Highly Efficient Luminescent Solar Concentrators Based on Benzoheterodiazole Dyes with Large Stokes Shifts

Article abstract of DOI:10.1002/chem.202001210

Five extended π-conjugated systems with electron donor (D) and acceptor (A) moieties have been synthesized. Their basic D-A-D structural motif is a benzothiadiazole unit symmetrically equipped with two thiophene rings (S2T). Its variants include 1) the same molecular framework in which sulfur is replaced by selenium (Se2T), also with four thiophene units (Se4T) and 2) a D’-D-A-D system having a N-carbazole donor moiety at one end (CS2T) and a D’-D-A-D-A’ array with a further acceptor carbonyl unit at the other extremity (CS2TCHO). The goal is taking advantage of the intense luminescence and large Stokes shifts of the five molecules for use in luminescent solar concentrators (LSCs). All of them exhibit intense absorption spectra in the UV/Vis region down to 630 nm, which are fully rationalized by DFT. Emission properties have been studied in CH₂Cl₂ (298 and 77 K) as well as in PMMA and PDMS matrices, measuring photoluminescence quantum yields (up to 98 percent) and other key optical parameters. The dye–PMMA systems show performances comparable to the present state-of-the-art, in terms of optical and external quantum efficiencies (OQE=47.6 percent and EQE=31.3 percent, respectively) and flux gain (F=10.3), with geometric gain close to 90. LSC devices have been fabricated and tested in which the five emitters are embedded in PDMS and their wave-guided VIS luminescence feeds crystalline silicon solar cells.

Full text of DOI:10.1002/chem.202001210

Accepted Article
Accepted Article
Title: Highly efficient luminescent solar concentrators based on
benzoheterodiazole dyes with large Stokes’ shifts
Authors: Nicola Armaroli, Sheng Gao, Bamisha Balan, Karuvath
Yoosaf, Filippo Monti, Elisa Bandini, and Andrea Barbieri
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202001210
Link to VoR: https://doi.org/10.1002/chem.202001210
01/2020

10.1002/chem.202001210
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Highly efficient luminescent solar concentrators based on
benzoheterodiazole dyes with large Stokes’ shifts
Sheng Gao,[a] Bamisha Balan,[b,c] Karuvath Yoosaf,*[b,c] Filippo Monti,[a] Elisa Bandini,[a] Andrea Barbieri,*[a] and
Nicola Armaroli*[a]
[a]
S. Gao, Dr. F. Monti, Dr. E. Bandini, Dr. A. Barbieri, Dr. N. Armaroli
Istituto per la Sintesi Organica e la Fotoreattività (ISOF)
Consiglio Nazionale delle Ricerche (CNR)
Via Gobetti 101, 40129 Bologna, Italy
E-mail: andrea.barbieri@isof.cnr.it; nicola.armaroli@isof.cnr.it
B. Balan, Dr. K. Yoosaf
[b]
Photosciences and Photonics Section, Chemical Sciences and Technology Division
CSIR-National Institute for Interdisciplinary Science and Technology
Thiruvananthapuram 695019, Kerala, India
E-mail: yoosafk@niist.res.in
[c]
B. Balan, Dr. K. Yoosaf
Academy of Scientific and Innovative Research (AcSIR)
CSIR-NIIST Campus
Thiruvananthapuram 695019, Kerala, India
Supporting information for this article is given via a link at the end of the document.
Abstract: Five extended p-conjugated systems with electron donor
(D) and acceptor (A) moieties have been synthesized. Their basic D-
A-D structural motif is a benzothiadiazole unit symmetrically equipped
with two thiophene rings (S2T). Its variants include (i) the same
molecular framework where sulfur is replaced by selenium (Se2T),
also with four thiophene units (Se4T) and (ii) a D’-D-A-D system
having a N-carbazole donor moiety at one end (CS2T) and a D’-D-A-
D-A’ array with a further acceptor carbonyl unit at the other extremity
(CS2TCHO). The goal is taking advantage of the intense
luminescence and large Stokes’ shifts of the five molecules for use in
luminescent solar concentrators (LSCs). All of them exhibit intense
absorption spectra in the UV-VIS region down to 630 nm, which are
fully rationalized by DFT. Emission properties have been studied in
CH2Cl2 (298 and 77 K) as well as in PMMA and PDMS matrices,
measuring photoluminescence quantum yields (up to 98%) and other
key optical parameters. The dye-PMMA systems show performances
comparable to the present state-of-the-art, in terms of optical and
external quantum efficiencies (OQE = 47.6% and EQE = 31.3%,
respectively) and flux gain (F = 10.3), with geometric gain close to 90.
LSC devices in which the five emitters are embedded in PDMS and
their waveguided VIS luminescence feeds amorphous silicon cells
have been fabricated and tested.
wavelengths. A fraction of these photons is guided by total internal
reflection towards photovoltaic (PV) cells that are positioned at
the edge of the LSC. Since the edge area is smaller than the
receiving one, it turns out to be possible to concentrate sunlight
without the need for solar tracking. The advantages of LSC-PV
devices include simple fabrication, substantial reduction of PV
surface due to photon concentration, versatile building integration
and low manufacturing costs.[2]
The performance of LSCs is dictated and essentially limited by
four mechanisms: (i) the transmission loss, due to incomplete light
absorption by the fluorophores, (ii) the photoluminescence
quantum yield of the emitter, (iii) the escape cone loss of the
waveguide system, (iv) and the re-absorption of emitted photons.
Transmission and escape cone losses can be addressed,
respectively, by increasing the concentration of the dye and/or the
thickness of the LSC, and by using matrices with high refractive
index.[3] Recently, bright fluorophores with large Stokes’ shift have
raised interest for technological application as active materials in
LSC, which also resulted in the publication of some patents.[4-8]
Introduction
Building-integrated photovoltaics (BIPV) are an attractive solution
for renewable electricity production in urban settings, where
energy demand cannot be met by roof surfaces, due to high
power density requirements.[1] In the area of BIPV, luminescence
solar concentrators (LSC) are a promising and intensively
investigated solution. In these devices, sunlight penetrates the top
surface of an inexpensive and transparent thin plastic plate,
where luminescent molecules (e.g., organic dyes, inorganic
phosphors, quantum dots) are dispersed. Incident solar photons
absorbed by the dyes are then isotropically re-emitted at longer
Chart 1. Molecular structures of the investigated luminescent molecules.
