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

Article abstract of DOI:10.1039/c6ra00999a

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.

Full text of DOI:10.1039/c6ra00999a

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DOI: 10.1039/C6RA00999A.
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AꢀfacileꢀstrategyꢀforꢀnewꢀorganicꢀWhiteꢀLEDꢀ
hybridꢀdevices:ꢀdesign,ꢀfeaturesꢀandꢀ
engineering.ꢀ
1
,2
1,2
2,3
2
Alberto Luridiana , GianLuca Pretta , Daniele Chiriu , Carlo Maria Carbonaro , Riccardo
2
1
1
2
2,3
Corpino , Francesco Secci , Angelo Frongia, Luigi Stagi and Pier Carlo Ricci*
1
Dipartimento di Scienze Chimiche e Geologiche, Università d Cagliari, SS 554 bivio Sestu, I-09042 Monserrato (CA),
Italy
2
Dipartimento di Fisica, Università degli Studi di Cagliari, S.P. Monserrato-Sestu Km 0.700, 09042 Monserrato (CA),
Italy
3
E-laboRad s.r.l., spin-off, Dip Fisica, Università degli Studi di Cagliari, S.P. Monserrato-Sestu Km 0,700 09042
Monserrato (CA), Italy
*
Corresponding author: carlo.ricci@dsf.unica.it
Abstract
A facile and ecofriendly strategy to project and engineering new organic white LED is tested. The hybrid
system was implemented by combining a commercial blue LED, with emission at 405 nm, with an hybrid
transparent organic film (Polycarbonate, PMMA, PVC) containing two selected organic dyes. The emitting
molecules,
a
home designed push-pull based Coumarin
1
and the DCM ([2-[2-[4-
(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile), were selected for their
optical features and properly mixed to obtain the eye perception of white light through metamerism effect.
The proposed strategy evidences the critical parameters to account for in the selection of the organic dyes,
bearing in mind to preserve the dye emission properties from the solution to the solid state. To this
purpose, the contribution of dynamic and static quenching effects were analyzed in detail, to single out the
optimum concentrations to assure the compliance of the respective sphere of interaction and guarantee
high optical performaces. Different hybrid new white-emitting LEDs with tunable color rendering index,
from warm to cold CCD features, high energy efficiency, with a quantum yield larger than 90%, and high
photostability were fulfilled to prove the proposed strategy.
1

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Introduction
Each year more than 3500 TeraWatt hours, about 20 % of the total electricity production is exploited for
lighting, from residential to commercial and industrial use. The impact of wide use of lighting devices is also
shocking from the environmental point of view, with more than 2000 Mt of CO2 worldwide emission. In
these regards, the progress in solid-state lighting (SSL) has been remarkable in the past decade 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 the actual 25 % (2015) and
1
estimation foreseen more than 90% of the total market in 2020.
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
3
+
2,3
(
YAG:Ce or Ce:YAG). In these light devices, the blue pump light of InGaN LED is combined with the
phosphor yellow emission to provide metameric white light, supplying a convincing feasibility proof for a
semiconductor-based source of white LEDs (WLEDs) and launched the race for 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
4
diodes which has enabled bright and energy-saving white light sources".
There are two basic ways to produce a 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 throughout 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 is based on the application of rare earth elements (REE) as emitting
3
+
sources. The paradigmatic example is the ubiquity of YAG:Ce 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
5
melting point and stable thermal and chemical properties of the system. The typical drawback of low color
3
+
rendering index (CRI) of YAG:Ce lighting devices was partially overcome by the use of additional REE as
dopants, such as Tb and Eu. Other possible strategies concern the use of different matrixes and/or
phosphors with different efficiency in the visible region. As a consequence, blending of alternative matrices
2
+
and different phosphors were developed and applied in the LED structure: (Ca, Sr)8Mg(SiO4)4Cl2:Eu ,
2

