White light-emitting diodes (WLEDs) are important solid-state light sources for lighting, display, and emerging flexible optoelectronic technologies because they can provide high efficiency, tunable spectra, and good color quality^[1-4]. Among different material and device strategies, emissive films that generate broadband white electroluminescence within a single deposited layer are particularly attractive because they can simplify device structures, reduce interfacial complexity, and improve spectral stability^[5-8]. Despite substantial progress in perovskites^[9,10], organic emitters^[11,12], metal complexes^[13], and colloidal quantum dots^[14], it remains challenging to combine broad visible emission, efficient electrical excitation, and flexible device performance in one material system.

Lanthanide-based materials (e.g., Ce³⁺ and Eu²⁺) are attractive candidates for white electroluminescence due to their parity-allowed 5d–4f transitions, broadly tunable emission across the visible spectrum, near-unity exciton utilization efficiency, and nanosecond-scale decay lifetimes^[15-17]. However, the direct implementation of lanthanide ions in electrically driven LEDs faces several fundamental limitations. On the one hand, the emission of individual lanthanide centers is often insufficiently broad for balanced white-light generation. On the other hand, direct electrical excitation remains difficult because the inner 4f orbitals are shielded by the outer 5s/5p electrons^[18,19], which limits carrier injection and energy utilization efficiency. These issues make it difficult to simultaneously achieve broad spectra and efficient electroluminescence in Lanthanide-based systems^[20].

To address these limitations, it is desirable to construct co-deposited inorganic–organic films in which the lanthanide-based component provides the luminescent center, while the organic component helps regulate the local coordination environment and excitation process^[21,22]. In this context, vacuum thermal evaporation is particularly suitable because it offers solvent-free processing, precise thickness control, and excellent reproducibility^[23]. More importantly, multi-source co-evaporation enables accurate regulation of inorganic and organic components in the same film, which is valuable for building compositionally tunable lanthanide-based halide-organic composites and integrating them into light-emitting devices^[24-27]. These features make vapor-deposited emissive films promising for flexible optoelectronic applications.

In this work, we develop vapor-deposited cerium-based halide-organic composites for warm-white electroluminescence. By combining the Cs₃CeI₆ with 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) in a co-evaporated film, dual-band visible emission is achieved, including a blue band centered at 405 nm and a yellow band centered at 560 nm. Spectroscopic results suggest that the blue emission arises from the combined contributions of Cs₃CeI₆ and CBP fluorescence, whereas the yellow emission is associated with Ce³⁺-centered 5d–4f emission under a modified local environment induced by interfacial interaction with CBP. Based on these films, warm-WLEDs with Commission Internationale de l’Eclairage coordinates of (0.38, 0.39), a peak external quantum efficiency of 2.1%, and a maximum luminance of 2,678 cd/m² are realized. In addition, the flexible electroluminescent device demonstration highlights the potential of these vapor-deposited cerium-based halide-organic composites for flexible light-emitting applications^[28].

To evaluate the luminescent properties of vapor-deposited cerium-based halide-organic composites, we first carried out steady-state photoluminescence (PL), PLQY, and TRPL measurements. A host-screening study was initially performed to identify suitable organic components for co-evaporated Cs₃CeI₆ films. Six conventional organic charge-transport materials were selected, including hole-transport and electron-transport molecules with representative diphenylamine, carbazole, pyrazole, and benzimidazole motifs. These host candidates were co-evaporated with CsI and CeI₃ to prepare a series of Cs₃CeI₆@host films using a three-source co-evaporation system [Figure 1A]. During this screening process, a high-throughput co-evaporation strategy was adopted to rapidly optimize the host-to-inorganic composition for each candidate. The deposition rates were controlled at 0.04 ± 0.002 Å/s for CsI, 0.02 ± 0.002 Å/s for CeI₃, and 1.00 ± 0.200 Å/s for the organic host during the initial comparison. Under 350 nm excitation, films containing carbazole-based hosts showed substantially stronger emission than the other systems [Supplementary Figure 1], indicating that carbazole derivatives are more favorable for constructing efficient emissive films in this material platform.

Among the carbazole derivatives, CBP, mCBP, and TCTA were selected for further comparison. Their molecular structures are shown in Supplementary Figure 1. As presented in Figure 1B, the corresponding Cs₃CeI₆@CBP, Cs₃CeI₆@mCBP, and Cs₃CeI₆@TCTA films all exhibit broad visible emission with two distinct peaks, consistent with white-light emission. Among them, the Cs₃CeI₆@CBP film shows the shortest PL lifetimes at both 405 and 560 nm under 350 nm excitation [Figure 1C and D]. This behavior indicates faster excited-state decay dynamics in the CBP-based film. The higher PLQY of Cs₃CeI₆@CBP further supports this conclusion [Supplementary Figure 2 and Supplementary Table 1]. Considering its favorable spectral profile, higher PL efficiency, and suitable electronic properties, CBP was selected as the optimal host for subsequent investigation. We therefore focused on the Cs₃CeI₆@CBP film to examine its emission behavior in detail.

