Thermally activated delayed fluorescence materials for nanomaterial-based light-emitting diodes

Abstract

The emergence of materials that exhibit thermally activated delayed fluorescence (TADF) has revolutionized the development of light-emitting diodes (LEDs). TADF involves a reverse intersystem crossing transition of excitations from a triplet to a closely lying singlet state, followed by radiative emission. This phenomenon enables the effective utilization of triplet excitations for light generation, achieving up to 100% internal quantum efficiency in devices. The ability to efficiently harvest all excitations has spurred the rapid development of new types of LEDs, in which TADF-active materials enhance or sensitize emission from other luminophores. This review focuses on the development and application of hybrid systems comprising TADF materials and inorganic emitters, such as semiconductor quantum dots and perovskite nanostructures. After providing a concise overview of these materials’ properties, we systematically examine how TADF materials can improve the optical, electronic, and morphological characteristics of inorganic emissive layers. This ultimately improves the performance and stability of devices based on these systems and paves the way for next-generation LED systems.

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1. Introduction

The subject of thermally activated delayed fluorescence (TADF) has attracted increased attention from researchers in recent years due to its potential applications in various fields involving light, including light-emitting devices, photodynamic therapy, and photocatalysis. This phenomenon can be defined as the redistribution of excited states, singlet and triplet, facilitated by thermal energy. This process consequently alters the substance's emissive properties. In the context of TADF materials, the excited triplet state, which is characterised by non-radiative relaxation, can undergo intersystem crossing, thereby facilitating the reuse of the state. This process leads to augmentation in the population of the singlet state, which, in turn, results in delayed fluorescence. Since this process allows for achieving a theoretical internal quantum efficiency (IQE) of 100%, TADF materials form the basis for the development of 3^rd generation OLEDs.^1–4 Despite the external quantum efficiency (EQE) of these devices reaching 30%,^5–8 there still exist challenges for their further mass production and utilisation. It is evident that the material exhibits broad emissive bands, which are characteristic of TADF materials.⁹ Furthermore, exciton quenching^8,10 and performance stability,¹¹ including sensitivity to moisture and heat⁸ and material degradation due to electrochemical reactions,¹² are also evident.

In recent years, TADF materials have been implemented as auxiliary materials for other emissive layers. This development has led to significant advancements in LED performance, giving rise to the 4^th generation of OLEDs.^4,13 OLEDs based on a TADF-sensitized host and a v-DABNA fluorescent dopant exhibited an exceptional EQE of 38.8% accompanied by narrow blue emission.¹⁴ A similar approach was employed in the fabrication of orange-red emissive OLEDs, which exhibited an EQE of 39.2% at a peak with a wavelength of 588 nm.¹⁵ Interested readers may refer to recent reviews for further information on advancements in TADF-based OLEDs.^16–18

Other emerging materials for advanced LEDs include semiconductor quantum dots (QDs)^19–21 and perovskite nanomaterials.^22–24 Despite the rapid increase in EQE observed over the past decade, these devices continue to encounter challenges, including Joule heating,^25–28 Auger recombination,²⁹ imbalanced electron and hole injection,³⁰ poor uniformity,^31,32 and stability upon increased humidity and oxygen levels.^33,34 The integration of TADF materials within LED structures based on inorganic nanomaterials has been demonstrated to enhance the homogeneity of emissive layers and facilitate charge carriers’ injection via FRET, thereby promoting equilibrium between hole and electron populations. Additionally, this integration has been shown to suppress Auger recombination, among other benefits. These examples have demonstrated that the integration of TADF materials into existing LED structures based on inorganic nanomaterials is a successful approach for enhancing LED performance.

The present review aims to demonstrate recent advances in the field of LEDs based on hybrid materials incorporating nanomaterials, such as quantum dots and perovskite nanomaterials, in conjunction with TADF materials. QDs and perovskite nanomaterials are renowned for their high color purity, tuneable emission wavelengths, and exceptional brightness, making them ideal candidates for enhancing the chromaticity of LEDs. Meanwhile, TADF materials can harvest triplet excitons effectively through reverse intersystem crossing, leading to potentially high IQE. By combining these materials, researchers aim to develop LEDs with improved energy efficiency, longevity, and environmental compliance compared to traditional technologies. The synergy between the properties of these advanced materials offers a promising route for creating next-generation optoelectronic devices optimized for utilization in solid-state lighting and display technology. This review begins with an explanation of the TADF phenomenon, subsequently progressing to a discourse on hybrid materials for LEDs, their performance, and a summarisation of future directions of LED development.

2. TADF mechanism, materials, and devices

2.1. TADF phenomenon

Thermally activated delayed fluorescence represents a unique photophysical process where triplet excitons are converted to singlet excitons through reverse intersystem crossing (RISC), enabled by thermal energy from the surrounding environment. The mechanism relies on a small energy gap between the lowest excited singlet state (S₁) and the lowest triplet state (T₁), typically denoted as ΔE_ST, which must be sufficiently small (<0.3 eV) to allow the transfer of the thermal population of S₁ from T₁ (see Fig. 1a). In conventional organic semiconductors, the formation of excitons following charge recombination follows spin statistics, yielding 25% singlet and 75% triplet excitons. Singlet excitons can radiatively decay to the ground state (S₀) through fluorescence, while triplet excitons typically undergo non-radiative decay or phosphorescence (if heavy atoms are present). The key innovation of TADF materials is their ability to convert “dark” triplet excitons into “bright” singlet excitons, theoretically enabling 100% IQE.

Fig. 1 (a) The schematic illustration of a material exhibiting the TADF mechanism: the lowest excited singlet (S₁) and triplet (T₁) energy states are separated by a small energy gap (ΔE_ST), which allows both intersystem conversion (ISC) and reverse intersystem conversion (RISC) transitions. Radiative transitions to the ground singlet state (S₀) occur as prompt and delayed emission. Due to a small ΔE_ST, efficient RISC followed by delayed emission allows utilization of 75% of excitations, which normally occupy the lowest triplet state. (b) Conjugation of donor and acceptor units to obtain spatial distribution between the HOMO and LUMO, which aims at minimizing the singlet–triplet energy separation. Engineering of ΔE_ST is possible, e.g., by increasing the dihedral angle between donor and acceptor units, which is marked as a solid arrow, or by introducing a spacer unit. (c) Realization of spatial distribution between the HOMO and LUMO using the “multiple resonance effect” by including the electron donor nitrogen and the electron acceptor boron in the π-conjugated system.