1
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In this work, we deal with transmission, quantum yield and re-
absorption issues by making LSC-PV devices based on bright
benzoheterodiazole dye molecules with large Stokes’ shifts; their
chemical structures are reported in Chart 1. They consist of
extended p-conjugated architectures with donor (D) and acceptor
(A) moieties of varying strengths. S2T is a D-A-D system which
constitutes the basic motif of the series, whilst the other molecules
can be approximated as larger or heavier homologues. Se2T also
exhibits a D-A-D structure, the only difference being the
replacement of S with Se. In Se4T p-conjugation has been
extended with two more additional thiophene units. Finally, CS2T
is obtained by equipping an additional stronger donor moiety (N-
carbazole) at one end of S2T, originating a D’-D-A-D system.
Similarly, functionalization of CS2T with another acceptor unit
(carbonyl group) at the other extremity yields a D’-D-A-D-A’ type
system, CS2TCHO.
The Se analogue of S2T (Se2T) was prepared by mechanical
grinding of o-phenylinediamine with SeO2 followed by
dibromination
and
Stille
coupling
with
2-
(Tributylstannyl)thiophene. Se2T was then converted to Se4T
through its further dibromination and Stille coupling reaction.
Photophysical properties. The photothermal stability of all the
dyes was assessed with UV-VIS absorption spectroscopy in
CH2Cl2 before further characterization. The results show that all
the dyes are stable at room temperature for several days (Figure
S1).
Absorption in solution. Absorption spectra of S2T, Se2T, Se4T,
CS2T and CS2TCHO are reported in Figure 1, and related key
data are collected in Table 1. All samples exhibit strong
absorption in the UV (peaking in the range 308-330 nm) and in
the visible (400-600 nm) region, with molar absorption coefficients
in the range 16-24,000 and 8-20,000 M-1cm-1, respectively (Table
1). For all of the dyes, the wide band in the visible region is entirely
attributable to the S0 ® S1 transition, corresponding to an almost
pure HOMO ® LUMO excitation having a strong charge-transfer
character (Figure S2). In such transition, one electron is promoted
from the oligo-thiophene-phenylene moiety to the central
benzoheterodiazole core (Tables S1a-e). On the other hand, the
higher-energy band envelope around 308-330 nm is attributable
to an admixture of electronic excitations involving both charge-
transfer and locally-excited states (Tables S1a-e). Compared to
the sulfur derivative S2T, the absorption spectrum of the selenium
analogue Se2T is red-shifted. The lowest energy band in the VIS
region is displaced by about 30 nm (1,510 cm-1, i.e. 0.18 eV) to
longer wavelengths (Figure 1). This is a common feature of
benzodiazole derivatives including Se vs. S atoms, and it is mainly
due to the stabilization of the LUMO (centered on
benzoheterodiazole core) caused by the heavier selenium atom
with respect to lighter sulphur (Figure S2), as also described in
the literature.[9-12] In the case of Se4T, which incorporates four
thiophene units, the absorption spectrum moves further to the red,
basically due to the strong destabilization of HOMO and HOMO–
1 caused by the increased delocalization of the oligo-thiophene-
phenylene moiety (Figure S2). This causes a considerable red-
shift of the S0 ® S1 absorption band (1,925 cm-1, i.e. 0.24 eV)with
respect to Se2T. Such experimental evidence is in line with TD-
DFT calculations, which indicates a shift of 2,296 cm–1 (i.e., 0.285
eV as in Tables S1b-c and Figure S3). On the other hand, if a
carbazole moiety is attached to the S2T scaffold, as in CS2T, a
slight reduction in the HOMO–LUMO gap is observed, despite no
direct contribution of the carbazole to these frontier orbitals (i.e.,
carbazole π orbitals mainly contribute to HOMO–1, HOMO–2 and
to LUMO+2, see Figure S2). The addition of an aldehyde terminal
units to CS2T, as in CS2TCHO, further reduces the HOMO–
LUMO gap by 0.15 eV, mainly due to LUMO stabilization (Figure
S2). Accordingly, the absorption spectra of CS2TCHO displays a
slightly red-shifted S0 ® S1 absorption band, compared to CS2T
(Figure 1).[13] Moreover, the presence of the aldehyde group in
CS2TCHO has a strong influence on the intensity of the lower
energy absorption band, whose absorption coefficient is almost
Results and Discussion
Synthesis. Schemes 1 and 2 illustrates the synthetic pathway
adopted for the preparation of the D-A-D molecular systems. The
base unit S2T was obtained by the Stille coupling reaction of the
4,7-dibromobenzo[c][1,2,5]thiadiazole
with
2-
(Tributylstannyl)thiophene. S2T was then brominated by the
reaction with N-bromosuccinimide, followed by the Ullman-type
cross coupling reaction of the monobromo derivative (1) with 9H-
carbazole, affording CS2T. Eventually, the latter was converted to
the D’-D-A-D-A’ system CS2TCHO by formylation.
Br
S
S
(a)
(b)
Br
Br
S
S
N
N
N
N
N
N
S2T
S
S
S
1
(c)
N
N
S
S
(d)
S
S
OHC
N
N
N
N
CS2TCHO
CS2T
S
S
Scheme 1. Synthetic strategy adopted for the preparation of D-A systems S2T,
CS2T and CS2TCHO; (a) 2-(Tributylstannyl)thiophene, Pd(PPh3)4, 90 °C, 24 h,
(b) NBS, 0 °C, 24 h, (c) 9H-carbazole, (+)-trans-1,2- diaminocyclohexane, t-
BuONa, (d) POCl3, DMF, sodium acetate.
NH2
S
(b)
(c)
(a)
Br
Br
S
NH2
N
N
N
N
N
N
Se2
T
Se
Se
Se
2
3
(d)
Br
4
S
S
S
(e)
S
S
S
doubled with respect to the aldehyde-free CS2T analogue (emax
=
Br
N
N
N
N
19,400 vs. 10,500 M-1cm-1, Table 1). These findings are in line
with TD-DFT results, which properly model the gradual decrease
of the S0 ® S1 transition energy along the series S2T > CS2T >
CS2TCHO (Figure S3) and predict the corresponding oscillator
strength to be the highest for CS2TCHO (Tables S1a,d,e).
Se4
T
Se
Se
Scheme 2. Synthetic strategy adopted for the preparation of D-A systems Se2T
and Se4T; (a) SeO2, 75 °C, (b) Br2, Ag2SO4, H2SO4, hr, (c) 2-
(Tributylstannyl)thiophene, Pd(PPh3)4, (d) acetic acid, NBS, (s) 2-
(Tributylstannyl)thiophene, Pd(PPh3)4.
