[image] [image] Ressource protégée - Archive ouverte HAL Accéder directement au contenu

*[image] [image]*

* Documentation

• FR
Français (FR)

• Anglais (EN)
Se connecter

- <u>HAL science ouverte</u>

* Recherche
Loading...

Recherche avancée

Information de documents

Titres

○ <u>Titres</u>

○ <u>Sous-titre</u>

○ <u>Titre de l'ouvrage</u>

○ <u>Titre du volume (Série)</u>

○ <u>Champ de recherche par défaut (multicritères) + texte intégral des PDF</u>

○ Résumé

○ <u>Texte intégral indexé des documents PDF</u>

○ <u>Mots-clés</u>

○ <u>Type de document</u>

○ <u>Sous-type de document</u>

○ <u>Tous les identifiants du document</u>

○ <u>Identifiant HAL du dépôt</u>

○ <u>Langue du document (texte)</u>

○ Pays (Texte)

○ Ville

O À paraître (true ou false)

Ajouter

Auteur
High-performance rare-earth-free fluorophores:
toward white LEDs

Anthony Barros<sup>a</sup>, Nicolas Ledos<sup>b</sup>, Rodolphe Valleix<sup>a</sup>, Jérémy Cathalan<sup>a</sup>, Geneviève Chadeyron<sup>a</sup>,

Pierre-Antoine Bouit<sup>b</sup>, Muriel Hissler<sup>b</sup>, Damien Boyer<sup>a*</sup>

a. Université Clermont Auvergne, Clermont Auvergne INP, CNRS, ICCF, Clermont-
Ferrand, 63000, France.

b. Université de Rennes, CNRS, ISCR - UMR 6226, Rennes, 35000, France.

Keywords: organic fluorophores, photoluminescence, luminescent composites, WLEDs, silicone

ABSTRACT

A series of down-converting organic fluorophores were synthesized in order to cover the full range
of the visible spectrum, two π-extended phospholes with blue (B) and red (R) emissions and two
difluoroboron β-diketonates with green (G) and yellow (Y) emissions. Their excellent optical

<footnote>*Corresponding author : damien.boyer@sigma-clermont.fr</footnote>
## Auteur (multicritères)

○ Auteur (multicritères)

○ Auteur : Nom complet

○ Auteur : Nom de famille

○ Auteur : Prénom

○ Auteur : Complément de nom, deuxième prénom

○ Auteur : Organisme payeur

○ Auteur : IdHal (chaîne de caractères)

○ Auteur : Fonction

○ Auteur : personID (entier)

○ Auteur : Identifiant de l'organisme payeur

○ Auteur : Identifiant de la structure

○ Directeur de thèse

○ Éditeur

○ Éditeur scientifique

○ Éditeur de la série

## Ajouter

### Structure

### Structure (multicritères)

○ Structure (multicritères)

○ Structure : Acronyme

○ Structure : Nom

○ Structure : Code

○ Structure : Pays

○ Structure : Type

○ Structure : État dans le référentiel

○ Structure : Identifiant HAL de la structure

○ Structure/équipe de recherche : Pays

○ Structure/regroupement d'équipes : Pays

○ Structure/laboratoire : Pays

○ Structure/regroupement de laboratoires : Pays

○ Structure/institution : Pays

○ Structure/regroupement d'institutions : Pays

## Ajouter

## Autres
performances were evidenced by recording their emission spectra and measuring their
photoluminescent absolute quantum yields in both liquid and solid states. Subsequently, these
fluorophores were incorporated into silicone at 10 wt.% to prepare luminescent composite films.

Their photoluminescent properties were analyzed at room temperature and their CIE chromaticity coordinates were determined. By strategically combining these films emitting different visible lights (B, G, Y, R) with commercial blue or violet LED chips, we were able to produce white light exhibiting a high color rendering index (90) and color correlated temperature close to 4500 K.

Finally, to assess the durability of these composite films and to demonstrate the potential of these rare-earth-free fluorophores as down-converters phosphors for LED based devices, the stability of their optical performances was investigated upon thermal and photonic stresses.

# 1. INTRODUCTION

The emergence of white light-emitting diodes (WLEDs) has resulted in a tremendous technological shift of lighting and display applications because of their high efficiency, long lifetime and environmental friendliness compared to fluorescent lamps or incandescent bulbs.

Currently, most WLEDs consist of a blue-emitting InGaN light-emitting diode (LED) chip coated by a down-converter (DC) phosphor, $Y_3Al_5O_{12}:Ce^{3+}$ (YAG:Ce), which converts a part of the blue
Champ de recherche par défaut (multicritères)