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2
+
5Error! Reference source not
(
Sr,Ba)2SiO4:Eu , operating in the green region (emission at 500-550 nm),
found.,6
2+
3+
(
Ca,Sr,Ba)2SiO4:Eu and (Y,Gd,Tb,Lu)3(Al,Ga)5O12:Ce , emitting in the yellow region (550-600 nm),
8
,9
2+
2+
10
(
Sr,Ba,Ca)2Si5N8:Eu and (Sr,Ca)AlSiN3:Eu working in the red region (600-680 nm). However, beside
the phase stability glitches and the difficulties related to the growing procedure of these matrices, all these
alternative solutions suffer the drawbacks related to the combined use of multiple REE and their possible
interactions.
The presented scenario briefly illustrates how REE 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 are no or very few efficient alternatives, and the world
demand of REE materials for lighting application is still growing. However REE are potentially subjected to a
high supply disruption, because of the high geographic localization of REE mines and refineries: for these
1
1
reasons REE were recently inserted in the list of critical raw materials by the European Commission.
Indeed, the overcome of the use of REE in most technologies, not only in lightening but also, for example,
as magnetic or superconducting materials, represents a great and ambitious scientific and technologic
challenge.
One possible solution for the lightening market that fits with the request of high efficiency, tunability of the
emission, reduced cost and fast integration in industrial production is the use of organic compound to
substitute the REE. 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 concentration of phosphors are
typically required, but the luminescence quenching effect poses 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 shown able to reach a quite high thermal stability (up to 130 °C), good quantum yield, and
1
2
optimal perspective of integration with the emission at 370 nm of AlGaAsN LED.
In the last years,
different approaches were proposed to efficiently exploit the emission properties of selected dye
molecules to produce white light, such as the organic LED (OLED) with high-refractive-index substrates or
1
4-16
13
graphene anode,
or the hybrid system proposed in the pioneering work of Di Martino et al. where a
suitable dye doped shell covered a commercial blue emitting InGaN LED. Based on this ground, 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 to protect the
organic dyes and should provide high chemical and mechanical stability, allowing enhancing the
photophysical properties of the encapsulated dyes. This approach can be easily applicated in the
development of large area emitting devices, such as LED tube or screens.
3

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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 lightening purposes: by characterizing the dye in
solution we pinpoint the crucial parameters to the final hybrid film and those features that lose their
importance once the dye is brought from the solution to the solid state matrix. In particular our attention is
focused on the dynamic and static quenching of the chosen molecules, and their relationship to the optical
properties in solution or when encapsulated into the solid matrix. Tailoring of the system can be obtained
by selecting the dye as a function of the requested final optical properties and by modelling the emission
geometry in different shape 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 1 and the DCM ([2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-
ylidene]-propanedinitrile),), chosen for their optical features, mainly blue absorption/emission for
Coumarin and blue/red absorption/emission for DCM. The selected dyes were encapsulated at proper
relative concentration in a polycarbonate matrix to obtain hybrid films. The final devices are composed by
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 lightening applications.
Experimental Details
General procedure for the synthesis of coumarin Dye -1
A solution of 4-(diethylamino)salicylaldehyde (5.0g, 0.026 mol), diethylmalonate (4.16g, 0.026 mol),
pyperidine (0.26mL) and acetic acid (0.026mL) in EtOH (200 mL) was stirred at 50°C for 16h and followed
TLC until completion. The reaction mixture was treated with hot water up to crystallization. The solid was
recovered and crystallized in i-PrOH, affording a pale yellow crystalline solid in 67% yield.
Preparation of the films
A commercial polycarbonate (0,67 g) was dissolved in CHCl (10mL) to obtain a PC dispersion. 200 uL of PC
3
-2
-3
-4
dispersion was added at 1 mL of the Dye solution at different concentration (0.1 M, 10 M, 10 M and 10
4

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M) and the mixture was put in a glass support until the complete evaporation of solvent. By the same
procedure we obtained four polymeric films at different concentration of dye in PC.
Scheme 1 Preparation of the hybrid films
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 (0,02g/mL) mixed with a 300 ꢀL of different ratio solution of dye 1 and dye 2 (0,1 M). The led
was extracted and dry 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
4
05 nm excitation wavelength was obtained by 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
5

