Novel high band gap pendant-borylated carbazole polymers with deep HOMO levels through direct +NB- interaction for organic photovoltaics

Article abstract of DOI:10.1039/c6tc01208f

In this communication, we investigate the direct and still conjugated intramolecular ⁺NB^- interactions in novel high band gap borylated carbazole containing polymers, namely, poly(3,6-(N-di(2,4,6-trimethyl)phenylboryl-carbazole)-alt-

Full text of DOI:10.1039/c6tc01208f

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Jo uP rl ne aa sl eo fd oM na ot et r ai ad l jsu sCt hme amr gi si nt rs y C
View Article Online
DOI: 10.1039/C6TC01208F
Journal Name
ARTICLE
Novel high band gap pendant-borylated carbazole polymers with
+
-
deep HOMO level through direct N=B interaction for organic
photovoltaics
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a,c,d
e
a,b
a,b
Rasmus G. Brandt , Steffen G. Sveegaard , Manjun Xiao , Wei Yue , Zhengkun Du , Meng
DOI: 10.1039/x0xx00000x
e
f
f,g
f
e*
Qiu , Dechan Angmo , Thomas R. Andersen , Frederik C. Krebs , Renqiang Yang , and Donghong
www.rsc.org/
a,b*
Yu
+
-
In this paper, we investigate the direct and still conjugated intramolecular N=B interactions (Scheme 1) in novel high
band gap borylated carbazole containing polymers, namely, poly(3,6-(N-di(2,4,6-trimethyl)phenylboryl-carbazole)-alt-4,8-
di(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene
P(3,6-BCBDT)
and
poly(3,6-(N-di(2,4,6-
trimethyl)phenylboryl-carbazole)-alt-3,3”’-didodecyl-2,2’:5’,2”:5”,2”’-quarter thiophene) (P(3,6-BCQT), which results in
ambipolarity, high electron affinity, and deep HOMO levels. A quasi-donor-acceptor nature of the two polymers was
confirmed by UV-Vis absorption, electro-chemical property study, and computer modelling. Band gaps of 2.07 eV for
P36BCBDT and 2.23 eV for P36BCQT were obtained. P(3,6-BCQT) afforded a power conversion efficiency of 1.44%, with a
J
J
sc of 4.82 mAcm-2, a Voc of 0.79 V and FF of 37%, and (P(3,6-BCBDT) performed better with an efficiency of 3.82%, with a
sc of 8.31 mAcm-2, a Voc of 1.0 V based on it low lying HOMO level, and FF of 45%.
V
OC is inevitable due to a raise of HOMO level of the donor
Introduction
materials, since it is empirically recognized as a correlation
between the HOMO of the donor and the LUMO of the
acceptor. One of the great efforts on keeping low lying HOMO
energy level of the p-type materials for lower band gap is to
introduce boron atom to the polymers thereby maintaining
The rational design and engineering of structural conjugation
in backbones of conjugated polymers for improved light
harvesting, alignment of frontier molecular orbitals, and
charge transportations have been a major focus in the field of
organic photovoltaics (OPVs) in the last decade. For a solar cell
device, it has been commonly accepted that the short-circuit
current density (Jsc) and the open-circuit voltage (Voc) are the
two key factors that play profound impacts on power
conversion efficiency (PCE) of the OPVs. For the former, the
photo current strongly relies on the energy gap between the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO). As the band gap is
narrowing for a better fitting to the solar radiation, a loss of
1
–3
z
high electron affinityboron . Possessing a vacant p orbital
and being able to significantly delocalize when conjugated with
an adjacent organic π-system, such three coordinated boron
moiety becomes electron deficient and a useful π-electron
acceptor, despite of being able to function as an σ-donor4
within B-C bonding due to its less electronegativity when
comparing to carbon. Lying on the ground of recently
5
–7
developed organoboron chemistry
,
boron containing
conjugated polymers has received great attention5,6,8,9
Lorbach et al. presented a diborananthracene polymer given
rise to new luminescent materials. Chujo et al.
.
1
0
1
1,12
introduced
their findings on the photo- and/or electrical–luminescence
obtained from a series of studies on polymers involving
cyclodiborazane, boron-quinolate, boron-diketonates, and
boron dipyrromethene (BODIPY), and attributed the atypical
properties and excellent performance into the conjugation
a.
Department of Chemistry and Bioscience, Aalborg University, Fredrik Bajers Vej
7
H, DK-9220, Aalborg, Denmark
b.
c.
Sino-Danish Center for Education and Research (SDC), Niels Jensens Vej 2, DK-
000, Aarhus, Denmark
8
Danish National Research Foundation: Centre for DNA Nanotechnology,
Department of Chemistry and iNANO, Aarhus University, Gustav Wieds Vej 14,
DK-8000, Aarhus C, Denmark
between the single electron in the p
atom of either vinylene or phenylene and the vacant p
of the boron atom (C-B interactions). Bazan et al.13–15
z
-orbital of the carbon
orbital
d.
e.
Unisense Environment A/S, Tueager 1, DK-8200 Aarhus N, Denmark
CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and
Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
Department of Energy Conversion and Storage, Technical University of Denmark,
Frederiksborgvej 399, DK-4000, Roskilde, Denmark
Centre for Organic Electronics, University of Newcastle, University Drive,
Callaghan, NSW 2308, Australia
z
f.
g.