○ <u>Champ de recherche par défaut (multicritères)</u>

○

○ <u>Revues (multicritères)</u>

○ <u>Revue : Éditeur</u>

○ <u>Revue : Titre abrégé</u>

○ <u>Revue : Titre</u>

○ <u>Revue : Identifiant interne</u>

○ <u>Revue : Date de début de publication</u>

○ <u>Revue : ISSN électronique</u>

○ <u>Revue : ISSN</u>

○ <u>Revue : Éditeur</u>

○ <u>Revue : Couleur dans SHERPA/RoMEO</u>

○ <u>Revue : État dans le référentiel</u>

○

○ <u>Colloque (multicritères)</u>

○ <u>Colloque : Titre</u>

○ <u>Colloque : Organisateur</u>

○ <u>Colloque : date de début (Année)</u>

○ <u>Colloque : date de fin (Année)</u>

○ <u>Conférence invitée (oui/non)</u>

○

○ <u>Projets ANR (multicritères)</u>

○ <u>Projet ANR : Acronyme</u>

○ <u>Projet ANR : Acronyme du programme</u>

○ <u>Projet ANR : Code décision (référence)</u>

○ <u>Projet ANR : Nom</u>

○ <u>Projet ANR : Identifiant interne</u>

○ <u>Projet ANR : État dans le référentiel</u>

○

○ <u>Projets européens (multicritères)</u>

○ <u>Projet européen : Acronyme</u>

○ <u>Projet européen : Identifiant de l'appel à projet</u>

○ <u>Projet européen : Référence</u>

○ <u>Projet européen : Nom</u>

○ <u>Projet européen : Date de fin</u>

○ <u>Projet européen : Financement</u>

○ <u>Projet européen : Date de début</u>

○ <u>Projet européen : État dans le référentiel</u>
light into yellow light. White light is then produced by the combination of the LED chip and phosphor emissions ¹, ². Even though YAG:Ce exhibits several advantages like a high quantum efficiency (QE), broad yellow emission spectrum and a high stability upon thermal or photonic stresses, the use of this single phosphor presents some drawbacks. Indeed, the obtained white light is characterized as cold (or cool) white light with a colour correlated temperature (CCT) higher than 4500 K and a poor colour rendering index (CRI) lower than 80 which does not fulfil the specifications for most lighting and display applications. Thus, to reach a higher CRI and a warmer white light, a red-emitting phosphor is commonly added to YAG:Ce ³. A few DC phosphors are able to convert blue emission of LED chip into red light like sulphide phosphors (SrS:Eu²⁺ or CaS:Eu²⁺), nitride phosphors (CaAlSiN₃:Eu²⁺, Sr₂Si₅N₈:Eu²⁺ or Ba₂Si₅N₈:Eu²⁺) or fluoride phosphors (K₂SiF₆:Mn⁴⁺ or KSF: Mn⁴⁺). But the development of a red-emitting phosphor with high quantum efficiency and stability upon an excitation wavelength range which does not overlap the YAG:Ce emission is still a major challenge.

As can be seen in their chemical formula, most DC phosphors for LED applications contain lanthanide ions and hence have full dependence on rare-earth elements (REEs) that are potentially facing a serious supply shortage, and their lack of recyclability is destructive to the environment
○ Projet européen : Identifiant interne

○

○ Date de production : année

○ Date de mise en ligne : année

○ Date de publication : année

○ Date d'écriture : année

○ Date de modification du dépôt : année

○ Date de dépôt : année

○ Date de publication électronique : année

○

○ Collection HAL (multicritères)

○ Collection HAL : catégorie

○ Collection HAL : Code

○ Collection HAL : Nom

○ Collection HAL : Identifiant interne

○

○ Identifiant interne du contributeur/déposant

○ Nom complet du contributeur/déposant

○

○ Domaines

○ Domaine primaire

○ Domaine racine

○ Sous-domaine niveau 1

○ Sous-domaine niveau 2

○ Sous-domaine niveau 3

○

○ Statut du document

○ Version du document

○ Type de dépôt

○ Type de document

○ ISBN

○ Numéro - référence

○ Identifiant DOI

○

○ Classification

○ Audience

○ Vulgarisation

○ Comité de lecture - texte (oui ou non)

○ Actes de colloque

○ Référence interne
⁴⁻⁷. Today, about 20% of global REEs are used in clean energy technologies. Among them, europium, terbium, and yttrium are essential ingredients of phosphors used in general lighting. Since the demand for REEs will continue to increase dramatically for various technologies, their prices have been rising constantly, making the cost issue even more crucial.

Thus, the development of rare-earth-free phosphors is highly important. In this context, the use of organic fluorophores appears as an appealing strategy. Hence, thanks to the power of organic synthesis, there exists a wide range of DC fluorophores with tuneable emission wavelengths, high photoluminescence quantum yields and satisfying chemical/thermal stability. Based on their successful insertion into various optoelectronic devices such as Organic Light-Emitting Diodes (OLEDs) or Lasers, we decided to focus on π-extended phospholes and difluoroboron β-diketonates for this study (Table 1) ⁸⁻¹⁰.

Regarding difluoroboron β-diketonate complexes, they display an easy molecular engineering, excellent solid-state luminescent as well as satisfying stability illustrated by their successful insertion OLEDs, solar cells or organic lasers ¹¹⁻¹³. For these reasons, 1-2 (Table 1) were therefore chosen to be combined with commercial LEDs.
○ Financement

○ Collaborations

Ajouter

Lancer la recherche

Recherche experte (SolR)

Recherche experte (SolR)

Lancer la recherche
Vers la recherche avancée

• Déposer

X

X

X

Loading...

X

Le document que vous demandez est en access restreint
jusqu'au 2024-11-03 !

Revenir a la notice

Retourner à l'accueil

[image]

[image]

[image]

[image]

Contact

• Support

Ressources

• Documentation

• FAQ

• API
Besides, the luminescence of π-extended phospholes has been widely exploited in the context of sensing, bioprobes and OLEDs ¹⁴⁻¹⁷.

Their intense luminescence, combined with an easy molecular engineering that allows fine-tuning color and an excellent thermal, chemical and photochemical stability make that π-extended phospholes such **3-4** (Table 1) are compounds of choice for being used in WLEDs as rare-earth-free phosphors ¹⁸.

