Strategies for Enhancing Efficiency and Operational Stability in Green Phosphorescence‐Sensitized Fluorescence (PSF) Organic Light‐Emitting Diode (OLED) Technology

ABSTRACT

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have gained considerable attention to solve the color purity issue in organic light‐emitting diode (OLED) commercialization. However, their slow reverse intersystem crossing rate (k_RISC) leads to the severe efficiency roll‐off and reduced operational stability. To address this limitation, phosphorescence‐sensitized fluorescence (PSF) technology has been widely adopted. Nevertheless, the external quantum efficiency (EQE) of PSF OLED is relatively low compared to TADF‐sensitized fluorescence (TSF) OLED, where the relatively low radiative rate (k_r) (∼10⁵ – 10⁶ s⁻¹) of phosphorescence sensitizer (PS) limits the Forster resonance energy transfer (FRET) rate and exciton utilization in PSF OLEDs. This review, describes the techniques to improve the FRET rate by optimizing the physical parameters related to it. From the analysis, the FRET rate of the PSF OLEDs can be enhanced by improving the terminal emitters' (TE's) extinction coefficient (ε) and a large spectral overlap (J) with PS emission. Further, utilizing the dual‐channel energy transfer process, combined with high T₁ exciplex host system to suppress the back energy transfer (BET) from PS to the host, resulting in high EQE. Furthermore, the stability of the PSF OLED can be improved by the utilization of a highly stable intrinsic ET‐type host, and TE materials.

Efficiency and stability of PSF OLEDs can be obtained by improving the terminal emitters (TE's) extinction coefficient (ε) and a large spectral overlap (J) with PS emission, combined with a dual‐channel energy transfer process, with a high T₁ exciplex host system to suppress the back energy transfer (BET) from PS to the host and a high intrinsic stable ET‐type host.

1. Introduction

Since the first appearance of organic light‐emitting diodes (OLEDs) by Ching Tang and Van Slyke, the OLED technology has gained significant attention in both academia and industry due to its high performance, low power consumption, and flexibility. In addition, OLED pixels emit their own light, which distinguishes them from liquid crystal displays (LCDs) that rely on a backlight. In the last couple of decades, OLED technology has attained a noteworthy level of development in terms of efficiency and operational stability, especially at a relatively low luminescence (∼10³ cd m⁻²) [1, 2]. The development mainly focuses on the conceptual engineering of both emitting materials and carrier‐transporting materials, as well as OLED device architectures. After the development of phosphorescent and thermally activated delayed fluorescence (TADF) emitters, the internal quantum efficiency (IQE) of OLEDs has reached 100%, in contrast to the conventional fluorescent emitters, which has an IQE of 25% [3, 4, 5]. The four‐fold enhancement of IQE in phosphorescent emitters is due to the harnessing of both the singlet and the triplet excitons into radiative decay through spin orbit coupling (SOC) involving heavy metals [6, 7, 8, 9, 10]. Whereas, in case of TADF emitters, 100% IQE is achieved from the up conversion of triplet excitons into radiative singlet excitons through the reverse intersystem crossing (RISC) process [11, 12, 13]. Upon extensive research on the three primary colors, both red and green phosphorescent OLEDs are incorporated in OLED panels for the commercialization of large‐screen displays and smartphones [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]. Although blue phosphorescent materials exhibit high efficiency, their operational stability is comparatively lower than that of fluorescent materials. Therefore, conventional fluorescent emitters are still used in commercial blue OLEDs due to their reliability and long operational lifetime [10].

Despite the commercialization of red and green OLEDs based on phosphorescence emitters, there remain several technical challenges, including the enhancement of color purity and the extension of device operational lifetime, which are critical for the advancement of ultra‐high definition (UHD) displays across diverse applications. One of the promising strategies for enhancing the color purity of OLEDs involves the intrinsic improvement of organic emission characteristics, specifically by achieving a narrow full‐width at half‐maximum (FWHM). In this regard, Hatakeyama et al. proposed a sub‐class of TADF materials known as multi‐resonance (MR) TADF materials in 2016 [25]. Generally, MR‐TADF materials possess a fused planar polycyclic aromatic hydrogen (PAH) framework, which typically includes electron‐ donating units such as nitrogen and electron‐withdrawing units like boron. As a result, the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are separated by atom by atom, resulting in the short‐range charge transfer (SRCT) phenomenon. This SCRT phenomenon leads to a reduction in reorganization energy (λ) for both the S₀ and S₁ states, which in turn results in narrow FWHM, high photoluminescence quantum yield (PLQY) and high horizontal orientation (θ) [26, 27, 28, 29, 30, 31, 32, 33, 34]. Regardless of these advantages, most of the reported MR‐TADF emitters exhibit relatively low k_RISC rate compared to the conventional donor‐acceptor (D‐A) type TADF emitters, typically in the range of 10⁴ –10⁵. Similar to phosphorescence emitters, MR‐TADF emitters also possess longer triplet exciton lifetime because of low RISC rates, which eventually leads to bimolecular quenching processes like triplet‐triplet annihilation (TTA) and triplet‐polaron annihilation (TPA), resulting in severe efficiency roll‐off at high current density or high brightness [35].

To address the limitations and harness the benefits of MR‐TADF emitters, two sensitization technologies have been proposed: the TADF‐sensitized fluorescent (TSF) technology and the phosphorescence‐sensitized fluorescence (PSF) technology. These technologies have recently attracted considerable attention because of their unique merits, such as the combination of excellent stability, high color purity, and high exciton utilization [36, 37, 38, 39, 40]. The TSF and PSF technologies make use of a sensitized‐mediated energy transfer process to transfer the singlet exciton energy from the TADF sensitizer or the triplet energy from the phosphorescent metal‐ligand complex to the terminal emitter (TE) via efficient Forster resonance energy transfer (FRET) [41, 42, 43, 44]. The schematics of emission mechanism and exciton dynamics in TSF and PSF technologies are presented in Figure 1. The primary distinction between the two technologies lies in the process by which the T₁ excitons of the sensitizers are utilized. In the TSF system, the triplet excitons on TADF materials are upconverted back to the emissive singlet excitons (S₁) through the process of RISC, subsequently transferring the singlet exciton energy to the S₁ state of TE for light emission. During the RISC process in TADF sensitizer, triplet excitons may accumulate in the T₁ state and subsequently involves in various processes, including TTA, TPA, and non‐radiative decay, resulting in high efficiency roll‐off and relatively low operational stability. Whereas in the PSF system, the triplet excitons on the phosphorescence sensitizer (PS) are directly transferred to the S₁ state of the TE, due to the robust SOC in PS which leads to singlet–triplet mixing, allowing the typically triplet excited state to gain some singlet characteristics and consequently participate in dipole‐allowed FRET to the singlet excited state of the fluorescent emitter. This process effectively reduces the accumulation of triplet excitons and minimized non‐radiative losses [43, 44]. Therefore, compared to the TSF OLEDs, PSF OLEDs demonstrate enhanced operational stability and reduced efficiency roll‐off at higher current density, making them well‐suitable for the commercialization [45, 46, 47].

