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Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes
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Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes Qing-Dong Ou, Chi Li, Yan-Qing Li ∗ , Jian-Xin Tang ∗ Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based Functional Materials & Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, Jiangsu, PR China
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Article history: Available online xxx Keywords: Tandem OLEDs Photoemission spectroscopy Energy level alignment Charge generation layer Transition metal oxides n-Type doping
a b s t r a c t Organic light-emitting diodes (OLEDs) using a tandem structure offer a highly attractive option for the applications of next-generation flat panel displays and solid-state lighting due to the extremely high brightness and efficiency along with the long operational lifetime. In general, reliable information about interface energetics of the charge generation layers (CGLs), which plays the central role in charge generation and carrier injection into the stacked emission units, is highly desirable and advantageous for interface engineering and the performance optimization of tandem OLEDs. In this review, our recent studies on tandem OLEDs are overviewed, especially from interface energetics perspective via photoemission spectroscopy. The electronic structures of various transition metal oxide (TMO)-based CGLs and their role in charge generation process are reviewed, addressing the n-type doping impact of organic layers in CGLs, thermal annealing-induced oxygen vacancy in TMOs, and the interfacial stability of CGLs on the device operational lifetime. The resulting energy level alignments are summarized in correspondence with tandem OLED performance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are gaining the increasing importance in the applications of next-generation full-color display panels and solid-state lighting sources because they have a wide range of merits in mechanical flexibility, optical transparency, thin and light form factor, and superior color quality [1–3]. OLEDs with a tandem structure are considered to be of great promise for future general lighting systems, since multiple emission units are stacked electrically in series, featuring extremely high brightness at low current density with the dramatic boost in device stability and efficiency [3,4]. To realize highly efficient tandem OLEDs, the intermediate connectors functioning as charge generation layers (CGLs) play a pivotal role in facilitating the injection of electrons and holes into suitable molecular energy levels of adjacent emission units, which determine the electron-to-photon conversion process in tandem OLEDs [5–7]. Correspondingly, several criteria for the formation of an effective CGL are required, such as high charge-generation capability, low optical absorption in the visible spectral range, low series
∗ Corresponding authors. Tel.: +86 512 65880942; fax: +86 512 65882846. E-mail addresses: [email protected] (Y.-Q. Li), [email protected] (J.-X. Tang).
resistance for a minimal electrical loss, good operational stability and deposition compatibility [8–10]. Accordingly, an amount of approaches to design an efficient CGL have been proposed for realizing high-performance tandem OLEDs. The CGLs in tandem OLEDs typically consist of a bilayer structure with various materials, including a metal–metal (or metal oxide) bilayer [11], an organic–metal (or metal oxide) bilayer [5–9,12], or an organic–organic bilayer [4,7,13]. For the commonly used CGLs with an organic–metal oxide bilayer, transition metal oxides (TMOs) such as tungsten trioxide (WO3 ) [11,12], molybdenum trioxide (MoO3 ) [9,14], and vanadium oxide (V2 O5 ) [15], are widely incorporated adjacent to the hole transport layer (HTL) of the neighboring emission unit. Meanwhile, the organic layers of a bilayer CGL are placed adjacent to the electron transport layer (ETL) of another neighboring emission unit, and commonly n-type doped by alkaline metals or metal compounds [4,6–12,16–18]. Regardless of the excellent device performance demonstrated in tandem OLEDs, the working mechanisms of charge generation and separation process in CGLs remain a subject of debate. Several models have been proposed for various CGL structures [13–22]. For the doped organic–organic bilayer CGL, a temperature-independent tunneling process was assumed for the charge generation, which was enabled by electric-field-induced electron transfer from the highest occupied molecular orbital (HOMO) of a p-type doped organic layer to the lowest unoccupied molecular orbital (LUMO)
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Please cite this article in press as: Q.-D. Ou, et al., Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes, J. Electron Spectrosc. Relat. Phenom. (2015), http://dx.doi.org/10.1016/j.elspec.2015.02.011
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Fig. 1. Cs2 CO3 -induced n-type electrical doping. (a) UPS spectra of 10 nm-thick Cs2 CO3 -doped BPhen films as a function of Cs2 CO3 doping concentration. (b) XPS spectra of C 1s, N 1s, and Cs 3d5/2 core levels of the corresponding Cs2 CO3 -doped BPhen films [25]. Copyright 2011, American Institute of Physics.
of an n-type doped organic layer [7,19]. However, different mechanisms have been put forward to unravel the charge carrier generation and separation process in TMO-based CGLs. One model suggested that charge generation and separation process was supposed to occur at the TMO/n-type doped organic interface by regarding TMOs as a p-type semiconductor [22]. On the contrary, another model for TMO-based CGLs attributed the thermally assisted charge generation at the TMO/HTL interface via electron transfer from the adjacent HTL’s HOMO into the TMO’s conduction band, since TMOs were demonstrated with n-type semiconducting character [21]. Consequently, the reliable information about interface energetics of the CGLs is highly desirable for understanding the working mechanisms of CGLs for interface engineering and the performance optimization of tandem OLEDs. Ultra-violet and X-ray photoemission spectroscopies (UPS and XPS) are the widely used techniques that provide the direct determination of energy level alignments (ELAs) at the organic interfaces. In this review, we focus on our recent advances on TMOs-based CGLs in tandem OLEDs, especially from interface energetics perspective via photoemission spectroscopy. The electronic structures of various TMO-based CGLs and their role in charge generation process are reviewed. Here, the TMO-based CGLs consist of an ntype doped organic layer and a TMO layer, where WO3 and MoO3 are the most widely used TMOs in tandem OLEDs due to their favorable electronic properties, highly optical transparency in the visible region for photon output, and excellent fabrication compatibility. Section 2 discusses the n-type doping effect of organic layers with cesium compounds (i.e., CsF, CsN3 , and Cs2 CO3 ). In Section 3, energetics at TMO/organic interfaces and the impact of annealing-induced oxygen vacancy on hole-injection barriers are overviewed as determined via photoemission spectroscopy. The ELAs and charge generation mechanisms of these TMO-based CGLs are addressed in Section 4. Finally, Section 5 provides a review of the influence of TMO-based CGLs on the performance characteristics of tandem OLEDs, particularly their stability on the device operational lifetime. 2. n-Type doping of organic layers with alkaline metal compounds For commonly used CGLs consisting of a TMO-organic bilayer, the charge generation and separation process in tandem OLEDs was proposed to occur and operate effectively with the presence of an n-type doped organic layer [22,23]. Traditionally, electrical doping of organic layers is an efficient strategy to overcome their intrinsic
limitations, e.g., low carrier transport conductivity and large charge injection barriers. For instance, p–i–n structures consisting of a ptype doped HTL, an intrinsic emission layer and an n-type doped ETL are generally considered as the state-of-the-art architecture for high-performance OLEDs due to the dramatically enhanced carrier transport properties with the highly reduced operating voltage [24]. Here, we address the n-type doping effect of organic materials with metal and metallic compounds in TMO-based CGLs and their influence on device performance of tandem OLEDs. Compared to reactive metals as n-type dopants, the incorporation of alkaline metal compounds, e.g., cesium carbonate (Cs2 CO3 ), cesium fluoride (CsF), cesium azide (CsN3 ), etc., provides an effective approach for overcoming the limitations of low bulk conductivity and high electron injection barriers in organic layers due to the advantages for material handling and operational stability [4,6–12,16–18]. Despite the extensive utilization of alkaline metal compounds as n-type dopants in OLEDs to facilitate electron injection and reduce the Ohmic losses in organic layers, the nature of the incorporated species, the electronic structures of doped materials, and the working mechanisms of alkaline metal compounds as an n-type dopant in organic layers are not yet fully clarified. We have systematically studied the electronic structures and underlying physical mechanisms of n-type doped organic layers with various alkaline metal compounds via photoemission spectroscopy [17,25–27]. The doping effect of Cs2 CO3 on electronic structures of 4,7diphenyl-1,10-phenanthroline (BPhen), a typical organic ETL, was characterized with UPS and XPS measurements [25]. It can be observed from the UPS spectra in Fig. 1(a) that the increase of the Cs2 CO3 doping concentration in BPhen films results in a gradual shift of the UPS spectral features toward higher binding energy (BE) region together with a vacuum level (VL) shift of 1.4 eV. Accordingly, it can be inferred that the Fermi level (EF ) in the Cs2 CO3 :BPhen layers moves within the energy gap toward the BPhen’s LUMO, showing an n-type doping process. Fig. 1(b) displays the related evolution of XPS C 1s, N 1s, and Cs 3d5/2 core level spectra. Taking into account the original molecular ratios in pristine Cs2 CO3 , it is verified here that Cs2 CO3 in n-type doped BPhen films is partially decomposed into cesium oxide during thermal evaporation, which is in agree with previous reports on thermal decomposition of Cs2 CO3 [28]. The doping of Cs2 CO3 into BPhen films imposes a progressive shift in general for C 1s and N 1s core levels toward higher BE region with respect to the increase in Cs2 CO3 doping concentration, which is consistent with the behavior observed in the UPS spectra in Fig. 1(a). Compared to the case of pristine
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Cs2 CO3 , the Cs 3d5/2 core level position located at lower BE region in doped systems can be ascribed to the different environment for Cs atoms surrounded by additional negative charges due to electron transfer to the BPhen host, since the Cs 3d5/2 core level is commonly localized at lower BE for Cs complex with higher oxidation state [29]. However, strong chemical reaction between Cs2 CO3 and BPhen in the doping system would be ruled out because of the absence of observable feature in the energy gap region [Fig. 1(a)] and the unchanged spectral shapes of C 1s and N 1s core levels [Fig. 1(b)]. The similar phenomena were observed for the Cs2 CO3 -doped tris(8-hydroxy-quinoline) aluminum (Alq3 ) as shown in Fig. 2. However, when comparing the difference in electronic structures of doped Alq3 layers with various alkaline metal compounds (i.e., CsF, CsN3 , and Cs2 CO3 ) as an n-type dopant, the UPS and XPS results clearly reveal the different doping mechanisms, which are sensitive to the doping constituent materials [17,25–27]. It is evident in Fig. 2(a) that all three kinds of alkaline metal compounds in the Alq3 layer resulted in the shifts of UPS spectral features towards high BE region relative to the case of pristine Alq3 layer. The shift of the secondary electron cutoff in the high BE region corresponds to a decrease in the work function of the doped Alq3 layer and the EF movement towards the Alq3 ’s LUMO within the energy gap. However, the doping of CsF and CsN3 was accompanied with the formation of a gap state in the forbidden energy gap of Alq3 , which was contrary to the case of Cs2 CO3 doping. In addition, the Mg doping in Alq3 was measured as a comparison, showing the formation of a gap state. The appearance of such a gap state indicates the presence of strong chemical reaction for Alq3 molecules with Mg, CsF or CsN3 in the doping system, which is further identified from the evolution of XPS spectra in Fig. 2(b) and (c). It is clear that the XPS O 1s and N 1s core level signals originating from Alq3 molecules exhibit splitting, with a new peak appearing at the lower BE region upon the doping of Mg, CsF or CsN3 into the Alq3 layer [Fig. 2(b) and (c)]. According to previous reports [17,27,30], the new features observed in UPS and XPS spectra are in accordance with the formation of negatively charged Alq3 radical anion induced by charge transfer to one of the triplets of LUMO in the neutral Alq3 molecule with the chemical bonding of reactive dopants with N and O in quinolate ligands of Alq3 . The difference in the electronic structures of various doping systems originates from the different doping mechanisms of alkaline metal compounds in the organic layer. It has been demonstrated that the incorporation of Cs2 CO3 , CsF or CsN3 by thermal evaporation results in decomposition during the deposition process [22,25,27]. For instance, Cs2 CO3 will partially decompose with the product of cesium oxide (Cs2 O) during the thermal evaporation [25], while CsN3 and CsF have been confirmed to generate Cs [22,27]. Therefore, no generation of metallic Cs can be found during the evaporation process of Cs2 CO3 , leading to the absence of strong chemical reaction between Cs atom and Alq3 molecule in the Cs2 CO3 :Alq3 doping system.