<table><caption>**Table 1.** Structure and label of the different fluorophores</caption><thead><tr><th>Structures of fluorophores</th><th>R</th><th>Name in DCM solution</th><th>Name of the powders</th></tr></thead><tbody><tr><td rowspan="2">MeO-tert-butyl-4-(4-(tert-butylphenoxy)phenyl)-2,4,6-triisopropylbenzenesulfonate</td><td><sup>1</sup>Bu</td><td>1</td><td>P<sub>1</sub></td></tr><tr><td>H</td><td>2</td><td>P<sub>2</sub></td></tr><tr><td rowspan="2">Ph-tert-butyl-4-(4-(tert-butylphenoxy)phenyl)-2,4,6-triisopropylbenzenesulfonate</td><td>NPh(OMe)<sub>2</sub></td><td>3</td><td>P<sub>3</sub></td></tr><tr><td>H</td><td>4</td><td>P<sub>4</sub></td></tr></tbody></table>
• OAI-PMH

• AuréHAL

## Informations

• À propos

• Données personnelles

• Mentions légales

• Accessibilité

• Conformité RGAA

## Questions juridiques

• Je publie, quels sont mes droits ?

• Loi pour une République numérique

• Stratégie de non-cession des droits

• SHERPA/RoMEO

## Portails

• Portails institutionnels

• HAL

• HAL SHS

• HAL Thèses

• MédiHAL

## CCSD

• CCSD

• Episciences

• Sciencesconf
In this study, four rare-earth-free fluorophores were examined in solution, in solid state, and in a composite mixture. The optical characteristics of these materials were investigated using UV spectroscopy, PLQY<sub>abs</sub> (Absolute Photoluminescence Quantum Yield), emission spectra, and CIE chromaticity coordinates were also recorded. Luminescent composite films prepared from powders of these different fluorophores dispersed in silicone were combined with commercial blue and UV chips to produce white light with tunable color temperature. Finally, the stability under thermal and photonic stresses of these fluorophores in both powder and composite film forms was investigated.
## 2. EXPERIMENTAL

### 2.1. Fluorophores synthesis

1, 2 and 4 were synthesized according to described procedure <sup>19-21</sup>. To provide 3 in moderate yield, the silver mediated cyclization between 4'-(ethyne-1,2-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) and diphenylphosphine oxide was used (see SI-section 1: Experimental section). The structure of this latter was confirmed by 1H, 13C and 31P NMR characterizations (Fig. S1-S3). The thermal behaviour of these four fluorophores was investigated by TGA and DSC analyses (Fig. S4-S11).

### 2.2. Fabrication of silicone composite films

The two-component silicone polymer (BluesilTM RTV 141 A&B) was composed of a viscous liquid (part A, 90 wt.%), cured by polyaddition reaction with a catalyst (part B, 10 wt.%). Free-standing fluorophore/silicone composite films with 10 wt.% load of fluorophore were obtained as follows. First, the suitable amount of fluorophore was dispersed in part A of silicone with a mechanical mixer (Planetary Centrifugal Vacuum Mixer "Thinky Mixer") for 10 min at 1200 rpm. The mixture was further homogenized by passing twice through a three-roller mill (Exakt80E). Then, part B of silicone was added to the mixture and homogenized using the mechanical mixer
for 10 min at 1200 rpm. Finally, the fluorophore/silicone film (labelled Si-Fx (x =1-4) film (10 wt.%)) was casted on a Teflon surface called Elcometer 4340 automatic film applicator at 40°C (the knife blade height was 200 µm and the casting speed was 30 mm/s) then cured at 70°C for another 2 h. The film thickness was measured with an Elcometer 456 coating thickness gauge.

## 2.3. Characterization techniques

UV-Visible spectra in solution were recorded at room temperature on a Specord 205 UV/Vis/NIR spectrophotometer. The UV-Vis emission and excitation spectra measurements in solution were recorded on a FL 920 Edinburgh Instrument equipped with a Hamamatsu R5509-73 photomultiplier for the NIR domain (300-1700 nm) and corrected for the response of the photomultiplier. Quantum yield was calculated relative to Rhodamine 6G (Ethanol), Φref= 0.94. The photoluminescence quantum yield (PLQY), photoluminescence spectra (PL), and trichromatic coordinates (CIE x,y) of fluorophores in solid state were recorded with PLQY measurement software (U6039-05) using an integrating sphere measurement system (C9920-02G) from Hamamatsu photonics. The setup consisted of a 150 W monochromatic Xe lamp, an integrating sphere (Spectralon coating, ø = 3.3 in.) and a high-sensitivity CCD camera. The internal photoluminescence quantum yield (PLQYint) and the absorption coefficient (Abs) defined by
formulae (1) and (2) were obtained directly from the measurements made in the integrating sphere.

The absolute photoluminescence quantum yield (PLQY<sub>abs</sub>) was calculated from the product of PLQY<sub>int</sub> x Abs, and corresponds to the equation (3).

$$PLQY_{int} = \frac{\text{number of emitted photons}}{\text{number of absorbed photons}} \qquad (1)$$

$$Abs = \frac{\text{number of absorbed photons}}{\text{number of incident photons}} \qquad (2)$$

$$PLQY_{abs} = \frac{\text{number of emitted photons}}{\text{number of incident photons}} \qquad (3)$$

Photometric parameters were collected in an integrating sphere with a diode array rapid analyzer system (GL Optic integrating sphere GLS 500) at room temperature. Si-Fx films were placed on a UV or violet or blue LED emitting respectively at 385 nm, 405 nm, 450 nm and 465 nm. The testing condition is under a forward current of 800 mA related to an applied voltage of 3.5 V.