FIGURE 1.

(a) Energy transfer mechanism of PSF technology consists of a phosphorescence sensitizer and MR‐TADF emitter, and (b) energy transfer mechanism of TSF technology consists of a TADF sensitizer and MR‐TADF emitter.

Nevertheless, the external quantum efficiency (EQE) of PSF OLED is relatively low compared to the TSF OLED, as the efficiency of the sensitized OLED primarily depends on the FRET rate between the sensitizer and TE. This variation in EQE is attributed to the direct relation between the radiative rate (k_r) of sensitizer and the FRET rate between sensitizer and TE, where the relatively low k_r (∼10⁵–10⁶ s⁻¹) of PS in comparison to the TADF sensitizers limits the FRET efficiency and exciton utilization in PSF OLEDs. In contrast, the TADF sensitizers, relatively possess high k_r (∼10⁶–10⁷ s⁻¹), enabling more efficient and rapid energy transfer to the TE. This accelerated exciton harvesting with the EML, resulting in enhanced FRET efficiency and high EQE in TSF OLEDs [48, 49, 50, 51, 52, 53]. Furthermore, extensive molecular engineering strategies have been reported to simultaneously minimize the ΔE_ST and enhance the spin orbit coupling (SOC) in TADF sensitizers, consequently achieved the TADF sensitizers with high RISC rates approaching 10⁷ s⁻¹ _. Such high RISC rates substantially improve exciton utilization in TSF OLEDs. However, the delayed emission in TADF sensitizers, which typically occurs on the microsecond timescale, continues to cause severe efficiency roll‐off and poor operational stability. This behavior arises from enhanced bimolecular annihilation processes under high exciton density, ultimately impeding the practical commercialization of TSF OLEDs technologies. In this context, enhancing EQE in the PSF technology will be a viable solution for the development of ulta‐high definition (UHD) displays across different applications. Therefore, enhancing the FRET rate between the PS and the TE, will be a promising strategy for constructing highly efficient and stable PSF OLED.

Furthermore, integrating an efficient PSF system into a top‐emission OLED (TEOLED) technology, which simultaneously combines the efficient exciton harvesting with microcavity enhanced light extraction, results in higher current efficiency (CE), superior color purity, reduced efficiency roll‐off, and significantly improved operational lifetime compared to bottom‐emission OLED (BEOLED) technology. Unlike conventional BEOLEDs, TEOLED employ a semi‐transparent top electrode and a reflecting bottom electrode to generate an optical microcavity that alters the optical density of states and redistributes spontaneous emission into favorable optical modes. One significant effect of the microcavity is the amplification of the radiative decay rate via the Purcell effect, enabling faster exciton decay and reducing the residence time of excitons within the emissive layer. As TTA and TPA as strongly dependent on exciton density and lifetime, accelerated radiative rate can effectively suppress these degradation pathways, thereby improving both efficiency and operational lifetime. Moreover, the TE in PSF OLEDs typically exhibit high PLQY, coupling these emitters with an optimized optical cavity allows an additional optical enhancement beyond the intrinsic exciton utilization. Another advantage of TEOLED is improving the CE and the color purity, where the resonance induced by the microcavity, results in spectral narrowing which minimizes light loss by reducing non‐resonance emitting wavelengths and amplifying the cavity resonance wavelength, leads to enhanced CE and color purity of TEOLED. Thus, molecular engineering for the narrowband MR‐TADF emitters together with suitable cavity‐length modification can further enhance the performance of PSF based TE‐OLEDs [54, 55, 56, 57, 58].

One of the possible ways to fully utilize the merits of PSF technology in both BEOLED and TEOLED is by enhancing the EQE of the PSF technology by optimizing the EML layer, which consists of host, PS and TE. In previously reported review articles on PSF technology, the design strategies, challenges, and future directions are discussed only from the TE and PS point of view. However, understanding of the physical parameters required for the enhancement of FRET rate and EQE within the PSF system is necessary. In this review, we aim to present a comprehensive perspective on efficient and stable green PSF OLEDs. We first discuss the key photophysical parameters required for an efficient and stable PSF system and their influence on EQE. Subsequently, we summarize the interplay between photophysical processes and device engineering strategies that together determine the device efficiency and operational lifetime. Beyond the fundamental FRET mechanism responsible for efficient exciton utilization, this review also highlights the synergistic role of molecular design, sensitizer engineering, host and device architecture optimization, exciton management, and degradation mitigation strategies in realizing high‐performance green PSF OLEDs.

2. Results and Discussion

2.1. Required Parameters for an Efficient PSF Technology

To construct highly efficient and stable PSF OLED devices, the four main parameters that need to be considered are as follows: (i) suppressed Dexter energy transfer (DET), (ii) reducing charge trapping on the TE, (iii) high horizontal orientation of TE, and (iv) enhanced FRET rate between PS and TE. Since the FRET rate depends on various physical parameters, understanding and optimize the parameters associated with FRET rate is necessary. The FRET rate can be theoretically determined through the application of the Forster equation. The energy transfer model through dipole‐dipole coupling was formulated by T. Förster in the following manner [59].

$$k_{FRET}=\frac{1}{{\tau }_D}{\left(\frac{R_0}{R}\right)}^6$$ (1)

In this context, τ _D represents the triplet radiative decay lifetime of the phosphorescence sensitizer; R denotes the center‐to‐center intermolecular distance, R_o indicates the Förster radius, which is defined as the radius at which half of the energy transfer efficiency occurs, and k_FRET refers to the FRET rate coefficient. According to Equation (1), k_FRET exhibits an inverse relationship with the decay lifetime of PS as well as the intermolecular distance separating the PS from the TE. The calculation of the center‐to‐center intermolecular distance (R) between the PS and TE are given as follows:

$$R={\left[\left(moleculardensityinfilm\right)\times \left(mol\%oftheTEinafilm\right)\right]}^{-1/3}$$ (2)

Were, the molecular density in the film (host and dopant) can be obtained from the molecular dynamics (MD) simulations and mol % of the TE in a film, refers to the percentage of the molecular number density occupied by the TE in a film. From this equation, the doping concentration of TE is directly proportional to R, therefore increasing doping concentration of TE, results in reducing R, thereby increasing the FRET rate. In addition to the R, the k_FRET is also directly proportional to the Förster radius (R_o), with the calculation of R_o is carried out as follows:

$$R_o=\sqrt[6]{\frac{9000\left(\ln 10\right)k^2\phi }{128{\pi }^2{N}_A{n}^4}J}$$ (3)

