A facile strategy for new organic white LED hybrid devices: design, features and engineering

Abstract

A facile and ecofriendly strategy to design and engineer new organic white LEDs is tested. The hybrid system was implemented by combining a commercial blue LED, with emission at 405 nm, with a hybrid transparent organic film (polycarbonate, PMMA, PVC) containing two selected organic dyes. The emitting molecules, a home-designed push–pull based coumarin and DCM ([2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile), were selected for their optical features and appropriately mixed to obtain white light, perceptible by the naked eye, through the metamerism effect. The proposed strategy shows the critical parameters to account for in the selection of organic dyes, bearing in mind the preservation of the dye emission properties from the solution to the solid state. To this purpose, the contributions of dynamic and static quenching effects were analyzed in detail, to single out the optimum concentrations to assure the compliance with the respective sphere of interaction and guarantee high optical performances. Different new hybrid white-emitting LEDs, with tunable color rendering index (from warm to cold CCT), high energy efficiency (quantum yield larger than 90%), and high photostability, were produced to prove the proposed strategy.

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Introduction

Each year more than 3500 terawatt hours, about 20% of total electricity production, is exploited for lighting, from residential to commercial and industrial uses. The impact of the wide use of lighting devices is also shocking from an environmental point of view, with more than 2000 Mt of CO₂ emitted worldwide. In these regards, the progress in solid-state lighting (SSL) in the past decade has been remarkable and its future will be even brighter! Actually, conventional incandescent bulbs generally convert less than 5% of the consumed energy into visible light as compared to up to 35% for light emitting diode (LED) systems, which boosted the LED market from 2.9% of the total lighting market in 2010 to 25% (2015) and an estimated more than 90% share of the total market in 2020 is foreseen.¹

Since 1995, Nichia, the world largest supplier of LEDs, has been using efficient InGaN blue LED chips in conjunction with a broad-spectrum yellow-emitting phosphor of cerium-doped yttrium aluminum garnet (YAG:Ce³⁺ or Ce:YAG).^2,3 In these light devices, the blue pump light of the InGaN LED is combined with the phosphor’s yellow emission to provide metameric white light, supplying convincing proof of feasibility for a semiconductor-based source of white LEDs (WLEDs) and launching the race for an LED-based solution for the SSL market. The impact of this development has been witnessed through the Nobel Prize in Physics in 2014, jointly awarded to I. Akasaki, H. Amano and S. Nakamura “for the invention of efficient blue light-emitting diodes, which has enabled bright and energy-saving white light sources”.⁴

There are two basic ways to produce white light, by mixing red, green, and blue emissions or by simply combining blue and yellow lights as previously mentioned, the colored emission being provided by specific phosphors. “White” light is then perceived using multiple colors through the visual phenomenon known as metamerism. The efficient exploitation of the metamerism phenomenon is the key point to appreciate the differences among different white light sources, such as LEDs or fluorescent lamps, in particular regarding the correlated color temperatures (CCTs) and the system efficiency, in the effort to design and engineer new lighting devices.

Up to now, the general solution has been based on the application of rare earth elements (REEs) as emission sources. The paradigmatic example is the ubiquity of the YAG:Ce³⁺ phosphor in commercial devices, due to its very simple and cost-effective fabrication, as well as its long term stability, which is related to the high melting point and stable thermal and chemical properties of the system.⁵ The typical drawback of the low color rendering index (CRI) of YAG:Ce³⁺ lighting devices was partially overcome by the use of additional REEs as dopants, such as Tb and Eu. Other possible strategies concern the use of different matrices and/or phosphors with different efficiencies in the visible region. As a consequence, the blending of alternative matrices and different phosphors was developed and applied in the LED structure: (Ca,Sr)₈Mg(SiO₄)₄Cl₂:Eu²⁺ and (Sr,Ba)₂SiO₄:Eu²⁺,⁷ operating in the green region (emission at 500–550 nm),^5,6 (Ca,Sr,Ba)₂SiO₄:Eu²⁺ and (Y,Gd,Tb,Lu)₃(Al,Ga)₅O₁₂:Ce³⁺, emitting in the yellow region (550–600 nm),^8,9 and (Sr,Ba,Ca)₂Si₅N₈:Eu²⁺ and (Sr,Ca)AlSiN₃:Eu²⁺, working in the red region (600–680 nm).¹⁰ However, besides the phase stability glitches and the difficulties related to the growth procedure of these matrices, all these alternative solutions suffer drawbacks related to the combined use of multiple REEs and their possible interactions.