Reliability studies were carried out on powders (P<sub>1</sub>-P<sub>3</sub>) using a home-made set-up consisting of a power-controlled blue LED emitting at 465 nm as the excitation source and a HR4000 high resolution spectrometer (Ocean Optics) as the PL analyser. The samples were positioned on a heating element whose temperature was adjusted to the desired value (room temperature (RT) or 100°C). The emission spectra of the fluorophores were acquired every 30 min for 170 hours. Their area was integrated to obtain the total emission intensity. LED power was measured using a Coherent PowerMax USB PS19Q (19 mm aperture). The measurement was performed by centring
the head unit over the LED source and measuring the power of the LED light emitted through the
aperture. The blue LED power was 183 mW. The LED power density can be expressed in W/m2
and was calculated by LED power (in watt) per unit surface of the sample (0.25 cm2). The power
density of the blue LED was 7300 W/m2; for our experiments, a bandpass filter was used in order
to reduce this value to 3080 W/m2. The measurements of fluorophores were also carried out with
a UV LED at 375 nm as excitation source to compare their durability to the fourth one (P₄) which
cannot be excited suitably by the blue LED. The complete system (UV LED + collimating lenses
+ filters) irradiates the sample under a photon flux of 900 W/m2.

3. RESULTS AND DISCUSSIONS

**3.1. Optical characterization of fluorophores in solution in dichloromethane at 298K**

The spectroscopic properties of the four fluorophores **1-4** (**Table 2**) were investigated in diluted dichloromethane (DCM) solutions (c ≈ 1.10⁻⁶ mol.L⁻¹). As described by Songpan Xu et al ²², **1-2** display similar absorption properties with an intense band between 350 nm and 425 nm (λmax(2) = 398 nm; λmax(1) = 403 nm) (**Fig. 1a**) attributed to π-π* transition. Diphenylbenzophosphole oxide (sample 4) exhibits a band at 346 nm, which is attributed to π-π* transition centered on the
benzophosphole core <sup>23</sup>. Fluorophore 3 displays a different absorption spectrum compared to 4, with a band centered at 300 nm which is attributed to π-π* transition centered on the benzophosphole core and another much wider and less intense band in the visible range (λmax(3) = 422 nm) attributed to Internal Charge Transfer (ICT) from the arylamine donor to the benzophosphole oxide acceptor as it has been reported for similar compound <sup>9, 24</sup>. As can be seen in **Table 2**, chromophores 1 and 2 show intense fluorescence with maximum emission wavelengths at 431 nm and 430 nm respectively (**Fig. 1b**), with high PLQY<sub>int</sub>, close to 100%. 4 displays fluorescence with a maximum wavelength at 462 nm with a low PLQY<sub>int</sub> of less than 1%, in adequation with its “aggregation induced emission” character <sup>23</sup>. Due to the presence of electron-rich ICT-inducing moieties, 3 displays a red-shifted fluorescence with a maximum wavelength at 632 nm with a PLQY<sub>int</sub> of 17% <sup>25</sup>. Furthermore, we can point out that the emission profile of **3-4** is rather broad compared to **1-2**.
<figure><img src="image_1.png" /><figcaption>**Fig. 1.** (a) UV-vis absorption and (b) emission spectra of 1-4 in diluted DCM solution</figcaption></figure>

($\lambda_{exc} = \lambda_{abs max}$ reported in the **Table 1** for each fluorophore)

The excitation spectra of fluorophores 1-4 in diluted DCM solution are presented in **Fig. S12**.

<table><caption>**Table 2.** Spectroscopic data of 1-4 in diluted DCM ($c \approx 1.10^{-6} mol.L^{-1}$)</caption><thead><tr><th>Fluorophores</th><th>λ<sub>abs max</sub> (nm)</th><th>ε (L.mol<sup>-1</sup>.cm<sup>-1</sup>)<sup>a</sup></th><th>λ<sub>em max</sub> (nm)</th><th>PLQY<sub>abs</sub><sup>e</sup>(%)</th></tr></thead><tbody><tr><td>1<sup>b</sup></td><td>403</td><td>64000</td><td>430</td><td>99</td></tr><tr><td>2<sup>c</sup></td><td>398</td><td>55000</td><td>431</td><td>99</td></tr><tr><td>3</td><td>422</td><td>15000</td><td>632</td><td>23</td></tr><tr><td>4<sup>d</sup></td><td>346</td><td>5900</td><td>462</td><td>&lt;1%</td></tr></tbody></table>

<sup>a</sup> ε : molar absorption coefficient, <sup>b</sup> data from reference<sup>20</sup>, <sup>c</sup> data from reference<sup>22</sup>, <sup>d</sup> data from reference<sup>23</sup>, <sup>e</sup> Absolute Quantum yield measured on the same integration sphere

## 3.2. Optical characterization of fluorophores in solid state
In order to assess the ability of fluorophores 1-4 to be used as DC phosphors in LED devices, their absolute photoluminescent quantum yields (PLQY<sub>abs</sub>) were recorded in solid state (powder sample) from 250 nm to 550 nm (Fig. 2). All of them exhibit high PLQY<sub>abs</sub> between 30% and 70% in the UV/blue excitation range, except P<sub>4</sub> whose PLQY<sub>abs</sub> drops drastically after 420 nm, in agreement with its UV-vis absorption spectrum. Thus, regarding their PLQY<sub>abs</sub> values (Table 3), fluorophores P<sub>x</sub> (x=1-3) can be combined efficiently with commercial UVA/violet/blue LED chips whereas P<sub>4</sub> can only be excited by UVA/violet LED chips (365 nm, 385 nm or 405 nm). As expected, based on the literature reports, all those fluorophores display good luminescent quantum yields in the solid-state.
<figure><img src="image_2.png" /><figcaption>**Fig. 2.** PLQY<sub>abs</sub> of solid-state fluorophores P<sub>x</sub> (x = 1-4) recorded upon excitation from 250 to 550 nm</figcaption></figure>

