Organic–Inorganic Hybrid Dielectric Layers for Low-Temperature Thin-Film Transistors Applications: Recent Developments and Perspectives

1. Introduction
Nowadays, thin-film transistors (TFTs) are fundamental devices for the development
of modern electronics, particularly for their applications in display technologies and large-
area electronic systems. TFTs are essential components of flat-panel displays, such as
LCDs and OLEDs, where they switch on and off each pixel independently, ensuring
precise control and high-quality visual output [1–3]. This has enabled the actual display
technology transforming the user interactions with devices like smartphones, tablets,
and smart TVs. TFTs also have applications beyond displays in diverse fields, including
digital radiography, sensors, and emerging areas like neuromorphic computing [4–6]. TFTs
offer cost-effectiveness and processing efficiencies, which make them ideal for large-scale
manufacturing, particularly in cost-sensitive applications. Their compatibility with flexible
substrates can smooth the way for innovative, wearable, and stretchable electronics [7–9].
Furthermore, the TFT technology contributes to sustainability through the production of
low-energy-consumption electronic devices, thereby reducing the environmental impact
of electronic manufacturing [10]. For these reasons, there has been intensive research on
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TFTs in the last recent decades, which is currently ongoing. This is crucial for achieving
device performance metrics like high electron mobility and stability, low operation voltages,
among others, which are essential for the next-generation electronic devices [11–13].
The literature about TFTs is so extensive that a number of review papers have been
published over the years. These review papers provide a comprehensive overview of
the current state of TFT research and development, highlighting the progress made in
materials, device structures, fabrication techniques, and circuit applications. There are very
few review papers touching all the aspects relevant to TFTs; instead, they focus on specific
aspects of TFTs. For example, some recent review papers focused on low-temperature
solution-processed metal oxide channel layers [14], 2D transition metal dichalcogenides [15],
solution-processed inorganic p-channels [16], etc. On the other hand, some other review pa-
pers focused on the gate dielectric layer like high-k polymeric gate insulators for OTFTs [17],
high-k gate dielectrics for flexible and stretchable electronics [18], high-k zirconia-based ma-
terials for gate dielectrics [19], hybrid polymer metal oxide layers for flexible TFTs [20], etc.
Most of the papers in the literature are related to the development of enhanced properties
of semiconductor materials for channel layer applications. Regarding the gate dielectric
layer, a general classification of dielectric materials into organic and inorganic categories
can be inferred from the literature, each category with its advantages and disadvantages,
and, above all, its own niche of applications. Among the inorganic dielectric materials, the
metal oxide compounds such as SiO2, TiO2, ZrO2, HfO2, Al2O3, Y2O3, etc. are well known
for their excellent electrical insulation, high dielectric constants, and good thermal stabil-
ity [21,22]. These properties make them ideal gate dielectric materials for conventional TFT
applications, including displays and integrated circuits, where durability and reliability are
critical. However, to achieve excellent thermal and electrical properties, the deposition of
these dielectric materials requires the use of vacuum-based techniques like physical vapor
deposition (PVD) and chemical vapor deposition (CVD). These techniques are associated
with high equipment costs, which increase the overall expense of device manufacturing [22].
There are also some challenges of maintaining a stable vacuum environment and precise
control of deposition parameters to achieve the proper thin-film quality required for the
efficient device performance. In addition, high temperatures are often required during thin-
film deposition, to achieve proper film densification and thus superior dielectric properties,
which limit the compatibility of inorganic dielectrics with temperature-sensitive substrates,
such as flexible plastics, restricting their use in flexible devices. Also, the inherent brittle-
ness of inorganic dielectric layers increases the risk of cracking or delamination, especially
after bending flexible substrates, degrading the device performance [18]. Environmental
and safety issues also arise from the generation of hazardous waste and the handling of
precursor materials during these vacuum-based processes. Therefore, all these factors
collectively represent significant challenges to overcome during the processing of inorganic
dielectric layers through vacuum-based deposition techniques.
Organic gate dielectric materials have also gained importance for the development of
TFTs, particularly in organic TFTs (OTFTs). They provide as good electrical insulation as
their inorganic counterpart, enhancing charge transport by improving the interface with
semiconductor channel layers, which positively affects the device performance [18,23].
These dielectric materials, unlike inorganic ones, also offer important processing advan-
tages, such as solution processability at low-temperature conditions. This feature makes
organic dielectric materials compatible with deposition on flexible substrates that can-
not tolerate high temperatures, which is a key factor for wearable and flexible devices
manufacturing [24]. Furthermore, organic dielectrics, such as poly(methyl methacrylate)
(PMMA) [25,26], poly (vinyl phenol) (PVP) [27,28], (p-xylylene) (PPx) [29], poly (vinyl
alcohol) (PVA) [30,31], and parylene [32,33], among others, offer excellent mechanical
Technologies 2025, 13, 20 3 of 36
flexibility, enabling TFTs to perform reliably under bending and stress, which is critical
for devices in dynamic environments. With such characteristics, organic dielectrics have
shown promising achievements when used with oxide semiconductors, evidencing their
excellent potential for high-performance flexible electronic applications. However, these
dielectric materials also have some drawbacks, including low dielectric constant, low di-
electric strength, and sensitivity to environmental factors like moisture and oxygen, which
can degrade their performance over time [18]. Additionally, their lower thermal stability
compared to inorganic dielectrics restricts their use under high-temperature conditions.
