The Role of the Bottom Oxide Layer in Oxide‐Metal‐Oxide (OMO) Electrode for Stretchable Organic Light‐Emitting Diodes

The challenges for stretchable organic light‐emitting diodes (SOLEDs) have led research into advanced manufacturing processes. Several electrodes have been researched to replace conventional indium tin oxide in SOLEDs due to its brittleness, indium scarcity in earth, and poor deformation capabilities. Oxide–metal–oxide (OMO) electrodes are promising alternatives for flexible/stretchable electronics owing their excellent charge injection and optical transparencies, including mechanical compliance. In this study, two oxides (i.e., MoO3 and V2O5) with different surface energies in an OMO structure to effectively inhibit the island growth of the ultra‐thin Au (5 nm) metal is incorporated. The morphology and interfacial coordinate covalent bonds between the seed layer and ultra‐thin Au film are extensively studied. The improved ultra‐thin Au growth in OMO structure together with figure‐of‐merit have been employed as the anode for a phosphorescent SOLED structure. The SOLEDs with OMO electrode under V2O5 as bottom oxide remain stable after peeling‐off and sustain a >50% uniaxial strain with a negligible reduction in luminance and current efficiencies. The surface energy and interface of the bottom oxide in the OMO structure are crucial for thin metals to attain superior optical, structural, electronic, and mechanical stability in SOLEDs.


Introduction
Organic light-emitting diodes (OLEDs) under mechanical strain are essential for extended technologies, like stretchable display and wearable electronics.Since the demonstration of single crystal silicon components on a pre-strained polydimethylsiloxane (PDMS) substrate, [1] several endeavors have been reported to realize efficient materials and devices for stretchable applications.Significantly, the fabrication of active-matrix organic light-emitting devices (AMOLEDs) on single-wall carbon nanotube (SWNT)incorporated fluorinated rubbers [2] was demonstrated in 2009.The elastomer filled with SWNT printed on a PDMS substrate was acted as an electrode and provide intrinsic stretchability to the AMOLEDs.The OLED fabricated on this elastomer with PDMS substrate exhibited almost constant brightness under the strain of 20-50%.However, materials employed for intrinsic stretchable electrode and substrate are limited [3 , 4,5,6] and is difficult to implement for large area display or lighting.
On the contrary, geometrical engineering-based LEDs can be applicable in large-area and wearable display applications. [7]owever, effectively engineering wrinkles or buckles to withstand the applied strain on the LED/OLED architecture is a considerable challenge. [8]7a,b,9] In our earlier study, [7c] we pre-stretched a Norland optical adhesive (NOA 63) as a substrate to create wrinkles to demonstrate geometrically stretchable OLEDs.7c,10] Moreover, the utilization of conventional electrodes like indium tin oxide (ITO) are limited in stretchable devices owing to chemical instability, [11] indium scarcity, [12] high temperature processing, and low mechanical compliance. [13]Therefore, various alternatives such as AgNW, [14] metal composites, [15] PEDOT:PSS, [16] ZnO, [17] graphene, [18] carbon nanotubes (CNTs), [19] and oxide/metal/oxide (OMO) structures have tested as an anode in OLEDs.AgNW, metal composites or other carbon nanostructures can be employed as an alternative to ITO, however scaling up the devices in large area will become serious issue due to the deposition processes.On the other hand, the conductivity and work function limitations play a decisive role in ZnO and other wide band gap semiconductor electrodes.Whereas, OMO electrode usually be fabricated via physical evaporation under room temperature conditions.As compared to AgNW, CNT or other solution processed electrodes, OMO can be effectively applied as electrode in large area deposition as well as for mass production of optoelectronic devices.In this scenario, the room temperature processed OMO structure [20] have been proposed in literature as one of a popular replacement to ITO due to its high transparency, low sheet resistance and suitable work function parameters.Here, a thin metal film (< 7 nm) is sandwiched between two anti-reflective dielectric surfaces that helps to attain high transparency.However, the conductivity of the OMO electrode is proportional to the thickness of the middle metal layer thickness.Hence, a tradeoff between conductivity and transparency in the OMO electrode could be addressed to attain a good anode.Several reports on OMO electrode employs similar oxide as both top and bottom layers. [21]20a,22] Therefore, a choosing a suitable bottom and top dielectric oxides are crucial for superior OMO electrodes.Moreover, certain metal oxides (e.g., tungsten oxide (WO 3 )) exhibits different wettability owing to different phase formation which might detrimental to metal film formation. [23]The metal electrode employed in OMO structure is usually very thin < 10 nm.20b,24] Significantly, OMO electrode with different metal layer thickness have been tried with MoO 3 as bottom and top oxide layers in stretchable transparent OLEDs. [25]However, the transparency of the devices was lower even with the metal thickness of 7 nm.To overcome the trade-off between transparency and conductivity of the OMO electrode, [20a] and to avoid island formation, an ultrathin Au film (<7 nm) is preferred.For better current injection and improved transmittance, it is important to consider the surface energy to enhance adhesion between metal and bottom oxide layer.This approach could prevent the inclination of metal to agglomerate together.Therefore, understanding the surface energy distribution of bottom oxide in OMO structure is crucial to attain ultra-smooth metal electrode.
In this scenario, we screened the two oxides (i.e., vanadium oxide (V 2 O 5 ) and molybdenum oxide (MoO 3 )) as bottom layer in OMO structure, and optimized the growth of ultra-smooth Au thin film.The transparency, root mean square value and sheet resistance of OMO electrode is optimized and a phosphorescent OLED architecture was built on it as stretchable device.Meanwhile, the surface energy variation of two oxides with respect to Au film was evaluated using X-ray photoelectron spectroscopy (XPS) and morphological studies.The performance of stretchable phosphorescent OLED (SPhOLED) under different OMO electrode configurations have been tested with different strain ratios.