3. Energetics at TMO/organic interfaces Although TMOs (e.g., MoO3 and WO3 ) are commonly used in OLEDs for enhancing hole injection, recent reports on their electronic structures have demonstrated that these materials exhibit an n-type semiconducting property due to oxygen vacancies [19,31,32], which rule out the misunderstandings of p-type characteristics in previous reports [33,34]. It is thus crucial to estimate the ELAs at the TMO/organic interfaces, which is of importance to clarify the charge generation and separation in TMO-based CGLs for tandem OLEDs and especially the impacts of constituent materials
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on the functional effectiveness of TMO-based CGLs. In the following, we will firstly present the electronic structure evolutions at the TMOs (i.e., MoO3 and WO3 )/organic interfaces via photoemission spectroscopy.
3.1. Electronic structures at TMO/organic interfaces As determined in Fig. 3(a) of HeI UPS spectra of the incremental MoO3 deposition on CsN3 :BPhen (10 nm, 20 wt%), the deposition of a MoO3 overlayer results in a progressive shift of emission features of CsN3 :BPhen and MoO3 towards lower BE region, which is accompanied by a VL shift of 4.6 eV [22]. Meanwhile, the original gap states in the forbidden gap of CsN3 :BPhen disappear upon the MoO3 deposition. It is noted that the spectral features become saturated at 5 nm-thick MoO3 layer with the appearance of a MoO3 -derived deep-lying gap state, which is attributed to the oxygen vacancy and partial reduction of Mo6+ cation in MoO3 as evidenced by XPS measurements [30,32]. The HOMO edge of CsN3 :BPhen and the valence band maximum (VBM) of the MoO3 layer can be extracted by the intercept of the tangent of the leading edges of the low BE feature and the background level, and are found at 4.4 and 2.8 eV below the substrate EF , respectively. Given that the energy gap of MoO3 is ∼3.1 eV, it is indicative that EF is pinned slightly below the conduction band minimum (CBM) of MoO3 , showing an ntype semiconducting character [19]. In addition, it is shown that a 10 nm-thick MoO3 layer possesses a high work function (WF) of 6.7 eV. Upon the interface formation between N,N -bis(1-naphthyl)N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (NPB) and MoO3 as shown in Fig. 3(b), the frontier molecular orbitals of NPB gradually shift toward higher BE region, which is accompanied by a VL downward shift of ∼2 eV. Energy level shifts indicate energy level bending at the MoO3 /organic interface. Additionally, there is no observable feature in the organic energy gap region, suggesting the absence of chemical interaction at the interface. For comparison, Fig. 3(c) displays the UPS spectra of a 10 nmthick NPB layer deposited on MoO3 , BPhen, and CsN3 :BPhen substrates, respectively. No evidence of chemical reaction or formation of new interfacial electronic states can be observed for these three interfaces [22]. On the other hand, these interfaces exhibit two different trends upon interface formation. The VL shift to higher BE region represents the formation of an interface dipole upon deposition of NPB on MoO3 , whereas the UPS spectra evolutions of CsN3 :BPhen/NPB and BPhen/NPB interfaces imply the absence of interface dipole at the corresponding interfaces. The interface energetics between WO3 and organic layers are similar to that of MoO3 /organic interfaces. As shown in Fig. 4(a), the incremental deposition of a WO3 overlayer on Mg:Alq3 (10 nm, 10 wt%) substrate results in a progressive shift of UPS spectral features towards the lower BE region [20]. In accordance to MoO3 , WO3 is confirmed as an n-type semiconducting material [35]. In addition, the formation of new state can be observed in UPS spectra at the WO3 /Mg:Alq3 interface, which is contrary to the WO3 /Alq3 interface without additional spectral structures [as shown in Fig. 4(b)]. Fig. 4(c) and (d) display the evolution of W 4f core level spectra for the NPB/WO3 /Mg:Alq3 and NPB/WO3 /Alq3 systems, respectively. These spectral evolutions coincide with the partial reduction of W6+ cations at the WO3 /Mg:Alq3 interface and the chemically inert nature for WO3 /Alq3 interface, which is consistent with the appearance of a new state in the forbidden band gap as shown in Fig. 4(a). On the other hand, upon NPB deposition onto WO3 films, the W 4f doublet peak only shows changes in intensity without additional peak or peak broadening, implying no chemical reaction between NPB and WO3 .
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Fig. 2. N-type doping effects of various metallic compound dopants. (a) UPS spectra of 10 nm-thick dopedAlq3 films using Mg, Cs2 CO3 , CsF, and CsN3 dopants with a doping ratio of 10 wt% [17,27]. (b) Comparison of the corresponding XPS O 1s spectra. Copyright 2014, Elsevier.
Fig. 3. HeI UPS spectra of (a) MoO3 /CsN3 :BPhen [22] and (b) NPB/MoO3 interfaces [32] as a function of the deposited overlayer thickness. (c) UPS spectra of a 10 nm-thick NPB layer deposited on MoO3 , BPhen, and CsN3 :BPhen substrates, respectively [22]. Ref. [22]: copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Ref. [32]: copyright 2010, American Institute of Physics.
3.2. Effects of oxygen vacancy on electronic structures It has recently been reported that oxygen vacancies in TMOs arising from partly filling empty d-states can cause the presence of gap states in the forbidden energy gap and significantly affect the chemical and electronic properties of these nonstoichiometric oxides [31,36,37]. For example, oxygen vacancy-induced gap states in MoOx play a crucial role in the hole transport properties of MoOx , which can act as n-dopant and reduce the WF with the EF shift in the energy gap [31]. It indicates that oxygen vacancies in MoOx are advantageous by increasing the gap-state-assisted hole transport and disadvantageous by increasing the hole injection barrier for the device performance, and a balanced interplay has to be found for highly efficient devices. Therefore, it is expected that the ELAs and charge exchange at the TMO/organic interfaces
can be tuned under the control of oxygen vacancies upon chemical or physical treatments. To gain a deep insight into the oxygen vacancy in TMOs and the corresponding effects on hole injection barriers in TMO-based CGLs, electronic structure evolutions at the TMO/organic interfaces were systematically investigated via UPS and XPS. Here, the TMOs (i.e., MoOx and WOx ) were thermally annealed under nitrogen atmosphere to control the oxygen stoichiometry. Fig. 5(a) and (b) display the XPS spectra of the Mo 3d and W 4f core levels for MoOx and WOx films on ITO substrate annealed at various temperatures, indicating that the the spectral features are highly dependent on the annealing temperature [38,39]. Obviously, the Mo6+ cation is the majority presence in the as-grown MoOx layer as an almost single Mo 3d3/2 and 3d5/2 doublet centers at the BEs of 235.8 and 232.7 eV, respectively. Similarly, the
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Fig. 4. UPS and XPS spectra of WO3 /organic interfaces. HeI UPS spectra of (a) WO3 /Mg:Alq3 and (b) WO3 /Alq3 interfaces formed on ITO glass substrates by firstly depositing a 10-nm thick doped or undoped organic layer. XPS W 4f core level spectra for (c) NPB/WO3 /Mg:Alq3 and (d) NPB/WO3 /Alq3 interfaces. The dashed lines represent the least-square peak fitting for a 10 Å-thick WO3 on Mg:Alq3 and Alq3 substrates, respectively [20]. Copyright 2010, Elsevier.
