Navigating the ZnO/metal phthalocyanine interface in OLEDs: challenges, perspectives, and engineering strategies

Abstract

The interface between inorganic charge injection/transport layers and organic functional layers is crucial for the performance and stability of organic light-emitting diodes (OLEDs), since this hybrid structure is the most strategic area in the operation of OLEDs. Zinc oxide (ZnO) as an efficient electron injection/transport layer and metal phthalocyanines (MPcs) as hole injection/transport and emission layers are individually well-established materials in OLED architectures; however, the direct integration and optimization of their interface suffer from a lack of consideration. This perspective addresses the potential challenges and opportunities associated with the ZnO/MPc interface in OLEDs. It analyzes potential issues such as work function (WF), interface morphology, chemical stability, exciton quenching, and charge trapping by seeking the related material systems and considering the electronic/structural properties of ZnO and MPcs. Additionally, we present a comprehensive view on the promising strategies for interface engineering for ZnO-based interfaces to enhance device performance, aiming to outline the potential device architectures leveraging the unique properties of the ZnO/MPc interface. Finally, we propose key future research directions to show some hidden potential of this material combination for OLED fabrication.

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1. Introduction

High-performance electroluminescence derived from organic light-emitting diodes (OLEDs) has revolutionized display and illumination technologies. The intrinsic advantages of OLEDs, such as vivid colors, wide display angles, potential for transparent and flexible devices, and high efficiency, have driven significant advancements in the optoelectronics field.^1–5 The operational characteristics of these advanced multilayer devices, e.g., turn-on voltage and stability, are fundamentally governed by a delicate interplay between the progressively applied inorganic and organic materials.^6–8 Accordingly, the interface formed at the inorganic–organic junctions critically dictates the performance of the OLED devices. In particular, the inorganic–organic interface governs crucial processes, for instance, the potential barrier, which in turn affects charge injection/transport and exciton management.⁸ Any mismatch or instability at this interface can lead to significant energy losses, low operational lifetime, and device degradation, pointing to the importance of meticulously engineering these inorganic–organic heterojunctions.^9–11

Among the explored inorganic electron-injecting and electron transporting (EIL/ETL) materials applicable to OLED architectures, zinc oxide (ZnO) has emerged as a promising candidate, since it has attractive behaviors, including high electron mobility, solution processability, and low cost due to its abundance.^12–14 ZnO has another unique property that makes it a promising candidate for the next generation of optoelectronic devices – the tunability of its electronic properties through doping^12,15 and surface modifications.¹⁶ For these reasons, ZnO has been successfully introduced into various OLED architectures, as shown in Fig. 1a and b, demonstrating its potential to facilitate efficient electron injection and transport.^17–19

Fig. 1 Schematic view of the energy band diagram of two reported inverted OLEDs in which a ZnO thin layer has been introduced as the electron injecting/transporting layer (EIL/ETL). Figures recreated from the literature: (a) ref. 18 and (b) ref. 20.

Metal phthalocyanines (MPcs), which represent symmetric organic semiconductors comprising of a macrocyclic structure, complement the electron transporting properties of ZnO.²¹ The process of replacing the central metal ion coordinated by four ligands in the MPc macrocyclic structure by substituent ions, using the Amsterdam Modelling Suit (AMS) and as presented in Fig. 2a–f, allows for tailoring of the highest occupied molecular orbital (HOMO level) and the lowest unoccupied molecular orbital (LUMO level), charge transport (hole mobility), and optical (absorption and emission spectra) properties.^22,23 This substitution of the metallic coordinated ion which alters the chemical structure of the organic molecule, in particular in emitter compound cases, plays a crucial role in determining the optical properties of the OLEDs including color purity, affecting the efficiency and operational stability of the device.²⁴ This demonstrates the critical relationship between the chemical structure of an emitter and its resulting optoelectronic performance, a key consideration for device design. As an example, the tunability and control over the electronic and optical properties make MPcs suitable for diverse applications in OLEDs, including hole transporting/injecting layers (HTLs/HILs)^25–29 and emissive layers (EMLs).^30–32 Thus, they often form crucial interfaces with other organic or inorganic layers within the sandwiched structure of the OLEDs.

Fig. 2 Schematic view of (a) H₂Pc or base phthalocyanine, (b) CoPc, (c) CoPcF₁₆, (d) MgPc, (e) CuPc, and (f) CuPcF₁₆ molecules extracted from the Amsterdam Modelling Suite (AMS) software.

Due to the individual success and particular functionalities of ZnO^8,19,33 and MPcs^26–29 in OLEDs, a systematic study of the direct ZnO/MPc interface has significant importance. Gains from merging the materials can be considerable. For instance, combining the efficient electron transport of ZnO with the tailored hole transport or emissive properties of MPcs could lead to high-performance devices with simplified architectures. In addition, exploring the basic interactions at the ZnO/MPc interface can reveal novel strategies for optimizing charge injection/transport, enhancing device stability, and potentially enabling new device functionalities. Despite the individual advantages of these materials, a comprehensive understanding of the interfacial phenomena, potential challenges arising from their integration, and effective engineering strategies for the ZnO/MPc interface remains relatively unknown. We propose a new paradigm that shifts the focus from optimizing the OLED structure by introducing individual ZnO and MPc layers within the device structure to meticulously engineering the synergistic interactions at the ZnO/MPc interface, unlocking a new level of device optimization. Therefore, this perspective aims to propose some rational potential solutions for this knowledge gap by considering the fundamental properties of ZnO and MPcs concerning their interface formation and analyzing potential challenges such as WF, potential barrier, interface morphology, chemical stability, and exciton quenching. Here, we will also discuss promising interface engineering strategies and potential device architectures to benefit from the unique properties of the ZnO/MPc interface. Finally, we will highlight key future research directions that hold the potential to reveal the full potential of the ZnO/MPc hybrid structure for next-generation OLED technology. This perspective will focus specifically on the electronic properties and chemical interactions at the direct ZnO/MPc interface. Broader topics, such as the bulk properties of the transport layers, optical outcoupling effects, or technical issues including hybrid layer deposition and appropriate techniques for device fabrication, are considered outside the scope of this work.

2. Fundamental properties of zinc oxide and metal phthalocyanines

2.1. Zinc oxide (ZnO)

At the nanoscale, metal oxide semiconductor materials are well recognized for their impressive applications in optoelectronic devices such as photodetectors, light-emitting diodes (LEDs), and solar cells.³⁴ Particularly, ZnO is a critical material to be involved in inverted OLED architectures due to its significant advantages in electronic, optical, and structural properties.^35–38 In the following subsection, these properties will be presented in detail.

2.1.1. Remarkable properties.
2.1.1.1. Electronic properties. ZnO possesses a direct band gap of 3.4 eV, notable charge transport properties (∼150 cm² V⁻¹ s⁻¹), and a favorable energy level alignment resulting from its potential for trap inducing and band gap engineering.^13,16,38,39 Most importantly, it has been considered a promising candidate to be replaced with gallium nitride (GaN) in optoelectronic devices due to its significantly large exciton binding energy of 60 meV at room temperature (RT).⁴⁰ While both materials have a similar band gap, the larger exciton binding energy of ZnO (60 meV) compared to that of GaN (25 meV) makes it an attractive candidate for OLED applications due to the possibility of exciton recombination at RT and above.³⁶
2.1.1.2. Optical properties. The high transparency of ZnO in the visible range of light, due to the intrinsic defects of lower energy states,^41–43 is a crucial characteristic for applications such as transparent conductive electrodes in OLEDs and solar cells. Moreover, possessing a wide band gap makes ZnO a strong UV absorption material, which is appropriate for UV photodetectors and UV protective layers.³⁶ Also, its efficient near-band-edge emission in the UV or near-UV region is an essential property to fabricate UV LEDs and lasers.⁴⁴ Finally, the appropriate refractive index of ZnO, 2.0041,⁴⁰ allows for efficient light extraction and management in optoelectronic devices.
2.1.1.3. Structural properties. Driven by the oxygen's high ionization energy, a bonding forms between the O 2p orbital and the Zn 3d orbital to form a hexagonal wurtzite and cubic (zinc blende and rock salt) lattice, depending on the pressure and temperature parameters.^45–47 The lattice parameters, at 300 K, for the hexagonal structure of ZnO include a = b = 0.32495 nm and c = 0.52069 nm.⁴⁰ A schematic representation of the stoichiometric and non-stoichiometric ZnO surface ([0001]) with a wurtzite structure generated using the AMS software is presented in Fig. 3a and b.