*
pioneered reversible intermolecular N-B coordination between
N-containing conjugated polymers and tri- fluorinated
phenylborane functioning as strong Lewis acids for fine color
Corresponding authors, Email: yu@bio.aau.dk, yangrq@qibebt.ac.cn Electronic
This journal is © The Royal Society of Chemistry 20xx
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Page 2 of 10
ARTICLE
Journal Name
characterization of two borylated carbazole based copolymers
View Article Online
DOI: 10.1039/C6TC01208F
poly(3,6-(N-di (2,4,6-trimethylphenyl)-boryl-carbazole)-alt-4,8-
di(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-
R
1
B
N
S
B
b′]dithiophene (P(3,6-BCBDT)) and poly(3,6-(N-di-(2,4,6-
trimethylphenyl)-boryl-carbazole)-alt-3,3”’-didodecyl-
,2’:5’,2”:5”,2”’-quaterthiophene (P(3,6-BCQT)), as shown in
N
R2
S
S
S
S
S
S
n
n
R
2
24
S
Scheme 2. The design was based on Taniguchi et al. in order
P(3,6-BCBDT)
P(3,6-BCQT)
1 2
R =Ethylhexyl R =dodecyl
R
1
to enhance the direct N-B interaction at the same time keeping
the conjugation (as illustrated in Scheme 1). Their photo-
physical and electrochemical properties were studied, and the
Scheme
1: Direct intramolecular +N=B- interaction within P(3,6-BCBDT)
(
bottom, left) and P(3,6-BCQT)
tuning in polymer light-emitting diodes (PLEDs). This design PV performance of the spin coated solar cells from them was
idea was further elevated into intramolecular N-B
interactions by the discovery by Liu et al. , replacing C-C unit
also investigated.
2
with a B ← N unit (a coordination between lone pair electrons
from N and a vacant orbital from B) and obtained a
cyclopenta-[2,1-b:3,4-b’]-dithiophene (CPDT) and thieno[3,4-
c]pyrrole-4,6-dione (TPD) donor-acceptor polymer with high
electron affinity and deep HOMO level. To date, very few
reports on boron containing materials have been tested for
their PV properties. All devices were prepared with BODIPY
based small molecules having a PCE ranging from 1.3% to 4.7%
Experimental
Materials and syntheses
All chemicals used were of commercial grade from Sigma-
Aldrich and used without any further treatment, otherwise
noted. 2,6-bis(trimethyltin)-4,8-di(5-(2-ethylhexyl)thiophene-
2
-yl)benzo[1,2-b:4,5-b′]dithiophene was purchased from
Sunatech, China. 3,3”’-didodecyl-2,2’:5’,2”:5”,2”’-
quaterthiophene (QT) was synthesized according to
1
6–21
and one BODIPY-based polymer with a PCE of 2.0%.
Best
performance was obtained on devices prepared by Thompson
2
5,26
literature
; and was obtained as yellow crystals. Melting
2
2
et al. containing a benzannulated BODIPY which reached a
PCE of 4.5%. A very recent report from Liu et al. pronounced a
new double B<-N bridge bypyridyl, as electron deficient
building block for a non-fullerene acceptor, yield a PCE of
point: 59.5 °C and decomposition temperature: 342.1°C
1
(Figure S1 in in electronic supplementary information (ESI)). H
NMR (600 MHz, CDCl
3
, δ, ppm, as shown in Figure S2, in ESI):
.18 (d, 2H), 7.13 (d, 2H), 7.02 (d, 2H), 6.95 (d, 2H), 2.78 (t, 4H),
.65 (m, 4H), 1.22-1.41 (m, 36 H), 0.87 (t, 6H). Molecular
24: 666.34; MALDI-TOF MS found:
7
1
2
3
3
.38% when paired with PTB7 as a donor material. However,
in all of the above-introduced boron materials, the conjugative
nature of the B atom must have been destroyed by converting
weight calcd. for C40
66.36 (Figure S3, ESI)
,6-dibromo-N-(di(2,4,6-trimethyl)phenylboryl-carbazole (3,6-BC)
58
H S
6
2
3
sp into sp hybridization with its tetrahedral configuration in
3
2
order to form the new σ-bonding between the sp lone pair of
electrons of N and vacant sp hybridized orbital of B due to the
In a dry two-necked flask, magnesium turnings (1.38 g, 56.7
mmol, 2.84 eq.) was added and activated with a small piece of
iodine crystal, followed by addition of dry THF (50.0 mL) and 2-
bromomesitylene (8.50 mL, 11.1 g, 55.5 mmol, 2.78 eq.) under
argon atmosphere. The reaction was initiated and maintained
at gentle reflux until all magnesium was consumed. Boron
trifluoride diethyletherate (2.80 mL, 3.22 g, 22.7 mmol, 1.14
3
demanding criteria on both symmetry and the overlapping of
atomic orbitals for forming MOs. The formation of such a four-
coordinated borate species will no longer participate
effectively in conjugation with adjacent π-systems.