In Equation (3), the variable k² denotes the relative orientation between the sensitizer and the TE. φ represents the PLQY of the phosphorescence sensitizer, N_A is Avogadro's number, while n indicates the refractive index of the medium, which is assumed to be 1.8 for the organic layer. J signifies the spectral overlap between the phosphorescence emission and the absorption of TE. As mentioned earlier, k_FRET is intrinsically associated with R _o, with R_o being directly proportional to both the relative orientation (k²) and the spectral overlap (J). The J value can be obtained as follows:

$$J=\int f_H\left(\lambda \right).{\epsilon }_D\left(\lambda \right)\lambda 4d\lambda$$ (4)

where, f_H (λ) represents phosphorescence emission spectrum, while λ denotes the wavelength. Additionally, ε _D (λ) refers to the molar absorption coefficient of the TE. Equation (4) indicates that a significant spectral overlap is achievable when the molar absorption coefficient of TE is increased, resulting in an increased R_o and a higher k_FRET rate [60, 61, 62].

In summary, the four main factors to enhance the FRET rate in PSF system are influenced by the PS and TE, are as follows: (i) a high molar absorption of the TE (ε _D ), which enables the absorption of the most of energy from the sensitizer with minimal loss, alongside a significant spectral overlap (J) value. (ii) The relative orientation of the sensitizer and final emitter, defined as a high horizontal dipole (k²) value. (iii) the intermolecular distance (R) between PS and TE for an efficient energy transfer, and (iv) the phosphorescence sensitizer should exhibit a short decay lifetime (τ _D ). Theoretically, improving all the above‐mentioned four main parameters will lead to an efficient FRET rate, thereby enhancing the efficiency of the PSF OLED.

Although the FRET efficiency in PSF systems is mostly controlled by the four factors stated above, the charge balance in the device should also be taken into account as a critical aspect in determining the overall efficiency and operational stability of PSF OLEDs. Efficient and balanced electron and hole injection and transport are needed to extend the exciton recombination zone and to minimize the accumulation of carriers on the phosphorescent sensitizer or terminal emitter. An imbalance of the carrier transport can cause carrier trapping, TTA and TPA processes, leading to efficiency roll‐off and faster device degradation. Thus, in order to realize efficient and stable PSF OLEDs, not only the optimized FRET dynamics but also the right charge management via rational host selection, energy‐level alignment, and balanced carrier transport are required.

2.2. Strategies to Achieve Efficient PSF Technology

2.2.1. Suppression of Dexter Energy Transfer (DET)

One of the strategies for achieving efficient green PSF technology is by suppressing DET from the triplet excited state of the PS to the triplet (T₁) state of the TE, as DET is one of the nonradiative energy loss channel in PSF technology. Unlike FRET, DET process necessitate the exchange of electrons, requires subsequential overlap of both spectral and orbital between the sensitizer and the TE. Consequently, restricts the DET process to shorter distances, approximately around 1 nm [63, 64, 65]. Suppression of DET is not only important for maximizing the exciton utilization efficiency, but also plays a major role in mitigating efficiency roll‐off and enhancing operational stability. The triplet exciton transfer to the TE via DET results in the accumulation of long‐lived triplet excitons on the TE. This accumulation increases the probability of bimolecular annihilation processes, including TTA and TPA, thereby resulting in high efficiency roll‐off at high current densities and induces chemical degradation under electrical stress. To effectively suppress DET while preserving efficient FRET, it is therefore essential to increase the average spatial separation (R) between the PS and TE beyond the effective DET interaction range, as schematically illustrated in Figure 2 and the DET equation presented below.

$$k_{DET}=KJ\exp \left(\frac{-2R_{DA}}{L}\right)$$

where K is the experimental factor, J refers to the normalized donor‐acceptor spectral overlap integral, R_DA is the center‐to‐center intermolecular distance between donor (PS) and acceptor (TE), and L is the sum of Van der Waals radius. From the above equation, the k_DET exhibits exponential dependance on R, indicates even a slight increase in intermolecular distance between PS and TE will suppress the DET.

FIGURE 2.

The schematic illustration of FRET and DET energy transfer process in relation with intermolecular distance between PS and TE, (a) without any peripheral bulky units and (b) with peripheral bulky units.

The commonly employed strategies to achieve this include the engineering of molecular structures and device architectures. At the molecular level, the introduction of bulky steric substituents or rigid three‐dimensional structures into either the PS or TE can physically restrict the close intermolecular interactions, thereby reducing orbital overlap and suppressing DET. At the device level, lowering the doping concentration of the TE within the EML layer increases the average intermolecular distance between the PS and TE, further reducing the probability of short‐range electron exchange. Further to confirm the dependency, we consider L as 0.5 nm, and calculated the k_DET for R values 1.0 and 1.5 nm. The resulting k_DET values are 1.83 × 10⁻² and 2.48 × 10⁻³ for R values 1.0 and 1.5 nm, respectively. Since DET is a short‐range interaction (<1 nm), increasing R by only 0.5 nm can reduce the Dexter transfer rate approximately by 7.4 times. In contrast, FRET remains efficient over longer distances because it follows R⁻⁶ dependence. Therefore, these strategies effectively suppress DET, thereby simultaneously enhances EQE, reducing efficiency roll‐off and extended operational lifetime in green PSF OLEDs [35, 36, 37, 38, 66, 67].

2.2.2. Reducing the Charge Trapping on the TE

The exciton recombination on the TE due to carrier trap within the TE is another main channel for nonradiative energy loss in PSF technology. The direct recombination on TE, leads to an increased triplet exciton density on TE, results in reduced the exciton utilization efficiency and bimolecular annihilations processes like TTA and TPA. Most commonly used strategy to reduce the carrier trap in TE and maximize exciton utilization is by utilizing an effective host‐dopant system as shown in Figure 3. To achieve an efficient host‐dopant system for PSF system, the following requirements must be satisfied: (i) the energy levels of the frontier orbitals (HOMO/LUMO) of the host materials should align with respective transporting layer to ensure smooth injection of carriers without barriers at low driving voltage, (ii) the host material should exhibit bipolar transporting capabilities to broaden the emission recombination zone, (iii) the host material must possess higher singlet (E_S) and triplet (E_T) energy levels compared to the dopant energy levels (S₁/T₁) to prevent the back energy transfer (BET) from dopant to the host and confine the triplet excitons within the EML, and (iv) the host material needs to exhibit good thermal stability and film forming ability to enhance the operational lifetime of an OLED [68, 69, 70]. Therefore, the utilization of wideband gap hosts or exciplex host systems are the successful approach for reducing carrier trap on TE. Such device architectures facilitate good charge balance, results in exciton recombination on the sensitizer and transfers energy to the TE. Implementing exciplex host/mixed host technology to the PSF system, reduces the nonradiative losses, suppresses efficiency roll‐off and significantly improves the operational stability of PSF OLEDs.

