Star-shaped electron acceptors containing a truxene core for non-fullerene solar cells

Article abstract of DOI:10.1016/j.orgel.2017.10.009

A series of new electron acceptors containing a truxene core with intense optical absorption were synthesized and used for non-fullerene organic solar cells. Due to the weak electron-donating characteristic of truxene core and thereby weak intramolecular charge transfer interaction from electron-donating core to electron-withdrawing end groups, the resulting new acceptors show relatively wide optical band-gap and high-lying lowest unoccupied molecular orbitals (LUMOs), which consequently lead to complementary light spectra with narrow band-gap donor polymers and high open circuit voltage (V_oc) in solar cells. Particularly, Tr(Hex)₆-3BR, a star-shaped planar acceptor, produced the highest power conversion efficiency of 2.1% with a high V_oc of 1.02 V when blended with PTB7-Th.

Full text of DOI:10.1016/j.orgel.2017.10.009

OrganicElectronics52(2018)42–50
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Star-shaped electron acceptors containing a truxene core for non-fullerene
solar cells
Kaiwen Lin1, Boming Xie1, Zhenfeng Wang, Ruihao Xie, Yunping Huang, Chunhui Duan∗,
Fei Huang∗∗, Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou
510640, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Truxene
Star-shaped molecules
Electron acceptors
Non-fullerene solar cells
A series of new electron acceptors containing a truxene core with intense optical absorption were synthesized
and used for non-fullerene organic solar cells. Due to the weak electron-donating characteristic of truxene core
and thereby weak intramolecular charge transfer interaction from electron-donating core to electron-with-
drawing end groups, the resulting new acceptors show relatively wide optical band-gap and high-lying lowest
unoccupied molecular orbitals (LUMOs), which consequently lead to complementary light spectra with narrow
band-gap donor polymers and high open circuit voltage (Voc) in solar cells. Particularly, Tr(Hex)6-3BR, a star-
shaped planar acceptor, produced the highest power conversion efficiency of 2.1% with a high Voc of 1.02 V
when blended with PTB7-Th.
1. Introduction
tetrakis(4-hexylpheny)-dithieno[2,3-d:2′,3′-d']-s-indaceno[1,2-b:5,6-b']
dithiophene (ITIC) [6], and (Z)-5-{[5-(15-{5-[(Z)-(3-Ethyl-4-oxo-2-
Over the past two decades, organic solar cells (OSCs) composed of
electron donors and fullerene-based acceptors have attracted enormous
attentions and made remarkable progress. The fullerene-based accep-
tors have high electron affinity and electron mobility, ensuring efficient
exciton dissociation and charge transport. Therefore, high performance
OSCs with power conversion efficiencies (PCEs) surpassed 11% have
been achieved in fullerene-based devices [1,2]. However, fullerene-
based acceptors suffer from a few drawbacks such as weak light ab-
sorption in the visible spectral region, limited variability in the energy
levels, high cost of synthesis, and poor morphological stability in the
blend films [3–6]. Recently, non-fullerene acceptors have made great
achievements because of their tunable absorption spectra and energy
levels, good chemical stability and photostability, as well as better
compatibility with donors to form appropriate morphology [7–10].
Encouragingly, impressive PCEs over 13% have been realized by non-
fullerene acceptors [11,12].
thioxo-1,3-thiazolidin-5-ylidene)methyl]-8-thia-7.9-diazabicyclo
[4.3.0]nona-1(9),2,4,6-tetraen-2-yl}-9,9,18,18-tetrakis(2-ethylhexyl)-
5.14-dithiapentacyclo[10.6.0.03,10.04,8.013,17]octadeca-
1(12),2,4(8),6,10,13(17),15-heptaen-6-yl)-8-thia-7.9-diazabicyclo
[4.3.0]nona-1(9),2,4,6-tetraen-2-yl]methylidene}-3-ethyl-2-thioxo-1,3-
thiazolidin-4-one (EH-IDTBR) [14–16] have shined in organic photo-
voltaics. Following this A-D-A strategy, we are interested in star-shaped
planar acceptors rather than linear planar acceptors. Star-shaped mo-
lecules are capable of harvesting incident light efficiently owing to their
extended dimensionality [17,18], which is beneficial to improving
photocurrents in OSCs [19,20]. In addition, star-shaped molecules are
able to achieve a proper balance between the size of molecular ag-
gregation in solid state and exciton diffusion length scale to obtain good
electronic communication due to moderate intermolecular interactions
[20,21].
