Circularly polarized electroluminescence from light-emitting diodes: mechanisms, materials, and applications

Abstract

Circularly polarized luminescence (CPL) has gained significant interest in applications ranging from quantum computing and optical communications to data encryption and bioimaging. Light-emitting diodes (LEDs) that directly emit CPL offer clear advantages over chiroptical approaches, which rely on external optical elements to impart handedness to otherwise unpolarized light. In this review, we first outline the working principles of the two leading CPL-emitting LED architectures: CPLEDs, which do not require spin injection, and spin-LEDs, which rely on spin injection. We then summarize recent material advances—from organic and inorganic semiconductors to hybrid systems—that enable high-performance CPLEDs, alongside the latest developments in spin-LEDs. We analyse the dissymmetry factors of these device systems and discuss strategies to enhance both dissymmetry and overall device efficiency. By combining advances in materials design and device architectures, the field is poised to deliver high-performance CPL sources for next-generation photonic and spintronic applications.

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Tianjun Liu

Dr Liu, Tianjun is a researcher in the Optoelectronics Group at the Cavendish Laboratory, University of Cambridge. He currently holds a Leverhulme Early Career Fellowship under the supervision of Prof. Sir Richard H. Friend. His research focuses on emerging semiconductor materials for optoelectronic applications, with particular emphasis on perovskite semiconductors and chiral compounds. Prior to this, he received his PhD degree from the Queen Mary University of London and conducted postdoctoral research in the Prof. Feng Gao group at Linköping University, where he worked on perovskite light-emitting diodes.

Yuqing Huang

Dr Huang, Yuqing is a joint professor at the Institute of Semiconductor (IOS) and College of Materials Science and Optoelectronic technology at the University of Chinese Academy of Sciences. He obtained his PhD degree from Linköping University, supervised by Prof. Weimin Chen. Later on, he worked as postdoc fellows in Nanyang Technological University under the advisement of Prof. Qihua Xiong and Linköping University before he joined the IOS. Prof. Huang's primary research interest is the spin-dependent phenomena and light–matter interactions in semiconductor materials with the focus on spintronic and quantum information applications. He has published over 30 papers in renowned journals like Nat. Photonics, Nat. Mater., Phys. Rev. Lett., etc.

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1. Introduction

Circularly polarized light (CPL) has emerged as a powerful tool in modern photonic technologies, with unique applications ranging from chiral sensing and 3D displays to quantum communication and spin-based optoelectronics.^1–3 Unlike linearly polarized light, CPL carries angular momentum and interacts differently with chiral matter, making it particularly valuable for probing and manipulating molecular and electronic chirality. Over the past decade, the development of CPL-emissive materials and devices has advanced significantly, driven by the demand for compact, efficient, and tunable CPL sources.^4–11

The generation of CPL can be achieved through multiple physical mechanisms, which can be broadly categorized into approaches breaking time reversal symmetry and effects involving breaking of spatial symmetry. In the former case, the photon helicity is converted from electron spin through spin–orbit coupling (SOC) or magnetic dipole moments, which is achieved in a so-called spin-LED that employs magnetic materials or chiral matter as sources for generation of imbalanced spin population.^12–14 In the other case, structures, e.g. metamaterials^15,16 and supramolecular assemblies,^17–19 which break mirror symmetry, give rise to chiral interaction with the emitting electromagnetic field. CPL is then obtained from the interplay with such chiral optical modes, which we denote as a chiral-LED. In some other cases, even though the mirror symmetry is preserved, structural symmetry is lowered such that opposite CPL is separated and emitted at different emission directions. This form is often termed extrinsic chiral emission.

In terms of device structure, as the chiral-LED explores mainly photonic effects, the active layers for generation of chirality and electroluminescence (EL) are the same as those illustrated in Fig. 1. This leads to a relatively simplified device structure that is similar to a conventional LED with the active layer replaced by chiral organic materials, such as conjugated polymers, or a LED with surface functionalized by chiral metasurfaces. In contrast, spin-LEDs operate by polarizing electron spins, which can be achieved either directly in the emissive layer, e.g. in chiral small molecules, magnetic semiconductors, etc. or via an additional spin aligner, such as magnetic electrodes,²⁰ or a chiral interfacial layer,²¹ which then injects spin into the emissive layer to emit CPL via spin dependent recombination. Such a spin injection scheme favours convenient control of the emission helicity in the way that the electron spin polarization can be tuned independently of the EL. However, it requires a relatively complicated device structure that not only increases fabrication barriers but also compromises emission helicity due to spin relaxation along the injection pathway.