FIGURE 3.

The schematic illustration of trap‐assisted recombination and Langevin recombination within the EML, (a) electron will be trapped on the TE due the deep LUMO, and undergo trap‐assisted recombination with the hole accumulated on the TE due to the shallow HOMO level compared to the host, along with the similar HOMO energy levels of PS and TE, (b) both the hole and electron are transported through the respective p‐type and n‐type materials and undergo Langevin recombination, due to the similar HOMO‐LUMO energy levels of host, PS and TE.

2.2.3. High Horizontal Orientation of TE

In addition to the suppression of DET and carrier trap on TE, high horizontal orientation (θ) of TE is also necessary to enhance the efficiency of the PSF technology, as the outcoupling efficiency of an OLED can be increased significantly if emitters are horizontally oriented, such that transition dipole moments (TDMs) align horizontally with respect to the plane of the device. To improve the horizontal orientation of TE, mainly used strategies include the optimization of emitters towards more planar and linear, utilizing the host materials with suitable glass transition temperatures (T_g) to stabilize favorable orientations during film formation, and tuning the polarity of the host material as it slightly disturbs the dipole‐dipole interactions between the dopant molecules and results in the enhancement of the horizontal TDM orientation of emitters as shown in Figure 4. Among these approaches, more research occurred on the improvement of molecular shape by increasing the planarity due to the extension of π‐conjugation, particularly for MR‐TADF based TE.

FIGURE 4.

(a) General relationship between the anisotropies of molecular shape and molecular orientation in vacuum‐deposited amorphous organic films. Reproduced with permission [71]. Copyright 2011, The Royal Society of Chemistry. (b) Schematics of film formation processes for emitter: host systems, including (1) nonpolar emitter: low‐T _g host, (2) nonpolar emitter: high‐T _g host, (3) polar emitter: nonpolar host, and (4) polar emitter: polar host. Dotted and broken lines indicate emitter–emitter dipole interactions for (3), and emitter–emitter, host–host, and emitter–host dipole interactions for (4), respectively. Black arrows indicate permanent dipole moments (PDMs) of emitter and host molecules. Reproduced (Adapted) with permission [72]. Copyright 2020, The Authors, published by AIP Publishing.

2.3. Techniques to Improve Förster Energy Transfer Rate (FRET) in Green PSF OLEDs

2.3.1. High Extinction Coefficient Of Final Emitter (ε_A) and Spectral Overlap (J)

As discussed earlier, the FRET rate is directly proportional to the Forster radius (R _o), where R _o being directly proportional to both the k ² and the spectral overlap (J). Initially to understand the influence of ε _A on spectral overlap, thereby on FRET rate. In this context, investigation of five recently reported narrowband green MR‐TADF emitters, namely tBN‐DNPTrz [73], tBuICzBN‐Q [74], tCzphB‐Fl [75], Me‐NapBF and Ph‐NapBF [76] with different molar extinction coefficients are conducted. The chemical structures of these materials, along with the chemical structures of PS are presented in Figure 5.

FIGURE 5.

Chemical structures of Phosphorescence sensitizers and narrowband MR‐TADF terminal emitters utilized in this review.

Further, the calculated molar extinction coefficient (ε _A ) for all five MR‐TADF emitters, which exhibit absorption peak maximum at 529, 516.5, 533, 512, 516 nm, and the calculated ε _A are 30 305, 31 200, 43 906, 53 330, and 63 889 M⁻¹ cm⁻¹ for tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl, Me‐NapBF and Ph‐NapBF, respectively. The spectral overlaps (J) were calculated by selecting Ir(mppy)₃ as PS for tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl, and Ir(ppy)₂acac for Me‐NapBF and Ph‐NapBF. The calculated J values between Ir(mppy)₃ and tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl are 8.20 × 10⁻¹⁴, 12.22 ×, 18.39 × 10⁻¹⁴ M⁻¹ cm⁻¹, respectively. Whereas the calculated J values between Ir(ppy)₂acac and Me‐NapBF, Ph‐NapBF are 16.71 × 10⁻¹⁴, 23.35 × 10⁻¹⁴ M⁻¹ cm⁻¹, respectively. The spectral overlap between PS and TE are presented in Figure 6. In case of tBN‐DNPTrz and tCzphB‐Fl, the absorption maximum is in the similar range of 530 nm, however, tCzphB‐Fl has higher ε_A value, results in increased J value. Similar trend was also observed in case of Me‐NapBF and Ph‐NapBF, where the absorption maximum is around 515 nm range, but Ph‐NapBF showed higher ε _A value, results in the higher J value due to the extended π‐conjugation and t‐butyl phenyl substitution in Ph‐NapBF.

FIGURE 6.

(a) Spectral overlap between the emission of Ir(mppy)₃ and the absorbance of tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl, and (b) Spectral overlap between the emission of Ir(ppy)₂acac and the absorbance of Me‐NapBF, Ph‐NAPBF.

The theoretically calculated FRET rate for 5wt.% Ir(mppy)3 and 1wt.% of tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl are 2.61 × 10⁶, 4.71 × 10⁶, 6.36 × 10⁶ s⁻¹, respectively. In case of 6wt.% Ir(ppy)₂acac and 1wt.% of Me‐NapBF, Ph‐NapBF are 5.75 × 10⁶ s⁻¹, 7.57 × 10⁶ s⁻¹ respectively. The relation between the spectral overlap vs FRET rate is shown in Figure 7a. This analysis confirms that the FRET rate can be improved through the enhancement of TE's molar extinction coefficient via molecular engineering, results in increased spectral overlap. To understand the relationship between the FRET rate and EQE, the comparison of EQE for the optimized PSF OLEDs and summarized in Table 1 and illustrated in Figure 7b. From Figure 7b, the EQE of tBuICzBN‐Q based PSF OLED was relatively low compared to the other emitters based PSF OLED, even though tBuICzBN‐Q based PSF system showed higher FRET rate than tBN‐DNPTrz based PSF system. This dispensary can be attributed to the variation in charge balance within the device, as it is one of the key parameters which determines the EQE of the OLED. Other than tBuICzBN‐Q, remaining 4 emitters exhibited direct relation between FRET rate and EQE. The Ph‐NapBF based PSF OLED exhibited relatively higher EQE of 37.7%, which can be attributed to the high molar extinction co‐efficient, results in improved FRET rate.