2
2
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plane and the nearby thiophene passes from ≈70° in S0 to ≈40° in
S1 minimum, Figure S5). Because of these excited-state
relaxations, all compounds display remarkable Stokes’ shifts,
ranging from 5,340 to 4,880 cm-1 for S2T, Se2T and Se4T (i.e.,
0.66–0.60 eV) and up to 6,350 cm-1 (i.e., ≈ 0.79 eV) in the
carbazole derivatives CS2T and CS2TCHO. Such results are in
line with TD-DFT calculations, which estimate a larger Stokes’
shift for CS2T and CS2TCHO (i.e., 0.98 ± 0.02 eV), if compared
to the carbazole-free compounds S2T, Se2T and Se4T (i.e., 0.86
± 0.02 eV).
3.0
S2T
Se2T
Se4T
CS2T
2.0
1.0
0.0
CS2TCHO
1.0
298 K
200
300
400
500
(nm)
600
700
λ
0.8
0.6
0.4
0.2
Figure 1. Absorption spectra of the five dyes in CH2Cl2 solution at 298 K.
Table 1. Main absorption parameters in CH2Cl2 solution at 298 K.
l
max, nm (emax, M-1cm-1)
S2T
Se2T
255 (12,350), 308 (22,500), 444 (11,700)
263 (8,100), 316 (24,400), 476 (9,750)
270 (5,700), 352 (16,200), 524 (8,050)
291 (19,350), 318 (18,600), 459 (10,500)
291 (24,100), 333 (21,500), 467 (19,400)
1.0
S2T
Se2T
Se4T
77 K
Se4T
0.8
0.6
0.4
0.2
0.0
CS2T
CS2T
CS2TCHO
CS2TCHO
Emission in solution. The five dyes exhibit intense fluorescence in
CH2Cl2 at 298 K (Figure 2, top and Table 2) and all emission
profiles are broad and unstructured, as expected for charge-
transfer (CT) transitions. The photoluminescence quantum yields
(FPL) are very high for all the luminophores (close to unity for S2T)
and decrease as the emission shifts to lower energies, due the
energy-gap law, but still displaying FPL as high as 0.22 for Se4T
(Table 2), the weakest and most red-shifted emitter of the series.
In line with the absorption behavior, by replacing the sulfur atom
with selenium, the fluorescence spectra are red-shifted (Figure 2,
top),[9, 12] with emission maxima spanning from the visible (e.g.,
582 nm for S2T) to the NIR (i.e., 704 nm for Se4T see Table 2).
Similarly, the addition of the carbazole moiety on the S2T scaffold
– with or without the presence of an extra aldehyde group as in
CS2T and CS2TCHO, respectively – is also able to red-shift the
emission of the related luminophores by approx. 1,900 cm–1 (i.e.,
0.24 eV), but without affecting the nature of the emitting state. As
suggested by TD-DFT calculations, all the molecules of the series
emit from the same excited state, which is also the one
responsible for the lowest energy absorption band (i.e., S1). Upon
excitation, such charge-transfer state undergoes important
structural relaxations, inducing a full planarization of the oligo-
thiophene lateral moiety with respect to the central
benzoheterodiazole core (Figure S4). Therefore, in their fully
relaxed S1 minimum, all the symmetric molecules (i.e., S2T, Se2T
and Se4T) increase their symmetry from the initial C2 point group
to the highly symmetrical C2v. The same relaxation mode also
occurs in the carbazole-equipped molecules (i.e., CS2T and
CS2TCHO), where also the carbazole moiety experiences a
partial flattening (e.g., the dihedral angle between the carbazole
500
600
700
800
λ (nm)
Figure 2. Intensity normalised emission spectra of the five dyes in CH2Cl2
solution at 298 K (top), and in CH3OH:CH2Cl2 1:1 (v/v) glass at 77 K (bottom).
Table 2. Main emission parameters. [a]
298 K [a]
77 K [b]
ems (nm)
l
ems (nm)
FPL
t (ns)
l
t (ns)
S2T
Se2T
582
634
704
648
663
0.92
0.54
0.22
0.58
0.56
13.8
14.3
5.7
562
603
669
594
601
11.6
14.2
7.1
Se4T
CS2T
8.7
8.9
CS2TCHO
6.1
5.8
[a] In CH2Cl2 solution at 298 K. [b] In CH2Cl2:CH3OH (1:1) glass solution.
Low temperature fluorescence spectra were recorded in a
transparent glass mixture (CH3OH:CH2Cl2 1:1 v/v) at 77 K (Figure
3
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2, bottom). All the emission profiles remain broad and
unstructured, displaying a blue-shift of the emission maxima with
respect to those recorded at room temperature. These findings
are a further indication of the predicted charge-transfer character
of the emitting states. Excited state lifetimes (τ) of the five dyes
are not much affected by the temperature and the nature of the
solution, liquid or frozen. This behavior, together with the lifetime
values in the ns range, is indicative of the singlet nature of the
luminescence (Table 2).
Photophysics in PMMA matrix. The photophysical properties of
the five dyes were investigated in PMMA matrix.[14-19] The films
were prepared at three different dye concentrations (0.2%, 0.5%
and 1.0% w/w) from evaporation of CH2Cl2 solutions (see
Experimental part for fabrication details). Figure 3 shows the films
under daylight and UV light; the films exhibit strong fluorescence
and excellent transparency. The thermal stability of the dye-
PMMA films was checked by transmission spectroscopy (Figure
S6); all samples prepared are stable for months.
100
80
60
40
20
0
S2T
Se2T
Se4T
CS2T
CS2TCHO
PMMA
200
300
400
500
(nm)
600
700
λ
Figure 4. Transmittance spectra of the five dyes in PMMA matrix at 298 K (conc.
0.2% w/w).
The fluorescence spectra of dye-PMMA films are displayed in
Figure 5. An apparent shift of the emission maxima to longer
wavelengths in the concentrated samples is observed, due to re-
absorption effects.[20-21] In the case of CS2TCHO, the sample
prepared at the highest concentration (1.0% w/w) displays a
transmittance (hence, transparency) much lower than the others.