Experimental Section
Figure 1a represented the schematic illustration of the SPhOLED device fabricated on the OMO structure.The OMO structure consists of bottom oxide (15 nm)/Au (3, 5, and 7 nm)/MoO 3 (5 nm), where the bottom oxide will be either MoO 3 or V 2 O 5 .

Results & Discussion
Before incorporated as anode in the SPhOLED architecture, the optical and electrical properties of the OMO electrodes were examined as shown in Figure 1c-e.Figure 1c indicates the transmittance of the OMO electrode deposited on glass substrate.A 5 nm of thin Au film was deposited either directly on glass or intercalated between two oxides (i.e., M5M -MoO 3 /Au 5 nm/MoO 3 and V5M -V 2 O 5 /Au 5 nm/MoO 3 ).The OMO electrode with different bottom oxides shown similar transmittance values, however bare Au deposited glass exhibited relatively lower transmission.The inset in Figure 1c is the camera image of the V5M electrode, which shows <80% transmittance for a green plant.Further, the sheet resistance of the OMO electrodes with different Au thickness are displayed in Figure 1d.Here, glass/Au film/MoO 3 (AM) samples represents the growth Au film without bottom oxide or seed layer, so it clearly suffered with higher sheet resistance and it attributed the island formation of the Au. [28]While employing a bottom oxide layer, the sheet resistance gradually reduced even for a 3-nm-thick Au film.Interestingly, the lowest sheet resistances were obtained when V 2 O 5 was used as the bottom oxide, instead of MoO 3 .Thus, the V 2 O 5 bottom oxide layer acts as a better seed layer for the growth of uniform thin Au films, and accordingly, very low sheet resistances can be achieved.Since the 3-nm-thick Au film showed a higher sheet resistance with or without the seed layers, 5-nm-thick Au films were used to fabricate the devices.Therefore, the figure-of-merit (FoM, T 10 /sheet resistance) [29] of the electrodes with 5-nm-thick Au films with or without the bottom seed layers are compared in Figure 1e.Here, the highest FoM was achieved for the electrode prepared with the V 2 O 5 bottom oxide seed layers.The complete transmittance and sheet resistance analysis of OMO electrodes with different Au thickness (i.e., 3, 5, and 7 nm) and different seed layers are represented in Figure S1 (Supporting Information).The V 2 O 5 bottom seed layer is believed to promote the growth of uniform ultra-thin Au film by inhibiting the Volmer-Webber mechanism. [28]Hence, the surface energy of the V 2 O 5 films might favorable than MoO 3 to attain uniform thin Au films.To clarify the role of surface energy of bottom seed layers on the growth of Au film, the wetting ability of ultra-thin Au films (3-7 nm) on MoO 3 and V 2 O 5 seed layers were studied using a contact angle measurement as depicted in Figure S2 (Supporting Information).The variation of contact angles with respect to different seed layer under 3, 5, and 7 nm ultra-thin Au films were evaluated.The measured contact angles irrespective of the thickness are lower with V 2 O 5 seed layer, whereas the contact angle measured with MoO 3 seed layer showed a relatively increased contact angle.Thus, the surface energy of V 2 O 5 is confirmed to be favorable to the formation of ultra-thin Au films.
To cross examine the morphology of above proposed electrodes, the Atomic Force Microscope (AFM) images were taken for thin Au films deposited without or different oxide seed layers.Figure 2a compares the surface features of the thin Au film through Field emission scanning electron (FE-SEM) images deposited on Si substrate with or without seed layers and the corresponding surface coverage/roughness are depicted in Figure 2b,c.The Au film deposited directly on the Si (i.e., without seed layer) exhibited the lowest surface coverage and highest surface roughness values.To examine the growth kinetics on seed layers, Au films with three different thickness (i.e., 3, 5, and 7 nm) are deposited on MoO 3 as well as V 2 O 5 .The highest surface coverage and lowest roughness were attained with the V 2 O 5 seed layer even with a 5-nm-thick Au film.Whereas, MoO 3 seed layer attained this high surface coverage only at 7 nm of Au.A highperformance OMO structure should have a very thin highly uniform metal film, and the V 2 O 5 seed-layer-based OMO facilitated a uniform coverage even with a 5-nm-thick Au film.Thus, the surface energy of V 2 O 5 in the OMO structure is favorable to achieve thin uniform Au films with a thickness of 5 nm.The transmittance (@ 520 and 550 nm), sheet resistance, surface coverage, and surface roughness of the OMO electrode with and without a seed layer are compared in Table 1.
Further, Figure 3a-c replicate the role of surface energy of the seed layer on growing thin metal films.The thin Au film deposited on NOA63, with or without seed layer was significantly affected in terms of optical, structural, and electronic properties.Hence, a detailed analysis is required to understand the surface energy contribution of seed layer on Au film growth.In this respect, we studied the high resolution XPS of thin Au film on Si substrate with and without seed layers (i.e., MoO 3 and V 2 O 5 ) as represented in Figure 3d.The survey spectrum of Figure 3d indicated the elements presented in both seed layers and Au.Interestingly, the Au 4f intensity is compared in Figure 3e confirming the highest counts observed with Au films deposited on V 2 O 5 seed layer.Therefore, we confirmed that the coverage of Au film on V 2 O 5 is better than the one on MoO 3 which is coincide with