The endothermic intersystem transition from the lowest triplet to the lowest excited singlet state is a crucial step that largely determines the performance of a TADF material. The rate constant of this process, k_RISC, must be competitive with the rate of non-radiative decay from the lowest triplet to the ground state and can be expressed as follows: , where A is a constant, k_B is the Boltzmann constant, and T is the temperature. As seen from the above equation, the achievement of a high RISC rate constant requires minimization of the energy gap between the lowest excited singlet and triplet states, ΔE_ST. Hence, the achievement of small ΔE_ST is a basic requirement for the synthesis of an efficient TADF material. A small ΔE_ST can be achieved when the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a molecule are spatially separated.³⁵

2.2. Design principles

Two common strategies to obtain spatial distribution between the HOMO and LUMO and, consequently, a small ΔE_ST energy gap and a high RISC rate constant, are shown in Fig. 1b and c. The primary and most effective strategy for the design of TADF materials is the utilization of a donor–acceptor (D–A) architecture,^35,36 schematically illustrated in Fig. 1b. The design of such materials intentionally utilizes electron-donating (D) and electron-accepting (A) units, interconnected by various linkers, in order to create a system in which the HOMO is predominantly localized on the D unit and the LUMO on the A unit. Common D units are strong electron-donating groups, including carbazoles, acridanes, phenoxazines, and diarylamines. The electron-rich nature of these compounds ensures that the HOMO is stabilized on them. Common A units are electron-deficient groups, including triazines, cyanobenzene, pyrimidines, quinoxalines, and benzophenones. The electron-poor structure of the latter compounds localizes the LUMO. The degree of HOMO–LUMO separation is controlled by the steric and electronic connection between the D and A units. It has been demonstrated that a near perpendicular (approx. 90°) dihedral angle between the D and A planes optimizes spatial separation and minimizes ΔE_ST (Fig. 1b). This objective can be achieved through diverse arrangements of materials, including TADF molecules, acceptor-core/donor-shell dendrimers, main chain D–A polymers, backbone-donor/pendant-acceptor polymers, side chain polymers, and self-emission polymers.^1,37 For these materials, a variety of mechanisms of D–A interaction are observed, namely direct linkage of D–A pairs, Through-Bond Charge Transfer (TBCT), and Through-Space Charge Transfer (TSCT). For the first mechanism, one can distinguish twisted structures, where directly bonded D and A units exhibit rotational freedom, resulting in a twisted conformation, and spiro-linkages, utilizing a spiro-configured core to force orthogonality between the D and A units, thereby providing rigidity and preventing excessive conformational relaxation. The more sophisticated examples of the first mechanism are D–A–D and A–D–A symmetric systems. These structures have been demonstrated to enhance the rigidity of the molecule, reduce non-radiative decay, and balance charge transport in the OLED device.³⁸ The second mechanism, TBCT, is realized in D–π–A systems, where the incorporation of a π-spacer between the D and A units can modulate the degree of charge transfer, offering a trade-off between a small ΔE_ST and a high photoluminescence quantum yield (PLQY).³⁹ It has been shown that a D–π–A type structure is capable of suppressing the racemisation of planar chirality, forming a basis for circularly polarized organic light-emitting diodes fabricated by vacuum deposition processing. The third mechanism, TSCT, has been observed in TADF materials, wherein D and A units are not directly connected but are positioned in close spatial proximity, with a separation of 0.3–0.5 nm, found predominantly in polymers.^4,40 The HOMO and LUMO separation is achieved through space rather than through bonds, which can yield remarkably small ΔE_ST values while preserving large oscillator strength.

However, a small ΔE_ST is a necessary but not sufficient condition for efficient RISC. The reverse intersystem crossing rate k_RISC is also governed by the spin–orbit coupling (SOC) between the S₁ and T₁ states at the minimum energy crossing point of their potential energy surfaces.⁴¹ Furthermore, k_RISC can be significantly enhanced by a second-order mechanism involving vibronic coupling to a higher-lying triplet state (³LE), which mediates the transition.^42,43 Therefore, optimal TADF performance requires a delicate balance between a small ΔE_ST, efficient SOC, and favorable coupling to vibrational and other electronic states.^44,45 Interested readers may find exhaustive analysis of different factors affecting efficient RISC in recent reviews.^18,35

Another approach for TADF material design is based on a HOMO–LUMO separation by the multiple resonance effect^16,46,47 based on the incorporation of electron-donating and electron-withdrawing atoms (typically N and B) into rigid aromatic frameworks, as demonstrated schematically in Fig. 1c. This approach has the potential to yield both small ΔE_ST values and narrow emission spectra. More details can be found in recent reviews.^48–50

2.3. TADF materials in LEDs

The progress made in OLEDs utilizing non-TADF emitters has been thoroughly analyzed in recent reviews.^51–55 The emergence of materials capable of effectively harvesting both singlet and triplet excitations through the TADF mechanism has catalyzed the rapid development of a new class of OLEDs. The pioneering electroluminescent device featuring an EML with TADF, specifically a tin porphyrin complex (SnF₂OEP) in a PVCz host matrix, was reported by Endo et al. in 2019,⁵⁶ achieving an external quantum efficiency (EQE) of less than 1%. Two years later, Endo et al.⁵⁷ introduced a pure aromatic compound, PIC-TRZ, characterized by a minimal ΔE_ST and efficient TADF, with a total PLQY of 39%. The OLED utilizing PIC-TRZ demonstrated an EQE of 5.3% and luminance reaching 10 000 cd m⁻². The higher EQE compared to conventional fluorophores with similar PLQY underscored the advantages of TADF materials in OLED systems. In 2012, Uoyama et al.⁵⁸ unveiled a new class of purely organic TADF materials, cyanobenzene-carbazole derivatives, and developed TADF-OLEDs with an EQE of 19.3%, comparable to the highly efficient phosphorescence-based OLEDs of the time.

Despite the IQE of TADF-OLEDs rapidly approaching the theoretical limit of 100%, several critical challenges remained unresolved. Notably, the accumulation of triplet excitons limited the operational stability and durability of these devices, and the relatively broad emission spectra of most TADF materials necessitated the exploration of new design approaches. To overcome these issues, Nakanotani et al.⁵⁹ and Zhang et al.⁶⁰ independently introduced the concept of TADF-sensitized fluorescence. This innovative approach, illustrated schematically in Fig. 2a, involves a material exhibiting the TADF mechanism serving as an energy donor (Fig. 2b) to another fluorescent molecule, which functions as the emitter. Within this device architecture, the TADF process enables the utilization of both singlet and triplet excitons generated in the donor material, resulting in a high EQE. Additionally, incorporating an extra emitter as an energy acceptor facilitates the rapid utilization of excitations, preventing their accumulation in a triplet state. Furthermore, the emission wavelength and spectral width of the LED can be precisely tuned by altering the additional emitter. As demonstrated in Fig. 2c, a strategic selection of acceptor/donor (emitter/assistant) pairs allows for the fabrication of LEDs that cover nearly the entire visible spectral range. Moreover, various types of emitters, including carbon dots,⁶¹ semiconductor QDs,⁶² and dye-sensitized down-conversion nanoparticles,⁶³ can be effectively sensitized using materials that support the TADF mechanism.

Fig. 2 (a) The schematic diagram of ISC, Förster resonant energy transfer (FRET), and radiative relaxation processes in the “emitter dopant”/“assistant dopant”/“host matrix” system designed for the realization of the TADF-sensitized fluorescence mechanism. The assistant dopants dispersed in a host matrix at a much higher concentration than the emitter dopant act as the main reservoir for both singlet and triplet excitations. Triplet excitations can be upconverted to the singlet state by RISC and then resonantly transferred (alongside previously existing singlet excitons) to the emitter dopant, where radiative recombination occurs. (b) Molecular structure of assistant dopants, exhibiting TADF, used for the fabrication of blue-to-red LEDs, whose electroluminescence spectra are shown in (c). Reproduced from ref. 59 with permission from Springer Nature, Copyright [2014].

3. Advances in hybrid systems with TADF materials

3.1. Emerging materials for advanced LEDs

Colloidal semiconductor QDs are nanometer-sized particles that exhibit unique optical and electronic properties due to the quantum confinement effect. Recent advancements in colloidal synthesis techniques have enabled the production of monodisperse QDs with a size-tunable bandgap, exceptional optical characteristics, high photostability, and versatile processability. These attributes have made them highly valuable across various applications, including optoelectronics, photovoltaics, biomedicine, and sensing.^64–68 The pioneering work on QD-based electroluminescent devices (QLEDs) by the Alivisatos group⁶⁹ in 1994 paved the way for a new emerging research and technological area. The initial QLED featured a hybrid organic/inorganic architecture, utilizing CdSe QDs in conjunction with semiconducting poly(p-phenylene vinylene). The capability to precisely tune the emission color by adjusting the QD size, along with achieving high color purity, represented a significant breakthrough in LED technology. Solution processing, encompassing colloidal QD synthesis and device fabrication, offered a substantial technological advantage over traditional methods of forming semiconductor nanostructures, such as molecular beam epitaxy. Nonetheless, the low PLQY of QDs, attributed to multiple surface defect states, significantly constrained device performance.