This is due to its limited solubility in the matrix, that causes
precipitation of the dye with formation of micro-crystals that
scatter the incoming light. Absolute photoluminescence quantum
yield (FPL) and excited state lifetimes t of all the dye-PMMA films
are gathered in Table 3. The most concentrated samples exhibit
apparently low FPL, which is clearly due to re-absorption, because
excited state lifetimes are not affected. Noteworthy, the relative
decrease in the apparent quantum yield with a 5-times increase
of the concentration of the dye is limited to 12% and 18% for S2T
and CS2T, respectively. Indeed, the two thiadiazole derivatives
display some of the largest Stokes’ shift in the series (5,110 and
5,490 cm-1, for S2T and CS2T).
Figure 3. Pictures of the dye-PMMA films (0.5% w/w) under day (a and b) and
UV light (c).
In Figure 4, the transmission spectra of the dye-PMMA films
(0.2% w/w) are reported. The transmittance outside the
absorption range of the dyes is over 90% (Figure 4 and S7a).
From the measured transmittance and diffuse reflectance spectra
(Figure S7b) it is possible to calculate the absorption factor of the
films, A%:
Table 3. Main photophysical parameters of dyes in PMMA matrix at 298 K.
conc. (%) [a]
l
abs (nm)
l
ems (nm)
F
PL (%) [b]
t (ns)
!% = 100 − '% − (%
where A%, T% and R% are the absorption factor, transmittance
and reflectance, respectively. The relevant spectra are reported
0.2
0.5
1.0
445
447
443
576
585
588
83.2
77.9
68.5
13.4
13.6
13.7
S2T
Se2T
in the ESI (Figure S7c) and the main parameters, lmax and Amax
,
0.2
0.5
1.0
477
476
474
621
630
636
65.4
54.3
47.7
14.5
15.8
15.8
are collected in Table 3, 4 and S2. All spectra show two
absorption bands, one in the UV (280-400 nm) and a second at a
lower energy in the visible (400-760 nm) range with similar
absorption factor. The absorption band maxima are very close to
those already observed in CH2Cl2 solution, indicating that the
electronic properties of the systems are not significantly affected
by the change in the nature of the matrix.
0.2
0.5
1.0
523
523
522
694
705
718
28.6
21.9
16.1
6.0
7.1
6.9
Se4T
0.2
0.5
1.0
458
458
450
612
623
626
76.9
73.2
67.9
10.2
10.5
10.4
CS2T
0.2
0.5
1.0
466
466
472
633
642
733
72.7
64.2
21.1
7.3
7.4
6.3
CS2TCHO
[a] w/w. [b] Absolute quantum yield.
4
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1.0
50
40
30
20
10
50
40
30
20
10
0
S2T
CS2T
Se2T
CS2TCHO
Se4T
S2T
Se2T
Se4T
0.8
0.6
0.4
0.2
0.0
500
600
700
800
λ (nm)
Figure 5. Emission spectra in PMMA matrix at 298 K at different dye
concentration: 0.2% w/w (full lines), 0.5 and 1.0% w/w (dotted lines).
Photoluminescence intensity is proportional to the quantum yield.
Concentration-dependent red-shifts are due to reabsorption effects, which are
maximized when dye loads are higher.
The geometric gain (G) of the devices (dimensions: 25 x 25 x
0.071 mm) is defined as:
* = +,-./
+
0120
0.0
0.2
0.4
0.6
c (%)
0.8
1.0
1.2
where Ssurf and Sedge are the area of the surface and edges of the
film, respectively. For our devices, it turns out to be 88.
The optical quantum efficiency is defined as the ratio between the
edge output and the absorbed photons:
Figure 6. Optical (OQE, bottom) and external (EQE, top) quantum efficiency as
a function of dye concentration.
345 = 60120 678,
where nedge and nabs are the number of photons collected from all
the edges of the devices and absorbed by the sample,
respectively. It has been determined by comparing the absolute
quantum yield of the films inside and in front of an integrating
sphere (see Experimental section for details).
Table 4. Main optical parameters in PMMA matrix at 298 K.
c (%) [a]
Amax (%)
OQE (%) [b]
htrap
EQE (%) [d]
[c]
The flux gain, F, is the geometric gain, G, corrected for the
efficiency losses in the concentrator:
0.2
0.5
1.0
64.3
73.4
76.2
47.6
42.7
35.1
0.572
0.548
0.512
30.6
31.3
26.7
S2T
Se2T
:
1
0120
9 =
=
345×*
:
4
78,
0.2
0.5
1.0
49.3
78.2
84.6
36.2
28.1
25.5
0.554
0.517
0.535
17.9
22.0
21.6
where Iedge and Iabs are the intensity of light output at the edge and
absorbed at the surface, respectively.
The experimental trapping efficiency (htrap) can be calculated from
the ratio between OQE and FPL:
0.2
0.5
1.0
35.9
64.8
87.0
7.4
5.1
1.3
0.259
0.233
0.081
2.7
3.3
1.1
Se4T
=
= 345 ΦAB
>.7?
Finally, the external quantum efficiency EQE is defined as the
ratio between the photons emitted at the edges and the overall
incident photons:
0.2
0.5
1.0
54.6
73.9
77.4
39.5
37.6
35.1
0.514
0.514
0.517
21.6
27.8
27.2
CS2T
545 = 345×!%
0.2
0.5
1.0
65.6
77.7
82.0
37.7
31.2
0.7
0.519
0.486
0.033
24.7
24.2
0.6
CS2TCHO
where A% is the absorption rate in percentage (absorptance or
absorption factor). The values obtained for the dye-PMMA films
at different concentrations are collected in Table 4.
The htrap of the samples containing the dyes S2T, Se2T and CS2T
in the PMMA matrix remains slightly above 0.5 at all
concentrations investigated (Table 4). This is lower than the
theoretical limit of 0.745 for polymeric luminescent solar
concentrators, with refractive index n = 1.5, embedding randomly
oriented chromophores that emit isotropically in the absence of
light-scattering defects.[22] The main reason for this deviation lies
in the non-optical quality of the surface of the film, whose
roughness introduces escape losses in the waveguide.
[a] w/w. [b] Optical quantum efficiency OQE = nedge / nabs. [c] Trapping efficiency
trap = OQE / FPL. [d] External quantum efficiency EQE = OQE·A%.
h
The drop of htrap in the Se4T and CS2TCHO containing samples
at the highest concentration (1% w/w) can be ascribed to the low
solubility of the dye in the polymer matrix. This induces
precipitation of microcrystals, as already commented above, that
behaves as scattering centers, reducing the trapping efficiency.
5
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10.1002/chem.202001210
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The optical quantum efficiency, OQE (Figure 6, top), displays an
almost linear decrease with increasing concentration of the dye.
