Metal oxide doped organic thin film transistors: a comprehensive review

Abstract

In recent years, organic thin film transistors (OTFTs) have been gaining widespread interest for electronic displays, circuits and sensors over traditional silicon-based transistors due to their unique properties such as low cost, mechanical and electrical stability, low-temperature processability and large-area processability. They are widely used in applications pertaining to flexible displays, wearable devices, radio frequency identification (RFID) tags, e-skin, biosensors, and flexible integrated circuits. However, organic thin film transistors are still inferior to silicon-based technologies, trailing behind in several critical performance metrics such as low mobilities and high operational voltages. These challenges can be mitigated using metal oxides, which, owing to their high work function and stability, can enhance the parameters of OTFT devices. This review aims to provide insights into the usage of metal oxides in organic thin film transistors and highlight their contribution as hole injection layers (HILs), charge transport complexes (CTCs), bilayer source–drain (S–D) electrodes and gate dielectrics.

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1. Introduction

Organic thin film transistors (OTFTs) have emerged as an important economical transistor technology in various applications due to their unique properties. Organic thin film transistors are a type of field effect transistors containing an organic semiconductor layer. Organic thin film transistors provide advantages over inorganic silicon-based semiconductors for their flexibility,¹ light weight,² biocompatibility,³ solution processability,⁴ printed fabrication,⁵ large-area processability,⁶ and cost effectiveness⁷. They are used in various applications such as flexible displays,⁸ wearable devices,^3,9 radio frequency identification (RFID) tags,¹⁰ e-skin,^2,11 biosensors,¹² and flexible integrated circuits.^13,14 Despite the remarkable progress in organic thin-film transistor (OTFT) technology, achieving high-performance and stable devices remains a significant challenge. Compared to silicon-based thin-film transistors, OTFTs continue to be inferior, encountering limitations such as inconsistent physical models, lower charge carrier mobilities, higher contact resistance, and reduced environmental stability, hampering their widespread implementation in practical applications. Thus, to address these issues, doping has emerged as a key strategy for enhancing the electronic properties of OTFTs. By introducing charge carriers via p-dopants and n-dopants, significant improvements in the performance and stability of organic semiconductors can be achieved. Metal oxide interlayers improve device parameters by easing hole injection, tuning contact engineering, and even by their excellent dielectric properties.

Among the p-dopants, one of the most widely used molecular dopants is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ),¹⁵ a strong electron acceptor with a low-lying LUMO (electron affinity) of 5.2 eV capable of oxidising the HOMO of most organic semiconductors. Molecular doping with the F₄-TCNQ molecule is desirable due to its efficient blending with various organic host materials and ordered arrangement, leading to improved charge transport characteristics such as enhanced mobilities, high on/off current ratios, reduced threshold voltage and decrease in contact resistance.¹⁶ Beyond molecular dopants, elemental species such as diatomic halogens (I₂, Br₂, and Cl₂) have been investigated as p-dopants¹⁷. These species facilitate the formation of charge transfer complexes (CTCs), which enhance the hole transport in organic semiconductors.¹⁸ Another category of p-dopants includes Brønsted and Lewis acids, such as tris(pentafluorophenyl)borane (BCF),¹⁹ iron chloride (FeCl₃),²⁰ tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue),²¹ antimony pentachloride (SbCl₅)²² and molybdenum tris(dithiolene) complexes (Mo(dt)₃),²³ to obtain higher device performances such as higher mobilities, thermoelectric efficiency and optoelectronic applications. n-Dopants have been utilised in organic electronics including elemental dopants such as lithium (Li), sodium (Na), potassium (K), and caesium (Cs)²⁴ and inorganic alkali salts such as lithium carbonate (Li₂CO₃),²⁵ sodium carbonate (Na₂CO₃),²⁶ and caesium carbonate (Cs₂CO₃),²⁷ which are applied to obtain a reduction in the electron injection barrier.

Organic semiconductor devices extensively employ metal oxides as p-dopants due to their high work function, high electron affinity, high sensitivity, and transparency. Interlayers of metal oxides also reduce the issue of high contact resistance at the interface between the organic semiconductor and the electrode. The differences in work function between the electrode and highest occupied molecular orbitals (HOMO) can result in barriers and increase the contact resistance. The high work function of metal oxides enhances the energy level alignment between the metal electrode and organic semiconductor, allowing Schottky to Ohmic conversion. Metal oxides can serve as electron acceptors, undergoing electron transfer with the HOMO of organic materials, generating holes in the organic material, and effectively p-doping. Thus, metal oxides provide an efficient method to decrease the barriers and contact resistance to obtain high-performance OTFTs.

This review focuses on the different metal oxides employed in enhancing the efficiency of OTFTs and their role as dopants in OTFTs. Finally, it is concluded with the limitations and scope of this research area.

2. Transition metal oxides as dopants in OTFTs

Transition metal oxides (TMOs) have been gaining widespread interest in organic thin film transistors (OTFTs) due to their high work function, stability and role as an interfacial layer. They are ideal for forming ohmic contacts at the metal oxide/organic semiconductor interface. They play an essential role as hole injection layers (HILs), charge transport complexes (CTCs), source–drain (S–D) electrodes and gate dielectrics. The commonly used metal oxides include MoO₃, V₂O₅, WO₃, ZnO, CuO, and ReO₃, which can help realise applications for the next generation of organic electronics.

Among them, molybdenum oxide (MoO₃)²⁸ is an p-type dopant commonly used in organic thin film transistors. MoO₃ facilitates efficient charge transfer at the metal oxide/organic semiconductor interface owing to its high work function. Thus, it is used as hole injection layers, enhancing organic thin film transistors by increasing their charge injection, stability, and conductivity and reducing their contact resistance.^6,29 MoO₃ helps in energy level alignment (ELA) due to its high work function and reducing the energy level mismatch with organic semiconductors. Additionally, it form charge transport complexes (CTCs), enhancing the generation of free carriers and carrier mobilities. It is also employed in source–drain (S–D) electrodes due to its electrical properties as an inexpensive alternative to Au and Ag electrodes.^30–32

Similarly, vanadium oxide (V₂O₅)³³ and tungsten oxide (WO₃)³⁴ are also employed extensively in organic thin film transistors as p-type dopants in source–drain (S–D) electrodes and hole injection layers. V₂O₅ and WO₃ are used to reduce contact resistance, enhance the charge carriers, increase the field effect mobility and match with the highest occupied molecular orbital (HOMO) of organic semiconductors, thereby providing energy level alignment (ELA).^35,36

Owing to the versatile nature of zinc oxide (ZnO), it has been employed as a dopant in organic thin film transistors due to its wide band gap, transparency, and high field-effect mobility. ZnO nanoparticles are used in organic devices as n-type dopants, generating electrons and enhancing the free charge carriers and electron acceptors in p-type organic semiconductors, enhancing hole transport. They improve the device performance due to their capability to produce a low sub-threshold voltage, low amount of hysteresis, enhanced current ratio (I_ON/I_OFF), and reduced trap density, thereby forming good ohmic contact. ZnO nanoparticles have also been used in hybrid multilayer organic–metal oxide heterojunctions, showing high field effect mobility and high operational stability.^37,38

Another transition metal oxide that has been extensively used is copper oxide (CuO),³⁹ which owing to its high work function serves as a means to reduce the threshold voltage, resulting in low operational voltages and hole injection layers to enhance the conductivity in organic thin film transistors. CuO films in organic thin film transistors have been investigated owing to their ability to shift the threshold voltages to lower values by controlling the trap densities in the OTFT channel layers.⁴⁰ Similarly, CuO is employed as a source–drain (S–D) bilayer electrode in organic thin film transistors, enhancing the device performance.⁴¹

Rhenium oxides have been explored to provide an alternative method to overcome the compatibility issues observed during co-evaporation at the high evaporation temperatures for metal oxides. Rhenium oxides are known to have a low evaporation temperature, improving the compatibility in low melting point organic semiconductors. ReO₃ has a melting point of 340 °C and Re₂O₇ has a melting point of 225 °C, thus improving the compatibility with organic semiconductors. Rhenium oxides showed a considerable improvement in the mobilities of organic devices and utilized as hole injection layers (HILs).^42–44

High dielectric constant (κ) metal oxides such as zirconium oxide (ZrO₂),⁴⁵ hafnium oxide (HfO₂),⁴⁶ and aluminium oxide (Al₂O₃)⁴⁷ and rare earth metal oxides such as neodymium oxide (Nd₂O₃)⁴⁸ and lanthanum oxide (La₂O₃)⁴⁹ are used in the gate dielectric in organic thin film transistors. The use of high κ metal oxides results in organic thin film transistors with low operational voltages, high operational stability, low leakage current density, and high capacitance.^50–52

3. Function of metal oxides in OTFTs

3.1 Metal oxides as hole injection layers (HILs)

Transition metal oxides (TMOs), such as CuO, WO₃, MoO₃, ReO₃, Ni₂O₃, Co₃O₄ and V₂O₅, owing to their high work function, reduced contact resistance, wide band gap, high stability, ease of processing and low cost, have been gaining significant attention as hole injection layers (HILs) (Fig. 1). Transition metal oxide interlayers are employed in organic thin film transistors, serving as an ohmic contact and increasing hole injection, thus enhancing the device performance. By inserting an injection layer electrode between a metal and an organic semiconductor, a reduced hole injection barrier and suppressed contact resistance are achieved.^53–55

Fig. 1 Schematic representation of hole injection layer in an OTFT and schematic of the hole injection mechanism between the conduction band of metal oxide and highest occupied molecular orbital (HOMO) of an organic semiconductor.