In this paper we present the design, synthesis and
H
N
Br
Mes
Mes
B
N
O
Mg/THF
reflux
Br
BF3
Br
3
,6-BC
reflux
n-BuLi
/
-78 o
C
Br
(
Br-Mes)
Br
R1
Mes
Mes
B
N
R1
Mes
Mes
B
S
N
Pd2dba3
o-tol
S
S
P(3,6-BCBDT)
Sn
+
Br
Sn
S
S
Tolune
S
Br
S
S
n
R1
S
R1
Mes
Mes
B
Mes Mes
B
R2
R2
S
N
TMPMgClLiCl
Ni(dppp)Cl2
N
S
+
S
n
S
S
S
S
Br
R2
Br
R2
P(3,6-BCQT)
R : ethylhexyl, R : dodecyl
1
2
Scheme 2: Chemical structure and synthesis of P(3,6-BCBDT) and P(3,6-BCQT
2
| J. Name., 2012, 00, 1-3
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Jo uP rl ne aa sl eo fd oM na ot et r ai ad l jsu sCt hme amr gi si nt rs y C
Journal Name
ARTICLE
eq.) was added dropwise and the solution was refluxed for (m, 4H), 7.06 (s, 2H), 6.91-6.99 (m, 4H), 6.71 (s, 2VHie)w, A2r.ti1cl1e-O2n.l8in5e
three hours and then cooled to room temperature, yielding a (m, 18H), 1.28-1.68 (m, 44H), 0.91 (t, 6H).DOI: 10.1039/C6TC01208F
white precipitate. 6.51 g 3,6-dibromocarbazole (20.0 mmol,
1
.00 eq.) was dissolved in 50.0 mL of dry THF under argon
atmosphere and cooled to -78 °C. n-BuLi in hexane (8.50 mL,
.50 M, 21.3 mmol, 1.06 eq.) was added dropwise to the Measurement
2
carbazole and kept stirring for 2.5 hours. Afterwards the
above-prepared dimesitylboron fluoride solution (cooled to -
1H NMR spectra were recorded on a Bruker Avance III 600 MHz
spectrometer. UV-vis absorption spectra were measured on a
Varian Cary 50 UV/vis spectrophotometer. Size exclusion
chromatography (SEC) was conducted at 35 °C with tetrahydrofuran
78 °C) was added under vigorous stirring. The resulting mixture
was extracted with pentane and dried in vacuo. The obtained
solid was recrystallized from ethyl acetate, yielding 3,6-
(
THF) as an eluent against polystyrene standards. The used column
dibromo-N-di-(2,4,6-trimethylphenyl)-boryl-carbazole 5.58
g
4
was a Phenomenex Phenogel™ 10 Å, 4.6 mm, range 5k-500k Da,
(9.73 mmol, 48.7 %) as snow white, glinting crystals. Melting
w
the molecular weight (M ) and polydispersity index (PDI) was
point: 244.4 °C and decomposition temperature: 335.2°C
determined by Dionex Chromeleon™ software 6.8. Differential
scanning calorimetry (DSC) and thermogravimetric analyses (TGA)
were carried out on a Perkin-Elmer STA 6000 under nitrogen at a
1
(
Figure S1, ESI). H NMR (600 MHz, CDCl
3
, δ, ppm (Figure S2,
ESI)): 8.06 (s, 2H), 7.22(d, 2H), 6.84 (s, 4H), 6.77 (d, 2H), 2.35 (s,
1
1
13
6
(
1
H), 2.01 (s, 12H). B NMR (CDCl
CDCl , δ, ppm): 142.41, 141.14, 139.93, 129,90, 129,10,
28.87, 122.66, 117.15, 116.24, 22.02, 21.47. Molecular
N: 571.07; MALDI-TOF MS found:
3
, δ, ppm): 58.80. C NMR
-
1
heating rate of 20 °Cmin . Matrix assisted laser
desorption/ionisation time of flight mass spectrometry (MALDI-TOF
MS) analyses were carried out on a ReflexTMIII from Bruker,
Germany. All samples were measured in reflector mode. Samples
were applied to a target plate coated with nitrocellulose, also called
a FENC layer. Surface roughness and morphology of thin films were
characterized by atomic force microscopy (AFM) on an Agilent
3
weight calcd. for C30
72.03 (Figure S3, ESI).
Poly(3,6-dibromo-N-di(2,4,6-trimethyl)phenylboryl-carbazole-alt-
,8-di(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-
H28BBr
2
5
4
b′]dithiophene) P(3,6-BCBDT) A mixture of 3,6-dibromo-N-
di(2,4,6-trimethylphenyl)boryl-carbazole (50 mg, 87µmol, 1.00
eq), 2,6-bis(trimethyltin)-4,8-di(5-(2-ethylhexyl)thiophene-2-
yl)benzo[1,2-b:4,5-b′]dithiophene (79 mg, 87 µmol 1.00eq),
5
400. Cyclic voltammetry (CV) was performed by using a CHI 660D
System with a three-electrode cell in a 0.10 M solution of
tetrabutylammonium hexafluorophosphate (TBAPF ) in acetonitrile
6
-1
as the electrolyte at a scan rate of 100 mVs , the electrodes
tris(dibenzylideneacetone)dipalladium (Pd2dba3, 2 mg, 1.75 consisted of a glassy carbon electrode (working electrode) on which
µmol, 2 mol%), and tri-o-tolylphosphine (P(o-Tol)3, 14 mg, 13 chloroform solutions of polymers were drop-casted, a platinum
µmol, 14 mol%) was degassed twice via vacuum and then wire (counter electrode), and a saturated calomel reference
consecutive nitrogen atmosphere. The mixture was then electrode. The film thickness was measured by a Veeco Dektak 150
dissolved in toluene (15 mL), degassed and heated to 120 ᵒC surface profiler. The potential was calibrated against the
+
ferrocene/ferrocenium redox couple (Fc/Fc , 4.8 eV below the
for 48 h under nitrogen atmosphere. The product was then
precipitated in methanol. The obtained polymer was further
purified via two consecutive precipitations from chloroform,
affording final product P(3,6-BCBDT)as a yellow solid (70 mg).