The presented scenario briefly illustrates how REEs are most commonly used as phosphors in multiple combinations, the color of the emitted light and the efficiency of the re-emitted radiation depending on the combination of phosphors. Up to now there have been no or very few efficient alternatives, and the world demand of REE materials for lighting applications is still growing. However, REEs are potentially subject to significant supply disruption, because of the high geographic localization of REE mines and refineries: for these reasons, REEs were recently added to the list of critical raw materials by the European Commission.¹¹ Indeed, to overcome the use of REEs in most technologies, not only in lightening but also, for example, as magnetic or superconducting materials, represents a great and ambitious scientific and technological challenge.

One possible solution for the lightening market that fits with the requirements of high efficiency, tunability of emission, reduced cost and fast integration in industrial production is the use of organic compounds to substitute the REEs. There have been a number of attempts to use organic luminescence converters for LED applications, the main concern being their physical and chemical stability and their integration within the actual devices: indeed, to achieve high lightening performances, large concentrations of phosphors are typically required, but the luminescence quenching effect imposes severe limits. Up to now, all these glitches hampered the development of WLEDs where the REE phosphors are substituted in a suitable and efficient way by organic ones. Recently, a metal–organic system exploiting iridium-complex encapsulation has been proposed and was shown to be able to reach a quite high thermal stability (up to 130 °C), with good quantum yield and optimal possibility of integration, with the emission at 370 nm of an AlGaAsN LED.¹² In recent years, different approaches have been proposed to efficiently exploit the emission properties of selected dye molecules to produce white light, such as organic LEDs (OLEDs) with high-refractive-index substrates or graphene anodes,^14–16 or the hybrid system proposed in the pioneering work of Di Martino et al.¹³ where a suitable dye-doped shell covered a commercial blue emitting InGaN LED. Based on these grounds, the idea considered in the present paper is to design and develop an efficient hybrid WLED system by encapsulating selected organic dye molecules into a transparent inert material. The latter has the function of protecting the organic dyes and should provide high chemical and mechanical stability, allowing enhanced photophysical properties of the encapsulated dyes. This approach can be easily applied in the development of large area emitting devices, such as LED tubes or screens.

We propose a general strategy, here applied to suitable model dyes, to identify the appropriate organic compound and to develop the hybrid film system for lighting purposes. By characterizing the dye in solution, we pinpoint the parameters that are crucial to the final hybrid film and those features that lose their importance once the dye is brought from the solution into the solid state matrix. In particular, our attention is focused on the dynamic and static quenching of the chosen molecules, and their relationship with the optical properties in solution or when encapsulated in the solid matrix. Tailoring of the system can be achieved by selecting the dye as a function of the required final optical properties and by modelling the emission geometry in different shapes depending on the final application and/or device.

As a proof of concept we tailored the hybrid film to the emission of low cost LED devices to achieve a wide emission in the visible region of the spectrum, achieving organic-based WLEDs. The dye molecules are a push–pull based coumarin dye-1 and DCM ([2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile), chosen for their optical features, mainly blue absorption/emission for the coumarin and blue/red absorption/emission for DCM. The selected dyes were encapsulated at appropriate relative concentrations in a polycarbonate matrix to obtain hybrid films. The final devices are composed of commercial GaN-based blue LEDs and the hybrid composite film as the emitting phosphor, the latter fulfilling the requirement of high luminescence quantum yield and high absorption cross section to grant promising lighting applications.

Experimental details

General procedure for the synthesis of coumarin dye-1

A solution of 4-(diethylamino)salicylaldehyde (5.0 g, 0.026 mol), diethylmalonate (4.16 g, 0.026 mol), piperidine (0.26 mL) and acetic acid (0.026 mL) in EtOH (200 mL) was stirred at 50 °C for 16 h and followed by TLC until completion. The reaction mixture was treated with hot water up to the point of crystallization. The solid was recovered and crystallized in i-PrOH, affording a pale yellow crystalline solid in 67% yield.

