Quantized electrode for robust organic devices

doi:10.21203/rs.3.rs-4902453/v1

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Quantized electrode for robust organic devices

https://doi.org/10.21203/rs.3.rs-4902453/v1

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The contact between inorganic electrodes and organic layers serves as both a charge-carrier conduit and a crucial topological link in organic optoelectronic devices, significantly impacting carrier injection/extraction efficiency and long-term stability. A barrier-free contact for hole injection with thermodynamic stability at the inorganic electrode/organic interface is essential for efficient devices. However, this has not yet been realized between metal electrodes and organic materials with high ionization energy (IE). Here we demonstrate a quantized electrode with surface charge-transfer states formed by chemisorbed ultrathin organic layer on an aluminum surface. This quantized electrode, featuring a new injection mechanism, forms universal ohmic hole contacts with organic semiconductors possessing high IEs up to 6.1 eV, rendering the conventional rule of ohmic hole contact that the work function of the anode must match the IE of the organic semiconductor invalid. The resultant organic light-emitting diode (OLED) achieves ultrahigh luminance at low voltage and extremely low efficiency roll-off. Moreover, the quantized electrode exhibits excellent thermal stability, with a seventeen-fold increase in operational lifetime compared to the reference device. Quantized electrodes will facilitate the development of OLED microdisplays and have potential applications in other advanced semiconductors requiring ohmic hole contact and robust stability.

Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Organic LEDs

Physical sciences/Materials science/Materials for devices

Inorganic electrode/organic interface is widely recognized as the weakest link in organic optoelectronic devices, primarily due to interfacial barriers for hole and electron contact arising from significant energy level offsets, and surface energy mismatches between inorganic electrodes and organic semiconductors. Despite extensive efforts to achieve ohmic hole contacts^5,6, a truly barrier-free contact with the commonly used deep highest occupied molecular orbital (HOMO) (~6.0 eV) of organic materials has not yet been realized.

Due to their high conductivity and/or high reflectivity, metals are commonly used as the anode in organic semiconductor devices^7-9. Surface buffer layers such as indium-tin-oxide (ITO)¹⁰, molybdenum oxide (MoO_X)¹¹, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ)¹², 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN)¹³, and others^14-16 are utilized to increase the metal work function to some extent. Nonetheless, a mismatch remains between the modified metal work function and the ionization energy (IE) (~6.0 eV) of widely used active-layers or hosts. To improve hole injection, multiple hole transport layers (HTLs) arranged in a stepwise mode^17,18 or p-doped HTL scheme^9,15,19 is commonly employed. However, the introduction of these additional layers, along with the presence of hetero-junctions, not only complicate fabrication processes, but also adversely affect device lifetime^20,21.

As the only organic semiconductor technology to have achieved large-scale commercialization, establishing an ohmic hole injection directly into the deep HOMO level of HTL in organic light-emitting diodes (OLEDs) holds significant application value, as it would greatly facilitate simplified device structure and enhances overall device performance. This is especially critical for top-emitting OLEDs (TEOLEDs) used in microdisplay applications for augmented reality/virtual reality headsets, where ultrahigh luminance exceeding 10,000 cd m^-2 is essential under 8 V^9,19,22,23, a voltage constrained by the standard sub-micron complementary metal-oxide-semiconductor (CMOS) technologies. More seriously, to ensure ultrahigh luminance, the commonly used p-/n-doped charge transport layers (CTLs) can cause electrical cross-talk between neighboring pixels²⁴. Additional barrier layers are implemented to spatially and electrically isolate pixels²⁵, complicating the fabrication process.

In addition to pursuing ohmic hole injection, ensuring a long device operational lifetime is essential²⁶. There is an urgent need to improve the thermal stability of the inorganic electrode/organic interface, which consistently suffers from large surface energy mismatches. Under conditions of thermodynamic instability, elevated temperatures caused by Joule heating during operation, primarily originating from interfacial barriers for charge injection and charge carriers dissipate kinetic energy in collisions, can lead to inorganic/organic interfacial degradation^27,28. Therefore, realizing an ohmic hole contact with a thermally robust topological link at the inorganic electrode/organic interface is both highly challenging and demanding.

In this study, we propose a novel concept of quantized electrode with surface charge-transfer states formed by a chemisorbed ultrathin HAT-CN layer on Al. The conventional rule for ohmic hole contact, which requires the Fermi energy level (E_F) of the anode to align with the HOMO of the organic semiconductor, does not apply to quantized electrodes. Instead, the surface charge-transfer states create a new pathway for hole injection. The quantized electrode achieves universal ohmic hole injection into organic semiconductors with IEs as high as 6.1 eV and excellent inorganic/organic interfacial thermal stability. This breakthrough enables a robust TEOLED without the use of p-/n-doped CTLs, resulting in ultrahigh luminance at limited voltage, significantly improved efficiency roll-off, and outstanding operational stability.