GPC: Mw = 10,130 Da, PDI = 1.54. 1H NMR (600 MHz, CDCl3, δ,
ppm): 7.90-8.05 (s, 2H), 7.62-7.75 (m, 4H), 7.34 (d, 2H), 7.17 (s,
vacuum level). The highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) were estimated by
onset
the equations: HOMO =-(4.80 + E
ox ), and LUMO = HOMO +
opt
E
bg
.
Solar cells fabrication and characterization
2H), 7.07 (s, 2H), 7.00 (s, 2H), 6.77-6.83 (m, 2H), 2.96-3.02 (d,
4H), 2.15-2.50 (m, 16H), 1.60-1.81 (m, 2H), 1.30-1.50 (m, 18H),
0.96-1.05 (m, 12H).
Photovoltaic devices were fabricated on 15 mm × 15 mm pre-
patterned indium tin oxide (ITO) coated glass substrates with a
layered structure of ITO/PEDOT:PSS/polymer:PC71BM blend
Poly(3,6-dibromo-N-di(2,4,6-trimethyl)phenylboryl-
carbazole-alt-3,3”’-didodecyl-2,2’:5’,2”:5”,2”’
quaterthiophene) P(3,6-BCQT) QT (300.1 mg, 0.450 mmol,
(∼80-100 nm)/Ca(10 nm)/Al(100 nm). The ITO coated glass
substrates were cleaned subsequently in acetone, toluene,
methanol, and isopropyl alcohol in an ultrasonic bath. The
substrates were then oxygen plasma treated for 20 min, spin
coated with PEDOT:PSS at 5000 rpm, and dried at 120 °C for 20
min. The polymer:PC71BM blend was spin coated with a layer
thickness of 80-100 nm from dichlorobenzensolution with a
total dry content of 30 mg mL . Subsequently, the metal
electrodes were thermally evaporated on top of the active
layer, firstly the Ca layer (10 nm) then the Al electrode
~100nm). The current density-voltage (J-V) characteristics
were recorded with a Keithley 2420 sourcemeter under
simulated 100 mWcm (AM 1.5G) irradiation from a Newport
solar simulator. The external quantum efficiency (EQE) was
1
0
.00 eq.), 3,6-dibromo-N-(dimesitylboryl)carbazole (57.7 mg,
.450 mmol, 1.00 eq.), and Ni(dppp)Cl (2.50 mg , 4.61 µmol, 1
2
mol%) were added into dry THF (20.0 mL) under argon
atmosphere. TMPMgCl · LiCl (1.10 mL, 0.9 M, 0.990 mmol, 2.20
eq.) was added dropwise and the solution was refluxed for 48
hours. The solution was poured into 100 mL methanol and the
precipitated red solid was filtered and washed with methanol.
The solid was dried in vacuo at 50 °C. The obtained product
P(3,6-BCQT) was a deep red solid with a yield of 59% (169
-1
(
1
mg). GPC: Mw = 11416 Da, PDI = 2.04. H NMR (600 MHz,
-2
3
CDCl , δ, ppm): 7.82-7.91 (m, 2H), 7.39-7.51 (m, 2H), 7.10-7.22
This journal is © The Royal Society of Chemistry 20xx
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ARTICLE
Journal Name
conducted by means of a source meter, silicon photodiode,
and a computer-controlled light source-monochromator-lock-
in system. The space-charge-limited current (SCLC)
measurements for electro- and hole-mobility were performed
View Article Online
DOI: 10.1039/C6TC01208F
on
hole only device (ITO/PEDOT:PSS/active layer/MoO/Ag)
and the electron only device (ITO/ZnO/active layer/Ca/Al).
P(3,6-BCQT) - solution
P(3,6-BCQT) - film
P(3,6-BCBDT) - solution
P(3,6-BCBDT) - film
1
,0
,8
0
Results and discussion
Synthesis and characterization
0,6
The synthetic routes to the monomer 3,6-BC and polymers are
outlined in Scheme 2. 3,6-BC was obtained from the reaction
between n-BuLi lithiated 3,6-dibromocarbazole in THF and the
freshly prepared di(2,4,6-trimethyl)phenylboryl fluoride
0
,4
0,2
0,0
(dimesitylboryl fluoride, synthesized from the fresh Grignard
reagent of 2-bromomesitylene reacting with boron trifluoride)
at -78°C. As can be seen in Figure S1 (ESI), the differential
scanning calorimetry (DSC) studies combined with
simultaneous thermal gravimetric analysis (TGA) on the
obtained snow-white crystals, pronounces a distinct melting
point of 244.4 °C without any mass loss. While another peak
400
600
800
Wavelength (nm)
Figure 1: The UV-Vis absorbance spectra of P(3,6-BCBDT) and P(3,6-BCQT) in
both CHCl solutions and films.
3
Photophysical properties
temperature of 380.4 °C presents its complete chemical The UV-vis absorbance spectra of P(3,6-BCQT) and P(3,6-
decomposition corresponding to an almost 100% mass loss of BCBDT) in both solution and film can be seen in Figure 1. It
the monomer 3,6-BC, indicating the high purity and high should be noted that the absorbance of both polymers in going
thermal stability of this small molecule monomer prior to the from solution to film exhibit bathochromic phenomena
polymerization. From NMR studies, as found in the ranging from 9 nm for P(3,6-BCBDT) to 28 nm for P(3,6-BCQT),
experimental section, the molecular structure of the monomer because the local planar configurations of the polymer
3,6-BC has been proven by: a) The co-existence of mesitylene backbones facilitate their interactions (and also charge
protons (with their chemical shifts at 6.84 (s, 4H), 2.35 (s, 6H), transport) in the solid state. Meanwhile such relatively short
and 2.91 (s, 12H)) and the rest of the protons from carbazole shifts suggest the strong sterically hindered effects on the π-π
moiety (with their right chemicals shifts and integrals); b) The stacking in solid states caused by the bulky mesitylene groups
disappearance of secondary amine protons (H-N(Ar)
starting material of 3,6-dibromo-9H-carbazole measured in from 300 to 400 nm for P(3,6-BCQT) and P(3,6-BCBDT) in both
CDCl , which would otherwise be seen around 8.10 ppm (s, solutions and films are assigned to n-π* and π-π* transitions.
