High Stokes shift perylene dyes for luminescent solar concentrators

Article abstract of DOI:10.1039/c2cc38708e

Highly efficient plastic based single layer Luminescent Solar Concentrators (LSCs) require the design of luminophores having complete spectral separation between absorption and emission spectra (large Stokes shift). We describe the design, synthesis and characterization of a new perylene dye possessing Stokes shift as high as 300 meV, fluorescent quantum yield in the LSC slab of 70% and high chemical and photochemical stability. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cc38708e

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High Stokes shift perylene dyes for luminescent solar
concentrators†
Cite this: DOI: 10.1039/c2cc38708e
Alessandro Sanguineti, Mauro Sassi, Riccardo Turrisi, Riccardo Ruffo,
Gianfranco Vaccaro, Francesco Meinardi and Luca Beverina*
Received 4th December 2012,
Accepted 4th January 2013
DOI: 10.1039/c2cc38708e
www.rsc.org/chemcomm
Highly efficient plastic based single layer Luminescent Solar Concen-
Amongst the main limits of this established concept, the
trators (LSCs) require the design of luminophores having complete re-absorption of the emitted light due to incomplete spectral
spectral separation between absorption and emission spectra (large separation between the dye absorption and emission spectra is
Stokes shift). We describe the design, synthesis and characterization of particularly relevant.10 The use of bi-layer structures consisting
a new perylene dye possessing Stokes shift as high as 300 meV, of a thin film of organic dyes, vacuum deposited on a high-
fluorescent quantum yield in the LSC slab of 70% and high refractive-index glass, efficiently reduces re-absorption losses,
chemical and photochemical stability.
at an additional cost.11 Low cost, single layer LSCs require the
design of efficient and robust luminophores having very large
The ever-increasing quest for sustainable and renewable Stokes shift.12 Amongst the various classes of chromophores
sources of energy boosted enormous interest in the solar cell considered for LSCs,8,12–15 the perylenediimides,16,17 some of them
technology. Alongside the well-established field of silicon and commercially available under the brand name Lumogens, repre-
thin film solar cells, a number of alternative organic and hybrid sent the state of the art in this field, even though their Stokes shift
technologies are believed to play an important role in the near is still very small and their absorption does not extend to the
future.1–6 Alongside their use in photocurrent generating whole solar spectrum.8 In this communication we describe the
devices, luminescent organic materials can play a relevant role synthesis, characterization and incorporation in LSC devices of
in the Luminescent Solar Concentrator (LSC) technology. The two new unsymmetrical peryleneimide derivatives possessing
LSC is a device able to reduce the cost and limits of standard improved Stokes shift, efficient absorption even at high energies
silicon based photovoltaics by the exploitation of the most (where standard perylenediimides do not absorb, see Fig. 1),
favourable properties of organic materials – strong absorption,
flexibility, and low cost – without facing their limits.7
LSCs are slabs of transparent, high optical quality materials
(mostly PMMA) doped with luminescent molecules.8 The mole-
cules absorb sunlight and emit it into the slab. Since the
refractive index of the slab (n = 1.491 at 588 nm for PMMA) is
higher than that of air, most of the emitted light is trapped by
total internal reflection, guided to the edges and there collected
by small area solar cells. The advantages of such structures over
standard silicon cells are: (1) the slabs can be easily integrated
with buildings, resulting in aesthetically pleasing structure.8 (2)
Strong reduction in the amount of silicon required, as the PV
cells need to cover only the edges.9 (3) The concentration effect
enables the PV cells coupled at the slab edges to operate better
under diffused illumination.10
´
Dipartimento di Scienza dei Materiali and INSTM, Universita di Milano-Bicocca,
via Cozzi 53, 20125, Milano, Italy. E-mail: luca.beverina@unimib.it
† Electronic supplementary information (ESI) available: Synthetic procedures and
1H and 13C NMR spectra for all new compounds. Details of electrochemical,
photochemical and optical characterizations. Pictures of LSC demonstrators. See
DOI: 10.1039/c2cc38708e
Fig. 1 UV-Vis absorption (line) and emission (dots) of: (a) derivative 1, 10À5
CHCl3 solution; (b) derivative 2, 10À5 M CHCl3 solution; (c) derivative 3, 10À5
CHCl3 solution.
M
M
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Table 1 Absorption and emission maxima, Stokes shift and fluorescence quantum
yield for derivatives 1–3 and f205 in CHCl3 solution
ec
Stokes shiftd
Ffe (%)
a
b
labs,max
lem,max
f205
527
528
534
577
535
535
550
674
80 300
80 200
65 700
32 000
35
31
68
99
99
55
70e
1
2
3
309
a
b
Wavelength of absorption band maximum (nm). Wavelength
c
of emission band maximum (nm). Extinction molar coefficient