the surface coverage estimations in Figures 2 and 3a-c.As derived from the contact angle measurements, we believed that the surface energy of V 2 O 5 is more favorable for uniform Au film growth than of MoO 3 .Meanwhile, the formation of ultra-thin 2D continuous metal films can be studied by the energy difference (Δ) [24] between substrate and metal (here, Au film).However, Δ is influenced by three factors [30] namely, surface free energy  of the substrate ( s ), which already been addressed by contact angle measurements, free energy of the film ( f ) (it is negligible for ultra-thin films) and free energy of the interface ( i ) between substrate and film (MoO 3 or V 2 O 5 /Au).In this perspective, the role of  i can be studied with the photoelectronic spectrum of the bottom seed layer and the metal films.The core level 4f spectra of thin Au films deposited on Si with and without seed layers have been compared in Figure 3f.The Au 4f spectra were divided in to two spin-orbit components [31] namely Au 4f 5/2 and Au 4f 7/2 at 87.67 and 84 eV respectively.Moreover, if seed layers such as MoO 3 or V 2 O 5 have been employed, the corresponding Au 4f spectrum exhibiting a positive chemical shift toward higher binding energies (i.e., both spinorbit components of Au 4f shifted 224 and 185 meV pertaining to V 2 O 5 and MoO 3 seed layers respectively).Meanwhile, the core electronic spectrum pertaining to seed layers such as V 2p and Mo 3d are displayed in Figure 3g,h, and they exhibited a negative chemical shift attributed to the electron donation from Au via coordinate covalent bond.As a result of this bonding between seed layer and Au, the photoelectronic spectrum corresponding to V 2p and Mo 3d encountered a positive chemical shift of 600 and 100 meV respectively as depicted in Figure 3f-h and Figure S3 (Supporting Information).This confirms the fact that the electron transfer occurred from Au to the seed layer helps to attain a coordinate covalent bond [20b] that reduces the free energy in the interface ( i ).
This reduction of free energy of the interface is beneficial to attain uniform ultra-thin Au films.In quantitatively, among the Au (5 nm)/V 2 O 5 and Au (5 nm)/MoO 3 samples, the chemical shift experience in V 2p spectrum is higher than that in Mo 3d, hence it is confirmed that the electron transfer from V 2 O 5 is stronger and it helps to achieve lower  i .The schematic diagram illustrating the coordinate covalent bonding mechanism between the seed layer and the ultra-thin Au film growth is mentioned in Figure 3i.The number of electrons involved in the bonding is more in V 2 O 5 as compared to MoO 3 seed layers, hence the bonding between Au film and V 2 O 5 is stronger owing to reduced interface surface energy as illustrated in Figure S4 (Supporting Information).In other words, this implies that the surface energy of V 2 O 5 surpasses that of MoO 3 . [32]This phenomenon is beneficial to attain the highest surface coverage and an improved FoM in the V 2 O 5 -based OMO electrode as shown in Figure 2. Furthermore, the above deduced phenomenon coincided with the findings extracted from the contact angle measurements.
Before applying the OMO electrodes, we tested the mechanical compliance of the electrode, and the corresponding results are shown in Figure 4a,b.Based on the morphological images displayed in Figure 2, we tested the electrical properties of the OMO electrode with MoO 3 (M5M) and V 2 O 5 (V5M) seed layers under mechanical deformation.Initially, the sheet resistance of the OMO electrodes with different bending radii were estimated as in Figure 4a.The sheet resistance becomes almost stable until the bending radius increases up to 1 mm.When the bending radium become lower than 1 mm, M5M electrode normalized resistance was increases significantly compared to V5M electrode.The bending nature and schematic of inner bending is illustrated in the inset of Figure 4a.Furthermore, we analyzed the normalized sheet resistance variation of the OMO electrodes under various uniaxial stresses as shown in Figure 4b.When the uniaxial stress is >40%, the normalized sheet resistance of the M5M electrodes increases, whereas the V5M electrodes exhibit a stable performance.The stable performances and the high mechanical compliance of V 2 O 5 based OMO is attributed to the improved surface energy of V 2 O 5 and the stronger electron transfer/coordinate covalent bond between the V 2 O 5 /ultra-thin Au film.
To validate the characteristics of OMO electrode as anode, a hole-only device (HOD) with an OMO/TAPC (100 nm)/Au structure was fabricated.For comparison, HODs were fabricated on conventional ITO electrodes as well.The hole injection barrier available between the work function of the anode with the highest occupied molecular orbital (HOMO) of the HTL materials plays a crucial role in charge balance in OLEDs.Hence, we calculate the injection barrier using the Richardson-Schottky model and the corresponding I-V characteristics are displayed in Figure 4c,d.Interestingly, V5M electrode displayed an improved hole transport than M5M and conventional ITO.The calculated zero field hole injection barrier is also lower (≈0.82eV) for V5M compared to other anodes.The stronger electron transfer from ultra-thin Au film to V 2 O 5 is expected to enable a higher work function for V5M which revealed through the reduced hole injection barrier.Thus, indicating the superior energy level alignment between V5M electrode with HTL TAPC for improved hole injection into PhOLED architecture.