H); C) The found resonance of boron (58.80 ppm) in the Such transitions (pronounced by the low absorptions in this
2
) in the around B atom. Weak absorption bands observed in the range
3
1
1
1
compound when probing B. Even though the carbon atom region) could be limited due to the delocalization of the lone-
directly attached to boron was hidden in C spectrum due to pair electrons at N atom to the vacant π orbital of B atom. The
1
3
the rapid quadrupolar relaxation, we can firmly confirm the peak absorption of the lower energy band at 437 nm for P(3,6-
succeeded synthesis of this target monomer.
The two polymers of P(3,6-BCBDT) and P(3,6-BCQT) were (poly(carbazole-quarterthiophene)) as reported by Lim et. al.27
BCQT) in our work correlates well to that at 447nm for PCzT4
.
synthesized by standard Pd catalyzed Stille coupling Furthermore, all lower energy absorptions at 437nm and 465
polymerization of the monomer 3,6-dibromo-N-(di(2,4,6- nm for P(3,6-BCQT) in solutions and films and at 472 nm and
trimethyl)phenylboryl-carbazole
(3,6-BC)
with
2,6- 480 nm for P(3,6-BCBDT) in solutions and films pronounce
bis(trimethyltin)-4,8-di(5-(2-ethylhexyl)thiophene-2-
yl)benzo[1,2-b:4,5-b′]dithiophene and Kumada cross-coupling atom, resulting an ambipolar
effective intrachain charge transfers (ICT) from N atom to B
+
-
N=B moiety through such a
of the Grignard reagent of QT on 5,5’’’-positions made by direct/still conjugated N-B interaction. Thus the electrical
TMPMgCl·LiCl with 3,6-BC at the presence of nickel catalyst, properties of P(3,6-BCQT) and P(3,6-BCBDT) turn into quasi
respectively. The correct backbone structure of both P(3,6- donor-acceptor materials instead of being major donor
BCBDT) and P(3,6-BCQT) was also evidenced by the chemical molecules like polythiophene/polycarbazole/polyBDTs. Their
1
shifts of all the protons in their H-NMR data (in the optical band gaps were calculated respectively as 2.2 eV and
experimental section), while the broadening of the aliphatic 2.07 eV from their absorption onset at 557 nm and 600nm in
hydrogens suggested macromolecular long-chain like structure films, which are lower than any of the three types of
due to the conformational isomerization of the methylene homopolymers of thiophene (2.3eV)28, 3,6-carbazole ( 3.23
hydrogen atoms caused by the energy-barrier limited free eV)29, and BDT (2.13eV)30, suggesting at least a pseudo donor-
rotation of C-C bonds in the aliphatic segments.
acceptor nature from less electron efficient property from
borylated carbazole caused by direct N-B electron interactions.
4
| J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/C6TC01208F
HOMO
LUMO
P(3,6-
BCBDT)
P(3,6-
BCQT)
Figure 2: HOMO and LUMO of the geometrically optimized structure of P(3,6-BCBDT) and P(3,6-BCQT) at the B3LYP/6-361g (d,p) level.
Molecular simulations
P(3,6-BCQT) are mainly distributed in the donor units, BDT and
QT. While LUMOs were expanded into B atom in carbazole
moiety (also proved by reports from Jäkle et. al and Yuan et.
Molecular simulations (density-functional theory (DFT)
calculations) on the model compound of 3,6-BCBDT and 3,6-
BCQT were conducted at the B3LYP level using the basis set 6-
+
- 33,34
al.), i.e., forming the quinoid form N=B .
This furthermore
supports our discussion on their optical absorptions regarding
the charge transfer from N-B, indicating a less electron rich
property of BC unit, which also improve the electron affinity of
the orbitals, and leads to possible deeper HOMO level (as will
be presented further): the experimental -5.56 eV and -5.52 eV
for the HOMO of P(3,6-BCBDT) and P(3,6-BCQT) are all deeper
3
1G(d,p), with Gaussian 09.D01 and visualized by Gaussian
Viewer 5 (GW5). The geometrically optimized structures are
illustrated in Figure 2. The planarity of the backbone in both
P(3,6-BCBDT) and P(3,6-BCQT) is evidenced, as the large
dihedral angle between carbazole unit and BDT and QT , being
ᵒ
ᵒ
25.00 and 25.05 , respectively, account for the twisting and
2
7
than -5.22 eV for poly(carbazole-QT) and -5.44 eV for
certain coil-like polymer conformation, and furthermore lead
3
0
3
1
polyBDT.
