coordination bond would be and broken (Figure The by this process is to with molecules to a Therefore, the energy difference between the two states is expected to the stability of the (Figure the highly state is to the state, the phosphorescent emitters might reach the by the from at and are at of the activation energy of the molecule to generate could be effectively by the structure and substituent the sky blue phosphorescent emitter the dissociation of the in excited is considered to be the for device Consequently, one to a long operational lifetime from is the modification of the structure for energy Figure 3 | (a) of and in and coordination of complexes at the dissociation and (b) of the (left image), and of the energy dissociation activation energy for complexes (right image). Figure created with data from Copyright American Chemical Society. (c) energy of four complexes in their and states and EL as a of the device time under a current corresponding to initial of cd/m2 image). pathway in a fluorescence Adapted with permission from ref Download figure Download PowerPoint Recently, et al. three blue-emitting and were from by the addition of an to the of in (Figure which the energy level without the energy the devices employing these four device exhibits the operational which is three of magnitude than the devices employing the other three emitters. to the energy between state and states is small 5 for and In contrast, the energy of MS2 and is calculated to be 15 and 12 (Figure a under device activation from to states is reduced for MS2 than for the other The different stability of the PHOLEDs based on different of complexes were found to be to the → → S0 by et In the are more and less emissive than to the this is caused by the of three different triplet and the small activation to these the that only one pathway active and high activation The excited states are responsible for breaking the bond as mentioned and is by the activation process from the state. Therefore, it is that the formation and of the excited states are confirmed to an in such dissociation reactions in for phosphorescent emitters to reach the state is it is still exciton–exciton interaction the lifetime of FOLEDs, it has been widely that a in the degradation of This is based on the that the excited state lifetime of a is three of magnitude than that of a et proposed that two triplet excitons one exciton energy to the other a ground state molecule and a high-energy exciton (Figure The high-energy exciton was expected to cause the decomposition of In addition to such a annihilation also to the degradation of In a one triplet exciton energy to a which is excited (Figure This excited is active and will with a host or guest materials to defects in the The shows that the of formation is to the energy and the lifetime of the excited et found that in devices with an EML of the of host mCBP is unstable and with excited and of the Degradation is due to the formation of which in the and the over They