<table><caption>**Table 3.** PLQY<sub>abs</sub> (%) of P<sub>x</sub> (x = 1-4) upon excitation at wavelengths of commercial LED chips</caption><thead><tr><th rowspan="2">λ<sub>exc</sub></th><th colspan="2">UVA LED</th><th>Violet LED</th><th colspan="2">Blue LED</th></tr><tr><th>365 nm</th><th>385 nm</th><th>405 nm</th><th>450 nm</th><th>465 nm</th></tr></thead><tbody><tr><td>P1</td><td>51</td><td>50</td><td>49</td><td>52</td><td>49</td></tr><tr><td>P2</td><td>39</td><td>38</td><td>39</td><td>42</td><td>42</td></tr><tr><td>P3</td><td>39</td><td>38</td><td>37</td><td>36</td><td>36</td></tr><tr><td>P4</td><td>60</td><td>58</td><td>58</td><td></td><td></td></tr></tbody></table>
Emission spectra were recorded at RT upon excitation at 405 nm for Pₓ (x=1-4) samples (Fig. 3). All those compounds display strong luminescence in powder with a gradual red-shift in the series P₄-P₁-P₂-P₃ (λem max(1) = 460 nm; λem max(2) = 500 nm; λem max(3) = 560 nm; λem max(4) = 616 nm). The solid-state luminescence of the boron difluoride complexes 1-2 markedly differ from the diluted solution, due to intermolecular interactions in the condense phase. This aspect has been previously investigated experimentally and theoretically ¹², ²⁰, ²⁶. On the contrary, for benzophospholes 3-4, there is no significant modification between the solution and the solid-state, as usually observed ⁸, ⁹, ²³. We should also point out that the emission spectra of the Pₓ (x=1-4) fluorophores do not change with the change in excitation wavelength, as shown in Fig. S13. The excitation spectra of fluorophores P₁-P₄ are displayed in Fig. S14.
<figure><img src="image_3.png" /><figcaption>**Fig. 3.** Emission spectra recorded at RT from solid state fluorophores in solid curve (P₁–P₄) and in diluted DCM solution in dashed curve (1-4) upon excitation at 405 nm</figcaption></figure>

We thus have in hand a series of fluorophores emitting respectively in the Blue, Green, Yellow and Red wavelength range as powders. The associated CIE chromatic coordinates (Fig. 4) highlight the bathochromic emission shift occurring when switching from powders P₄ to P₁, P₂ and P₃. As a result, by mixing these fluorophores in solid state (Pₓ (x=1-4)) in the right proportion, we should be able to produce white light with controlled photometric parameters.
<figure><img src="image_4.png" /><figcaption>**Fig. 4.** Chromaticity coordinates of Pₓ (x=1-4) fluorophores in solid state with the CIE chromaticity coordinates excited by a 465nm blue light for P₁, P₂ and P₃, and excited by a 405 nm UV light for P₄.</figcaption></figure>

Interestingly, the emission spectrum of compound P₂ recorded upon excitation at 465 nm (Fig. 5) overlaps very well with the emission spectrum of YAG:Ce (Ce³⁺ doped Y₃Al₅O₁₂) which is the reference yellow phosphor used in all white light emitting diodes (WLEDs).
<figure><img src="image_5.png" /></figure>

**Fig. 5.** Comparison of normalized emission spectra of P₂ and commercial YAG:Ce upon excitation at 465 nm

## 3.3. Optical characterization of luminescent composite films

### 3.3.1. Emission spectra and colorimetry

To further investigate the suitability of the selected fluorophore powders for LED applications, composite films were prepared by dispersing the powder in a silicone elastomer (PDMS-type) matrix (according to the protocol described in the experimental part) (Fig. 6). Four composite films, named Si-Fₓ (x = 1,2,3 or 4) (Si-F₁ for example refers to the films elaborated with 10wt.% of fluorophore P₁) were prepared. For each film, the measured thickness is of the order of 110 ± 5 µm.
<figure><img src="image_6.png" /><figcaption>**Fig. 6.** Pictures of compounds Si-Fx (x=1-4) composite films under (a) daylight and (b) UV light at 365nm</figcaption></figure>

The emission spectra of the Si-Fx (x=1-3) and Si-F4 composite films were recorded under excitation at 465 nm and 405 nm respectively (Fig. 7a solid lines) and were compared with those of powders (Fig. 7a dotted lines).

Analysis of Fig. 7a reveals a shift in the emission maximum towards shorter wavelengths for the four composite films compared with their powder counterparts. This behaviour has already been observed for composite films produced using the same procedure as that described in this paper²⁷ and can be explained by a disaggregation effect of the powders induced by the rolling step during film preparation. We can also point out that the differences between the emission spectra are more marked for blue and green powders and films. This suggests that fluorophores 4 and 1 are more
sensitive to roll milling-induced shear forces than fluorophores 2 and 3. As a result, the chromaticity diagram shows slightly different color points for red and yellow powders and films.