It is the direct conjugated N-B interaction
to enhanced torsional relaxation along the backbone. This
could be part of the reason for the relatively small red-shift (9
nm) of absorption maxima between solution and films for
P(3,6-BDBDT), as discussed above. Furthermore, the dihedral
angles of 55 between mesityl units and the carbazole moiety
prevent the overall planarity of the polymer. This is expected
to have a negative influence on the band gap properties, as
Table 1: Theoretical calculation of frontier orbitals calculated at the level of
B3LYP using the basis set 6-31G(d,p)
ᵒ
Sample name
P(3,6-BCBDT)
P(3,6-BCQT)
HOMOtheo
-5.08
-4.83
LUMOtheo
-1.61
-1.92
Ebg-theo
3.48
2.92
the spatial steric effect will affect the π-π stacking. It is promoting low lying HOMO orbital level that determines a high
important to mention that all side-chain substituents were
oc of 1.00 V (be presented below) for solar cell made from
replaced with a methyl group in order to simplify the P(3,6-BCBDT) and moderately high value of 0.80 V for P(3,6-
V
3
4
calculations, nevertheless, the side chains typically play an BCQT).
important role in modeling torsion angles of conjugated Electrochemical properties
3
2
polymers. Herein we used the same simplified side group for
both BDT and QT moieties and therefore the results regarding
the torsion angles between BC and BDT/QT should be
reasonable. When observing the orbital distribution (Figure 2),
it becomes evident that HOMOs of both P(3,6-BCBDT) and
The electrochemical characteristics of P(3,6-BCBDT) and P(3,6-
BCQT) were studied by cyclic voltammetry. The HOMO and
LUMO levels, together with the electrochemical band gaps
푒푐푐
(
퐸
) are listed in Table 2. The experimental results have the
푔
Table 2: Electro and photo physical parameters of the P(3,6-BCQT) and P(3,6-BCBDT)
Sample name
Thickness
λ
onset
Eoptbg
[eV]b
2.23
2.07
E
oxonset
E
HOMO
E
LUMO
µ
electron
µ
hole
nm]a
85
[nm]
557
600
[V]c
[eV]d
-5.56
-5.52
[eV] e
-3.33
-3.45
[10 cm V s ]
1.72
-4
2
-1 -1
f
[10 cm V s ]
1.79*10-2
5.33
-4
2
-1 -1 g
[
P(3,6-BCQT)
P(3,6-BCBDT)
0.76
0.72
100
1.54
a: determined by a Dektak profilometer b: calculated using Ebg=1240/λonset c: determined from CV curve d: EHOMO=-(Eoxonset+4.8 eV) e: LUMO = HOMO + Eoptbg f: measured on
electron only device (see experimental) g: measured on hole only device (see experimental)
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-
2
same tendency as the modulated above (Table 1), the (polymer to PC71BM), with a VOC of 1.00 V, a JSC of 8.31 mAcm , and
View Article Online
oxidation onset of P(3,6-BCBDT) and P(3,6-BCQT) afford a a FF of 45 % affording a total power conversDioOnI:e10ff.i1c0ie39n/cCy6oTfC301.8220%8F.
HOMO level of -5.56 eV and -5.52 eV, respectively. Such deep The optimized weight ratio of P(3,6-BCQT) was also 1:3 (P(3,6-
HOMO levels imply that introduction of direct N-B interaction BCQT) to PC71BM), having a VOC of 0.79 V combined with a JSC of
5
0
5
5
0
5
55
P(3,6-BCQT) 1:1
P(3,6-BCQT) 1:2
P(3,6-BCQT) 1:3
P(3,6-BCQT) 1:4
P(3,6-BCBDT) 1:1
P(3,6-BCBDT) 1:2
P(3,6-BCBDT) 1:3
P(3,6-BCBDT) 1:4
P(3,6 -BCQT)
P(3,6 -BCBDT)
50
45
40
35
30
25
20
15
10
5
0
-
-
-
10
0.0
0.5
1.0
0.0
0.5
1.0
400
600
800
Potential (V)
Potential (V)
Wavelength (nm)
Figure 3: The Performance of P(3,6-BCQT) (left) and P(3,6-BCBDT) (middle) at four different ratios between donor (P(3,6-BCQT) and P(3,6-BCBDT), respectively)
and acceptor (PC71BM): 1:1 (black), 1:2 (red), 1:3 (green), and 1:4 (blue). The right figure shows external quantum efficiencies (EQE) of the best performing devices
for both P(3,6-BCQT) (black) and P(3,6-BCBDT) (red) with the ratio 1:3 (D:A)
-
2
endow both P(3,6-BCBDT) and P(3,6-BCQT) with high electron 4.82 mAcm and a FF of 37 % affording a total efficiency of 1.44%.
affinity, and also show high oxidation stability which is For P(3,6-BCBDT, the JSC values of the devices increased from 1.05
-
2
-2
desirable for the application in devices. Combining the HOMO mA cm at 1:1 blend ratio to 5.42 mA cm at a 1:2 ratio, and to
-
2
levels with the E
affords a LUMO level of -3.33eV for P(3,6-BCQT) and a LUMO upon increasing the PC71BM concentration in the blend for both
level of -3.45 eV for P(3,6-BCBDT). P(3,6-BCBDT) and P(BCQT) was related to the increase of total
g
determined from the absorbance curves, 8.31 mA cm at a 1:3 ratio. Such enhancements of the photocurrent
3
7
The high band gap, 2.23 eV for P(3,6-BCBDT) and 2.07 eV for interfacial area at the heterojunction for more sufficient exciton
P(3,6-BCQT), and such deep HOMOs indicate, ambipolar dissociations and possibly charge transport properties as well.