Despite these disadvantages, organic dielectrics are particularly well-suited for flexible dis-
plays, wearable devices, and large-area electronics, where their low-temperature processing
on flexible substrates provides significant advantages.
Organic–inorganic hybrid dielectrics, constituting the blending of dielectric polymers
with high-k inorganic materials, offer a promising approach to develop advanced gate
dielectric materials for TFTs [23,34–36]. This combination of organic and inorganic phases
linked through diverse types of chemical bonds, including hydrogen bridges, van der
Waals forces, and covalent or ionic interactions, allows for the tunability of properties, such
as mechanical flexibility, optical transparency, and electrical performance, making these
materials highly desirable for a range of applications, including gate dielectrics. These
hybrid dielectrics combine the superior dielectric properties of inorganic dielectrics with
the flexibility and lightness of polymers, resulting in materials with high dielectric con-
stants, low leakage currents, and mechanical robustness. Like organic dielectrics, their
low-temperature, solution-processable nature makes them ideal for deposition on large-
area, flexible substrates. Homogeneous, highly transparent, with a very smooth surface,
hybrid dielectric materials can be obtained by the simultaneous condensation of the in-
organic phase and polymerization of the organic one, taking place in a hybrid precursor
solution. The resulting hybrid material is an amorphous network with crosslinked nano-
metric domains of both organic and inorganic phases, whose properties depend on its
organic-to-inorganic content ratio and the type of linking between them [37–40]. This fea-
ture enables the design of hybrid materials with targeted properties for specific applications
including gate dielectrics in TFTs [41–46]. The characteristics of hybrid dielectrics are quite
appropriate for their application in flexible electronic devices where the processing tem-
perature must be low and the materials capable of enduring deformation and mechanical
stress without degrading. These characteristics are very important to fulfil the conditions to
integrate the TFTs as fundamental parts in flexible, stretchable, wearable, and implantable
devices. The organic–inorganic hybrid gate dielectric approach has been applied in several
configurations. The simplest one consists of a hybrid organic–inorganic bilayer gate dielec-
tric, where the organic layer passivates surface defects of the inorganic one, providing a
smooth surface at the dielectric–semiconductor interface. Meanwhile, the inorganic layer
increases the capacitance of the gate dielectric [47–49]. Another approach is the embedding
of high-k inorganic nanoparticles in a polymeric matrix to achieve polymer nanocomposite
gate dielectric layers, where the interaction between the inorganic and organic phases is
rather low [50–52]. A third, less explored approach involves inorganic–organic hybrid
gate dielectric layers with phases much stronger, linked by covalent and/or ionic bonds.
In this case, the synergistic combination of the phases, through stronger bonds, not only
meets the mechanical and electrical requirements for flexible TFTs but also offers smooth,
low-roughness surfaces that are essential for the growth of additional dielectric and semi-
conductor layers [53–60]. Recent research reports have demonstrated the potential of these
hybrid materials in gate dielectric applications, showing promising electrical performance
in both n-type and p-type TFTs. However, although the advantages of the organic–inorganic
hybrid dielectrics for TFTs applications are great, it is still challenging to address some
Technologies 2025, 13, 20 4 of 36
issues like high-leakage currents, complexity to achieve uniformity and compatibility be-
tween organic and inorganic components, thermal instability, etc., to improve even more
their performance and stability.
The development of gate dielectric materials is fundamental for the emerging trends
in TFT technologies because they have a direct impact on the electrical performance of the
devices, including threshold voltage, leakage current, and overall stability. Because there
is currently an increasing demand for TFTs for flexible, wearable, and high-performance
electronics, the research on dielectric materials featuring high capacitance, low process-
ing temperatures, and mechanical flexibility has considerably increased. In this regard,
organic–inorganic hybrid dielectric materials are gaining attention because they fulfill the
requirements for their application as gate dielectric in TFTs for flexible electronics.
In this paper, we review the development, characterization, and application of
organic–inorganic hybrid dielectric materials for gate dielectric layer applications in TFTs.
We focus on the sol-gel process as the layer deposition method, the role of coupling agents
in enhancing phase compatibility, and the performance of several types of hybrid dielectric
layers in TFTs, both rigid and flexible. Our findings contribute to the growing body of knowl-
edge on hybrid materials and their potential to revolutionize the field of flexible electronics.