Fig. 1 (a) and (b) Generation of CPL emission by breaking the time reversion symmetry (spin-LEDs) and spatial symmetry (chiral-LEDs). (c) and (d) Typical vertical device structures of CPLEDs.

One of the key figures-of-merit for chiral-/spin-LEDs is the emission helicity. It is typically measured with either the degree of circular polarization (P_cir) or the electroluminescence dissymmetry factor (g) in different research communities.

Both P_cir and g_lum quantify the relative difference in intensities of right- and left-handed circularly polarized emission and are linked through the relationship P_cir = g/2. In eqn (1) and (2), I_L and I_R are the intensity of left-handed and right-handed circularly polarized emission, respectively.

In addition to circular polarization metrics, standard optoelectronic performance parameters such as external quantum efficiency (EQE), brightness (for visible electroluminescence), and radiance (for infrared emission) are also critical for evaluating the overall performance of CPL-emitting LEDs. These metrics provide insight into the light-emitting efficiency and practical applicability of the devices across different spectral regions.

The development and control of CPL-emitting devices has been an intensely pursued topic in the field for a long time. In 1999, circularly polarized electroluminescence has been demonstrated in GaAs spin-LEDs by injection of electron or hole spin from dilute magnetic semiconductors (DMSs) at cryogenic temperature.^22,23 After this, extensive efforts have been made to improve P_cir as well as the operational temperature towards practice room-temperature applications. This includes employment of tunnelling spin injection,^24,25 growth of perpendicular magnetic heterostructures,^26–28etc. These developments establish optimized III–V spin-LED structures that are capable of emitting CPL with a P_cir of >30% at room temperature.²⁰ Developing in parallel, the first CPL-emitting organic LED was introduced in 1997 using a chiral conjugated polymer.²⁹ Here, the main challenge is caused by the trade-off between the brightness, affected by both the EL quantum efficiency and the oscillator strength, and emission helicity g_lum. With combined advances in chiral metamaterials and metasurfaces, the chiral-LED has now expanded the spatial extent of the chiral light–matter interaction from molecular orbitals (of ∼nm) to ∼μm scale that is comparable to the emission wavelength. Recently, the effect of chiral induced spin selectivity (CISS) has been explored to incorporate chiral molecules for spin generation in an organic–inorganic hybrid spin-LED,³⁰ which combines the initiative from both communities.

This review provides a comprehensive overview of the state-of-art chiral- and spin-LEDs, including detailed discussion of the emerging CPL-active materials,^31–34 the underlying spin physics and mechanisms for CPL emission and device integration. This article further summarizes a critical evaluation of existing device architectures and highlights the challenge and strategies for improving the device performance. We conclude with a summary of application opportunities, and an outlook on future directions for materials and technologies enabling high-performance circularly polarized light emission.

2. Mechanisms for CPL emission

In molecular systems, the quantum mechanics description of the CPL emission is derived by Richardson et al.³⁵ The dissymmetry factor for a completely randomized and relaxed molecular ensembles can be quantitatively described using the following eqn:which is received from the interplay between the electric ([small mu, Greek, vector]) and magnetic (m⃑) transition dipole moments associated with a given electronic transition (i → j). Eqn (3) indicates that both magnetic and electric dipoles are required for the emission of CPL, which can only be achieved in low symmetry molecular structures, e.g. chiral chromophores, such that magnetic- and electric-dipole transitions are mixed. Organic semiconductors show great advantages in CPL materials due to their flexible structure design for chiral chromophores. Nevertheless, achieving a high g_lum while maintaining strong emission efficiency remains a significant challenge. This is fundamentally limited by eqn (3) as in most organic semiconductors electric dipole transitions are orders of magnitude stronger than magnetic dipole transitions. There are two main types of measurement setups for CPL. One is built by using quarter-wave plates, and another is using photoelastic modulators. When the material system or LEDs show a high degree of polarization or high g factor, the quarter-wave plate setup is suitable for measuring the CPL emission. However, if the emitters or LEDs show very weak degree of polarization or low g factors, the setup by using photoelastic modulators will be the best choice.

The chiral light–matter interaction can be greatly enhanced if one goes beyond the point-like molecule to larger structures with size comparable to the helical pitch of the CPL.³⁵ This includes well-aligned chiral polymers,³⁶ twisted-stacked liquid crystals³⁷ and photonic and plasmonic chiral metasurfaces.^15,38 In contrast to the randomized molecular ensembles, these structures additionally benefit from the circular birefringence effect,⁸ which converts the linearly polarized emission to CPL. The optical chiral effect is given by:³⁹

Eqn (4) is in a way similar to the molecular dissymmetry factor in eqn (3) but with the transition dipole replaced by the imaginary electric–magnetic dipole polarizability G′′ and electric polarizability α′′.