Fig. 1 illustrates how holes are injected from the valence band or conduction band of a metal oxide into the highest occupied molecular orbital (HOMO) of an organic semiconductor due to the charge transfer that occurs by electron extraction from the HOMO of the organic semiconductor to the valence band or conduction band of a metal oxide. Metal oxides such as MoO₃, V₂O₅ and WO₃, which are known for their high work functions, are efficient at creating minimal energy barriers when used as an interface layer with organic semiconductors that have deeply positioned HOMO levels. The utilization of metal oxides is crucial in optimizing the device properties in organic thin film transistors. By reducing the injection barrier height at the interface, they enhance the amount of charge that can be introduced into the channel. Consequently, a reduction in the injection barrier promotes lower power consumption. These metal oxides also exhibit the ability to suppress short channel effects, offering scalability and stability for the development of low-voltage and short-channel organic devices. Energy level alignment by metal oxides minimizes the energy mismatch, and thus helps in efficient hole injection, allowing efficient operation and improving the working of organic devices.^56,57

The presence of a large charge injection barrier in an OTFT is due to the mismatch in the work function or formation of interfacial states, which is seen to critically hamper the device performance. The consequent non-linear relation between the current and applied voltage and reduction in effective gate electric field weakens the ability of the gate to induce an accumulation layer and reduces the field effect mobility compared to the intrinsic mobility. Yun et al.⁵⁸ investigated the use of molybdenum oxides (MoO_x) as a hole injection layer in a DBTTT semiconductor OTFT, functioning to reduce the contact resistance and increase the conductivity. A typical metal/organic semiconductor is prone to non-linear I–V characteristics at low voltages, which is attributed to its large charge injection barriers. The deposition of MoO_x as a hole injection layer resulted in a linear relationship in the I–V characteristics of a device, indicating a transition from Schottky to Ohmic contact. The presence of MoO_x removes the inconsistency that persists between intrinsic mobility and field effect mobility in the linear regime. A field effect mobility of 3.68 cm² V⁻¹ s⁻¹ was obtained using a bare Au electrode, whereas when an MoO_x film layer was incorporated in the Au electrode, the obtained field effect mobility in the linear regime was 6.47 cm² V⁻¹ s⁻¹ for the MoO_x (5 nm)/Au layer and 6.72 cm² V⁻¹ s⁻¹ for the MoO_x(10 nm)/Au layer. These values are consistent with the estimated intrinsic mobility, indicating a significant decrease in contact resistance, and thus increase in the field effect mobility. The use of an Al electrode as a cheaper alternative to Au was also explored, which required 75 nm-thick MoO_x films. A similar enhanced performance was obtained for an MoO_x/Al layer with a field effect mobility of 5.77 cm² V⁻¹ s⁻¹ in the linear regime. Liu et al.⁵⁹ found that the interfacial contact resistance is diffusion limited. Experimental analysis of the charge injection in the metal/organic semiconductor showed that non-Ohmic contacts have a characteristic hook-shaped conductance/drain voltage relationship. To overcome this effect, Liu et al. utilised metal oxides, MoO₃ and V₂O₅, as charge injection layers, as well as other strategies such as bulk doping, interlayer and high κ dielectric. The metal oxide charge injection layer increased the mobilities and reduced the contact resistance. At gate voltage V_g = −80 V, the bare metal/organic semiconductor of Mo/P3HT obtained mobilities of 4.5 × 10⁻³ cm² V⁻¹ s⁻¹, while the OTFTs with MoO₃ and V₂O₅ obtained mobilities of 2.1 × 10⁻² cm² V⁻¹ s⁻¹ and 1.9 × 10⁻² cm² V⁻¹ s⁻¹, respectively. The recombination rates also are enhanced due to the decrease in injection barrier from the metal oxide injection layer. These results show that contact resistance engineering goes hand in hand with enhancing the mobility in OTFTs and the decrease in interfacial contact resistance as the carrier mobility increases confirms that both the charge injection and interfacial contact resistance are diffusion limited.

The use of high work function metal oxides does not guarantee the ability to form an Ohmic contact. There are various factors that determine the formation of an ohmic contact, especially the energy level alignment. For a highly efficient ohmic contact, the bulk-limited current should be close to the injection limited current. Yoo et al.⁶⁰ demonstrated the formation of ohmic contacts by the deposition of thin film metal oxides such as MoO₃ and ReO₃ between metal/organic semiconductor interfaces, obtaining a near perfect ohmic contact with the use of ReO₃ injection layer. MoO₃ and ReO₃ were used due to their high electron affinity and hole injection properties. The current density vs electric field curve, i.e. J–F characteristics of the hole-only devices, were measured with and without a metal oxide interlayer. The J–F curve of the hole-only devices without a metal oxide interlayer showed non-linear characteristics, whereas the hole only devices with a metal oxide interlayer showed comparable linear characteristics, showing that the insertion of an interlayer enhanced the current densities. The ReO₃ interlayer obtained a near perfect hole injection efficiency of close to 100% and gave a significant increase in current density compared to the MoO₃ interlayer, with one magnitude larger than MoO₃ interlayer. Ultraviolet photoelectron spectroscopy (UPS) was the method employed to measure the hole injection barriers, which disclosed that the hole injection barriers obtained for MoO₃/organic semiconductor was 0.41 eV and that for ReO₃/organic semiconductor was 0.38 eV, showing a considerable reduction in barriers compared to the substrate/organic semiconductor devices without metal oxides (1.64 eV). The enhanced device properties is due to the p-type doping caused by the formation of a charge transfer complex, which can be confirmed by the increase in the Mo^v and Re^iv peaks as a result of the reduction of the metal oxide films by NBP.

The interface structure of the device, whether metal oxide is deposited on an organic semiconductor (inverted deposition) or an organic semiconductor is deposited on metal oxide (non-inverted deposition) plays an important role in the device performance. When an organic semiconductor is deposited onto a transition metal oxide structure, its characteristics closely resemble those of only the organic semiconductor film and a larger part of the organic semiconductor remains undoped. This is attributed to the high density and stability of the underlying metal oxide layer, preventing any diffusion. In contrast, a structure that employs a transition metal oxide on an organic semiconductor layer results in the diffusion of the metal oxide onto the organic semiconductor and is shown to depend on the thermal stability of the organic semiconductor. Beck et al.⁶¹ studied inverted deposition and non-inverted deposition at organic (CBP)/metal oxide (MoO₃) interfaces using IR spectroscopy. The intensities of the CBP cations were higher in the inverted structure of MoO₃ deposited on CBP compared to the non-inverted deposition of CBP on MoO₃. This was principally due to the difference in mechanism, where in the inverted structure, the diffusion of metal oxide is extended to up to 10 nm, while in the non-inverted structure, the space charge region is found to be up to 2 nm, and thus only limited CBP molecules are affected. Zhao et al.⁶² systematically investigated the impact of deposition order on the optical and electronic properties of MoO₃ and NPB interfaces. The absorption spectrum of the non-inverted deposition of NBP (10 nm) on MoO₃ (3 nm) closely resembled that of the pure NBP layer. Alternatively, the absorption spectrum of the inverted deposition of MoO₃ (3 nm) on NBP (10 nm) exhibited two additional absorption peaks compared to pure NBP films, proving the diffusion of MoO₃ into NBP. These spectral features resembled those of MoO₃-doped NBP films, suggesting the effective intermixing of MoO₃ on the NBP layer. The diffusion of MoO₃ studied with NBP, TCTA and CBP showed an increase in diffusion as a function of their evaporation temperature. Given that the evaporation temperatures of metal oxides is much higher than those of organic semiconductors, the inverted interface always results in the diffusion of metal oxide in organic semiconductors. White et al.⁶³ further reported that diffusion does not occur into metal oxides in the non-inverted deposition order due to the higher stability, higher density and significant difference in evaporation temperatures between the organic materials and MoO₃. In contrast, in the inverted structure, the deposited metal oxides have higher kinetic energy to be able to diffuse into the organic layer. The diffusion of metal oxide into the organic semiconductor results in electron transfer, resulting in the p-doping of organic semiconductors. This leads to band bending, which can be characterised by the shifts in the C1s peak to a lower binding energy after CBP donates charge to MoO₃, while the Mo3d peak shifts to a higher binding energy after receiving the charge. The formation of new core–shell peaks, C^x+ and Mo^x+, provided evidence for charge transfer, and consequently it was observed that ratio of C^x+ to C–C bond state showed a decrease for molecules with a higher molecular mass compared to molecules with a lower molecular mass. Diffusion was shown to be a function of molecular mass, as observed in the comparison of mCP, CBP, and mCBP, which exhibited the most diffusion, with molecules of larger molecular mass, such as NPB, TCTA, and MTDATA, exhibiting the least diffusion. This result is consistent with the high kinetic energy model, given that the thermal stability of organic molecules with weak intermolecular forces is largely dictated by their molecular mass. Although the diffusion of transition metal oxides onto organic semiconductors can be utilised to obtain enhanced hole injection and is a simpler alternative to the complex process of controlled doping of metal oxides in organic semiconductors, it has been associated with device instability and poor reproducibility. The diffusion of metal oxides into organic semiconductors is often unpredictable and results in variable device parameters such as change in threshold voltage and on/off current ratio. Recent advances involve a multilayer interface that employs a blocking layer between the organic semiconductor and metal oxide to prevent diffusion, and thus improve the device stability. Yang et al.⁶⁴ utilised a metal/metal oxide/organic multilayered interface contact (MIC) source–drain electrode, which achieved higher device stability. They proposed an organic multilayer interface containing an organic buffer layer (OBL) such as such as 1,3-bis(carbazol-9-yl)benzene (mCP), 4,4′-bis(carbazol-9-yl)-biphenyl (CBP), 4,4′-bis(carbazole-9-yl)-2,2′-dimethylbiphenyl (CDBP), N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), 1,4-bis(triphenylsilyl)benzene (UGH-2), di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC), and 4,4′,4′′-tris(carbazol-9-yl)triphenylamine (TCTA) among the organic semiconductor, copper phthalocyanine and metal oxide MoO₃ to prevent the diffusion of the metal oxide. The devices with OBL obtained a higher on/off current given that the off current in these devices did not intensely increase, as was shown in the devices without OBL due to the diffusion of metal oxides in the organic semiconductors. Hence, the operation stability also increased considerably. The on/off current ratio of the devices without OBL was 1.10 × 10², while that of the devices with organic buffer layers such as mCP was 1.33 × 10³, CBP was 1.90 × 10³, CDBP was 2.67 × 10³, NBP was 3.72 × 10³, UGH-2 was 2.89 × 10³, TAPC was 1.07 × 10² and TCTA was 4.94 × 10². The obtained contact resistance in the devices with the MoO₃/organic semiconductor and device with OBL NBP was 0.51 MΩ and 0.76 MΩ, respectively, showing that MoO₃ can still efficiently reduce the contact resistance, even in the presence of OBL. However, due to the lack of literature on the use of OBL, the selection of an appropriate OBL for OTFT devices is limited.