characteristic of 3,6-BC, which prevents the narrowing of the Therefore it is notable that both materials showed a significantly
bandgap normally associated with presence of both a donor lower efficiency when the ratio between donor polymer (P(3,6-
and acceptor moiety in the backbone. Comparing the HOMO BCBDT) and P(3,6-BCQT)) and acceptor (PC71BM) was 1:1 and 1:2,
level of the polymer containing BDT moiety with polymers this could be an indication of a low exciton mobility as the efficiency
containing a similar moiety, this ambipolar characteristics are increases significantly with the concentration of acceptor. In
again evident, as the HOMO level is expected to be around - particular, such remarkably high VOC of 1.00 V obtained for P(3,6-
3
5
4
.95 to -5.30 (reported by Hou et al. ) when comparing to BCBDT): PC71BM (1:3, w/w) should benefit from the deep HOMO
large array of polymers, but combined with the 3,6-BC unit this energy level of P(3,6-BCBDT) that was caused by direct N-B
is lowered by 0.25 eV. The HOMO energy levels calculated for interaction in the 3,6-BC moieties as above-discussed. To the best
P(3,6-BCBDT) and P(3,6-BCQT) are -5.08 eV and -4.83 eV, of our knowledge, this is one of the highest VOC values reported to
3
0,38–
which are 0.48 eV and 0.69 eV higher than the values from date for high-performance polymer:PCBM-based BHJ-type PSCs
electrochemical measurements. We speculate that this
4
2
. The remarkable high efficiency of both P(3,6-BCBDT) and P(3,6-
mismatch might derive from both the lack of consideration for BCQT) at 3.82 % and 1.44 %, respectively, is especially interesting
the π-π stacking of the polymers, and also to some extent an when considering the unfavorable absorbance spectrum (Figure 1)
under estimation of the acceptor ability of the N-B bond.
Photovoltaic performance
and band gap of 2.23 eV and 2.07eV, respectively. Therefore an EQE
measurement was conducted (Figure 3, right), showing a maximum
performance of 53 % for P(3,6-BCBDT) and 27 % for P(3,6-BCQT).
The EQE has an onset of approximately 675 nm, which falls in line
with the absorbance spectrum (Figure 1). This shows that despite
high bandgap the morphology and charge separation of the
polymers are of an utmost importance. The performance of the
P(3,6-BCQT) is also superior to the unborylated (N-alkylated)
BHJ OPVs were fabricated using P(3,6-BCBDT) or P(3,6-BCQT)
as donor in different ratios with PC71BM as the acceptor. The
device structure applied was ITO/PEDOT:PSS/Polymer: PC71BM
/
Ca/Al. PC71BM was chosen as electron acceptor instead of
PC61BM due to its better light absorption in the visible range.3
J-V curves for the both P(3,6-BCBDT) and
6
27
carbazole-QT polymer reported by Lim et al. , having a PCE of
.40%, which is only around one third of the value reported in this
P(3,6-BCQT) can be seen in Figure 3 (left and middle) and the PV
performance data are summarized in Table 3. For P(3,6-BCBDT) the
best performance obtained was with the weight ratio of 1:3
0
work. Remarkable point is it that Lim et al. also found an optimal
D:A ratio of 1:3 which is consistent with findings of this work.
Table 3: Photovoltaic performance of P(3,6-BCBDT) and P(3,6-BCQT)
-2
Sample name
P(3,6-BCBDT)
Composition[D:A]
1:1
J
sc[mA cm ]
1.05
5.42
8.31
7.76
1.46
3.59
4.82
4.87
V
oc[V]
FF[%]
22
33
45
44
PCE [%]
0.21
1.87
3.82
3.32
0.30
0.98
1.44
0.87
1.00
1.00
0.98
0.66
0.75
0.79
0.79
1
1
1
:2
:3
:4
P(3,6-BCQT)
1:1
31
36
37
36
1
1
1
:2
:3
:4
6
| J. Name., 2012, 00, 1-3
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1.41
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DOI: 10.1039/C6TC01208F
Figure 4: The AFM Tapping mode topography (a,d), phase (b,e) images (4 µm x 4 µm), and TEM image (c,f) of the polymer blends 1:3 P(3,6-BDT):PC71BM (a,b and c)
and 1:3 P(3,6-BCQT):PC71BM (d,e and f)
Free charge carrier mobility
a Rsq of 0.75 nm, while P(3,6-BCQT) showed a Rsq of 0.45 nm.
To assess the quality of the synthesized material and to explain The AFM pictures (Figure 4, a. and d. respectively) show some
both the difference and the overall good performance of the irregularities, with very local extremities at around 8 nm for
polymers, the free charge carrier mobility was evaluated. The P(3,6-BCBDT) and 10 nm for P(3,6-BCQT). The phase AFM
electron mobility was evaluated using an electron only device picture for P(3,6-BCQT) shows a much uniform film, which is
and the hole mobility was investigated using a hole only device also agreed by the reported roughness. The TEM images of
as described in the experimental section. The charge carrier both P(3,6-BCBDT) and P(3,6-BCQT) show that there are no
mobility is determined by using the space-charge limited large phase separated domains created, which again supports
current (SCLC) method and the mobility is the described using the notion that the films are rather smooth.