In the aforementioned mechanism, electron spin is not necessarily needed to generate CPL. The effective magnetic dipole is provided by orbital states as well as optical magnetic resonances. In semiconductors with strong spin orbital coupling (SOC), the electron spin is coupled to the orbital angular momentum such that the spin orientation determines the emission helicity.⁴⁰ Therefore, generation of spin polarized electron or hole spins by spin injection from ferromagnetic materials/electrodes, and excitation with CPL or via interaction with an external magnetic field can also lead to CPL emission. Such mechanism does not require chiral arrangement of materials or atoms. The emission helicity of the spin-LED can be conveniently controlled by manipulating the spin orientation with the magnetic field or field-free switching of the magnetic spin aligner.^20,41

Spin generation and hence the spin-LED can also be achieved in non-magnetic structures or devices by exploring the spin splitting with SOC. Structural asymmetry in conjunction with SOC leads to k-dependent spin splitting or spin accumulation in the presence of an electric field/current. Chiral-SOC, which is found in chiral small molecules or chromophores, give rise to the CISS effect, which has recently been used to demonstrate CPL in perovskite spin-LEDs.⁴¹

3. Emerging emitters for chiral-LEDs

Various molecular designing strategies have been explored to improve both emission intensity and g_lum. For instance, chiral emitters can be constructed by incorporating both chiral chromophores—which possess inherently asymmetric, conjugated π-systems that give rise to circularly polarized luminescence—and chiral non-chromophores, which lack extended conjugation but still impart overall molecular handedness.³¹ In practice, a chiral chromophore (for example, a helicene or a binaphthyl unit bearing an extended π-conjugation) is embedded within the emitter's backbone so that its asymmetric electronic transitions directly generate circularly polarized emission. Simultaneously, one can append or fuse chiral non-chromophoric moieties (such as chiral alkyl side chains, stereogenic centers in saturated linkers, or chiral auxiliaries) to the periphery of the molecule; although these non-chromophoric fragments do not themselves absorb or emit light in the visible region, they induce a defined three-dimensional twist or a helical environment. This combined strategy ensures that the electronically active chromophore operates within a rigid, enantio-enriched scaffold, which stabilizes a single handedness and enhances dissymmetry in the emitted light.

3.1. Small molecules

Fluorescent small molecules always present a low g_EL of around 10⁻³ due to the restriction of low m⃑; moreover, the EQEs of these LEDs are limited because the contributed excitons are from the singlet.³¹ Although the low brightness and electroluminescence efficiency is obtained from OLEDs with singlet emission, the long-term stability is the key feature for OLEDs with singlet emission, which is also crucial for the development of commercial CPOLEDs. Phosphorescence emitters based on heavy metals can overcome the triplet-exciton bottleneck via strong SOC, enabling efficient CPL through radiative triplet decay. Circularly polarized phosphorescent LEDs (CP-PhOLEDs) incorporating such chiral complexes have achieved EQEs near 20–30%, but the magnitude of g is often modest—generally in the order of 10⁻³—due to limitations in the alignment and the relative strength of μ and m.^18,42–48 Norel et al. reported the first emitter materials of helicene derivatives with a transition metal incorporated into their ortho-annulated π-conjugated backbones without the demonstration of LEDs.⁴⁷ Fuchter et al. demonstrated a CP-PHOLED that achieves both high brightness of 200 cd m⁻² and a high g_EL factor of 0.38 using platinahelicene molecules.⁴⁹ Yan et al. reported a platinahelicene enantiomer, CP-PhOLED, with a high EQE of 18.8% and a g_EL of 5.1 × 10⁻³.⁴³ Qian et al. demonstrated a strategy by making phosphorescent and liquid-crystalline cyclometalated platinum complexes to enhance the g_EL of up to 0.02.⁴⁵ Additionally, recent research has explored more sustainable alternatives using earth-abundant metals like copper, zinc, and chromium.^50–54 Chen et al. reported CP-EL based on a salen-Zn(II) complex chromophore with a g_EL of 0.044.⁵³ Jiménez et al. reported the chiral Cr(III) based complexes with a g_PL of up to 0.2.⁵⁰ Encouraging results have been achieved in terms of device efficiency and optical dissymmetry.

