# Deep Blue Fluorescent OLEDs Based on a Solution and Solid-State Emitter (SSSE): Investigations with 2-Phenyl-Naphthoxazole

Houssein El Housseiny, Suzanne Fery-forgues, Marc Ternisien, David Buso, Georges Zissis, Cédric Renaud

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Houssein El Housseiny, Suzanne Fery-forgues, Marc Ternisien, David Buso, Georges Zissis, et al. Deep Blue Fluorescent OLEDs Based on a Solution and Solid-State Emitter (SSSE): Investigations with 2-Phenyl-Naphthoxazole. Advanced Electronic Materials, 2025, 10.1002/aelm.202500151. hal-05100085

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# Deep Blue Fluorescent OLEDs Based on a Solution and Solid-State Emitter (SSSE): Investigations with 2-Phenyl-Naphthoxazole

Houssein El Housseiny,* Suzanne Fery-Forgues, Marc Ternisien, David Buso, Georges Zissis, and Cédric Renaud*

Blue fluorescent materials play a crucial role in the advancement of blue organic light-emitting diodes (OLEDs) with high stability and high color purity. Fluorescent molecules known as Solution and Solid-State Emitters (SSSEs) garner significant interest due to their unique optical properties. Most fluorophores exhibit strong emission in solution and typically lose their emissive properties in the solid state due to the emergence of intermolecular interactions in the aggregated phase. In contrast, SSSEs retain their fluorescence both in solution and in the solid state, making them highly valuable for various applications. This study focuses on 2-phenyl-naphth[2,3-d]oxazole (Nzx) that exhibits SSSE-like properties. This fluorescent molecule is characterized by deep blue emission near the UV range (300–450 nm). It is investigated with the goal of developing stable and high-quality blue fluorescent OLEDs. To achieve this, Nzx is integrated as both an emissive layer and a dopant in OLED devices. The stability of the OLEDs under electrical stress is analyzed, and the device structure along with the doping concentration is investigated to optimize OLED performance. As a result, near deep blue-emitting devices with an emission wavelength of 438 nm and an external quantum efficiency (EQE) of 1% are successfully achieved.

## 1. Introduction

Due to their low energy consumption, flexibility, light weight, and exceptional image quality,¹⁻⁴¹ organic light emitting diodes

(OLED) are used in various fields, including smartphones, televisions, and display panels.⁵⁻⁸¹ Specifically, over the past decades, blue emitting OLEDs (< 460 nm) with high efficiency and stability have emerged as a major research focus due to their potential applications in various fields, such as lighting, displays, and biological sensing. Among these applications, we can mention excitation sources for highly sensitive DNA microarray analysis,⁹¹ tissue sterilization without harming healing cells,¹⁰¹ and their role in the development of display screens and white OLEDs (RGB).¹¹

Extensive research on OLEDs has progressively enhanced device performance through the development of first-, second-, and third-generation emitters based on fluorescence (FL), phosphorescence (Phos), and Thermally Activated Delayed Fluorescence (TADF), respectively.¹²⁻¹⁵¹ Both phosphorescent and TADF emitters have achieved high efficiency by using both singlet and triplet excitons, with an internal quantum efficiency (IQE) of 100%.¹⁶,¹⁷¹ However, blue phosphorescent and TADF OLEDs remain unstable and exhibit limited lifetimes.¹⁸¹ This instability primarily results from the longer lifetime of triplet excitons compared to singlet excitons. Blue luminophores have large energy gaps (≈3 eV). A blue emitting triplet exciton (≈2.8 eV) interacts with a polaron (having an energy of ≈3.3 eV) to form a hot polaron (with the energy of ≈6 eV).¹⁹⁻²¹¹ The high energy of these hot polarons exceeds the bond dissociation energies (BDE) of most organic materials, leading to the break-up of the molecular framework, which leads to material degradation and reduced lifetime of blue PhOLEDs. This issue is less pronounced in red and green PhOLEDs, as their triplet energies are significantly lower. TADF materials also possess high triplet energy levels and exciton lifetimes comparable to those of phosphorescent materials.¹⁹,²²,²³¹ This makes them vulnerable to triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ), which leads to performance instability and reduces the lifetime of TADF devices.²⁴¹ Although blue fluorescent OLEDs exhibit lower efficiencies, they offer several advantages over their phosphorescent and TADF counterparts, such as performance stability and

<footnote>H. El Housseiny, M. Ternisien, D. Buso, G. Zissis, C. Renaud
LAPLACE, CNRS
Université de Toulouse
118 route de Narbonne, Toulouse 31062 Toulouse cedex 9, France
E-mail: housseiny@laplace.univ-tlse.fr; renaud@laplace.univ-tlse.fr</footnote>

<footnote>S. Fery-Forgues
SPCMIB, CNRS UMR 5068
Université de Toulouse
118 route de Narbonne, Toulouse 31062 Toulouse cedex 9, France</footnote>

<footnote>ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aelm.202500151</footnote>

<footnote>© 2025 The Author(s). Advanced Electronic Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.</footnote>

<footnote>DOI: 10.1002/aelm.202500151</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Molecular structure and energy levels (HOMO and LUMO) of Nzx, estimated from DFT calculations in dichloromethane using the B3LYP functional.</figcaption></figure>

color purity. Consequently, they are currently employed in many commercial OLED displays.

Organic luminophores are typically composed of planar aromatic rings and exhibit efficient emission in highly dilute solutions. However, in the aggregated or solid states, their photoluminescence quantum yields can decrease significantly or become completely quenched due to the well-known phenomenon

<table><caption>**Table 1.** Structures of the fabricated OLEDs.</caption><thead><tr><th>Device</th><th>Structures</th></tr></thead><tbody><tr><td>A</td><td>ITO/ NPD (5 nm)/ TCTA (90 nm)/ BCP (35 nm)/ Ca (80 nm)</td></tr><tr><td>B</td><td>ITO/ NPD (5 nm)/ TCTA (90 nm)/ Nzx (10 nm)/ BCP (35 nm)/ Ca (80 nm)</td></tr><tr><td>C</td><td>ITO/ NPD (5 nm)/ TCTA: 10 wt% Nzx (90 nm)/ BCP (35 nm)/ Ca (80 nm)</td></tr><tr><td>D</td><td>ITO/ TCTA (130 nm)/ Ca (80 nm)</td></tr><tr><td>E</td><td>ITO/ TCTA: 10 wt% Nzx (130 nm)/ Ca (80 nm)</td></tr><tr><td>F</td><td>ITO/ NPD (95 nm)/ BCP (35 nm)/ Ca (80 nm)</td></tr><tr><td>G</td><td>ITO/ NPD: 10 wt% Nzx (95 nm)/ BCP (35 nm)/ Ca (80 nm)</td></tr><tr><td>H</td><td>ITO/ NPD (130 nm)/ Ca (80 nm)</td></tr><tr><td>I</td><td>ITO/ NPD: 10 wt% Nzx (130 nm)/ Ca (80 nm)</td></tr><tr><td>J</td><td>ITO/NPD: 27 wt% Nzx (95 nm)/BCP (35 nm)/Ca (80 nm)</td></tr><tr><td>K</td><td>ITO/NPD: 27 wt% Nzx (95 nm)/Bphen (35 nm)/Ca (80 nm)</td></tr><tr><td>L</td><td>ITO/NPD: 27 wt% Nzx (95 nm)/TPBi (35 nm)/Ca (80 nm)</td></tr></tbody></table>

<table><caption>**Table 2.** Maximum absorption wavelength ($\lambda_{abs}$), the maximum emission wavelength ($\lambda_{em}$), fluorescence ($\Phi_F$) and photoluminescence ($\Phi_{PL}$) quantum yields of Nzx in ethanol, solid state, and thin film.</caption><thead><tr><th colspan="3">Solution in Ethanol</th><th colspan="2">Solid state (Powder)</th><th>Thin film</th></tr><tr><th>$\lambda_{abs}$ [nm]</th><th>$\lambda_{em}$ [nm]<sup>a)</sup></th><th>$\Phi_F$ [%]<sup>a)</sup></th><th>$\lambda_{em}$ [nm]<sup>**a)</sup></th><th>$\Phi_{PL}$ [%]<sup>**a)</sup></th><th>$\lambda_{em}$ [nm]<sup>***a)</sup></th></tr></thead><tbody><tr><td>216, 268, 276, 328</td><td>428</td><td>24</td><td>420, 441</td><td>25</td><td>428–455</td></tr></tbody></table>

of aggregation-caused quenching (ACQ).[25,26] In 2001, Tang et al. identified a phenomenon opposite to ACQ, known as aggregation-induced emission (AIE).[27–29] They reported a series of fluorogens with propeller-like conformations that exhibit negligible emission in dilute solutions but become highly luminescent upon aggregation in the solid state. Since then, many AIE materials have been synthesized for a variety of applications.[30–32] In parallel, materials that exhibit strong emissions both in solution and solid states have been highly sought after. This advancement bridges the gap between the contrasting phenomena of ACQ and AIE, leading to the development of a new class of luminescent compounds that show dual-state emission (DSE).[33] However, the term “dual-state emission” can be ambiguous, as it also refers to molecules that can emit from two different excited states.[34] Hence, the term “solution and solid-state emitters” (SSSEs) is preferred to avoid confusion.[35,36]

Commonly used in spectroscopy and for various applications in the biological and biomedical fields,[37,38] SSSEs remain insufficiently investigated for their potential use in OLEDs, where they have nonetheless demonstrated their potential.[39] Moreover, their physicochemical properties provide an advantage for thin film deposition. In addition to their optoelectronic and thermal properties, these molecules are relatively small and generally soluble in organic solvents, making them suitable for deposition via both types of technological approaches (dry and wet processes), which is of particular interest for their integration into OLEDs.