Kim et al.⁶⁵ used VO₂ as a hole injection layer (HIL) and studied the electrical properties by using photoemission spectroscopy. The measured UPS spectra of the normalized secondary electron cutoff (SEC) and background removed the HOMO region during NPB deposition on the VO₂/FTO substrate, which was similar to previous reports, showing a shallow valence band maximum (VBM) close to Fermi level (E_F). The valence band maximum (VBM) is measured to be 0.45 eV below the Fermi level (E_F), corresponding to the hole injection barrier (ϕ_h) from fluorine-doped tin oxide (FTO) to VO₂. According to the obtained UPS spectrum, it is evident that there is an increase in work function (ψ) from 4.02 eV for only FTO to 5.66 eV for VO₂/FTO, aligning with the highest occupied molecular orbital (HOMO) of NBP, and consequently reducing the hole injection barrier. Thus, VO₂ acts as a hole injection layer (HIL), reducing the original hole injection barrier (ϕ_h) with a step-like energy level alignment called the ladder effect, wherein the large barrier is divided into two smaller barriers, i.e. the first hole injection barrier (ϕ_h) from FTO to VO₂ is 0.45 eV and the second ϕ_h from VO₂ to NPB is 0.14 eV. XPS measurements further helped confirm the VO₂ phase to be consistent with the valence band shape obtained in the UPS measurements. The areal ratio was determined to be 1.0 : 1.9 between the V 2p and O 1s and is consistent with the stoichiometric VO₂ phase expected. The V 2p_3/2 peak observed at 516.2 eV is characteristic of V⁴⁺, similar to the reported values such as V⁵⁺ (V₂O₅) at 517.2 eV and V³⁺ (V₂O₃) at 515.3 eV for vanadium. The hole injection layer of VO₂ provides p-doping due to the electron transfer from the NBP HOMO and conduction band as well as the valence band of VO₂, leaving behind mobile holes. This double hole injection of device shows the versatile nature of VO₂ and can considerably enhance the device metrics, showing enhanced J–V characteristics for VO₂/FTO compared to the devices without hole injection, providing potential for transparent anodes.

Another key consideration is the ability to fine tune the work function of metal oxides to obtain the best possible performance in OTFTs. Metal oxides are prone to air oxidation, resulting in a decrease in work function.⁵⁶ Thus, different work function values are obtained based on the conditions, requiring the work function to be tuned for specific values. Yao et al.⁶⁶ was able to modify the work function of MoO₃ by employing different deposition techniques, during which either the ratio of O₂ to the total sum of O₂ and Ar was varied or the time of oxygen plasma treatment was varied. The work function increased from 4.85 eV to 5.80 eV, wherein a linear increment was observed in the ratio of O₂ to the total sum of O₂ and Ar of 0.1 to 0.2, after which there were only small changes in work function in the range of 0.2 to 0.3 O₂ to the total sum of O₂ and Ar. Alternatively, the work function based on oxygen plasma treatment after Mo deposition increased from 4.66 eV to 5.30 eV for a treatment time in the range of 0 to 30 seconds. Similar to varying the ratio of O₂ to the total sum of O₂ and Ar, the work function reached saturation after an oxygen plasma treatment time of 35 to 45 seconds. The surface smoothness obtained by atomic force microscopy revealed that as the ratio of O₂ to the total sum of O₂ and Ar increased from 0.1 to 0.3, the RMS roughness decreased from 0.71 nm to 0.33 nm. A similar decrease in RMS roughness from 0.83 nm to 0.51 nm was observed upon varying the oxygen plasma treatment time from 0 to 45 seconds. The XPS data shows that on increasing the ratio of O₂ to the total sum of O₂ and Ar, the peak of the Mo⁴⁺ state decreases, while there is an increase in the Mo⁶⁺ and Mo⁵⁺ peaks. As the gas ratio increased from 0.1 to 0.2, the Mo⁵⁺ peak area decreased from 61.2% to 1%, while simultaneously the Mo⁶⁺ peak increased from 70% to 84.7%. This was attributed to the increase in oxygen anion during deposition by increasing the gas ratio, thus increasing the work function. The XPS data for oxygen plasma treatment shows a similar shift in peaks towards higher binding energies as the treatment time increases. As the treatment time increased from 5 to 45 seconds, the peak area of Mo⁵⁺ decreased from 23.5% to 13.8%, while peak area of Mo⁶⁺ increased from 70.0% to 84.7%. The OTFT device parameters such as on/off current ratio, subthreshold slope and the threshold voltage obtained with and without oxygen plasma treatment were 1 × 10⁶ and 1 × 10⁴, 1 and 1.4, and −3.8 V and −14.5 V, respectively, showing that the device could be optimized by modifying its work function.

Metal oxides can also be explored as the hole injection layer in organic phototransistors. The properties of metal oxides such as wide bandgap, optical properties and transparency with optical transmittance greater than 70% in the visible region enable the preparation of low cost and efficient organic phototransistor devices. Park et al.⁶⁷ reported that transition metal oxide-based (TMOs) buffer layers significantly enhanced the device parameters in organic phototransistors. A transparent indium zinc oxide (IZO) electrode with molybdenum oxide (MoO₃) buffer layer was used as the hole injection layer to improve the electrical conductivity as well as due to its optical transparency in the visible region. The presence of metal oxide was shown to reduce the hole injection barrier, which was confirmed by measuring the hole injection barrier values using ultraviolet photoelectron spectroscopy (UPS). The measured values of the hole injection barrier between IZO and the pentacene layer without the presence of buffer MoO₃ was found to be 0.65 eV, whereas the hole injection barrier between IZO/MoO₃ and the pentacene layer was found to be 0.42 eV, with a decrease of 35% in the presence of a metal oxide buffer layer. The device showed excellent electrical properties with a mobility of 1.40 cm² V⁻¹ s⁻¹, threshold voltage of −13.8 V, subthreshold slope of −4.54 V decade⁻¹, on/off current ratio of 10⁵, and optical properties with high transparency of 70.4% average transmittance in the visible region and photoresponsivity of 54.8 A W⁻¹. The device prepared on a cellulose nanofibrillated fibre substrate was also confirmed to be biodegradable, providing an environmentally friendly electronic device. Jung et al.⁶⁸ investigated the use of MoO₃-doped pentacene to enhance the electrical performance of UV photodetectors. A 10 nm MoO₃ layer enhanced the OTFT device metrics with a hole mobility of 1.71 × 10⁻¹ cm² V⁻¹ s⁻¹, threshold voltage of −16.69 V and on/off current ratio of 2.5 × 10³ compared to the pristine pentacene OFET devices, exhibiting the hole mobility 5.16 × 10⁻³ cm² V⁻¹ s⁻¹, threshold voltage of −54.26 V and on/off current ratio of 1.7 × 10⁻⁴. Again, this was attributed to the considerable reduction in contact resistance, which was approximately half of that of the pristine pentacene OFET devices. The MoO₃-doped OFETS also showed increased photosensitivity, with an increase in photoresponsivity from 3.2 × 10² to 1.0 × 10⁴ A W⁻¹ upon the introduction of MoO₃. Although the device performance was considerably shown to increase, the use of OFET devices as photodetectors has been restricted due to their high power consumption. High power dissipation of 1.0 × 10⁻⁴ W cm⁻² was calculated for MoO₃-doped OFETs, leading to the lower energy efficiency of the photodetectors. In comparison, an MoO₃-doped suboptimal source gated transistor (sSGT) device showed a significantly lower P_diss of 3.04 × 10⁻⁷ W cm⁻², providing an enhanced optical power efficiency in photodetectors. Thus, further improvements in power efficiency need to be realised for the application of OFETs in photodetectors.

3.2 Metal oxides as charge transfer complexes (CTCs)

Transition metal oxides (TMOs) having high electron affinity can interact with wide band gap organic semiconductors to form a charge transfer complex. As shown in Fig. 2, these complexes arise from the interaction between the electron-rich organic semiconductors and the electron-deficient transition metal oxides (TMOs), leading to partial electron transfer. Transition metal oxides (TMOs) play a role in organic thin-film transistors (OTFTs) by forming charge transfer complexes (CTCs) with organic semiconductors, and the corresponding interaction provides significant enhancement of the electronic properties of the devices such as lowering the injection barrier for holes, increasing the current density and conductivity and improving the charge injection efficiency and mobility in organic semiconductor devices. This mechanism of charge transport complex is capitalized by bulk doping of metal oxides, as well as interlayers such as hole injection layers, and even source/drain bilayer electrodes to obtain high-performance OTFTs. Yan et al.⁶⁹ utilized MoO₃ as a buffer layer, enhancing the OTFT device parameters by capitalizing on charge transfer complex formation. OTFT devices were constructed using pentacene as the organic semiconductor and 4,4′′-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) and molybdenum oxide (MoO₃) as buffer layers. According to the output curves in the saturation regime (V_ds = −100 V), the pentacene:m-MTDATA:MoO₃ device demonstrated an outstanding effective mobility (μ_eff) of 0.72 cm² V⁻¹ s⁻¹ and a reduced threshold voltage (V_th) of −13.4 V, while devices consisting pentacene:MoO₃ obtained μ_eff of 0.36 cm² V⁻¹ s⁻¹ and V_th of −33.7 V. The m-MTDATA:MoO₃ device obtained μ_eff of 0.40 cm² V⁻¹ s⁻¹ and V_th of −20.7 V. At a fixed gate voltage of −100 V, the obtained I_dsvs. V_ds curves indicated that the pentacene:m-MTDATA:MoO₃ device exhibited the best drain current of 38.05 μA compared to the drain current values in pentacene:MoO₃ of 12.93 μA and m-MTDATA:MoO₃ of 23.91 μA. This significant enhancement in drain current is primarily due to the formation of charge transfer complexes (CTCs) in the buffer layer. These CTCs are critical, given that they greatly increase the hole density and improve the alignment of energy levels. AFM measurements convey that the application of metal oxide resulted in smoother films with a considerable reduction in RMS roughness from 4.33 nm for the pure pentacene film to 3.21 nm in pentacene:m-MTDATA:MoO₃. Thus, the OTFT devices exhibited a significant improvement in device parameters such as high effective mobility, reduced threshold voltage, increased hole injection to organic semiconductors and lower total resistance.