Fig. 3 Schematic view of the (a) stoichiometric and (b) non-stoichiometric ZnO surface, and their respective valence bands (VB) in (c) and (d), all calculated using AMS software. (e) Experimental VB of a ZnO layer deposited by plasma-assisted molecular beam epitaxy (substrate temperature during the deposition was 190 °C). The inset represents the electronic band structure (inspired by Fig. 2 in ref. 36). (f) O 1s spectrum for the as-received ZnO layer. The observed O_vacs at the spectrum can be attributed to the imperfection sites on the ZnO surface such as dangling bonds. The inset illustrates O_vac levels and luminescence color associated with the energy level (inspired by Fig. 3 and 4 in ref. 48).

2.1.1.4. Defect tolerance. ZnO maintains functional electronic properties despite the presence of atomic-level surface defects or non-stoichiometry, e.g. zinc interstitials (Zn_i) or oxygen vacancies (O_vac).^16,49 While in conventional semiconductors, defects often create detrimental mid-gap states that trap charges,⁵⁰ these native defects in ZnO primarily introduce shallow energy levels near the valence band (VB) edge.¹⁶ O_vacs have been often mentioned as the accepted mechanism for the visible emission of ZnO with three different charge states:⁴⁸ doubly ionized O_vac (O⁺⁺_vac), singly ionized O_vac (O⁺_vac), and neutral O_vac (O^x_vac), as illustrated in the inset of Fig. 3f. Fig. 3e and f show our observation of the native O_vacs at the as-received ZnO surface derived from He I (hν = 21.22 eV) ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), respectively (obtained at the Electron Spectroscopy for Functional Materials Laboratory (ESpeFuM), at the Silesian University of Technology). It is noticeable that before providing the UPS and XPS VB results, the cleanness of the ZnO layer was checked by XPS. To examine the presence of the defect-induced energy levels in the ZnO band gap, we provided both stoichiometric and non-stoichiometric ZnO VBs employing DFT based quantum calculation using AMS software. By comparing Fig. 3c and d, the presence of O_vacs near the edge of the VB of the ZnO substrate became clear. To do so, first, we optimized the geometries of both stoichiometric and non-stoichiometric ZnO structures by applying the generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional with Grimme 3 dispersion correction, spin–orbit coupling for relativistic effects, Slater type orbital (STO) basis set, type Triple Zeta, 1 polarization function (TZP) with a small frozen core and an automatic k-space grid for good numerical quality. Then, DOS spectra were calculated using optimized geometries as an input for which single point calculations were performed with the TZ2P basis set. The calculations were performed without the frozen core and for k-space sampling within very good numerical accuracy. These observed shallow trap states present two benefits for device performance. First, the O_vacs act as electron donors boosting intrinsic carrier concentration without requiring extrinsic doping and resulting in enhancing n-type conductivity.⁴⁰ Second, shallow defect states which usually are placed near the edge of the valence band allow the trapped electrons to escape thermally, reducing losses from charge trapping and minimizing trap-assisted recombination.⁴⁰ Thus, the defect tolerance behavior is crucial for ZnO to act efficiently as the ETL within the OLED structure, where conventional organic semiconductors degrade under similar imperfections.^51,52 However, there should be a limit for defect concentration since excessive defects can lead to exciton quenching,⁵³ which makes controlled synthesis essential (e.g., sol–gel methods with annealing¹⁶) to optimize defect density.
2.1.1.5. Surface properties. ZnO naturally provides a smooth and defect-tolerant surface morphology, resulting in uniform thin-film formation and intimate interfacial contact with adjacent organic layers.¹⁷ This minimizes charge trapping and recombination losses. The ZnO hydrophilic surface character enables compatibility with solution processing, while post-deposition treatments (e.g., UV-ozone, plasma treatment) can further tune WF and oxygen vacancy concentrations to optimize electron injection.^17,54 Most importantly, compared to organic ETLs, the ZnO surface exhibits low densities of deep-level traps, leading to reduced non-radiative recombination and enhancing device efficiency.^55,56 The material's inherent chemical stability also suppresses interfacial degradation during device operation, contributing to improved operational lifetime.⁵⁷
2.1.1.6. Low-temperature processability. ZnO can be processed by applying low-temperature solution-based methods such as sol–gel¹⁶ and nanoparticle dispersion,^33,58 as well as sputtering technology,⁵⁹ suitable for temperature-sensitive organic layers and flexible substrates. These depositing advantages make it particularly attractive for developing high-performance flexible OLED displays.

2.1.2. Role of ZnO as the EIL and ETL in OLEDs. In OLEDs, ZnO plays the role of both EIL and ETL, offering distinct advantages compared to the organic alternatives due to its high electron mobility, tunable WF (∼3.6–4.2 eV), and remarkable stability.^36,60 Conventional OLED structures mainly rely on organic ETLs with limited electron mobility, typically lower than 10⁻⁵–1 cm² V⁻¹ s⁻¹.^35,61 However, ZnO is a metal oxide with an electron mobility ranging from 5 to 150 cm² V⁻¹ s⁻¹,^35,36 which offers remarkable electron transport characteristics, enabling ZnO to play the role of an ETL as well.⁸ This can significantly enhance the charge balance and reduce the turn-on voltage in OLEDs.

ZnO as an EIL has an interface with common electrodes, such as Al and Ag, and reduces the electron injection barrier by forming an ohmic or near-ohmic contact.⁸ For example, for the Al/Bphen interface, ZnO with a WF of ∼4.0 eV (after mild UV-ozone treatment) bridges the gap between Al (WF ∼ 4.3 eV) and the LUMO of the Bphen (3.0 eV), a common ETL, minimizing voltage losses. Additionally, ZnO/organic interlayers, e.g. blended ZnO/polyethylenimine (PEI), can further decrease the WF to ∼3.7 eV, enhancing electron injection into emissive polymers like Super Yellow.⁶²

In an ETL role, ZnO transports electrons while blocking holes, improving charge balance in the EML. For phosphorescent OLEDs, ZnO's conduction band (∼4.1 eV) aligns well with the LUMO of hosts such as CBP (2.9 eV) or TPBi (2.7 eV), ensuring smooth electron transfer.^34,62,63 In OLED structures, where ZnO acts as both EIL and ETL at the same time, a ZnO/EML interface is possible. In solution-based OLEDs, ZnO nanowires or sol–gel films interface with PFN to reduce exciton quenching at the inorganic–organic junction.^64–66

A schematic view of conventional and inverted structures is presented in Fig. 4a and b. In contrast to the stack of conventional OLEDs, where the anode is placed on the substrate (bottom of the structure), in inverted OLEDs, the cathode is positioned at the bottom of the stack, mainly to prevent the interaction of the low WF cathode with the ambient atmosphere. The inverted OLED fabrication technique is especially useful in the manufacturing of low-cost active matrix OLED displays (AMOLEDs) where the inverted OLEDs are connected to n-type thin film transistors, so-called n-channel TFTs.^67,68 The relatively low WF of ZnO (4.05 eV; as presented in Fig. 4c) facilitates effective electron injection from common cathode materials such as aluminum (Al; WF = 4.3 eV⁸) and silver (Ag; WF = 4.6 eV⁶⁹).