as-grown WOx layer with the chemical state of W6+ shows an almost single W 4f5/2 and 4f7/2 doublet centered at BEs of 38.1 and 36.0 eV. However, thermal annealing causes the appearance of new features at the lower BE region for both MoOx and WOx films. Upon the least-squared fitting analysis by means of different main doublets with a Shirley background, the peak positions and the relative contributions of different oxidation states can be accurately determined in all the Mo 3d and W 4f core level spectra. It is clear in Fig. 5(a) that the intensity of a new doublet at lower BE region (e.g., 234.9 eV for Mo 3d3/2 and 231.8 eV for Mo 3d5/2 ) increases
with respect to the rise in the annealing temperature from room temperature (RT) to 150 ◦ C, while the peak positions of the Mo6+ doublet remain almost constant under all annealing temperatures. When the annealing temperature is beyond 200 ◦ C, another new doublet is observed at lower Bes (e.g., 233.9 for Mo 3d3/2 and 230.8 eV for Mo 3d5/2 ). According to previous reports [32,40], these new peaks can be assigned to the chemical states of Mo5+ and Mo4+ , respectively, which indicate the partial reduction of Mo6+ cations to Mo5+ and Mo4+ due to the annealing-induced oxygen vacancy in MoOx films [41]. On the other hand, the similar phenomena can be
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Fig. 5. Annealing-induced oxygen vacancy in TMO films at various temperatures in a nitrogen atmosphere [38,39]. XPS spectra of (a) Mo 3d and (b) W 4f core levels measured for MoOx and WOx films with a thickness of 5 nm deposited on ITO substrates and annealed at various temperatures in a nitrogen atmosphere. Solid lines in (a) and (b) represent Gaussian/Lorentzian mixing functions for fitting analysis and resultant curves of the fitting on the experimental data. The dependence of WF and the stoichiometry x of (c) MoOx and (d) WOx layers versus annealing temperature. Copyright 2013, American Institute of Physics and The Japan Society of Applied Physics.
observed in the annealed WOx films as shown in Fig. 5(b), where the signal of an additional doublet at lower BEs of 37.4 eV (W 4f5/2 ) and 35.3 eV (W 4f7/2 ) increases with the increase of annealing temperature from RT to 250 ◦ C. These new features can be assigned to the reduced chemical states of W5+ from W6+ cations due to the annealing-induced oxygen vacancies in the WOx films [35]. Correspondingly, the stoichiometry (x) of annealed MoOx and WOx films can be calculated by taking the relative contribution of different oxidation states into account. As shown in Fig. 5(c) and 5(d), the x values varied from ∼3.0 to ∼2.7 and ∼2.8 for MoOx and WOx , respectively, with respect to the annealing temperature from RT to 250 ◦ C. Moreover, the evolution of oxygen vacancies in the annealed MoOx and WOx layers under nitrogen atmosphere basically follows the same trend as these films annealed in vacuum [37]. The corresponding electronic structures of MoOx and WOx films treated at various annealing temperatures are consistent with the reduction of stoichiometry, showing a progressive decrease in WF with the increase of oxygen vacancy-induced gap states. For example, the WF of annealed MoOx film was reduced from 5.05 eV at RT to 4.55 eV at 250 ◦ C [Fig. 5(c)], while the corresponding WF of the annealed WOx film decreased from 4.67 to 4.47 eV with the increase of oxygen vacancies [Fig. 5(d)]. The influence of annealing-induced oxygen vacancies on energetics at TMO/organic interfaces (i.e., MoOx /NPB and WOx /NPB) is identified by incrementally depositing a 10 nm-thick NPB layer onto various annealed TMO films [38,39]. As summarized in Fig. 6, a clear dependency of WF and hole injection barriers on the annealing temperature at both MoOx /NPB and WOx /NPB interfaces can be observed. According to the proposed energy level diagram depicted in the inset of Fig. 6(a), the hole-injection barriers for MoOx /NPB interfaces range from 0.9 eV (on MoOx at RT) to 1.2 eV (on MoOx annealed at 250 ◦ C), whereas the WF of NPB layers varies from 4.5 eV to 4.1 eV. For WOx /NPB interfaces [Fig. 6(b)], the hole-injection barrier ranges from 0.6 eV (on as-fabricated WOx substrate) to 1.0 eV (on WOx annealed at 250 ◦ C), which is accompanied with the change of NPB’s WF from 4.8 eV to 4.4 eV. However, the overall changes of WF and hole injection barrier are almost
Fig. 6. Dependence of WF and hole-injection barriers of NPB layers deposited on annealed MoOx (a) and WOx (b) substrates as a function of the annealing temperature. Inset shows the schematic energy level diagram at the annealed TMO/NPB interface. Copyright 2013, American Institute of Physics and the Japan Society of Applied Physics.
rigid, keeping the constant ionization energy of NPB within the experimental error. The dependence of hole injection barriers at the MoOx /NPB interfaces are correlated with the degree of thermal annealing-induced oxygen vacancies in MoOx . Therefore, the degree of oxygen vacancies in TMOs can be controlled by thermal annealing, which results in the partial reduction of transition metal cations and a decrease in WF. The interfacial ELA and thus hole-injection barrier at TMO/organic interfaces increase as a consequence of the increase in oxygen deficiency.
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Fig. 7. Schematic energy level diagrams of MoO3 -based CGLs. (a) BPhen/CsN3 :BPhen/MoO3 /NPB, (b) BPhen/MoO3 /NPB, (c) BPhen/CsN3 :BPhen/NPB, and (d) BPhen/NPB interfaces on ITO substrate. All the values shown are in the unit of eV [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
4. ELAs and charge generation mechanism of TMO-based CGLs 4.1. ELAs of TMO-based CGLs On the basis of electronic structures characterized by UPS and XPS measurements, ELAs at various TMO-based CGLs can be extracted, which favor the understanding of charge generation and separation process in CGLs. Fig. 7 depicts the energy level diagrams for the MoO3 -based CGLs by taking into account interface dipole and energy level bending [22]. Here, the EF of the underlying ITO substrate was used as the reference. The ELAs of CGLs without MoO3 or n-type doped organic layer are also shown for comparison. As shown in Fig. 7(a) and (b), a p–n junction is formed between NPB and MoO3 with a small hole-injection barrier. For the CGLs without MoO3 , typical flat vacuum level alignment is observed at CsN3 :BPhen/NPB and BPhen/NPB interfaces as displayed in Fig. 7(c) and (d). Besides, the n-type doped organic layer incorporated into TMO-based CGLs also plays a crucial role in tandem OLEDs. As shown in Fig. 7(a) and (c), the EF in CsN3 :BPhen is significantly tuned only 0.1 eV below the BPhen’s LUMO, which will be favorable for electron injection and transport through CsN3 :BPhen into the adjacent BPhen ETL. Moreover, the CsN3 :BPhen layer could exhibit superior hole-blocking capability due to the large ionization potential value and the wide bandgap. In contrast, there is a large LUMO offset at the MoO3 /BPhen interface, restricting the effective electron injection from MoO3 into the BPhen layer. In addition, the large energy level offset exists between NPB’s HOMO and CsN3 :BPhen’s (or BPhen’s) LUMO, implying the difficulty in charge generation and separation at CsN3 :BPhen/NPB and BPhen/NPB interfaces under
weak electric field. According to the energy level diagrams of the TMO-based CGLs with neighboring HTL and ETL layers depicted in Fig. 7, the impact of the constituent materials on the functional effectiveness of TMO-based CGLs and thus the device performance of the corresponding tandem OLEDs can be understood, which will be discussed in details in Section 5. 4.2. Charge generation mechanism of TMO-based CGLs As reported previously [15,19,42,43], an electric-field-assisted charge separation process was proposed to occur at the TMO/organic interface. However, the charge transfer process in TMO-based CGLs remains a subject of debate. It was assumed that under applied bias such a charge transfer occurred from the TMO’s VBM to the LUMO state of n-type doped organic layer, since the TMOs were usually regarded as a p-type material [15]. Nevertheless, as demonstrated by Kröger et al. [19] and observed in Section 2, MoO3 acts as n-type semiconductor with deep-lying defect states below EF derived from oxygen vacancies. As a result, the charge generation process was ascribed to the hole injection from TMO’s CBM into the adjacent organic HTL’s HOMO [19,42]. According to the experimentally determined ELAs and auxiliary capacitance characteristics demonstrated recently [22], the working principle of TMO-based CGLs has been proposed that charge carriers are generated within the TMO layer by exciting electrons from the oxygen vacancy-derived gap states into its conduction band. Fig. 8 represents the schematic of charge generation and separation process in the TMO-based CGLs. Electrons in TMOs are expected to spontaneously transfer via thermal diffusion from various defect states to the conduction band [denoted as process (1) in Fig. 8]. Under
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Fig. 8. Schematic electric field-assisted charge generation and separation processes under forward bias, including (1) thermal excitation of electrons from defect levels to the CBM within MoO3 ; (2) electron injection from MoO3 ’s CBM into the CsN3 :BPhen’s LUMO; (3) hole injection from MoO3 ’s defect levels to NPB’s HOMO [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
the external electric field, electrons and holes are injected through tunneling from conduction band and defect states of TMOs, respectively, into the LUMO of n-type doped organic layer and the adjacent HTL’s HOMO [processes (2) and (3)]. Here, electrons generated in TMOs can easily inject from the TMO’s CBM into the LUMO of ntype doped organic layer due to the small injection barrier at the TMO/organic interface. Then, electrons will immediately be driven away from CsN3 :BPhen to the BPhen layer by the external electric field, while holes are in the same way moved away from the interface. 5. Impact of CGLs on device characteristics The impacts of the constituent materials on the functional effectiveness of TMO-based CGLs in tandem OLEDs can be verified on a basis of device performance characteristics such as electrical and spectral properties, luminance efficiency, and operational stability, etc.