Fig. 2 Schematic representation of the charge transport complex between a metal oxide and an organic semiconductor.

As mentioned before, metal oxides serve as an effective tool to enhance the performance of organic phototransistors owing to their wide band gap and transparency. The formation of charge transfer complexes between a heterojunction of an organic semiconductor and metal oxide can be an effective solution to enhance the performance of organic phototransistors for fiber communication, especially in the second near infrared region. Wang et al.⁷⁰ used the CTC principle to enhance the device properties of on organic phototransistor, obtaining near infrared detection greater than 1000 nm. A heterojunction CTC was prepared using 4,4′′-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) as an electron donor and tungsten oxide (WO₃) as an electron acceptor in a pentacene organic phototransistor. The CTC mechanism is explained by intermolecular charge transfer transitions from the highest molecular orbital of m-MTDATA (−5.1 eV) into the corresponding conduction band of WO₃ (−4.6 eV). Thus, an electron from the HOMO of m-MTDATA can transfer into the conduction band of WO₃, while holes can be injected from WO₃ to the HOMO of m-MTDATA, causing band bending. The device properties were investigated by studying the output characteristics of organic phototransistors with varying CTC layer thicknesses. The devices exhibited enhanced mobilities as the thickness of the CTC layer increased from 20 nm to 30 nm, functioning as an enhancing layer up to a thickness of 30 nm, after which it decreases, functioning as a depletion layer. The devices with an organic phototransistor comprised of a 20, 30, 40, 50 and 60 nm CTC layer provided the mobility of 2.79 × 10⁻², 4.39 × 10⁻², 4.18 × 10⁻², 2.45 × 10⁻², and 1.60 × 10⁻² cm² V⁻¹ s⁻¹, respectively. On increasing the thickness of CTC, the absorption of near-infrared light increased, as well as the defect states; however, an increase in thickness can cause a decrease in the charge transport quality. Hence, the highest photoresponsivity of 36.76 mA W⁻¹ was obtained with the CTC thickness of 40 nm and highest mobility and dark current at the thickness of 30 nm by balancing the respective trade-offs. Yin et al.⁷¹ utilised CTC formation between V₂O₅ and 2T-NATA to obtain copper phthalocyanine (CuPc) organic phototransistors, which allows photodetection in the near infrared-II region of 1000–1700nm. Intermolecular charge transfer from the HOMO of 2T-NATA at −5.0 eV and conduction band of V₂O₅ at −4.8 eV resulted in a photosensitive film. Consequently, electron extraction from the HOMO of 2T-NATA to the conduction band of V₂O₅ enables the formation of CTC, which greatly enhances the metrics of organic phototransistor devices by p-doping, enabling absorption in NIR-II. Similar charge transfer can also occur between V₂O₅ and F₁₆CuPc, resulting in electron transfer from the LUMO to conduction band and charge transfer between V₂O₅ and CuPc, resulting in electron transfer from the HOMO to V₂O₅ through carrier tunnelling and heterojunction accumulation with holes in CuPc and electrons in F16CuPc. The devices prepared with different fabrication methods utilising these mechanisms exhibit p-type, n-type and ambipolar characteristics, allowing the optimization of CTC organic phototransistors. It was seen that the V₂O₅ and 2T-NATA CTC resulted in inhibition of electron transport, causing weak n-type characteristics. Thus, to realise ambipolar nature, the n-type characteristics were enhanced by incorporating F16CuPc as well as optimizing the ratio of CTC. A device with ambipolar nature was attained by CTC of V₂O₅ : 2TNATA (1 : 2), showing field effect mobilities of 7.18 × 10⁻⁵ cm² V⁻¹ s⁻¹ and 6.60 × 10⁻⁵ cm² V⁻¹ s⁻¹, as well as the maximum photoresponsivity of 0.115 A W⁻¹ and 2.600 A W⁻¹ in n-type and in p-type, respectively.

At the interface between an organic semiconductor, acting as an electron donor, and p-type metal oxide, acting as an electron acceptor, the corresponding charge transport complex formed greatly helps reduce the activation energy for the conduction of charge carriers, improving the device performance. Lee et al.⁷² demonstrated that employing electron acceptor metal oxides such as MoO₃ to form charge transfer complexes (CTCs) with organic hole semiconductors enhances the charge transport, and thus the overall mobility of organic semiconductors. Bulk doped NBP and the MoO₃/NBP heterostructure exhibited additional absorption peaks at the wavelength of 500 nm, indicating that a CTC is formed between MoO₃ and NBP. The heterojunction of MoO₃/NBP provided enhanced conductivity, which was attributed to the increased charge carrier density due to the formation of CTC. The Beer–Lambert law analysis at 350 nm shows an initial high absorption coefficient value due to the vertical orientation of the NBP molecules. As the thickness increases, the increase in randomly aligned molecules can explain the constant absorption coefficient. Alternatively, at the 500 nm absorption band, a constant absorption coefficient is observed, indicating the independent nature of the molecular orientation with thickness. By utilising the principle of the CTC in a metal oxide/organic semiconductor interface, a significant reduction in the activation energy (E_a) of MoO₃ from 0.23 eV to 0.18 eV was achieved. This decrease in E_a was attributed to the formation of charge transfer complexes, which generate additional charge carriers, consequently filling the deep trap states associated with the electronic and structural defects in MoO₃ and increasing the mobilities. Lim et al.⁷³ studied the formation of charge transfer complexes (CTC) and the generation of free carriers in the heterojunctions of several organic semiconductors and transition metal oxides (TMOs) using UV-visible-near IR (UV-VIS-NIR) spectroscopy and the transmission line method (TLM). The formation of a charge transfer complex was confirmed by studying the absorption spectrum of the organic semiconductor and transition metal oxide bilayer, which shows intrinsic absorption peaks of its individual materials (MoO₃, 2TNATA, NPB, TCTA, and CBP) at specific wavelengths and additional peaks in the short wavelength region (400–600 nm) and long wavelength region (1000–2000 nm), proving the charge transfer from the organic semiconductor to transition metal oxide. The short wavelength peak obtained for MoO₃/2TNATA was located at 474 nm, MoO₃/NPB at 496 nm, MoO₃/TCTA at 420 nm and MoO₃/CBP at 441 nm and the long wavelength peak obtained for MoO₃/2TNATA at 1329 nm, MoO₃/NPB: at 1483 nm, and MoO₃/TCTA at 1560 nm and MoO₃/CBP had no distinct peak. The CTC peaks are obtained because of the charge transfer from the HOMO of the organic semiconductor (donor) to the LUMO of the transition metal oxides (acceptor), resulting in the formation of radicals. The electrical characteristics of the organic semiconductor-transition metal oxide bilayer showed that there is an enhancement in current and conductivity, which is attributed to the generation of free charges from CTC. The currents measured at 100 V were 1.5 × 10⁻⁹ A for MoO₃/2TNATA, 2.4 × 10⁻⁹ A for MoO₃/NBP, 2.4 × 10⁻⁹ A for MoO₃/TCTA and 7.4 × 10⁻⁹ A for MoO₃/CBP, which are one order greater than that of intrinsic MoO₃ having a value of 3.9 × 10⁻¹⁰ A. The conductivity of the bilayers was measured to be 6.2 × 10⁻⁶ S cm⁻¹ for MoO₃/2TNATA, 7.7 × 10⁻⁶ S cm⁻¹ for MoO₃/NBP, 1.4 × 10⁻⁵ S cm⁻¹ for MoO₃/TCTA, and 3.8 × 10⁻⁵ S cm⁻¹ for MoO₃/CBP, which was considerably enhanced compared to that of intrinsic MoO₃ of 2.6 × 10⁻⁶ S cm⁻¹. On comparing the MoO₃/organic semiconductor bilayers, a trend of decreasing conductivity is seen with an increase in CTC density at the interface. This trend was also exhibited by MoO₃/NBP, ReO₃/NBP and WO₃/NBP with conductivity of 7.7 × 10⁻⁶ S cm⁻¹, 1.1 × 10⁻⁶ S cm⁻¹ and 5.2 × 10⁻⁶ S cm⁻¹, respectively. Similar results were also obtained while measuring the activation energy, wherein the interfaces with high CTC densities exhibited hinderance and lower lateral conductivity. Thus, even when the energy difference between the HOMO of the organic semiconductor and LUMO of TMO is large, other factors such as density of CTC in the interface can reduce the conductivity.