The rapid advancement in colloidal synthesis methods has laid a robust foundation for the continued development of QLEDs. Specifically, the emergence of colloidal heterostructured semiconductor QDs has significantly enhanced their optoelectronic properties. These heterostructured semiconductor QDs are nanomaterials that integrate different semiconductor materials within a single nanoparticle, typically forming a core/(multi)shell architecture. This design allows for precise modulation of electronic and optical properties by spatially separating charge carriers, thereby enhancing quantum confinement effects and reducing nonradiative recombination through surface trap states, achieving near-unity PLQY. Additionally, the engineered shells offer further tunability of emission wavelengths and improve the stability of QDs.

The most common types of heterostructures realized in QDs are illustrated in Fig. 3a. The specific design of a heterostructure allows the achievement of desired distributions of wave functions of charge carriers. The selection of materials with suitable band offsets is crucial for determining the required spatial localization of photogenerated charge carriers in a heterostructured QD. In a type-I heterostructure, both electron and hole wave functions are confined within a core, which is enveloped by a wider bandgap semiconductor material, with CdSe/ZnS, CdS/ZnS, and InP/ZnS serving as representative examples. Conversely, in a type-II heterostructure, such as CdTe/CdSe or CdSe/ZnTe QDs, the electron and hole wave functions are spatially separated. In the quasi-type-II (type I1/2) heterostructure, one charge carrier is localized in either the core or shell, while the other is delocalized across the entire QD volume, as seen in CdSe/CdS or ZnSe/CdSe systems.⁷⁰ The formation of multiple shells over a QD core allows for even more precise engineering of their optoelectronic properties, making them highly promising for modern QLEDs with superior efficiency and operational stability.^71–74

Fig. 3 (a) Three types of core/shell semiconductor quantum dots (type I, type quasi-II (I_1/2), and type II exemplified with CdSe/Zns, CdSe/CdS, and CdSe/CdTe QDs, respectively). The upper panel displays the corresponding distributions of wave functions, which demonstrate the charge carriers’ localization. Reproduced from ref. 70 under Creative Commons CC BY. (b) Photographs of operating blue, green, and red LEDs with an active layer composed of non-toxic (blue-emitting 6.9 nm ZnTeSe/ZnSe/ZnS, green-emitting 8.6 nm InP/ZnSe/ZnS, and 7.1 nm red-emitting InP/ZnSe/ZnS, respectively) heterostructured semiconductor QDs. Reproduced from ref. 75 with permission from Springer Nature, Copyright [2020]; from ref. 76 under Creative Commons CC BY; from ref. 77 with permission from Springer Nature, Copyright [2019]. (c) Three types of perovskite structures most frequently used in PeLEDs: polycrystalline film, quasi-2D film, and 3D NCs (nanoparticles). Reproduced from ref. 23 with permission from John Wiley and Sons, Copyright [2025].

The incorporation of non-toxic materials is another crucial step in the application of QDs for commercial-grade LEDs. For two decades, QDs based on cadmium (Cd) and lead (Pb) have been prevalent due to their superior optical properties in the visible and near-infrared spectral regions, respectively. However, recent advancements have enabled the development of highly efficient and stable QLEDs in all three primary colors using non-toxic QDs (Fig. 3b).

In the last decade, perovskite-type nanomaterials have emerged as a prominent focus in optoelectronic applications, including the development of LEDs, solar cells, photodetectors, and other devices. Metal-halide perovskites, characterized by the general structural formula ABX₃—where A represents a monovalent cation namely Cs, FA (formamidine), MA (methylammonium), or Rb; B is a divalent metal cation like Pb or Sn; and X is a monovalent halide anion namely Cl, Br, or I—exhibit unique optoelectronic properties, high tunability of optical responses, and excellent processability. Three types of perovskite structures most frequently used in PeLEDs are illustrated in Fig. 3c. In 2014, Tan et al.⁷⁸ reported an LED with an active layer composed of an organometal halide perovskite (PeLED) achieving an EQE of only 0.76%. Despite this initial modest efficiency, the straightforward fabrication of high-quality semiconductor active layers has spurred significant interest and rapid advancement in this field. Over the past decade, PeLED efficiency has surged to approximately 30%, nearing the threshold for commercial viability.^79–83 The initial generation of devices utilized a polycrystalline perovskite film as the active layer, formed during the manufacturing process by applying a solution of suitable precursors to a substrate, followed by crystallization induced through thermal annealing or exposure to an antisolvent.

Soon after, the protocols for the formation of high-quality colloidal perovskite NCs (nanoparticles) were developed.^84–86 Due to these significant advances, the ease of fine-tuning the spectral position of the emission band and the high color purity, inherent to most metal-halide perovskites, were coupled with high photoluminescence quantum yield and excellent processability, offering extensive technological potential for synthesis, property modification, and the formation of thin layers and ordered structures. The initial reports on LEDs incorporating organic–inorganic⁸⁴ and fully inorganic⁸⁷ perovskite NCs marked the inception of a novel research and technological trajectory. Beyond varying the halide composition to adjust the emission wavelength, metal-halide perovskites in NC form also allow for the exploitation of the quantum confinement effect. Specifically, synthesizing ultra-small nanocrystals with bromine and iodine as halogens facilitates the production of pure-blue and red emissions. Unlike NCs with mixed anionic compositions, single-halide NCs do not suffer from phase segregation, which can adversely affect LED color purity. An alternative strategy involves the use of two-dimensional colloidal perovskite nanoplatelets (NPls).^88,89 Due to quantum and dielectric confinement effects, the synthesis of bromine-based NPls with a thickness of 3 monolayers and iodine-based NPls with a thickness of 4–5 monolayers enables the production of highly efficient emitters of pure-blue and pure-red emission, respectively.^90–94 Besides, the synthesis and application of lead-free perovskite NCs attract much attention for the development of non-toxic materials and devices.^95,96

Quasi-2D perovskites represent a distinct category of perovskite-type materials that have recently attracted significant interest for application in LEDs.^97,98 These materials exhibit a combination of 2D and 3D phases, whose contributions can be modulated by altering the A-site cation composition. Specifically, the incorporation of relatively large organic cations, such as phenethylamine, isopropylamine, and tert-butylamine, intercalates the perovskite framework, leading to the formation of multiple quantum wells with varying thicknesses (number of octahedral slabs, n), alongside a 3D (n = ∞) phase. The n value is crucial in determining the key physical properties of the different phases in quasi-2D perovskites, including bandgap width and exciton binding energy. Due to the presence of phases with different n values, an energy funneling process occurs from low-n phases (with a larger bandgap) to higher-n phases (with a smaller bandgap), facilitating efficient radiative recombination in the higher-n phases. Controlling the phase distribution and managing the energy transfer process between them with suitable additives are promising strategies for developing efficient quasi-2D perovskite light emitters.²³ In this context, the use of TADF materials as additives for quasi-2D perovskites can be considered a multifunctional strategy, as they may exert multiple effects on their optical properties.