To explore the performances of OMO electrodes with and without seed layers, typical phosphorescent green OLEDs have been fabricated as shown in Figure 1a, and the corresponding current density-voltage-luminescence (J-V-L) plot displayed in Figure 5a-d.All the PhOLEDs showed a turn-on voltage of 3 V, however the maximum luminescence of the electrode without seed layer (i.e., Au on NOA 63, 5 M) is significantly reduced.It is attributed to the poor coverage of Au film without seed layer causing leakage current and drastically limits the brightness of the device.Interestingly, the current injection is superior in V5M than M5M, and is attributed to the high surface coverage and the lowest hole injection barrier estimated in Figure 4c.The corresponding energy level alignment in OMO and current inject mechanism at OMO/HTL interface is described clearly in Figure 5b.The interface of seed layer (either MoO 3 or V 2 O 5 ) and ultra-thin Au film is dictated by a coordinate bond, [20b] in which the higher electron transfers from V 2 O 5 to Au film rather than MoO 3 case.Since, the top layer of the OMO is similar in both cases, an enhanced work function is expected from a V5M electrode than M5M.Hence, a reduced hole injection barrier between V5M/HTL interface is expected and is corroborated by the HOD analysis in Figure 4c,d.As a result of reduced hole injection barrier at V5M/HTL interface, a relatively higher current density is observed in SPhOLED devices as illustrated in Figure 5a.
Despite this scenario, both OMO electrodes with either MoO 3 or V 2 O 5 seed layers demonstrated the stable operation in PhOLED, compared to device without bottom seed layer.The current and quantum efficiencies of the V5M electrode-based device were superior to those of the M5M-based device.This result can be attributed to the improved coverage, low sheet resistance, and low hole injection barrier of the V5M electrodes.We speculated that the efficiency enhancement in V5M electrode could be influenced by the charge balance of this device architecture.Since the hole injection barrier in the case of V 2 O 5 is smaller than that of M5M, the hole injection rate in V5M is slightly higher than that of M5M anodes.Hence, V5M with higher work function expected to inject more holes, which speculated to contribute to better charge balance, whereas M5M is hard to attain charge balance in EML due to insufficient carrier injection.Hence, the V5M anode exhibited enhanced efficiency in SPhOLED architecture.
The OLED parameters such as turn on voltage, current efficiency, power efficiency, quantum efficiency and peak emission wavelength with respect to different seed layers are listed in Table 2.Moreover, the thickness of the Au film was further increased to 7 nm, and its performances in the V 2 O 5 -based PhOLED was extensively evaluated; the corresponding results are depicted in Figure S5 (Supporting Information).Furthermore, the V 2 O 5 seed-layer-based OMO electrode was employed to fabricate large-area OLEDs, which uniformly emitted light in a 13 × 13 mm 2 area (Figure S6, Supporting Information).Figure 5d shows the electroluminescence plot of the SPhOLED, which resembles the conventional green phosphorescent emission.Further, the angular distribution of photons mentioned in the inset of Figure 5d confirms an almost Lambertian-type emission, which further supports the OMO electrode uniformity.
Figure 6 displays the performances of the peeled-off SPhO-LED with the V5M electrode under different uniaxial strains.The V5M-based SPhOLED demonstrated a stable current and quantum efficiencies under different uniaxial strains with negligible fluctuations as shown in Figure 6a.The electroluminescence spectrum of the SPhOLED, shown in Figure 6b, remain identical under different strains, indicating a stable emission.Figure 6c reflects the change in luminance in percentage (%) with respect to the applied uniaxial strain; evidently, the SPhOLED retain its original luminance value until 50% strain.The observed variation in the luminance after 50% strain is also negligible.The SPhOLED fabricated with the V5M electrode exhibits a variation of 0.08% and 5.9% in its luminance and current efficiency, respectively, under a 50% uniaxial strain.The V5M electrode-based SPhOLED exhibits a stable performance under different uniaxial  strains owing to the improved surface coverage and enhanced coordinate covalent bond between V 2 O 5 and the ultra-thin Au film in the anode.Conversely, the performance of the SPhOLED with M5M have suffered with severe damages and poor performances.During the peel-off process, the ultra-thin Au film in the M5M might develops voids or islands, which may cause device failure.Further, the large-area M5M-based SPhOLED (13 × 13 mm 2 area) underwent similar peel-off-induced damages and exhibited a poor performance (Figure S6, Supporting Information), which can be attributed to the variation in the resistance of the M5M electrode as depicted in Figure 2b.Thus, the resistance of the M5M electrode significantly decreases even after a 20% strain is applied.The ultra-thin 5 nm Au film expected to attach with MoO 3 poorly which incurred severe damages during the peel-off.The pixel image of SPhOLED under various uniaxial strains is displayed in Figure 6d.Therefore, the V5M electrode provides a decent efficiency as well as a high mechanical stability to the SPhOLED.The V5M electrode and its OLED performances have been compared with recent reports in literature as depicted in Table S1 (Supporting Information).