We then investigated the optical properties of the Cs₃CeI₆@CBP film in detail. As shown in Figure 2A, the film exhibits two emission bands centered at 405 and 560 nm. The overall PL profile remains nearly unchanged under different excitation wavelengths, indicating that the two bands are generated within the same emissive system. Consistently, the photoluminescence excitation (PLE) spectra monitored at 390, 400, and 410 nm, as well as at 550, 560, and 570 nm, show very similar shapes, suggesting that the blue and yellow emissions are associated with closely coupled excitation pathways.

TRPL measurements further reveal distinct decay dynamics for the two bands [Figure 2B]. The blue emission at 405 nm exhibits a short lifetime of 7 ns, whereas the yellow emission at 560 nm shows a longer lifetime of 42 ns. This clear difference indicates that the two bands originate from different radiative processes. In addition, both lifetimes are much shorter than those reported for some other single-component white-emissive systems, such as Cs₂Na_0.4Ag_0.6InCl₆ with microsecond-scale decay^[9], which indicates faster excited-state decay dynamics.

To clarify the origin of the dual-band emission, we measured the PL spectra of individually evaporated CBP and Cs₃CeI₆ films [Supplementary Figure 3] and performed peak-deconvolution analysis on the Cs₃CeI₆@CBP PL spectrum. The fitted peak positions are consistent with those experimentally observed [Figure 2C], revealing that both Cs₃CeI₆ and CBP contribute to the blue emission, and that the 405 nm peak arises from the superposition of these contributions. Biexponential fitting of the Cs₃CeI₆@CBP blue-band decay yields two lifetimes, τ₁ = 1.6 ns and τ₂ = 14.4 ns, which agree well with the lifetimes of pure CBP (2.2 ns) and pure Cs₃CeI₆ (14.6 ns), respectively [Figure 2D]. The shortened CBP lifetime in the hybrid film confirms the presence of resonant energy transfer from CBP to Cs₃CeI₆, with a calculated energy-transfer efficiency of 27.2%. according to

$$ \eta_{ET}=1- \tau_{DA}/ \tau _D $$

where τ_D and τ_DA represent the lifetimes of pristine CBP and the CBP component in the composite film, respectively.

To gain further insight into the emissive film, we next examined its structural and chemical characteristics. TEM shows that Cs₃CeI₆ is formed within the co-evaporated film, with an average size of about 15 nm and uniform distributions of Cs, Ce, and I [Supplementary Figure 4]. High-resolution TEM indicates a preferred orientation along the (2, -1, 2) plane. This result is consistent with the XRD pattern of the film, which agrees well with the simulated pattern derived from Cs₃CeI₆ CIF data [Figure 3A], confirming the formation of crystalline Cs₃CeI₆ nanodomains in the deposited film.

XPS was further employed to probe the chemical environment of Ce³⁺ in pristine Cs₃CeI₆ and Cs₃CeI₆@CBP films. The appearance of an N 1s signal in the survey spectrum of Cs₃CeI₆@CBP confirms successful incorporation of CBP [Figure 3B]. For the pristine Cs₃CeI₆ film, characteristic Ce 3d_5/2 and 3d_3/2 peaks are observed at 886.4 and 904.7 eV, respectively. In contrast, the Cs₃CeI₆@CBP film exhibits additional Ce 3d_5/2 and 3d_3/2 components at 882.4 and 901.2 eV [Figure 3C], suggesting the formation of a modified Ce chemical environment associated with interfacial interaction with CBP. Simultaneously, the I 3d_5/2 peak shifts from 619.1 to 619.7 eV after CBP incorporation [Figure 3D], indicating that the local iodide environment is also affected by CBP incorporation. Together, these results support the presence of interfacial interaction between the inorganic Cs₃CeI₆ component and the organic CBP host.

We next evaluated the influence of CBP concentration on the emissive properties by varying the CBP deposition rate from 1 to 4 Å/s while keeping the CsI and CeI₃ rates fixed at 0.04 and 0.02 Å/s, respectively. As shown in Supplementary Figure 5, increasing the CBP deposition rate leads to a gradual decrease in the blue-emission intensity and a simultaneous increase in the yellow-emission contribution. When the CBP content is too high, intrinsic CBP fluorescence becomes more evident and exciton transfer becomes less complete. When the CBP content is too low, the relative fraction of Cs₃CeI₆ increases, which is unfavorable because it can aggravate concentration quenching. Based on these trends, the deposition rates of 0.04 Å/s for CsI, 0.02 Å/s for CeI₃, and 2 Å/s for CBP were identified as the optimal condition for achieving balanced dual-band emission.