Fig. 4 Schematic view of the organic light emitting diode (OLED) structure for (a) conventional and (b) inverted OLEDs. (c) Work function of zinc oxide (ZnO) obtained by He I (hν = 21.22 eV) UPS at ESpeFuM laboratory.

2.1.3. Common modification techniques. Several common modification methods are applied to optimize the electronic properties, interfacial compatibility, and stability of the ZnO layer. Doping is a widely used method to enhance conductivity and tune WFs.⁷⁰ Elements such as aluminum, gallium, or magnesium have been commonly used as dopants in the ZnO lattice. The results revealed that Al and Ga doping increases electron concentration and narrows the band gap; conversely, Mg doping widens it. This band gap engineering leads to energy-level alignment tuned toward particular application.

Surface treatments such as UV-ozone exposure, oxygen plasma, argon plasma, or argon ion sputtering can modify surface stoichiometry by controlling oxygen vacancies and regulating electron injection barriers.⁷¹ Organic ligand passivation, for instance with ethanolamine or thiols, decreases the surface defects and improves adhesion to adjacent organic layers. Additionally, nanostructuring controls morphology and results in improved charge transport.^72,73

Moreover, the intrinsic electrical properties of ZnO can be controlled during synthesis, impacting its performance at organic interfaces.¹⁶ As an example, the resistivity of the electrodeposited ZnO layers can be adjusted by controlling growth parameters, such as cathodic current density. Studies on hybrid ZnO/H₂Pc photovoltaic devices revealed that preparing ZnO layers with different resistivity ranging from 1.8 × 10³ to 1 × 10⁸ Ω cm influences the electrical characteristics of the rectification feature, which additionally is a powerful approach for tailoring the electronic properties such as WF and charge transport.⁷⁴

The combined ZnO attributes, including morphological uniformity, tunability of its electronic properties with a wide range of modification techniques, low-temperature solution-processability, and role versatility (EIL and ETL), make ZnO an ideal charge-transporting interface for high-performance inverted OLEDs, particularly in demanding applications such as flexible and transparent displays.

2.2. Metal phthalocyanine (MPc)

In the last few decades, organic materials have received huge attention for the fabrication of organic optoelectronic devices. In contrast to their inorganic counterparts, organic semiconductors present attractive properties such as environmental friendliness, unlimited availability, cost-effectiveness, chemical stability, tunability, and possibility for large-scale device fabrication.^{25,28,75–78} Among the organic semiconductors, phthalocyanine (Pc) exhibits promising photovoltaic and photoconductive responses, and thus is attractive for fabricating optoelectronic devices such as solar cells, light-emitting diodes, and transistors.⁷⁹

2.2.1. MPc's molecular structure. Pc ((C₈H₄N₂)₄H₂) is a large, symmetric, organic 2D compound consisting of four isoindoles (C₈H₄N₂) connected through nitrogen atoms.^79–81 Pcs can host various single or compound metal atoms to produce metallophthalocyanines (MPcs, M stands for metallic elements such as Mg, Mn, Fe, Cu, Pb, Co, Zn, Ni, and Ag). Due to the symmetrical structure of the MPcs, they can form an organic thin layer or even a mono-layer semiconductor of high quality, giving them the chance to be considered as a promising candidate for a vast range of applications, ranging from catalyzers to organic optoelectronics devices.⁸² MPcs can be employed in different key roles of an optoelectronic device architecture such as the hole injecting/transporting layer (HIL/HTL), electron blocking layer (EBL)^{20,35,82–84} or EML^30,31,85 due to the variety of their energy band gaps. A schematic view of the MPc’s band diagram, applying different measurement techniques, is presented in Fig. 5a and b. Fig. 5a shows the HOMO/LUMO energy lines of MPcs obtained through various experimental setups and with different MPc layer thicknesses, primarily greater than 20 nm. These values are sourced from Table 1 in this perspective. Fig. 5b displays the HOMO/LUMO energy lines of MPcs gathered using the same UV-visible spectrophotometer setup, with a consistent MPc layer thickness of 30 nm, as derived from Table 1 in ref. 86.

Fig. 5 HOMO/LUMO energy level diagram of MPcs with different method applicability. (a) Literature-derived energy levels (Table 1) from different experimental setups and different thicknesses, mainly >20 nm. The exact thickness value is reported in Table 1. (b) Energy levels derived from Table 1 in ref. 86, all collected with the same UV-visible spectrophotometer setup and the same thicknesses (30 nm).

Table 1 The HOMO/LUMO energy levels of ZnO and MPcs applied in different OLED architectures

No.	Phthalocyanine or ZnO	Conductivity type	HOMO	LUMO	Role	OLED type	Device structure	Ref.
1	H2Pc (100 nm)	p-Type	—	—	EML (red emitter)	Conventional	ITO/H2Pc/In–Mg alloy	30
2	ZnPc (15 nm)	—	5.0	3.4	HIL	Conventional	ITO/CuPc/NPB/Alq3/Al	29
3	CuPc (15 nm)	—	5.2	3.6	HIL	Conventional	ITO/CuPc/NPB/Alq3/Al	29
4	MgPc (15 nm)	—	5.4	3.9	HIL	Conventional	ITO/CuPc/NPB/Alq3/Al	29
5	VOPc (100 nm)	p-Type	5.4	3.6	—	p–n diode	ITO/ZnO/VOPc/MoO3/Al	35
6	VOPc (20 nm)	—	5.4	3.9	HTL	Conventional	ITO/VOPc/TPD/Alq3/Mg–Ag	88
7	F10-SiPc (30 nm)	—	5.7	3.8	Dopant (red emitter)	Conventional	ITO/PEDOT:PSS/F8+F10-SiPc/TPBi/Ca/Ag	31
8	(3MP)2-SiPc (30 nm)	—	5.5	3.6	Dopant (red emitter)	Conventional	ITO/PEDOT:PSS/F8+(3MP)2-SiPc/TPBi/Ca/Ag	31
9	FePc (30 nm)	P-Type	5.0	3.34	HTL	Conventional	ITO/FePc/NPB/Alq3/Lif/Al	86
10	NiPc (20 nm)	p-Type	5.3	3.6	HTL	Conventional	ITO/NiPc/TPD/Alq3/Mg–Ag	88
11	CoPc (20 nm)	p-Type	5.0	3.4	HTL	Conventional	ITO/CoPc/TPD/Alq3/MG-Ag	88
12	CuPc (10 nm)	p-Type	5.3	—	HIL	Conventional	ITO/Ts-CuPc:F4-TCNQ/TCTA/TCTA:Flrpic/TmPyPB/Liq/Al	28
13	CuPc (35–40 nm)	p-Type	5.4	3.3	HTL	Conventional	ITO/CuPc/CBP/TPBi/LiF/Al	27
14	CuPc (35–40 nm)	—	5.4	3.3	HIL/HTL	Conventional	ITO/CuPc/CBP:4CzIPN/TPBi/LiF/Al	27
15	CoPcF16 (30 nm)	n-Type	6.1	4.5	—	—	—	89
16	ZnPcF4 (12 nm)	n-Type	5.46	4.0	—	—	—	90
17	ZnPc (60 nm)		4.9	3.4	EML	Conventional	ITO/PEDOT:PSS/ZnPc/Ca/Al	85
18	ZnO (30 nm)	n-Type	7.4	4	EIL	Inverted	ITO/ZnO/Pd3O8-Py5/TrisPCz/NPD/HATCN/AL	20
19	ZnO (— nm)	n-Type	—	—	Buffer layer	Inverted	FTO/ZnO+dopants/2-ME+EA/F8BT/MoO3/Au	19
20	ZnO (200 nm)	n-Type	7.5	4.2	—	Electron only	ITO/ZnO/Al	35
21	H2Pc (20 nm)	p-Type	5.9	—	HTL	Conventional	ITO/H2Pc/TPD/Alq3/Mg–Ag	88
22	SnPc (20 nm)	p-Type	5.7	4.2	HTL		ITO/SnPc/TPD/Alq3/Mg–Ag	88
23	PbPc (20 nm)	p-Type	5.2	3.9	HTL	Conventional	ITO/PbPc/TPD/Alq3/Mg–Ag	88
24	FePc (20 nm)	p-Type	5	—	HTL	Conventional	ITO/FePc/TPD/Alq3/Mg–Ag	88
25	ZnPc (20 nm)	p-Type	5.0	3.4	HTL	Conventional	ITO/ZnPc/TPD/Alq3/Mg–Ag	88