Fig. 9. Device performance of tandem OLEDs using different TMO-based CGLs. (a) Current density-voltage and (b) luminance efficiency-current density characteristics of tandem OLEDs with double emission units and reference device with single emission unit [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
5.1. Electrical and optical properties
into BPhen can greatly move EF to only 0.1 eV below the LUMO, resulting in the small injection barrier at the CsN3 :BPhen/MoO3 interface and the enhanced electron injection through CsN3 :BPhen into the BPhen ETL. Consequently, the driving voltage of tandem OLED with CsN3 :BPhen/MoO3 in Fig. 9(a) is almost twice that of the reference device with a single unit at a constant current density, which is predictable for an effective tandem OLED. In contrast, other tandem devices exhibit poorer electrical properties and thus require higher driving voltages due to the large energy level offset between NPB’s HOMO and CsN3 :BPhen’s (or BPhen’s) LUMO. It indicates that charge generation process is hard to occur at CsN3 :BPhen/NPB and BPhen/NPB interfaces unless a higher driving voltage is applied. As a result, the luminance efficiencies of these inferior devices are just comparable with that of the reference device with single emission unit [Fig. 9(b)]. In addition, the lower driving voltage and higher luminance efficiency compared with the TMO/organic bilayer structure can be realized with the adoption of a hybrid CGL structure involving p-type doping (i.e., n-type doped organic ETL/TMO/TMO-doped organic HTL) [44], which is attributed to more favorable ELA for charge generation and separation in tandem OLEDs. Fig. 10 compares the electroluminescent (EL) spectra of two series of tandem OLEDs combining green and red emission units (EL-G and EL-R) in different stacking sequences, which helps to identify the correlation between carrier recombination processes and various CGLs [22]. The EL-G comprises NPB layer
Fig. 9 summarizes the current density versus voltage characteristics and luminance efficiency of tandem OLEDs together with that of a reference device with single emission unit, in which four different CGLs are used with the structures of (1) CsN3 :BPhen/MoO3 , (2) CsN3 :BPhen, (3) MoO3 , and (4) none, respectively. The emission unit consists of the NPB layer as HTL, BPhen layer as ETL, and green emitting layer of 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl1H,5H,11H-(1)-benzopyropyrano(6,7,8-i,j)quinolizin-11-one (C545T) doped Alq3 . Obviously, constituent materials of CGLs play critical parts in their functional effectiveness and the carrier recombination processes for light emission in tandem OLEDs. It is noted that the device characteristics of tandem OLEDs using TMO-based CGLs in Fig. 9 can be understood based on the ELAs of various CGLs shown in Fig. 7. The driving voltages of tandem OLEDs are consistent with the energy level offsets between the n-type doped organic layer’s LUMO and the adjacent HTL’s HOMO. The use of both TMO (e.g., MoO3 in Fig. 7) and n-type doped organic layer in the CGLs can significantly reduce this energy level offsets. As shown in Fig. 7(a) and (b), a p–n junction is formed between NPB and MoO3 with a small hole injection barrier height. In addition, the incorporation of an n-type doped organic layer in TMO-based CGLs is also essential for tandem OLEDs to function efficiently with enhanced EL efficiency. As shown in Fig. 7(a) and (c), doping CsN3
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Fig. 10. Electroluminescent spectra of tandem OLEDs composed of various CGLs with two different emission units. Devices in (a): the bottom emission unit with red emission and the top green emission; devices in (b): the bottom EL unit with green emission and the top red emission, respectively [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
as HTL, BPhen layer as ETL, and Alq3 layer as the green emitting layer, while the EL-R is formed between NPB and BPhen by inserting red emitting layer of 4-(dicyanomethylene)-2-t-butyl6-(1,1,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) doped Alq3 . As shown in Fig. 10, tandem OLEDs using the CGL of CsN3 :BPhen/MoO3 exhibit clearly both green and red emission components, implying that such a CGL functions well and emissions in both EL-G and EL-R units are mutually independent on the tandem structure. However, when only MoO3 or CsN3 :BPhen is used in the CGL, these tandem OLEDs do not show the fully unlocked emission pattern in comparison to that using CsN3 :BPhen/MoO3 , indicating that charge generation and separation process is prohibited due to the reduced electron or hole injection into the neighboring ETL or HTL, respectively [22,30]. Therefore, the asymmetric contribution of two emission units to the EL spectra validates that the MoO3 layer is essential to charge generation in the CGL. It is believed that the electron–hole recombination in the cathode-side emission unit for tandem OLEDs using CGLs without MoO3 is eliminated due to the lack of charge generation by MoO3 and leakage current of holes from the ITO side. This is the reason why intensive emission from the cathode-side EL unit can only be observed for tandem OLEDs with MoO3 in the CGLs, while other tandem devices behave like an OLED with a single EL unit as shown in Fig. 10.
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Fig. 11. Operational stability of tandem OLEDs with various CGLs. (a) Normalized luminance (luminance/initial luminance) and (b) driving voltage as a function of the operational time for tandem devices encapsulated with an epoxy resin-sealed glass cap and tested at a constant current density [17]. Copyright 2014, The Royal Society of Chemistry.
5.2. Operational stability Long lifetime is the prerequisite for tandem OLEDs as a cost competitive and sustainable technology in commercial applications. It has been demonstrated that device degradation can be induced by various factors, including dark spots caused by an unstable interface between electrode and organic [45], the morphological instability of organic films [46], electrical shorts due to structure defects [47], electrochemical aging at organic/cathode interface [48], metal atom/ion diffusion [49], and oxidization of functional layers with the diffusion of moisture and oxygen molecules [50]. Inevitably, the insertion of CGLs will introduce more interfaces, which are strongly related to an additional voltage drop and charge accumulation across each CGL in tandem device. Therefore, it is desirable to elucidate the impact of CGLs on device lifetime of tandem OLEDs and the corresponding degradation mechanism. Fig. 11 illustrates the luminance and driving voltage as a function of the operational time under a constant current condition. It is obvious that tandem OLEDs with various CGLs exhibit totally different degradation behavior, and the degradation of luminance and driving voltage of these devices varied in a non-monotonic style during continuous operation. For example, tandem device with Mg:Alq3 /MoO3 exhibits a slow luminance degradation [Fig. 11(a)] and a minimum increase in driving voltage [Fig. 11(b)], while the use of Cs2 CO3 :Alq3 /MoO3 and CsN3 :Alq3 /MoO3 as a CGL causes a rapid initial decrease in luminance and a dramatic increase in driving voltage. It is verified that various degradation modes of tandem devices could mainly be attributed to the interface instability of the CGLs due to the
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different doping mechanisms in n-type doped organic ETLs [17,51]. These findings confirm that the constituent materials of CGLs play an important role in the operational stability of tandem OLEDs. For practical applications, the long operating lifetime of tandem OLED devices is undoubtedly a prerequisite in terms of massive production. 6. Conclusions In summary, we have reviewed previous investigations on TMO-based CGLs for tandem OLEDs, especially from interface energetics perspective via photoemission spectroscopy. TMOs such as MoO3 , WO3 exhibit n-type semiconductor property with deeplying defect states due to the presence of oxygen vacancies. It has been demonstrated that the energetics of TMO/organic interfaces plays an important role in charge generation process for tandem OLEDs. The doping mechanisms of n-type doped organic layers in CGLs with various cesium compounds have been identified by intensive comparison of their influences on electronic structures. Energy level alignment and charge separation mechanism of TMO-based CGLs have been proposed, which can provide a deeper understanding of charge generation and separation process in CGLs. Moreover, the impacts of TMO-based CGLs on device performance have been clarified by checking the electrical and optical properties and operational stability of tandem OLEDs. The resulting energy level alignments are summarized in correspondence with tandem OLED performance. The correlation between the operational stability of CGLs relating to different n-dopants and the device degradation behaviors are also discussed. Acknowledgments The authors acknowledge financial support from the 973 Program by Ministry of Science and Technology of China (Nos. 2014CB932600, 2011CB808404), the National Natural Science Foundation of China (Nos. 91433116, 11474214), Jiangsu Science and Technology Department (No. BK20140053), Bureau of Science and Technology of Suzhou Municipality (No. ZXG201422), and the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References [1] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (2009) 234. [3] H. Sasabe, J. Kido, J. Mater. Chem. C 1 (2013) 1699. [4] L.-S. Liao, W.K. Slusarek, T.K. Hatwar, M.L. Ricks, D.L. Comfort, Adv. Mater. 20 (2008) 324. [5] T. Matsumoto, T. Nakada, J. Endo, K. Mori, N. Kawamura, A. Yokoi, J. Kido, SID Int. Symp. Digest Tech. Papers 34 (2003) 979. [6] L.S. Liao, K.P. Klubek, C.W. Tang, Appl. Phys. Lett. 84 (2004) 167. [7] J.-X. Tang, M.-K. Fung, C.-S. Lee, S.-T. Lee, J. Mater. Chem. 20 (2010) 2539. [8] C.W. Chen, Y.J. Lu, C.C. Wu, E.H.E. Wu, C.W. Chu, Y. Yang, Appl. Phys. Lett. 87 (2005) 241121. [9] H. Kanno, R.J. Holmes, Y. Sun, S.K. Cohen, S.R. Forrest, Adv. Mater. 18 (2006) 339. [10] J. Meyer, S. Hamwi, M. Kröger, W. Kowalsky, T. Riedl, A. Kahn, Adv. Mater. 24 (2012) 5408. [11] H.M. Zhang, Y.F. Dai, D.G. Ma, H.M. Zhang, Appl. Phys. Lett. 91 (2007) 123504. [12] C.C. Chang, S.W. Hwang, C.H. Chen, J.F. Chen, Jpn. J. Appl. Phys., 1 43 (2004) 6418.