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monochromator for the spectral and time resolved measurements (typical operative system time response
shorter than 50 ps, spectral resolution 1 nm).
Steady time photoluminescence measurements were carried out using a 405.0 nm line by a wavelength
stabilized diode modules (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 absorpꢀon measurements were carried out by means of a spectrophotometer Perkin-Elmer Lambda
1
5 (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 the interaction with the surrounding
1
7
environment, both in solution and in solid state systems. Successful engineering of new organic dye based
phosphor materials relies on the 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 requested 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 on
the optical performances of different parameters, such as the concentration and the kind of solvent to end
up with a suitable solid matrix able to host and preserve the dye 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 is in general
dramatic on the dye performances 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 performances, it is mandatory to separate the contribution of dynamic and static
quenching, the latter being detrimental at the solid state.
6

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Coumarins are a group of heterocyclic benzopyrone naturally derived compounds. Most of these molecules
possess interesting biological activity including cytoprotective and modulatory functions and are used as
1
8,19,20
drugs in many diseases.
Moreover, coumarins were studied as industrial tools in particular in polymer
2
1, 22
23
science
high temperature resistance to degradation,
notes cited therein), coumarins attracted different research groups as potential organic-dyes.
and as laser dye-sensitized photoinitiators. Due to their chemical-physics properties, including
1
9,23,24
solubility and fluorescence (see for example ref 25 and
2
6,27
Also,
these compounds can be easily synthetized in multigram scale reaction through ecofriendly and low cost
preparation procedures by using commercially available starting materials. In this class of compound,
coumarins bearing electron donating groups (EDG) such as N,N-dialkyl amines, or coumarin push-pull
systems, obtained by the introduction on the coumarin scaffold of an electron donating- and an electron
withdrawing group (such as a carbonyl groups) (EWG), have been recently studied as organic dyes, showing
good to excellent photophysic properties. These characteristics can be rationalized by taking in
consideration the coumarin skeleton structure. In fact, the benzopyranone aromatic moiety allows the
electron transfer from EDG groups to electron demanding districts or specific groups of the molecules,
extending the electronic conjugation. This process increases the fluorescence properties of the coumarin
2
5
derivatives also influencing the absorption and the relative emission spectrum.
O
O
O
EWG
R'
R'
R
R
EDG
O
O
N
R
O
O
N
R
O
Scheme 2 Representative resonance species on a push-pull coumarin 1 system
Following the scheme above reported a modified version of Coumarin 1 was obtained (Dye 1 see the
Experimental section). In principle, the use of dye 1 and DCM permits to obtain an overall spectral emission
perceived as white light from the human eye. By exciting the 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 (Figure 1). To
exploit these optical properties in a WLED device requires the wise mix of the two dyes at the right
concentrations and the detailed knowledge of the concentration related performances. Since DCM dye is
2
7
not affected by static quenching, the selection of the proper solvent and the suitable dye concentration is
carried on dye 1. So, the first characterization reported in Figure 2 is the emission properties of coumarin
-
6
dye 1 in chloroform in a wide concentration range (from 10 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 limit
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-
1
(
[C]>5 x 10 ). The concentration effect on the absorption spectra is less dramatic, as can be observed for
-
5
the two different concentrations (10 M and 2 M in CHCl ) compared in Figure 3: the absorption peak,
3
recorded at 420 nm at the lowest concentration, broadens and undergoes a hypsochromic shift down to
4
00 nm at the highest ones.
Dye 1
Abs.
DCM
Abs.
DCM
Em.
LED
em.
Dye 1
Em.
4
00
500
600
700
800
Wavelength (nm)
Figure 1 Emission spectrum of 405 nm pumping LED (blue), absorption (pale blue) and emission (green)
spectra of dye 1 and DCM (pink)
8

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1
1
1
40000
20000
00000
-
6
9
,0 x 10
x10
x10
x10
-
-
5
4
9,0 x 10
,0 x 10
9
-
3
9,0 x 10
-
1
1
,2 x 10
-
1
8
6
4
2
0000
0000
0000
0000
0
1,5 x 10
-
-
1
1
,8 x 10
1
2,3 x 10
-
1
3
,1 x 10
-
1
5
,0 x 10
-
1
8
,0 x 10
,0
2
4
00
450
500
550
600
650
Wavelenght (nm)
-
4
Figure 2 Emission spectra of dye 1 in CHCl3 (excitation wavelength at 405 nm); the main bands (9x 10 M,
-
5
-6
9
x 10 M and 9 x 10 M) are divided for a factor 10. The quantum yields for these solutions were
estimated in 1. The picture reports the naked eye emissions from representative solutions, from right to
-
1
-2
-3
-4
left: 1M, 1∙10 M, 1∙10 M, 1∙10 M, 1∙10 M.
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1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
-5
Solution 10 M
Solution 2 M
2
50
300
350
400
450
Wavelength (nm)
-
5
Figure 3 Absorbance spectra of dye -1 in CHCl for the two different concentrations (10 M and 2 M)
3
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.
1
0