Naphthoxazole derivatives are small heterocyclic compounds structurally related to benzoxazoles. Substitution by an aromatic moiety at the 2-position endows them with outstanding fluorescence properties. They can be synthesized in high yields through one-pot protocols, are thermally stable, and exhibit good stability under light exposition.[40] This family comprises two types of isomers, depending on how the naphthalene moiety is fused with the oxazole ring. In solution, derivatives of the [1,2-d] isomer have been studied as fluorescent cation,[41] DNA,[42] micellar,[40] and cellular[43] probes. They are scintillators for β-ray counting.[44] Recently, one of them has been reported to emit white fluorescence both in solution and solid phases.[45] As for the [2,3-d] isomer, the 2'-hydroxy derivative has been incorporated into heteroleptic Ir(III) complexes, yielding yellow phosphorescent compounds used as an emissive material in OLEDs.[46] For the same application, Kim et al. have used phosphine oxide-naphthoxazole derivatives that emit greenish-blue light with a maximum emission between 498 and 528 nm.[47,48] Ghobane et al.[49] have shown that very simple derivatives of the [2,3-d] isomer are excellent SSSE. They are characterized by their deep blue emission near the UV range (between 300 and 450 nm) and their remarkable efficiency. Among them, 2-phenyl-naphth[2,3-d]oxazole, denoted as Nzx (Figure 1), was selected for the present work because it was synthesized with the best yield and available in large quantities.

The objective of this work is to develop stable and high-quality blue fluorescent OLEDs based on SSSE emitters. We investigated the integration of Nzx as an emissive layer and as a dopant in OLED devices. Two host matrices were tested, namely Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPD). The stability of the OLEDs under electrical stress was studied to identify the most stable matrix. The device structure and the doping concentration were optimized to achieve the highest performance. As

<footnote><sup>a)</sup> $\lambda_{ex}$ = 330 nm; <sup>**a)</sup> $\lambda_{ex}$ = 303 nm; <sup>***a)</sup> $\lambda_{ex}$ = 365 nm</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Normalized emission spectra of Nzx dissolved in ethanol, in the solid state (microcrystalline powder), and in thin film.</figcaption></figure>

a result, near deep blue-emitting devices with a wavelength of 438 nm and an external quantum efficiency (EQE) of 1% were achieved.

## 2. Experimental Section

OLED devices were fabricated on 100 nm of Indium tin oxide (ITO) coated glass substrates from Kintec with a sheet resistivity of 15 Ω/square used as the anode. The ITO glass substrates were cleaned sequentially with deionized water, acetone, ethanol, and isopropanol and treated with UV ozone right before organic deposition. In this study, 2-Phenyl-naphth[2,3-d]oxazole (Nzx) was used as the emitter, synthesized using previously reported methods by condensing 2-aminonaphthol with an acyl chloride in stoichiometric proportions.[49] Nzx was purified by column chromatography on Machery-Nagel silica gel. Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and N,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)benzidine (NPD) were employed as hole transporting layers (HTLs) and host materials. For the electron transport layers (ETLs), 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and 4,7-Diphenyl-1,10-phenanthroline (BPhen) were utilized. The cathode was composed of calcium (Ca, 99.99% from Sigma Aldrich) with a thickness of 80 nm. All organic materials and the cathode were deposited by thermal evaporation in a multiple-source chamber under high vacuum (<10⁻⁵ mbar). The doping of TCTA and NPD was performed by co-evaporation of the matrix and the guest (Nzx). All the thicknesses and doping ratios of the co-doped devices were monitored by an in situ quartz crystal microbalance (QCM). The active area of the device was 18 mm². The structures of the fabricated OLEDs, along with their thicknesses, are presented in Table 1. It should be noted that for each device structure mentioned in Table 1, at least three batches of five OLEDs were fabricated and characterized under similar experimental conditions. Characterizations of the OLEDs were obtained under an inert atmosphere. Electrical characterization (I-V) of devices was performed using a Current-Voltage source (Keithley). The electroluminescence (EL) and the photoluminescence (PL) spectra were measured by using a spectroradiometer (JETI SpecBos 1201). The turn-on voltage, at which the OLED starts emitting, was evaluated for a luminance of 1 cd.cm⁻². The external quantum efficiency (EQE) was measured using a photodetector positioned at an appropriate distance from the device to measure the emitted photons. Measurements were repeated multiple times to verify the stability of the devices. The surface morphology was probed by Atomic Force Microscopy (AFM Bruker Multimode 8) in tapping mode.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Crystal cell of Nzx (from ref. [49]).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Energy level diagram of device B.</figcaption></figure>

## 3. Results and Discussion

### 3.1. Spectroscopic Properties of 2-Phenyl-Naphthoxazole (Nzx) in Solution, Microcrystalline State, and Thin Film

All spectroscopic data are summarized in Table 2.

The optical properties of Nzx in ethanol solution and as microcrystalline powder were studied in a previous work.[49] The absorption spectrum in ethanol solution lies in the UV region, the band at the longest wavelengths has a maximum of 328 nm (see Figure S1, Supporting Information).

The emission spectra are located in the blue spectral region, with a maximum between 420 and 428 nm (Figure 2). The spectrum in ethanol was broader than the solid-state spectrum at short wavelengths. The emission quantum yields of 24% and 25%, respectively, were very close.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** a) Current density (J)-Voltage (V) plot, b) Luminance (L)-Current (I) plot, and c) Normalized electroluminescence (EL) spectra of devices A and B.</figcaption></figure>

No strong spectroscopic effect resulting from intermolecular interactions[50,51] can be distinguished in the microcrystalline state. This may be explained by the fact that, in crystals, the planar molecules of Nzx exhibit a cross-arrangement (Figure 3), which limits intermolecular π-π stacking interactions. This arrangement is very favorable to light emission.

To obtain a thin film of Nzx, this material was deposited by vacuum thermal evaporation onto a glass substrate following the same protocol as that used for OLED fabrication. A thickness of 30 nm was sufficient to measure the PL spectrum, which showed a maximum of 428 nm with a shoulder ≈455 nm (Figure 2).

The spectrum shape was very close to that observed for the microcrystalline state. The wavelength shift may be enhanced by the fact that the spectra have been recorded on different apparatus. Notably, the X-ray diffraction (XRD) analysis of the Nzx powder showed that most of the peaks except three are identical to those of the monocrystalline structure, revealing that in the microcrystalline powder of Nzx, most of the molecules adopt an intermolecular cross orientation. Thin films were analyzed using grazing incidence X-ray diffraction (GIXRD) that showed a very strong texturation (only some crystallographic planes are in diffraction position), as currently observed for this type of material. Only two peaks were clearly indexed by the monocrystalline structure. The formation of nanodomains with a molecular arrangement similar to that identified in the microcrystalline state may occur within the thin films of Nzx, driving photoluminescence emission. It is also possible that a different molecular arrangement, also favorable to light emission, takes place in the thin films.

## 3.2. Integration of Nzx into OLEDs as the Emissive Layer

The reference OLED structure ITO/NPD (5 nm)/TCTA (90 nm)/BCP (35 nm)/Ca is referred to as OLED A (as detailed in Table 1). To study the integration of Nzx as the emissive layer (EML), the following structure was selected: ITO/NPD (5 nm)/TCTA (90 nm)/Nzx (10 nm)/BCP (35 nm)/Ca, referred
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Energy level diagram of device C.</figcaption></figure>

to as OLED B (Figure 4). The thickness of the Nzx layer was set at 10 nm, which corresponds to the exciton diffusion length in amorphous organic layers.[52,53] For this OLED, we selected a thickness of 5 nm for NPD, 90 nm for TCTA, and 35 nm for BCP, based on the results reported in a previous work.[54] In this structure, the NPD layer reduces the energy barrier (for hole injection) between the work function of the ITO anode (−4.7 eV) and the HOMO level of TCTA (−5.83 eV). An NPD thickness of 5 nm is sufficient to reduce the hole injection barrier without emitting or contributing to emission.

**Figure 5a** shows the J–V characteristics of OLEDs A and B. It can be observed that OLED B exhibits a lower current density than OLED A, indicating that the presence of the Nzx layer reduces charge carrier conduction within the device. At a current density of 30 mA, device B exhibits a luminance of 197 cd m⁻², whereas device A achieves a luminance of 97.2 cd m⁻² (Figure 5b). This suggests that the presence of the

<figure><img src="image_8.png" /><figcaption>**Figure 8.** Schematic illustration showing the various layers in OLED devices. a) In bilayer planar-heterojunction (PHJ) OLED, the active layer is composed of an electron donor material and an electron acceptor material. b) The electron donor and acceptor are mixed at the nanoscale, forming a structure referred to as a bulk heterojunction (BHJ).</figcaption></figure>

Nzx layer enhances the luminance of the OLED. The normalized electroluminescence spectra of devices A and B show blue emission with peaks centered at 421 and 444 nm, respectively (Figure 5c). The emission of OLED A is attributed to the formation of emissive complexes, such as exciplexes, at the TCTA/BCP interface.[54] In contrast, the electroluminescence spectrum of OLED B closely resembles the photoluminescence spectrum of the Nzx layer, suggesting that the emission primarily originates from this layer.