As an excellent building block for the construction of larger poly-
arenes, star-shaped molecule of truxene has been recognized and widely
used in optoelectronic fields, such as dye-sensitized solar cells [22],
perovskite solar cells [23,24], two-photon absorption materials [25],
photocatalysts [26], organic light emitting diodes [27], and organic
field effect transistors [28]. Moreover, truxene has been successfully
So far, state-of-the-art non-fullerene acceptors are based on linear
acceptor-donor-acceptor (A-D-A) framework with planar conjugated
backbone, which contain an electron-rich core and two electron-with-
drawing terminal groups [5,6,9–16]. Among these successful materials,
3,9-bis(2-methylene-(3-(1,1-dicyano-methylene)-indanone))-5,5,11,11-
∗
Corresponding author.
Corresponding author.
∗∗
E-mail addresses: duanchunhui@scut.edu.cn (C. Duan), msfhuang@scut.edu.cn (F. Huang).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.orgel.2017.10.009
Received 18 August 2017; Received in revised form 1 October 2017; Accepted 7 October 2017
Availableonline10October2017
1566-1199/©2017ElsevierB.V.Allrightsreserved.

K. Lin et al.
OrganicElectronics52(2018)42–50
Scheme 1. Device structure employed in this study and the
chemical structures of truxene-based acceptors.
employed in constructing electron donors for OSCs [29]. However,
truxene has never been used to construct electron acceptors for OSCs.
The truxene unit has rigid planar and C3h symmetric structure. As a
result, the resulting star-shaped acceptors will be capable of harvesting
incident light efficiently due to the extended molecular dimensionality
and well-delocalized conjugated structure [30]. Besides, the weak
electron-donating characteristic of truxene core and thereby weak in-
tramolecular charge transfer interaction from electron-donating core to
electron-withdrawing end groups will endow the resulting new accep-
tors relatively wide optical band-gap and high-lying lowest unoccupied
molecular orbitals (LUMOs), which consequently lead to com-
plementary light spectra with narrow band-gap donor polymers and
high open circuit voltage (Voc) in OSCs [31–33].
Based on these considerations, herein we developed three new ac-
ceptors with a truxene core, namely Tr(Hex)6-3IC, Tr(Dec)6-3IC, and
Tr(Hex)6-3BR (Scheme 1). Among these molecules, Tr(Hex)6-3IC and
Tr(Dec)6-3IC possess the same thiophene π-bridges and terminal elec-
tron-withdrawing units of (2-(3-oxo-2,3-dihydroinden-1-ylidene)mal-
ononitrile. Different side chain of hexyl and decyl were introduced onto
these two materials because alkyl chain length is proven to be an im-
portant factor that determines device performance [34,35]. These two
star-shaped materials can be regarded as analogues of ITIC, because
they possess the same terminal acceptor units of (2-(3-oxo-2,3-dihy-
droinden-1-ylidene)malononitrile. Tr(Hex)6-3BR possesses hexyl side
chains and double electron withdrawing units of benzo[2,1,3]thiadia-
zole and 3-ethylrhodanine. This material is a star-shaped analogue of
another famous acceptor of EH-IDTBR. Among the three new acceptors,
Tr(Hex)6-3BR displayed a maximum absorption coefficient as high as
2.9 × 105 cm−1 in film. When combined with the widely used narrow
band-gap donor polymer of PTB7-Th, the resulting OSCs based on Tr
(Hex)6-3BR acceptor achieved a PCE of 2.1% with a high Voc of 1.02 V.
Moreover, the structure-property relationship with these truxene-based
acceptors are discussed in detail in this contribution.
and washed with water, acetone, and dichloromethane to give an off-
white powder (11 g, 65%). 1H NMR (500 MHz, CDCl3) δ: 7.98 (d, 3H),
7.71 (d, 3H), 7.51 (t, 3H), 7.40 (t, 3H), 4.29 (s, 6H). 13C NMR
(125 MHz, CDCl3) δ: 144.21, 142.13, 137.54, 135.71, 127.35, 126.73,
125.56, 122.31, 36.98.
2.1.2. Compound 2a
In a three-necked flask containing compound 1 (10 g, 29 mmol)
under argon protection, anhydrous THF (200 mL) was added and the
suspension was stirred at −78 °C. n-BuLi (115.2 mL, 2.5 M) was added
dropwise and the mixture was kept at −78 °C for 2 h. 1-Bromohexene
(48.2 g) was injected slowly. The mixture was allowed to warm to room
temperature and stirred overnight. After that, the mixture was poured
into 1 L saturated NH4Cl aqueous solution to quench the excess n-BuLi.
The water phase was exacted with ethyl acetate, and then the combined
organic phase was dried over MgSO4. After the solvent was removed,
the crude product was subjected to silica gel column with hexane as
eluent to give an off-white powder 2a (R = C6H13, 22 g, 95%). It was
used directly in the next step without characterization.
2.1.3. Compound 2b
Compound 2b was synthesized similarly to compound 2a with a
yield of 92% by using 1-bromodecane to react with compound 1.