Conclusion
In summary, OMO structures with thin Au metal layers were applied as anodes in SPhOLEDs.To understand the growth kinetics of the thin Au layer, V 2 O 5 and MoO 3 were employed as bottom seed layers in the OMO architecture.FE-SEM, AFM, and XPS analyses confirmed that both the seed layers inhibited Volmer-Webber growth, and the V 2 O 5 seed layer provided an optimum surface energy to realize uniform ultra-thin Au films.Moreover, XPS and contact angle measurements corroborated the improved surface coverage of Au (5 nm) on the V 2 O 5 seed layer; this result can be attributed to the reduced interface free energy between the seed and metal films.The HOD fabricated using the OMO electrode with a V 2 O 5 seed layer exhibited the highest current density and lowest injection barrier for holes.However, the SPhOLED fabricated on the OMO electrode with the V 2 O 5 seed layer demonstrated a superior current efficiency of 132 cd A −1 , which was higher than that of MoO 3 (100 cd A −1 ).The improved charge balance and appropriate hole injection barrier in this architecture paves the way for higher efficiencies of V5M-based OLEDs.Furthermore, the mechanical robustness of the V5Mbased SOLED was better than that of the M5M-based device.The V5M-based SOLED sustained a >50% uniaxial strain with a negligible variation in its luminance and current efficiency values.
The improved surface energy of the bottom layer in OMO electrode plays a crucial role in dictating the mechanical stretchability of the SOLEDs.