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sternVolmermodified (Us
er)
Model
3
2
2
1
1
00
50
00
50
00
(1+KD*x)*exp(x*VN/1000
Equation
)
Reduced
Chi-Sqr
48.67928
Adj. R-Square
0.98842
Value
Standard Error
60.16741
Io/I
Io/I
VN
KD
727.33679
91.65776
6.07642
50
0
0
.0
0.2
0.4
0.6
0.8
1.0
1.2
C
Figure 4 Stern-Volmer graph of dye -1 in CHCl3. The fitting parameter are reported in the text and in the
graph.
1
7
The dynamic quenching trend can be described by the equation:
I0
=
1+ K Q
D
I
where I
o
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 case of simple dynamic quenching. However the experimental
data drew a non-linear relationship with an upward curvature plot, indicating that not only a collisional
quenching occurs but also the formation of non-luminescent aggregates (static quenching). In this case the
0
fractional fluorescence (I /I) must take into account both the fraction of quenched by collision (dynamic)
and the fraction of complexed (static) dye molecules, and the general Stern-Volmer equation now reads:
I0
=
(1+ K Q)⋅(1+ K Q)
D
S
I
where K is the static fluorescence quenching constant and it is related to the critical concentration of the
s
dye above which the static quenching occurs.
1
1

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The analysis on the static quenching effect can be patterned 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 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:
QVNA
I0
1000
=
(1+ K Q)⋅e
(1)
D
I
where N is Avogadro number and I is the fluorescence intensity at the Q concentration of the dye, being KD
A
the dynamic Stern-Volmer quenching constant.
Figure 4 reports the experimental points as well the fitting curve by using the equation 1 with K and V as
D
-
1
3
free parameters. The fitting results indicate K
sphere of action is about 6.60
D
= 91 M and V= 120.7 cm and, hence the radius of the
Å.
d
As already mentioned the K 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
constant,
1
1
1
=
+
τexp τrad τnonrad
The shortening of the decay time constant by increasing the concentration suggests the creation of new
non radiative channels. On this basis, we can assume
K = kqτ0
D
where τo is the decay time at low concentration (without quenching) and k is the bimolecular dynamic
q
quenching constant, respectively.
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, being
τ
o
=2.90 ns,
1
0
-1
we get k
q
= 3.13 x 10 M s . 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 causing the formation of dark complexes, whilst the remaining recorded fluorescence is still
from the emitting unperturbed not-complexed fluorophores; in this circumstances, the recorded
fluorescence lifetime keeps constant to τ , in contrast to the dynamic quenching.
0
1
2

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Whilst the static quenching depends on the sphere of action, i.e. on distance between dye molecules, and
hence relies on the concentration and is virtually not dependent on 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. Figure 5 reports the luminescence spectra obtained at the same
concentration and in 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 reference since the quantum efficiency, measured at the same concentration, is almost 1 (0.91,
see Supplementary Materials). Concerning the spectral shape, we observe that only the emission intensity
changes in different solvents, but no new bands at lower energy, such as the ones previously reported at
5
50 nm and related to the formation of aggregates, appear.
30000
25000
20000
15000
10000
CHCl3
DCM
THF
DMSO
5000
0
4
00
450
500
550
Wavelength (nm)
-
5
Figure 5 Emission spectra of dye-1 with concentration 10 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 them by increasing the solvent viscosity. These two parameters were the main criticism that
hampered to efficiently exploit organic dyes in most optical up-to-date applications such as CFL and LED. 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 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.
1
3