The Nzx layer exhibits an emission band similar to that observed in the microcrystalline configuration, and related to the formation of X-type aggregates. It can be concluded that Nzx in thin-film configuration should present a crossed-stacking molecular arrangement leading to blue emission between 420 and 460 nm. Although this arrangement is favorable to emission, the reduction of π–π overlap also limits charge conduction.[55–58] The optical and electrical properties of Nzx-based OLEDs, particularly

<figure><img src="image_9.png" /><figcaption>**Figure 7.** AFM surface scan of a) a TCTA film and b) a TCTA: 10 wt% Nzx film.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 9.** a) Current density (J)-Voltage (V) plot, b) Luminance (L)-Current (I) plot, and c) Normalized electroluminescence (EL) spectra of device A (undoped) and C (doped).</figcaption></figure>

the increase in luminance and the decrease in conductivity, can therefore be explained by the crossed molecular structuring of Nzx in thin films.

### 3.3. Integration of Nzx as a Host in the TCTA Guest Matrix

Since Nzx emits efficiently in thin-film form, it does not show aggregation-caused quenching (ACQ). However, it is possible that some bimolecular interactions are responsible for non-radiative losses. The Nzx molecules were thus dispersed within an organic host matrix, with the hope of increasing emission efficiency.

#### 3.3.1. Surface Analysis by Atomic Force Microscopy

**Figure 6** shows the band diagram of the studied OLEDs. The comparison of the HOMO and LUMO energy levels of TCTA and

Nzx indicates that the energy levels of Nzx are located between those of the TCTA matrix. Therefore, the transfer of electrons from the LUMO level of TCTA to that of Nzx is promoted. Similarly, hole transfer from the HOMO level of TCTA to that of Nzx is also facilitated. Furthermore, a comparison of the emission spectrum of TCTA and the absorption spectrum of Nzx (see Figure S2, Supporting Information) shows a slight overlap between 350 and 375 nm, indicating that Förster-type energy transfer from the TCTA matrix to Nzx molecules is unlikely. Consequently, excitons are directly formed on the Nzx molecules by charge transfer from TCTA to Nzx. For a low doping concentration (<10%), an emission band at 420 nm, characteristic of exciplex emission from the TCTA/BCP interface, was observed via PL and EL. Therefore, Nzx emission was not detected at low doping ratios (<10%). In contrast, for Nzx doping ratios higher than 10%, the OLEDs exhibited an emission band ≈440 nm, corresponding to the Nzx emission. The EL behavior can be explained by considering that a low doping ratio does not allow efficient charge transfer from TCTA to Nzx. The absence of a significant PL signal for low doping ratios
is more difficult to understand. One hypothesis is that detrimental intermolecular interactions occur between isolated molecules of Nzx and the matrix, or between Nzx molecules that can be oriented in the same direction in the matrix (vide infra) and undergo π-π stacking. Such interactions may disappear at a high Nzx ratio, with the formation of crystalline nano-domains.

To clarify this point, AFM measurements were performed to observe the influence of Nzx dispersion on the morphology of the TCTA layer.

**Figure 7** shows the AFM images of a TCTA layer and a TCTA:10 wt% Nzx layer. It can be observed that the dispersion of 10% Nzx in the TCTA layer modifies its morphological structure. The TCTA layer is uniform with a granular morphology, whereas the TCTA:10 wt% Nzx layer exhibits a sponge-like morphology with the formation of bicontinuous nanodomains. This morphological structure is characteristic of phase separation by spinodal decomposition.[59] This type of morphology is observed in bulk heterojunction (BHJ) structures, which are found, for example, in polymer blend-based OPVs (Organic Photovoltaics).[60] Spinodal decomposition is a transition between a dispersed phase and a condensed phase. Molecules tend to aggregate due to concentration effects. This phase separation thus induces a TCTA-rich phase and an Nzx-rich phase. Phases separated by spinodal decomposition are generally non-spherical (sponge-like appearance) and exhibit a high degree of connectivity, whereas separated particles tend to be spherical and display low connectivity. The spinodal morphology is suitable for ensuring balanced charge transport (between electrons and holes) and for promoting efficient charge transfer between two materials. Indeed, TCTA is known to be an electron-donating molecule, while previous studies have shown that oxazole derivatives such as Nzx offer efficient electron transport.[61,62] Nzx is an electron-accepting molecule. Thus, the TCTA:Nzx layer can be considered a BHJ structure similar to those found in OPV systems. This blend enables the creation of interpenetrating “donor” and “acceptor” regions throughout the entire volume of the active layer (Figure 8b). The blend of two-electron donor and acceptor materials tends to form percolation pathways for charge transport in the BHJ layer. The donor material primarily transports positive charges, while the acceptor material mainly transports negative charges.

### 3.3.2. Electrical and Optical Characteristics of TCTA:10 wt% Nzx-Based OLEDs

Two types of “host-guest” OLEDs were fabricated. The multilayer OLEDs are labeled **OLED C**, while the single-layer OLEDs are labeled **OLED E**. A single-layer structure of TCTA (device **D**) corresponds to the reference OLED (as detailed in Table 1).

**Figure 9a** shows the *J*–*V* characteristics of OLEDs **A (undoped)** and **C (doped)**. A slight decrease in current density can be observed upon doping. This could be explained by the modification of the electronic structure of TCTA. Upon doping, trap states can be introduced in the bandgap of the matrix, which reduces hole mobility. The luminance-current (*L*–*J*) characteristics of **A and C** are shown in Figure 9b.

It can be observed an increase in the luminance of device **C (doped–BHJ OLED)** compared to the reference device **A (undoped)** and device **B (PHJ OLED)**. At a current of 30 mA, the

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Normalized electroluminescence (EL) spectra of single-layer device D (undoped) and device E (doped).</figcaption></figure>

luminance was 97.2 cd m⁻² (for OLED **A**) while it reached 197 and 221 cd m⁻² for Nzx based OLEDs, namely devices **B (PHJ structure)** and **C (BHJ structure)**, respectively. For BHJ devices, the luminance values were higher than those obtained with PHJ devices.

The normalized electroluminescence spectra of **A and C** showed blue emission with peaks centered at 421 and 443 nm, respectively (Figure 9c). Adding Nzx as an emissive layer (PHJ structure) and as a dopant (BHJ structure) induced a red shift in the EL spectra of the devices. For Nzx based OLEDs, a similar emission band centered at 443 nm was observed, which is characteristic of Nzx emission.

To confirm this interpretation, electroluminescence (EL) spectra of the single-layer **D and E** structures were measured. As shown in Figure 10, the EL spectrum of **D-type** device exhibits an emission peak at 392 nm, attributed to the TCTA emission.[63] In contrast, the doped device (**E**) showed an emission in the blue region between 443 and 460 nm, similar to the emission band of Nzx. The absence of TCTA emission at 390 nm confirmed that the emission band ≈450 nm in the host-guest OLEDs originates from Nzx emission.

### 3.3.3. Emission stability of TCTA-Based OLED

**Figure 11a** shows the EL spectrum of device **C** for different current values. We observed a redshift and broadening in the EL spectrum as well as the appearance of a new band at higher current values. A new emission band, centered at 644 nm and accompanied by a decrease in OLED performance, appeared at polarization currents exceeding 55 mA. Successive measurements on the same sample indicated that the change in the emission spectrum and the degradation of performance under the influence of the driving current are irreversible. Furthermore, a study of the evolution of the chromatic coordinates (CIE) as a function of current showed that the emission color of the OLEDs was unstable. Figure 11b shows that at low currents, the emission color of the OLEDs was blue with a CIE of (0.17, 0.17), whereas it
<figure><img src="image_12.png" /><figcaption>**Figure 11.** a) Electroluminescence spectra at different driving currents and b) variation of CIE coordinates as a function of the current of device C (doped).</figcaption></figure>

shifted toward cool white (bluish white) with a CIE of (0.30, 0.27) after the apparition of the red band at higher current values.

This phenomenon was also observed for the reference OLED A. It is, therefore, specific to OLEDs based on TCTA and BCP. Thus, the appearance of these new emission bands is not related to the degradation of Nzx. An in-depth study showed that the appearance of these new wavelengths is caused by the degradation of the TCTA matrix. The accumulation of excitons in the TCTA layer induces molecular aggregation, leading to the formation of electrically active electronic states, namely, electroplexes and electromers, which lead to the appearance of additional emission bands.[64] Due to the instability of TCTA-based OLEDs, the TCTA matrix was replaced with NPD (Figure 12).