Preparation of the films

Commercial polycarbonate (0.67 g) was dissolved in CHCl₃ (10 mL) to obtain a PC dispersion. 200 μL of the PC dispersion was added to 1 mL of the dye solution at different concentrations (0.1 M, 10⁻² M, 10⁻³ M and 10⁻⁴ M) and the mixture was put in a glass support until complete evaporation of solvent. By the same procedure, we obtained four polymeric films with different concentrations of dye in PC (Scheme 1).

Scheme 1 Preparation of the hybrid films, (a = CHCl₃, b = THF, c = ethyl acetate).

Preparation of polymer covering commercial LED

A commercial 405 nm LED Bivar UV5TZ-405-15 was immersed for 4 h in 1 mL of CHCl₃ polycarbonate dispersion (0.02 g mL⁻¹) mixed with 300 μL of different ratio solutions of dye-1 and dye-2 (0.1 M). The LED was extracted and dried in air.

Optical characterization

Time resolved photoluminescence (TR-PL) measurements were performed in the picosecond to nanosecond time range with fs light pulses generated by a kilohertz ultrafast Ti:Sapphire Regenerative Amplifier System (800 nm Spectra-Physics Hurricane, pulse energy > 750 microJ, pulse width < 130 fs). The 405 nm excitation wavelength was obtained using a second harmonic generator system (Eksma Optics). The PL signal was detected with a Hamamatsu streak camera (Model C5680) coupled to an Arc-Spectra-Pro 275 monochromator for the spectral and time resolved measurements (typical operative system time response shorter than 50 ps, spectral resolution 1 nm).

Steady state photoluminescence measurements were carried out using a 405.0 nm line by a wavelength stabilized diode module (Ondax Inc. Surelock LM series) coupled with a Reflecting Bragg Grating (Optigrate-Braggrade 405) to narrow the laser line or, where indicated, a Bivar UV5TZ-405-15 standard LED (emission peak 405 nm). The signal was collected through a fiber coupled AvaSpec-ULS2048LTEC spectrometer.

UV-vis absorption measurements were carried out by means of a Perkin-Elmer Lambda 15 spectrophotometer (spectral bandwidth 2 nm) in the 250–700 nm range.

Results and discussion

It is well known that the properties of organic dyes greatly depend on their interaction with the surrounding environment, both in solution and in solid state systems.¹⁷ Successful engineering of new organic dye-based phosphor materials relies on knowledge of the most relevant chemical–physical characteristics, focusing on the photonic target and in connection with the interaction kinetics of the dye molecules with the neighboring elements. In order to achieve the required state of the art and aiming to transfer the optical properties of the synthesized coumarin from solution to a solid state system, we investigated the effects of different parameters on the optical performances, such as the concentration and kind of solvent, to end up with a suitable solid matrix able to host and preserve the dye’s optical features.

The first key parameter is the concentration of the dye itself, since even at relatively low concentration it is possible to observe a drastic quenching of the emission properties. Obviously, this effect in general has a dramatic influence on the dye performance in the solid phase, preventing direct application in LED technology where a solid state system is mandatory. Fluorescence quenching is mainly due to the molecular interactions (static and dynamic) that yield quenching as a result of the contact between fluorophores. In the case of dynamic collisional quenching, the contact between molecules takes place during the lifetime of the excited state, causing the return of the fluorophore to the ground state without emission of a photon. In general, dynamic quenching occurs without any permanent change in the molecules, that is, without a photochemical reaction. On the contrary, in static quenching a non-fluorescent complex is formed between the molecules at higher concentration. Bearing in mind that the goal of our study is to model the transfer of the optical properties of selected dyes from the solution to a suitable hybrid solid matrix for lighting purposes without loss of performance, it is mandatory to separate the contributions of dynamic and static quenching, the latter being detrimental in the solid state.