Device performance

We fabricated TEOLEDs based on a novel anode structure consisting of an ultrathin HAT-CN layer (2 nm) deposited on Al and bottom-emitting OLEDs (BEOLEDs) employing the widely used ITO/HAT-CN (2 nm). To highlight superiority of the Al/HAT-CN (2 nm) anode structure, our previous work related to chlorinated ITO (ITO-Cl)-based device²⁹, where a surface chlorination process modified the work function of ITO electrode up to 6.1 eV, was also incorporated. Figure 1a (inset) shows schematic diagram of all comparative devices that adopt the same HTL, emission layer, and electron transport layer, which corresponds to 4,4’-N,N’-dicarbazole-biphenyl (CBP), CBP: 8% Bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)₂(acac)), and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), respectively. The detailed device structures are described in the Methods.

Despite the huge difference between the work function of Al (4.2 eV) and the HOMO of CBP (6.1 eV), the Al/HAT-CN-based device (red line) exhibits superior current density versus voltage (J-V) and luminance versus voltage (L-V) characteristics (Fig. 1a,b). Electroluminescence (EL) with an ultrahigh luminance reaching 10,000, 100,000, and 200,000 cd m^–2 is observed at voltage as low as 5.1, 6.7, and 7.4 V, respectively. At the same voltage, the corresponding luminance for ITO-Cl-based device, which is regarded as one of the best performing OLEDs, is only 3,400, 25,000, and 41,000 cd m^–2(dash line in Fig. 1b). Figure 1c compares the current efficiency (CE) as a function of luminance. The maximum CE for the Al/HAT-CN anode structure is up to 113.2 cd A^-1. Particularly, high CEs of 110.5, 88.6, and 74.5 cd A^-1 were achieved at 10,000, 100,000, and 200,000 cd m^-2, respectively. A retained 98% of maximum CE at a luminance of 10,000 cd m⁻² indicates an extremely low efficiency roll-off³⁰^,3¹, which is critical for meeting the low-power consumption requirements of lightweight, battery-operated wearable microdisplays. A comparison of the roll-offs performance with other Ir(ppy)₂(acac)-based OLEDs is summarized in Extended Data Table 1 and Extended Data Fig. 1. Additionally, a seventeen-fold increase in device lifetime at 90% of the initial luminance (LT₉₀) has been obtained compared to the ITO/HAT-CN-based device using the conventional stepwise injection structure (Fig. 1d and Extended Data Fig. 2). The angle-dependent normalized emission patterns and normalized EL spectra of TEOLED are shown in Extended Data Fig. 3. To illustrate the universal applicability of the Al/HAT-CN anode structure, we explored red and blue TEOLEDs with different HOMO level HTLs. Both types demonstrated excellent performance, achieving luminance levels of 10,000 cd m^-² at voltages below 5 V (Extended Data Fig. 4, Extended Data Table 2,3).

Ohmic hole injection and energy level alignment

To understand the origin of significant differences in device performance caused by various anode structures, hole-only devices were fabricated to evaluate hole injection capability (Fig. 2a). Compared to ITO-based hole-only device, the excellent J-V characteristic indicates superior hole injection from Al/HAT-CN into the deep HOMO level of CBP. To quantify the hole injection capability, the charge-carrier mobility of CBP is derived from the Mott–Gurney equation,

where J is the current density, ε is the dielectric constant, ε₀ is the vacuum permittivity, µ is the carrier mobility, V is the applied voltage, V_bi is the built-in voltage, and L is the layer thickness. The calculated charge-carrier mobility of CBP, based on space-charge-limited current (SCLC), is much higher than previously reported values⁴ and accords well with high-field values obtained using the time-of-flight (TOF) technique (Fig. 2b)^32,33. These results suggest that the Al/HAT-CN anode structure facilitates ohmic hole injection into CBP over a wide voltage range, thus enabling ultrahigh luminance at limited voltages and extremely low roll-off. Furthermore, if the hole injection were ohmic, the curve of JL³ as a function of V-V_bi would be independent of organic layer thickness. To verify this, another set of hole-only devices with varying CBP thickness (100 and 200 nm) was fabricated. Both devices collapse onto a single curve (Extended Data Fig. 5), providing additional strong evidence for the obtainment of ohmic hole injection.