FIGURE 7.

(a) FRET rate vs. spectral overlap plot between PS and TE (b) Relation between FRET rate and EQE of the PSF OLED using Ir(mppy)₃, Ir(ppy)₂acac as PS and tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl, Me‐NapBF, Ph‐NapBF as TE's.

TABLE 1.

Summary of the photophysical and experimental results for tBN‐DNPTrz, tBuICzBN‐Q, tCzphB‐Fl, Me‐NapBF, Ph‐NapBF.

PSF system	PLmax/FWHM (nm)	ε A M−1 cm−1	J × 10−14 M−1 cm−1	k FRET × 106 s−1	EQEmax (%)	ELmax/FWHM (nm)
5 wt.% Ir(mppy)3: 1 wt.% tBN‐DNPTrz	529/22	30 305	8.20	2.61	29.3	531/35
5 wt.% Ir(mppy)3: 1 wt.% tBuICzBN‐Q	536/23	31 200	12.22	4.71	28.9	536/23
5 wt.% Ir(mppy)3: 2 wt.% tCzphB‐Fl	533/25	43 906	18.39	6.36	36.2	535/31
5 wt.% Ir(ppy)2acac: 0.5 wt.% Me‐NapBF	526/23	53 330	16.71	5.75	35.5	530/28
5 wt.% Ir(ppy)2acac: 3 wt.% Ph‐NapBF	531/21	63 889	23.35	7.57	37.7	533/25

2.3.2. High Relative Orientation Between the Sensitizer and the Final Emitter (k²)

Another factor that has a direct influence on the Forster radius is the relative dipole orientation (k²). To understand the effect of phosphorescence sensitizer orientation on FRET rate, Jun Yeob Lee et al. reported the effect of the orientation of the PS, where two difference PS are compared with different horizontal orientation. The two different PS are Ir(ppy)₃ and Ir(pctpy)₃ with horizontal orientations of 66% and 89%, resulting in different k² values of 0.66 and 0.75, respectively [77]. The theoretically calculated k_FRET between 10wt.% of Ir(ppy)₃ and 1wt.% TBRb is 6.6 × 10⁶ s⁻¹, whereas the calculated k_FRET between 10wt.% of Ir(pctpy)₃ and 1wt.% TBRb is 1.1 × 10⁷ s⁻¹. However, in addition to k² value, the other parameters like PLQY, J, R₀ and R, are also varied due to the two different PS systems. Therefore, to understand the effect of k² on FRET rate, the FRET rate was calculated for the 10wt.% of Ir(pctpy)₃ and 1wt.% TBRb by changing only k² value to 0.66, and the calculated FRET rate is 9.6 × 10⁶ s⁻¹. This analysis confirms that the higher k² value results in a higher FRET rate. As a result, the EQE of the device was improved for Ir(pctpy)₃ based PSF with maximum EQE of 24.0% with minimal efficiency roll of 23.9% (EQE) at 3 000 cd m⁻², compared with Ir(ppy)₃ based PSF which showed maximum EQE of 20.8% with a relatively higher efficiency roll of 18.2% (EQE) at 3 000 cd m⁻².

As the PS sensitizer orientation plays a key role in enhancing the k² value, as shown in Figure 8, it's important to understand the factors that influence the orientation of PS. As mentioned in earlier section that, the emitting dipole orientation (EDO) of the Heteroleptic Ir complex varies from the horizontal to isotropic, or even to the vertical, direction depending on T _g of host molecules. The mobility of the molecular surface decreases on a host with a high T _g, which inhibits the randomization of the molecular orientation during vacuum deposition. Further, host material's static dipole moment and polarizability also influence the EDO of PS. The CBP and UGH‐2 host materials possess nearly zero static dipole moments due to their symmetric structures and less polar groups, whereas TSPO1 has large static dipole moment due to the polar phosphine oxide and the asymmetric structure. The resulted orientation of Ir(ppy)₂tmd are 60%, 75% and 78% for UGH‐2, CBP and TSPO1 respectively. This enhancement of PS orientation is attributed to the attractive interactions between pyridines of a phosphor and CBP (quadrupole‐induced dipole interaction) or TSPO1 (quadrupole‐dipole interaction), anchors the phosphor onto host molecules with horizontal iridium‐pyridine alignment, thereby increasing the horizontal EDO of PS. Therefore, by carefully selecting the host materials which favors the high orientation of PS which results in increased k², thereby increasing FRET rate and EQE [59]. In addition, the horizontal EDO of TE also effects the k² by altering the relative orientation between the emission transition dipole moment of the PS and the absorption transition dipole moment of the TE. Both the PS and the TE with high horizontal EDO leads to a favorable in‐plane alignment of the two dipoles, resulting in k² value bigger than isotropic value (k² = 2/3), which results in an increased FRET efficiency. Hence, both the PS and TE orientation is crucial to improve the energy transfer efficiency and the device performance.

FIGURE 8.

(a) Contour plot of k² as a function of the horizontal EDOs of the donor (Θ_D) and acceptor (Θ_A). The k² isovalues are marked as a solid black square. Isotropic k²‐values are marked as dotted black lines. The 𝜅 [2]‐values are 0.66 and 0.75 for Ir(ppy)₃: TBRb and Ir(pctpy)₃: TBRb, respectively. Reproduced with permission [77], copyright 2023, Wiley‐VCH GmbH. (b) Horizontal emitting dipole ratio of coumarin 6 doped into different host materials. Reproduced (Adopted) with permission [59], copyright 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

2.3.3. Optimizing Intermolecular Distance (R) Between PS and TE

The center‐to‐center intermolecular distance (R) exhibited an inverse relationship with the FRET rate, as indicated by Equation (1), means if the R is small then k_FRET can be high. The intermolecular distance primarily depends on the doping concentration of the PS and TE, higher the doping concentration, the lower the intermolecular distance. Dongdong Zhang et al. experimentally investigated the effect of TE doping concentration on FRET rate, where the TE (TBRb) doping concentration was varied as 0.5, 1.0, and 1.5 wt.%. The obtained k_FRET using 20wt.% of Ir(ppy)₂acac with varying doping concentrations of TBRb yielding results of 1.41 × 10⁶, 3.20 × 10⁶, and 6.41 × 10⁶ s⁻¹ for concentrations of 0.5, 1.0, and 1.5 wt.%, respectively [48]. Subsequently, the same research team conducted measurements of k_FRET  utilizing a 10wt.% of Ir(ppy)₂acac while varying the doping concentrations of tCzphB‐Fl at 0.5 and 1.0 wt.%. The results yielded values of 3.8 × 10⁶ and 8.0 × 10⁶ s⁻¹, respectively [78]. These results indicate that an increase in the doping concentration of TE, which results in low R value, leads to higher k_FRET  between PS and TE. However, along with the increased k_FRET , higher doping concentration of TE causes disadvantages like carrier trapping on TE and concentration quenching as well as the DET between the PS triplet energy to the FD triplet energy, which results in poor device performance. Therefore, optimizing TE doping concentration plays a crucial role for achieving higher k_FRET , reducing DET and enhancing overall device performance.