Coumarins are a group of naturally derived heterocyclic benzopyrone compounds. Most of these molecules possess interesting biological activity, including cytoprotective and modulatory functions, and are used as drugs in many diseases.^18–20 Moreover, coumarins have been studied as industrial tools, in particular in polymer science^21,22 and as laser dye-sensitized photoinitiators.²³ Due to their chemical and physical properties, including high temperature resistance to degradation,^19,23,24 solubility and fluorescence (see for example ref. 25 and notes cited therein), coumarins have attracted different research groups for their potential use as organic-dyes.^26–28 Also, these compounds can be easily synthesized in multigram scale reactions through ecofriendly and low cost preparation procedures by using commercially available starting materials. In this class of compound, coumarins bearing electron donating groups (EDGs) such as N,N-dialkyl amines, or coumarin push–pull systems (Scheme 2), obtained by the introduction of an electron donating- and an electron withdrawing group (such as carbonyl groups) (EWG) on the coumarin scaffold, have been recently studied as organic dyes, showing good to excellent photophysical properties. These characteristics can be rationalized by taking into consideration the structure of the coumarin skeleton. In fact, the benzopyranone aromatic moiety allows electron transfer from EDG groups to electron demanding areas or specific groups of the molecule, extending the electronic conjugation. This process increases the fluorescence properties of coumarin derivatives, also influencing the absorption and the relative emission spectrum.²⁵

Scheme 2 Representative resonance species of a push–pull coumarin system.

Following the scheme shown previously, a modified version of coumarin was obtained (dye-1, see the Experimental section). In principle, the use of dye-1 and DCM allows acquisition of an overall spectral emission perceived as white light by the human eye. By exciting dye-1 with a commercial blue LED, its emission at about 450 nm is partially absorbed by the DCM dye and re-emitted in the red visible range (Fig. 1). To exploit these optical properties in a WLED device requires the wise mixing of the two dyes at the right concentrations and detailed knowledge of the concentration-related performances. Since the DCM dye is not affected by static quenching,²⁷ the selection of an appropriate solvent and suitable dye concentration is carried out with dye-1. Therefore, the first characterization reported in Fig. 2 is the emission properties of coumarin dye-1 in chloroform over a wide concentration range (from 10⁻⁶ to 2 M). At low concentration, the emission peak (excited at 405 nm) is located at 450 nm; by increasing the dye concentration in the investigated solution, the overall emission intensity decreases, whilst a second band centered at 550 nm is observed. Its relative PL intensity increases, being the only contribution detected at the highest dye concentration ([C] > 5 × 10⁻¹). The concentration effect on the absorption spectra is less dramatic, as can be observed for the two different concentrations (10⁻⁵ M and 2 M in CHCl₃) compared in Fig. 3; the absorption peak, recorded at 420 nm at the lowest concentration, broadens and undergoes a hypsochromic shift down to 400 nm at the highest concentrations.

Fig. 1 Emission spectrum of 405 nm pumping LED (blue), absorption (pale blue) and emission (green) spectra of dye-1, and absorption and emission of DCM (pink).

Fig. 2 Emission spectra of dye-1 in CHCl₃ (excitation wavelength at 405 nm); the main bands (9 × 10⁻⁴ M, 9 × 10⁻⁵ M and 9 × 10⁻⁶ M) are divided by a factor 10. The optical quantum yield is estimated to be 100%. The picture shows the naked eye emissions from representative solutions, from right to left: 1 M, 1 × 10⁻¹ M, 1 × 10⁻² M, 1 × 10⁻³ M, 1 × 10⁻⁴ M.

Fig. 3 Absorbance spectra of dye-1 in CHCl₃ for the two different concentrations (10⁻⁵ M and 2 M).

The trend of the PL intensity as a function of the dye concentration is summarized in Fig. 4, where a classic Stern–Volmer graph is reported.

Fig. 4 Stern–Volmer graph of dye-1 in CHCl₃. The fitting parameters are reported in the text and in the graph.

The dynamic quenching trend can be described by the equation:¹⁷

where I₀ and I are the fluorescence intensities in the absence and presence of quenching, respectively, Q is the concentration of the dye and K_D is the dynamic Stern–Volmer quenching constant, related to the lifetime of the fluorophore at low concentration (absence of quenching). According to this equation, a linear relationship in the plot is expected in the case of simple dynamic quenching. However the experimental data showed a non-linear relationship with an upward curving plot, indicating not only the occurrence of collisional quenching but also the formation of non-luminescent aggregates (static quenching). In this case, the fractional fluorescence (I₀/I) must take into account both the fraction quenched by collision (dynamic) and the fraction of complexed (static) dye molecules, and the general Stern–Volmer equation now reads:
where K_S is the static fluorescence quenching constant and is related to the critical concentration of the dye above which static quenching occurs.