Thermally activated delayed fluorescence (TADF) offers a promising route for maximizing exciton utilization by converting non-radiative triplet states into emissive singlets through reverse intersystem crossing. TADF emitters can, in principle, achieve 100% IQE without relying on heavy-metal centers. By introducing chirality either within the TADF core or via peripheral groups, several chiral TADF systems have been developed, including donor–acceptor and multiple-resonance types (Fig. 2). Zheng's group has made excellent progress in efficient CPLEDs based on chiral TADF by the rational design of the chirality structure in terms of point, axial and planar chirality. For point chirality TADF molecules, they reported a pair of spiro-type blue chiral TADF enantiomers (R/S-1,2,3,4), which showed symmetrical CPL spectra with the highest g_PL factor of −1.6 × 10⁻³. Moreover, CP-OLEDs exhibited an EQE of 20.0% with a g_EL factor of around 3 × 10⁻³. By replacing the electron-withdrawing units and prolonging the length of electron-deficient units, the CPL signals could be amplified while exhibiting good device performances.^55,56 For axial chirality, Zheng et al. reported the bidibenzo[b,d]furan and bidibenzo[b,d]thiophene units (R/S-5), and two pairs of axial MR-TADF enantiomers showed a g_PL of 1.8 × 10⁻³. Correspondingly, CP-OLEDs exhibited a g_EL of 1.6 × 10⁻³ and an EQE of 35.7%.⁵⁷ For the planar chirality, they reported three MR-TADF materials by the face-to-face arrangement of indolo[3,2,1-jk]carbazole and MR-TADF fluorophores sterically on the naphthalene bridge. Because of the asymmetric and steric hindrance structures, both m-6 and m-7 were separated into planar enantiomers, exhibiting g_PL factors of 1.1 × 10⁻³ and 2.3 × 10⁻³, respectively. CP-OLEDs displayed g_EL factors of 1.88 × 10⁻³ and 1.89 × 10⁻³, respectively.⁵⁸ Further exploration is needed to understand how molecular design can better align the mechanisms of TADF and circular polarization to achieve both high efficiency and strong dissymmetry.

Fig. 2 The reported luminescent molecules with different types of chiralities used as emitters in CPOLEDs. (a) Point chirality. Reproduced with permission from ref. 56. Copyright 2021, Wiley-VCH. (b) Axial chirality. Reproduced with permission from ref. 57. Copyright 2024, Wiley-VCH. (c) Planar chirality. Reproduced with permission from ref. 58. Copyright 2025, Wiley-VCH.

Radical luminescence emitters show excellent optoelectronic properties with near unit IQE due to efficient doublet emission. Chiral radical emitters have shown dissymmetry factors of 0.5–0.8 × 10⁻³.^59–61 Burrezo et al. reported organic free radicals as intrinsic circularly polarized luminescence emitters with a g_PL of up to 0.8 × 10⁻³.⁶¹ Wang et al. reported that the radical anion exhibited an inversed CPL signal with a significantly enhanced g_PL of 0.1, which provide a way to control the emission helicity in one LED device.⁵⁹Only a limited number of chiral radical systems have been reported, and several key performance aspects—such as absorption efficiency and g—require significant improvement to meet the demands of advanced optoelectronic applications.

Chiral lanthanide complexes are among the most effective CPL emitters, utilizing magnetic-dipole (m-allowed) and electric-dipole (μ-forbidden) f–f transitions to achieve exceptionally high dissymmetry.^62–68 These complexes generate emission through an energy transfer process from chiral organic ligands to the lanthanide ion, in which both the singlet and triplet excitons transfer from the ligand to the metal center, potentially achieving an internal quantum efficiency of up to 100%. Di Bari et al. reported the high performance CPLED constructed with the Eu(III) organic complex with a g_EL of up to 1.4.⁶⁹Most chiral lanthanide emitters have focused on europium complexes, which are predominantly emitted in the red region of the spectrum. However, extending these systems to cover a broader spectral range remains challenging. Additionally, the long excited-state lifetimes typical of lanthanide emission (in the order of 0.1 to 1 ms) can reduce the overall device efficiency due to increased susceptibility to non-radiative quenching.

3.2. Chiral-assembled polymers and supramolecular systems

Apart from small molecules, polymers have been developed for CPL active systems. First, the CPOLED was constructed using a polymer with a chiral side chain, poly{[2,5-bis[(S)-2-methylbutoxy]phenylene]vinylene} (BMB-PPV).²⁹ The pronounced optical activity in the π–π* transition of BMB-PPV arises from the regioselective placement of enantiomerically pure side groups along the backbone, which induces the formation of small chiral aggregates. Although the CPOLEDs based on polymer systems exhibit relatively low device performance such as EQE, they present high dissymmetry g in the LEDs owing to the enhanced birefringence in the cholesteric order of polymers (Fig. 3). Nuzzo et al. reported a chiral and enantiomerically pure poly(fluorene-alt-benzothiadiazole) (S-8) polymer.³⁶ The nonlocal influence of cholesteric ordering in S-8 produced highly a g_EL of up to 0.8 by adjusting the emitting layer thickness. Upon thermal annealing into the liquid crystalline regime, the polymer self-assembles into a cholesteric phase; the resulting films exhibit a multidomain cholesteric order with domain sizes in the order of a few hundred nanometers. The strong g_EL arises from circular and selective scattering and birefringence effects within these multidomain structures.