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Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes Qing-Dong Ou, Chi Li, Yan-Qing Li ∗ , Jian-Xin Tang ∗ Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-based Functional Materials & Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, Jiangsu, PR China
a r t i c l e
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Article history: Available online xxx Keywords: Tandem OLEDs Photoemission spectroscopy Energy level alignment Charge generation layer Transition metal oxides n-Type doping
a b s t r a c t Organic light-emitting diodes (OLEDs) using a tandem structure offer a highly attractive option for the applications of next-generation flat panel displays and solid-state lighting due to the extremely high brightness and efficiency along with the long operational lifetime. In general, reliable information about interface energetics of the charge generation layers (CGLs), which plays the central role in charge generation and carrier injection into the stacked emission units, is highly desirable and advantageous for interface engineering and the performance optimization of tandem OLEDs. In this review, our recent studies on tandem OLEDs are overviewed, especially from interface energetics perspective via photoemission spectroscopy. The electronic structures of various transition metal oxide (TMO)-based CGLs and their role in charge generation process are reviewed, addressing the n-type doping impact of organic layers in CGLs, thermal annealing-induced oxygen vacancy in TMOs, and the interfacial stability of CGLs on the device operational lifetime. The resulting energy level alignments are summarized in correspondence with tandem OLED performance. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Organic light-emitting diodes (OLEDs) are gaining the increasing importance in the applications of next-generation full-color display panels and solid-state lighting sources because they have a wide range of merits in mechanical flexibility, optical transparency, thin and light form factor, and superior color quality [1–3]. OLEDs with a tandem structure are considered to be of great promise for future general lighting systems, since multiple emission units are stacked electrically in series, featuring extremely high brightness at low current density with the dramatic boost in device stability and efficiency [3,4]. To realize highly efficient tandem OLEDs, the intermediate connectors functioning as charge generation layers (CGLs) play a pivotal role in facilitating the injection of electrons and holes into suitable molecular energy levels of adjacent emission units, which determine the electron-to-photon conversion process in tandem OLEDs [5–7]. Correspondingly, several criteria for the formation of an effective CGL are required, such as high charge-generation capability, low optical absorption in the visible spectral range, low series
∗ Corresponding authors. Tel.: +86 512 65880942; fax: +86 512 65882846. E-mail addresses: [email protected] (Y.-Q. Li), [email protected] (J.-X. Tang).
resistance for a minimal electrical loss, good operational stability and deposition compatibility [8–10]. Accordingly, an amount of approaches to design an efficient CGL have been proposed for realizing high-performance tandem OLEDs. The CGLs in tandem OLEDs typically consist of a bilayer structure with various materials, including a metal–metal (or metal oxide) bilayer [11], an organic–metal (or metal oxide) bilayer [5–9,12], or an organic–organic bilayer [4,7,13]. For the commonly used CGLs with an organic–metal oxide bilayer, transition metal oxides (TMOs) such as tungsten trioxide (WO3 ) [11,12], molybdenum trioxide (MoO3 ) [9,14], and vanadium oxide (V2 O5 ) [15], are widely incorporated adjacent to the hole transport layer (HTL) of the neighboring emission unit. Meanwhile, the organic layers of a bilayer CGL are placed adjacent to the electron transport layer (ETL) of another neighboring emission unit, and commonly n-type doped by alkaline metals or metal compounds [4,6–12,16–18]. Regardless of the excellent device performance demonstrated in tandem OLEDs, the working mechanisms of charge generation and separation process in CGLs remain a subject of debate. Several models have been proposed for various CGL structures [13–22]. For the doped organic–organic bilayer CGL, a temperature-independent tunneling process was assumed for the charge generation, which was enabled by electric-field-induced electron transfer from the highest occupied molecular orbital (HOMO) of a p-type doped organic layer to the lowest unoccupied molecular orbital (LUMO)
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Fig. 1. Cs2 CO3 -induced n-type electrical doping. (a) UPS spectra of 10 nm-thick Cs2 CO3 -doped BPhen films as a function of Cs2 CO3 doping concentration. (b) XPS spectra of C 1s, N 1s, and Cs 3d5/2 core levels of the corresponding Cs2 CO3 -doped BPhen films [25]. Copyright 2011, American Institute of Physics.
of an n-type doped organic layer [7,19]. However, different mechanisms have been put forward to unravel the charge carrier generation and separation process in TMO-based CGLs. One model suggested that charge generation and separation process was supposed to occur at the TMO/n-type doped organic interface by regarding TMOs as a p-type semiconductor [22]. On the contrary, another model for TMO-based CGLs attributed the thermally assisted charge generation at the TMO/HTL interface via electron transfer from the adjacent HTL’s HOMO into the TMO’s conduction band, since TMOs were demonstrated with n-type semiconducting character [21]. Consequently, the reliable information about interface energetics of the CGLs is highly desirable for understanding the working mechanisms of CGLs for interface engineering and the performance optimization of tandem OLEDs. Ultra-violet and X-ray photoemission spectroscopies (UPS and XPS) are the widely used techniques that provide the direct determination of energy level alignments (ELAs) at the organic interfaces. In this review, we focus on our recent advances on TMOs-based CGLs in tandem OLEDs, especially from interface energetics perspective via photoemission spectroscopy. The electronic structures of various TMO-based CGLs and their role in charge generation process are reviewed. Here, the TMO-based CGLs consist of an ntype doped organic layer and a TMO layer, where WO3 and MoO3 are the most widely used TMOs in tandem OLEDs due to their favorable electronic properties, highly optical transparency in the visible region for photon output, and excellent fabrication compatibility. Section 2 discusses the n-type doping effect of organic layers with cesium compounds (i.e., CsF, CsN3 , and Cs2 CO3 ). In Section 3, energetics at TMO/organic interfaces and the impact of annealing-induced oxygen vacancy on hole-injection barriers are overviewed as determined via photoemission spectroscopy. The ELAs and charge generation mechanisms of these TMO-based CGLs are addressed in Section 4. Finally, Section 5 provides a review of the influence of TMO-based CGLs on the performance characteristics of tandem OLEDs, particularly their stability on the device operational lifetime. 2. n-Type doping of organic layers with alkaline metal compounds For commonly used CGLs consisting of a TMO-organic bilayer, the charge generation and separation process in tandem OLEDs was proposed to occur and operate effectively with the presence of an n-type doped organic layer [22,23]. Traditionally, electrical doping of organic layers is an efficient strategy to overcome their intrinsic
limitations, e.g., low carrier transport conductivity and large charge injection barriers. For instance, p–i–n structures consisting of a ptype doped HTL, an intrinsic emission layer and an n-type doped ETL are generally considered as the state-of-the-art architecture for high-performance OLEDs due to the dramatically enhanced carrier transport properties with the highly reduced operating voltage [24]. Here, we address the n-type doping effect of organic materials with metal and metallic compounds in TMO-based CGLs and their influence on device performance of tandem OLEDs. Compared to reactive metals as n-type dopants, the incorporation of alkaline metal compounds, e.g., cesium carbonate (Cs2 CO3 ), cesium fluoride (CsF), cesium azide (CsN3 ), etc., provides an effective approach for overcoming the limitations of low bulk conductivity and high electron injection barriers in organic layers due to the advantages for material handling and operational stability [4,6–12,16–18]. Despite the extensive utilization of alkaline metal compounds as n-type dopants in OLEDs to facilitate electron injection and reduce the Ohmic losses in organic layers, the nature of the incorporated species, the electronic structures of doped materials, and the working mechanisms of alkaline metal compounds as an n-type dopant in organic layers are not yet fully clarified. We have systematically studied the electronic structures and underlying physical mechanisms of n-type doped organic layers with various alkaline metal compounds via photoemission spectroscopy [17,25–27]. The doping effect of Cs2 CO3 on electronic structures of 4,7diphenyl-1,10-phenanthroline (BPhen), a typical organic ETL, was characterized with UPS and XPS measurements [25]. It can be observed from the UPS spectra in Fig. 1(a) that the increase of the Cs2 CO3 doping concentration in BPhen films results in a gradual shift of the UPS spectral features toward higher binding energy (BE) region together with a vacuum level (VL) shift of 1.4 eV. Accordingly, it can be inferred that the Fermi level (EF ) in the Cs2 CO3 :BPhen layers moves within the energy gap toward the BPhen’s LUMO, showing an n-type doping process. Fig. 1(b) displays the related evolution of XPS C 1s, N 1s, and Cs 3d5/2 core level spectra. Taking into account the original molecular ratios in pristine Cs2 CO3 , it is verified here that Cs2 CO3 in n-type doped BPhen films is partially decomposed into cesium oxide during thermal evaporation, which is in agree with previous reports on thermal decomposition of Cs2 CO3 [28]. The doping of Cs2 CO3 into BPhen films imposes a progressive shift in general for C 1s and N 1s core levels toward higher BE region with respect to the increase in Cs2 CO3 doping concentration, which is consistent with the behavior observed in the UPS spectra in Fig. 1(a). Compared to the case of pristine
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Cs2 CO3 , the Cs 3d5/2 core level position located at lower BE region in doped systems can be ascribed to the different environment for Cs atoms surrounded by additional negative charges due to electron transfer to the BPhen host, since the Cs 3d5/2 core level is commonly localized at lower BE for Cs complex with higher oxidation state [29]. However, strong chemical reaction between Cs2 CO3 and BPhen in the doping system would be ruled out because of the absence of observable feature in the energy gap region [Fig. 1(a)] and the unchanged spectral shapes of C 1s and N 1s core levels [Fig. 1(b)]. The similar phenomena were observed for the Cs2 CO3 -doped tris(8-hydroxy-quinoline) aluminum (Alq3 ) as shown in Fig. 2. However, when comparing the difference in electronic structures of doped Alq3 layers with various alkaline metal compounds (i.e., CsF, CsN3 , and Cs2 CO3 ) as an n-type dopant, the UPS and XPS results clearly reveal the different doping mechanisms, which are sensitive to the doping constituent materials [17,25–27]. It is evident in Fig. 2(a) that all three kinds of alkaline metal compounds in the Alq3 layer resulted in the shifts of UPS spectral features towards high BE region relative to the case of pristine Alq3 layer. The shift of the secondary electron cutoff in the high BE region corresponds to a decrease in the work function of the doped Alq3 layer and the EF movement towards the Alq3 ’s LUMO within the energy gap. However, the doping of CsF and CsN3 was accompanied with the formation of a gap state in the forbidden energy gap of Alq3 , which was contrary to the case of Cs2 CO3 doping. In addition, the Mg doping in Alq3 was measured as a comparison, showing the formation of a gap state. The appearance of such a gap state indicates the presence of strong chemical reaction for Alq3 molecules with Mg, CsF or CsN3 in the doping system, which is further identified from the evolution of XPS spectra in Fig. 2(b) and (c). It is clear that the XPS O 1s and N 1s core level signals originating from Alq3 molecules exhibit splitting, with a new peak appearing at the lower BE region upon the doping of Mg, CsF or CsN3 into the Alq3 layer [Fig. 2(b) and (c)]. According to previous reports [17,27,30], the new features observed in UPS and XPS spectra are in accordance with the formation of negatively charged Alq3 radical anion induced by charge transfer to one of the triplets of LUMO in the neutral Alq3 molecule with the chemical bonding of reactive dopants with N and O in quinolate ligands of Alq3 . The difference in the electronic structures of various doping systems originates from the different doping mechanisms of alkaline metal compounds in the organic layer. It has been demonstrated that the incorporation of Cs2 CO3 , CsF or CsN3 by thermal evaporation results in decomposition during the deposition process [22,25,27]. For instance, Cs2 CO3 will partially decompose with the product of cesium oxide (Cs2 O) during the thermal evaporation [25], while CsN3 and CsF have been confirmed to generate Cs [22,27]. Therefore, no generation of metallic Cs can be found during the evaporation process of Cs2 CO3 , leading to the absence of strong chemical reaction between Cs atom and Alq3 molecule in the Cs2 CO3 :Alq3 doping system.