Despite significant advancements in the fabrication of highly efficient QLEDs and PeLEDs, further efforts are necessary for their optimization. The synergistic integration of QDs and perovskite nanomaterials, alongside other nanostructures, may offer innovative concepts for device architectures. In particular, the development of hybrid devices that combine inorganic nanosized emitters with fully organic materials exhibiting the TADF effect could lead to improvements in charge injection, exciton utilization, color purity, and roll-off suppression.

3.2. Design principles

LEDs utilizing inorganic materials such as quantum dots (QDs) and perovskite nanostructures typically feature a multilayered architecture between the cathode and anode. The active emitting layer (EML) is sandwiched between auxiliary layers that facilitate the injection and transport of charge carriers, as illustrated in Fig. 4a–c, which depicts common device structures. These auxiliary layers, including hole-transporting (HTL), hole-injection (HIL), electron-transporting (ETL), and electron-injection (EIL) layers, can be deposited through thermal evaporation or liquid processing techniques. Consequently, various strategies are available for integrating TADF materials into QLED and PeLED structures.

Fig. 4 (a) Device structure and the corresponding energy level diagram for the QD-based LED with the HTL composed of PVK and the TADF material (dCF₃5CzOXD). dCF₃5CzOXD captures excess electrons from the EML to form excitons, which are further transferred back to the EML via FRET. Reproduced from ref. 100 with permission from American Chemical Society, Copyright [2025]. (b) Incorporation of a TADF-based exciton harvesting layer (EHL) between the HTL and EML. Reproduced from ref. 101 with permission from John Wiley and Sons, Copyright [2023]. (c) Mixing of the TADF material (CzAcSE) with green-emitting carbon dots to form a hybrid EML for the LED whose energy diagram is shown on the right. Reproduced from ref. 61 with permission from John Wiley and Sons, Copyright [2025]. (d) Embedding of the TADF material (DTC-mBPSB or BTBC-DPS) into the HTL, followed by deposition of the PEA₂Cs_n−1Pb_nBr_3n+1 quasi-2D perovskite layer as the EML. Reproduced from ref. 102 with permission from American Chemical Society, Copyright [2024]. (e) Mixing of the TADF material (Cz-4CzCN) with CsBr, PbBr₂, and PEABr precursors for deposition of a hybrid EML by spin-coating followed by thermal annealing. Reproduced from ref. 103 with permission from American Chemical Society, Copyright [2021].

A longstanding challenge of unbalanced carrier injection stems from disparities in charge mobilities between the ETL and HTL, as well as improper energy level alignment due to a deeply positioned QD valence band. Preferential trapping of one carrier type within the QD-based EML leads to QD charging and subsequent Auger recombination, ultimately constraining device performance.⁹⁹ To address this, an additional electron-harvesting material can be embedded into the HTL or HIL to capture excess electrons accumulating within the QD-based EML. These captured electrons can form excitons within this multifunctional layer and transfer them back to the EML via the Förster resonant energy transfer (FRET) mechanism, as illustrated in Fig. 4a. Due to the nature of materials supporting the TADF mechanism, these captured electrons are expected to be utilized more efficiently compared to traditional fluorescent or phosphorescent dyes, owing to an effective RISC process and subsequent FRET. Consequently, the integration of TADF materials within the HTL or HIL is anticipated to prevent a decline in the device's EQE and enhance operational stability. Furthermore, modifying the HTL may result in higher hole mobility and improved charge injection due to better energy alignment and a reduced hole injection barrier. Additionally, altering the HTL in a direct LED structure may influence the formation of an EML during subsequent deposition. A similar modification of an HTL with TADF materials has been developed for PeLEDs with polycrystalline or quasi-2D perovskite EMLs (Fig. 4d).

Instead of mixing with a charge-transporting layer, TADF materials may be incorporated as an additional interlayer (exciton-harvesting layer, EHL) between an EML and an HTL, as illustrated in Fig. 4b. This approach enables step-wise energy alignment, thereby facilitating charge transfer and injection. It also allows for the use of TADF materials that may not be compatible with the HTL/ETL solutions employed in device fabrication. Similar to TADF materials dispersed within HTL/ETL layers, the EHL is designed to capture leaked or excess electrons in the EML, forming excitons that are subsequently transferred back to the EML to enhance overall emission. It is important to note that an additional buffer layer (BL) may be introduced between the TADF material and the EML in both scenarios. Kim et al.¹⁰¹ demonstrated that the inclusion of such a BL can significantly enhance the luminance efficiency of LEDs by ensuring that the EHL effectively supplies excitons to the EML. In systems where both Förster and Dexter (Dexter energy transfer, DET) mechanisms coexist, and FRET is the preferred process, the insertion of a BL can block DET, thereby optimizing exciton recycling.

An alternative strategy for incorporating TADF materials into LEDs involves blending them with luminescent nanomaterials within an EML (Fig. 4c). This approach necessitates a thorough evaluation of the TADF material's impact on both the photophysical and morphological properties of the EML. In such a hybrid EML, the TADF material can serve as a host matrix, facilitating a more uniform distribution of luminescent QDs. This spatial separation of QDs mitigates luminescence quenching, which can occur at defective QDs following multiple excitation hops, potentially enhancing the PLQY of the luminescent film. Moreover, TADF materials can be deliberately functionalized with specific chemical groups to provide additional passivation of defect states on the QD surface. From a morphological perspective, the integration of luminescent and TADF materials can result in a smoother, pinhole-free EML, promoting improved interfaces and eliminating current leakages. Additionally, the electrical conductivity, dielectric permittivity, and work function can be fine-tuned in a hybrid EML. To optimize the performance of LEDs utilizing such a hybrid EML, the proportion of luminescent and TADF materials must be meticulously adjusted to ensure pure emission, appropriate electrical properties, and optimal morphology.

In contrast to the traditional approach of mixing TADF materials with pre-synthesized luminescent nanocrystals, their application in PeLEDs utilizing perovskite thin films or quasi-2D perovskites allows the in situ crystallization of a hybrid EML. Similar to various additives commonly employed in perovskite-based multilayer structures for LEDs and solar cells,^104–106 TADF materials can be easily integrated with perovskite precursors during the deposition of a luminescent thin film (Fig. 4e). Alternatively, TADF materials can be dispersed in an antisolvent to induce the crystallization of a thin perovskite film. The combination of these two methods allows for the incorporation of different types of TADF additives that are compatible with both polar and non-polar solvents. Moreover, any TADF additive may influence the crystallization and growth of perovskites, thereby altering film morphology and optical properties. From this perspective, employing TADF materials in polycrystalline or quasi-2D PeLEDs represents a highly versatile strategy to enhance their EQE and operational stability. Beyond the modification of electroluminescence properties observed in hybrid EMLs composed of TADF materials and preformed nanocrystals, improvements in thin film morphology, grain boundary passivation, crystallinity, suppression of low-n phase formation, and enhanced hydrophobicity have been reported in quasi-2D and polycrystalline perovskite films modified with TADF species.