Fig. 3 (a) Polymer emitter with a chiral side chain. (b) Illustration of the polycrystalline cholesteric film in the device stack, and the cholesteric domains are represented by groups of black lines, with z being the direction of the cholesteric helical axis. The direction of detection of EL with respect to the substrate is also shown. (c) Calculated g_EL. Reproduced with permission from ref. 36. Copyright 2017, American Chemical Society. (d) Molecular structures of 1-aza [6] helicene right- (P-9) and left- (M-9) handed enantiomers. (e) CP-PL spectra of F8BT doped with P/M-9. (f) CP-PL spectra of F8BT doped with 7% (by weight) P-9 (filled symbols) and 6% (by weight) M-9 (open symbols). Reproduced with permission from ref. 8. Copyright 2013, Wiley-VCH. (g) Molecule structure of R-10. (h) Schematic diagrams of the twisted stacking of rigid rods describing F8BT in a sublayer for the CPL. Reproduced with permission from ref. 70. Copyright 2017, Wiley-VCH.

Instead of adding chiral side chains to polymers, an effective way is to use chiral molecules as inducers to dope in the achiral polymers, forming lend systems with luminescence and chiral properties. In this part, the chiral molecules are not emissive, which is distinct from the above-mentioned section. In 2013, Fuchter's group reported that a chiral molecule (1-Aza[6]helicene, P/M-9) is used as an dopant to induce CPEL from a polymer LED (poly[9,9-dioctylfluorene-co-benzothiadiazole] (F8BT)) (Fig. 3).⁸ The blends show an increasing g_EL of up to 0.5 with the adding amount of the dopant ratio of up to 53% (by weight). More importantly, the overall PL spectra of the blends are well presented as the emission spectra of neat F8BT, in which the emission from chiral helicene is not presented. To further enhance the CPEL from the blend polymer LEDs, Kim et al. reported that high CPEL with a g_EL of up to 1.13 was generated from the helical stacking of an achiral conjugated polymer (F8BT) induced by a non-emissive chiral dopant (R-10) with high helical twisting power.⁷⁰ Theoretical analysis using Stokes parameters shows that the polymer's twisting angle, its birefringence, and the degree of linear polarization in the emitted light each contribute roughly equally to both the CPEL and the CPPL. This work highlighted the design rule for polymer blends in which the chiral small molecule inducer tunes that the twisted angle of polymer chains can efficiently enhance CPEL.^11,19,71

A host–guest system by co-evaporating chiral molecules and matrix also shows advantages compared to that in polymer blends with a solution process.^72–75 This host–guest system exhibits high performance of LEDs such as brightness and EQE, which is much higher than that in polymer blend CPLEDs. A recent study by Friend's group has reported the CPEL with green emission with a 10% dissymmetry factor in a triazatruxene (TAT)-based supramolecular assembly, in which TAT molecules stack into six-unit helices.⁷⁶ By co-sublimating TAT as a “guest” within a structurally mismatched “host,” the thin films showed helical formation occurring in situ: thermal annealing drives nanoscale phase segregation of the dopant and host while maintaining film uniformity. CPLED devices fabricated from these films achieve an EQE of up to 16%. These supramolecular systems demonstrate the ability to combine a high electroluminescence dissymmetry factor with excellent LED performance.

3.3. Chiral organic–inorganic frameworks

In another CISS-based spin-LED design, the chiral and emissive functions are combined in a single layer, with a chiral perovskite serving as both the spin-filtering and light-emitting materials. Chiral perovskite semiconductors are emerging materials for CPL emission due to their structural tunability, solution processability, and intrinsic chiroptical activity.^77,78 These materials are usually composed of chiral organic cations and lead halide frameworks. The CPL from chiral perovskites originates primarily from spin-polarized exciton recombination in the distorted lattice environment. This process does not require external magnetic fields or circularly polarized excitation, making these materials particularly attractive for compact, integrated CPL sources.⁷⁹ Xiao et al. reported a chiral perovskite nanocrystal with chiral ligands.⁸⁰ The fabricated LEDs presented a g_EL of around 6 × 10⁻³ in which the spin polarized carriers were filtered by the chiral ligands. Most importantly, the overall chiral nanocrystals give high photoluminescence quantum efficiency, resulting in a considerable EQE of 3.6%. Chiral ligand optimization and device optimization have boosted the g_EL up to 0.285 and EQE up to 16.8%.⁸¹

4. Emerging spin-LEDs

Spin generation, manipulation and relaxation are critical processes that determine the efficiency and functionality of spin-LEDs. Significant advancements have been achieved through the introduction of a novel device structure, materials for both the spin aligner and emitters and spin-dependent processes, which together contribute to the substantial improvement of the device performance that may lead to room-temperature applications.