As the trapping efficiency is not much affected by the
concentration, with the exception discussed above, the variation
in OQE can be attributed to the variation in the measured
apparent quantum yield. This is affected by a re-absorption effect,
as evidenced by the emission spectra in Figure 5. It should be
noted that, despite the non-optimal flatness of the film surface that
severely limits the trapping efficiency of the devices, an OQE as
high as 48% is observed for S2T at 0.2% in PMMA.
Photophysics in PDMS matrix. The dyes can be easily doped into
PDMS matrix to form soft devices.[25-27] In Figure 7 and S10 the
pictures of the five dye-PDMS devices under daylight and UV light
are gathered. These have been prepared in three different dye
concentrations (0.004%, 0.010% and 0.020% w/w). S2T, Se2T
and Se4T are perfectly soluble into PDMS matrix to form the
colored transparent devices, whereas CS2T displays a lower
solubility and forms transparent devices only at the lowest
concentration tested (0.004% w/w). CS2TCHO showed only
limited solubility in PDMS, affording samples of lower quality,
where the dispersion of dye microcrystals is evident.[28]
The decrease in OQE, due to re-absorption, is somehow offset by
the increase in the absorption factor A% by increasing the
concentration of the dye. This results in an external quantum
efficiency, EQE, that maximizes at intermediate values of dye
concentration (Figure 6, bottom). Notably, EQE above 30% at G
≈ 88 is reached for the S2T-PMMA samples. Besides, a flux gain
over 10 has been observed for the same sample (Table S2). This
performance is comparable or exceeds that reported for state-of-
the-art single layer LSCs. For instance, Wong and coworkers
reported few systems comprising highly fluorescent perylene
diimides with simulated OQEs between 45 and 55% and flux
gains around 10 at G ≈ 90.[23] On the other hand, the system with
the record power conversion efficiency (h = 6.8%) reported by
Baldo and coworkers displays higher flux gain and EQE at low G
values.[24] But the performances rapidly drop at higher geometric
gains well below the values observed in the present work.
From the integration of the absorption spectra (Figure S7c) it is
possible to obtain the overall fraction of photons absorbed in the
UV, Vis and UV-Vis range (Table S2). Since OQE is wavelength-
independent, then we can derive the EQE(l) spectra (Figure S8)
from the equation reported above. As expected on the basis of
the absorption spectra (Figure S7c), the main contribution to EQE
is from the UV region. Even for the highest concentration of the
dye analyzed, the situation does not improve significantly, mainly
due to the absence of absorption above 600 nm (Figure S9).
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
λ (nm)
700
800
Figure 8. Normalised absorption factor (A%, full lines) and emission (I, dotted
lines) spectra in PDMS matrix at 298 K (conc. 0.004% w/w). S2T (blue), Se2T
(green), Se4T (red), CS2T (cyan), CS2TCHO (orange).
Table 5. Main photophysical parameters of the dyes in PDMS matrix at 298 K.
c (%) [a]
l
abs (nm)
l
ems (nm)
F
PL (%) [b]
SS (cm-1) [c]
0.004
0.010
0.020
446
446
444
549
553
550
90.8
89.5
81.4
4,210
4,340
4,340
S2T
Se2T
0.004
0.010
0.020
478
478
474
600
601
604
62.0
61.1
54.6
4,250
4,280
4,540
0.004
0.010
0.020
516
513
510
647
642
642
34.7
24.8
19.8
3,920
3,920
4,030
Se4T
0.004
0.010
0.020
458
449
450
588
571
567
79.8
80.0
55.1
4,830
4,760
4,590
CS2T
0.004
0.010
0.020
469
473
470
603
607
601
61.4
44.3
35.0
4,740
4,670
4,640
CS2TCHO
[a] w/w. [b] Absolute quantum yield. [c] Stokes’ shift.
Normalised absorptance (A%) spectra of the dye (0.004 % w/w)
in PDMS matrix are shown in Figure 8 and the relevant
transmittance, reflectance and absorptance spectra are collected
in Figure S12a-c. The transparency of the PDMS device in the
non-absorbing range is again above 90% with the exception of
some concentrated samples exhibiting lower transparency, due to
the presence of dye crystals or nanocrystals inside the PDMS
Figure 7. Pictures of the dye-PDMS devices (0.004% w/w) under day (a) and
UV light (b).
6
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matrix. All systems exhibit good stability within two weeks with the Conclusion
exception of Se4T (Figure S11). Its transmittance changes with
time, probably due to the growth of nanocrystals or aggregates
inside the PDMS matrix, but the emission spectrum and excited
state lifetime do not change significantly (Figure S13).
A series of donor-acceptor benzoheterodiazole derivatives has
been prepared to serve as the dye in luminescent solar
concentrator devices. The selection of emitters with large Stokes’
shift, up to 5,660 cm-1, and high photoluminescence quantum
yield, up to 83.2% in PMMA matrix, has allowed to achieve an
optical quantum efficiency OQE as high as 47.6% at geometric
gain G = 88, with EQE maximizing at the intermediate dye
concentration of 0.5% (w/w). The OQE value can be predictably
enhanced to ≈70% by improving the quality of the PMMA film
surface and, hence, the observed trapping efficiency. Overall, the
performances reached by the best performing systems in the
series are comparable to those reported for benchmark
devices.[23-24, 31]
The fluorescence spectra of S2T, Se2T, Se4T and CS2TCHO at
the three different concentrations tested are virtually overlapping
(Figure S14). Compared with the emission spectra of 0.01% and
0.02% w/w, the diluted sample of CS2T-PDMS (0.004% w/w)
undergoes an apparently large red-shift (Figure S14). This is
because CS2T is completely dissolved into the PDMS matrix at
the low concentration (no crystals inside). Absolute fluorescence
quantum yields and emission lifetimes of these dye-PDMS
devices were collected in Table 5 and S3. The most diluted
samples exhibit the highest quantum yields, and the lifetimes are
also influenced by the PDMS matrix.