2.1.4. Compound 3a
A mixture of compound 2a (2.4 g, 2.8 mmol), anhydrous FeCl3
(5 mg) and chloroform (15 mL) was stirred. And then a solution of
bromine (0.5 mL, 10 mmol) in 5 mL of chloroform was added dropwise
under stirring at 0 °C, then kept overnight. Na2SO3 aqueous solution
(50 mL) was added to remove excess bromine. The mixture was exacted
with dichloromethane for three times, and the organic phase was dried
over MgSO4. After the solvent was removed, the yellow residue was
recrystallized from ethanol to yield an off-white powder (2.65 g, 89%).
1H NMR (500 MHz, CDCl3) δ: 8.18 (d, 3H), 7.56 (d, 3H), 7.52 (dd, 3H),
2.87 (m, 6H), 2.04 (m, 6H), 0.93 (m, 36H), 0.63 (m, 18H), 0.46 (m,
14H). 13C NMR (125 MHz, CDCl3) δ: 156.02, 145.04, 139.00, 137.77,
129.51, 126.04, 125.66, 121.18, 56.13, 36.95, 31.59, 29.51, 24.02,
22.40, 14.01.
2. Experimental section
2.1. Materials and synthesis
All reagents were obtained from commercial sources and used
without further purification, unless otherwise specified. Solvents were
dried before use (tetrahydrofuran (THF) from sodium/benzophenone,
toluene was washed with H2SO4 and then treated with CaCl2, and
CH3CN from CaH2). Scheme 2 shows the synthetic routes of the
truxene-based acceptors. The detailed synthesis procedures are de-
scribed as following.
2.1.5. Compound 3b
Compound 3b was synthesized similarly to compound 3a from
compound 2b. 1H NMR (500 MHz, CDCl3) δ: 8.17 (d, 3H), 7.72 (d, 3H),
7.56 (dd, 3H), 2.83 (m, 6H), 2.04 (m, 6H), 0.91 (m, 102H), 0.51 (m,
12H). 13C NMR (125 MHz, CDCl3) δ: 156.22, 145.11, 139.02, 137.55,
129.55, 126.24, 125.67, 121.28, 56.35, 36.98, 32.29, 29.99, 29.97,
29.80, 29.70, 29.61, 24.32, 23.10, 14.28.
2.1.1. Compound 1
1-Indanone (20.0 g, 151 mmol) was dissolved in the mixture solu-
tion of acetic acid (120 mL) and concentrated hydrochloric acid
(60 mL). The solution was heated to 120 °C and refluxed for 24 h. The
hot mixture was poured into 1 L ice water, then sodium carbonate was
added slowly with stirring for 1 h. The yellow precipitate was filtered,
2.1.6. Compound 4a
A mixture of compound 3a (0.615 g, 0.568 mmol), (5-(1,3-dioxolan-
2-yl)thiophen-2-yl)trimethylstannane (1.136 g, 2.556 mmol), tetrakis
(triphenylphosphine)palladium(0) (146 mg) were dissolved in toluene
(70 mL) and degassed with argon for 30min. The mixture was refluxed
43

K. Lin et al.
OrganicElectronics52(2018)42–50
Scheme 2. Synthetic routes of truxene-based acceptors.
Reagents and conditions: (a) HCl, HOAc, 120 °C; (b) n-BuLi,
THF, −78 °C; 1-bromohexene/1-bromodecane; (c) FeCl3,
CHCl3, Br2,
0 °C; (d) Pd(PPh3)4, toluene, 100 °C; (e)
chloroform, pyridine, 65 °C; (f) bis(pinacolato)diboron,
CH3COOK, DMF, Pd(dppf)2Cl2, 90 °C; (g) toluene, Pd
(PPh3)4, K2CO3, 95 °C.
for 48 h, then cooled to room temperature. HCl solution (1 M, 90 mL)
was added into the mixture and stirred overnight. The mixture was
exacted with dichloromethane, and the organic phase was dried over
MgSO4. After solvent evaporation the product was purified by flash
7.85 (s, 3H), 2.95 (m, 6H), 2.16 (m, 6H), 1.42 (s, 36H), 0.89 (m, 36H),
0.59 (t, 18H),0.45 (m, 12H). 13C NMR (125 MHz, CDCl3) δ: 152.76,
146.45, 143.24, 138.33, 132.87, 128.21, 123.95, 83.73, 55.77, 36.84,
31.53, 29.52, 25.00, 23.97, 22.30, 13.89.
column
chromatography
on
silica
gel
(eluent,
di-
chloromethane:petrolem ether = 1:1) to afford compound 4a as a
yellow solid (409 mg, 65%). 1H NMR (500 MHz, CDCl3) δ: 9.94 (s, 3H),
8.43 (d, 3H), 7.82 (d, 3H), 7.77 (d, 6H), 7.57 (d, 3H), 2.99 (m, 6H),
2.18 (m, 6H), 0.93 (m, 36H), 0.87 (m, 30H). 13C NMR (125 MHz,
CDCl3) δ: 155.34, 155.31, 146.98, 142.93, 142.07, 138.58, 138.24,
138.07, 125.88, 125.58, 124.70, 120.59, 56.71, 37.72, 32.13, 30.10,
24.67, 22.93, 14.55.