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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 matrix of the final hybrid phosphors. Different solvents (ethylacetate,
THF, Chloroform) were utilized to dissolve both dyes and the organic polymer such as Poly(methyl
methacrylate) (PMMA), Polyvinyl chloride (PVC), 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 were utilized to use an environmental friendly solvent like THF to embed the dye in the
polymeric matrices, without any loose of quantum efficiency, avoiding the use of chloroform. Actually
opposite to the results obtained in solutions (see figure 5), we point out that any variation in the optical
features of the hybrid matrices were obtained dependent from the starting solvent.
Steady time and absorption measurements showed the same behavior independently from the starting
solvent, time resolved luminescence spectra as well do not reveal any drastic solvent related variation. On
the contrary, a time decay slower with respect to the most diluited solution was observed in all the hybrid
film samples. Figure 6 reports the absorption spectra of two paradigmatic hybrid films (dye + PC) obtained
starting form a ethil acetate solution and a chloroform solution and, for comparison, the absortpion
spectrum registred on the diluited solution of dye in chloroform. In Figure 7 the luminescence spectra
recorded with excitation at 405 nm in a dye-PC hybrid and in the reference diluited chloroform solution of
the dye are reported. Finally Figure 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 result 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 polimeric matrix
without forming new compounds and that there is not residual solvent nested within the matrix itself. Time
resolved measurements give important insights on the quantum efficiency of the process. As we already
point out, static quenching weights on the spectral profile of absorption and emission properties and for
diluited solution and films no difference were observed. Only the dynamic quenching (DQ) affects the
temporal behaviour of the luminescence, hence considering only DQ effects we can have an estimation of
the quantum yield in the film from the ratio of the time decay constant:
τ Film
Q.Y. = τ Sol Φsol
where
for the film and
is faster in solution (2.9 ns solution, 3.2 in PC film), the Q.Y. in the solid film is higher with respect the value
calculated for a diluted solution of CHCl , approaching 100 % (91% in solution, see Supplementary
τsol is the time decay constant in a diluted solution, while
τsol is the time decay constant calculated
Φ
Sol is the known quantum efficiency calculated in a diluted solution. Since the time decay
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Materials). An increase of the Q.Y. in the solid state was already observed in previous work and it is related
1
3
to the total removal of the dynamic component of the quenching among molecules.
1.25
1.00
0.75
0.50
0.25
0.00
dye 1 in CHCl solutions
3
dye 1 in PVC film (chloroform)
dye 1 in PVC film (ethalon)
350
400
450
Wavelength (nm)
Figure 6 Absorption spectra of two paradigmatic films (dye 1+PC) obtained in different starting solution
-
5
(
ethyl acetate solution and a chloroform). The absorption spectrum of dye -1, 10 M in CHCl
3
is reported for
comparison.
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3
2
2
1
1
0000
5000
0000
5000
0000
dye 1 in CHCl Solution
3
dye 1 in PC Film
5
000
0
4
00
450
500
550
600
Wavelenght (nm)
-5
Figure 7 Luminescence spectra (excitation at 405 nm) of dye 1+Polycarbonate film and dye -1, 10 M in
CHCl
3
.
1
0.1
Film PVC
Film PMMA
Film PolyCarbonate
-- τ= 3.2 ns
-- τ= 3.0 ns
-- τ= 3.2 ns
-- τ= 2,90 ns
-5
Solution CHCl 1 x 10
M
3
5
10
Time (ns)
Figure 8 Time resolved luminescence measurements of different dye 1 + film and solution monitored at 460
nm, excitation 405 nm.
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As already discussed, one of the main parameters that spoilts the optical properties is the concentration of
the dye. In solid state hybrid case the concentration we are dealing with is not referred to the 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 dye 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 hybid films, since the dye molecules are trapped within the matrix and no residual solvent was
detected. Figure 9A reports the luminescence spectra at four different concentration.
30000
B
30000
25000
20000
15000
10000
A
0
0
1
1
,01 M Dye / on PC
,1 M Dye / on PC
20000
M
Dye / on PC
0 M Dye / on PC
10000
PL Integrated Area
5000
0
0
4
00
450
500
Wavelenght (nm)
550
600
0.01
0.1
1
10
C (dye on PC)
Figure 9 Emission spectra (excitation 405 nm) of dye 1 in PC film in four different dye concentration. On the
right picture of the film excited with a broad spectrum UV lamp (A); Dependence of the PL intensity of as a
function of the dye 1 concentration in Polycarbonate films (B)
On this basis we report in Figure 9B the dependence of the PL intensity as a function of the dye
concentration in PC films. The PL intensity keeps 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 treshold 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 mols of
dye in PC matrix) the average distance between each molecule can be estimated in 3.2 nm (weigthing 0.11