2.3.4. Short Decay Lifetime (τ_d) of Phosphorescence Sensitizer

According to Equation (1), the other parameter which inversely influences the k_FRET is the triplet decay lifetime of the PS, higher the decay time lower the k_FRET and vice versa. This trend was experimentally validated in the report by Jun Yeob Lee et al. where they investigate the FRET dynamics in two different PSF systems [77]. The triplet exciton (τ _D ) decay time for Ir(ppy)₃ is around 1µs and for Ir(pctpy)₃ i's 0.65µs, and the calculated experimental FRET rate based on the following equation are [79]:

$$k_{FRET}=\frac{1}{{\tau }_x}-\frac{1}{{\tau }_0}$$ (6)

The parameters τ _x and τ₀ denote the triplet radiative lifetimes of the phosphorescent sensitizer after and before doping TE, respectively. The calculated FRET rates between 10 wt.% of Ir(ppy)₃ and 1 wt.% TBRb is 4.34 × 10⁶ s⁻¹, whereas the calculated k_FRET between 10wt.% of Ir(pctpy)₃ and 1wt.% TBRb is 38.3 × 10⁶ s⁻¹. As a result, the EQE of the device was improved for Ir(pctpy)₃ based PSF with maximum EQE of 24.0% with minimal efficiency roll off of 23.9% (EQE) at 3000 cd m⁻², compared with Ir(ppy)₃ based PSF which showed maximum EQE of 20.8% with relatively higher efficiency roll of 18.2% (EQE) at 3 000 cd m⁻². Although in the report it's mentioned that the observed variation in EQE is related to the emission dipole orientation of the PS. Nevertheless, there is currently no study that directly examines the impact of PS decay lifetime. To verify this, we have computed the FRET rate utilizing constant R and R_o values of 2.6 and 3.6 nm, respectively, for both Ir(ppy)₃ and Ir(pctpy)₃ systems, which are the values documented for Ir(pctpy)₃. By implementing this approach, we can remove the impact of other parameters on FRET and focus exclusively on examining the effect of PS decay lifetime. For example, we chose the decay lifetimes of Ir(ppy)₃ and Ir(pctpy)₃, which are 1.04 µs and 0.65 µs, respectively. The calculated FRET rates for these compounds are 6.78 × 10⁶ and 1.08 × 10⁷ s⁻¹, respectively. These findings validate that the PS decay lifetime influences the k _FRET rate. In another report, Jun Yeob Lee et al. conducted the energy transfer analysis for the three different PSF systems, consist of Ir(ppy)₂acac, Ir(ppy)₃, Ir(tms‐py)₃ as PS and TBRb as TE [80]. The three PS showed radiative rates (k_r) of 7.13 × 10⁵, 6.44 × 10⁵, and 7.81 × 10⁵ s⁻¹ for Ir(ppy)₂acac, Ir(ppy)₃ and Ir(tms‐py)₃, respectively. Since radiative rate is inversely proportional to the τ _D of PS, the higher (k_r) of PS results in higher k_FRET . The calculated experimental k_FRET between 5wt.% Ir(ppy)₂acac and 2 wt.% TBRb, 5 wt.% Ir(ppy)₃ and 2 wt.% TBRb, 5 wt.% Ir(tms‐py)_3, and 2 wt.% TBRb are, 5.86 × 10⁶, 5.72 × 10⁶, and 6.03 × 10⁶ s⁻¹, respectively. Compared to the above‐mentioned work of using same PS (Ir(ppy)₃) and TE (TBRb), the current work showed slightly reduced k_FRET from 6.6 × 10⁶ to 5.72 × 10⁶ s⁻¹, even though the doping concentration of TE is increased from 1 to 2 wt.%. This can be attributed due to the reduction of PS doping concentration from 10 wt.% in the previous work to the 5 wt.% in current work, results in reducing the number of donors available to transfer the energy to the TE. The fabricated PSF devices using these three different PS exhibited maximum EQE of 12.7%, 14.6% and 17.8% for Ir(ppy)₂acac, Ir(ppy)₃ and Ir(tms‐py)₃, respectively. The observed trend of EQE_max for Ir(ppy)₂acac and Ir(ppy)₃ is not correlating with the observed k_FRET rates whre, Ir(ppy)₂acac showed slightly higher k_FRET than Ir(ppy)₃ but EQE_max was higher for Ir(ppy)₃ than Ir(ppy)₂acac. This confirms that along with the higher k_FRET the carrier trap on TE should be minimum.

2.4. Dual Channel Energy Transfer Process for Efficient Green PSF Technology

Most of the reported green PSF systems, achieved through the development of novel MR‐TADF emitters, exhibit an EQE around 30%, due to the FRET dynamics between the PS and TE. The relatively low EQE of PSF system is attributed to the exciton loss during the single channel energy transfer process. To minimize the exciton loss and construct an efficient and stable green PSF OLEDs, Recently, Dongdong Zhang et al. reported an innovative strategy based on multiple sensitizations called pTSF [48], where a TADF sensitizing host, a PS and a TE were utilized in the EML as shown in Figure 9.

FIGURE 9.

(a) Energy transfer mechanism of PSF technology consists of a phosphorescence sensitizer and MR‐TADF emitter and (b) Energy transfer mechanism of pTSF technology consists of, TADF host, phosphorescence sensitizer and MR‐TADF emitter.