Given that ITO possess a relatively higher work function of 4.7 eV than Al, theoretically, the E_F of ITO modified by HAT-CN should align more closely with the HOMO of CBP than that of Al/HAT-CN, resulting in better hole injection efficiency. However, the ITO/HAT-CN-based hole-only device demonstrates significantly low current density (Fig. 2a). To reveal the relative energy level position, in-situ photoemission studies were conducted. The work functions of ITO/HAT-CN and Al/HAT-CN are 5.12 eV and 5.68 eV, respectively, as revealed by the secondary electron cutoff (SECO) of ultraviolet photoelectron spectroscopy (UPS) (Fig. 2c,d). The noticeably different IEs of HAT-CN absorbed on ITO (8.12 eV) and Al (8.78 eV) can be explained by the presence of two different types of molecule orientations: face-on and edge-on configurations (insets of Fig. 2c,d), as corroborated by both theoretical and experimental studies on orientation-dependent IEs^34-36. The UPS spectrum at the HAT-CN/CBP interface for both anode structures is displayed in Extended Data Fig. 6, with a schematic representation of the corresponding energy level alignment shown in Fig. 2e,f. The substantial energy offset (2.24 eV) reflects how problematic hole injection into CBP is. Despite the much lower energy offset (0.77 eV) benefiting from the face-on HAT-CN molecular orientation (Fig. 2f), nevertheless, it contradicts the evidence of ohmic hole injection presented above. In other words, the conventional rule that the electrode E_F should match the HOMO of organic semiconductor for ohmic hole contact is overturned. Therefore, it is necessary to further explore the influence of the Al/HAT-CN interface on hole injection.

Charge transfer induced quantum states and their role on ohmic hole injection

To elucidate the interfacial energy structures at Al/HAT-CN interface, valence band (VB) of UPS for increasing HAT-CN coverage on Al substrate were obtained (Fig. 3a). Upon the formation of an ultrathin HAT-CN layer on Al, a new spectral feature was detected, as illustrated in the inset of Fig. 3a. As the coverage increased, the new features diminish and eventually disappear. These new spectral features, formed between the HOMO of HAT-CN and the E_F of Al/HAT-CN. Since these new quantum states only appear in HAT-CN molecules closest to the Al substrate, it can be deduced that these quantum states originate from charge transfer, as suggested by previous studies^37,38. Direct experimental evidence for this hypothesis is provided by x-ray photoemission spectroscopy (XPS) measurements of a 1 nm HAT-CN layer on Al, which showed two peaks in the N 1s region at 398.62 and 399.70 eV binding energy (BE) (Fig. 3b). The higher BE N 1s peak is assigned to neutral HAT-CN, while the lower BE peak is attributed to anion species, signifying charge transfer from Al to HAT-CN molecule. A further proof for charge transfer from Al to HAT-CN is provided by the appearance of the anion HAT-CN species and cation Al species in the C 1s and Al 2p spectra (Extended Data Fig. 7). Meanwhile, density functional theory (DFT) calculated density of state (DOS) supports the presence of these charge-transfer states at the HAT-CN/Al interface (Fig. 3c). All these confirm that the HAT-CN molecules accept electrons from Al, indicating HAT-CN molecules form primary chemical bonds with the Al substrate. For this reason, the chemisorbed HAT-CN can thus be regarded as an integral part of the anode, rather than an independent organic functional layer. Thereby, we refer to this anode, with new surface quantum states formed by the chemisorption between Al and HAT-CN, as a composite anode.

Having established that the quantum states are induced by charge transfer from Al to HAT-CN, we now examine their influence on hole injection. The UPS spectrum of 2 nm HAT-CN/Al, along with subsequent depositions of 2 and 5 nm CBP, is shown in Fig. 3d. The sample work function decreases from 5.7 eV to 5.1 eV (Fig. 3d, left). Notably, there is a broad peak of quantum states at around 1.9 eV below the E_F(Fig. 3d, right). This UPS data is summarized in a schematic illustration of the electrode surface quantum states and the CBP HOMO (Fig. 3e). The energy level distribution of quantum states completely overlaps with the HOMO energy levels of CBP. Consequently, these quantum states participate in the charge injection process, allowing hole injection to proceed via these states into the HOMO level of CBP. We named the composite electrode that utilizes quantum states to provide a novel hole injection pathway as a “quantized electrode”.