The analysis of the static quenching effect can be fitted to a general steric model, with the introduction of the concept of the sphere of action of the quenching mechanism, by defining the volume V surrounding the dye molecule where there is a unity quenching if other dye molecules are within V, and there is no quenching with other molecules outside of the quenching sphere. The modified Stern–Volmer equation that takes into account the action sphere is the following:

where N_A is Avogadro’s number and I is the fluorescence intensity at dye concentration Q, K_D being the dynamic Stern–Volmer quenching constant.

Fig. 4 reports the experimental points as well the fitting curve using eqn (1) with K_D and V as free parameters. The fitting results indicate that K_D = 91 M⁻¹ and V = 120.7 cm³ and hence the radius of the sphere of action is about 6.60 Å.

As already mentioned, the K_D constant is related to the observed decay time constant of the radiative emission, being the experimental decay time related to the effective radiative and non-radiative rate constants,

The shortening of the decay time constant when increasing the concentration suggests the creation of new non-radiative channels. On this basis, we can assume

K_D = k_qτ₀

where τ₀ is the decay time at low concentration (without quenching) and k_q is the bimolecular dynamic quenching constant.

By combining the estimated K_D constant and the decay time photoluminescence measurements, it is possible to give an estimation of the value of the bimolecular quenching constant. Indeed, with τ₀ = 2.90 ns, we get k_q = 3.13 × 10¹⁰ M s⁻¹. It is worth noting that the calculated value is also relevant when static quenching is considered; indeed, the latter removes a fraction of the fluorophores from the observed emission signal caused by the formation of dark complexes, whilst the remaining recorded fluorescence is still from the emitting, unperturbed, not-complexed fluorophores. In these circumstances, the recorded fluorescence lifetime keeps constant to τ₀, in contrast to the dynamic quenching.

Whilst the static quenching depends on the sphere of action, i.e. on the distance between dye molecules, and hence relies on the concentration and is virtually independent of the kind of solvent, the bimolecular quenching constant (and therefore the dynamic quenching) largely depends on the characteristics of the solvent, such as viscosity and polarity. Fig. 5 reports the luminescence spectra obtained at the same concentration and under the same experimental conditions for different solvents. As can be observed, the efficiency drastically decreases in a polar aprotic solvent like dimethyl sulfoxide as compared to chloroform, taken as a reference since the quantum efficiency, measured at the same concentration, is almost 1 (0.91, see ESI†). Concerning the spectral shape, we observe that only the emission intensity changes in different solvents, and no new bands at lower energy, such as the ones previously reported at 550 nm and related to the formation of aggregates, appear.

Fig. 5 Emission spectra of dye-1 with concentration 10⁻⁵ M in different solvents (excitation 405 nm).

It is well known that both the temperature of the solution and the viscosity of the solvent strongly affect the dynamic quenching processes, increasing the collisional quenching by increasing the temperature and decreasing it by increasing the solvent viscosity. These two parameters were the main problems that hampered the efficient exploitation of organic dyes in most up-to-date optical applications, such as Compact Fluorescence Light (CFL) and LEDs. As already mentioned, one of the key issues for the development of new phosphors based on organic compounds is transferring the optical properties gathered in solution to the solid state. The goal is reproducing the conditions of emission observed in the solution of a chosen dye by doping a suitable solid matrix with the same dye, thus obtaining a new class of hybrid phosphor in the solid state.

In the present case, the hybrid films were obtained by mixing in solution both the selected dye and different organic polymers suitable to act as matrices for the final hybrid phosphors. Different solvents (ethylacetate, THF, chloroform) were utilized to dissolve both dyes and the organic polymers, such as poly(methyl methacrylate) (PMMA), polyvinyl chloride (PVC) and polycarbonate (PC), but no effects of the solvent were observed in the study of the final phosphors after the formation of the hybrid film.

This effect was utilized to use a more environmentally friendly solvent like THF to embed the dye in the polymeric matrices, without any loss of quantum efficiency, avoiding the use of chloroform. Actually, opposite to the results obtained in solution (see Fig. 5), we point out that any variations in the optical features of the hybrid matrices were obtained independent from the starting solvent.