3
mg of dye in 4.4 mg of policarbonate with density of 1.20 mg/cm ). This value is twice the radius of
interaction of each dye molecule we calculated for the quenched solution case (0.6 nm), so that preventing
the static quenching even at such a large concentration. On the contrary, when the concetration is
increased up to 1 M (corresponding to 1.1 mg dye in 4.4 mg of policarbonate) the average distance
decreases of one order of magnitude (0.32 nm), therefore two molecules are inside the sphere of action
one of the other, generating non fuorescent aggregates.
To test the possible application of new phosphors in photonic field, accounting for an easy and ecofriendly
procedure implementing the hybrid system in the actual devices, we covered a commercial, low cost LED
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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 of the respective sphere of interaction (we also
verified that the two dye molecules do not react or interact the one with the other). 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 achieve
the white emission. By changing the relative ratio of the two dyes (but still keeping both concentration
below the quenching threshold) complete tuning can be achieved, and different perceived white light with
hot to cold CCT performances can be reached, Figure 10 reports the new devices obtained by the 405
pumping LED covered with co-doped PC film with different relative concentration between dye 1 and dye 2.
Different white shades can be obtained from the combination of the two dyes from warm white in Figure
1
)
0A (CIE x=0.4670 ,y=0.4820, CCT=2779, CRI= 65 ), Figure 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 ) Figure 10 C.
Figure 11 reports the relative luminous intensity as function of the forward current, showing a linear
behavior and a unaltered spectral profile.
Finally, in order to verify the stability of our devices, we collected the PL spectrum along 10 working days in
continuous operation at the typical electrical power condition (3.4 V - 15 mA) and at room conditions.
Remarkable stability was observed without any variation in the emitted spectra during the testing time
(Supporting information, figure S3).The emission properties of the film were also studied as function of the
temperature up to 80 °C, showing thermally induced degradation less than 5% at 50 °C (Supporting
information, figure S4).
It is worth pointing out that the synthesis of the hybrid film is very easy and ecofriendly, allowing covering
large surface thanks to the easy to handle procedure of the conventional transparent matrix utilized.
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A
B
C
Figure 10 Emission spectra, CIE diagram and pictures of 405nm LED covered with co-doped PC film with
different relative concentration: a) dye 1 /dye 2 =1; b) dye 1/dye 2 =2 and c) dye 1 /dye 2 = 10. The spectra
were recorded in the typical working condition of the pumping LED (emission 405 nm, I=15 mA, V=3.4 V).
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14000
12000
10000
Integrated Intensity
8000
6000
4000
2000
0
400
500
600
700
nm
0
2
4
6
8
10
12
14
16
18
20
Forward Current (mA)
Figure 11 Integrated emission intensity vs applied forward current (device C in Figure 10). The emission
spectra did not change in the operating condition (inset).
Conclusions
In this work a detailed study on the critical parameters related to the emission properties in organic dyes
(efficiency and spectral response) from the solution to the solid state was performed. In particular the
contribution of dynamic and static quenching effects were analyzed in detail, in order to single out the
optimum concentrations, to assure the compliance of the respective sphere of interaction and guarantee
high optical performaces. The analysis reveals that the concentration treshold in the film is about one order
of magnitude higher than the value at which the quenching starts in solution.
On these 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 an hybrid transparent organic Polycarbonate film containing two
selected organic dyes.
The typical optical characterization tests (as a function of the electrical power and temperature) confirm
the stability of both efficiency and spectral emission of the final device over several days of operation.
The procedure and the device reported opens new possibilities to the use of the emission from InGaN
based LED to tune the overall luminescence in a wide spectral range just changing the outer packaging of
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the LED devices, and to avoid the use of inorganic phosphors (Ce:YAG) inside the LED device itself.
Moreover, just changing the dye in the hybrid film, it is possible to extend the applications to other fields
where excitation photon needs to be redshifted by choosing the absorption and emission suitable for the
identified application and verify the maximum concentration before static quenching occurs from standard
measurements in solution.
Acknowledgments
The authors acknowledge the EIP commitment on raw materials “RESET”.
L.S. gratefully acknowledges 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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