In the pTSF system, the additional DET between the triplet state of the TADF host and the PS offers a one‐way exciton utilization consumption pathway for the TADF host, breaking the singlet‐triplet spin‐flip cycles and accelerating exciton dynamics compared to the conventional PSF system. In this work, a TADF material, namely DPT‐IC, was developed with a higher RISC rate of 1.4 × 10⁶ s⁻¹. Further, the calculated R_o and k_FRET between DPT‐IC to 1 wt.% of tCzphB‐Fl and 10 wt.% of Ir(ppy)₂acac to 1 wt.% of tCzphB‐Fl were 3.9, 3.7 nm and 1.0 × 10⁸, 8.6 × 10⁶ s⁻¹, respectively. The k_FRET between DPT‐IC to 1 wt.% of tCzphB‐Fl is ten folds higher than k_r and k_ISC of DPT‐IC, suggesting a favorable energy transfer process. In addition, the high horizontal orientation of tCzphB‐Fl (92%) can greatly enhance the light extraction of pTSF devices for achieving high EQE. The fabricated pTSF device exhibited EQE_max of 39.3% with very low efficiency roll‐off of EQE 35.4% at 100 000 cd m⁻² and exceeding EQE_max values for TSF (17.8%), PSF and TADF sensitized phosphorescene (TSP) (25.7%) systems. The operational stability of pTSF demonstrated an LT₉₀ of 114 h at an initial luminescence of 10 000 cd m⁻², whereas TSP, and TSF, systems exhibited an operational lifetime at same conditions are 76, and 45 h, respectively. The enhanced performance of pTSF OLED mainly depends on the kinetics of TADF host. Consequently, it is essential to select an appropriate TADF host that exhibits a high RISC rate and possess anti‐ACQ (aggregation caused quenching) characteristics. Despite the advantages like high efficiency and color purity by utilizing TADF host in a pTSF technology, it also possesses a disadvantage: (i) Limited structural availability of the TADF host as it's a proper alignment of donor and acceptor within a single molecule, (ii) The charge balance issue within the device as the hole mobility or electron mobility varies depending on the structure of the TADF host and (iii) Due to the weak C─N bonds presents in most of the TADF hosts, causing them less stable compare to conventional hosts, resulting in lower operational lifetime of the OLED.

To address these challenges, recently our group proposed the utilization of exciplex systems as hosts in pTSF system, as it offers several advantages: (i) wide structural availability for donor and acceptor materials, resulting in numerous exciplex systems, (ii) Due to the combination of both donor and acceptor materials, the exciplex systems possess better charge balance within the device compared to the single molecule TADF host, and (iii) The inherent stability of ET‐type or HT‐type materials results in longer operational device lifetime. In our recent works, our group developed ET‐type host materials to construct high efficiency and stable PSF OLED with triazine as central core as shown in Figure 10 [68, 81].

FIGURE 10.

Chemical structure of our recently developed electron type (ET) host materials for constructing high efficiency and stable green PSF OLEDs.

In these both works, 9 ([1,1′‐biphenyl]‐4‐yl)‐9′‐phenyl‐9H,9′H‐3,3′‐bicarbazole (BPP‐BCz) is used as HT‐type material to form exciplex with our newly developed materials. The BPP‐BCz: DTTrzDFph‐3 based exciplex showed PL emission at 514 nm with T₁ energy level of 2.51 eV and photoluminescence quantum yield (PLQY) of 37.1%, whereas BPP‐BCz: DPSiTrz exhibited PL emission at 497 nm with T₁ energy level of 2.53 eV and PLQY of 55.1%. The higher T₁ of BPP‐BCz: DPSiTrz and BPP‐BCz: DTTrzDFph‐3 exciplex systems back energy transfer (BET) from PS to exciplex host system. In case of BPP‐BCz: DTTrzDFph‐3, Ir(mppy)₃ is used as PS, and for BPP‐BCz: DPSiTrz, Ir(ppy)₂acac is used as PS, however in both cases tCzphB‐Fl is used as a TE. The calculated experimental FRET rate between 5 wt.% Ir(mppy)3 and 2 wt.% tCzphB‐Fl using BPP‐BCz: DTTrzDFph‐3 exciplex is 4.74 × 10⁶ s⁻¹, whereas the FRET rate between 6 wt.% Ir(ppy)₂acac and 2.5 wt.% tCzphB‐Fl is 5.65 × 10⁶ s⁻¹. Further, the optimized green PSF OLED utilizing BPP‐BCz: DPSiTrz and BPP‐BCz: DTTrzDFph‐3 as exciplex hosts reached impressive maximum EQE of 39.4% and 36.2%, respectively. The improved efficiencies of the BPP‐BCz: DPSiTrz and BPP‐BCz: DTTrzDFph‐3 based PSF OLEDs compared to BPP‐BCz: diDBFTrz exciplex based PSF OLED can be attributed to the high triplet (T₁) energy of the exciplex host, which effectively prevents back energy transfer (BET) from the PS to the exciplex host. Furthermore, the BPP‐BCz: DPSiTrz and BPP‐BCz: DTTrzDFph‐3 based exciplex devices exhibited extended operational lifetimes (LT₉₅) of 134 and 380 h at 5000 cd m⁻², respectively. The extended lifetime of the BPP‐BCz: DTTrzDF‐ph‐3 results from the robust stability of the ET‐type host material, which mitigates the device degradation. The summary of these PSF devices is presented in Table 2.

TABLE 2.

Comparison of exciplex‐based PSF OLED results.

EML	Exciplex T1 (eV)	EQE (%)	Lifetime (h)
BPP‐BCz: diDBFTrz: 5 wt.% Ir(mppy)3: 2 wt.% tCzphB‐Fl	2.29	32.5	195 h (LT97 @ 5000nits)
BPP‐BCz: DTTrzDF‐ph‐3: 5 wt.% Ir(mppy)3: 2 wt.% tCzphB‐Fl	2.41	36.2	380 h (LT95 @ 5000nits)
BPP‐BCz: diDBFTrz: 6 wt.% Ir(ppy)2acac: 2wt.% tCzphB‐Fl	2.29	36.1	—
BPP‐BCz: DPSiTrz: 6 wt.% Ir(ppy)2acac: 2 wt.% tCzphB‐Fl	2.53	39.4	134.4 h (LT95 @ 5000nits)

From all this analysis, the efficiency of the PSF OLED can be enhanced by improving the TE's extinction coefficient (ε) and a large spectral overlap (J) with PS emission, at the molecular level, which are necessary for improving FRET rate and exciton utilization efficiency. Whereas at the device level, utilizing the dual channel energy transfer process within the PSF system, in addition with the incorporation of sufficiently high T₁ exciplex host system to suppress the BET from PS to the host, thereby resulting in high EQE.