These differences are more pronounced for green and blue powders and films (Fig. 7b).
<figure><img src="image_7.png" /><figcaption>**Fig. 7.** a) Emission spectra of the **Si-Fx** (x=1-4), b) CIE chromaticity coordinates of powders and **Si-Fx** composite film upon 465 nm for powders P₁, P₂ and P₃ and the corresponding films and 405 nm for powder P₄ and **Si-F4**.</figcaption></figure>

### 3.3.2. PLQY<sub>abs</sub> of Si-Fx composite film

PLQY<sub>abs</sub> of the **Si-Fx** (x=1-4) composite films were recorded for excitation wavelengths from 250 to 550 nm (Fig. 8). The films exhibit PLQY<sub>abs</sub> comparable to those of powders, meaning that the dispersion in silicone did not modify photoluminescence behaviour. A more detailed analysis of the evolution of the PLQY<sub>abs</sub> even shows a slight increase in the performance of the films compared to the powders. This can be explained by the dispersion effect generated by the polymer
matrix, which leads to a reduction in non-radiative energy transfer and, consequently, to an
improved internal quantum yield, as illustrated in Figs. S15 and S16.
<figure><img src="image_8.png" /><figcaption>**Fig. 8.** PLQY<sub>abs</sub> of composite films Si-F<sub>x</sub> (x = 1-4) recorded upon excitation from 250 to 550 nm.</figcaption></figure>

### 3.3.3. White light emission produced by the association of a blue or a UV LED chip and several Si-F<sub>x</sub> (x=1-4) films

As these composites emit in the blue, green, yellow, or red parts of the spectrum, they can be combined to generate white light under blue or UV excitation. Consequently, several suitable combinations of these films with commercial blue or UV LEDs were tested. The three devices that yielded the highest Color Rendering Index (CRI) are presented (Fig. 9). Many other combinations were explored by varying the concentration of phosphors in the composite coating and also by adjusting the thickness of these coatings. The photometric parameters related to these different prototypes are summarized in Table S1.
➤ Blue LED emitted at 465 nm + Si-F₃ (5%wt / 100 µm) + Si-F₂ (5%wt / 200 µm) + Si-F₁
(5%wt / 100 µm) (Fig. 9a).

➤ Blue LED emitted at 450 nm + Si-F₃ (5%wt / 100 µm) + Si-F₂ (5%wt / 100 µm) + Si-F₁
(5%wt / 100 µm) (Fig. 9b).

➤ Violet LED emitted at 405 nm Si-F₃ (5%wt / 100 µm) + Si-F₂ (2%wt / 100 µm) + Si-F₁
(2%wt / 100 µm) + Si-F₄ (10%wt / 200 µm) (Fig. 9c).

The three configurations studied lead to characteristic emission spectra. This is explained in
particular by the greater or lesser absorption of the emission from the LED used and by the
reabsorption phenomena that will take place given the overlap of the emission spectra.

As shown in Fig. 9d, these specific characteristics allow the photometric parameters to be
modulated including white light emission.

This study thus show that a combination of the fluorophores described in this article can be
successfully used as REEs free DC phosphors.
<figure><img src="image_9.png" /><figcaption>**Fig. 9.** Spectrometric and photometric properties of white LED prototypes designed using a combination of different commercial LEDs (465 nm (a), 450 nm (b) and 405 nm (c)) with a superposition of composite films: one red, one yellow, one green and one blue for the 405 nm LED and (d) CIE chromaticity coordinates of the three combinations (LED/Si-Fₓ Films)</figcaption></figure>
### 3.4. Stability study of fluorophores upon thermal and photonic stresses

Stability under photonic and thermal stresses is an important parameter for LED applications.

To study aging phenomena, samples $P_x$ (x=1-4) were exposed for several hours to the radiation from a commercial UV LED emitting at 375 nm. The behavior of powders $P_x$ (x=1-3) was also studied under irradiation with a commercial blue LED emitting at 465 nm.

Under excitation at 465 nm with a flux of 3080 W/m², it is observed that the fluorophore $P_3$ (based on an organophosphorus emitter) degrades much more rapidly than the boron difluoride complexes ($P_1$ and $P_2$) (Fig. 10a). Indeed, after 20 hours of irradiation, $P_3$ is almost extinguished, whereas $P_1$ and $P_2$ retain approximately 60% and 70% of their initial emission intensity, respectively, and their performance remains nearly constant over the studied exposure time.

The same study conducted under excitation with a UV LED emitting at 375 nm. (a wavelength suitable for studying $P_4$), highlights that $P_2$ remains the most stable fluorophore (Fig. 10b). It retains more than 80% of its emission after 7 days of aging. $P_1$ and $P_4$, on the other hand, retain nearly 50% of their initial emission over the same period. The red fluorophore $P_3$ experiences a significant decrease in its emission intensity, similar to that observed under blue excitation, and is nearly extinguished after only 40 hours of irradiation. The emission spectra recorded during the
aging process for these powders are shown in **Fig. S17**. They exhibit the same spectral profile
throughout aging, with only a decrease in intensity. This leads us to conclude that the studied
molecules are being destroyed rather than modified under the influence of the photon flux.