2.1.9. Compound 6
A mixture of compound 5 (0.613 g, 0.5 mmol) 2,1,3-benzothiadia-
zole-4-carboxaldehyde (0.546 g, 2.25 mmol) in anhydrous toluene
(40 mL) was degassed for 30 min before Pd(PPh3)4 (58 mg, 0.05 mmol)
and K2CO3 aqueous solution (2 M, 10 mL) was added. The solution was
heated at 95 °C for 48 h. Water and dichloromethane were added, and
the organic layer was dried over MgSO4. After removal of the solvent,
the crude product was chromatographically purified on silica gel
column (eluted with ethyl acetate:petrolem ether = 1:20) to afford
compound 6 as a yellow solid (0.46 g, 55%). 1H NMR (500 MHz, CDCl3)
δ: 10.85 (s, 3H), 8.61 (d, 3H), 8.40 (d, 3H), 8.23 (d, 3H), 8.20 (d, 3H),
3.11 (m, 6H), 2.33 (m, 6H), 0.97 (m, 36H), 0.60 (m, 30H). 13C NMR
(125 MHz, CDCl3) δ: 189.44, 154.62, 154.50, 154.42, 147.28, 141.99,
140.89, 138.46, 134.97, 133.06, 128.38, 127.22, 126.59, 125.31,
124.02, 56.55, 37.46, 31.93, 29.94, 24.52, 22.72, 14.33.
2.1.7. Compound 4b
Compound 4b was synthesized similarly to compound 4a from
compound 3b. 1H NMR (500 MHz, CDCl3) δ: 9.94 (s, 3H), 8.43 (d, 3H),
7.82 (d, 3H), 7.79 (d, 6H), 7.57 (d, 3H), 2.99 (m, 6H), 2.17 (m, 6H),
1.13 (m, 84H), 0.87 (m, 18H), 0.52 (m, 12H). 13C NMR (125 MHz,
CDCl3) δ: 155.05, 155.02, 146.67, 142.60, 141.81, 138.27, 137.90,
131.75, 125.59, 125.22, 124.33, 120.25, 56.38, 37.30, 32.26, 30.08,
29.97, 29.76, 29.67, 29.61, 24.36, 23.00, 14.42.
2.1.10. Tr(Hex)6-3IC
2.1.8. Compound 5
Compound 4 (0.15 g, 0.1275 mmol) and 2-(3-oxo-2,3-dihydroinden-
1-ylidene)malononitrile (0.371 g, 3.825 mmol) were dissolved in
chloroform (15 mL). A drop of pyridine was added and the solution was
left to stir at 65 °C overnight. The reaction was dissolved in ethanol and
stirred for another 10 min. The product was extracted with di-
chloromethane and dried over MgSO4. After removal of the solvent, the
crude product was chromatographically purified on silica gel column
(eluted with chlorobenzene) to afford dark purple solid (105 mg, 85%).
1H NMR (500 MHz, CDCl3) δ: 8.95 (s, 3H), 8.75 (m, 3H), 8.50 (m, 3H),
8.01 (m, 3H), 7.98 (m, 6H), 7.91 (s, 3H), 7.83 (m, 6H), 7.69 (d, 3H),
3.04 (m, 6H), 2.28 (m, 6H), 0.95 (m, 36H), 0.62 (m, 30H). 13C NMR
A mixture of compound 3a (0.88 g, 0.82 mmol), bis(pinacolato)
diboron (1.4 g, 5.5 mmol), CH3COOK (0.61 g, 6.2 mmol), and N,N-
dimethylformamide (30 mL) was carefully degassed before Pd
(dppf)2Cl2 (50 mg, 0.082 mmol) was added. The mixture was stirred for
48 h at 90 °C under argon. Water and dichloromethane (300 mL) were
added, and the organic layer was separated and washed with water
three times. After removal of the solvent, the crude product was chro-
matographically purified on silica gel column (eluted with ethyl acet-
ate:petrolem ether = 1:20) to afford compound 5 as a white solid
(0.51 g, 51%). 1H NMR(500 MHz, CDCl3) δ: 8.39 (d, 3H), 7.87 (d, 3H),
44

K. Lin et al.
OrganicElectronics52(2018)42–50
(125 MHz, CDCl3) δ: 187.44, 159.75, 159.54, 153.77, 145.88, 145.39,
141.05, 139.05, 137.13, 136.97, 135.96, 135.41, 134.29, 133.62,
130.51, 124.42, 124.40, 124.27, 123.99, 122.85, 121.62, 119.14,
113.53, 113.46, 70.06, 56.40, 37.23, 31.63, 29.57, 24.22, 22.41, 14.02.