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2.2.2. Electronic energy levels (HOMO/LUMO). The electronic energy levels of MPcs, specifically the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), are critical in determining the charge injection/transport properties of MPc molecular layers. For different commonly applied MPcs, the energy levels vary with the central metal ion, added functional groups, and fluorination.^83,87 For instance, the HOMO and LUMO levels of different MPc molecules applied in OLEDs are presented in Fig. 5 and Table 1. These distinct energy levels of the molecules play a key role in establishing the potential energy barriers at the interfaces with the adjacent layers, which affect the efficient electron/hole injection and transport in OLEDs.

2.2.3. Optical properties. Strong absorption in the visible and near infrared regions of the electromagnetic spectrum, coupled with high molar extinction coefficients, is the key optical property of the phthalocyanines that makes them highly attractive for OLED and light sensing applications.⁹¹ Beyond their remarkable absorption characteristic, they exhibit tunable emission properties. It is possible to control their emission wavelength by modifying the central metal atom (e.g. CuPc, CoPc, ZnPc) or by introducing peripheral substituents to the phthalocyanine ring.^27,92 For example, alkyl chains (hydrocarbon chains) with the primary role of enhancing solubility of the Pcs in common organic solvents,³¹ or halogen atoms such as fluorine and chlorine which are highly electronegative can lead to a shift in the absorption and emission spectra and modify the energy levels particularly ionization energy,^22,93 allowing for emission of various colors.

Many Pc derivations boast high photoluminescence quantum yields, ensuring that a portion of absorbed energy is converted into emitted light, thus enhancing device efficiency. For instance, Zn and Al are diamagnetic metals with no unpaired electrons, facilitating efficient intersystem crossing from the excited singlet state to the triplet state, minimizing non-radiative pathways that would otherwise quench the fluorescence.^94–97 Unsubstituted ZnPc in different solvents (such as DMSO, DMF, or THF) shows fluorescence quantum yields (FQY) in the range of 0.20 to 0.25.⁹⁸ Additionally, its derivation that can prevent aggregation can maintain or slightly improve the FQY value.⁹⁸ Moreover, AlClPc and AlPcS are known for their high quantum yields, which depend on the solvent and specific structural modification. These yields have been reported in the range of 0.50 to 0.60 or even higher.^97,99 Among the MPcs, SiPc is notable because the Si atom can accommodate two axial ligands, particularly effective in preventing π–π aggregation of planar phthalocyanine molecules in solution or film which is a major cause of fluorescence quenching. For various axially substituted SiPcs, depending on the axial ligand and solvent, FQY has been reported ranging from 0.2 to 0.35.^100,101 For specific bis[2-(2-pyridyl)ethoxy]-SiPc derivatives, the FQY value around 0.31 has been reported.¹⁰⁰

2.2.4. Charge transport characterization and properties. The rapid growth of interest in organic electronics is attributable to π-conjugated materials, where the interplay between the π-electronic structure and the geometric structure facilitates crucial properties for applications like flexible large-area OLEDs and OSCs.82 These studies [ref. ⁸²] show that the performance of these devices depends on the efficiency of the charge carrier transport within the π-electronic cloud. Over the years, organic semiconductor materials have been designed and synthesized to preferentially transport electrons or holes. However, in most cases, the actual charge transport properties of the materials, e.g. charge mobility, reflect the charge injection properties of the metal or metal oxide (MO) electrodes in the device architectures.⁸² From this perspective, semiconductor materials can be divided into three different categories: (1) hole transporters in which the ionization energy closely matches the Fermi level of the electrode; (2) electron transporters in which the electron affinity is close to the Fermi level; and (3) ambipolar transporters which are able to transfer both electrons and holes.

Charge mobility, μ, of the thin layer is the main factor for characterizing charge transport and is expressed in cm² V⁻¹ s⁻¹, corresponding to velocity over the electric field. Diffusion is the dominant charge transport process in the absence of an external potential representing a local charge displacement around an average position, as described by the diffusion equation:

where e, D, k_B, and T are, respectively, the electron charge, diffusion coefficient, Boltzmann constant, and temperature.

Applying the external electric field, as an external potential, induces charge carrier drift by the displacement of the charge's average position, resulting in charge migration across the thin organic layer. Then, the charge mobility is defined by the following equation:

where v and F are the charge velocity and applied electric field, respectively.

It is also possible to determine the charge mobility through experimental methods, allowing measurement of the mobility over macroscopic and microscopic distances. The widely applied techniques include time-of-flight (TOF), diode configuration, field-effect transistor configuration (FET), and pulse-radiolysis time-resolved microwave conductivity (PR-TRMC). Here, we will explain TOF and diode configuration, to narrow our perspective on the topic of this paper, and the last two techniques are briefly summarized in Table 2.

Table 2 Additional experimental techniques to determine the charge mobility in organic semiconductors

Method's name		Approach	Equation	Parameters	Ref.
Field-effect transistor configuration	FET	Processing speed of field-effect transistors by utilizing field-effect mobility (μFET)	In a linear regime:	I SD: current between source and drain	102 and 104
				V SD: voltage bias between source and drain
			In a saturated regime:	V G: gate voltage
				V D: drain voltage
				V T: threshold voltage
				C: capacitance
				W: width of the channel
				L: length of the channel
Pulse-radiolysis time-resolved microwave conductivity	PR-TRMC	Exciting the sample with high-energy electrons (MeV), creating a low density of free carriers, and measuring the change in electrical conductivity	Δσ = e∑μNe–h	σ: electrical conductivity	102 and 105
				∑μ: sum of hole and electron mobilities
				N e–h: electron–hole pairs

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2.2.4.1. Time-of-flight. This method is used to measure the charge carrier mobility in organic materials applied in electronic devices where the organic layer is sandwiched between two electrodes, such as in OLEDs and OSCs. This technique measures the time (t) it takes for an electron/hole to travel across an organic layer thickness (d) under the influence of an electric field, so-called transit time. A short laser pulse with energy above the organic material band gap is shone through the transparent electrode and absorbed by the organic layer to produce photoelectrons and photoholes. At the same time, a uniform electric field (E) is generated by applying a DC voltage (V). Depending on the polarity of the applied V and corresponding E, the generated photoelectrons and photoholes start to drift across the organic layer toward the opposite electrode. The induced current in the external circuit is measured as a function of time, current transient, utilizing a fast oscilloscope. The transient shape can offer indirect insight into the ordering and structural characteristics of the organic layer. For the ordered material, the signal is sharp; in contrast, a disordered layer produces a broad signal due to the distribution of drift velocities. In this method, the drift mobility of the charge carriers is calculated by eqn (3), derived from eqn (2):It is important to highlight that this method allows us to investigate the impact of the available defects within the organic layer. Moreover, it has been the first method for charge mobility measurement.¹⁰²
2.2.4.2. Diode configuration. Electron and hole mobility in organic semiconductors can be determined by the current–voltage characteristics of a simple diode structure. In this technique, the organic layer must be sandwiched between two electrodes to primarily inject either electrons or holes at low voltage. Under a trap-free and low electric field, the current density (J) scales quadratically with voltage (V²), which is a characteristic of space charge limited current (SCLC), where injected charge accumulates near the injecting electrode and limits further current. However, the presence of traps within the organic semiconductor layer complicates this behavior. In the presence of traps, J–V curves initially show an injection-limited linear regime, followed by a sharp current increase, before eventually transitioning to a V² dependence resembling trap-free SCLC at higher biases. The specifics of this intermediate region provide information about the spatial and energetic distribution of charge traps within the organic semiconductor layer.^102,103