Bulk doping has an advantage over contact doping, given that co-evaporation of the dopant/host and not requiring complex patterning make it a much more simpler process.⁷⁴ Lee et al.⁷⁵ demonstrated that bulk doping with high work function metal oxides such as MoO₃ and ReO₃ as p-type dopants in organic semiconductor materials improved the generation of holes, current density, and conductivity and increased the carrier density and mobilities. The CTC formed in 25 mol% doped 2TNATA improved the electrical conductivities with 25 mol% ReO₃ and MoO₃, obtaining values of 2 × 10⁻⁷ and 3.3 × 10⁻⁸ S cm⁻¹, respectively. In the case of doping at the same concentration, the charge transfer complexation increases with an increase in energy difference between the HOMO of the organic semiconductor and Fermi level of the dopant, and thus ReO₃ having a higher energy difference shows higher conductivities. Chan et al.⁷⁶ investigated the effects of transition metal oxides (TMOs) as p-dopants on organic thin film transistors. Doping of the transition metal oxides MoO₃, V₂O₅ and WO₃ in an organic hole transporting semiconductor (NBP) results in the formation of a charge transfer complex and is shown to increase the conductivity of organic thin film transistors, which is attributed to an increase in the free carrier concentration. The electrical characteristics of TMO-doped OTFTs were determined and their I–V characteristics showed linearity, indicating the formation of ohmic contact in the transition metal oxide/organic semiconductor interface. The conductivity was measured to be 2.2 × 10⁻⁷ S cm⁻¹ for MoO₃-doped NBP, 5.1 × 10⁻⁸ S cm⁻¹ for WO₃-doped NBP and 2.8 × 10⁻⁸ S cm⁻¹ for V₂O₅-doped NBP, with an increase in conductivity observed with an increase in temperature. A decrease in mobilities was observed in the doped devices compared to the devices with only an NBP layer having linear mobility (μ_lin) of 1.1 × 10⁻⁵ cm² V⁻¹ s⁻¹ and saturation mobility (μ_sat) of 7.8 × 10⁻⁶ cm² V⁻¹ s⁻¹ to MoO₃-doped NBP, WO₃-doped NBP and V₂O₅-doped NBP having linear mobility (μ_lin) and saturation mobility (μ_sat) of 5 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 9.4 × 10⁻⁶ cm² V⁻¹ s⁻¹, 4.1 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 5.6 × 10⁻⁶ cm² V⁻¹ s⁻¹, 6.6 × 10⁻⁶ cm² V⁻¹ s⁻¹ and 7.4 × 10⁻⁶ cm² V⁻¹ s⁻¹, respectively. This has been attributed to the presence of anions, which have tendencies to trap holes, briefly causing a reduction in the hole mobilities. The extracted free hole concentration showed an enhancement on doping, with MoO₃-doped NBP exhibiting the highest carrier concentration of 1.4 × 10¹⁷ cm⁻³ compared to WO₃ and V₂O₅ with values of 5.7 × 10¹⁶ and 2.4 × 10¹⁶ cm⁻³, respectively. This is evidence that an increase in carrier concentration causes an enhancement in conductivity, while a decrease in the carrier mobility upon doping is not the reason for enhancement in conductivity. Furthermore, doping with transition metal oxides has also been associated with a lower activation energy, enhancing the carrier generation, with MoO₃-doped NBP having E_a of 138 meV, WO₃-doped NBP having E_a of 142 meV and V₂O₅-doped NBP having E_a of 166 meV. Yedikardes et al.⁷⁷ investigated the effects of various doping concentrations of WO₃ (10–50%) in P3HT to obtain significant OTFT device improvements. Doping concentrations in the range of 0 to 30% WO₃ resulted in an increase in the device parameters, reaching the maximum, and then showing a decline in performance with an increase in doping from 30 to 50%. The doping of WO₃ on P3HT results in the formation of a charge transfer complex by electron transfer via the HOMO of P3HT to the conduction band of WO₃, leaving behind mobile holes. This mechanism considerably enhances the field effect mobility, threshold voltage and on/off current ratio. In the case of doping concentrations of 0, 10%, 20%, 30%, 40%, and 50%, the field effect mobility values were 5.3 × 10⁻⁵, 2.8 × 10⁻⁴, 5.6 × 10⁻³, 1.1 × 10⁻³ and 9.0 × 10⁻⁴ cm² V⁻¹ s⁻¹, the threshold voltage values were 40.1, 37.2, 17.7, 32.7, and 45.1 V, and the on/off current ratio values were 2.5 × 10², 5.0 × 10², 2.3 × 10³, 1.0 × 10³ and 5.0 × 10¹, respectively.

Li et al.⁷⁸ utilized multiparticle Monte Carlo simulations to examine the effects of doping organic semiconductors with transition metal oxides (TMOs). The hole only devices face a major challenge due to the non-uniform electric field caused by the potential barriers at the anode, which is attributed to the lower electric fields near the anode due to space charge perturbation. Thus, the organic devices must be optimized to lower the injection barrier height, which is possible with the help of incorporating high work function transition metal oxides. Doping of the devices with transition metal oxides resulted in a redistribution of charge carriers and electric field, leading to a more uniform electric field, with a higher electric field near the anode compared to the undoped systems, while at the same time doping caused a broader density of state (DOS) distribution, leading to complicated charge mobility behaviour. Hence, the charge mobility is observed to decrease with the doping concentration at low energetic disorders, while at high energetic disorders, the charge mobility first decreases, and then increases with an increase in doping concentration and free charge carriers, while the current densities always increase with doping concentration regardless of the energetic disorder.⁷⁹ The Monte Carlo simulations provide insight into the complex mechanisms of charge transport and can validate the results obtained experimentally.

3.3 Metal oxides as bilayer electrode layer

The usage of metal oxides in bilayer source–drain (S–D) electrodes in organic thin film transistors has been shown to significantly enhance the organic device performance in terms of improved charge injection and mobility, high optical transparency, reduced contact resistance, smoother surfaces, improved threshold voltage, subthreshold slope and suppression of the short-channel effect. These bilayer electrodes typically consist of a metal oxide layer paired with a metal, optimizing the interface between the electrode and the organic semiconductor (Fig. 3). Various transition metal oxides (TMOs) have been utilized as bilayer source–drain (S–D) electrode layers owing to their high work function and wide band gap. Fig. 3 describes the resulting device enhancement on incorporating metal oxides as bilayer electrodes by comparing the mechanism of charge injection in metal/organic semiconductor and metal/metal oxide/organic semiconductor configurations. These bilayer metal oxides primarily reduce the barrier height required for charge injection, providing critical tuning of the contact. Thus, inserting a transition metal oxide layer such as MoO₃, V₂O₅, and WO₃ improved the field effect mobilities and on/off current ratios and reduced the contact resistance. These metal oxide layers protect from direct contact in the organic/metal interface, preventing diffusion or any unfavourable chemical reactions.⁸⁰

Fig. 3 Schematic representation of metal oxides as bilayer source–drain (S–D) electrodes in an OTFT and schematic comparing the mechanism of charge injection in metal/organic semiconductor and metal/metal oxide/organic semiconductor configurations.

Alam et al.⁸¹ investigated the use of germanium oxide (GeO) and titanium oxide (TiO₂)⁸² with Au as a bilayer electrode in pentacene top contact organic thin film transistors (OTFTs). GeO/Au as the bilayer electrode in the top-contact OTFTs resulted in a considerable enhancement in field-effect mobility (μ), threshold voltage (V_T), and on/off current ratio (I_ON/I_OFF) compared to the devices with only an Au electrode. The devices with 5 nm GeO exhibited the greatest improvement in electrical properties with the field-effect mobility (μ) of 0.96 cm² V⁻¹ s⁻¹, threshold voltage (V_T) of −4 V and on/off current ratio (I_ON/I_OFF) of 5.2 × 10⁴. The root mean square (RMS) roughness for the devices without a GeO layer is 5.94 nm and that with 5 nm GeO is 5.20 nm, indicating smoother surfaces, which are required for improving the ohmic contacts. The valence band of GeO lies below the highest occupied molecular orbital (HOMO) level of pentacene, which reduces the barrier for hole injection into the pentacene layer, facilitating better charge injection and improving the device performance. The GeO layer also protects pentacene from direct contact with the Au electrodes, minimizing Au penetration and preventing unfavourable chemical reactions between the organic and metal layers. The insertion of a titanium oxide (TiO₂) layer between the Au electrode and the pentacene layer in an organic thin-film transistor also is shown to improve the device performance by reducing the hole injection barrier and contact resistance at the Au/pentacene interface. The electrical properties measured in devices containing TiO₂ were comparatively better than that of the devices without a metal oxide bilayer electrode, exhibiting a field-effect mobility (μ) of 0.63 cm² V⁻¹ s⁻¹, threshold voltage (V_T) of −1.5 V, and on/off current ratio (I_ON/I_OFF) of 3.7 × 10⁴. The alignment of the highest occupied molecular orbital (HOMO) of pentacene with the valence band of TiO₂ reduces the barrier height for hole injection into the pentacene layer. Without the TiO₂ layer, there is a significant hole injection barrier of 0.8–1 eV at the Au/pentacene interface. By inserting the TiO₂ layer, this barrier is removed, leading to enhanced charge injection and improved mobility in the OTFTs. Additionally, the TiO₂ layer smoothens the surface of the pentacene film. The root mean square (RMS) roughness decreased from 7.45 nm for pentacene alone to 7.05 nm with a 5 nm TiO₂ layer. This smoother surface further contributes to lowering the barrier height, increasing charge injection, and ultimately improving the device performance.