Interfacial thermal stability

To gain insight into the high operational stability, we investigate the thermal stability of the Al/HAT-CN interface. The surface properties of the inorganic substrate, which serves as the electrode, are crucial for device performance, as the organic thin film directly contacts the inorganic electrode surface. Unfortunately, the inorganic/organic interface is thermodynamically unstable. That is because inorganic substrate has a high surface energy and its contact with low energy organic layer suffers from a large surface energy mismatch. Elevated temperatures caused by Joule heating accelerate inorganic/organic interfacial degradation during device operation, thereby deteriorating hole injection and device lifetime^1,39-41. To test the interfacial thermal stability of Al/HAT-CN, thermal annealing was performed on simple devices composed of Si/Al (100 nm)/HAT-CN (2 nm)/CBP (10 nm) and Si/HAT-CN (2 nm)/CBP (10 nm) in a vacuum, where CBP is employed to simulate the real device. The obviously poor interfacial stability of Si/HAT-CN is evident from the changes in the XPS spectrum, with weak peaks from N 1s and C 1s assumed to be residual amounts of HAT-CN (Fig. 4a,c,e). On the contrary, no noticeable changes in the features originating from the HAT-CN/Al were observed for the Si/Al/HAT-CN/CBP sample, even after annealing at 250 °C (Fig. 4b,d,f), which is higher than the sublimation temperature of HAT-CN.

In conclusion, we successfully established a quantized electrode strategy for achieving universal ohmic hole injection into organic semiconductors with HOMO levels up to 6.1 eV. This ohmic hole injection is attributed to the quantum states that provide more favorable energy levels for hole injection. Moreover, the quantized electrode possesses a thermally robust topological link between the inorganic Al and the organic overlayer. Such an electrode with surface quantum states presents numerous advantages, including independence from the HTL HOMO level to achieve ohmic hole contact, ultrahigh luminance at limited voltage, high efficiency, extremely low roll-off, excellent device stability, and simplified manufacturing process. We believe that the quantized electrode has potential applications in a wide range of ﬁelds, such as organic semiconductor laser diodes, organic solar cells, organic ﬁeld-effect transistors, perovskite LEDs, quantum dot LEDs, etc.

Data availability

The data that support the findings of this study are available from the corresponding authors on request.

Acknowledgements:

This research was supported by Basic Research Project of Yunnan Province of China (no. 202301AT070221), Scientific Research Fund of Education Department of Yunnan Province (no. 2024Y038), National Natural Science Foundation of China (nos. 61805211, 12064048, and 61905206).

Author contributions:

C. S. conceived the original idea. C. S. fabricated the OLEDs. N. C. conducted UPS and XPS measurements. C. Z. performed DFT calculations. C. S., L. T., D. Z., J. M., D. W., H. D., Y. Z. measured device performances. C. S., N. C., N. S., Z-H. L. contributed to data analysis. N. S. and Z.-H. L. supervised the project. N. S. wrote the first draft of the paper. C. S., N. S. and Z.-H. L. revised paper.

Competing interests:

The authors declare no competing interests.

Additional information:

Correspondence and requests for materials should be addressed to Changsheng Shi, Ning Sun, Zheng-Hong Lu.

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Device fabrication

Glass, ITO-coated glass and Si substrates were pre-cleaned successively via ultrasonication by turns in deionized water, acetone and ethanol for 6 min each. After blowing dry by nitrogen gas, the substrates were UV-ozone treated for 15 min followed by electrode and organic layers deposition. OLED devices for measurements were fabricated by using vacuum deposition (~ 2×10^-5 Pa).

Green TEOLED configuration (correspond to the red line in Fig. 1a-d and Extended Data Fig. 2 and 3): Al (100 nm)/HAT-CN (2 nm)/CBP (40 nm)/CBP: 8 wt% Ir(ppy)₂(acac) (15 nm)/TPBi (45 nm)/Liq (1 nm)/Mg:Ag (weight ratio = 7:3, 3 nm)/Ag (20 nm)/NPB (75 nm)

Green BEOLED configuration (correspond to the blue line in Fig. 1a-c): ITO (120 nm)/HAT-CN (2 nm)/CBP (40 nm)/CBP: 8 wt% Ir(ppy)₂(acac) (15 nm)/TPBi (45 nm)/Liq (1 nm)/Al (100 nm)

Green BEOLED configuration (correspond to the blue line in the operational stability measurements shown in Fig. 1d and Extended Data Fig. 2): ITO (120 nm)/HAT-CN (2 nm)/TAPC (35 nm)/TCTA (5 nm)/CBP: 8 wt% Ir(ppy)₂(acac) (15 nm)/TPBi (45 nm)/Liq (1 nm)/Al (100 nm)