the expression in the equation below:
9
8
푉2
ꢀ3
Conclusion
퐽푆퐶퐿퐶
=
휀 휀 휇
푟 푐푐
표
Based on the successful synthesis of the monomer 3,6-
Where JSCLC is the current density, 휀표 is the permittivity of free dibromo-N-(di(2,4,6-trimethyl)phenylboryl-carbazole (3,6-BC)
1
11
13
space, 휀푟 is the relative dielectric constant of the transport proven by H-, B-, and C-NMR studies, novel boron
medium, 휇푐푐 is the free charge carrier mobility (either hole or containing
polymers, poly(3,6-(N-di(2,4,6-
electron mobility), V is the internal potential in the device, and trimethyl)phenylboryl-carbazole)-alt-4,8-di(5-(2-
L is the thickness of the active layer (measured by a ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene
profilometer). The mobilities for P(3,6-BCQT) and P(3,6-BCBDT) (P(3,6-BCBDT)) and poly(3,6-(N-di(2,4,6-trimethyl)phenylboryl-
are listed in Table 2. For P(3,6-BCBDT) , the mobility of 5.33 × carbazole)-alt-3,3”’-didodecyl-2,2’:5’,2”:5”,2”’-quarter
−
4
2
−1 -1
−4
2
−1
1
0
cm V s falls in the same magnitude (1.04 × 10 cm V
thiophene) (P(3,6-BCQT)) have been designed and successfully
-
1
s ) as that of a BDT Quinoxaline based copolymer, which is polymerized via Stille & Kumada cross-coupling reactions,
also a relatively bulky polymer with pendant phenyl rings, as which were confirmed by H-NMR measurements. From the
1
4
3
Duan et al. reported . For mobilities for P(3,6-BCQT), an study on their photophysical- and electro-chemical properties
imbalance between mobility of electrons and holes by two and computer modelling, a quasi-donor-acceptor nature of
orders of magnitude (see Table 2) was found. Its lower PV P(3,6-BCBDT) and P(3,6-BCQT) were confirmed, and band gaps
performance when comparing to P(3,6-BCBDT), could be well of 2.07 eV for P36BCBDT and 2.23 eV for P36BCQT were
attributed into this large difference, while the latter exhibits obtained. The intrachain charge transfer between N-B on both
comparable charge carrier mobilities between the hole ( 5.33 × P(3,6-BCBDT) and P(3,6-BCQT) has been investigated, such
−4
2
−1 -1
−4
2
−1 -1
1
0
cm V
s
) and the electron ( 1.54 × 10 cm V s ).
certified direct/conjugated N-B interaction results in
ambipolarity, high electron affinity, and deep HOMO level. The
devices from P(3,6-BCBDT) and P(3,6-BCQT) have been
fabricated with the structure of glass/ITO/PEDOT:PSS/Active
Film morphology
2
layer/Ca/Al with an active area 0.1cm . Both polymers were
In Figure 4 the atomic force microscopy (AFM) (both phase and tested as donor materials with PC71BM as acceptor, and the
topography) and transmission electron microscopy (TEM) optimal weight ratio of 1:3 polymer to acceptor was
images of the optimized polymer/PC71BM blends are shown. concluded. P(3,6-BCQT) afforded an efficiency of 1.44%, with a
Generally all films are relatively smooth for P(3,6-BCBDT) with
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J
sc of 4.82 mAcm , a Voc of 0.79 V and FF of 37%. (P(3,6-BCBDT) 16.
B. Kim, B. Ma, V. R. Donuru, H. Liu and J. M. J. Fréchet,
View Article Online
Chem. Commun., 2010, 46, 4148.
DOI: 10.1039/C6TC01208F
polymer performed better with an efficiency of 3.82%, with a
sc of 8.31 mAcm , a Voc of 1.0 V based on it low lying HOMO 17.
level, and FF of 45%. We consider these preliminary results as
evidence of a large potential of the application of such direct 18.
N-B interacted boron containing good and robust donor
material for future OSCs.
-2
J
Y. Kubo, K. Watanabe, R. Nishiyabu, R. Hata, A. Murakami,
T. Shoda and H. Ota, Org. Lett., 2011, 13, 4574–4577.
T. Rousseau, A. Cravino, E. Ripaud, P. Leriche, S. Rihn, A. De
Nicola, R. Ziessel and J. Roncali, Chem. Commun., 2010, 46,
5082.
19.
S. Kolemen, O. A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-
Ela, M. Marszalek, J.-H. Yum, S. M. Zakeeruddin, M. K.
Nazeeruddin, M. Grätzel and E. U. Akkaya, Chem. Sci.,
Acknowledgement
2
011, 2, 949.
We gratefully acknowledge financial support from Innovation
Fund Denmark (IFD) (previously The Danish Council for
Strategic Research (DSF)) through the WAPART project (IFD file
No. 0603-00493B or DSF file No. 11-116380), the Danish
National Research Foundation (DNRF) for the Danish-Chinese
Center for Organic based Photovoltaic Cells with
Morphological Control. R. Q. Yang is deeply grateful to the
National Natural Science Foundation of China (21274161,
2
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2
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC01208F
Table of Contents Graph for
Novel high band gap pendant-borylated carbazole polymers with
+
-
deep HOMO level through direct N=B interaction for organic
photovoltaics
a,b
a,c,d
e
a,b
Rasmus G. Brandt , Steffen G. Sveegaard , Manjun Xiao , Wei Yue ,
a,b
e
f
f,g
Zhengkun Du , Meng Qiu , Dechan Angmo , Thomas R. Andersen , Frederik C.
Krebs , Renqiang Yang , and Donghong Yu
f
e*
a,b*
R
1
B
N
B
S
N
R
2
S
S
S
S
S
n
S
n
R
2
S
P(3,6-BCBDT)
R =Ethylhexyl R =dodecyl
1 2
P(3,6-BCQT)
R
1
+
-
Direct and still conjugated intramolecular N=B interactions in novel high band gap
borylated carbazole containing polymers, namely, P(3,6-BCBDT) and (P(3,6-BCQT),
results in ambipolarity, high electron affinity, and deep HOMO levels, with a PCE of
3
.82% and 1.44%.