3.3. TADF materials in QLEDs

In 2018, Zhang et al.¹⁰⁷ pioneered the integration of TADF materials into the QLED structure with CdSe/CdS QDs as the EML. They incorporated 4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN) into a hole injection layer (HIL) composed of poly(N-vinylcarbazole) (PVK). The authors hypothesized that the TADF additives dispersed in the HTL would capture and efficiently utilize electrons leaking from the QD-based EML. Their findings demonstrated that both EQE and luminance efficiency (LE) increased with the addition of 2CzPN, up to a weight concentration of 20%. To confirm the role of FRET in enhancing electroluminescence intensity, they analyzed transient photoluminescence (PL) responses from 2CzPN(20%):PVK/QD films in both 2CzPN and QD emission channels, comparing them with reference samples. The study revealed that while the PL decay of 2CzPN accelerated after QD deposition, the PL lifetime of QDs was prolonged when deposited onto a hybrid 2CzPN(20%):PVK layer compared to a pristine PVK surface. The quenching of 2CzPN PL and the delay in QD PL were attributed to FRET from excitons formed in 2CzPN to adjacent QDs. Subsequent studies have reported similar findings for other TADF material and QD-emitter combinations.^{100,108–110} Apart from the prolongation of a prompt component in the QD PL decay, Xiang et al.¹¹¹ observed the appearance of a remarkably delayed component (measured under degassed conditions) for the CdSe/ZnS QDs mixed with a TADF material 10,10′-(sulfonylbis(4,1-phenylene))bis(2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9,9-dimethyl-9,10-dihydroacridine) (4CzDMAC-DPS). The appearance of such a delayed component differed in the PL decay of QDs dispersed in a TADF material from those dispersed in a common host material, 1,3-bis(carbazol-9-yl)benzene (mCP).

It is worth noting that conventional phosphorescent emitters, which do not support the TADF mechanism, can also be employed to recycle excess excitons in QLEDs. However, the energy transfer mechanisms involved differ, and the overall device performance depends on the selected combination of harvesting and emitting materials and the distance between them. Kim et al.¹⁰¹ conducted a systematic study on the reuse of leaked electrons in LED structures featuring both organic (Ir(ppy)₃) and QD-based (InP/ZnSeS) EMLs. They incorporated either phosphorescent (bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium, Firpic) or TADF (9,9′,9″,9‴-((6-phenyl-1,3,5-triazine-2,4-diyl)bis(benzene-5,3,1-triyl))tetrakis(9H-carbazole), DDCzTRz) dopants into a host material, 3-(diphenylphosphoryl)-9-(4-(diphenylphosphoryl)phenyl)-9H-carbazole (PPO21). This was inserted as an additional EHL between the HTL and the corresponding EML, with or without an additional blocking layer. The possible energy transfer pathways between dopants and emitters are illustrated in Fig. 5a. When a TADF-EHL is employed, energy transfer to any EML occurs via a long-range FRET mechanism. In contrast, both FRET and short-range DET may occur when a phosphorescent EHL is used. In the latter case, short-range DET is the dominant mechanism, because the distance between the EHL and EML is small.

Fig. 5 (a) Energy transfer mechanisms between the phosphorescent (upper panel) and TADF (bottom panel) EHLs to the organic dye emitter (Ir(ppy)₃) and the QD-emitter (InP/ZnSeS). (b) Energy level diagram for the QD-based LEDs. Devices D, E, and F have no additional EHL, phosphorescent (Firpic) EHL, and TADF (DDCzTRz) EHL, respectively. (c) Energy transfer efficiencies for the devices with and without an additional BL utilizing phosphorescent (device E) and TADF (device F) EHLs. (d) CE curves obtained for devices D, E, and F. Reproduced from ref. 101 with permission from John Wiley and Sons, Copyright [2023].

However, when an additional blocking layer is present, DET is significantly inhibited. DET efficiency decreases exponentially with distance because it requires direct orbital overlap between the donor and acceptor units. For this reason, DET is effective over very short distances and requires physical contact or very close proximity between the donor and acceptor. An additional blocking layer increases the distance between the donor and acceptor and thus strongly reduces DET efficiency. The author demonstrated that the insulating shell on the QD surface acts as an intrinsic blocking layer, preventing DET from the phosphorescent EHL to the EML. Consequently, only minimal energy transfer occurs due to a small fraction of excitons in the singlet state within the phosphorescent EHL (Fig. 5c). Consequently, the use of the phosphorescent EHL results in insufficient enhancement of the efficiency of LEDs using QDs as the EML. Conversely, in the TADF EHL, excitons predominantly exist in the singlet state due to RISC, allowing the EML to be effectively populated through long-range FRET, even in the presence of a protective shell or an additional blocking layer (Fig. 5c). As a result, devices (whose energy level diagrams are shown in Fig. 5b) utilizing the QD EML and the TADF EHL exhibit superior efficiency (Fig. 5d). Specifically, the current efficiency (CE), EQE, and luminance increased from 14.3 to 21.3 cd A⁻¹, from 3.58% to 5.20%, and from 10 500 to 133 000 cd m⁻², respectively, for devices containing an additional PPO21/DDCzTRz interlayer.

In addition to the reutilization of excess excitons, TADF materials can enhance QLED efficiency by altering the electrical or morphological properties of the charge-transporting layers. For example, Zhou et al.¹⁰⁰ incorporated 0–30 wt% TADF material, specifically 2-(3,5-bis(trifluoromethyl)phenyl)-5-(2,3,4,5,6-penta-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole (dCF35CzOXD), into the PVK HTL of a QLED featuring a CdZnSe/ZnSe/ZnS EML. They studied fabricated hole-only devices and observed a significant impact of doping on the hole mobility within the HTL. As illustrated in Fig. 6a–d, the hole mobilities increased substantially when the TADF material concentration exceeded 20 wt% in PVK. Nevertheless, despite the enhanced hole mobility in the modified HTL, it remained considerably lower than the electron mobility of the ZnMgO ETL, indicating that an excess of electrons in the EML still requires effective management.

Fig. 6 (a–d) J–V curves obtained for the hole-only devices to estimate the hole mobility of the (a) pristine and (b–d) dCF₃5CzOXD-modified PVK-based HTLs. Reproduced from ref. 100 with permission from American Chemical Society, Copyright [2025]. SEM images of the (e) bare QD film and (f) poly(AcBPCz-TMP)/QD hybrid film. Scale bars are 100 nm. RMS was estimated using AFM measurements for the corresponding samples. Reproduced from ref. 108 with permission from Elsevier, Copyright [2022].

Furthermore, TADF additives significantly impact the morphology of the layers in which they are incorporated. Zhang et al.¹⁰⁷ observed no notable effect of 20 wt% 2CzPN on the morphology of a PVK layer. In contrast, Zheng et al.¹¹⁰ found that the root mean square (RMS) surface roughness of the same composite is dependent on the 2CzPN/PVK ratio. Specifically, the RMS varied from approximately 1.5 to 3.4 nm when 20–60 wt% 2CzPN was introduced into PVK. Given the RMS of 2.3 nm for the reference sample with a bare PVK HTL, one might anticipate differing effects on film roughness at various doping concentrations. Furthermore, TADF materials influence film morphology when combined with QDs in the EML.^108,111 Yang et al.¹⁰⁸ utilized scanning electron microscopy (SEM) and atomic force microscopy (AFM) to demonstrate that hybrid EMLs composed of CdSe/ZnS QDs and the TADF polymer poly(4-(9,9-dioctylacridin-10(9H)-yl))-(4-(9H-carbazol-9-yl))benzophenone (poly(AcBPCz-TMP)) exhibit superior morphology compared to the reference layer of bare QDs (Fig. 6e and f). The reduced RMS and absence of large aggregates indicate improved homogenization of QDs within the EML, which generally enhances the emission characteristics of the thin film. The advances in the application of TADF materials in QLEDs are detailed in Table 1.