As expected, the combined action of photon and thermal stress accelerates the degradation for
all studied samples. For instance, the P₁ powder, which was the most stable at 20°C, showing only
a 20% decrease in emission after 160 hours of irradiation, experiences a 60% intensity drop when
a temperature of 100°C is applied (Fig. S18).
<figure><img src="image_10.png" /><figcaption>**Fig. 10.** Evolution of the emission (integrated intensity) as a function of irradiation time under (a) blue LED (465nm) and (b) under UV LED (375 nm) at 20°C</figcaption></figure>

We deliberately worked under stringent conditions to highlight the stresses to avoid in order to preserve satisfactory emission properties. From this study, it is important to note that matrices $P_1$, $P_2$, and $P_4$ exhibit promising performance under blue and UV LED excitation, even under photon
stress. This suggests their potential use in various applications such as displays, anti-counterfeiting marking, and more.

4. CONCLUSIONS

This work explores the potential of rare-earth-free phosphors for white light-emitting diodes (WLEDs) as a sustainable and cost-effective alternative to conventional phosphors containing rare-earth elements. We focused our attention on organic fluorophores, particularly π-extended phospholes and difluoroboron β-diketonates. The different fluorophores (P₁-P₄) were characterized by a high photoluminescence absolute quantum yield and a wide range of emission colors (B, G, Y and R). The fabrication of silicone/fluorophores composite films, Si-Fₓ (x=1-4) exhibiting similar properties than phosphors powders, further validated their potential for LED applications. Various WLEDs prototypes with tunable color correlated temperature are presented, demonstrating the feasibility of producing white light emission through the careful combination of red, yellow, green, and blue-emitting films. Furthermore, the stability of their optical properties under thermal and photonic stresses highlighted the robustness of the fluorophores P₁, P₂ and P₄, especially under UV and blue LED excitation. In summary, the development of these rare-earth-
free organic fluorophores presents a compelling avenue for advancing the field of LED technology,
offering sustainable alternatives with tunable properties and enhanced stability.

5. SUPPORTING INFORMATION

Synthesis description and NMR spectra (¹H, ¹³C, ³¹P) of compound 3; TGA and DSC curves of all compounds; additional experimental details about emission spectra; PLQYint of powders and films; degradation evolution and all LED configurations tested.

6. AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors.

7. CONFLICTS OF INTEREST

There are no conflicts of interest to declare.

8. REFERENCES

1. Piquette, A.; Bergbauer, W.; Galler, B.; Mishra, K. C., On Choosing Phosphors for Near-UV and Blue LEDs for White Light. *ECS Journal of Solid State Science and Technology* **2016**, *5* (1), R3146.

2. Gao, T.; Tian, J.; Liu, Y.; Liu, R.; Zhuang, W., Garnet phosphors for white-light-emitting diodes: modification and calculation. *Dalton Transactions* **2021**, *50* (11), 3769-3781.

3. Li, J.; Yan, J.; Wen, D.; Khan, W. U.; Shi, J.; Wu, M.; Su, Q.; Tanner, P. A., Advanced red phosphors for white light-emitting diodes. *Journal of Materials Chemistry C* **2016**, *4* (37), 8611-8623.
4. Binnemans, K.; Jones, P. T.; Blanpain, B.; Van Gerven, T.; Yang, Y.; Walton, A.; Buchert, M., Recycling of rare earths: a critical review. *Journal of Cleaner Production* **2013**, *51*, 1-22.

5. Tkaczyk, A. H.; Bartl, A.; Amato, A.; Lapkovskis, V.; Petranikova, M., Sustainability evaluation of essential critical raw materials: cobalt, niobium, tungsten and rare earth elements. *Journal of Physics D: Applied Physics* **2018**, *51* (20), 203001.

6. Lukowiak, A.; Zur, L.; Tomala, R.; LamTran, T. N.; Bouajaj, A.; Strek, W.; Righini, G. C.; Wickleder, M.; Ferrari, M., Rare earth elements and urban mines: Critical strategies for sustainable development. *Ceramics International* **2020**, *46* (16, Part B), 26247-26250.

7. Ricci, P. C., Assessment of Crystalline Materials for Solid State Lighting Applications: Beyond the Rare Earth Elements. *Crystals* **2020**, *10*(7), 559.

8. Duffy, M. P.; Delaunay, W.; Bouit, P. A.; Hissler, M., π-Conjugated phospholes and their incorporation into devices: components with a great deal of potential. *Chemical Society Reviews* **2016**, *45*(19), 5296-5310.

9. Zhuang, Z.; Bu, F.; Luo, W.; Peng, H.; Chen, S.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z., Steric, conjugation and electronic impacts on the photoluminescence and electroluminescence properties of luminogens based on phosphindole oxide. *Journal of Materials Chemistry C* **2017**, *5*(7), 1836-1842.

10. Kim, D.-H.; D'Aléo, A.; Chen, X.-K.; Sandanayaka, A. D. S.; Yao, D.; Zhao, L.; Komino, T.; Zaborova, E.; Canard, G.; Tsuchiya, Y.; Choi, E.; Wu, J. W.; Fages, F.; Brédas, J.-L.; Ribierre, J.-C.; Adachi, C., High-efficiency electroluminescence and amplified spontaneous emission from a thermally activated delayed fluorescent near-infrared emitter. *Nature Photonics* **2018**, *12*(2), 98-104.

11. Archet, F.; Yao, D.; Chambon, S.; Abbas, M.; D'Aléo, A.; Canard, G.; Ponce-Vargas, M.; Zaborova, E.; Le Guennic, B.; Wantz, G.; Fages, F., Synthesis of Bioinspired Curcuminoid Small Molecules for Solution-Processed Organic Solar Cells with High Open-Circuit Voltage. *ACS Energy Letters* **2017**, *2*(6), 1303-1307.