3. Energetics at TMO/organic interfaces Although TMOs (e.g., MoO3 and WO3 ) are commonly used in OLEDs for enhancing hole injection, recent reports on their electronic structures have demonstrated that these materials exhibit an n-type semiconducting property due to oxygen vacancies [19,31,32], which rule out the misunderstandings of p-type characteristics in previous reports [33,34]. It is thus crucial to estimate the ELAs at the TMO/organic interfaces, which is of importance to clarify the charge generation and separation in TMO-based CGLs for tandem OLEDs and especially the impacts of constituent materials
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on the functional effectiveness of TMO-based CGLs. In the following, we will firstly present the electronic structure evolutions at the TMOs (i.e., MoO3 and WO3 )/organic interfaces via photoemission spectroscopy.
3.1. Electronic structures at TMO/organic interfaces As determined in Fig. 3(a) of HeI UPS spectra of the incremental MoO3 deposition on CsN3 :BPhen (10 nm, 20 wt%), the deposition of a MoO3 overlayer results in a progressive shift of emission features of CsN3 :BPhen and MoO3 towards lower BE region, which is accompanied by a VL shift of 4.6 eV [22]. Meanwhile, the original gap states in the forbidden gap of CsN3 :BPhen disappear upon the MoO3 deposition. It is noted that the spectral features become saturated at 5 nm-thick MoO3 layer with the appearance of a MoO3 -derived deep-lying gap state, which is attributed to the oxygen vacancy and partial reduction of Mo6+ cation in MoO3 as evidenced by XPS measurements [30,32]. The HOMO edge of CsN3 :BPhen and the valence band maximum (VBM) of the MoO3 layer can be extracted by the intercept of the tangent of the leading edges of the low BE feature and the background level, and are found at 4.4 and 2.8 eV below the substrate EF , respectively. Given that the energy gap of MoO3 is ∼3.1 eV, it is indicative that EF is pinned slightly below the conduction band minimum (CBM) of MoO3 , showing an ntype semiconducting character [19]. In addition, it is shown that a 10 nm-thick MoO3 layer possesses a high work function (WF) of 6.7 eV. Upon the interface formation between N,N -bis(1-naphthyl)N,N -diphenyl-1,1 -biphenyl-4,4 -diamine (NPB) and MoO3 as shown in Fig. 3(b), the frontier molecular orbitals of NPB gradually shift toward higher BE region, which is accompanied by a VL downward shift of ∼2 eV. Energy level shifts indicate energy level bending at the MoO3 /organic interface. Additionally, there is no observable feature in the organic energy gap region, suggesting the absence of chemical interaction at the interface. For comparison, Fig. 3(c) displays the UPS spectra of a 10 nmthick NPB layer deposited on MoO3 , BPhen, and CsN3 :BPhen substrates, respectively. No evidence of chemical reaction or formation of new interfacial electronic states can be observed for these three interfaces [22]. On the other hand, these interfaces exhibit two different trends upon interface formation. The VL shift to higher BE region represents the formation of an interface dipole upon deposition of NPB on MoO3 , whereas the UPS spectra evolutions of CsN3 :BPhen/NPB and BPhen/NPB interfaces imply the absence of interface dipole at the corresponding interfaces. The interface energetics between WO3 and organic layers are similar to that of MoO3 /organic interfaces. As shown in Fig. 4(a), the incremental deposition of a WO3 overlayer on Mg:Alq3 (10 nm, 10 wt%) substrate results in a progressive shift of UPS spectral features towards the lower BE region [20]. In accordance to MoO3 , WO3 is confirmed as an n-type semiconducting material [35]. In addition, the formation of new state can be observed in UPS spectra at the WO3 /Mg:Alq3 interface, which is contrary to the WO3 /Alq3 interface without additional spectral structures [as shown in Fig. 4(b)]. Fig. 4(c) and (d) display the evolution of W 4f core level spectra for the NPB/WO3 /Mg:Alq3 and NPB/WO3 /Alq3 systems, respectively. These spectral evolutions coincide with the partial reduction of W6+ cations at the WO3 /Mg:Alq3 interface and the chemically inert nature for WO3 /Alq3 interface, which is consistent with the appearance of a new state in the forbidden band gap as shown in Fig. 4(a). On the other hand, upon NPB deposition onto WO3 films, the W 4f doublet peak only shows changes in intensity without additional peak or peak broadening, implying no chemical reaction between NPB and WO3 .
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Fig. 2. N-type doping effects of various metallic compound dopants. (a) UPS spectra of 10 nm-thick dopedAlq3 films using Mg, Cs2 CO3 , CsF, and CsN3 dopants with a doping ratio of 10 wt% [17,27]. (b) Comparison of the corresponding XPS O 1s spectra. Copyright 2014, Elsevier.
Fig. 3. HeI UPS spectra of (a) MoO3 /CsN3 :BPhen [22] and (b) NPB/MoO3 interfaces [32] as a function of the deposited overlayer thickness. (c) UPS spectra of a 10 nm-thick NPB layer deposited on MoO3 , BPhen, and CsN3 :BPhen substrates, respectively [22]. Ref. [22]: copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Ref. [32]: copyright 2010, American Institute of Physics.
3.2. Effects of oxygen vacancy on electronic structures It has recently been reported that oxygen vacancies in TMOs arising from partly filling empty d-states can cause the presence of gap states in the forbidden energy gap and significantly affect the chemical and electronic properties of these nonstoichiometric oxides [31,36,37]. For example, oxygen vacancy-induced gap states in MoOx play a crucial role in the hole transport properties of MoOx , which can act as n-dopant and reduce the WF with the EF shift in the energy gap [31]. It indicates that oxygen vacancies in MoOx are advantageous by increasing the gap-state-assisted hole transport and disadvantageous by increasing the hole injection barrier for the device performance, and a balanced interplay has to be found for highly efficient devices. Therefore, it is expected that the ELAs and charge exchange at the TMO/organic interfaces
can be tuned under the control of oxygen vacancies upon chemical or physical treatments. To gain a deep insight into the oxygen vacancy in TMOs and the corresponding effects on hole injection barriers in TMO-based CGLs, electronic structure evolutions at the TMO/organic interfaces were systematically investigated via UPS and XPS. Here, the TMOs (i.e., MoOx and WOx ) were thermally annealed under nitrogen atmosphere to control the oxygen stoichiometry. Fig. 5(a) and (b) display the XPS spectra of the Mo 3d and W 4f core levels for MoOx and WOx films on ITO substrate annealed at various temperatures, indicating that the the spectral features are highly dependent on the annealing temperature [38,39]. Obviously, the Mo6+ cation is the majority presence in the as-grown MoOx layer as an almost single Mo 3d3/2 and 3d5/2 doublet centers at the BEs of 235.8 and 232.7 eV, respectively. Similarly, the
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Fig. 4. UPS and XPS spectra of WO3 /organic interfaces. HeI UPS spectra of (a) WO3 /Mg:Alq3 and (b) WO3 /Alq3 interfaces formed on ITO glass substrates by firstly depositing a 10-nm thick doped or undoped organic layer. XPS W 4f core level spectra for (c) NPB/WO3 /Mg:Alq3 and (d) NPB/WO3 /Alq3 interfaces. The dashed lines represent the least-square peak fitting for a 10 Å-thick WO3 on Mg:Alq3 and Alq3 substrates, respectively [20]. Copyright 2010, Elsevier.