Ablat et al.⁸³ studied the role of a metal/metal oxide bilayer electrode in organic field effect transistors (OFETs). Different OFETs were explored having bilayer electrodes with MoO₃ and WO₃ as metal oxides and Ag and Au as metals. The presence of metal oxides greatly reduces the contact resistance and the metal/metal oxide bilayer electrode showed a considerable increase in the output current of the devices compared to the devices with only Au/Ag metal electrodes. The output characteristics of the metal oxide bilayer exhibited enhanced linearity at low V_SD. The metal/metal oxide bilayers provided improved threshold voltages, on/off current ratios and subthreshold slopes. The best electrical characteristics were obtained in the devices that used MoO₃/Ag bilayer electrodes with a threshold voltage of −14.7 V, on/off current ratio of 5.1 × 10⁶, and subthreshold slope of 2.4 V dec⁻¹. The maximum field effect mobilities (μ_max) were found to be 1.35 cm² V⁻¹ s⁻¹ for only Ag, 1.13 cm² V⁻¹ s⁻¹ for WO₃/Ag, 1.30 cm² V⁻¹ s⁻¹ for MoO₃/Ag, 0.69 cm² V⁻¹ s⁻¹ for only Au, 0.96 cm² V⁻¹ s⁻¹ for WO₃/Au, and 0.76 cm² V⁻¹ s⁻¹ MoO₃/Au. Although the Ag-only devices gave the highest field effect mobility, it forms the highest contact resistance and largest threshold shift, making it unfavourable compared to the bilayer electrode. The contact resistance values at V_GS = −30 V were measured to be 860.6 kΩ cm for only Ag, 43 kΩ cm for WO₃/Ag, 29.5 kΩ cm for MoO₃/Ag, 742 kΩ cm for only Au, 669 kΩ cm for WO₃/Au, and 120.5 kΩ cm for MoO₃/Au, confirming the reduced contact resistance in the metal/metal oxide bilayer electrodes. Ultraviolet photoelectron spectroscopy (UPS) was used to the study energy level alignment in the bilayer electrode interface and metal-only electrode interface. Bare C₈-BTBT has a valence band edge of 2.13 eV below the Fermi level and estimated HOMO of 5.4 eV. In contrast, in the metal oxide bilayer, the valence band edges are at 0.64 eV for C₈-BTBT/MoO₃ and 1.0 eV for C₈-BTBT/WO₃, which is in contrast with that found in the literature. This confirms the presence of delocalized gap states that shifts the HOMO of C₈-BTBT to 0.64 eV by MoO₃ or 1 eV by WO₃. Thus, the presence of MoO₃ and WO₃ interlayers reduces the injection barrier compared to the Ag- or Au-only electrodes. The lower injection barrier with MoO₃ explains its superior performance compared to WO₃ in OFETs. The study of the surface morphology using AFM demonstrated the impact of the morphology of the interface between the metal/metal oxide bilayers. The larger RMS roughness of WO₃ compared to MoO₃ results in inhomogeneity, causing large grains and voids. However, the RMS roughness measured for the bilayer electrode shows that the metal can diffuse when in direct contact with the metal oxide and fill the voids. This results in a similar RMS roughness for both MoO₃ and WO₃ in the bilayer electrode system with metal.

Borthakur et al.⁸⁴ investigated organic thin film transistors (OTFTs) using a transition metal oxide, MoO₃, layer at the organic/electrode interface. The devices with the metal oxide bilayer showed an improved device performance, which was confirmed by the measured field effect mobilities, with the oxidized MoO₃/Au bilayer S–D electrode having a field effect mobility of 1.7 cm² V⁻¹ S⁻¹ and laboratory-produced MoO₃/Au bilayer S–D electrode having a field effect mobility of 1.03 cm² V⁻¹ S⁻¹ compared to the field effect mobility of 0.18 cm² V⁻¹ S⁻¹ for the Au-only electrode OTFTs. The on/off current ratio of the oxidized MoO₃/Au bilayer S–D electrode was obtained to be 2 × 10⁶ and that of the laboratory-produced MoO₃/Au was 2 × 10⁵, while the threshold voltage of −3.5 V was obtained in the oxidized MoO₃/Au bilayer S–D electrodes compared to −1.1 V obtained in the laboratory-produced MoO₃/Au. Thus, the oxidized MoO₃/Au bilayer S–D electrode was shown to have a better field effect mobility, on/off current ratio and threshold voltage compared to the laboratory-produced MoO₃/Au bilayer S–D electrode, inferring that the combination of molybdenum oxides ranging from MoO₃ to MoO as well as a small amount Mo can provide a better interface compared to pure MoO₃. Similar enhancements in OTFT device performance was obtained when a V₂O₅/Au bilayer electrode was employed.⁸⁵ The V₂O₅/Au devices obtained a field effect mobility of 0.77 cm² V⁻¹ s⁻¹ compared to the bare Au device with a value of 0.18 cm² V⁻¹ s⁻¹. The threshold voltage and on/off current ratio also showed comparable improvements, with the bilayer source–drain devices obtaining values of −2.9 V and 7.5 × 10⁵, while the bare Au electrode device obtained −1.2 V and 7.5 × 10⁴, respectively. Thus, efficient OTFT devices could be realised by the application of metal oxides in a bilayer electrode system through reduced contact resistance and injection barrier.

The practical application of metal oxides towards engineering better organic electronics can pave way for superior performances. Developments in OTFT sensor applications have incorporated the use of metal oxide interlayers for enhanced performance. Jain et al.⁸⁶ investigated the use of a bilayer electrode in a dielectric-modulated OTFT biosensor to obtain high performances. A bilayer electrode system of Au (20 nm)/TiO₂ (5 nm) provides significant reductions in injection barrier, providing an enhanced on/off current ratio. The bilayer electrode interface provides enhanced sensitivity, which is an operation critical for any sensor application. The dielectric-modulated bilayer electrode OTFT biosensor achieved a significantly enhanced device performance in terms of sensitivity, achieving a drain current sensitivity of 50.91.

3.4 Metal oxides as gate dielectrics

Low-voltage operation in organic thin film transistors is a desirable trait, which can be attained at high capacitance by either reducing the dielectric thickness or increasing the dielectric constant. Metal oxides such as aluminium oxide (Al₂O₃), hafnium oxide (HfO₂), zirconium oxide (ZrO₂) and rare earth metal oxides such as neodymium oxides (Nd₂O₃) and lanthanum oxides (La₂O₃) offer much higher dielectric constants compared to the commonly used silicon oxides (SiO₂), thus achieving greater capacitance values. Compared to SiO₂, metal oxides serve as promising gate dielectric materials for organic thin film transistors due to their high dielectric constants, relatively high bandgap, high mechanical stability, and high electric field strength. Fig. 4 shows a schematic representation of a metal oxide gate dielectric in OTFT. Metal oxides are used in OTFTs to achieve low voltage operation, high operational stability, smooth surfaces, optimized surface modification, high current on/off ratio, low subthreshold swing, and suppressed current leakage.

Fig. 4 Schematic representation of a metal oxide gate dielectric in an OTFT.

One of the major drawbacks associated with utilising inorganic metal oxides as gate dielectrics in OTFTs is the requirement of expensive and sophisticated instruments for deposition including high-vacuum processes such as sputtering, atomic layer deposition and vacuum evaporation, high-temperature annealing processes and anodized processes. Hence, there is a need for alternative methods as low-cost and efficient deposition techniques. Byun et al.⁸⁷ employed a simple and cost-effective sol–gel method to fabricate high dielectric UV curable hafnium oxide (HfO_x) compared to thermally annealed HfO_x. Phosphonic acid-based n-dodecylphosphonic acid (C12PA) served as a self-assembled monolayer (SAM), functioning to optimize the surface energy for enhanced crystallization of the organic semiconductor as well as enhance the device performance by increasing the leakage barrier. The morphological study of the dielectric layer by atomic force microscopy provided insights into the smooth surfaces obtained for both processing methods, with the maximum RMS roughness of 0.3 nm. Although UV-cured HfO_x provides an alternative to high temperature processes, a trade-off resulting in lower dielectric properties is seen. Excellent OTFT device parameters were obtained with the UV-cured HfO_x dielectric along with pentacene organic semiconductors, with the capacitance of 540 nF cm⁻², leakage current of 7.2 × 10⁻⁷ A cm⁻², hole mobility of 0.31 cm² V⁻¹ s⁻¹, on/off current ratio of 10⁵, threshold voltage of −0.3 V and low operating voltage of −3 V. Gong et al.⁸⁸ utilised the solution-processed, low-temperature deposition of UV-cured zirconium oxide (ZrO₂) as well as post-deposition thermal annealing (PDA) and compared the different techniques. The thickness of the films prepared by PDA was larger than that of the UV-cured films, confirming that the UV-cured films exhibited better densification. Low-temperature UV-cure processing showed comparable dielectric properties with the leakage current density, capacitance, and dielectric constant of 1.8 × 10⁻⁶ A cm⁻², 116 nF cm⁻², and 8.7 for the UV-irradiated device (exposed for 60 min) compared to that of 3.6 × 10⁻⁵ A cm⁻², 261 nF cm⁻², and 17.8 for the films thermally annealed at 160 °C, respectively. Additionally, the UV-cured device achieved a high performance with the field effect mobility of 0.88 cm² V⁻¹ s⁻¹, threshold voltage of −1.16 V and high on/off current ratio of 3.1 × 10⁶. Zhao et al.⁸⁹ fabricated low-temperature, solution-based processed UV-cured lanthanum oxide (La₂O₃) as a gate dielectric in OTFTs. Thermal annealing of the films at the extremely high temperature of 500 °C caused the films to become polycrystalline, making them thinner compared to the amorphous UV-irradiated films. Although the UV-irradiated films possessed a thickness three-times that of the film prepared via the high-temperature process, the amorphous films provide better mechanical flexibility, smoother surfaces and electrical insulation stability, as well as allow for large-scale fabrication on flexible substrates. Moreover, these UV-lanthanum oxide dielectrics possess a large band gap (5.78 eV), high breakdown electric field (>3 MV cm⁻¹) and high dielectric constant of 12.68. Excellent OTFT device parameters were obtained with UV-treated La₂O₃ with a thin PαMS polymer as an interface modification layer, obtaining the low-voltage operation of below ±3 V for both n-type (PTCDI-C8) and p-type (pentacene) OTFTs, low threshold voltage of −2.368 V and 0.579 V, field effect mobility of 0.815 cm² V⁻¹ s⁻¹ and 0.026 cm² V⁻¹ s⁻¹ and subthreshold slope of 122.9 and 68.8 mV dec⁻¹ for n-type and p-type, respectively.