2.5. Factors Affecting Operational Lifetime in PSF Technology

Along with enhancing FRET rate in PSF OLEDs to improve EQE, achieving long operational lifetime is also important for practical display applications. In general, the OLED device degradation originates from a combination of exciton‐induced chemical reactions, charge imbalance, material instability, and interfacial deterioration. Thus, understanding degradation mechanisms is of great importance to improve both efficiency and operational stability simultaneously. One of the primary degradation pathways is exciton‐induced molecular decomposition. Under electrical excitation, the accumulation of high‐energy singlet and triplet excitons can trigger irreversible bond cleavage and chemical rearrangement in the emissive layer. The high exciton density also increases the probability of exciton–exciton and exciton–polaron interactions, such as TTA, TPA and singlet–triplet annihilation (STA). Such non‐radiative processes not only reduce device efficiency but also generate localized heat and chemically reactive excited states that accelerate material degradation. The PSF mechanism inherently contributes to improved stability by rapidly transferring singlet excitons from the phosphorescent sensitizer to the terminal fluorescent emitter through highly efficient FRET. Fast FRET decreases the exciton residence time on the sensitizer, hence suppressing triplet accumulation and reducing the probability of triplet‐induced degradation. Consequently, minimizing exciton density on long‐lived excited states is an effective strategy for extending operational lifetime.

The selection of PS is also very crucial to improve the operational stability. PS with high PLQY, fast radiative decay, and effective exciton harvesting reduce the non‐radiative energy losses and lower the exciton accumulation. Similarly, the terminal emitters with strong photochemical stability can better withstand long term electrical operation. Along with that, host material engineering is also equally important for stable PSF OLEDs. Hosts with high triplet energy effectively confine excitons within the emissive layer while preventing exciton leakage to adjacent transport layers. The balanced electron and hole transport of bipolar host materials may improve the charge balance, thus minimizing the local accumulation of charges and suppressing the exciton‐polaron annihilation. In recent years, exciplex hosts and mixed‐host systems have attracted considerable attention because they provide balanced carrier transport, broaden the exciton recombination zone, and reduce local exciton density. Electrically induced degradation and the operational lifetime of PSF OLEDs can be increased by utilizing chemically robust intrinsically stable ET‐type or exciplex host systems possessing strong molecular bonding characteristics. Additionally, the optical microcavity of top‐emitting OLEDs may improve the radiative decay rate via the Purcell effect, shortening exciton lifetime and thereby suppressing degradation processes associated with long‐lived excited states [82, 83].

Despite the remarkable progress of PSF OLEDs, each performance‐enhancement strategy involves inherent trade‐offs. A larger spectrum overlap and a shorter intermolecular distance can greatly boost the FRET rate. But high doping concentration of terminal‐emitter may cause concentration quenching, carrier trapping and DET. Molecular extension of MR‐TADF emitters can achieve a high molar absorption coefficient and exciton utilization efficiency, although it usually increases synthetic complexity and can affect color purity and molecular packing. Likewise, the employment of sensitizers with a high degree of horizontal orientation and short triplet lifetimes is effective in suppressing exciton accumulation and efficiency roll‐off, nevertheless the identification of materials that are simultaneously high PLQY, strongly oriented, and chemically robust is still a challenge. Exciplex and TADF host systems can enhance charge balance and exciton management. However, their performance is highly sensitive to energy‐level alignment and the suppression of reverse energy transfer. Therefore, the main problem to be tackled in PSF OLED research is the simultaneous optimization of FRET efficiency, charge balancing, color purity, and operational stability, while keeping the material processability and the device manufacturability intact (Table 3).

TABLE 3.

Advantages, limitations and remaining challenges of performance‐enhancement strategies in green PSF OLEDs.

Strategy	Advantages	Limitations/Trade‐offs	Remaining Challenges
Large spectral overlap and high extinction coefficient	Higher (R0) and faster FRET	Requires precise spectral matching	Comprehensive molecular design principles
Increased TE concentration	Higher FRET efficiency	Carrier trapping, concentration quenching, DET	Identification of the optimal doping range
MR‐core extension	Higher absorption coefficient, higher PLQY	Synthetic complexity, possible spectral broadening	Maintaining narrowband emission
High horizontal orientation	Enhanced out‐coupling and FRET channels	Difficult molecular design	Simultaneous orientation and stability
pTSF architecture	Near‐complete exciton utilization, low roll‐off	Complex energy alignment requirements	Long‐term operational stability
Exciplex/TADF hosts	Improved charge balance and suppressed BET	Host compatibility and stability issues	General design principles for robust hosts

3. Summary and Outlook

This review has carefully analyzed the photophysical requirements and device‐engineering approaches necessary for enhancing PSF performance, focusing on the essential Förster resonance energy transfer (FRET) mechanism that governs exciton dynamics in the emitting layer. Through comprehensive analysis, four key factors influencing FRET efficiency have been identified: (i) Significant spectral overlap (J) and higher molar absorption coefficients of the terminal emitter contribute to increase the Förster radius. (R₀); (ii) The high horizontal dipole orientation of the phosphorescent sensitizer contributes to an increased orientation factor. (k²); (iii) Short decay lifetime of the sensitizer (τ_d), facilitating quick exciton transfer in comparison to both radiative and non‐radiative decay pathways; and (iv) The optimized donor–acceptor intermolecular distance (R) is crucial for achieving a high FRET rate, while simultaneously minimizing DET and mitigating concentration‐induced quenching effects.

From the perspective of TE molecular design, MR‐TADF emitters have been identified as optimal terminal emitters owing to their naturally narrowband emission, elevated PLQY, and pronounced localized short‐range charge transfer properties. To achieve an efficient and stable PSF OLED, it is essential that the MR‐TADF emitter exhibits certain properties. (i) Significant spectral overlap exists with the emission of the sensitizer, (ii) A high molar absorption coefficient can be achieved via controlled MR‐core extension, (iii) High PLQY and horizontal orientation are necessary, and (iv) The bandgap should be smaller than that of the sensitizer. These characteristics of FD are expected to lead to an increased FRET rate and an enhanced EQE, potentially reaching or surpassing 35%.

At the sensitizer level, the presence of PS with high orientation promotes enhanced FRET channels and diminishes exciton accumulation, which contributes to a reduction in efficiency roll‐off and extends the operational lifetime. In the context of PS, the essential properties for efficient and stable PSF OLED are: (i) Large bandgap than final emitter, to suppress the back energy transfer, (ii) low triplet exciton lifetime, (iii) High PLQY and horizontal orientation, (iv) High chemical stability for high operational stability. Moreover, the host material, which holds equal significance to the sensitizer and FD, must exhibit specific properties to ensure the efficiency and stability of PSF OLEDs: (i) Balanced carrier transport for good charge balance, (ii) Proper energy level alignment with transporting materials and emitting materials to reduce the carrier trap, (iii) High triplet energy level to suppress the back energy transfer from the PS to the host system, (iv) intrinsic stability of host for longer operational stability of OLED and (v) Host material with TADF/exciplex characteristics for dual channel energy transfer.

The comprehensive insights obtained from this review indicate that the future trajectory of PSF and pTSF OLED technologies will hinge on the synergistic fusion of sophisticated photophysical design and device‐level engineering.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

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