MS (MALDI-TOF) calculated for C114H108N6O3S3, 1705.77; found,
1705.94.
acceptors were prepared by simultaneously dissolving both materials
with weight ratio of 1:1 in chloroform and spin-coated on the ITO/
PEDOT:PSS electrode (at 1600 rpm for 60 s) to form active layer with
thickness of 100 nm. Then a 10 nm Ca and a 100 nm Al layer were
thermally deposited onto the active layer through a shadow mask at a
vacuum of 5 × 10−5 Pa. During the test, an aperture with an area of
3.14 mm2 was used. The current density–voltage (J–V) curves were
measured using AM1.5G solar simulator (Class AAA solar simulator,
Model 94063A, Oriel) with an irradiation light intensity of
100 mW cm−2. The external quantum efficiency (EQE) spectra were
determined from a QEX10 Solar Cell IPCE measurement system (PV
measurement, Inc).
2.1.11. Tr(Dec)6-3IC
Tr(Dec)6-3IC was synthesized similarly to Tr(Hex)6-3IC. The pro-
duct was obtained as a dark purple solid in 80% yield. 1H NMR
(500 MHz, CDCl3) δ: 8.95 (s, 3H), 8.75 (m, 3H), 8.50 (d, 3H), 8.02 (m,
3H), 7.98 (m, 6H), 7.92 (d, 3H), 7.83 (m, 6H), 7.70 (d, 3H), 3.03 (m,
6H), 2.27 (m, 6H), 1.01 (m, 84H), 0.75 (m, 18H), 0.61 (m, 12H). 13C
NMR (125 MHz, CDCl3) δ: 187.42, 159.75, 159.56, 153.81, 145.92,
145.38, 141.08, 139.05, 137.11, 136.96, 135.97, 135.39, 134.28,
133.62, 130.49, 124.42, 124.34, 124.28, 123.92, 122.84, 121.59,
119.11, 113.54, 113.46, 70.28, 56.62, 37.35, 32.27, 30.10, 29.99,
29.79, 29.68, 29.67, 24.47, 23.00, 14.42. MS (MALDI-TOF) calculated
for C138H156N6O3S3, 2042.14; found, 2042.33.
2.4. Fabrication and characterization of single-carrier devices
The charge carrier mobilities of PTB7-Th:truxene-based acceptor
blend films were determined from single-carrier devices with space-
charge-limited current (SCLC) model. The device structures of the
electron only and hole only devices are Al/PTB7-Th:truxene-based ac-
ceptor/Ca/Al and ITO/PEDOT:PSS/PTB7-Th:truxene-based acceptor/
MoO3/Ag, respectively. The mobilities were determined by fitting the
dark J–V current to the model of a single carrier SCLC using the
equation: J = 9ε0εrμV2/8d3 [37], where J is the current density, d is the
thickness of the blend films, ε0 is the permittivity of free space, εr is the
relative dielectric constant of the transport medium, and μ is the charge
carrier mobility. V = Vapp–Vbi, where Vapp is the applied voltage and Vbi
is the offset voltage. The carrier mobility can be calculated from the
slope of the J1/2–V curves.
2.1.12. Tr(Hex)6-3BR
Tr(Hex)6-3BR was synthesized similarly to Tr(Hex)6-3IC. The
product was obtained as a dark purple solid in 80% yield. 1H NMR
(500 MHz, CDCl3) δ: 8.61 (m, 6H), 8.21 (m, 6H), 8.05 (d, 3H), 7.88 (d,
3H), 4.28 (m, 6H), 3.11 (m, 6H), 2.31 (m, 6H), 1.37 (m, 12H), 1.34 (m,
9H), 0.97 (m, 36H), 0.63 (m, 30H). 13C NMR (125 MHz, CDCl3) δ:
193.04, 167.42, 154.60, 154.12, 153.40, 146.57, 141.18, 137.98,
136.48, 134.53, 131.03, 127.58, 127.43, 127.22, 125.51, 125.41,
124.79, 123.18, 56.01, 39.87, 36.99, 31.44, 29.46, 24.02, 22.22, 13.83,
12.24. MS (MALDI-TOF) calculated for C99H111N9O3S9, 1762.63; found,
1763.76.
3. Results and discussion
3.1. Synthesis and characterization
2.2. Instruments and characterization
Scheme 2 shows the synthetic routes of the truxene-based acceptors.