as-grown WOx layer with the chemical state of W6+ shows an almost single W 4f5/2 and 4f7/2 doublet centered at BEs of 38.1 and 36.0 eV. However, thermal annealing causes the appearance of new features at the lower BE region for both MoOx and WOx films. Upon the least-squared fitting analysis by means of different main doublets with a Shirley background, the peak positions and the relative contributions of different oxidation states can be accurately determined in all the Mo 3d and W 4f core level spectra. It is clear in Fig. 5(a) that the intensity of a new doublet at lower BE region (e.g., 234.9 eV for Mo 3d3/2 and 231.8 eV for Mo 3d5/2 ) increases
with respect to the rise in the annealing temperature from room temperature (RT) to 150 ◦ C, while the peak positions of the Mo6+ doublet remain almost constant under all annealing temperatures. When the annealing temperature is beyond 200 ◦ C, another new doublet is observed at lower Bes (e.g., 233.9 for Mo 3d3/2 and 230.8 eV for Mo 3d5/2 ). According to previous reports [32,40], these new peaks can be assigned to the chemical states of Mo5+ and Mo4+ , respectively, which indicate the partial reduction of Mo6+ cations to Mo5+ and Mo4+ due to the annealing-induced oxygen vacancy in MoOx films [41]. On the other hand, the similar phenomena can be
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Fig. 5. Annealing-induced oxygen vacancy in TMO films at various temperatures in a nitrogen atmosphere [38,39]. XPS spectra of (a) Mo 3d and (b) W 4f core levels measured for MoOx and WOx films with a thickness of 5 nm deposited on ITO substrates and annealed at various temperatures in a nitrogen atmosphere. Solid lines in (a) and (b) represent Gaussian/Lorentzian mixing functions for fitting analysis and resultant curves of the fitting on the experimental data. The dependence of WF and the stoichiometry x of (c) MoOx and (d) WOx layers versus annealing temperature. Copyright 2013, American Institute of Physics and The Japan Society of Applied Physics.
observed in the annealed WOx films as shown in Fig. 5(b), where the signal of an additional doublet at lower BEs of 37.4 eV (W 4f5/2 ) and 35.3 eV (W 4f7/2 ) increases with the increase of annealing temperature from RT to 250 ◦ C. These new features can be assigned to the reduced chemical states of W5+ from W6+ cations due to the annealing-induced oxygen vacancies in the WOx films [35]. Correspondingly, the stoichiometry (x) of annealed MoOx and WOx films can be calculated by taking the relative contribution of different oxidation states into account. As shown in Fig. 5(c) and 5(d), the x values varied from ∼3.0 to ∼2.7 and ∼2.8 for MoOx and WOx , respectively, with respect to the annealing temperature from RT to 250 ◦ C. Moreover, the evolution of oxygen vacancies in the annealed MoOx and WOx layers under nitrogen atmosphere basically follows the same trend as these films annealed in vacuum [37]. The corresponding electronic structures of MoOx and WOx films treated at various annealing temperatures are consistent with the reduction of stoichiometry, showing a progressive decrease in WF with the increase of oxygen vacancy-induced gap states. For example, the WF of annealed MoOx film was reduced from 5.05 eV at RT to 4.55 eV at 250 ◦ C [Fig. 5(c)], while the corresponding WF of the annealed WOx film decreased from 4.67 to 4.47 eV with the increase of oxygen vacancies [Fig. 5(d)]. The influence of annealing-induced oxygen vacancies on energetics at TMO/organic interfaces (i.e., MoOx /NPB and WOx /NPB) is identified by incrementally depositing a 10 nm-thick NPB layer onto various annealed TMO films [38,39]. As summarized in Fig. 6, a clear dependency of WF and hole injection barriers on the annealing temperature at both MoOx /NPB and WOx /NPB interfaces can be observed. According to the proposed energy level diagram depicted in the inset of Fig. 6(a), the hole-injection barriers for MoOx /NPB interfaces range from 0.9 eV (on MoOx at RT) to 1.2 eV (on MoOx annealed at 250 ◦ C), whereas the WF of NPB layers varies from 4.5 eV to 4.1 eV. For WOx /NPB interfaces [Fig. 6(b)], the hole-injection barrier ranges from 0.6 eV (on as-fabricated WOx substrate) to 1.0 eV (on WOx annealed at 250 ◦ C), which is accompanied with the change of NPB’s WF from 4.8 eV to 4.4 eV. However, the overall changes of WF and hole injection barrier are almost
Fig. 6. Dependence of WF and hole-injection barriers of NPB layers deposited on annealed MoOx (a) and WOx (b) substrates as a function of the annealing temperature. Inset shows the schematic energy level diagram at the annealed TMO/NPB interface. Copyright 2013, American Institute of Physics and the Japan Society of Applied Physics.
rigid, keeping the constant ionization energy of NPB within the experimental error. The dependence of hole injection barriers at the MoOx /NPB interfaces are correlated with the degree of thermal annealing-induced oxygen vacancies in MoOx . Therefore, the degree of oxygen vacancies in TMOs can be controlled by thermal annealing, which results in the partial reduction of transition metal cations and a decrease in WF. The interfacial ELA and thus hole-injection barrier at TMO/organic interfaces increase as a consequence of the increase in oxygen deficiency.
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Fig. 7. Schematic energy level diagrams of MoO3 -based CGLs. (a) BPhen/CsN3 :BPhen/MoO3 /NPB, (b) BPhen/MoO3 /NPB, (c) BPhen/CsN3 :BPhen/NPB, and (d) BPhen/NPB interfaces on ITO substrate. All the values shown are in the unit of eV [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
4. ELAs and charge generation mechanism of TMO-based CGLs 4.1. ELAs of TMO-based CGLs On the basis of electronic structures characterized by UPS and XPS measurements, ELAs at various TMO-based CGLs can be extracted, which favor the understanding of charge generation and separation process in CGLs. Fig. 7 depicts the energy level diagrams for the MoO3 -based CGLs by taking into account interface dipole and energy level bending [22]. Here, the EF of the underlying ITO substrate was used as the reference. The ELAs of CGLs without MoO3 or n-type doped organic layer are also shown for comparison. As shown in Fig. 7(a) and (b), a p–n junction is formed between NPB and MoO3 with a small hole-injection barrier. For the CGLs without MoO3 , typical flat vacuum level alignment is observed at CsN3 :BPhen/NPB and BPhen/NPB interfaces as displayed in Fig. 7(c) and (d). Besides, the n-type doped organic layer incorporated into TMO-based CGLs also plays a crucial role in tandem OLEDs. As shown in Fig. 7(a) and (c), the EF in CsN3 :BPhen is significantly tuned only 0.1 eV below the BPhen’s LUMO, which will be favorable for electron injection and transport through CsN3 :BPhen into the adjacent BPhen ETL. Moreover, the CsN3 :BPhen layer could exhibit superior hole-blocking capability due to the large ionization potential value and the wide bandgap. In contrast, there is a large LUMO offset at the MoO3 /BPhen interface, restricting the effective electron injection from MoO3 into the BPhen layer. In addition, the large energy level offset exists between NPB’s HOMO and CsN3 :BPhen’s (or BPhen’s) LUMO, implying the difficulty in charge generation and separation at CsN3 :BPhen/NPB and BPhen/NPB interfaces under
weak electric field. According to the energy level diagrams of the TMO-based CGLs with neighboring HTL and ETL layers depicted in Fig. 7, the impact of the constituent materials on the functional effectiveness of TMO-based CGLs and thus the device performance of the corresponding tandem OLEDs can be understood, which will be discussed in details in Section 5. 4.2. Charge generation mechanism of TMO-based CGLs As reported previously [15,19,42,43], an electric-field-assisted charge separation process was proposed to occur at the TMO/organic interface. However, the charge transfer process in TMO-based CGLs remains a subject of debate. It was assumed that under applied bias such a charge transfer occurred from the TMO’s VBM to the LUMO state of n-type doped organic layer, since the TMOs were usually regarded as a p-type material [15]. Nevertheless, as demonstrated by Kröger et al. [19] and observed in Section 2, MoO3 acts as n-type semiconductor with deep-lying defect states below EF derived from oxygen vacancies. As a result, the charge generation process was ascribed to the hole injection from TMO’s CBM into the adjacent organic HTL’s HOMO [19,42]. According to the experimentally determined ELAs and auxiliary capacitance characteristics demonstrated recently [22], the working principle of TMO-based CGLs has been proposed that charge carriers are generated within the TMO layer by exciting electrons from the oxygen vacancy-derived gap states into its conduction band. Fig. 8 represents the schematic of charge generation and separation process in the TMO-based CGLs. Electrons in TMOs are expected to spontaneously transfer via thermal diffusion from various defect states to the conduction band [denoted as process (1) in Fig. 8]. Under
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Fig. 8. Schematic electric field-assisted charge generation and separation processes under forward bias, including (1) thermal excitation of electrons from defect levels to the CBM within MoO3 ; (2) electron injection from MoO3 ’s CBM into the CsN3 :BPhen’s LUMO; (3) hole injection from MoO3 ’s defect levels to NPB’s HOMO [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
the external electric field, electrons and holes are injected through tunneling from conduction band and defect states of TMOs, respectively, into the LUMO of n-type doped organic layer and the adjacent HTL’s HOMO [processes (2) and (3)]. Here, electrons generated in TMOs can easily inject from the TMO’s CBM into the LUMO of ntype doped organic layer due to the small injection barrier at the TMO/organic interface. Then, electrons will immediately be driven away from CsN3 :BPhen to the BPhen layer by the external electric field, while holes are in the same way moved away from the interface. 5. Impact of CGLs on device characteristics The impacts of the constituent materials on the functional effectiveness of TMO-based CGLs in tandem OLEDs can be verified on a basis of device performance characteristics such as electrical and spectral properties, luminance efficiency, and operational stability, etc.