4.1. Electric spin generation in spin-LEDs

In conventional spin-LEDs, electron or hole spins are injected from magnetic materials or electrodes, which then give rise to CPL through spin-dependent recombination.^{20,22,23,41,82–89} Typical emitters include III-nitride (e.g. GaN), III–V semiconductors (e.g. GaAs, InGaAs) and emerging two-dimensional materials (e.g. transition metal dichalcogenide (TMD)). Low-dimensional materials such as quantum wells and quantum dots are preferred due to the suppressed spin relaxation and concentrated oscillator strength in the electron-confined system.⁴⁰ Furthermore, quantum confinement and strain effect in these nanostructures may result in splitting of heavy-hole and light-hole states, for instance in III–V materials, leading to an efficient conversion from spin polarization to CPL.

Dilute magnetic semiconductors (DMS) or ferromagnetic metals (FM) are commonly employed as the spin aligner to generate electrical spin injection (Fig. 4). The use of DMS for spin injection to the semiconductor is driven by the need to overcome the resistance mismatch between metal electrodes and semiconductors.⁴⁰P_cir values of > 40% and 16.8% have been achieved by spin injection from GaMnAs to GaAs and WS₂ at cryogenic temperature.⁸⁷ However, the magnetic doping is usually considered inhomogeneous, leading to formation of FM clusters and phase segregation, which may lead to severe scattering that not only compromises the spin injection efficiency but also leads to poor luminescence properties.^92,93 Moreover, to date, the highest Curie temperature reported in GaMnAs has been 200 K, which limits the room-temperature operation of the DMS-based spin-LED.⁹⁴ Replacing DMS with the intrinsic 2D magnetic semiconductor, e.g. monolayer CrX₃ (X = Cl, Br, and I), may help to improve the material quality and further enhance the spin-LED performance.^94–96 In Van der Waals (VdWs) heterostructures combining the 2D magnetic semiconductor, e.g. CrI₃, CrBr₃, and CrSBr, and the 2D semiconductor, e.g. TMDC, the magnetic proximity effect leads to enhancing of spin/valley Zeeman splitting in the semiconductor emitter and boost the emission helicity.^97–101 PL emission polarization reaches 60% in CrI₃/WSe₂ heterojunctions at low temperature.¹⁰² Using CrI₃ as the spin aligner, Dang et al. achieved a 10%–20% EL P_cir in a spin-LED based on the CrI₃/hBN/WSe₂ heterostructure. Nevertheless, the Curie temperatures of the available magnetic semiconductors are still far from room temperature.

Fig. 4 Electric spin generation and injection in spin-LEDs. (a) Schematic illustration of the spin injection from FM to a semiconductor (SC) via a tunneling barrier (TB). (b) Schematic illustration of the spin accumulation with chiral SOC. Illustration of the spin injection with DMS (c), 2D FM materials (d) and topological magnetic structures (e) in different spin-LEDs. (c) Reproduced with permission from ref. 87. Copyright 2016, Nature Publishing Group. (d) Reproduced with permission from ref. 90. Copyright 2022, Nature Publishing Group. (e) Reproduced with permission from ref. 88. Copyright 2023, Nature Publishing Group. (f) and (g) Illustration of the spin-LED with spin generation via the CISS mechanism. (f) Reproduced with permission from ref. 30. Copyright 2021, American Association for the Advancement of Science. (g) Reproduced with permission from ref. 91. Copyright 2024, Nature Publishing Group.

For spin injection with FM, perpendicular magnetic anisotropy (PMA), e.g. CoPt and CoFeB electrodes, is usually required to generate out-of-plane orientated spins.^85,103 Additionally, a tunneling structure for spin injection is mandatory to deal with the resistance mismatch. To avoid spin loss in the interface of a spin injector and an emitter is crucial for high performance spin-LEDs. By implementing a sharp tunneling injection interface in a CoFeB/MgO/GaAs structure and the spin filtering effect with the MgO tunneling barrier, Dainone et al. recently achieved high circularly polarized EL with P_cir up to 30% at room temperature.¹⁰⁴ They estimate that the spin injection efficient is close to 100% and the EL P_cir is only limited by the spin polarization in CoFeB (about 65%) and spin relaxation inside the semiconductor. Meanwhile, a VdWs heterostructure provides an atomic sharp interface for spin injection. Using a gate-tunable room-temperature FM Fe₃GeTe₂ as the spin injector, Li et al. demonstrated the hole spin injection and chiral emission in a VdWs spin-LED. The EL P_cir reaches 8% in the absence of the magnetic field and can be controlled through the gate induced spin density of state change.^90,105 Besides conventional magnetic materials, materials with topological spin texture,¹⁰⁶e.g. merons, antimerons, and skyrmions, can also contribute to spin injection and CPL in a spin-LED. This is shown recently by Wu et al. that a large scale meron lattice created by preparing the FM spin injector under a high magnetic field enhances the GaN spin-LED EL polarization from 0.9% to 22.5%.⁸⁸