The synthetic routes include cyclization of 1-indanone to produce
truxene (compound 1) [27–31,34,35], lithium-hydrogen exchange re-
action to obtain alkyl chain modified truxene (compound 2) [29,34],
bromization reaction to yield compound 3 [29], Pd-catalyzed Stille
cross-coupling to introduce π-bridge of thiophene, Pd-catalyzed Suzu-
kicross-coupling to introduce benzothiadiazole, and Knoevenagel con-
densation between terminal acceptor groups and truxene-based alde-
hydes to produce the target acceptor materials [11–15]. The purity of
the final compounds were characterized by 1H-, 13C-NMR spectroscopy,
and MALDI-TOF MS, which indicates the high purity of these com-
pounds (Figs. S13–S21, Supporting Information). All these acceptors are
readily soluble in common organic solvents at room temperature, such
as dichloromethane, and chloroform, which ensures the preparation of
smooth and uniform films. The thermal stability of the acceptors was
investigated by TGA under a nitrogen atmosphere. All materials show
good thermal stability with 5% decomposition temperature (at 5%
weight-loss) (Td) over 350 °C (Fig. S22, Supporting Information).
Among them, Tr(Hex)6-3BR shows the best thermal stability with Td
above 400 °C.
1H and 13C NMR spectra were tested on a Bruker AV-500 with tet-
ramethylsilane (TMS) as an internal reference. MALDI-TOF-MS was
performed by using
spectra were measured on
a
Bruker Agilent1290/maXis impact. UV–vis
HP 8453 spectrophotometer.
a
Thermogravimetric (TGA) analysis was measured on a NETZSCH TG
209 at a heating rate of 10 °C min−1 with a nitrogen flow rate of
20 mL min−1. Cyclic voltammetry data were measured on a CHI600D
electrochemical workstation with Bu4NPF6 (0.1 M) in acetonitrile as the
electrolyte, glass carbon electrode and a saturated calomel electrode as
the working and reference electrodes, respectively. The thin films were
coated on a glassy carbon working electrode. The scan rate was
100 mV s−1. The onset potential of Fc/Fc+ was measured to be 0.36 V,
and the HOMO and LUMO levels from the onset oxidation (Eoxonset) and
reduction (Eredonset) potentials were calculated by equations: EHOMO = -
e(Eoxonset-EFc/Fc++4.8) eV and ELUMO = -e(Eredonset-EFc/Fc++4.8) eV,
respectively [36]. The geometry was optimized by Density Functional
Theory (DFT) calculations performed at the B3LYP/6-31G(d) level to
optimize the ground state geometries of the acceptor molecules using
the Gaussian 09. The atom force microscopy (AFM) images were ob-
tained from a NanoMan VS microscopy under tapping mode. The
transmission electron microscopy (TEM) images were characterized
with a JEM-2100F instrument.
3.2. Theoretical calculations
Density functional theoretical (DFT) calculations were performed at
B3LYP/6-31G(d) level to study the molecular geometries and electron
structures of the acceptors. To simplify the calculation, hexyl and decyl
chains were replaced with methyl unit and the model structures were
named as Tr-3IC and Tr-3BR. As shown in Fig. 1, the optimized mo-
lecular geometry reveales a quite planar structure for the star-shaped
Tr-3BR with minimal torsion of the backbone. This highly planar
geometry would facilitate π-electron delocalization and enhance charge
transport property [11,12]. Tr-3IC is slight twisted with dihedral angles
of 22.53° between truxene and thiophene units, 0.80° between
2.3. Fabrication and characterization of solar cells
The device structure of ITO/PEDOT:PSS/PTB7-Th:acceptor/Ca/Al
were fabricated as the following procedure. The ITO-coated glass sub-
strate was cleaned in an ultrasonic bath with deionized water, acetone,
and isopropanol, each process was approximately 15 min, and then
dried under a stream of dry nitrogen. PEDOT:PSS (Heraeus Clevios P VP
A 4083) was spin-coated on top of the above ITO and annealed in air at
150 °C for 10 min. Subsequently, the blend solutions of PTB7-Th and
45

K. Lin et al.
OrganicElectronics52(2018)42–50
Fig. 1. DFT calculated molecular geometries
and frontier molecular orbital distributions
for the model structures (Tr-3IC and Tr-
3BR).
46

K. Lin et al.
OrganicElectronics52(2018)42–50
Fig. 2. Nomalized absorption spectra of truxene-based acceptors in
chloroform solutions (A) and as thin films (B).
thiophene and the terminal electron-withdrawing units. The distorted
configuration may help to reduce intermolecular interactions and re-
strict the excessive self-aggregation and large-scale phase separation in
the blend films [38]. To further study the alkyl chain effect on dihedral
angles, Tr(Hex)6-3IC and Tr(Dec)6-3IC were calculated separately (Fig.