Fig. 9. Device performance of tandem OLEDs using different TMO-based CGLs. (a) Current density-voltage and (b) luminance efficiency-current density characteristics of tandem OLEDs with double emission units and reference device with single emission unit [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
5.1. Electrical and optical properties
into BPhen can greatly move EF to only 0.1 eV below the LUMO, resulting in the small injection barrier at the CsN3 :BPhen/MoO3 interface and the enhanced electron injection through CsN3 :BPhen into the BPhen ETL. Consequently, the driving voltage of tandem OLED with CsN3 :BPhen/MoO3 in Fig. 9(a) is almost twice that of the reference device with a single unit at a constant current density, which is predictable for an effective tandem OLED. In contrast, other tandem devices exhibit poorer electrical properties and thus require higher driving voltages due to the large energy level offset between NPB’s HOMO and CsN3 :BPhen’s (or BPhen’s) LUMO. It indicates that charge generation process is hard to occur at CsN3 :BPhen/NPB and BPhen/NPB interfaces unless a higher driving voltage is applied. As a result, the luminance efficiencies of these inferior devices are just comparable with that of the reference device with single emission unit [Fig. 9(b)]. In addition, the lower driving voltage and higher luminance efficiency compared with the TMO/organic bilayer structure can be realized with the adoption of a hybrid CGL structure involving p-type doping (i.e., n-type doped organic ETL/TMO/TMO-doped organic HTL) [44], which is attributed to more favorable ELA for charge generation and separation in tandem OLEDs. Fig. 10 compares the electroluminescent (EL) spectra of two series of tandem OLEDs combining green and red emission units (EL-G and EL-R) in different stacking sequences, which helps to identify the correlation between carrier recombination processes and various CGLs [22]. The EL-G comprises NPB layer
Fig. 9 summarizes the current density versus voltage characteristics and luminance efficiency of tandem OLEDs together with that of a reference device with single emission unit, in which four different CGLs are used with the structures of (1) CsN3 :BPhen/MoO3 , (2) CsN3 :BPhen, (3) MoO3 , and (4) none, respectively. The emission unit consists of the NPB layer as HTL, BPhen layer as ETL, and green emitting layer of 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl1H,5H,11H-(1)-benzopyropyrano(6,7,8-i,j)quinolizin-11-one (C545T) doped Alq3 . Obviously, constituent materials of CGLs play critical parts in their functional effectiveness and the carrier recombination processes for light emission in tandem OLEDs. It is noted that the device characteristics of tandem OLEDs using TMO-based CGLs in Fig. 9 can be understood based on the ELAs of various CGLs shown in Fig. 7. The driving voltages of tandem OLEDs are consistent with the energy level offsets between the n-type doped organic layer’s LUMO and the adjacent HTL’s HOMO. The use of both TMO (e.g., MoO3 in Fig. 7) and n-type doped organic layer in the CGLs can significantly reduce this energy level offsets. As shown in Fig. 7(a) and (b), a p–n junction is formed between NPB and MoO3 with a small hole injection barrier height. In addition, the incorporation of an n-type doped organic layer in TMO-based CGLs is also essential for tandem OLEDs to function efficiently with enhanced EL efficiency. As shown in Fig. 7(a) and (c), doping CsN3
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Fig. 10. Electroluminescent spectra of tandem OLEDs composed of various CGLs with two different emission units. Devices in (a): the bottom emission unit with red emission and the top green emission; devices in (b): the bottom EL unit with green emission and the top red emission, respectively [22]. Copyright 2012, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
as HTL, BPhen layer as ETL, and Alq3 layer as the green emitting layer, while the EL-R is formed between NPB and BPhen by inserting red emitting layer of 4-(dicyanomethylene)-2-t-butyl6-(1,1,7,7,-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) doped Alq3 . As shown in Fig. 10, tandem OLEDs using the CGL of CsN3 :BPhen/MoO3 exhibit clearly both green and red emission components, implying that such a CGL functions well and emissions in both EL-G and EL-R units are mutually independent on the tandem structure. However, when only MoO3 or CsN3 :BPhen is used in the CGL, these tandem OLEDs do not show the fully unlocked emission pattern in comparison to that using CsN3 :BPhen/MoO3 , indicating that charge generation and separation process is prohibited due to the reduced electron or hole injection into the neighboring ETL or HTL, respectively [22,30]. Therefore, the asymmetric contribution of two emission units to the EL spectra validates that the MoO3 layer is essential to charge generation in the CGL. It is believed that the electron–hole recombination in the cathode-side emission unit for tandem OLEDs using CGLs without MoO3 is eliminated due to the lack of charge generation by MoO3 and leakage current of holes from the ITO side. This is the reason why intensive emission from the cathode-side EL unit can only be observed for tandem OLEDs with MoO3 in the CGLs, while other tandem devices behave like an OLED with a single EL unit as shown in Fig. 10.
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Fig. 11. Operational stability of tandem OLEDs with various CGLs. (a) Normalized luminance (luminance/initial luminance) and (b) driving voltage as a function of the operational time for tandem devices encapsulated with an epoxy resin-sealed glass cap and tested at a constant current density [17]. Copyright 2014, The Royal Society of Chemistry.
5.2. Operational stability Long lifetime is the prerequisite for tandem OLEDs as a cost competitive and sustainable technology in commercial applications. It has been demonstrated that device degradation can be induced by various factors, including dark spots caused by an unstable interface between electrode and organic [45], the morphological instability of organic films [46], electrical shorts due to structure defects [47], electrochemical aging at organic/cathode interface [48], metal atom/ion diffusion [49], and oxidization of functional layers with the diffusion of moisture and oxygen molecules [50]. Inevitably, the insertion of CGLs will introduce more interfaces, which are strongly related to an additional voltage drop and charge accumulation across each CGL in tandem device. Therefore, it is desirable to elucidate the impact of CGLs on device lifetime of tandem OLEDs and the corresponding degradation mechanism. Fig. 11 illustrates the luminance and driving voltage as a function of the operational time under a constant current condition. It is obvious that tandem OLEDs with various CGLs exhibit totally different degradation behavior, and the degradation of luminance and driving voltage of these devices varied in a non-monotonic style during continuous operation. For example, tandem device with Mg:Alq3 /MoO3 exhibits a slow luminance degradation [Fig. 11(a)] and a minimum increase in driving voltage [Fig. 11(b)], while the use of Cs2 CO3 :Alq3 /MoO3 and CsN3 :Alq3 /MoO3 as a CGL causes a rapid initial decrease in luminance and a dramatic increase in driving voltage. It is verified that various degradation modes of tandem devices could mainly be attributed to the interface instability of the CGLs due to the
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different doping mechanisms in n-type doped organic ETLs [17,51]. These findings confirm that the constituent materials of CGLs play an important role in the operational stability of tandem OLEDs. For practical applications, the long operating lifetime of tandem OLED devices is undoubtedly a prerequisite in terms of massive production. 6. Conclusions In summary, we have reviewed previous investigations on TMO-based CGLs for tandem OLEDs, especially from interface energetics perspective via photoemission spectroscopy. TMOs such as MoO3 , WO3 exhibit n-type semiconductor property with deeplying defect states due to the presence of oxygen vacancies. It has been demonstrated that the energetics of TMO/organic interfaces plays an important role in charge generation process for tandem OLEDs. The doping mechanisms of n-type doped organic layers in CGLs with various cesium compounds have been identified by intensive comparison of their influences on electronic structures. Energy level alignment and charge separation mechanism of TMO-based CGLs have been proposed, which can provide a deeper understanding of charge generation and separation process in CGLs. Moreover, the impacts of TMO-based CGLs on device performance have been clarified by checking the electrical and optical properties and operational stability of tandem OLEDs. The resulting energy level alignments are summarized in correspondence with tandem OLED performance. The correlation between the operational stability of CGLs relating to different n-dopants and the device degradation behaviors are also discussed. Acknowledgments The authors acknowledge financial support from the 973 Program by Ministry of Science and Technology of China (Nos. 2014CB932600, 2011CB808404), the National Natural Science Foundation of China (Nos. 91433116, 11474214), Jiangsu Science and Technology Department (No. BK20140053), Bureau of Science and Technology of Suzhou Municipality (No. ZXG201422), and the project of the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References [1] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [2] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 459 (2009) 234. [3] H. Sasabe, J. Kido, J. Mater. Chem. C 1 (2013) 1699. [4] L.-S. Liao, W.K. Slusarek, T.K. Hatwar, M.L. Ricks, D.L. Comfort, Adv. Mater. 20 (2008) 324. [5] T. Matsumoto, T. Nakada, J. Endo, K. Mori, N. Kawamura, A. Yokoi, J. Kido, SID Int. Symp. Digest Tech. Papers 34 (2003) 979. [6] L.S. Liao, K.P. Klubek, C.W. Tang, Appl. Phys. Lett. 84 (2004) 167. [7] J.-X. Tang, M.-K. Fung, C.-S. Lee, S.-T. Lee, J. Mater. Chem. 20 (2010) 2539. [8] C.W. Chen, Y.J. Lu, C.C. Wu, E.H.E. Wu, C.W. Chu, Y. Yang, Appl. Phys. Lett. 87 (2005) 241121. [9] H. Kanno, R.J. Holmes, Y. Sun, S.K. Cohen, S.R. Forrest, Adv. Mater. 18 (2006) 339. [10] J. Meyer, S. Hamwi, M. Kröger, W. Kowalsky, T. Riedl, A. Kahn, Adv. Mater. 24 (2012) 5408. [11] H.M. Zhang, Y.F. Dai, D.G. Ma, H.M. Zhang, Appl. Phys. Lett. 91 (2007) 123504. [12] C.C. Chang, S.W. Hwang, C.H. Chen, J.F. Chen, Jpn. J. Appl. Phys., 1 43 (2004) 6418.
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Please cite this article in press as: Q.-D. Ou, et al., Photoemission spectroscopy study on interfacial energy level alignments in tandem organic light-emitting diodes, J. Electron Spectrosc. Relat. Phenom. (2015), http://dx.doi.org/10.1016/j.elspec.2015.02.011