In experimental research devoted to fabricating OLEDs, where MPcs are employed as hole injection layers (HILs),^30,85,86,88 the performance of OLEDs is noticeably tied to the HOMO level of the MPc HIL, since it dictates the potential energy barrier for hole injection. For instance, in an Alq₃-based OLED with ITO/NPD(45 nm)/Alq₃ (80 nm)/LiF (0.5 nm)/Al architecture, inserting a single layer of ZnPc, MgPc, or CuPc as an alternative HIL resulted in a lower potential energy barrier for hole injection from ITO to the organic structure, leading to improved current efficiency and decreased turn-on voltage.²⁹ Furthermore, double HIL structures incorporating MPcs have been explored to fine-tune hole injection and transport. The results showed that a specific combination, CuPc/ZnPc double HIL, led to improved luminance and electron–hole balance due to hole confinement.²⁹ Doping MPcs as p-type dopants, e.g. TS-CuPc with F4-TCNQ through an aqueous process, in the HIL layer noticeably affected the hole mobility and facilitated efficient carrier injection in an OLED with the ITO/HIL/TCTA/TCTA:Flrpic/TmPyPB/Liq/Al structure.²⁸

2.2.5. MPc applications in OLEDs (HTL/HIL/emissive layer). MPcs have been utilized as the hole transport layer (HTL), HIL, and emissive layer (EL), mainly due to their tunable energy band gap^{26,28,29,32,74,106} in OLEDs. As the HIL, they could significantly improve device performance in green Alq₃-based OLEDs, leading to a 22% increase in current efficiency and a 67% increase in power efficiency compared to the reference device.²⁹ This strategic placement of MPc layers further underscores their importance in facilitating efficient charge injection/transport at critical interfaces within OLEDs. More advanced approaches have recently shown that solution-processed nanocrystalline CuPc can be applied as an effective injection/transport layer, a promising alternative to conventional HILs such as PEDOT:PSS, yet enabling the fabrication of highly efficient OLEDs.²⁷ The replacement resulted in an increase in EQE_max from 13.0% to 16.2% in OLEDs, fabricated based on a thermally activated delayed fluorescence (TADF) emitter, 4CzIPN.²⁷ This advancement in HILs may increase the performance of next-generation emissive materials, including those exhibiting TADF and aggregation-induced emission (AIE) that are often processed from solution for deep-red and NIR emission.¹⁰⁷

While the electron-deficient nature of some MPcs (such as CuPcF₁₆, ZnPcF₁₆, and axially substituted SiPcs) makes them attractive n-type candidates,^108–110 they have not been widely used as electron injection/transport layers in OLEDs due to their critical performance limitations. The most common strategy to induce n-type character is through peripheral fluorination leading to materials like CuPcF₁₆ and ZnPcF₁₆, as well as novel axially substituted SiPcs. However, fluorination lowers the LUMO energy level to enable electron injection, but their overall performance is often insufficient. A key bottleneck is their low electron mobility, which is significantly lower than the hole mobility of their p-type counterparts, thereby creating an imbalanced charge flow and limiting overall device efficiency.¹¹⁰ For instance, a dibenzoate-substituted SiPc showed a field-effect electron mobility of roughly 6 × 10⁻⁴ cm² V⁻¹ s⁻¹ in a vacuum, which improved to over 0.01 cm² V⁻¹ s⁻¹ with further optimization.¹⁰⁸ Furthermore, a major challenge is obtaining high-quality thin films of n-type MPc materials since they have a tendency to grow in island mode on MO substrates, as shown in Fig. 7b and c. These issues prevent their widespread use despite potential.

3. Potential challenges and considerations for the ZnO/MPc interface

As is the case with all inorganic–organic heterostructures applied within OLEDs, the possible primary challenges, such as WF mismatch (charge injection barriers), lattice mismatch, interface morphology (particularly surface roughness), chemical stability, and exciton quenching, could potentially be observed at the ZnO/MPc interfaces. Precise control over these parameters can result in a well-optimized ZnO/MPc interface and, consequently, efficient OLEDs with low turn-on voltages. Particularly, energy level alignment between ZnO and MPc layers dictates the potential barriers that carriers must overcome.

An additional challenge is the lack of n-type MPc materials for the electron-injecting or electron-transporting layer (EIL and ETL). While MPcs, especially CuPc (as can be observed in Table 1), are very well-known as the HIL and HTL in OLEDs due to their appropriate HOMO levels and hole mobility, their application as the EIL and ETL is less common. EILs/ETLs require materials with low LUMO energy levels to facilitate electron injection from the cathode into the EML and to block holes (HBL) from reaching the cathode to avoid leakage current. To address this, MPcs can be engineered to possess a lower LUMO through functionalization, such as with strong electron-withdrawing groups, e.g. substitution of hydrogen atoms by fluorine atoms. As a result, the p-type CoPc molecule can be converted to n-type CoPcF₁₆^89,111 through such modification. Derivatives of SiPc have also been explored for n-type or electron transport behavior in organic thin film transistors (OTFTs) with a sufficient electron mobility of 7.2 × 10⁻² cm² V⁻¹ s⁻¹.¹¹² Since OTFT is a related research field, this material can be further investigated for its electron mobility and stability as an ETL in OLED applications.

3.1. Work function mismatch and charge injection barriers

Similar to any other inorganic–organic heterojunction in OLEDs, ZnO/MPc interfaces encounter a WF mismatch, impacting charge injection in a device, which is evident by analyzing similar systems.²⁸ Thus, there is a need for precise management of WF and the resulting charge injection barriers. In conventional indium tin oxide (ITO)-based OLEDs, the ITO/ZnPc interface exhibited a lower energy barrier (0.3 eV) compared to the ITO/CuPc interface (0.5 eV), which is correlated with more efficient hole injection and a beneficial shift in J–V characteristic toward lower voltages.²⁹ In contrast, the ITO/MgPc interface offered a larger potential energy barrier and higher turn-on voltage. Achieving balanced charge injection and efficient exciton confinement within the emission layer prevents recombination zone shift and ensures long-term spectral stability, as observed in high-performance deep-red OLEDs utilizing zinc complexes as host materials.¹¹³

It is also possible to lower the WF mismatch in the ZnO/organic (e.g. ZnO/MPc) structures by manipulating the properties of the ZnO layer and doping strategies for MPcs (e.g. p-type doping of CuPc²⁸). For instance, in a ZnO/phthalocyanine-based photovoltaic device, tailoring the resistivity of ZnO can impact the rectification properties of the device, confirming control over the potential barrier at the interface.¹⁰⁶

These findings in the literature underscore the strategic need for WF tuning of ZnO and careful selection of MPcs to regulate the potential barrier, aiming to achieve optimal electron–hole balance within the emissive layer and gain maximum possible efficiency. Our experimental results for WF measurements of the ZnO/CuPc, ZnO/MgPc, ZnO/CoPc, and ZnO/CoPcF₁₆ hybrid structure can further elaborate on interface energetics and contribute to the development of optimized energy level alignment (Fig. 6a and b). The results show a significant increase in WF for fluorinated CoPc compared to the non-fluorinated MPc molecules. Additionally, the observed higher background in the ZnO/CoPcF₁₆ UPS spectrum (Fig. 6a) is attributed to increased inelastic scattering. The increased inelastic scattering broadens the kinetic energy distribution of the emitted electrons, contributing to higher background. Moreover, different film morphology (molecular packing) caused by fluorination, as shown in Fig. 7a and b for non-fluorinated and fluorinated CoPc, on ZnO, also contributes to the increased background. The rougher surface of CoPcF₁₆ provides more opportunities for electrons to scatter, further elevating the background signal.