S23, Supporting Information). The dihedral angles are not changed
along with alkyl chain changes. The frontier molecular orbital dis-
tributions for the model structures were also calculated. The highest
occupied molecular orbitals (HOMOs) are mainly distributed on the
truxene core and the adjacent π-bridge, while the LUMOs are localized
on the terminal units and benzothiadiazole unit due to their strong
electron-withdrawing effect. It is expected that this large electron-ac-
cepting area will benefit charge transfer. The theoretical HOMO/LUMO
energy levels of Tr-3IC and Tr-3BR are predicted to be −5.87/–3.27
and −5.74/–2.94 eV, respectively.
a higher Voc, due to Voc is proportional to the offset between the LUMO
level of the acceptor and the HOMO level of the donor [41].
3.4. Photovoltaic properties
The photovoltaic properties of the truxene-based acceptors were
evaluated in OSCs with
a structure of ITO/PEDOT:PSS/PTB7-
Th:acceptor/Ca/Al (Scheme 1). PTB7-Th was used as the donor because
its strong optical absorption at long-wavelength region can achieve
complementary absorption with our truxene-based acceptors. The
photoactive layers were formed by spin-coating the blend solutions of
PTB7-Th:truxene-based acceptors (1:1, w/w) in chloroform without
additives and post-processing. The optimum thickness of the photo-
active layer is 100 nm (Table S1, Supporting Information). Fig. 4A
shows the current density-voltage (J-V) curves and Table 2 lists the
corresponding photovoltaic parameters. The acceptors produce a high
Voc in the range from 0.74 to 1.0 V. However, the acceptors can only
afford moderate PCEs due to the relatively low short circuit current (Jsc)
and fill factor (FF). Tr(Hex)6-3BR-based devices offer the maximum
PCE of 2.10% with a Jsc of 5.92 mA cm−2, a high Voc of 1.02 V and an
FF of 33.4%. The high Voc can be ascribed to the high LUMO energy
level of Tr(Hex)6-3BR [41]. The change of alkyl chains from hexyl (Tr
(Hex)6-3IC) to decyl (Tr(Dec)6-3IC) is detrimental to device perfor-
mance. The Tr(Hex)6-3IC- and Tr(Dec)6-3IC-based solar cells show the
maximum PCE of 1.28% and 0.55%, respectively. Obviously, the pho-
tovoltaic parameters of Tr(Hex)6-3IC- and Tr(Dec)6-3IC-based solar
cells are lower than other reported (2-(3-oxo-2,3-dihydroinden-1-yli-
dene)malononitrile end-capped electron acceptors [42–44]. Processing
additives such as 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN)
have been widely used for non-fullerene-based OSCs to improve PCEs
[10–15]. However, DIO is not able to improve the performance of Tr
(Hex)6-3BR based devices (Table S2, Supporting Information).
Fig. 4B shows the corresponding external quantum efficiency (EQE)
spectra of the solar cells based on Tr(Hex)6-3IC, Tr(Dec)6-3IC, and Tr
(Hex)6-3BR. For each device, the integration from the EQE curve
matches well with the Jsc value acquired from the J–V curves. The Tr
(Hex)6-3IC- and Tr(Dec)6-3IC-based devices exhibit a maximum EQE
of 22% and 15% at 595 nm, and showed similar profile. The Tr(Hex)6-
3BR-based devices exhibit an EQE exceeding 20% in the range from
340 to 730 nm with the maximum value reaching 31% at around
505 nm. The broad EQE of Tr(Hex)6-3BR devices indicate the efficient
electron transfer from PTB7-Th to Tr(Hex)6-3BR and efficient hole
transfer from Tr(Hex)6-3BR to PTB7-Th, which is beneficial to gen-
erating a higher PCE in devices [45].
3.3. Optical and electrochemical properties
The UV–vis absorption spectra of the acceptors in solution and as
thin film are displayed in Fig. 2. All acceptors show intense absorption
in the range of 400–650 nm that can be ascribed to the intramolecular
charge transfer (ICT) transitions between central truxene and terminal
acceptor units [39,40]. In solutions, the absorption coefficients of all
acceptors exceed 106 L g−1 cm−1 (1.8 × 105 M−1 cm−1). In particular,
Tr(Hex)6-3BR exhibits a maximum absorption coefficient as high as
165 L g−1 cm−1 (2.9 × 105 M−1 cm−1) at 482 nm (Fig. S24A, Sup-
porting Information). The high absorption coefficients of these mole-
cules are possibly owing to their extended dimensionality, which is
consistent with other star-shaped molecules based on triphenylamine,
triazatruxene, and benzotrithiophene that presented high molecular
absorption coefficients over 1.0 × 105 M−1 cm−1 [17,19,20]. The
absorption bands of the truxene-based acceptors in film (Fig. 2B) are
significantly red-shifted and broadened compared to that in chloroform
solution, which can be attributed to the aggregation of the molecules in
solid state [10,11]. Interestingly, the changes in the alkyl chain length
on truxene lead to considerable spectral shifts. In solution, Tr(Hex)6-
3IC shows significantly red-shifted absorption relative to Tr(Dec)6-3IC.