Besides magnetic materials, a nonmagnetic chiral material can create imbalanced spin population through the so-called chiral induced spin selectivity (CISS). When the electrons propagate in a chiral medium such as molecules, the electron spin will be filtered by the chiral molecules. Spin polarization depends on the chirality of the molecules. To confirm the CISS effect in chiral molecules, a general approach is to use electrical measurements to validate the current differences such as conductive atomic force microscopy¹⁰⁷ and sandwich diode devices.³⁰ The first demonstration of the spin-LED using chiral molecules as the spin filter was reported by Beard's group in a solution-processed device.³⁰ The CISS layer consisted of oriented, self-assembled chiral organic–inorganic metal-halide hybrid perovskites, while the emission layer is made from CsPbI₃ nanocrystals. The spin-LED achieved a modest P_cir of ±2.6% EL at room temperature. Following a similar strategy, Yao et al. reported an EL P_cir of 3.9% and an EQE of 13.5% in a spin-LED made with chiral quasi-2D perovskites. They identified the rapid spin transfer from 2D chiral perovskites to 3D perovskites and attribute the weak P_cir to the indirect and inefficient spin injection process.¹⁰⁸ Recently, by replacing the perovskite emitting layer with the epitaxy-grown high-quality AlGaInP quantum well that has a longer spin relaxation time, Hautzinger et al. managed to improve the P_cir to 15% in a hybrid chiral perovskite/III–V spin LED.⁹¹

4.2. Controlling the emission helicity of a spin-LED

The unique benefit of a spin-LED is that the emission helicity can be potentially controlled with the orientation of the spin. This is commonly addressed with an external magnetic field, which aligns the magnetization and hence the injected carrier spins in the ferromagnetic or paramagnetic spin injector.⁴¹ However, for practical applications that requires high density integration and independent control of the device, an electrical approach is favored. The technique of spin-orbital torque (SOT) is an ultrafast and energy-efficient method for switching the magnetization in FM with the electric current, which is well-established in the new generation of magnetoresistive random access memory devices.^109–111 Dainone et al. demonstrated a pioneering work that achieved magnetic-field-free switching of the emission helicity of a spin-LED using SOT (Fig. 5).²⁰ However, due to the use of the high symmetry structure, only 52% of total magnetization is flipped (corresponding to P_cir switching between −12% and 12%) in magnetic-field-free schema as compared to the field-assisted case, indicating that there is still room to improve. Electric control of the CPL emission can also be realized with the anti-ferromagnetic (AFM) spin aligner by exploring the gating effect. This is shown in ref. 86 where repeatable switching of EL P_cir between 7% and 21% is demonstrated by tuning the bilayer CrI₃ to the vicinity of the AFM-FM transition. In both approaches, although the magnetization switching as well as the emission helicity change is expected to be fast, the experimental assessment of the corresponding characteristic time still awaits to be demonstrated.

Fig. 5 Controlling the EL emission helicity with the electric approach. (a)–(c) First demonstration of SOT switching of the CPL emission helicity in III–V spin-LEDs. (d) Demonstration of the magnetic-field-free switching of the CPL emission helicity. Reproduced with permission from ref. 20. Copyright 2024, Nature Publishing Group. (e) Demonstration of the gate induced switching of the VdWs spin-LED. (f) and (g) Gating induced CPL emission polarization change at the vicinity of the magnetic phase transition for the device with bilayer and trilayer CrI₃. Reproduced with permission from ref. 86. Copyright 2024, Nature Publishing Group.

4.3. High emission helicity with spin relaxation

Spin relaxation is detrimental for efficient CPL emission in a spin-LED. Assuming one-to-one correspondence between spin and emission helicity, the emission circular polarization can be estimated using the following eqn:where P₀ is the injected spin polarization and τ and τ_s are the carrier lifetime and spin relaxation time. When spin relaxation dominates (τ_s ≲ τ), it gives rise to an additional decreasing factor of F = (1 + τ/τ_s)⁻¹ to P_cir. In the state-of-art spin-LED reported in ref. 20, F is estimated to be 0.53 at room temperature, contributing to almost half of the polarization loss of the device. The problem can be mitigated with spin filtering. By filtering out the minority spins, the spin filtering effect can potentially enhance P_cir to value even higher than P₀.