Fig. 6 (a) Secondary electron cutoff position (WF) for ZnO, ZnO/MgPc, ZnO/CuPc, ZnO/CoPc, and ZnO/CoPcF₁₆ systems obtained from He I (hν = 21.22 eV) UPS results. (b) WF values for ZnO and ZnO/MPc hybrid systems with an error bar value of 0.05 eV. The thickness of the MPc organic overlayer in all the hybrid systems is ∼9 nm.

Fig. 7 (a) AFM image of the ZnO/CoPc hybrid layer. AFM images of the CoPcF₁₆ molecule on (b) ZnO and (c) SiO₂ substrates, indicating the influence of the substrate on film growth. Comparison of panels (a) and (b) illustrates the effect of fluorination on molecular growth. Panel (c) is reprinted from ref. 89, copyright 2025, under the CCBY license.

3.2. Interface morphology and contact area

In the context of composites and hybrids, the morphology of the ZnO layer is crucial for achieving a functional and efficient interface with the organic material. Hence, it affects the orientation and organization of the MPc molecules on the ZnO surface, which is one of the most important factors in the charge injection process. In this context, studies have shown a direct correlation between ZnO surface roughness and device efficiency. For instance, decreasing the RMS roughness of the ZnO layer from 48 nm to 1.9 nm resulted in an increase in power conversion efficiency, from 2.7% to 3.9%, which was attributed to the larger and more uniform interfacial area between the layers, facilitating more efficient exciton dissociation and reducing trap-assisted recombination.¹¹⁴ Another study showed the effect of different ZnO nanoparticle sizes (5, 10, 15 nm) on the OLED metrics, e.g. luminance and efficiency.¹¹⁵ The OLED with the smallest ZnO nanoparticles (5 nm) exhibited a significantly higher performance. Specifically, at an operating voltage of 8 V, it showed a maximum luminance of 455.1 cd m⁻², which was much higher than that of the devices with 10 (194.4 cd m⁻²) and 15 (34.8 cd m⁻²) nm ZnO nanoparticles. Additionally, the device with 5 nm ZnO nanoparticles showed a lower luminance onset voltage, meaning it possesses a lower turn-on voltage and needs less power to emit light.

Therefore, since an inhomogeneous (rough) interface can create traps, reduce the contact area, or hinder charge flow, the interface morphology and effective contact area are critical parameters for charge injection and device efficiency.⁸⁷ This means that the solid-state organization of MPcs (crystallinity and polymorphism) is an effective factor influencing electronic properties and OLED performance. For instance, as a result of thermal treatment, CuPc can appear in different polymorphs, metastable α-phase and more stable β-phase. It has been observed that different thermal treatments lead to distinct grain organization in CuPc, impacting the current–voltage–luminance performance of the OLEDs.²⁶ Additionally, other parameters, such as the nature of the coordinated ion within the MPc structure (Fig. 8a–d), the choice of the substrate (Fig. 7b, c and 8a, b), and fluorination (Fig. 7a and b), can also lead to distinct interface morphology and roughness profiles.

Fig. 8 AFM topography images of ZnO/MPc hybrid systems: (a) O-terminated ZnO/VOPc (50 nm),¹¹⁶ (b) Zn-terminated ZnO/VOPc (50 nm),¹¹⁶ (c) ZnO/MgPc (28 nm) [our experimental results], and (d) ZnO/CuPc (28 nm). Panels (a) and (b) are reprinted from ref. 116, Copyright (2025), under CCBY license.

3.3. Chemical stability at the interface

Beyond electronic alignment and morphological considerations, the chemical stability of the interfaces plays a critical role in device longevity. Key degradation mechanisms include interdiffusion, where atoms or molecules from the inorganic (ZnO) and organic (MPc) layers intermix under operational conditions, disrupting electronic alignment and creating new trap states.¹¹⁷ As self-metalation has been observed in the case of phthalocyanines on the metallic surface,^87,118 it is needed to carefully consider this matter while applying ZnO/MPc hybrid systems within the OLED structures.

Another significant challenge to interfacial chemical stability can result from omnipresent environmental factors such as moisture and oxygen. The AFM results for ZnO/VOPc interfaces with different chemical terminations, O-terminated ZnO and Zn-terminated ZnO surfaces (Fig. 8a and b), are evidence to confirm the different chemical reaction of the MPc molecule and growth mechanism in the presence of the oxygen atom.¹¹⁶ Additional oxygen species adsorbed from the environment can react with the ZnO surface, leading to hydroxylation or other modifications that can alter WF and interface dipole. In such interfaces, by applying bias to the OLED, oxygen can induce an oxidative reaction within the organic molecule, which results in a non-conductive or trap-rich interface, severely degrading device performance.^7,119 Moreover, direct chemical reaction between ZnO and MPc molecules at their contact point can occur, altering the intrinsic properties of the layers. While theoretical insights into the relaxed molecule on the ZnO surface, as shown in Fig. 9, provide a fundamental understanding of ideal interactions, it is important to acknowledge that practical ZnO/MPc interfaces are constantly influenced by the aforementioned external environmental factors resulting in deviation from the relaxed state. Particularly, deviation from the thermodynamically relaxed state under operating conditions may result in accelerating the degradation process, consequently, increasing the turn-on voltage, reducing the efficiency, and decreasing the operational lifetime of the OLED. Here (Fig. 9), we performed the optimization of MPc adsorption geometries on ZnO in AMS software. The generalized gradient approximation (GGA) Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional with Grimme 3 dispersion correction, scalar ZORA for relativistic effects, STO (Slater type orbital) basis set, type DZP (DoubleZeta, 1 polarization function) with a small frozen core and an automatic K-space grid for good numerical quality were applied.

Fig. 9 Simulated molecular adsorption for (a) CuPc, (b) MgPc, (c) CoPc, and (d) CoPcF₁₆ on the stoichiometric ZnO substrate obtained using AMS.

3.4. Exciton quenching at the ZnO/MPc interface

The exciton quenching at the ZnO/MPc interface becomes a key concern, particularly when MPcs are employed as the EML in OLED architecture and in direct contact with ZnO. If the exciton (electron–hole pairs responsible for light emission) forms near the inorganic–organic interface or migrates near this interface, it will result in non-radiative emission, a decay pathway.¹²⁰ In this situation, the exciton's energy can be lost as heat rather than light, reducing device efficiency.

Exciton quenching at the inorganic–organic interface occurs through two principal mechanisms: energy transfer and charge transfer.¹²¹ The first, energy transfer, is a non-radiative process where an exciton in the MPc layer transfers its energy to the ZnO layer, often to acceptor states at the interface. This can happen through either Förster resonance energy transfer (FRET) or Dexter energy transfer, depending on the distance and the spectral overlap between the MPc's emission and the ZnO's absorption.¹²² The second mechanism is charge transfer. Upon reaching the interface, the exciton dissociates into a free electron and hole.¹²¹ These free charges are subsequently transported into the ZnO substrate or reinjected into the MPc molecule, but without radiative recombination. Both mechanisms lead to efficiency losses (such as internal/external quantum efficiency (IQE/EQE) reduction) of OLEDs.