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Energy level diagram of device C.</figcaption></figure>

### 3.4. Integration of Nzx as a Host in the NPD Guest Matrix

The same study was conducted on two types of OLEDs: bilayer and single-layer devices. The previous study on TCTA-based host-guest OLEDs showed Nzx emission at a doping ratio of 10%. Therefore, we initially chose to maintain this Nzx doping ratio in the NPD matrix.

#### 3.4.1. Emission Stability of NPD-Based OLED

To evaluate the stability of NPD based devices, the EL spectrum of device G (with an active layer of NPD:10 wt% Nzx, as detailed in Table 1) and the evolution of its chromatic coordinates (CIE) as a function of current were measured. As shown in Figure 13a, the EL spectra at different current values exhibit stability in the emission wavelength, centered at 438 nm with CIE of (0.16, 0.13). Furthermore, the CIE diagram (Figure 13b) indicates that the emission color of the OLEDs remains stable at high currents (above 55 mA).

#### 3.4.2. Electrical and Optical Characteristics of NPD:10 wt% Nzx-Based OLEDs

The L-I and J-V characteristics of type-F (undoped) and type-G (doped) devices are shown in Figure 14. The results show an increase in luminance upon doping. At 30 mA, a luminance of 162.5 cd m⁻² was achieved for the undoped type-F device, while this value increased to 346.9 cd m⁻² for the type-G device. The doping of the NPD matrix with Nzx appeared to improve charge balance within the NPD:10%Nzx layer. The doped devices exhibited a lower current density than the reference OLEDs. The decrease in current density suggests that the dispersion of Nzx in the NPD layer tends to alter the electronic properties of the NPD layer and, consequently, its charge transport properties.

The measured EL spectrum of the reference device F (undoped) showed an emission at 433 nm, attributed to NPD
<figure><img src="image_14.png" /><figcaption>**Figure 13.** a) Normalized electroluminescence spectra at different driving currents and b) CIE diagram for device G (doped).</figcaption></figure>

emission. Upon doping, this spectrum underwent a slight red shift and a broadening. An emission centered at 438 nm was measured (Figure 15). This spectrum is close to the EL spectrum of the reference OLED. Indeed, the emission of Nzx is very similar to that of the NPD matrix, both emit in the blue region, and the spectral shift induced by Nzx remains relatively small. In this case, the emission may originate from both NPD and Nzx. However, the emission of OLEDs (NPD:10% Nzx) predominantly originates from NPD. A more detailed analysis of the origin of emission in NPD:Nzx-based OLEDs is provided in the discussion section.

To evaluate the possibility of Förster-type energy transfer from NPD to Nzx, the emission spectrum of NPD and the absorption spectrum of Nzx were measured and compared (see Figure S3, Supporting Information). This comparison revealed a weak spectral overlap between 350

and 375 nm, suggesting that Förster-type energy transfer is unlikely.

To corroborate this interpretation, monolayer structures of NPD (labeled Device **H**, as detailed in Table 1) and doped NPD (labeled Device **I**, as detailed in Table 1) were fabricated, and their optoelectronic properties were studied.

The *J*–*V* and *L*–*I* characteristics are presented in **Figure 16**. We observed that the evolution of current densities and luminances follows the same trend as for bilayer devices. The current decreases upon doping, while the luminance increases. For the same current, device **H** exhibits a lower luminance than device **I** (0.419 cd m⁻² versus 1.79 cd m⁻², respectively, for I = 40 mA). These characteristics are consistent with previous results (obtained with bilayer structures), which suggest that doping with Nzx leads to an increase in optical performance and a decrease in current.

<figure><img src="image_15.png" /><figcaption>**Figure 14.** a) Luminance (L)-Current (I) plot and b) Current density (J)-Voltage (V) plot of device F (undoped) and device G (doped).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Figure 15.** Normalized electroluminescence (EL) spectra of device F (undoped) and device G (doped) measured at 30 mA.</figcaption></figure>

The EL spectrum of OLED **H** shows an emission at 430 nm, corresponding to the emission of NPD, while the EL spectrum of the doped device **I** exhibits an emission centered at 434 nm. To verify whether Nzx contributes to the emission, photoluminescence spectra of single-layer devices were measured under 365 nm excitation. As shown in **Figure 17b**, the PL spectrum for undoped device **H** exhibits an emission band at 435 nm associated with NPD emission. The PL spectrum obtained for doped device **I** is redshifted and slightly broader compared to that of the reference. It exhibits a peak centered at 442 nm. It can be assumed that this broadening and shift are clearly related to the emission originating from Nzx.

### 3.4.3. Optimization of Doping Ratio

In this section, we focused on optimizing the doping concentration of Nzx in the NPD matrix. Controlling the doping concentration of the emissive layer is a key parameter for achieving high-efficiency OLED devices. Various OLEDs with Nzx concentrations ranging from 10% to 37% were fabricated. At these concentrations, the system can be considered as a donor/acceptor blend. The NPD:Nzx layer can be considered as a bulk heterojunction (BHJ) structure.

**Figure 18a** shows the *L*-*I* characteristics of OLEDs for different Nzx concentrations in the NPD layer. At a current of 30 mA, the luminance of the OLED reaches 347 cd/m² for a concentration of 10% (Table 3). For the same current value, the luminance of the OLEDs increases as the Nzx concentration increases. At a current of 30 mA, the OLEDs exhibit luminance values of 473, 507, and 598 cd m⁻² for Nzx concentrations of 16%, 20%, and 27%, respectively. For concentrations above 27%, the luminance decreases. It drops to 392 and 380 cd m⁻² for Nzx concentrations of 31% and 37%, respectively. This decrease in luminance suggests a decrease in the density of emissive excitons. It could be caused by significant exciton quenching and/or by a modification in charge balance within NPD layer when the Nzx concentration exceeds 27%. For an Nzx concentration of 27%, optimal optical performance was achieved, with a maximum luminance

of 772 cd m⁻². Figure 18b shows the EL spectra obtained for different Nzx concentrations. It can be observed that increasing the Nzx concentration in the NPD layer induces a slight redshift as well as a broadening of the OLED emission spectrum. Thus, the OLEDs exhibit an emission band centered at 448 nm. In the literature, this spectral shift as a function of doping concentration was attributed to polarization phenomena.[65] However, in our case, we assume that this spectral shift is due to Nzx emission, which becomes more significant as the concentration of these molecules increases.

In a BHJ structure, the variation in the concentration of acceptor molecules (here Nzx) affects the electron density in the layer and, consequently, the charge balance. The optimal performance observed at 27% Nzx therefore suggests an optimal charge balance within the BHJ layer, promoting the formation of emissive excitons. However, a purely optical effect cannot be excluded. Indeed, studies on the dispersion of triazine-based TADF molecules in a CBP matrix have shown that at a TADF molecule concentration of ≈30%, OLEDs exhibit maximum luminance.[66–69] The variation in the quantum efficiency of this type of OLED as a function of concentration can be related to the orientation of the dipoles of the emitting molecules, which enhances the optical coupling of the emitted photons.[69] Each emissive molecule is associated with an emission transition dipole moment. Fluorophore molecules primarily emit photons in the direction normal to this dipole moment. Photon emission at normal incidence to the OLED's emitting surface minimizes reflection losses at the various interfaces (organic layer/ITO, ITO/glass, and glass/air) and enhances photon extraction to the exterior of the OLED.

A horizontal orientation (with respect to the OLED's emitting surface) of the dipole moments of the emissive molecules thus promotes photon extraction and consequently enhances the OLED's luminance.[70] Several studies have shown that when fluorophores are dispersed in an organic matrix, both the fluorophore concentration and the nature of the matrix influence the dipolar orientation of the fluorophores and the efficiency of OLEDs through optical coupling (i.e., light extraction).[70,71] Indeed, when fluorophores are dispersed in a matrix, their dipole moment tends to align with that of the matrix.[70–73] Furthermore, studies on the orientation of NPD molecules have shown a horizontal orientation of the NPD dipole moment in thin films.[74] When Nzx is dispersed in an NPD matrix, a dipolar alignment of Nzx following that of NPD (in a horizontal orientation) is highly likely, thereby enhancing optical coupling. Thus, the variation in the optical performance of OLEDs as a function of the Nzx concentration (in the NPD matrix) could be explained by a modulation of the charge balance in the BHJ layer, as well as by a change in the dipole moment orientation of Nzx, which enhances photon extraction from the OLEDs.

### 3.4.4. Influence of the ETL Layer

This section focuses on testing various ETL materials in order to optimize the structure and optimize performance and efficiency of Nzx-based OLEDs. To this end, three structures J, K, and L were fabricated using BCP, BPhen, and TPBi as ETL layers, respectively (as detailed in Table 1). The NPD layer was doped with Nzx at a concentration of 27%, a percentage at which optimal
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 16.** a) Current density (J)-Voltage (V) plot and b) Luminance (L)-Current (I) plot of monolayer-based device H (undoped) and device I (doped).</figcaption></figure>

performance was achieved. The thicknesses of the NPD:Nzx and ETL layers were consistently maintained at 95 and 35 nm, respectively. The optoelectronic performance of the three fabricated structures was measured and compared to **determine** the optimal structure with the highest performance.