Although the solution processing of metal oxides via methods such as sol–gel-based methods makes their processing easier, it commonly involves the use of toxic precursors and solvents that harm the environment. Thus, there is a growing need for green processing methods to comply with sustainability. Dacha et al.⁹⁰ investigated ultrathin DUV-cured and thermal-annealed aluminium oxide (AlO_x) films owing to their cost-efficient and eco-friendly qualities. Given that they are derived from non-toxic precursors, aluminium nitrate nonahydrate and utilising green solvents such as water, this method helps promote sustainable electronic manufacturing processes. Ultra-thin films of aluminium oxides (AlO_x) of 7 nm fabricated by solution shearing serve as a dielectric layer, making it possible to achieve high capacitance values of 750 nF cm⁻² and 600 nF cm⁻² and leakage current of 10⁻⁶ and 10⁻⁷ for the thermal-annealed and UV-cured processes, respectively. The DUV films showed excellent smoothness with a low RMS roughness of 89.4 pm compared to 423.3 pm for the thermal-annealed films. The organic devices are capable of low operation voltage at −0.5 V and obtained a field effect mobility of 6.1 cm² V⁻¹ s⁻¹, threshold voltage of −0.14 V and subthreshold slope of 96 mV dec⁻¹. Kumar et al.⁹¹ investigated a novel water-processed bilayer dielectric to provide an eco-friendly solution for preventing the use of toxic organic solvents. A water-induced bilayer dielectric of LiO_x/AlO_x in an OTFT of DPP-DTT transferred using the floating film transfer method achieved low-voltage operation with an enhanced device performance. The LiO_x/AlO_x dielectric had a dielectric constant of 7.2 at 1 kHz and capacitance density of 378 (± 24) nF cm⁻². The device metrics showed that the OTFT obtained the mobility of 0.34 cm² V⁻¹ s⁻¹, on/off current ratio of 10⁵, subthreshold slope of 90 mV decade⁻¹ and threshold voltage of −0.26 V.

Barium titanate has emerged as a metal oxide gate dielectric owing to its low cost, biocompatibility, ferroelectric properties and applications due to its temperature sensitivity, providing an excellent performance. Wang et al.⁹² studied the use of barium titanate as an effective gate dielectric in OTFTs. The surface roughness of the BTO films could be tailored by deposition at room temperature without post-deposition annealing or utilising both deposition and post-deposition annealing at high temperature. The amorphous barium titanate deposited by sputtering at 200 °C without post-deposition annealing achieved a hysteresis-free, high performance with a dielectric constant of 0.36 μF cm⁻² at 1 MHz. The device showed an excellent device performance with mobility of 2.91 cm² V⁻¹ s⁻¹, on/off current ratio of 1.16 × 10⁶, negligible hysteresis of 18 mV, subthreshold slope of 0.140 V decade⁻¹ and threshold voltage of −1.09 V. The temperature-sensitive nature of barium titanate could be exploited for sensor applications. Mandal et al.⁹³ utilised low temperature-processed hexagonal barium titanate nanocrystals (h-BTNC) as a gate dielectric, which showed extremely high temperature precision of 4.3 mK and response time of 24 ms, while consuming 1 μW at 1.2 V for a stable and flexible OTFT temperature sensor. The h-BTNC films reduced the surface roughness of Al₂O₃ from 3.87 to 0.4, significantly improving the interface for pentacene. The OTFT device parameters were field effect mobility of 1.46 cm² V⁻¹ s⁻¹, on/off current ratio of 10³ and threshold voltage of −1.05 V. The applicability of the devices for commercial scale was also possible by screen printing of the dielectric, obtaining parameters of field effect mobility, threshold voltage and on/off current ratio of 0.70 cm² V⁻¹ s⁻¹, −1.26 V and 10³, respectively.

The use of bilayer dielectrics has been studied to obtain smoother surfaces to improve the interface between the semiconductor and dielectric. These bilayer dielectrics also help tune the electronic properties of OTFTs, especially their threshold voltage. Kim et al.⁹⁴ explored the use of HfO₂ in a bilayer gate dielectric as a means of achieving high operational stability and low threshold voltage in organic thin film transistors. Thin films of HfO₂/CYTOP can be achieved in bottom gate-configured organic thin film transistors to obtain a high gate capacitance density value of 276 nF cm⁻² for BG OTFT. The threshold voltage in the HfO₂/CYTOP bilayer gate dielectric devices was measured to be below 1 V, showing a low operational voltage. The electrical parameters showed average values such as field effect mobility measured to be in the range of 0.3–0.8 cm²/V s and subthreshold slope as low as 95 mV dec⁻¹. The major advantage of using metal oxide/organic semiconductor bilayer gate dielectrics is the ability to achieve high operational stability for prolonged periods. The operational stability was studied for the devices with the gate bilayer dielectric of HfO₂ (20 nm)/CYTOP (5 nm), HfO₂ (20 nm)/CYTOP (7 nm) and HfO₂ (10 nm)/CYTOP (7 nm). Among them, the best operational stability was obtained with HfO₂ (10 nm)/CYTOP (7 nm) bilayer gate dielectric with a change in the drain-to-source current (|ΔI_DS(t)|) value of 9% and threshold voltage shift (|ΔV_TH|) of 0.1 V observed after 24 h, thereby showing high operational stabilities in OTFTs.

Khound et al.⁹⁵ studied the use of high κ dielectrics rare earth metal oxides such as neodymium oxide (Nd₂O₃) and lanthanum oxide (La₂O₃) as bilayer gate dielectrics in organic thin film transistors (OTFTs). Pentacene organic thin film transistors were fabricated with an La₂O₃/Nd₂O₃ bilayer gate dielectric. These La₂O₃ (118 nm)/Nd₂O₃ (100, 150, 175 nm) bilayer gate dielectrics were shown to reduce the current leakage compared to the La₂O₃-only gate dielectric, given that the addition of Nd₂O₃ to La₂O₃ smoothens the surface. The root mean square (RMS) value obtained from atomic force microscopy (AFM) was 1.82 nm for the first layer and 0.94 nm for the second layer. The addition of Nd₂O₃ to La₂O₃ caused the bilayer gate dielectric layer to reduce the threshold voltage to −1.1 V compared to −0.42 V for the La₂O₃-only gate dielectric, providing low operational voltages. The La₂O₃/Nd₂O₃ bilayer gate dielectric employed helped the improve electrical properties of OTFTs with La₂O₃ (118 nm)/Nd₂O₃ (150 nm), showing the best capacitance per unit area (C_i) of 28.45 nF cm⁻², current on/off ratio (I_ON/I_OFF) of 2.4 × 10⁵, subthreshold slope (SS) of 0.5 V decade⁻¹ and carrier mobility of 1.08 cm² V⁻¹ s⁻¹.

Recent advancements in organic–inorganic dielectric layers as potential high-quality dielectric films have resulted in high sensitivity, low leakage current and enhanced stability. Although inorganic metal oxides can attain excellent dielectric properties, their application is hindered by their high-temperature processability and lower mechanical stability compared to organic dielectrics. Thus, a hybrid organic–inorganic dielectric can be used to obtain the best of both worlds, with the organic layer exhibiting mechanical flexibility, while high dielectric constants of inorganic layer provide a reduced leakage current. Gallegos-Rosas et al.⁹⁶ reported the use of mixed metal oxides as a possible solution to achieve high-capacitance dielectric layers in organic light emitting transistors. Hafnium aluminate (HfAlO_x) was obtained by HfO₂ and Al₂O₃ and it showed excellent dielectric properties operational even under low voltage conditions. Reducing the thickness of the gate dielectric layer or using materials having large dielectric constants (κ) are two important methods to obtain high capacitance dielectrics. However, although polymeric dielectrics are attractive options given that they allow fabrication on plastic substrates, they are constrained due to their low-κ values, and thus an alternative approach of organic–inorganic dielectric layer was explored. The κ for a hafnium aluminate (HfAlO_x)/PMMA bilayer stack at 1 kHz was measured to be 7.1 compared to that of 3.3 for the devices containing PMMA as the dielectric. The hafnium aluminate (HfAlO_x) bilayer stack was operational even at low voltages, with a higher capacitance of 63 nF cm⁻² for the bilayer dielectric compared to 6.6 nF cm⁻² for PMMA, while exhibiting no dielectric breakdown up to 20 V. The transfer characteristics presented a V shape, indicating that the OLET operated in an ambipolar regime. The hole mobility of the HfAlO_x bilayer was measured to be 1.09 cm² V⁻¹ s⁻¹ compared to that of 0.19 cm² V⁻¹ s⁻¹ for a single PMMA layer, and the electron mobility of the bilayer was obtained to be 0.08 cm² V⁻¹ s⁻¹ compared to 0.003 cm² V⁻¹ s⁻¹ for PMMA. Thus, the bilayer of hafnium aluminate (HfAlO_x)/PMMA provided an effective dielectric layer with efficient power consumption and enhanced interface between the organic material and dielectric layers. Priya et al.⁹⁷ studied the use of high κ metal oxides such as AlO_x, TiO_x and TaO_x with poly vinyl phenol (PVP) as organic–inorganic dielectrics in OTFTs. The use of metal oxides in the PVP layer considerably improved the smoothness with a surface roughness of 0.33 nm obtained for AlO_x, 3.04 nm for TaO_x, and 3.74 nm for TiO_x. This improvement in smoothness led to improved uniformity in the films, providing an enhanced device performance. The electrical parameters of the device showed the high performance of the OTFT with a mobility of 1.3 cm² V⁻¹ s⁻¹ for the AlO_x/PVP dielectric, 1.5 cm² V⁻¹ s⁻¹ for TiO_x/PVP and 1.1 cm² V⁻¹ s⁻¹ for TaO_x/PVP. The enhancement could be attributed to the similar surface energies of pentacene and PVP, obtaining an optimized arrangement during crystallization, and thus providing an improved interface with the dielectric. Alternatively, the lower relative performance of TaO_x/PVP was due to the larger thickness of the TaO_x film reducing the charge transport. The capacitance values of the organic/inorganic films were 1.5 × 10⁻⁷ F cm⁻² for AlO_x/PVP, 1.9 × 10⁻⁷ F cm⁻² for TiO_x/PVP, and 2.7 × 10⁻⁷ F cm⁻² for TaO_x/PVP, providing low voltage application and reduction in leakage current. Although the metal oxide-organic hybrid proved to be an efficient dielectric layer, the high-temperature processing of metal oxides is still an obstacle. Thus, low temperature-based metal oxide nanoparticles can be used to mitigate the limitation of high temperature processing. Le et al.⁹⁸ prepared low-temperature TiO₂ nanoparticles to be used in an amphiphilic polymer as an organic–inorganic dielectric. Amphiphilic urethane polymer (AUP) served as a stabilizing agent for the colloidal TiO₂ nanoparticles, providing a mechanically flexible Uti (urethane–TiO₂) dielectric. The UTi films were highly uniform, while the TiO₂/poly(AUP) nanocomposite films were uneven and non-uniform due to the agglomeration of the metal oxide nanoparticles. The UTi films exhibited dielectric constants as high as 18.9 at 1 MHz, providing a reduced leakage current of 2.6 × 10⁻⁷ at 2 MV cm⁻¹ with the shortest hydrophilic polyethylene oxide chain (molecular weight of 550 g mol⁻¹). This was largely due to the better packing obtained with shorter hydrophilic PEO chains, while longer chains are more prone to aggregation. The OTFT performance of UTi-m550 exhibited the average effective field-effective mobility of 6.39 cm² V⁻¹ s⁻¹, on/off current ratio of 1.76 × 10⁴, subthreshold slope of 0.272 V dec⁻¹ and threshold voltage of −0.95 V (Tables 1 and 2).