As discussed in Section 2.1.1, ZnO as a metal oxide possesses intrinsic defect states (Fig. 3e), including O_vacs and Zn_is. If the traps are deep, then they can act as non-radiative recombination centers.¹²³ These defects at the ZnO surface can react with MPc molecules and create local energy levels within the bandgap, facilitating exciton dissociation, which is a direct pathway for non-radiative decay. It not only results in light output reduction but also affects charge carrier mobility near the ZnO/MPc interface, leading to recombination loss increment and localized heating. However, remarkably, H₂Pc-based electroluminescent (EL) diodes have shown two discrete peaks in their EL spectrum at approximately 480 nm and 800 nm.³⁰ This observation suggests the presence of two different excitons with distinct energies that have been produced within the phthalocyanine-based EML, correlating with the known Q and B energy bands characteristic of Pcs.³⁰ This dual-band emission underscores the complex dynamics of the exciton within these materials, critical for their application in OLEDs. Coupled with the availability of vacancy-based energy levels within the ZnO electronic structure, it is possible to engineer the interface to effectively lower the exciton quenching and increase devices’ efficiency.

4. Strategies for interface engineering and performance optimization of ZnO/MPc-based OLEDs

To overcome the challenges at the ZnO/MPc interfaces and optimize ZnO/MPc-based OLED performance, knowledge gained from analogous inorganic–organic hybrid layers has led to the recommendation of different interface engineering strategies: ZnO surface modification, doping of ZnO, self-assembled monolayers, and insertion of interfacial layers.^2,33,48,124 Particularly, the diversity of ZnO band gap tuning methods and the unique conjugated structure, along with the coordination properties of MPcs, offer advanced strategies for interface engineering.

The soft bonding between Zn and O atoms within the ZnO matrix makes it possible to tune the structure through simple energy level alignment by affordable setups at low temperatures (<350 °C).¹⁶ Engineering the properties of the ZnO layer, such as its resistivity by controlling growth parameters during electrode deposition, can tune the interface properties in ZnO/H₂Pc-based optoelectronic devices to manipulate charge transfer.⁷⁴ As demonstrated in recent work on zinc metal batteries, phthalocyanines can coordinate with metal ions and construct an electrostatic field layer at the interface.¹²⁴ This electric field layer can balance the interfacial electric field and charge carrier flux, which promotes uniform deposition. Applying this strategy to the OLED may result in reducing WF mismatch due to depositing MPcs on the ZnO by locally modulating the electric field and consequently optimizing charge injection. Additionally, as a powerful technique, solution-processed doped MPc layers could enhance MPcs’ conductivity and charge mobility for scalable and cost-effective fabrication of high-performance OLEDs.^27,28

Lattice mismatch in MO/MPc and metal/MPc hybrid layers has been compensated for by different methodologies. (1) Density functional theory (DFT)-based calculations have shown that the existence of the same atoms in both metal oxide and organic sides, e.g. ZnO/ZnPc, can result in strong chemical bonds, e.g. O–Zn bonds in the ZnO/ZnPc interface.^125,126 This interfacial O–Zn bond exhibits a length of 2.07 Å, remarkably close to the Zn–O bond length observed in bulk wurtzite ZnO (1.97–1.99 Å). Furthermore, these studies confirmed planar molecular aggregation which results in significant molecule-to-surface electronic coupling. Given the strong chemical integration with similar bond lengths and the planar aggregation (due to molecule's adsorption on the surface by the coordinated ion), it can be interpreted that ZnPc is adsorbed in a manner that promotes a high degree of structural continuity, effectively continuing the local lattice structure of ZnO at the interface, thereby leading to intimate contact and mitigating the effect of lattice mismatch. Therefore, we can hypothesize that if researchers apply ZnO/ZnPc, ZnO/ZnPcF_x, ZnMgO/MgPc, etc., in the OLED structure as one of the electrodes, then perhaps the efficiency of the device will increase. (2) Inserting an ultra-thin layer of inert graphite between the metallic substrate and the MPc layer has shown that for MPcs with the tendency of island growth on the metallic surface, e.g. CuPcF_x and VOPc, it can make the MPcs mimic the crystallinity of the structure of the ultrathin layer beneath and provide a more uniform MPc layer with lower roughness.^127,128 Therefore, to mitigate the tendency of island growth, we can consider the insertion of such buffer layers between the metal oxide and MPc materials, such as CoPcF₁₆, which showed this behavior on both ZnO (Fig. 7b) and SiO₂ substrates (Fig. 7c).⁸⁹

Lowering the roughness at an inorganic/organic interface has always been the center of attention in the OLED fabrication process since it can provide a better ohmic junction and more contact area at the interface, resulting in more uniform lighting of OLEDs.¹²⁹ It can also avoid short-circuiting by protecting the integrity of the organic overlayer. This has mostly been achieved by surface modification of MO utilizing plasma treatments (e.g., oxygen, argon, argon), use of self-assembled monolayers (SAMs) on MO, and introducing buffer layers.¹³⁰ However, in terms of large-area OLED manufacturing, it should be considered that the scalability of SAM deposition remains a significant challenge due to its batch-wise nature that must be overcome.

Control over exciton quenching in ZnO/MPc systems is possible through (1) tuning molecular energy levels of the planar MPc molecules, e.g. substitution of hydrogen ligands by electronegative fluorine or functional group such as CH₃,⁹⁰ and (2) introducing desired vacancy levels into the electronic structure of ZnO.³³

For enhanced performance and effective charge transport within ZnO/MPc-based OLEDs, it is also crucial to further develop and incorporate n-type phthalocyanine derivatives into the device structure as the EIL and ETL. Current ongoing research tries to engineer the MPcs to possess lower LUMO levels through functionalization.^89,111

While the engineering strategies discussed here aim to optimize initial device performance, their impact on the operational stability and long-term lifetime is a critical consideration. For instance, although doping with the Ga, Al, and Mg elements can effectively tune the WF and conductivity of the ZnO layer, this approach in not without its trade-offs. A key challenge is that the excessive doping can introduce interstitial and vacancy defects introducing charge trap states, which can hinder device performance by acting as a non-radiative recombination center and consequently lower the long term device stability¹²³ by accumulation of charges that cause a progressive increase in turn-on voltage and eventual device failure. Therefore, finding an optimal doping concentration is a critical balance between achieving high conductivity, maintaining structural integrity, and device stability. Similarly, while SAMs are effective at tailoring the interface energy profile, their own thermal stability is a significant factor. The long-term performance of these interfaces is dependent on the choice of a robust, thermally stable molecule and the quality of the SAM's packing density, which can be optimized through processes like annealing to ensure stability under continuous operation. The inferior performance seen at higher annealing temperatures is attributed to the inadequate number of –OH groups and a disordered dipole moment of the SAM on the coarse surface.¹³⁰ Therefore, any interface engineering strategy must be evaluated not only on its immediate benefits but also on its long-term implications for device degradation.

5. Potential device architectures incorporating the ZnO/MPc interface

ZnO as the EIL/ETL in the proximity of MPc-based HIL/HTLs has already been introduced to the OLED structure,^35,131 since the MPcs are very well known for their high hole mobility.¹³² In addition, the remarkably efficient emission of MPcs and metal-free Pcs in the visible range makes these organic molecules attractive for EML application.^30,31,85Fig. 10a shows the impact of different MPcs utilized as single and double HILs on the current and power efficiency of Alq₃-based OLEDs.²⁹ Specifically, the effects of CuPc, MgPc, and ZnPc had been investigated. The utilization of MPcs between the ITO anode and the HTL improved both current and power efficiency, attributed to a low energy barrier at the ITO/MPc interface for hole injection and hole confinement in double HIL structures.²⁹

Fig. 10 (a) Current efficiency and power efficiency characteristics of Alq₃-based OLEDs with different single and double MPc-based HILs, obtained at 20 mA cm⁻² (data were derived from Table 1 in ref. 29). (b) Turn-on voltages versus the HOMO level of the MPcs at 100 cd m⁻² luminescence. Panel (b) is reprinted from ref. 88, Copyright (2025), with permission from Elsevier.