Table 1 The use of TADF materials in QLEDs

Year	TADF	Place	Emitter	EL peak, nm	EQE, %	Luminance, cd m^−2	CE, cd A^−1	Impact	Ref.
a 4,5-Di(9H-carbazol-9-yl)phthalonitrile. b 10,10′-(Sulfonylbis(4,1-phenylene))bis(2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9,9-dimethyl-9,10-dihydroacridine). c 10,10′-(4,4-Sulfonylbis(4,1-phenylene))bis(9,9-dimethyl-9,10-dihydroacridine). d Poly(4-(9,9-dioctylacridin-10(9H)-yl))-(4-(9H-carbazol-9-yl))benzophenone. e 9,9′,9″,9‴-((6-Phenyl-1,3,5-triazine-2,4-diyl)bis(benzene-5,3,1-triyl))tetrakis(9H-carbazole). f 2-(3,5-Bis(trifluoromethyl)phenyl)-5-(2,3,4,5,6-penta-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole.
2018	2CzPN^a	HTL	CdSe/CdS	628	12.37	19 258	17.3	2CzPN captures excess electrons leaking from the EL and donates excitons to the EML via TADF and FRET. Improved device efficiency and stability	107
2019	4CzDMAC-DPS^b	EML	CdSe/ZnS	∼630	11.9	>5000	16.1	Suppression of non-radiative decay in the EML, smoother EML morphology, and improved device efficiency	111
2019	DMAC-DPS^c	Interlayer between the HTL and EML	CdSe/ZnS	∼530	2.54	63 458	10.4	Reducing the hole injection barrier, harvesting the leaked electrons, and alleviating the carrier recombination at high current densities Improved turn-on voltage and efficiency and suppressed roll-off	109
2020	2CzPN	HTL	CdSe/ZnS	∼625	9.82	35 352	11	Improved hole injection and the usage of the leaked electrons. Improved device efficiency	110
2022	Poly(AcBPCz-TMP)^d	EML	CdSe/Zns	630	14.9	21 446	16.7	Improved PLQY of the EML, improved hole injection/transport, and utilization of triplet excitons	108
2023	DDCzTRz^e	Interlayer between the HTL and EML	InP/ZnSeS	533	9.64	40 700	68.0	Use of leaked electrons by energy transfer from the exciton harvesting layer to the EML. Improved device performance and stability	101
2025	2CzPN	Interlayer between the ETL and EML	CsPbBr3	521	0.73	7660	2.8	Better charge transfer and energy alignment, harvesting of excitons	112
2025	dCF35CzOXD^f	HTL	CdZnSe/ZnSe/ZnS	632	35.8	>200 000	42.3	dCF35CzOXD enhances the hole mobility of the HTL and captures excess electrons. Improved device efficiency and stability	100

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3.4. TADF materials in PeLEDs

Several TADF materials have been effectively utilized to enhance the efficiency and operational stability of PeLEDs that employ both polycrystalline perovskite thin films and quasi-2D perovskites as an EML (refer to Table 2). As previously discussed, the incorporation of TADF materials can induce various modifications in the optoelectronic properties of perovskites by influencing crystallization and growth processes, passivation effects, exciton harvesting, and more. For example, the impact of TADF materials on perovskite crystallization was first identified by Gao et al.,¹¹³ who observed an increase in grain size for CsPbBr₃/2CzPN hybrid films compared to the reference sample. Since then, numerous studies have reported changes in the morphology of perovskite-based EMLs following the addition of TADF materials.^{102,114–117}

Table 2 The use of TADF materials in PeLEDs

Year	TADF	Place	Emitter	EL peak, nm	EQE, %	Luminance, cd m^−2	CE, cd A^−1	Improved stability	Impact	Ref.
a Introduced into a precursor solution, unless otherwise specified. b (2R)-2,4,6-Tris(4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazol-9-yl)benzonitrile. c (2R,3R,5S,6S)-2,3,5,6-Tetrakis(4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazol-9yl)benzonitrile. d Poly-(3-(4-((5-((9H-carbazol-4-yl)oxy)pentyl)oxy)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)benzonitrile). e Poly-(3-(4-((5-((9H-carbazol-4-yl)oxy)pentyl)oxy)-9H-carbazol-9-yl)-2,4,6-tris(4-((6-(9H-carbazol-9-yl)hexyl)oxy)-9H-carbazol-9-yl)benzonitrile). f 1,3-Bis((4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)sulfonyl)benzene. g BTBC-DPS 9,9′-(sulfonylbis(4,1-phenylene))bis(3,6-di-tert-butyl-9H-carbazole). h SO-DMAc – 9,9-dimethyl-10-(4-(phenyl sulfonyl)phenyl)-9,10-dihydroacridine. i 4CzIPN – (4,6S)-2,4,5,6-tetrakis(9H-carbazol-9-yl)dibenzo[b,d]furan-2,7,10,13-tetrahedron.
2018	2CzPN	EML^a	Polycrystalline CsPbBr3	522	2.26	22 063	8.7	n/d	Preferred orientation of a perovskite film and exciton harvesting	113
2021	Cz-3CzCN^b	EML	Quasi-2D (PEA)2Csn−1PbnBr3n+1	∼520	10.01	19 518	38.0	+	Enhanced film quality and defect passivation	114
2021	Cz-4CzCN^c	EML	Quasi-2D (PEA)2Csn−1PbnBr3n+1	511	10.1	18 000	39.0	+	54% of the enhancement in device efficiency was ascribed to the defect passivation, and 46% – to the retrieved energy	103
2023	P-5CzCN^d	EML	Quasi-2D (PEA)2Csn−1PbnBr3n+1	512	13.9	31 540	52.9	+	Improved film morphology, crystallinity, and defect passivation; enhanced PLQY, hydrophobicity, and thermal stability	115
	P-Cz5CzCN^e				17.4	20 710	66.1
2024	DTC-mBPSB^f	HTL	Quasi-2D (PEA)2Csn−1PbnBr3n+1	512	11.04	19 620	42.0	+	Control over the nucleation and growth of a perovskite film, suppression of the formation of low-n phases, defect passivation, and improved hole transport and injection	102
	BTBC-DPS^g				13.97	24 570	53.1
2024	SO-DMAc^h	EML	Quasi-2D (PEA0.4NMA0.6)2Csn−1PbnI3n+1	682	21.8	404	3.2	+	Surface passivation; coordination with the perovskite bottom upshifts the perovskite electronic structure and facilitates hole injection; accelerated charge transfer funneling. T50 of luminance and EQE exceeded 6 and 35 h, respectively	118
2024	DTC-mBPSB	EML	Quasi-2D (PEA)2Csn−1PbnBr3n+1	∼515	11.77	17 130	44.72	+	More uniform and denser EL, enhanced carrier transport, suppression of low-n phases, and defect passivation	116
	BTBC-DPS				17.94	26 340	68.18
2025	4CzIPN^i	EML	Quasi-2D (PEA)2Csn−1Pbn(Cl/Br)3n+1	495	5.44	745	n/d	n/d	Suppressed formation of low-n phases, improved film morphology, reduced defect densities, and improved device efficiency	117
		EML, introduced into an anti-solvent		496	4.84	958

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Yang et al.^102,116 investigated the effects of TADF additives (DTC-mBPSB and BTBC-DPS) dispersed in either the HTL (PVK) or EML ((PEA)₂Cs_n−1Pb_nBr3_n+1) on the performance of PeLEDs based on quasi-2D perovskites, noting significant improvements in film morphology in both scenarios. As illustrated in Fig. 7, the introduction of TADF additives into a perovskite EML resulted in the fabrication of smoother and less defective films. Specifically, SEM measurements (Fig. 7a–c) demonstrated that TADF-modified EMLs exhibited fewer pinholes, which are known to cause current leakages and reduce device performance. Furthermore, AFM analysis (Fig. 7d–f) indicated a reduced RMS for perovskite films modified with TADF additives. This morphological analysis was consistent with results obtained from photoluminescence (PL) microscopy (Fig. 7g–k). In addition to higher PL intensity in films containing TADF additives, the PL intensity distribution was clearly more homogeneous, indicating the formation of more uniform perovskite films.