The luminescent solar concentrators. Luminescent solar
concentrator devices were assembled starting from the PDMS
matrices, by placing four mini Si-c PV cells at the four edges of
the dye-PDMS devices. Incident photon-to-electron conversion
efficiency (IPCE) spectra (Figure 9 and S15) of the LSCs were
further investigated at three different concentrations (0.004%,
0.010% and 0.020% w/w). Transparent devices of the S2T and
Se2T show a similar trend. The samples with the highest and
lowest concentration display weak IPCE values, whereas
samples with intermediate concentration display the strongest
IPCE values. IPCE in far-red / near-infrared (NIR) region above
600 nm have almost the same values as the reference (PDMS
with no dye). Because of the scattering effects of the dye crystals
in the PDMS devices,[29-30] the IPCE values of the CS2T and
CS2TCHO in the NIR region are higher than the PDMS reference;
only the sample with 0.004% w/w CS2T (transparent) has the
same level as the blank (Figure S15). For Se4T, although the
device looks transparent by naked-eye, the IPCE values of the
concentrated sample (0.020% w/w) in the NIR region are
influenced by scattering effects (Figure S15). This is due to the
formation of Se4T nanocrystals in the PDMS matrix, which also
explains the transmittance increase of the Se4T-PDMS device.
Experimental Section
Materials and methods. The reagents and materials used for the
synthesis were procured from Sigma-Aldrich, Merck, and Spectrochem
Chemical suppliers, and used as received. Air and moisture sensitive
synthetic steps were performed under argon atmosphere. Before Stille’s
coupling, the reaction mixture was made devoid of dissolved oxygen via
freeze-pump-thaw method and low temperature was provided by using
liquid nitrogen (-198 °C).
1H and 13C NMR spectra of the synthesized compounds were recorded on
a Bruker Advance II spectrometer (500 MHz) and tetramethylsilane was
used as the internal standard. The FTIR measurements were carried out
after dispersing samples in KBr and spectra were acquired in diffuse
reflectance mode by using an IR Prestige-21 FTIR-8400s spectrometer
(Shimadzu Corporation, Japan). The final compounds and intermediates
were also characterized through mass spectroscopy using Thermo
Scientific Exactive Orbitrap Spectrometer in electrospray ionization (ESI)
mass spectrometric mode. For determining the melting points, Mel-Temp
II melting point apparatus was employed.
Synthesis of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (S2T).
4,7-Dibromobenzo[c][1,2,5]thiadiazole (0.76 g, 2.6 mmol) and 2-
(Tributylstannyl)thiophene (2.24 g, 6.0 mmol) were dissolved in dry toluene
(44 mL) and degassed with nitrogen flow for 30 minutes. Then Pd(PPh3)4
(72 mg, 0.06 mmol) was added and the reaction mixture was stirred at
90 °C for 24 h. The mixture was then cooled to room temperature and
poured into water, extracted with chloroform, concentrated and dried in
vacuum. Further purification was done through column chromatography
over silica (100-200 mesh) using 1:1 hexane: chloroform mixture as eluent
which yielded S2T as orange solid with 74 % yield. MP: 128-129 °C. ESI-
MS: 299.98, found 300.99 (MH+), 1H NMR (500 MHz, CDCl3) δ (ppm) =
8.12 (d, J = 4.5 Hz, 2H), 7.88 (s, 2H) 7.46 (d, J = 5 Hz, 2H), 7.22 (dd, J =
4.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz), d (ppm) = 152.67, 139.37, 128.04,
127.53, 126.83, 126.03, 125.82; IR FTIR (KBr) u = 3428, 3092, 3011, 1805,
1788, 1578, 1540, 1525, 1483, 1425, 1377, 1356, 1311, 1279, 1234, 1215,
1200, 1178, 1074, 1043, 924, 881, 849, 824, 706, 694, 640, 511 cm-1.
25
S2T
Se2T
Se4T
CS2T
20
15
10
5
CS2TCHO
PDMS
Synthesis
of
4-(5-bromothiophen-2yl)-7-(thiophen-2-yl)benzo[c]
0
[1,2,5]thiadiazole (1). A solution of NBS (0.29 g, 1.6 mmol) in 10 mL DMF
was added drop wise to a solution of S2T (500 mg, 1.7 mmol) in 15 mL
DMF kept at 0 °C and stirred for 24 h. Then the reaction mixture was
extracted with diethyl ether, washed organic phase with water and the
solvent was removed through vacuum evaporation. The crude product
thus obtained was further purified by column chromatography over silica
gel (100-200 mesh) Hexane:Chloroform 4:1 as eluent, and later
recrystallised from ethanol (379 mg, 59% yield). MP: 119-120 °C FTIR
(KBr) u = 3094, 3047, 3010, 2920, 2849, 1541, 1483, 1423, 1379, 1275,
200
400
600
λ (nm)
800
1000
Figure 9. IPCE spectra of the PDMS-based LSCs (dye concentration 0.010%
w/w).
7
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10.1002/chem.202001210
Chemistry - A European Journal
FULL PAPER
1215, 1074, 1043, 970, 920, 881, 825, 797, 702, 690, 670, 640, 600, 565,
509 cm-1.
127.51, 127.40, 127.14, 126.10; ESI-MS found 348.93(MH+), calculated =
347.93. FTIR (KBr) u = 3444, 3105, 3086, 3068, 1583, 1524, 1510, 1470,
1433, 1383, 1350, 1296, 1277, 1213, 1072, 1045, 910, 847, 816, 770, 708,
690.
Synthesis of CS2T. 9H-carbazole (45 mg, 0.27 mmol), compound 1 (100
mg, 0.26 mmol), (+)-trans-1,2- diaminocyclohexane (74 mg, 0.65 mmol), t-
BuONa (0.037 g, 0.39 mmol), and CuI (12.3 mg) were dissolved in 1,4-
dioxane (100 mL). The reaction mixture was refluxed for overnight. The
solvent was removed under reduced pressure and the crude mixture was
purified by column chromatography over silica gel (100-200 mesh) using
hexane as an eluent to get pure product, CS2T. (100 mg, 80 % yield). MP:
175-176 °C. Mass (ESI-MS): 465.04 found, 1H-NMR (500 MHz, CDCl3), δ
(ppm) = 8.2 (d, J = 4 Hz, 1H), 8.16 (d, J = 3.5 Hz, 1H), 8.14 (d, J = 7.5 Hz,
2H), 7.9 (s, , 2H), 7.64 (d, J = 8 Hz, 2H), 7.49-7.46 (m, 4H), 7.35-7.34 (m,
4H). FTIR (KBr) u: 3410, 3300, 3245, , 1534, 1490, 1336, 1260, 1220,
1040, 826, 806, 748 and 702 cm-1.