12. D'Aléo, A.; Sazzad, M. H.; Kim, D. H.; Choi, E. Y.; Wu, J. W.; Canard, G.; Fages, F.; Ribierre, J. C.; Adachi, C., Boron difluoride hemicurcuminoid as an efficient far red to near-infrared emitter: toward OLEDs and laser dyes. *Chemical Communications* **2017**, *53* (52), 7003-7006.
13. Li, P.; Liang, Q.; Hong, E. Y.-H.; Chan, C.-Y.; Cheng, Y.-H.; Leung, M.-Y.; Chan, M.-Y.; Low, K.-H.; Wu, H.; Yam, V. W.-W., Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices. *Chemical Science* **2020**, *11* (42), 11601-11612.

14. Fave, C.; Cho, T.-Y.; Hissler, M.; Chen, C.-W.; Luh, T.-Y.; Wu, C.-C.; Réau, R., First Examples of Organophosphorus-Containing Materials for Light-Emitting Diodes. *Journal of the American Chemical Society* **2003**, *125* (31), 9254-9255.

15. Fadhel, O.; Gras, M.; Lemaitre, N.; Deborde, V.; Hissler, M.; Geffroy, B.; Réau, R., Tunable Organophosphorus Dopants for Bright White Organic Light-Emitting Diodes with Simple Structures. *Advanced Materials* **2009**, *21* (12), 1261-1265.

16. Wang, C.; Fukazawa, A.; Taki, M.; Sato, Y.; Higashiyama, T.; Yamaguchi, S., A Phosphole Oxide Based Fluorescent Dye with Exceptional Resistance to Photobleaching: A Practical Tool for Continuous Imaging in STED Microscopy. *Angewandte Chemie International Edition* **2015**, *54* (50), 15213-15217.

17. Gaffen, J. R.; Bentley, J. N.; Torres, L. C.; Chu, C.; Baumgartner, T.; Caputo, C. B., A Simple and Effective Method of Determining Lewis Acidity by Using Fluorescence. *Chem* **2019**, *5*(6), 1567-1583.

18. Wang, C.; Taki, M.; Sato, Y.; Fukazawa, A.; Higashiyama, T.; Yamaguchi, S., Super-Photostable Phosphole-Based Dye for Multiple-Acquisition Stimulated Emission Depletion Imaging. *Journal of the American Chemical Society* **2017**, *139* (30), 10374-10381.

19. Cogné-Laage, E.; Allemand, J.-F.; Ruel, O.; Baudin, J.-B.; Croquette, V.; Blanchard-Desce, M.; Jullien, L., Diaroyl(methanato)boron Difluoride Compounds as Medium-Sensitive Two-Photon Fluorescent Probes. *Chemistry – A European Journal* **2004**, *10* (6), 1445-1455.

20. Zhang, G.; Lu, J.; Sabat, M.; Fraser, C. L., Polymorphism and Reversible Mechanochromic Luminescence for Solid-State Difluoroboron Avobenzone. *Journal of the American Chemical Society* **2010**, *132* (7), 2160-2162.

21. Unoh, Y.; Hirano, K.; Satoh, T.; Miura, M., An Approach to Benzophosphole Oxides through Silver- or Manganese-Mediated Dehydrogenative Annulation Involving C☐C and C☐P Bond Formation. *Angewandte Chemie International Edition* **2013**, *52* (49), 12975-12979.

22. Xu, S.; Evans, R. E.; Liu, T.; Zhang, G.; Demas, J. N.; Trindle, C. O.; Fraser, C. L., Aromatic Difluoroboron β-Diketonate Complexes: Effects of π-Conjugation and Media on Optical Properties. *Inorganic Chemistry* **2013**, *52* (7), 3597-3610.
23. Bu, F.; Wang, E.; Peng, Q.; Hu, R.; Qin, A.; Zhao, Z.; Tang, B. Z., Structural and Theoretical Insights into the AIE Attributes of Phosphindole Oxide: The Balance Between Rigidity and Flexibility. *Chemistry – A European Journal* **2015**, *21* (11), 4440-4449.

24. Ledos, N.; Tondelier, D.; Geffroy, B.; Jacquemin, D.; Bouit, P.-A.; Hissler, M., Reaching the 5% theoretical limit of fluorescent OLEDs with push-pull benzophospholes. *Journal of Materials Chemistry C* **2023**, *11* (11), 3826-3831.

25. Kuno, J.; Ledos, N.; Bouit, P.-A.; Kawai, T.; Hissler, M.; Nakashima, T., Chirality Induction at the Helically Twisted Surface of Nanoparticles Generating Circularly Polarized Luminescence. *Chemistry of Materials* **2022**, *34* (20), 9111-9118.

26. Wilbraham, L.; Louis, M.; Alberga, D.; Brosseau, A.; Guillot, R.; Ito, F.; Labat, F.; Métivier, R.; Allain, C.; Ciofini, I., Revealing the Origins of Mechanically Induced Fluorescence Changes in Organic Molecular Crystals. **2018**, *30* (28), 1800817.

27. Legentil, P.; Leroux, F.; Therias, S.; Boyer, D.; Chadeyron, G., Sulforhodamine B-LDH composite as a rare-earth-free red-emitting phosphor for LED lighting. *Journal of Materials Chemistry C* **2020**, *8* (34), 11906-11915.
<figure><img src="image_11.png" /><figcaption>Combining organic fluorophores for high performances WLEDs</figcaption></figure>

<figure><img src="image_12.png" /></figure>