Based on the favorable warm-white emission of the Cs₃CeI₆@CBP film, we fabricated electroluminescent devices using this film as the emissive layer. The device architecture is illustrated in Figure 4A, where TAPC serves as the hole-transport layer, CBP acts as an interfacial layer, and TPBi functions as the electron-transport layer. The initial device, D1, with the structure ITO/HAT-CN (3 nm)/TAPC (40 nm)/Cs₃CeI₆@CBP (30 nm)/TPBi (25 nm)/LiF (2 nm)/Al (100 nm), shows a relatively high turn-on voltage of 7 V [Figure 4B]. To improve charge injection and overall device performance, we further optimized both the transport structure and the emissive-layer composition.

By introducing MoO₃-doped TAPC and inserting a thin CBP transition layer, we obtained the optimized device D2 with the structure ITO/HAT-CN (3 nm)/TAPC:15% MoO₃ (40 nm)/CBP (5 nm)/Cs₃CeI₆@CBP (30 nm)/TPBi (25 nm)/LiF (2 nm)/Al (100 nm). This optimized device exhibits a reduced turn-on voltage of 4 V, a maximum luminance of 2,678 cd/m², and a peak EQE of 2.1% [Figure 4C]. The electroluminescence spectra remain nearly unchanged over the voltage range of 5 to 9 V and closely match the PL spectrum of the film [Figure 4D], indicating stable warm-white emission. The device shows warm-white emission with Commission internationale de l'éclairage (CIE) coordinates of (0.38, 0.39), corresponding to a correlated color temperature of approximately 4,062 K [Figure 4E]. Under constant-current operation at an initial luminance of 200 cd/m², the device shows a T₅₀ lifetime of 59.7 min [Figure 4F]. Devices with different emissive-layer ratios also exhibit differences in performance [Supplementary Table 2]. A comparison of device performance with previously reported WLEDs is summarized in Supplementary Table 3. The EQE and operational lifetime remain limited compared with practical requirements. These limitations are likely associated with the intrinsic difficulty of electrically exciting Ce³⁺ centers, since the localized 4f orbitals are shielded by outer 5s/5p orbitals, which can limit direct carrier injection and energy utilization. In addition, incomplete energy transfer from CBP to Cs₃CeI₆, nonradiative losses, and unbalanced electron–hole injection and transport arising from the present device architecture collectively degrade the overall device performance. To further illustrate the applicability of this material system in flexible optoelectronics, we constructed a flexible device concept on a polyethylene naphthalate (PEN) substrate based on the same functional layer configuration and Cs₃CeI₆@CBP emissive film [Figure 4G and H]. The corresponding flexible device exhibits bright warm-white electroluminescence under bending conditions [Figure 4I], serving as an initial proof-of-concept demonstration of the compatibility of the co-evaporated emissive layer with flexible substrates. The EL spectrum of the flexible device remains nearly unchanged after bending, suggesting good spectral stability under mechanical deformation [Supplementary Figure 6A]. Quantitative bending tests further show that the device retains over 94% of its initial luminance after 100 bending cycles at a bending radius of 6 mm, and still maintains approximately 85% after 200 cycles [Supplementary Figure 6B], with L₀ set to approximately 100 cd/m². These results, together with the solvent-free and thickness-controllable nature of vapor deposition, highlight the potential of these cerium-based halide-organic composites for future flexible light-emitting applications. By tuning the deposition-rate ratio of CsI, CeI₃, and CBP, the electroluminescence spectra and CIE coordinates can also be modulated, enabling color-adjustable devices [Supplementary Figures 7 and 8].

In summary, we have demonstrated vapor-deposited Cs₃CeI₆@CBP nanocrystal films for warm-white electroluminescence. The co-evaporated films exhibit dual-band visible emission, with a blue band centered at 405 nm and a yellow band centered at 560 nm. Spectroscopic analysis suggests that the blue emission arises from the combined contributions of the Cs₃CeI₆ component and CBP fluorescence, while the yellow emission is associated with Ce³⁺-centered 5d–4f emission under a modified local environment induced by interfacial interaction with CBP. Based on these films, warm-WLEDs were realized with Commission Internationale de l’Eclairage coordinates of (0.38, 0.39), a peak external quantum efficiency of 2.1%, a maximum luminance of 2,678 cd/m², and stable electroluminescence over a broad operating-voltage range. The relatively modest EQE is mainly attributed to incomplete energy transfer from CBP to Cs₃CeI₆, charge imbalance, and non-radiative recombination within the hybrid emissive layer. In addition, the flexible electroluminescent device demonstration further reveals the applicability of these nanocrystal films in flexible light-emitting devices. This work provides an effective vapor-deposition route for cerium-based halide-organic composites and offers useful guidance for the development of flexible warm-WLEDs.