Synthesis of 4,7-bis(5-bromothiophen-2-yl)benzo[c][1,2,5]selenadia-
zole (4). To a solution of Se2T (1.07 g, 3 mmol) in chloroform (24 mL) and
acetic acid (24 mL), N-bromosuccinimide (1.2 g, 6.7 mmol) was added and
the reaction mixture was stirred under N2 atmosphere for overnight at room
temperature. The red precipitate obtained was filtered and recrystallized
from DMF to yield compound 4 as brown needles (54%); MP: 220.3 °C,
ESI-MS found 503.75 (M+), 505.75 (M++2), calculated =503.75; FTIR (KBr)
u = 3416, 3092, 1745, 1640, 1618, 1475, 1423, 1373, 1308, 1215, 1202,
1097, 1068, 972, 846, 831, 783, 762, 731, 696, 683 cm-1.
Synthesis of Se4T. To a degassed solution of compound 4 (0.61 g, 1.2
mmol) and 2-(Tributylstannyl)thiophene (0.88 g, 2.4 mmol) in 20 mL of
toluene, Pd(PPh3)4 (5 mol percentage) was added and the reaction mixture
was refluxed for 20 h. The reaction mixture was cooled to room
temperature and poured into water, extracted with chloroform, dried,
concentrated in vacuum. Further purification was done by using column
chromatography over silica (100-200 mesh) using 1:4 hexane:chloroform
mixture as eluent. Yield: 19 %; MP: 181.2 °C 1H NMR (CDCl3, 500 MHz),
d (ppm) = 7.95 (d, J = 4 Hz, 2H) 7.8 (s, 2H); 7.3 (d, J = 3.5 Hz, 2H), 7.27
(2H), 7.26 (2H), 7.06 (dd, J = 4 Hz, 2H); 13C NMR (CDCl3, 125 MHz),
158.23, 139.68, 127.73, 127.51, 127.40, 127.14, 126.10; ESI-MS found
511.9 (M+), calculated = 511.9; FTIR (KBr) u = 3070, 1575, 1505, 1497,
1475, 1423, 1373, 1308, 1285, 1215, 1204, 1097, 1068, 972, 847, 831,
814, 804, 795, 783, 762, 731, 696, 682 cm-1.
Synthesis of CST2CHO. Vilsmeir reagent prepared by mixing POCl3
(18.39 mg, 1.2 mmol) and dry DMF (10.3 mg, 1.41 mmol) in dichoroethane
(5 mL) was added to an ice-cold solution of CS2T (200 mg, 0.43 mmol) in
the same solvent (30 mL), heated to reflux for 12 hr, hydrolysed with
saturated aqueous sodium acetate solution (50 mL) for 3 h and extracted
with DCM. The organic phase was collected and dried over anhydrous
Na2SO4. The compound was purified by column chromatography using
silica gel (100-200 mesh) and 1:1 dicholormethane-hexane mixture as
eluent to get red coloured solid. (26 mg, 12% yield). MP: 214-215 °C.
Mass(ESIMS) = 493.04 found, 494.04. 1H-NMR (500 MHz, CDCl3), δ
(ppm) = 10.00 (s, 1H), 8.29 (d, J = 4 Hz, 1H), 8.26 (d, J = 4 Hz, 1H), 8.14
(d, J = 7.5 Hz, 2H), 8.07 (d, J = 8 Hz, 1H), 7.97 (d, J = 7.5 Hz, 1H), 7.88
(d, J = 4 Hz, 1H), 7.65 (d, J = 8 Hz, 2H), 7.498-7.465 (m, 3H), 7.360-7.331
(m, 3H). 13C NMR: 183.05, 152.50, 143.60, 141.59, 136.84, 128.14,
126.43, 125.41, 125.12, 123.78, 120.94, 110.37. IR: 3410, 3230, 2922,
2813, 1665, 1520, 1442, 1337, 1234, 1220, 1067, 873, 848, 810, 755,
728cm-1.
Photophysics. The spectroscopic investigations of the dyes were carried
out in spectrofluorimetric grade dichloromethane (DCM) and methanol
(MeOH)
(Merck
Uvasol®).
A
Perkin-Elmer
Lambda
950
spectrophotometer was used to record the absorption spectra. In order to
carry out photoluminescence experiments, dye solutions were put into
fluorimetric Suprasil quartz gas-tight cuvettes (1 cm) in which oxygen was
removed by bubbling argon for 20 minutes. Uncorrected emission spectra
were recorded with an Edinburgh Instruments FLS920 spectrometer
equipped with a Peltier-cooled Hamamatsu R928P photomultiplier tube
(185-900 nm). Corrected spectra were obtained via a calibration curve
supplied with the instrument. The excitation light source was a 450 W
xenon arc lamp. Photoluminescence quantum yields (FPL) in solution were
determined from the corrected emission spectra, using Coumarin 153 (FPL
= 0.53) and Rhodamine 6G (FPL = 0.91) in ethanol solutions as reference
for S2T, CS2T, CS2TCHO, and Se2T, Se4T, respectively.[32]
Photoluminescence quantum yields were also determined with the
Synthesis of 2,1,3-benzoselenadiazole (2). o-phenylinediamine (1 g, 9.2
mmol, 1eq.) and SeO2 (1.02 g, 9.2 mmol, 1 eq.) were ground in a mortar.
The reaction mixture was heated to 75 °C for 30 minutes and then cooled
to room temperature. The crude product was washed with distilled water
and recrystallized from ethanol to yield compound 2 as brown solid. Yield
75% MP: 74.5 °C; 1H NMR, (CDCl3, 500 MHz), δ (ppm) = 7.8 (d, J = 10
Hz, 2H) 7.47 (d, J = 10 Hz, 2H). ESI-MS found 184.95 (MH+), calculated =
183.95. FTIR (KBr) u = 3082, 3059, 3038, 3009, 2972, 1952, 1933, 1911,
1828, 1805, 1745, 1709, 1610, 1508, 1475, 1437, 1354, 1290, 1219, 1142,
1132, 1076, 974, 949, 852, 798, 750, 743, 712, 596, 555, 489 cm-1.