Steady state and absorption measurements showed the same behavior, independent from the starting solvent; the time-resolved luminescence spectra also reveal no drastic solvent-related variation. On the contrary, a time decay slower with respect to the most diluted solution was observed in all the hybrid film samples. Fig. 6 reports the absorption spectra of two paradigmatic hybrid films (dye + PC) obtained starting from an ethyl acetate solution and a chloroform solution and, for comparison, the absorption spectrum registered for the diluted solution of the dye in chloroform. In Fig. 7 the luminescence spectra recorded with excitation at 405 nm for a dye–PC hybrid and for reference the diluted chloroform solution of the dye are reported. Finally Fig. 8 reports the time-resolved luminescence measurements recorded at the emission peak. The time decay constant of the luminescence from the dye (monitored at 460 nm) is about 3.0–3.2 ns in all the matrices. All these results pointed out that there is no memory of the starting solvent, or, from a different point of view, one could say that the dye is dispersed in the polymeric matrix without forming new compounds and that there is no residual solvent nested within the matrix itself. Time-resolved measurements give important insight into the quantum efficiency of the process. As we already pointed out, static quenching influences the spectral profile in emission and absorption, whilst the dynamic quenching (DQ) affects the temporal behaviour of the luminescence; hence, considering only DQ effects we can obtain an estimation of the quantum yield in the film from the ratio of the time decay constant:

where τ_sol is the time decay constant in a diluted solution, while τ_sol is the time decay constant calculated for the film and Φ_sol is the known quantum efficiency calculated in a diluted solution. Since the time decay is faster in solution (2.9 ns in solution, 3.2 ns in PC film), the Q.Y. in the solid film is higher with respect to the value calculated for a diluted solution of CHCl₃, approaching 100% (91% in solution, see ESI†). An increase of the Q.Y. in the solid state was already observed in previous work and it is related to the total removal of the dynamic component of the quenching among molecules.¹³

Fig. 6 Absorption spectra of two paradigmatic films (dye-1 + PC) obtained in different starting solutions (ethyl acetate and chloroform). The absorption spectrum of dye-1, 10⁻⁵ M in CHCl₃, is reported for comparison.

Fig. 7 Luminescence spectra (excitation at 405 nm) of dye-1 + polycarbonate film and dye-1, 10⁻⁵ M in CHCl₃.

Fig. 8 Time-resolved luminescence measurements of different dye-1 + matrix films and dye solution monitored at 460 nm, excitation 405 nm.

As already discussed, one of the main parameters that spoils the optical properties is the concentration of the dye. In the solid state hybrid case, the concentration we are dealing with is not referring to the solvent but should be considered with respect to the solid matrix. Bearing that in mind, we reported the emission properties not in terms of the dye concentration in the starting solvent but in terms of the dye concentration (mol) within the matrix volume in the final solid state film. With reference to the previous discussion concerning the dynamic and static quenching, it is clear that the latter is the only one effective in the hybrid films, since the dye molecules are trapped within the matrix and no residual solvent was detected. Fig. 9A reports the luminescence spectra at four different concentrations.

Fig. 9 Emission spectra (excitation 405 nm) of dye-1 in PC film at four different dye concentrations (A); dependence of the PL intensity of as a function of the dye-1 concentration in polycarbonate films (B). On the right is a picture of the film excited with a broad spectrum UV lamp.

On this basis we report in Fig. 9B the dependence of the PL intensity as a function of the dye concentration in PC films. The PL intensity remains constant up to 0.1 M, then it rapidly decreases to a very faint signal when the concentration is above 1 M. One should note that the concentration threshold is one order of magnitude higher than the value at which the quenching starts in solution. By considering the sphere of action for the static quenching case, we get that at the 0.1 M concentration (in terms of moles of dye in the PC matrix) the average distance between each molecule can be estimated to be 3.2 nm (weighing 0.11 mg of dye in 4.4 mg of polycarbonate with a density of 1.20 mg cm⁻³). This value is higher than the radius of interaction of each dye molecule we calculated for the quenched solution case (0.66 nm), preventing the static quenching even at such a large concentration. On the contrary, when the concentration is increased up to 1 M (corresponding to 1.1 mg dye in 4.4 mg of polycarbonate) the average distance decreases by one order of magnitude (0.32 nm); therefore, two molecules are inside the sphere of action of one another, generating non-fluorescent aggregates.