Blue TEOLED configuration (correspond to the blue TEOLED in Extended Data Fig. 4): Al (100 nm)/HAT-CN (2 nm)/TCTA (55 nm)/26DCzPPy: 8 wt% FIrpic (10 nm)/TmPyPB (50 nm)/Liq (1 nm)/Mg:Ag (weight ratio = 7:3, 3 nm)/Ag (20 nm)/NPB (75 nm)

Red TEOLED configuration (correspond to the red TEOLED in Extended Data Fig. 4): Al (100 nm)/HAT-CN (2 nm)/TAPC (55 nm)/46 wt% NPB: 46 wt% Bphen: 8 wt% Ir(MDQ)₂(acac) (15 nm)/Bphen (55 nm)/Liq (1 nm)/Mg:Ag (weight ratio = 7:3, 3 nm)/Ag (20 nm)/NPB (75 nm)

Here, Liq, Mg:Ag, NPB, TAPC, TCTA, 26DCzPPy, FIrpic, TmPyPB, BPhen, and Ir(MDQ)₂(acac) stands for 8-hydroxy-quinolinolato-lithium, magnesium and silver alloy, N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine, 4,4′-cyclohexylidenebis [N,N-bis(4-methylphenyl)benzenamine], 4,4′,4″-Tris(carbazol-9-yl) triphenylamine, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine, bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium(III), 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene, 4,7-diphenyl-1,10-phenanthroline, and bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III), respectively.

Measurement of EL characteristics

All OLEDs and hole-only devices were measured under ambient conditions without encapsulation.

J-V-L and EL spectrum. The current density-voltage characteristics were performed using an HP4140B picoammeter. And the luminance and EL spectrum were recorded by Minolta LS-110 Luminance meter and Ocean Optics USB-4000 spectrometer, respectively.

Angular-dependent. Angular-dependent EL spectra and intensity were measured by a light distribution measurement system (Hamamatsu, C9920-11).

Device thermal stability evaluation

The thermal stability of Al/HAT-CN/CBP interfaces were investigated with the structures of Si/Al (100 nm)/HAT-CN (2 nm)/CBP (10 nm) and Si/HAT-CN (2 nm)/CBP (10 nm). Both devices were subjected to heating treatment under vacuum at 150, 200, and 250 °C for 10 min. Subsequently, they were transferred to the characterization chamber of the photoemission spectroscopy system without exposure to air.

In-situ photoemission spectroscopy measurement

Both the preparation and measurement of the organic films were completed in a vacuum chamber. The sample was transferred in situ from the deposition chamber (base pressure of ~10⁻⁶ Pa) to the characterization chamber (base pressure of ~10⁻⁸ Pa) through a high-vacuum radical distribution chamber (RDC) (base pressure of ~10⁻⁷ Pa) to avoid air exposure. All organic films were prepared in the deposition chamber, with the deposition rate monitored using a quartz crystal microbalance. Photoemission spectroscopy, including XPS and UPS, was carried out using the Thermofisher ESCALAB Xi+ system. The chemical composition and reactions were analyzed using XPS, with data collected using a monochromated Al Kα source (hν = 1486.6 eV) at a pass energy of 20 eV for high resolution spectra. The work function and valence band were measured using UPS. The UV photon source is a non-monochromatic He Iα (hν = 21.22 eV) radiation with a pass energy of 1 eV. The sample was biased under -10 V during UPS measurements.

DFT calculation

The HAT-CN/Al interactions were calculated by DFT as implemented in the Vienna Ab Initio Simulation Package (VASP)⁴². The exchange-correlation potentials were treated with the generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerholf (PBE). The interaction between valence electrons and ion cores was described by the projected augmented wave (PAW) method⁴³. The wave functions were expanded in the plane wave basis up to a kinetic energy of 400 eV. The (111) surface of Alwere chosen to build the base, respectively. A vacuum thickness of 20 Å was adopted to eliminate interactions between the slab layers. Then the supercell was constructed based on an Al base. The final lattice parameters of the supercells are: a=22.9 Å, b=19.8 Å, c=30.0 Å, with α=β=γ=90°. The k-mesh was set to 1×1×1 for all calculations. Each supercell contained 192 Al atoms arranged in a three-layer base. A HAT-CN molecule was put on top of the base and allowed to be relaxed with the top two layers of the base until the total energy difference was no more than 0.0001 eV.

There is NO Competing Interest.

ExtendedDataNature.docx
Extended Data Fig.1,2,3,4,5,6,7 and Extended Data Table 1,2,3

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Quantized electrode for robust organic devices

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