Synthesis of 4,7-dibromobenzo[c][1,2,5]selenadiazole (3). Bromine
(0.53 mL, 10 mmol) was added to a solution of 2,1,3-benzoselenadiazole
(1 g, 5 mmol) and silver sulphate (1.6 g, 5 mmol) in 11 mL concentrated
H2SO4. The reaction mixture was shaken at room temperature for 2 h. The
precipitate of silver bromide formed was filtered off and the filtrate was
poured into ice-water. The precipitate, from ethyl acetate (500 mL) gave
4,7-dibromo-2,1,3-benzoselenadiazol as golden yellow needles. Then it
was washed with Na2S2O3 to remove excess bromine. Yield 14 %, MP:
263.3 °C; 1H NMR (CDCl3, 500 MHz), δ (ppm) = 7.64 (s, 2H). ESI-MS
found 340.78 (MH+), 342.78 (MH++2), calculated = 339.77; FTIR (KBr) u =
3167, 3070, 3050, 3030, 2952, 2912, 1870,1672, 1585, 1485, 1475, 1355,
1329, 1310, 1076, 945, 925, 914, 837, 797, 764, 750, 578 cm-1
absolute method, using
a barium sulfate-coated integrating sphere
(diameter of 3 in.), elaborating corrected emission spectra.[32] The excited
state lifetimes (τ) were obtained by the time-correlated single photon
counting (TCSPC) technique with an HORIBA Jobin Yvon IBH FluoroHub
equipped with a pulsed NanoLED (λexc = 465 nm) excitation source and a
TBX-05C Picosecond Photon Detection Module (300-850 nm). The
luminescence decay profiles were analyzed with the DAS6 Decay Analysis
Software, and the quality of the fit was assessed with the χ2 value (close
to unity) and with the residuals randomly distributed along the time axis.
Low temperature luminescence spectra (77 K) were recorded by means of
2 mm inner diameter quartz tubes (sample inside) fitted into a special
quartz cold finger Dewar filled with liquid nitrogen. Experimental
uncertainties are estimated to be ±8% for τ determinations, ±20% for FPL
and ±2 nm and ±5 nm for absorption and emission peaks, respectively.
,
Synthesis of Se2T. To a degassed solution of compound 3 (1 g, 2.9 mmol,
1 eq.) and 2-(Tributylstannyl)thiophene (2.1 g, 5. 6 mmol, ~2 eq.), in 20 mL
of dry toluene, Pd(PPh3)4 (5 mol percentage) was added, and the reaction
mixture was refluxed for 20 h. Then it was poured into water, extracted
with chloroform, dried, concentrated in vacuum. Further purification is
done by using column chromatography over silica (100-200 mesh) using
1:3 hexane: chloroform mixture as eluent to obtain Se2T as dark brown
solid with yield 21 %. MP 126.7 °C; 1H NMR (CDCl3, 500 MHz), δ (ppm) =
8.01 (d, J = 3.5 Hz, 2H) 7.8 (s, 2H); 7.47 (d, J = 5 Hz, 2H); 7.2 (dd, J = 4,
4.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz), d (ppm) = 158.23, 139.68, 127.73,
Preparation and optical characterization of dye-PMMA films and dye-
PDMS devices. Poly(methylmethacrylate) (PMMA) with average MW ~
350,000 (by GPC) was obtained from Sigma-Aldrich. Dye-PMMA films
(1.0% w/w, 0.5% w/w and 0.2% w/w) were prepared by dissolving 200 mg
of PMMA powder in 4 mL dye-DCM solutions (0.5 mg/mL, 0.25 mg/mL and
0.1 mg/mL, respectively); solvent was then evaporated overnight in a 5 cm
glass Petri dish. The resulted Dye-PMMA films were separated through
8
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10.1002/chem.202001210
Chemistry - A European Journal
FULL PAPER
sonication from the glass dish and the thickness of the films was measured
Acknowledgements
as 71±4 µm. For the preparation of the dye-PDMS device,
a
polydimethylsiloxane (PDMS) matrix from Dow (SYLGARD 184) was used.
Devices based on PDMS (0.02% w/w, 0.01% w/w and 0.004% w/w) were
fabricated by mixing 6.8 g of PDMS component A (elastomer) and 0.68 g
of PDMS component B (curing agent) with 1.5 mL of dye solution in DCM
(1.0 mg/mL, 0.5 mg/mL and 0.2 mg/mL, respectively) under stirring for 20
minutes in a 10 mL glass vial. After homogenizing the PDMS and dye
solutions, samples were put for 3 days into a 3×3 cm (bottom area) mold
to evaporate the solvents and form the dye-polymer device at room
temperature. Transmission and reflection spectra of dye-PMMA films and
dye-PDMS devices were measured with a Perkin-Elmer Lambda 950
spectrophotometer equipped with 100 mm diffuse reflectance and
transmission integrating sphere accessory, including PMT / PbS detectors.
Absolute fluorescence quantum yields of the dye-PMMA films and dye-
PDMS devices were calculated from the corrected emission spectra using
a barium sulfate-coated integrating sphere (diameter of 3 in.).[32]
We thank CNR (PHEEL) and CSIR network project NWP0054
for Financial Support. The authors thank the China Scholarship
Council for a PhD fellowship to Gao Sheng (file n.
201706870014).
Keywords: Luminescent solar concentrators •
benzoheterodiazole dyes • optical quantum efficiency • external
quantum efficiency • PDMS
[1]
V. Smil, Power Density: A Key to Understanding Energy Sources and
Uses, MIT Press, Cambridge, MA, USA, 2015.
[2]
[3]
M. G. Debije, P. P. C. Verbunt, Adv. Energy Mater. 2012, 2, 12-35.
M. Debije, in Photovoltaic Solar Energy: From Fundamentals to
Applications (Eds.: A. Reinders, P. Verlinden, W. v. Sark, A. Freundlich),
John Wiley & Sons, Chichester, UK, 2017, pp. 420-430.
Fabrication and characterization of LSCs. Edge emission light of the
LSCs was collected through four IXYS KXOB22-12X1F silicon PV cells
(6.5×20 mm) connected in a series/parallel network placed at the four
edges of the 30×30×8.5 mm PDMS matrix. The devices were illuminated
orthogonally at the center of the receiving surface with a monochromatic
beam of 10x5 mm2, avoiding direct irradiation of the side PVs. To
investigate the monochromatic incident photon-to-electron conversion
efficiency (IPCE) of the PDMS-LSCs, a 150 W Xenon arc lamp (LOT Oriel)
equipped with a MSH3101 150 mm monochromator (LOT Oriel) was used
as the light source. The power of the excitation light source at different
wavelengths was measured by a calibrated SM1PD2A silicon photodiode
(Thorlabs), and the current signals were recorded using a PDA200C
photodiode amplifier (Thorlabs). The values of the IPCE (including all
optical losses, PV interface losses and the quantum efficiency of the PV
cell) were calculated from the equation:[16, 33-35]
[4]
[5]
S. Flores Daorta, A. Proto, R. Fusco, L. Claudio Andreani, M. Liscidini,
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