Fig. 7 SEM (upper panel) and AFM (middle panel) images and PL intensity maps (bottom panel) for the (a, d, g) pristine (PEA)₂Cs_n−1Pb_nBr_3n+1 quasi-2D perovskite films and those fabricated with (b, e, h) DTC-mBPSB or (c, f, i) BTBC-DPS TADF additives. Reproduced from ref. 116 with permission from American Chemical Society, Copyright [2024].

Because quasi-2D perovskites are composed of multiple phases with varying layer thicknesses (number of [BX₆]⁴⁻ octahedra, n = 1, 2, 3…), alongside a 3D phase, the modulation of phase distribution is critical for optimizing optoelectronic devices based on them. Recent studies have demonstrated that the incorporation of TADF materials into the EML^117,118 or HTL¹⁰² can effectively inhibit the formation of low-n phases. Yang et al.¹⁰² conducted absorption and photoluminescence (PL) spectroscopy on (PEA)₂Cs_n−1Pb_nBr3_n+1 quasi-2D perovskite films deposited on either pristine or TADF-doped PVK layers serving as the HTL. As can be seen from Fig. 8a, the reference sample exhibited distinct high-energy peaks in the absorption spectrum, indicative of low-n phase formation (n < 4). Conversely, these peaks were absent in the absorption spectra of films deposited on TADF-modified surfaces. Similar trends were observed in the PL spectra (Fig. 8b), where peaks associated with low-n phases were suppressed in films formed on the TADF-doped HTL. Optical spectroscopy findings were corroborated by X-ray diffraction (XRD) analysis. As shown in Fig. 8c, in addition to the diffraction peaks at 15.5° and 30.8°, corresponding to Bragg's diffraction from the (100) and (200) planes, respectively, two additional peaks at lower 2θ values, indicative of low-n phase formation, were observed in the XRD spectrum of the (PEA)₂Cs_n−1Pb_nBr3_n+1 film deposited onto the pristine PVK layer. In alignment with optical data, the emergence of these peaks is diminished in films deposited on the TADF-doped PVK layer. The suppression of low-dimensional phases in quasi-2D perovskites is generally regarded as beneficial for their optoelectronic properties, attributed to a reduced density of trap states, a decreased proportion of the inorganic component, and a minimized energy transfer cascade necessary for efficient emission. Consequently, the modulation of phase distribution in quasi-2D perovskites through the use of TADF additives may further enhance the performance of hybrid PeLEDs.

Fig. 8 (a) Absorption, (b) PL, and (c) XRD spectra for the pristine (PEA)₂Cs_n−1Pb_nBr_3n+1 quasi-2D perovskite film and that fabricated with DTC-mBPSB and BTBC-DPS TADF additives. Reproduced from ref. 102 with permission from American Chemical Society, Copyright [2024]. (d) SEM images and water contact-angle measurement for the pristine (PEA)₂Cs_n−1Pb_nBr_3n+1 quasi-2D perovskite film and those fabricated with P-5CzCn and P-Cz5CzCnTADF additives. Reproduced from ref. 115 with permission from Elsevier, Copyright [2023].

Moreover, TADF materials have the potential to enhance the stability of perovskite-based EMLs by mitigating the adverse effects of moisture and heat, which are significant challenges for the long-term operational stability of PeLEDs.^119,120 For instance, Ban et al.¹¹⁵ evaluated the water contact angle of pristine (PEA)₂Cs_n−1Pb_nBr3_n+1 quasi-2D perovskite thin films and their hybrids with TADF materials. As illustrated in Fig. 8d, the contact angle increased from 37° to 51° and 63° with the addition of P-5CzCN and P-Cz5CzCN to the perovskite precursor solutions. The authors suggested that the incorporation of TADF polymers promotes the formation of a hydrophobic layer, potentially offering enhanced protection of the EML from moisture. Furthermore, they reported improved thermal stability of the hybrid films, with the P-Cz5CzCN modified film retaining 80% of its initial PL intensity after 5 minutes of exposure to 90 °C.

In this context, greater emphasis should be placed on the comprehensive investigation of the localization of TADF additives within quasi-2D and polycrystalline perovskite thin films. For example, Chen et al.¹¹⁸ have shown that the TADF additive (SO-DMAc), when incorporated into a precursor solution to form a (PEA_0.4NMA_0.6)₂Cs_n−1Pb_nI_3n+1 quasi-2D perovskite, becomes exclusively coordinated with the perovskite bottom after crystallization, which was directly identified by time-of-flight secondary ion mass spectrometry (Fig. 9a). This pronounced localization of the TADF additive results in an upward shift of the EML valence band minimum, thereby enhancing hole injection efficiency. Furthermore, the authors performed cross-sectional scanning Kelvin probe microscopy analysis to study the electric field (EF) distribution in the devices. The surface potential (Fig. 9b) and the corresponding electric field (Fig. 9c) profiles showed the strongest alteration of the EF distribution at the HTL/perovskite interface when the TADF material was incorporated. This example not only highlights the critical role of spatial localization of TADF-active additives in perovskite-based EMLs but also illustrates the multiple impacts they can have on device performance.

Fig. 9 (a) Time-of-flight secondary ion mass spectrometry analysis of the ITO/PVK/SO-DMAc modified perovskite thin film. The distribution of the signal related to SO₂⁻ indicates localization of the TADF material at the HTL/EML interface. (b) Surface potential and (c) the local electric field profiles for the SO-DMAc modified perovskite thin film as compared to the reference sample. Reproduced from ref. 118 with permission from John Wiley and Sons, Copyright [2024]. (d) Molecular structures of the TADF material (Cz-4CzCN) and similar normal (Cz-4CzPh) additives used in the EML of the quasi-2D perovskite film. (e) The scheme of exciton retrieval in a hybrid TADF/quasi-2D perovskite film. Reproduced from ref. 103 with permission from American Chemical Society, Copyright [2021].

A critical factor for the successful advancement of TADF materials in PeLEDs is the clear identification of the impact of the TADF mechanism on device performance. Although the addition of TADF materials has been shown to modify the optoelectronic and morphological properties of perovskite thin films, there remains a gap in understanding how the utilization of triplet excitons via the RISC process affects the overall performance and stability of PeLEDs. To address this, Ban et al.¹⁰³ conducted a comparative study on the effects of TADF (Cz-4CzCN) and conventional (Cz-4CzPh) additives on the efficiency of PeLEDs based on the (PEA)₂Cs_n−1Pb_nBr_3n+1 quasi-2D perovskite. Cz-4CzPh was synthesized to have a similar molecular structure to the TADF-active Cz-4CzCN (Fig. 9d). The authors hypothesized that these additives would predominantly localize at the grain boundary defects of the EML film, where charge carriers might become trapped and dissociate non-radiatively, thereby reducing device performance. The additives are designed to capture these trapped or dissociated excitons and transfer their energy back to the perovskite via FRET (Fig. 9e). Given their similar molecular structures, Cz-4CzCN and Cz-4CzPh were anticipated to have comparable effects on the passivation of trap states and grain boundaries. The Cz-4CzPh additive, which does not support the TADF mechanism, increased the CE from 7.3 Cd A⁻¹ to 25 Cd A⁻¹ compared to the reference device with a pristine (PEA)₂Cs_n−1Pb_nBr_3n+1 film. Notably, the TADF-active Cz-4CzCN further enhanced the CE to 39 Cd A⁻¹. This improvement, which counts as much as 45% of overall device efficiency enhancement, was attributed solely to the reuse of triplet excitons due to a RISC process.