Figure 1 .
Figure 1.Schematic and energy diagram of the SPhOLED and OMO electrode performances.a) Schematic view of the SPhOLED architecture on OMO electrodes under different bottom seed layers, b) energy level design of the SPhOLED, c) Transmittance, d) sheet resistance, and e) figure-of-merit of the OMO electrode with and without bottom seed layers.(i.e., 5 M -5 nm Au on MoO 3 , M5M -MoO 3 /5 nm Au/MoO 3, and V5M -V 2 O 5 / 5 nm/MoO 3 ).

Figure 2 .
Figure 2. Morphological study of the OMO electrode with different seed layers.a) FE-SEM and AFM images of Au film coated on Si substrate with (MoO 3 or V 2 O 5 ) or without seed layers.The root mean square values are mentioned on the inset of AFM images.b) Surface coverage percentages of Au film with different thickness estimated using FE-SEM images and c) surface roughness of Au film under different thickness with and without seed layers.

Figure 3 .
Figure 3. Understanding the interface physics of the seed layer and ultra-thin Au film.a-c) Schematic view of the 5 nm Au film growth on NOA 63 without and with MoO 3 and V 2 O 5 seed layers.d) XPS survey spectrum of Au (5 nm), MoO 3 /Au (5 nm), and V 2 O 5 /Au (5 nm) deposited on Si, e) intensity counts of Au 4f peaks, f) Au 4f spectra of Au (5 nm), MoO 3 /Au (5 nm), and V 2 O 5 /Au (5 nm).g,f) V 2p and Mo 3d core electronic spectrum respectively.i) dSchematic view of electron transfer and bonding mechanism of seed layers.

Figure 4 .
Figure 4. Evaluation of mechanical compliance and hole injection capabilities of the OMO electrode with MoO 3 and V 2 O 5 seed layers.a,b) Variations in the normalized sheet resistances of the M5M and V5M electrodes under different bending radii and uniaxial strains, respectively.c) Hole injection barrier estimation of ITO, M5M, and V5M electrodes.d) Current density-voltage graph of the HODs with different anodes.

Figure 5 .
Figure 5. Device performance of the SPhOLED.a) J-V-L performances of SPhOLEDs fabricated on NOA 63, current density-luminescence. b) Graphical view of Au growth mechanism and electron transfer within OMO and current injection at OMO/HTL interface, c) current and quantum efficiency, and d) electroluminescence intensity and Lambertian distribution of the SPhOLED fabricated with the M5M and V5M anodes.

Figure 6 .
Figure 6.Stretchable performances of the SPhOLEDs with OMO electrodes.a) Comparison of the current and quantum efficiencies and b) electroluminescence of the SPhOLED with the V5M electrode under different uniaxial strains.c) Luminance variation of the V5M-based SPhOLED with various uniaxial strains (inset pixel image comparison of SPhOLED with V5M and M5M electrodes).d) Physical images of the SPhOLED under different uniaxial strains.

Table 1 .
Sheet resistance, transmittance, Au surface coverage, and root mean square values (RMS) of the electrodes employed in this study.(*Au thickness is 5 nm).

Table 2 .
Performances of the OLEDs with different electrodes.(*Turn-on is 1 cd m −2 ).