### 3.4.5. Current Density-Voltage Characteristics (J-V)

**Figure 19** shows the J-V characteristics of J-type, K-type, and L-type structures. The turn-on voltages ($V_{ON}$) and optical/electrical features are summarized in **Table 4**. The J, K, and L OLEDs exhibit $V_{ON}$ of 6.5, 5.6, and 5.4 V, respectively. As shown in **Figure 20**, the energy barrier $\Delta_{HOMO}$ at the NPD/ETL interface (> 0.7 eV) limits hole injection from NPD layer to ETL, and hence, promotes hole accumulation in the NPD layer.

The variation in turn-on voltage depends on the nature of the ETL layer. Given that the hole injection is limited by the high $\Delta_{HOMO}$, it appears that the $V_{ON}$ variation can be related to the energy difference between the LUMO levels ($\Delta_{LUMO}$) at the NPD:Nzx/ETL interface. For the NPD/BCP interface, $\Delta_{LUMO}$ is 1.1 eV, while it is 0.6 eV at the NPD/BPhen interface. The reduction of the electron injection barrier impacts the lowering of the turn-on voltage by 0.9 V. For the L device, $\Delta_{LUMO}$ at the NPD/TPBi interface is ≈0.3 eV. Due to this energy reduction, the turn-on voltage is decreased by 0.2 V compared to that of the K device. Therefore, the reduction of the energy barrier $\Delta_{LUMO}$ at the NPD/ETL interface induces a decrease in the turn-on voltage of OLEDs. Indeed, a low energy barrier $\Delta_{LUMO}$ at the NPD/ETL interface favors electron injection from the ETL layer to the NPD:Nzx, thereby increasing the electron density in the NPD:Nzx layer.

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 17.** Normalized a) electroluminescence (EL) and b) photoluminescence (PL) spectra of device H (undoped) and device I (doped).</figcaption></figure>
<figure><img src="image_21.png" /><figcaption>**Figure 18.** a) Luminance (L)-Current (I) characteristics and b) Normalized electroluminescence (EL) spectra of the OLEDs with different Nzx concentrations.</figcaption></figure>

With an ETL layer of Bphen, we observe that the current density is higher than that obtained with BCP and TPBi, as shown in **Figure 19**. At a voltage of 10 V, a current density of 186 A m⁻²

<table><caption>**Table 3.** Luminance and maximum wavelength of the devices as a function of the Nzx doping ratio in the NPD matrix, measured at 30 mA.</caption><thead><tr><th>Nzx Doping [%]</th><th>Luminance [cd m<sup>-2</sup>]</th><th>Maximum wavelength [nm]</th></tr></thead><tbody><tr><td>0</td><td>163</td><td>433</td></tr><tr><td>10</td><td>347</td><td>438</td></tr><tr><td>16</td><td>473</td><td>445</td></tr><tr><td>20</td><td>507</td><td>449</td></tr><tr><td>27</td><td>598</td><td>448</td></tr><tr><td>31</td><td>392</td><td>440</td></tr><tr><td>37</td><td>380</td><td>446</td></tr></tbody></table>

<figure><img src="image_22.png" /><figcaption>**Figure 19.** Current density (J)-Voltage (V) characteristics of device J (with BCP), device K (with BPhen), and device L (with TPBi).</figcaption></figure>

<table><caption>**Table 4.** The electrical and optical performance of devices J, K, and L.</caption><thead><tr><th>Device</th><th>V<sub>ON</sub><br>[V]</th><th>L<sub>max</sub><br>[cd m<sup>-2</sup>]</th><th>Le<sub>max</sub><br>[W sr.m<sup>-2</sup>]</th><th>J<sub>max</sub><br>[A m<sup>-2</sup>]</th><th>λ<sub>max</sub><br>[nm]</th><th>EQE<sub>max</sub><br>[%]</th></tr></thead><tbody><tr><td>J (with BCP)</td><td>6.5</td><td>772</td><td>5.3</td><td>3009</td><td>448</td><td>0.34</td></tr><tr><td>K (with BPhen)</td><td>5.6</td><td>1511</td><td>10</td><td>3282</td><td>446</td><td>0.5</td></tr><tr><td>L (with TPBi)</td><td>5.4</td><td>3431</td><td>30.3</td><td>5416</td><td>438</td><td>1</td></tr></tbody></table>

was achieved for J (with BCP), while current densities of 626 and 300 A m⁻² were reached for the K (with Bphen) and L (with TPBi) structures, respectively. This difference in current density is due to electron mobility, which varies between ETL materials. Indeed, the Bphen layer exhibits electron mobility (μe evaluated at 3.9 × 10⁻⁴ cm²·V⁻¹·s⁻¹) higher than those of the BCP and TPBi layers (evaluated at 6 × 10⁻⁷ cm²·V⁻¹·s⁻¹ and 3.3 × 10⁻⁵ cm²·V⁻¹·s⁻¹, respectively).[75]

### 3.4.6. Optoelectronic Performances of OLEDs

The L-I characteristics and the external quantum efficiencies as a function of the applied current are shown in **Figure 21a,b**.

The highest performance was achieved with a TPBi layer. OLEDs with BCP exhibit the lowest performance. An increase in luminance and quantum efficiency is observed in the device with BPhen (Device K) compared to the device with BCP (Device J). A maximum luminance of 1511 cd/m² and an EQE of 0.5% were obtained with BPhen, compared to 772 cd m⁻² and 0.34% with BCP. The same applies to the radiance, which reaches a maximum of 5.3 W/sr·m² for structure J, then increases to 10 W/sr·m² and 30.3 W/sr·m² for structures K and L, respectively. For the same current value, the radiance of L is more than three times higher than that of J and more than twice that of K (12.1 W/sr·m² for L compared to 4.1 W/sr·m² and 6.3 W/sr·m² for J and K, respectively, measured at 30 mA).
<figure><img src="image_23.png" /><figcaption>**Figure 20.** Energy level diagram of NPD, BCP, BPhen, and TPBi.</figcaption></figure>

The lowest performance obtained for OLEDs with BCP based ETL can be attributed to the energy barrier Δ<sub>LUMO</sub> at the NPD:Nzx/ETL interface, which is highest for BCP based ETL (1.1 eV). This substantial barrier significantly limits electron injection into the NPD:Nzx layer, promoting electron accumulation in the BCP layer close to the NPD:Nzx/BCP interface. As a result, the electron density in the NPD:Nzx layer is reduced, leading to exciton formation in the NPD:Nzx layer near the NPD:Nzx/BCP interface. Charge accumulation at the vicinity of the exciton formation zone adversely impacts performance by favoring polaron-exciton interactions, which promote non-radiative de-excitation. Consequently, this reduces the luminance and EQE of the OLED.

With TPBi as the ETL, the maximum luminance and efficiency of the OLED increase to 3431 cd m⁻² and 1%, respectively. The energy barrier Δ<sub>LUMO</sub> at the NDP:Nzx/TPBi interface is 0.3 eV, allowing electrons to easily cross this barrier. Electron accumulation at the NPD:Nzx/ETL interface is reduced and electron density within the NPD:Nzx layer is enhanced. The exciton formation zone tends to occur within the bulk of the NPD:Nzx layer (rather than in the vicinity of the interface with the ETL layer, as is the case with a BCP layer). The non-radiative losses (particularly exciton-polaron interactions) occurring at the NPD:Nzx/ETL interface are limited.

SSSE-based devices display low-efficiency roll-off, as shown in Figure 21b. Although phosphorescent and TADF OLEDs exhibit higher efficiency than SSSE-based OLEDs, their efficiency decreases, particularly severely for the TADF OLEDs, as the current density increases.[76] The integration of Nzx within NPD matrix did not affect the roll-off feature (see Figure S4, Supporting Information). In literature, to evaluate the roll-off feature of devices, it was defined J<sub>90</sub> as the current density at which the external quantum efficiency (EQE) falls to 90% of its peak value. EQE-J curve indicates a high J<sub>90</sub> (> 200 mA cm<sup>-2</sup> for brightness > 1000 cd m<sup>-2</sup>) for SSSE-based OLEDs, while many blue emitting devices based on TADF display a J<sub>90</sub> of a few mA/cm<sup>2</sup>[76] and EQE drops to below 5%.[77]

### 3.4.7. Origin of Emission in NPD:Nzx-Based OLEDs

The electroluminescence spectra of structures J, K, and L are presented in Figure 22. These spectra clearly indicate emission in the blue region.

### 3.4.8. Observation of BHJ Structures

Nevertheless, a slight shift in the emission wavelength and a broadening of the EL spectrum can be observed as the energy barrier Δ<sub>LUMO</sub> at the NPD:Nzx/ETL interface increases. The EL spectrum peak of OLED L is centered at 438 nm with FWHM of 65 nm, whereas OLEDs K and J exhibit EL emission peaks centered at 446 and 448 nm with FWHM of 86 and 84 nm, respectively. Thus, the spectra of structures J and K are broader than that of structure L. The Commission Internationale de l'Eclairage (CIE) coordinates are (0.163, 0.155), (0.170, 0.171) and (0.165, 0.120) for devices J, K and L respectively. Under variation of current, the measurements of CIE coordinates show that the chrominance of devices J, K, and L remains stable (see Figure S5, Supporting Information).