To test the possible application of the new phosphors in the photonics field, accounting for an easy and ecofriendly procedure implementing the hybrid system in actual devices, we covered a commercial, low cost LED with emission at 405 nm (Bivar UV5TZ-405-15) with the hybrid film. The two dyes in the hybrid film had selected low concentrations to assure the compliance with the respective sphere of interaction (we also verified that the two dye molecules do not react or interact with one another). Since the thickness of the deposited hybrid film is quite small (not larger than 1 mm from absorption measurements at 405 nm as a function of film thickness), the residual pumping light of the LED is still visible and contributes to achieving the white emission. By changing the relative ratio of the two dyes (but still keeping both concentrations below the quenching threshold), complete tuning can be achieved, and different perceived white light with hot to cold CCT performances can be reached. Fig. 10 reports the new devices obtained with the 405 nm pumping LED covered with co-doped PC films with different relative concentrations between dye-1 and dye-2. Different white shades can be obtained from the combination of the two dyes from warm white in Fig. 10A (CIE x = 0.4670, y = 0.4820, CCT = 2779, CRI = 65) and Fig. 10B (CIE x = 0.4150, y = 0.0.3670, CCT = 3100, CRI = 75) to a cool white (CIE x = 0.3216, y = 0.3447, CCT = 5980, CRI = 81) in Fig. 10C.

Fig. 10 Emission spectra, CIE diagram and pictures of 405 nm LED covered with co-doped PC film with different relative concentrations: (A) dye-1/dye-2 = 1; (B) dye-1/dye-2 = 2 and (C) dye-1/dye-2 = 10. The spectra were recorded under the typical working conditions of the pumping LED (emission 405 nm, I = 15 mA, V = 3.4 V).

Fig. 11 reports the relative luminous intensity as a function of the forward current, showing a linear behavior and an unaltered spectral profile.

Fig. 11 Integrated emission intensity vs. applied forward current (device C in Fig. 10). The emission spectra did not change under the operating conditions (inset).

Finally, in order to verify the stability of our devices, we collected the PL spectrum over 10 working days in continuous operation under the typical electrical power conditions (3.4 V – 15 mA) and under ambient conditions. Remarkable stability was observed without any variation in the emission spectra during the testing time (ESI, Fig. S3†). The emission properties of the film were also studied as a function of the temperature up to 80 °C, showing thermally-induced degradation of less than 5% at 50 °C (ESI, Fig. S4†).

It is worth pointing out that the synthesis of the hybrid film is very easy and ecofriendly, allowing coverage of a large surface thanks to the easy to handle procedure of the conventional transparent matrix utilized.

Conclusions

In this work, a detailed study on the critical parameters related to the emission properties of organic dyes (efficiency and spectral response) from the solution to the solid state was performed. In particular, the contributions of dynamic and static quenching effects were analyzed in detail, in order to single out the optimum concentrations, to assure compliance with the respective sphere of interaction and guarantee high optical performances. The analysis reveals that the concentration threshold of the film is about one order of magnitude higher than the value at which the quenching starts in solution.

On this basis, a hybrid white-emitting LED with tunable color rendering index and high efficiency (Q.Y. over 90%) was designed and achieved. The hybrid system was obtained by combining a commercial blue LED, with emission at 405 nm, with a hybrid transparent organic polycarbonate film containing the two selected organic dyes.

The typical optical characterization tests (as a function of electrical power and temperature) confirm the stability of both the efficiency and spectral emission of the final device over several days of operation.

The procedure and the device reported opens new possibilities for the use of the emission from an InGaN-based LED to tune the overall luminescence over a wide spectral range, just by changing the outer packaging of the LED devices, and to avoid the use of inorganic phosphors (Ce:YAG) inside the LED device itself. Moreover, just by changing the dye in the hybrid film, it is possible to extend the applications to other fields where the excitation photon needs to be redshifted, by choosing the absorption and emission suitable for the identified application and verifying the maximum concentration before static quenching occurs from standard measurements in solution.

Acknowledgements

The authors acknowledge the EIP commitment on raw materials “RESET”. L. S. gratefully acknowledges the Sardinia Regional Government for the financial support of his Ph.D. scholarship (P. O. R. Sardegna F. S. E. Operational Programme of the Autonomous Region of Sardinia, European Social Fund 2007–2013-Axis IV Human Resources, Objective l.3, Line of Activity l.3.1.).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00999a

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