Spin filtering has been explored for the optical pumped spin laser in GaN/Fe₃O₄ nanostructures¹¹² and strong chiral emission in VdW heterostructures (Fig. 6).^101,113 In an edge-emission-type spin-LED consisting of the Fe spin aligner and the AlGaAs/GaAs p–i–n diode, Munekata et al. reported an EL polarization of 95% with an applied current density above 100 A cm⁻² at room temperature.⁴¹ The authors explained the magnificent EL P_cir with the spin filtering effect due to spin-dependent reabsorption from the band tail states as well as circular birefringence. We note that P_cir exceeds the maximum electron spin polarization injected from a Fe electrode (44%),⁴¹ which underlines the powerful effect of spin filtering. Apart from magnetic structures, spin filtering can be implemented in non-magnetic semiconductor nanostructures as well through the spin filtering defect. Using an interstitial defect in III–V dilute nitride semiconductors, Chen's group demonstrated the efficient generation of room-temperature electron spin polarization and CPL emission in thin films, quantum wells and etched photonic structures.^114–116 Recently, by designing a remote defect spin filtering scheme, they managed to improve the InGaAs quantum dot (QD) emission helicity from 2% to over 90% in a tunneling coupled GaNAs/QD nanostructure at room temperature.¹¹⁷ Using the similar structure as the active layer in a spin-LED, Etou et al. demonstrated over 2 times enhancement of the EL emission polarization.¹¹⁸

Fig. 6 Improving the CPL emission with spin filtering. (a) Schematic illustration of the spin filtering process. The minority spins are filtered while the majority spins are blocked. The spin filter can be either FM or defects. Strong CPL emission is achieved in semiconductor nanostructures at low temperature with FM (b) or at room temperature with defect spin filtering (c). (b) Reproduced with permission from ref. 117. Copyright 2021, Nature Publishing Group. (c) Reproduced with permission from ref. 117. Copyright 2021, Nature Publishing Group. A III–V edge-emission spin-LED (d) with spin filtering from the band tail state and circular birefringence (e) achieved strong CPL emission at room temperature under high current injection (f). Reproduced with permission from ref. 41 Copyright 2017, National Academy of Sciences of the United States of America.

5. Outlook

Circularly polarized light holds transformative potential across a broad range of applications. In display technologies, CPL enables glass-free 3D imaging and contrast enhancement. In biosensing and chiral discrimination, CPL-active emitters can enhance the sensitivity and selectivity of optical probes. Optical data storage and spintronic logic systems may also benefit from CPL-based components due to their inherent polarization-dependent logic states. Moreover, quantum communication protocols increasingly depend on the helicity of emitted photons, making CPL emitters crucial for secure information transfer and spin–photon interfaces.

Despite growing material diversity, CPL-active emitters still face key challenges. Many organic chiral luminophores suffer from a trade-off between emission efficiency and dissymmetry factor. Fluorescent small molecules typically offer high brightness but low g values, while supramolecular assemblies and chiral radicals yield stronger CPL signals but reduced quantum yields. Material optimization strategies include molecular design to align electric and magnetic transition dipole moments, chiral ligand engineering in metal complexes or perovskites, aggregation-induced CPL (AICPL), and chirality transfer in layered or host–guest architectures. Expanding the library of blue- and NIR-emitting CPL materials remains especially important for full-color displays and telecommunications.

From a device perspective, major bottlenecks include low external quantum efficiencies (EQEs) in systems with high g values, limited carrier balance, and interfacial degradation, especially in blue emitters. Additional issues involve insufficient brightness, limited operational stability, and challenges in scalability for commercial production. To address these, charge transport layers must be engineered for balanced injection and improved interfacial robustness. Chiral layer alignment and film morphology control are critical for maximizing polarization output. Device architectures like multi-layer stacks or spin-injection interfaces can be explored to enhance both optical and electronic performance. Scalable fabrication of CP-LEDs and spin-LEDs via solution processing remains an area of active development.

Conflicts of interest

There are no conflicts to declare.

Data availability

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

Acknowledgements

T. L. acknowledges financial support from Isaac Newton Trust by Trinity College University of Cambridge. T. L. is a Leverhulme Trust early career fellow (ECF-2024-217). Y. H. acknowledges financial support from the National Natural Science Foundation of China (No. 12374077) and the CAS Project for Young Scientists in Basic Research (No. YSBR-120).

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