### 3.4.9. Observation of Reference Structures (Undoped OLEDs)

Figure 22b shows the EL spectra of the reference OLEDs NPD/BCP, NPD/BPhen, and NPD/TPBi. The NPD/BCP and NPD/TPBi devices exhibit similar EL emissions centered at 434 nm, which corresponds to the emission of NPD, whereas the EL spectrum of the NPD/BPhen OLED is broader and features an emission band at 440 nm. This broadening is attributed to emissive complexes such as exciplexes and/or electroplexes that form at the NPD/BPhen interface in addition to the NPD emission. Indeed, the PL spectrum of the NPD/BPhen devices shows a narrower spectrum with an emission band at 435 nm, which corresponds to the emission of NPD (see Figure S6, Supporting Information). This feature confirms that the broadening of the spectrum under electrical excitation is due to electrically activated states (namely electroplexes).

### 3.4.10. Comparison Between BHJ and Reference Structures

The variation in EL emission as a function of the nature of the ETL layer can be explained by the influence of the ETL layer on the electrical activation of the molecules in the NPD:Nzx layer. The OLEDs (NPD/TPBi) and (NPD: 27 wt%Nzx/TPBi) exhibit similar EL spectra, suggesting that the emission of both devices has the same origin. Similar observations can be noted for the OLEDs (NPD/BPhen) and (NPD: 27 wt%Nzx/BPhen), which display similar EL spectra. Consequently, the emission of OLEDs with TPBi based ETL is predominantly dominated by the emission of NPD, while the emission of OLEDs with BPhen based ETL primarily arises from electroplexes (see Figure S7b,c, Supporting Information). The doped OLEDs (NPD: 27 wt%Nzx/BCP) exhibit a red-shifted and broader EL spectrum compared to the undoped OLEDs (NPD/BCP) (see Figure S7a, Supporting Information). This result indicates that the emission
<figure><img src="image_24.png" /><figcaption>**Figure 21.** Luminance (L)-Current (I) plot, b) external quantum efficiency (EQE), and c) Radiance-Current (I) plot of device J (with BCP), device K (with Bphen) and device L (with TPBi).</figcaption></figure>

of the OLEDs (NPD: 27 wt%Nzx/BCP) predominantly originates from Nzx.

Several studies on NPD/ETL interfaces have demonstrated that polar ETL layers, such as Alq₃ or BCP, play a role in charge accumulation at the NPD/ETL interface.[78-82] Indeed, polar ETL layers induce the formation of dipoles at the NPD/ETL interface. These dipoles lead to electron accumulation in the ETL layer in the vicinity of the NPD/ETL interface. The electrons accumulated in ETL at NPD/ETL interface is electrostatically compensated by cationic species distributed in the bulk of NPD layer. Hence, the interfacial electron accumulation in ETL results in the formation and accumulation of NPD⁺ cations (or hole accumulation) in the NPD layer. The density of NPD⁺ cations formed at this interface depends on the density of electrons accumulated in the ETL layer.

The ETL layers studied here, namely TPBi, Bphen, and BCP, are polar materials.[78,83,84] A similar effect is therefore possible for the three types of NPD/ETL interfaces. For these three materials, the energy barrier Δ<sub>HOMO</sub> at the NPD/ETL interface (> 0.7 eV) promote hole accumulation in the NPD layer, while the energy barrier Δ<sub>LUMO</sub> varies between 0.3 and 1.1 eV (as shown in Figure 20) depending on the nature of the ETL layer.

The low energy barrier Δ<sub>LUMO</sub> (0.3 eV) at the NPD:Nzx/TPBi interface tends to reduce the density of electrons accumulated at this interface. Therefore, this interface promotes electron injection into the NPD:Nzx layer. Consequently, the electron density at this interface has to be limited. Hence, the density of compensated cations at the NPD:Nzx/TPBi interface should be reduced, and the formation exciton within NPD:Nzx layer should be promoted. On the other hand, NPD is a hole transport material. In
<figure><img src="image_25.png" /><figcaption>**Figure 22.** Normalized electroluminescence (EL) spectra: a) of doped devices J, K, and L, and b) of the reference structures NPD/BCP, NPD/BPhen, and NPD/TPBi (undoped OLEDs).</figcaption></figure>

the NPD:Nzx layer, hole transport is, therefore, predominantly governed by NPD. Moreover, its HOMO level (−5.5 eV) is higher than that of Nzx (−5.63 eV). Hence, this electronic structure is more favorable for the formation of NPD⁺ cations than for the formation of Nzx⁺ cations. Thus, we can assume that during the electrical polarization of OLEDs (NPD:Nzx/TPBi), the density of NPD⁺ cations is higher than that of Nzx⁺ cations. Thus, this electronic configuration is more favorable to the formation of emissive excitons originating from NPD than from Nzx (see **Figure 23a**).

For high energy barrier Δ<sub>LUMO</sub> such as at the NPD:Nzx/BCP interface (estimated at 1.1 eV), the injection of electrons into the NPD:Nzx layer is limited and the electrons tend to accumulate in BCP layer close to the NPD:Nzx/BCP interface. Consequently, the density of accumulated electrons at this interface is significantly higher than that of the NPD:Nzx/TPBi interface. This induces a higher density of compensated cations in the vicinity of the NPD:Nzx/BCP interface (compared to TPBi

interface). Consequently, the accumulated electron density induces the accumulation of NPD⁺ cations. Moreover, the density of accumulated electrons can be sufficiently high to activate the accumulation of a greater quantity of Nzx⁺ cations (compared to the NPD:Nzx/TPBi interface). Thus, under electrical polarization, the injection of electrons from the BCP layer into the NPD:Nzx layer results in the formation and emission of excitons originating from both NPD and Nzx molecules (for NPD:27wt% Nzx/BCP based OLEDs).

The study on the concentration rate of Nzx showed that for low Nzx concentrations (<10%), the EL spectrum is dominated by NPD emission. Above 10% Nzx, the EL spectra shift toward the red, clearly indicating that the emission originates from both NPD and Nzx. However, the EL spectrum of OLEDs (NPD:10%Nzx) shows emission at 438 nm, very close to that of OLEDs without Nzx (which exhibit an emission at 433 nm), along with slight spectral broadening. This observation suggests that the emission of OLEDs (NPD:10%Nzx) arises mainly from

<figure><img src="image_26.png" /><figcaption>**Figure 23.** Illustration of the mechanisms that may explain the origin of emission in devices a) L (NPD: 27 wt% Nzx/TPBi), b) G (NPD: 10 wt% Nzx/BCP), and c) J (NPD: 27 wt% Nzx/BCP).</figcaption></figure>
NPD. The study focused on OLEDs with a BCP ETL layer (Sec-
tion 3.4.2). Based on previous interpretation, the accumulation
of electrons at the NPD:10%Nzx/BCP interface should induce
the formation of both NPD⁺ and Nzx⁺ cations. Consequently, the
emissions from both NPD and Nzx have to be favored. Neverthe-
less, the low concentration of Nzx compared to NPD suggests that
NPD⁺ cations are predominant relative to Nzx⁺ cations, which
may explain why the emission of this type of OLED is mostly due
to NPD.

Nonetheless, interactions between NPD⁺ cations and excitons formed on Nzx (such as the quenching type, specifically singlet-polaron annihilation, SPA) cannot be entirely excluded. Indeed, studies on the optoelectronic properties of NPD layers have shown that NPD⁺ cations exhibit a broad absorption band between 400 and 543 nm, leading to the quenching of excitons emitting in the blue and green regions.[85-90] The emission band of Nzx between 420 and 455 nm and the absorption band of NPD⁺ therefore exhibit a significant spectral overlap. Consequently, energy transfer from excitons formed on Nzx molecules (denoted as Nzx*) to NPD⁺ cations is highly likely, resulting in the quenching of Nzx⁺ excitons. For the same OLED structure, increasing the concentration of Nzx molecules, e.g. to 27%, implies, on the one hand, an enhancement in the density of Nzx⁺ cations (and therefore the density of Nzx⁺ excitons) and on the other hand the decrease in the density of NPD⁺ cations (and consequently the density of NPD* excitons). Thus, the interactions between NPD⁺ cations and Nzx⁺ excitons, i.e., SPA quenching, should be reduced, thereby promoting the emission from Nzx molecules (see Figure 23 b, c).