Weaknesses have also been reported in this research area. For instance, issues such as the absence of electroluminescence (without annealing the ZnPc as the EML) due to the lack of one type of charge carrier in the ZnPc-based diode and high turn-on voltage for both ZnPc- and H₂Pc-based OLEDs have been reported.^29,85 Mainly, these weaknesses resulted due to the high ionization potential energy of MPcs and low band gap, lower than 2 eV, making the MPcs very poor electron blocking materials.⁸⁸ This problem was overcome by inserting a TPD layer next to 9 different MPcs in the ITO/MPc/TPD/Alq₃/Mg–Ag OLED structure,⁸⁸ which resulted in a lower turn-on voltage than that observed in the case of ITO/VOPc/Alq₃/Al at ∼45 V.¹³³ The turn-on voltage of the ITO/MPc/TPD/Alq₃/Mg–Ag OLEDs is presented in Fig. 10b. It represents a linear relationship between the turn-on voltage of the MPc-based fabricated OLED and the HOMO level of MPc molecules.^86,88 However, MgPc did not fulfill the role.⁸⁸

As a potential solution, applying a layer of ZnO between ITO and ZnPc⁸⁵ can achieve two things: first, as discussed in Section 4, according to the DFT calculations, it can result in a strong O–Zn chemical bonding at the interface and planar molecular aggregation, thus more contact area;^125,126 and second, given that the annealed ZnPc layer in this OLED structure showed a hexagonal columnar order, it can contribute to reducing lattice mismatch since ZnO also generally shows hexagonal crystallinity.¹⁷ Additionally, to compensate for the lack of charge carriers, the energy band diagram of the fabricated device needs to be carefully considered to apply the appropriate MPc in its correct role.

The other challenge is that, mainly in the reported MPc-based OLEDs, ZnO has been applied within the conventional OLED structure, while there is significant interest in considering its role in inverted OLEDs (iOLEDs).²⁰ In this context, the acceleration of ongoing research on n-type MPc creation, mainly obtained by fluorination of MPc molecules (MPcF_x),^21,22 and characterization is crucial. Finally, we propose an MPc-based OLED with the architecture presented in Fig. 11, as a suggestion to be fabricated in the laboratory. Within the proposed structure, the materials have been chosen carefully based on the band diagram shown in Fig. 5a, to facilitate the charge transfer. Additionally, we do hope that all organic ZnPcF_x=0 or 4 materials stacked on the ZnO will show the least lattice mismatch.

Fig. 11 Schematic band diagram of the proposed MPc-based OLED.

6. Future perspectives and research directions

So far, inorganic–organic hybrid OLEDs have experienced a remarkable growth. However, there are still several challenges and promising avenues for future device optimization, particularly for ZnO/MPc-based OLEDs. A primary focus will be the interface engineering (through thermal annealing, doping, plasma treatment, and defect creation) to mitigate issues such as lattice mismatch, enhancing interface bonding, and aligning the energy levels, thereby improving charge injection and transport efficiency. Advanced characterization techniques such as photoelectron spectroscopy, microscopy, and diffraction, coupled with computational methods (such as DFT-based quantum calculations), are necessary to understand the complex interfacial phenomena and provide a rational design for OLEDs. Furthermore, understanding and controlling phenomena such as exciton quenching at the interface, and creating appropriate charge confinement within the emissive layer, remains essential.

Furthermore, the development of stable and efficient n-type MPc derivatives represents a critical research area here. The successful synthesis and integration of the n-type MPcs could boost device architectures by enabling all-MPc organic stacks, a novel pathway for balanced charge injection and reduced energy losses. Further innovation is expected in all-MPc iOLED structures, particularly by optimizing ZnO's role as the EIL or transparent cathode, since the WF of ZnO is comparable with that of conventional cathodes such as Ag and Al, which can make it possible to utilize such a structure in fabricating both side transparent OLEDs.

Finally, the alternative combination of these novel structures (device architecture engineering) will be essential to push the performance limits of ZnO/MPc-based OLEDs toward high-efficiency, long lifetime, and cost-effective display and lighting applications.

However it was not considered in this perspective that transition from lab-scale OLEDs to high volume manufacturing requires focusing on scalable fabrication techniques. In this regard, the exploration of continuous processes, particularly printing methods such as roll-to-roll processing (for high-volume and fast manufacturing), slot-die coating (for depositing uniform and continuous ZnO thin films), and inkjet printing methods (for precise, patterned organic layers), is recommended to be considered since they hold significant promise for realizing flexible and large scale ZnO/MPc-based OLEDs.

7. Conclusion

In this perspective, we highlighted the potential of direct integration of ZnO and MPcs in OLED architectures. However, it is accompanied by several interface-specific challenges, requiring precise engineering, including work function mismatch, charge injection barriers at the interface, interface morphology, chemical stability issues, exciton quenching, and lack of n-type MPc materials.

To address these challenges and optimize device performance, we proposed several engineering strategies such as modifying the ZnO surface through doping materials into ZnO and applying an interfacial layer, controlling ZnO layer properties, utilizing MPc molecules which include common coordinated ions with the metal oxide substrate such as ZnO/ZnPc and ZnMgO/MgPc, tuning the molecular energy levels along with control over vacancies and imperfection sites at the ZnO surface, and developing n-type MPcs to expand their roles as the EIL and ETL.

We also proposed an MPc-based OLED structure whose organic part includes only MPcs, ITO/ZnPc/ZnPcF₄/ZnO/Ag. This structure in turn leverages the strength of ZnO and MPcs (due to the role versatility of these molecules, tunable band gap, and charge transport properties) within the OLED architectures. The strategic integration of these materials, particularly HOMO/LUMO energy levels and charge confinement, aims to create a hybrid OLED structure that optimizes performance and stability.

This work serves as a foundational step toward a ZnO/MPc hybrid structure from an electronic and morphological point of view. In fact, we propose a new paradigm that shifts the focus from applying individual ZnO and MPc into the OLED structure, aiming to optimize the device efficiency, to meticulously engineering the synergistic interactions at the ZnO/MPc interface.

This perspective review is beneficial for a diverse range of researchers within the optoelectronic field, particularly materials scientists and chemists. Most importantly, researchers and engineers specializing in OLED and other hybrid device fabrication will find a relatively comprehensive analysis of challenges and proposed engineering strategies to optimize device performance and stability. Furthermore, academic society will benefit from this perspective as a valuable resource for understanding the current state of research, identifying key issues, and exploring future directions in the field.

Conflicts of interest

There are no conflicts to declare.

Data availability

The original data supporting this article (i.e. for Fig. 2–4, 6, 8 and 9) have been included as part of the SI as a compressed folder. See DOI: https://doi.org/10.1039/d5tc02786a.

Other presented data are not primary research results.

Acknowledgements

The authors acknowledge the ESpeFuM laboratory for access to all experimental setups. S. A. N. acknowledges the GRAND project (the publication was co-financed by the project no. FESL.10.25-IZ.01-07E7/23), BKM (project no. 14/030/BKM25/0243), new research topic (project no. 32/014/SDU/10-22-78), and computational grant from Poland's high-performance Infrastructure PLGrid ACC Cyfronet AGH (project no. PLG/2024/017434 and PLG/2025/018639). M. K. acknowledges the Institute of Physics statutory funding support through grant no. 14/030/BK_25/0236 and the support from Silesian University of Technology for the financial support under Rector's pro-quality grant no. 14/030/RGJ25/0238.

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