4. Outlook and perspectives

Although the development of hybrid composites incorporating TADF materials for LED applications is still in its infancy, several significant observations have already emerged. It is important to note that the effect of incorporating TADF materials is generally complex and extends far beyond improved exciton management. The impacts of TADF additives responsible for superior LED performance are listed in Tables 1 and 2.

In particular, integrating TADF materials into charge transfer layers or as additional interlayers has been shown to have a significant impact on the performance of QLEDs and PeLEDs. Beyond the reutilization of leaked or excess electrons through the effective combination of RISC and FRET processes, TADF additives significantly influence the morphological and electronic properties of auxiliary layers. Consequently, incorporating TADF materials improves energy level alignment, reduces injection barriers for charge carriers, and enhances charge carrier mobility. Additionally, there are observable effects on the optical and morphological properties of the subsequently deposited EML, which are attributed to improved surface roughness and wettability. Furthermore, incorporating TADF materials within the active layer of QLEDs can enhance the PLQY of the EML and improve its morphology and electronic properties. Overall, these strategies have been shown to enhance QLED performance, reduce efficiency roll-off, and improve device stability.

For devices utilizing polycrystalline or quasi-2D perovskites as an EML, the effect of incorporating TADF additives is even more pronounced. These additives have been demonstrated to have an impact on the nucleation and growth processes of perovskites, thereby affecting both the quality of thin films and the distribution of perovskite phases. Specifically, the development of superior perovskite layers with reduced pinhole density and smoother interfaces enhances device performance and minimizes current leakage. Concurrently, suppressing low-n phase formation and passivating additional surface defects facilitate improved energy funneling and augment radiative recombination within the EML. Furthermore, the application of TADF additives can enhance the moisture and thermal stability of perovskite layers, which are critical factors for the performance and longevity of PeLEDs.

At this point, we can also outline several prospective future directions for the advancement of hybrid systems incorporating TADF materials and inorganic nanoscale emitters. As emphasized previously, the introduction of TADF additives has multiple effects on the performance of QLEDs and PeLEDs. However, this multifaceted influence can obscure the contributions of specific effects. Therefore, more detailed experimental and theoretical analysis of such hybrid systems incorporating TADF materials is essential for advancing this field efficiently. In particular, when improvements in device performance and stability are observed, the role of the additive in facilitating the RISC process – the primary reason for employing TADF materials – may not be immediately apparent. This necessitates comprehensive experimental investigations and sophisticated control experiments. Furthermore, employing advanced experimental techniques to examine the properties of hybrid systems containing TADF materials and inorganic emitters at the micro/nanoscale could provide valuable insights into their interactions and potentially lead to innovative concepts for the development of these systems.

On the other hand, standardizing characterization protocols across various levels, from materials to devices, is crucial for the accurate interpretation of experimental data and the comparison of reported device parameters. A more detailed description of the fabrication processes for organic–inorganic hybrid systems, along with comprehensive accounts of experimental setups and conditions under which specific measurements are conducted, would enhance the credibility of reported results. Additionally, providing statistical analyses of obtained values would further strengthen these findings. In this context, adhering to the best practices and guidelines for characterizing energy-related nanomaterials is essential.^71,121–123 Detailed reporting of device performance is also critical for comparing the efficacy of different hybrid systems and demonstrating the advantages of material integration. While significant progress has been made in measuring and reporting solar cell performance,^124,125 similar advancements are needed in the development of novel LEDs.¹²⁶ For example, reporting maximum EQE for devices with a pronounced roll-off effect presents challenges that complicate the interpretation of results. High efficiency reported at very low current densities may not be practical for real operational conditions. Furthermore, standardizing device lifetime measurements to ensure that they are meaningful at an industrial application level is highly desirable.¹⁸

From a materials design perspective, a limited number of TADF-active materials have been evaluated in hybrid systems, primarily focusing on small molecules. However, utilizing high molecular weight species offers significant technological advantages. Solution-processable high molecular weight TADF materials, such as dendrimers and polymers, could serve as excellent host matrices for the incorporation of QDs and offer additional protection for polycrystalline and quasi-2D perovskite thin films. Consequently, the advancement of TADF polymers themselves is crucial, in particular, with emphasis on wide-bandgap materials that are suitable for the fabrication of blue LEDs.

In turn, only QDs and NCs with 3D confinement were employed in combination with TADF materials. However, various types of two-dimensional semiconductor colloidal nanoplatelets, such as core, core–(multi)shell, and core–(multi)crown CdSe-based,^127–129 ternary alloyed core/crown CdSeS/CdS,¹³⁰ quaternary alloyed core–shell CdZnSeS/ZnS,¹³¹ and perovskite-type ABX₃,^132,133 are significant for LED applications and may be explored in hybrid systems with TADF materials. These materials exhibit strong Auger recombination due to their two-dimensional nature and high exciton binding energies. Controlling Auger recombination is essential for enhancing LED efficiency and mitigating the roll-off effect. Therefore, managing excitations through materials exhibiting the TADF mechanism is of particular interest. Additionally, most QDs and perovskite nanostructures studied in conjunction with TADF materials contain toxic elements such as lead (Pb) and cadmium (Cd), which limits their use in commercial LEDs. Recent advancements in the fabrication of efficient, non-toxic inorganic nanosized light-emitters and devices^134,135 should be considered in the future design of hybrid systems.

Furthermore, several challenges related to color purity, device stability, and further commercialization remain pertinent. The efficient, tunable, and narrowband emission characteristics of QDs and perovskite nanostructures position them as exceptional materials for LED applications. However, intrinsic emission from TADF materials may contribute to the EL spectrum, thereby compromising color purity. To mitigate this issue, energy transfer processes in hybrid systems must be meticulously optimized. This necessitates a more precise design of hybrid structures across various spectral ranges, considering their detailed energy structures. Additionally, the moderate thermal stability, efficiency roll-off, and limited operational lifetime at high brightness levels, which are common to both TADF materials and perovskite nanostructures, still require careful management.

The necessity of further commercialization of the described hybrid systems raises additional challenges. Primarily, reducing costs and enhancing scalability demand significant effort. The relatively complex synthesis process for many TADF compounds, coupled with the high cost of certain precursors necessary for the synthesis of state-of-the-art inorganic light emitters, significantly restricts the widespread adoption of advanced hybrid systems for LED production. Moreover, advanced techniques for active layer deposition, such as spray-coating or inkjet printing, should be adapted for organic–inorganic hybrid systems, potentially requiring new design concepts for material integration. Additionally, the development of technologies for the formation of large-area LEDs based on hybrid systems is anticipated. Recent successes reported in the fabrication of efficient large-area LEDs using the individual components of these systems^136–141 provide some optimism in this area.

5. Conclusion

In this review, we explored the development of new hybrid systems incorporating TADF materials, semiconductor QDs and perovskite thin-film nanostructures. After the discussion of the fundamental properties of these materials, which are significant for LED technology, we examined recent advances in the design, characterization, and implementation of these systems. In particular, we examined how the optical, electronic, and morphological characteristics of an active layer of an LED can be enhanced by combining these materials. Finally, we outlined prospective future directions for advancing hybrid systems that incorporate TADF materials and inorganic nanoscale emitters for the fabrication of efficient and stable LEDs.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

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

Acknowledgements

This work was supported by the Russian Science Foundation (RSF 25-13-00300).

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