4. Conclusion

The SSSE 2-phenyl-naphthoxazole (Nzx) was used as emitting material for OLEDs, both as an emissive layer and as a dopant in a matrix. Initially, TCTA was employed as the HTL material and as the host matrix. The doped devices exhibited higher performance compared to the non-doped devices, and emission from Nzx was clearly observed in both structures. The PL and EL emissions of Nzx-based thin solid film were interpreted in terms of intermolecular arrangement. Indeed, the emission of Nzx in thin films was similar to that observed in the microcrys- talline solid state, suggesting that it is driven by the formation of nano-aggregates with molecular cross-arrangement (X-type ag- gregates). For low doping concentrations (below 10 wt%), emis- sion from Nzx was not detected. However, it was clearly observed for a doping ratio higher than 10 wt%. Morphological analysis of the TCTA:10wt%Nzx layer revealed phase separation by spin- odal decomposition. This morphology is characteristic of a bulk heterojunction, with the formation of Nzx nanodomains. The EL emission appeared to be related to the formation of these X- type nano-aggregates. At high bias currents, a spectral shift and the emergence of new bands in the red region were observed in TCTA-based OLEDs. This effect was attributed to the degradation of TCTA. NPD: Nzx-based OLEDs showed highly stable perfor- mance and emission in the near deep blue region (< 460 nm). Optimal performance was achieved at an Nzx doping ratio of 27 wt%. To further enhance the performance and efficiency of the OLEDs, three ETL materials were tested. The optimal perfor-

mance was achieved with the structure NPD: 27 wt% Nzx/TPBi, which exhibited near deep blue emission centered at 438 nm, along with a maximum luminance of 3430 cd/m² and an EQE of 1%. The origin of blue emission in Nzx-based OLEDs was discussed in terms of charge balance and interaction between cation accumulation and exciton. This work opens new perspectives for the use of Nzx derivatives in blue OLEDs.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

Acknowledgements

The authors are grateful to Dr. Nadine Leygue and Dr. Evelyne Delfourne (SPCMIB, Toulouse) for their help in synthesis.

Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the cor-
responding author upon reasonable request.

Keywords

blue fluorescent organic light-emitting diodes (OLED), oxazole derivatives, solution and solid-state emitters (SSSE), X-type aggregates

Received: February 27, 2025

Revised: May 1, 2025

Published online:

[1] C. W. Tang, S. A. VanSlyke, *Appl. Phys. Lett.* **1987**, *51*, 913.

[2] M. Singh, J.-H. Jou, S. Sahoo, S. S. S., Z.-K. He, G. Krucaite, S. Grigalevicius, C.-W. Wang, *Sci. Rep.* **2018**, *8*, 7133.

[3] A. Salehi, X. Fu, D. Shin, F. So, *Adv. Funct. Mater.* **2019**, *29*, 1808803.

[4] J.-H. Jou, S. Sahoo, D. K. Dubey, R. A. K. Yadav, S. S. Swayamprabha, S. D. Chavhan, *J. Mater. Chem.* **2018**, *6*, 11492.

[5] B. W. D'Andrade, S. R. Forrest, *Adv. Mater.* **2004**, *16*, 1585.

[6] Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R. Forrest, *Nature* **2006**, *440*, 908.

[7] H. Sasabe, J. Kido, *J. Mater. Chem. C* **2013**, *1*, 1699.

[8] R. Das, X. He, *Flexible & Foldable OLED Displays 2019–2029*, IDTechEX, Cambridge, UK **2018**.

[9] M. Manzano, F. Cecchini, M. Fontanot, L. Iacumin, G. Comi, P. Melpignano, *Biosens. Bioelectron.* **2015**, *66*, 271.

[10] U. Giovanella, C. Botta, F. Galeotti, B. Vercelli, S. Battiato, M. Pasini, *J. Mater. Chem. C* **2013**, *1*, 5322.

[11] H. Kanno, N. C. Giebink, Y. Sun, S. R. Forrest, *Appl. Phys. Lett.* **2006**, *89*, 023503.

[12] S. Reineke, T. C. Rosenow, B. Lüssem, K. Leo, *Adv. Mater.* **2010**, *29*, 3189.
[13] S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K. Leo, *Nature* **2009**, *459*, 234.

[14] K.-Y. Lu, H.-H. Chou, C.-H. Hsieh, Y.-H. O. Yang, H.-R. Tsai, H.-Y. Tsai, L.-C. Hsu, C.-Y. Chen, I.-C. Chen, C.-H. Cheng, *Adv. Mater.* **2011**, *23*, 4933.

[15] C. Fan, L. Zhu, T. Liu, B. Jiang, D. Ma, J. Qin, C. Yang, *Angew. Chem.* **2014**, *126*, 2179.

[16] M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, In *Electrophosphorescent Materials and Devices*, Jenny Stanford Publishing, Dubai, U.A.E **2023**.

[17] K. Goushi, K. Yoshida, K. Sato, C. Adachi, *Nat. Photonics* **2012**, *6*, 253.

[18] A. Chaskar, H.-F. Chen, K.-T. Wong, *Adv. Mater.*, **23**, 3876.

[19] N. C. Giebink, B. W. D'andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson, S. R. Forrest, *J. Appl. Phys.* **2008**, *103*, 044509.

[20] J.-H. Lee, C.-H. Chen, P.-H. Lee, H.-Y. Lin, M. Leung, T.-L. Chiu, C.-F. Lin, *J. Mater. Chem.* **2019**, *7*, 5874.

[21] D. Barah, S. Sahoo, N. S. M. Inaganti, H. Kesavan, J. Bhattacharyya, D. Ray, *Semicond. Sci. Technol.* **2022**, *37*, 035006.

[22] N. C. Giebink, B. W. D'Andrade, M. S. Weaver, J. J. Brown, S. R. Forrest, *J. Appl. Phys.* **2009**, *105*, 124514.

[23] M. Tanaka, H. Noda, H. Nakanotani, C. Adachi, *Adv. Electron. Mater.* **2019**, *5*, 1800708.

[24] W. Song, T. Kim, J. Y. Lee, Y. Lee, H. Jeong, *J. Ind. Eng. Chem.* **2018**, *68*, 350.

[25] J. E. Anthony, *Chem. Rev.* **2006**, *106*, 5028.

[26] P. Wen, Z. Gao, R. Zhang, A. Li, F. Zhang, J. Li, J. Xie, Y. Wu, M. Wu, K. Guo, *J. Mater. Chem.* **C** **2017**, *5*, 6136.

[27] J. Luo, Z. Xie, Y. Lam, *Chem. Commun.* **2011**, *0*, 1740.

[28] G. Yu, S. Yin, Y. Liu, J. Chen, X. Xu, X. Sun, D. Ma, X. Zhan, Q. Peng, Z. Shuai, B. Tang, D. Zhu, W. Fang, Y. Luo, *J. Am. Chem. Soc.* **2005**, *127*, 6335.

[29] J. Mei, N. L. Leung, R. T. Kwok, J. W. Lam, B. Z. Tang, *Chem. Rev.* **2015**, *115*, 11718.

[30] Y. Hong, J. W. Y. Lam, B. Z. Tang, *Chem. Commun.*, No. **2009**, *29*, 4332.

[31] J. Huang, Y. Jiang, J. Yang, R. Tang, N. Xie, Q. Li, H. S. Kwok, B. Z. Tang, Z. Li, *J. Mater. Chem.* **C** **2014**, *2*, 2028.

[32] K. Guo, Z. Gao, J. Cheng, Y. Shao, X. Lu, H. Wang, *Dyes Pigments* **2015**, *115*, 166.

[33] M. Huang, J. Zhou, K. Xu, X. Zhu, Y. Wan, *Dyes Pigments* **2019**, *160*, 839.

[34] N. A. Kukhta, M. R. Bryce, *Mater. Horiz.* **2021**, *8*, 33.

[35] S. T. N. Sailaja, I. Maisuls, J. Kösters, A. Hepp, A. Faust, J. Voskuhl, C. A. Strassert, *Beilstein J. Org. Chem.* **2020**, *16*, 2960.

[36] S. Riebe, S. Adam, B. Roy, I. Maisuls, C. G. Daniliuc, J. Dubbert, J. Voskuhl, *Chem.-Asian J.* **2021**, *16*, 2307.

[37] J. L. Belmonte-Vázquez, Y. A. Amador-Sánchez, L. A. Rodríguez-Cortés, B. Rodríguez-Molina, *Chem. Mater.* **2021**, *33*, 7160.

[38] M. Durko, G. Ulrich, D. Jacquemin, J. Mysliwiec, J. Massue, *Phys. Chem. Chem. Phys.* **25**, 15085.

[39] V. Trannoy, A. Léaustic, S. Gadan, R. Guillot, C. Allain, G. Clavier, S. Mazerat, B. Geffroy, P. Yu, *New J. Chem.* **45**, 3014.

[40] M. Curitol, X. Ragas, S. Nonell, N. Pizarro, M. V. Encinas, P. Rojas, R. P. Zanocco, E. Lemp, G. Günther, A. L. Zanocco, *Photochem. Photo-biol.* **2013**, *89*, 1327.

[41] C. Yuan, S. Li, Y. Wu, L. Lu, M. Zhu, *Sens. Actuators, B* **2017**, *242*, 1035.

[42] D. G. Guimarães, V. L. Santos, S. S. Simplicio, A. A. Gonsalves, J. J. Gouveia, F. B. Almeida, C. R. Araujo, *J. Braz. Chem. Soc.* **2024**, *36*, e.

[43] N. V. Slovesnova, A. S. Minin, L. T. Smolyuk, O. S. Taniya, A. N. Tsmokalyuk, G. A. Kim, V. N. Charushin, *Dyes Pigments* **2022**, *204*, 110410.

[44] C. Rai, U. Pradesh, T. H. Szawlowski, (2-Substituted Naphth(1,2)Oxazole Scintillators), US3526768A, **1970**

[45] R. P. Zanocco, S. Valdebenito, Y. Gidi, G. Zapata-Torres, E. Lemp, S. Nonell, A. L. Zanocco, *Photochem. Photobiol.* **2018**, *94*, 1092.