Table 1 OTFT device parameters utilising different metal oxides

Organic semiconductor	Metal oxide	Mobility (cm^2 V^−1 s^−1)	Subthreshold slope (V decade^−1)	Threshold voltage (V)	On/off ratio	Year^Ref.
Ph-BTBT-10	MoO3	7.9 (μFET)	N/A	−0.03	N/A	2024^6
Pentacene	MoO3	0.184 (μFET)	5 ± 0.4	N/A	N/A	2014^32
Pentacene	WO3	0.025 (μcarrier)	N/A	N/A	N/A	2019^34
Pentacene	VOx	0.80 (μFET)	N/A	N/A	4 × 10^5	2016^35
BOPAnt	VOx	1.56 (μFET)	N/A	N/A	5.6 × 10^7
C8-BTBT	MoO3	13.1 (μFET)	N/A	N/A	N/A	2018^36
	V2O5	11.6 (μFET)	N/A	N/A	N/A
	WO3	10 (μFET)	N/A	N/A	N/A
P3HT	ZnO	2 × 10^−3 (μFET)	6.25	10	73	2023^37
Pentacene	CuO	0.182 (μFET)	N/A	−8	1.37 × 10^−4	2019^39
Pentacene	CuO	0.011 (μFET)	N/A	7.91	10^4	2009^41
TIPS-pentacene	ReO3	0.7 (μh)	N/A	0.9	N/A	2019^43
C8-BTBT	MoOx	2.3 (μFET)	0.22	N/A	N/A	2009^57
DBTTT	MoOx	7.3 (μFET)	N/A	N/A	N/A	2017^58
P3HT	MoO3	2.1 × 10^−2 (μcarrier)	N/A	N/A	N/A	2016^59
	V2O5	0.9 × 10^−2 (μcarrier)	N/A	N/A	N/A
CuPc	MoO3/NBP (OBL)	2.74 × 10^−4 (μFET)	N/A	1.98	3.72 × 10^3	2023^64
Polythiophene	MoOx	N/A	1	−4.5	N/A	2025^66
Pentacene	MoO3	1.40 (μFET)	−4.54	−13.8	10^5	2018^67
Pentacene	MoO3	1.71 × 10^−1 (μh)	N/A	−16.69	2.5 × 10^3	2025^68
Pentacene	MoO3	0.36 (μh)	N/A	−33.7	N/A	2014^69
m-MTDATA		0.40 (μh)	N/A	−20.7	N/A
Pentacene/m-MTDATA		0.72 (μh)	N/A	−13.4	10^2
Pentacene	WO3/m-MTDATA (CTC)	2.69 × 10^−2 (μFET)	N/A	−0.0054	N/A	2020^70
CuPc/F16CuPc	V2O5/2T-NATA (CTC)	6.6 × 10^−5 (μFET)	N/A	N/A	N/A	2023^71
		7.18 × 10^−5 (μFET)	N/A	N/A	N/A
NBP	MoO3	9.4 × 10^−6 (μFET)	N/A	N/A	N/A	2011^76
	V2O5	7.4 × 10^−6 (μFET)	N/A	N/A	N/A
	WO3	5.6 × 10^−6 (μFET)	N/A	N/A	N/A
P3HT	WO3	5.6 × 10^−3 (μFET)	N/A	17.7	2.3 × 10^3	2020^77
Pentacene	MoO3	0.4 (μFET)	N/A	−12.1	3.8 × 10^4	2005^80
	V2O5	0.226 (μFET)	N/A	−10.43	1.8 × 10^4
	WO3	0.253 (μFET)	N/A	−12.88	4.1 × 10^4
Pentacene	GeO	0.96 (μFET)	N/A	−4	5.2 × 10^4	2013^81
Pentacene	TiO2	0.63 (μFET)	N/A	−1.5	3.7 × 10^4	2012^82
C8-BTBT	MoO3	1.30 (μFET)	1.7	−14.7	5.1 × 10^6	2019^83
	WO3	1.13 (μFET)	1.9	−17.8	8.2 × 10^5
Pentacene	MoO3	1.7 (μFET)	0.39	−3.5	2.5 × 10^6	2022^84
Pentacene	V2O5	0.77 (μFET)	0.36	−2.9	7.5 × 10^5	2017^85

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Table 2 Properties of different metal oxide gate dielectrics

Organic semiconductor	Metal oxide	Leakage current density (A cm^−2)	Capacitance (nF cm^−2)	Dielectric constant	Year^Ref.
Pentacene	ZrO2	1.89 × 10^−6	N/A	19.70	2016^45
FS11	HfO2	10^−7	N/A	26	2014^46
DNTT	AlOx	10^−6	700		2021^47
Pentacene	NdNbON	N/A	N/A	8.5	2022^48
Pentacene	La2O3/cPVP	N/A	65	9.2	2022^49
	La2O3	N/A	50.4	7.2
Pentacene	ZrOx	8 × 10^−8	188	15	2018^50
Pentacene	ZrO2/PI	7.32 × 10^−9	N/A	8.1	2017^51
Pentacene	PrOx	N/A	290	13	2023^52
	SmOx	N/A	390	22
	GdOx	N/A	880	11.5
	TbOx	N/A	600	15.1
	ErOx	N/A	580	14.1
	YbOx	N/A	1900	13.5
Pentacene	HfOx	7.2 × 10^−7	540		2019^87
Pentacene	ZrOx	1.8 × 10^−6	261	17.8	2018^88
Pentacene	La2O3	8.8 × 10^−7	N/A	12.68	2019^89
PTCDI-C8
C10-DNTT	AlOx	10^−6	750	N/A	2024^90
DPP-DTT	LiOx/AlOx	2.5 × 10^−9	378 (± 24)	7.2	2023^91
	LiOx	3.7 × 10^−8	195 (± 15)	7.94
Pentacene	a-BTO	9.75 × 10^−8	360	7.87	2024^92
Pentacene	h-BTNC/Al2O3	N/A	158	N/A	2019^93
DPP-DTT	HfO2/CYTOP	N/A	276	N/A	2020^94
Pentacene	La2O3/Nd2O3	10^−7	28.45	N/A	2019^95
C8-BTBT	HfAlOx	N/A	63	7.1	2024^96
DFH-4T
Pentacene	AlOx/PVP	N/A	150	N/A	2024^97
	TiOx/PVP	N/A	190	N/A
	TaOx/PVP	N/A	270	N/A
C10-DNTT	TiO2/poly(AUP)	2.6 × 10^−7	50	18.6	2024^98

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4. Conclusion

Metal oxides have emerged as versatile and effective components to enhance the performance of organic thin-film transistors (OTFTs). Their incorporation as hole injection layers significantly lowers the hole injection barrier, leading to improved carrier mobility and reduced contact resistance. The formation of charge transfer complexes (CTCs) at the metal oxide/organic semiconductor interface further optimizes the energy level alignment, facilitating efficient charge transfer. Additionally, when employed as part of bilayer source–drain electrodes, metal oxides not only provide enhanced field-effect mobility, on/off current ratios, smoother surfaces, and improved threshold voltages and subthreshold slopes but also offer crucial protection against undesirable chemical reactions at the interface between the metal/organic semiconductor. Metal oxides play an important role as gate dielectrics, contributing to high capacitance, dielectric constant, potential for low temperature and solution processability, and electrical and mechanical stability.

However, despite these advancements, organic thin film transistors are inferior to silicon-based transistors owing to their inconsistent physical models, lower charge carrier mobilities, higher contact resistance, diffusion at the interfaces, reproducibility and reduced environmental stability. The deposition of metal oxides requires high cost and sophisticated instruments, hindering their large-scale production. Nevertheless, organic thin film transistors can play a critical part in next-generation organic electronics, emerging as inexpensive, flexible, lightweight, biocompatible, low temperature-processible and solution-processible alternatives to silicon-based technology, and utilized in various applications including flexible displays, wearable devices, radio frequency identification tags (RFID), shelf tags, e-skin, biosensors, bio-implantable devices and flexible integrated circuits.

As research progresses, optimized materials and device architectures will drive the next generation of organic electronics, meeting the growing demand for high-performance, energy-efficient, and sustainable technologies.

Author contributions

Conceptualization: Poornima Bhagavath; methodology: Nikhil Pais, Manav Jeetendra Shirodkar and Poornima Bhagavath; formal analysis: Nikhil Pais, Manav Jeetendra Shirodkar and Poornima Bhagavath; investigation: Nikhil Pais, Manav Jeetendra Shirodkar; writing – original draft: Nikhil Pais, Manav Jeetendra Shirodkar and Poornima Bhagavath; writing – review & editing: Nikhil Pais, Manav Jeetendra Shirodkar and Poornima Bhagavath; funding acquisition: Poornima Bhagavath; resources: Poornima Bhagavath; supervision: Poornima Bhagavath.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

We thank Manipal Institute of Technology and Manipal Academy of Higher Education for providing the necessary facilities to pursue this review work with renewed vigor.

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