(l molÀ1 cmÀ1). Stokes shift (meV). Absolute fluorescence quantum
d
e
yield in CHCl3 and in PMMA slab.
Scheme 1 Molecular structure of perylene derivatives 1–3 and of Lumogen f
Orange 240.
high chemical stability, compatibility with the polymeric matrix
and high luminescence efficiency.
Scheme 1 shows the molecular structure of the commercially
available perylene derivative Lumogen f Orange 240 (f240)
considered as the benchmark,18 its fluorinated analog 1 and
the two new unsymmetrical derivatives 2 and 3.
Both 2 and 3 were prepared in order to convey the dipolar
character to the electronic structure of the perylene dye. A high
Stokes shift is commonly observed in dipolar structures, where
there is a major difference between the electronic structure of the
Fig. 2 CV plots (current density vs. potential) of derivatives 1–3 and of Lumogen
f240.
ground and first excited states.19 Also, the presence of the quinoxa- of the electron withdrawing capabilities of the peri-substituents
line residue in 2 and of the dibenzazepine one in 3 was expected to on the perylene core.
provide additional high-energy absorption bands, thus improving
Fig. 2 shows the CV plots of derivatives 1–3 and of Lumogen
the overall photon harvesting efficiency. Derivative 1, never inves- Orange in CH2Cl2 with tetrabutylammonium p-toluenesulfonate
tigated in the context of LSCs, was prepared starting from perylene as the supporting electrolyte. Derivatives 1–2 and f240 traces
dianhydride by a microwave enhanced modification of the proce- all show two reversible reduction waves, corresponding to the
dure reported by Bao and Wu¨rthner et al.20 The synthesis of first (radical anions) and second (dianions) reduction processes.
derivatives 2 and 3 is more complex and thoroughly described in Derivative 1 LUMO, estimated as the half wave potential of the
the ESI.† In short, derivative 2 was prepared by the Suzuki–Miyaura first reversible reduction process, is down shifted by 145 meV with
coupling of the corresponding naphthaleneimide-4-boronic deriva- respect to that of Lumogen Orange. The shift reflects the increase
tive and 3-bromoacenaphtho[1,2-b]quinoxaline, according to the in the peri-substituents electron accepting capabilities due to
method reported by Mu¨llen et al.21 followed by base promoted perfluorination. Conversely, derivative 2 LUMO is up shifted with
ring closure.22 The Buchwald–Hartwig amination of the key respect to Lumogen Orange by 184 meV, in agreement with the
intermediate 9-bromoperylenediimide with commercially avail- milder electron accepting capabilities of the quinoxaline ring with
able dibenzazepine gave derivative 3. We characterized all respect to the diimide residue.
derivatives by UV-Vis absorption and steady state emission as
well as by cyclic voltammetry.
The resulting polarization of ground state electronic struc-
ture effectively reflects in a sizeable increase in the Stokes shift
Fig. 1 shows the absorption and emission spectra of deriva- that goes from 30 to 70 meV. Moreover, the presence of the
tives 1 and 2. The spectra of the fluorinated derivative 1 are quinoxaline ring is associated with the relatively strong absorp-
almost superimposable with those of f240 (see Fig. S1 of the tion in the 300–400 nm region where common perylenediimide
ESI†). Indeed, although extremely efficient in terms of emission dyes do not absorb. It has been shown that the introduction of
(Table 1), derivative 1 and Lumogen are strongly affected by various kinds of donors at the 9 position,23,24 and recently even
reabsorption.
at both 8 and 9 positions,25 of the perylene core of electron
Derivative 2 was designed in order to introduce certain donating substituents, led to the pronounced dipolar character
directional charge transfer character to the HOMO–LUMO and high Stokes shifts. Derivative 3 features the very peculiar
transition of a perylene dye, without compromising the overall donor dibenzazepine (DBA). The central ring of DBA possesses
rigidity required in order to preserve a large emission efficiency. 8p electrons, making it an antiaromatic system according to the
Indeed, the quinoxaline ring is less pronouncedly electron poor Hu¨kel definition.26 As such, DBA behaves like a very strong
than the diimide residue, thus conveying the dipolar character. donor as its corresponding cation is aromatic. Also, DBA is a
Cyclic Voltammetry (CV) can efficiently gauge the overall effect rigid molecule, a particularly helpful feature for the suppression
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luminophore solution without degradation of the emission
properties. Our results show that this new perylene derivative
represents a strong improvement in the state of the art of
luminophores for single layer LSCs.8
Financial Support from the Fondazione Cariplo under the
Grant no. 2010-0564 ‘‘Luminescent Solar Concentrators for
Building Integrated Photovoltaics – LumiPhoto’’ is gratefully
acknowledged.
Fig. 3 Derivative 1 LSC (10 Â 4 cm2) under standard global AM 1.5 solar conditions.
Notes and references
´
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