[46] Y. You, J. Seo, S. H. Kim, K. S. Kim, T. K. Ahn, D. Kim, S. Y. Park, *Inorg. Chem.* **2008**, *47*, 1476.

[47] I.-H. Kim, K. H. Kim, H. Jo, B.-J. Lee, D.-E. Kim, H.-K. Shin, *J. Nanosci. Nanotechnol.* **2017**, *17*, 3298.

[48] I.-H. Kim, E. Kang, D.-E. Kim, H.-K. Shin, B.-J. Lee, *Mol. Cryst. Liq. Cryst.* **2014**, *597*, 88.

[49] A. Ghodbane, J. Colléaux, N. Saffon, R. Mahiou, J.-P. Galaup, S. Fery-Forgues, *ChemPlusChem* **2013**, *78*, 185.

[50] H. Wang, C. Liu, B. Mi, J. Sang, X. Li, Z. Gao, *Sci. China Chem.* **2014**, *57*, 849.

[51] C.-H. Chen, L.-C. Hsu, P. Rajamalli, Y.-W. Chang, F.-I. Wu, C.-Y. Liao, M.-J. Chiu, P.-Y. Chou, M.-J. Huang, L.-K. Chu, C.-H. Cheng, *J. Mater. Chem. C* **2014**, *2*, 6183.

[52] O. V. Mikhnenko, M. Kuik, J. Lin, N. Kaap, T.-Q. Nguyen, P. W. Blom, *Adv. Mater.* **2014**, *26*, 1912.

[53] O. V. Mikhnenko, P. W. Blom, T.-Q. Nguyen, *Energy Environ. Sci.* **2015**, *8*, 1867.

[54] H. El Housseiny, G. Mbarki, D. Buso, M. Ternisien, G. Zissis, C. Renaud, Organic Electronics and Photonics: Fundamentals and Devices IV, SPIE, Strasbourg, France **2024**.

[55] L. Ma, Y. Liu, Y. Zhao, X. Sun, Y. Zhang, A. Yin, D. Cao, Q. Chen, G. Gao, *Appl. Surf. Sci.* **2023**, *616*, 156422.

[56] Q. Miao, X. Chi, S. Xiao, R. Zeis, M. Lefenfeld, T. Siegrist, M. L. Steigerwald, C. Nuckolls, *J. Am. Chem. Soc.* **2006**, *128*, 1340.

[57] F. He, H. Xu, B. Yang, Y. Duan, L. ? L. Tian, K. ? K. Huang, Y. ? G. Ma, S. ? Y. Liu, S. ? H. Feng, J. ? C. Shen, *Adv. Mater.* **2005**, *17*, 2710.

[58] J. Zhang, B. Xu, J. Chen, S. Ma, Y. Dong, L. Wang, B. Li, L. Ye, W. Tian, *Adv. Mater.* **2014**, *26*, 739.

[59] L. Xue, J. Zhang, Y. Han, *Prog. Polym. Sci.* **2012**, *37*, 564.

[60] S. Kouijzer, J. J. Michels, M. van den Berg, V. S. Gevaerts, M. Turbiez, M. M. Wienk, R. A. Janssen, *J. Am. Chem. Soc.* **2013**, *135*, 12057.

[61] A. Irfan, A. G. Al-Sehemi, A. R. Chaudhry, S. Muhammad, *J. King Saud Univ. – Sci.* **2018**, *30*, 75.

[62] Z.-H. Xing, J.-Y. Zhuang, X.-P. Xu, S.-J. Ji, W.-M. Su, Z. Cui, *Tetrahedron* **2017**, *73*, 2036.

[63] Y.-T. Lin, L.-Y. Chen, C.-C. Wu, K.-T. Wong, R.-T. Chen, Y.-Y. Chien, *Int. J. Opt. Photonics* **2008**, *2*, 45.

[64] H. El Housseiny, S. Fery-Forgues, M. Ternisien, D. Buso, G. Zissis, C. Renaud, *ACS Appl. Mater. Interfaces* **16**, 62456.

[65] V. Bulović, A. Shoustikov, M. A. Baldo, E. Bose, V. G. Kozlov, M. E. Thompson, S. R. Forrest, S. Bright, *Chem. Phys. Lett.* **1998**, *287*, 455.

[66] W. Li, W. Li, L. Gan, M. Li, N. Zheng, C. Ning, D. Chen, Y.-C. Wu, S.-J. Su, *ACS Appl. Mater. Interfaces* **2020**, *12*, 27171.

[67] T.-A. Lin, T. Chatterjee, W.-L. Tsai, W.-K. Lee, M.-J. Wu, M. Jiao, K.-C. Pan, C.-L. Yi, C.-L. Chung, K.-T. Wong, C.-C. Wu, *Adv. Mater.* **2016**, *28*, 6976.

[68] W. Li, B. Li, X. Cai, L. Gan, Z. Xu, W. Li, K. Liu, D. Chen, S.-J. Su, *Angew. Chem.* **2019**, *131*, 11423.

[69] M. Liu, R. Komatsu, X. Cai, K. Hotta, S. Sato, K. Liu, D. Chen, Y. Kato, H. Sasabe, S. Ohisa, Y. Suzuri, D. Yokoyama, S.-J. Su, J. Kido, *Chem. Mater.* **2017**, *29*, 8630.

[70] D. Yokoyama, *J. Mater. Chem.* **2011**, *21*, 19187.

[71] B. A. Naqvi, M. Schmid, E. Crovini, P. Sahay, T. Naujoks, F. Rodella, Z. Zhang, P. Strohriegl, S. Bräse, E. Zysman-Colman, W. Brütting, *Front. Chem.* **2020**, *8*, 750.

[72] D. Yokoyama, A. Sakaguchi, M. Suzuki, C. Adachi, *Org. Electron.* **2009**, *10*, 127.

[73] F. Tenopala-Carmona, O. S. Lee, E. Crovini, A. M. Neferu, C. Murawski, Y. Olivier, E. Zysman-Colman, M. C. Gather, *Adv. Mater.* **2021**, *33*, 2100677.

[74] Y. Chen, Q. Sun, Y. Dai, D. Yang, X. Qiao, D. Ma, *Adv. Opt. Mater.* **2019**, *7*, 1900703.
[75] I. W. Wu, P. S. Wang, W. H. Tseng, J. H. Chang, C. I. Wu, *Org. Electron.* **2012**, *13*, 13.

[76] S. Diesing, L. Zhang, E. Zysman-Colman, I. D. W. Samuel, *Nature* **2024**, *627*, 747.

[77] S. Jung, W.-L. Cheung, S.-J. Li, M. Wang, W. Li, C. Wang, X. Song, G. Wei, Q. Song, S. S. Chen, W. Cai, M. Ng, W. K. Tang, M.-C. Tang, *Nat. Commun.* **2023**, *14*, 6481.

[78] Y. Noguchi, Y. Miyazaki, Y. Tanaka, N. Sato, Y. Nakayama, T. D. Schmidt, W. Brütting, H. Ishii, *J. Appl. Phys.* **2012**, *111*, 114508.

[79] L. Jäger, T. D. Schmidt, W. Brütting, *AIP Adv.* **2016**, *6*, 095220.

[80] S. Altazin, S. Züfle, E. Knapp, C. Kirsch, T. D. Schmidt, L. Jäger, Y. Noguchi, W. Brütting, B. Ruhstaller, *Org. Electron.* **2016**, *39*, 244.

[81] D. Y. Kondakov, J. R. Sandifer, C. W. Tang, R. H. Young, *J. Appl. Phys.* **2003**, *93*, 1108.

[82] Z. B. Wang, M. G. Helander, Z. W. Liu, M. T. Greiner, J. Qiu, Z. H. Lu, *Appl. Phys. Lett.* **2010**, *96*, 043303.

[83] S. He, E. Pakhomenko, R. J. Holmes, *ACS Appl. Mater. Interfaces* **2023**, *15*, 1652.

[84] J. S. Bangsund, J. R. Van Sambeek, N. M. Concannon, *Sci. Adv.* **2020**, *6*, abb2659.

[85] R. H. Young, J. R. Lenhard, D. Y. Kondakov, T. K. Hatwar, *SID Symp. Dig. Tech. Pap.* **2008**, *39*, 705.

[86] T. Miyamae, N. Takada, H. Ohata, T. Tsutsui, *Appl. Phys. Express* **2017**, *10*, 102101.

[87] Y. Furukawa, K. Seto, K. Nakajima, Y. Itoh, J. Eguchi, T. Sugiyama, H. Fujimura, *Vib. Spectrosc.* **2012**, *60*, 5.

[88] S.-U.-Z. Khan, G. Londi, X. Liu, M. A. Fusella, G. D'Avino, L. Muccioli, A. N. Brigeman, B. Niesen, T. C.-J. Yang, Y. Olivier, J. T. Dull, N. C. Giebink, D. Beljonne, B. P. Rand, *Chem. Mater.* **2019**, *31*, 6808.

[89] A. P. Marchetti, K. E. Sassin, R. H. Young, L. J. Rothberg, D. Y. Kondakov, *J. Appl. Phys.* **2011**, *109*, 013709.

[90] H. Wang, K. P. Klubek, C. W. Tang, *Appl. Phys. Lett.* **2008**, *93*, 325.