In film, this difference becomes less considerable but Tr(Hex)6-3IC still
exhibits slightly wider ICT bands than Tr(Dec)6-3IC. Encouragingly, Tr
(Hex)6-3BR displays a maximum absorption coefficient as high as
2.9 × 105 cm−1 in film (Fig. S24B, Supporting Information). The op-
tical band gaps (Eg) of the acceptors are calculated to be 1.90 eV for Tr
(Hex)6-3IC, 1.92 eV for Tr(Dec)6-3IC, and 2.15 eV for Tr(Hex)6-3BR,
respectively (Table 1).
The energy levels of truxene-based acceptors were determined by
cyclic voltammetry (CV) (Fig. 3). The relevant data are summarized in
Table 1. The HOMO/LUMO levels of Tr(Hex)6-3IC, Tr(Dec)6-3IC, and
Tr(Hex)6-3BR are −5.92/–3.94, −5.88/–3.90, and −5.79/–3.74 eV,
respectively. The electrochemical band gaps (EgCV) are thus calculated
to be 1.98, 1.98, and 2.05 eV, respectively. The higher LUMO level of Tr
(Hex)6-3BR compared to Tr(Hex)6-3IC and Tr(Dec)6-3IC may result in
The charge recombination of the devices were studied by measuring
the photocurrent as
a function of light intensity (from 1 to
100 mW cm−2). The relevant characteristics are plotted in Fig. S25
(Supporting Information). Generally, Jsc and the intensity of incident
light (Plight) follow the relationship of Jsc∝Plightα, where the power-law
component (α) will be unity when there is no bimolecular recombina-
tion [9,14]. The α values are 0.898 for Tr(Hex)6-3IC, 0.828 for Tr
47

K. Lin et al.
OrganicElectronics52(2018)42–50
Table 1
Optical and electrochemical properties of truxene-based acceptors.
sol
film
opta
onset
onset
b
c
CVd
Acceptors
λmax
λmax
Eg
Eox
Ered
(V)
EHOMO
(eV)
ELUMO
(eV)
Eg
(nm)
(nm)
(eV)
(V)
(eV)
Tr(Hex)6-3IC
Tr(Dec)6-3IC
Tr(Hex)6-3BR
570
570
482
588
588
491
1.90
1.92
2.15
1.48
1.44
1.35
−0.50
−0.54
−0.70
−5.92
−5.88
−5.79
−3.94
−3.90
−3.74
1.98
1.98
2.05
a
opt
film
Calculated from Eg = 1240/λonset
eV.
Calculated from EHOMO = –e(Eoxonse–EFc/Fc++4.8) eV.
Calculated from ELUMO = –e(Eredonset–EFc/Fc++4.8) eV;dCalculated from Eg = ELUMO–EHOMO
b
c
CV
.
This should be the main reason of low performance of these truxene-
based acceptors. Among the three acceptors, Tr(Hex)6-3BR exhibites
the highest electron mobility in blend film, which may be attributable
to its good planarity and favorable π−π stacking. As a result, Tr(Hex)6-
3BR-based devices achieve the highest Jsc, FF, and PCE (Table S3,
Supporting Information).
3.6. Morphology
Atomic force microscopy (AFM) was used to investigate the mor-
phology of blend films of PTB7-Th:truxene-based acceptor (Fig. 6). All
the films were prepared on top of PEDOT:PSS using the same fabrica-
tion conditions for solar cells. The film of PTB7-Th:Tr(Hex)6-3IC ex-
hibits obvious phase separation with large granulate features, resulting
in a relative coarse surface with a root-mean-square (RMS) surface
roughness of 8.0 nm. It is well recognized that excessive aggregation
and phase separation are detrimental to exciton dissociation [49]. The
blends based on Tr(Dec)6-3IC and Tr(Hex)6-3BR show homogeneous
films with a RMS roughness of 0.76 and 0.96 nm, respectively, in-
dicating intimately mixed blends without noteworthy phase separation.
With such a morphology, charge generation is impeded because of the
lack of pure domains. High domain purity is favorable for dissociating
photo-generated charges from the donor/acceptor interface, while low
domain purity often causes serious geminate recombination.
Fig. 3. Cyclic voltammograms of truxene-based acceptors in thin film.
(Dec)6-3IC, and 0.858 for Tr(Hex)6-3BR, respectively, which indicates
considerable bimolecular recombination losses. Typically, charge re-
combination is related to the FF of the devices. The lower bimolecular
recombination in the Tr(Hex)6-3IC-based OSCs (α = 0.898) agrees
well with the higher FF value of the devices (38.1%) as compared to the
other devices.
3.5. Charge carrier mobility
The morphology of the blend films were further studied by trans-
mission electron microscopy (TEM). The relevant images are shown in
Fig. 7. Consistent with the large RMS roughness observed in AFM
images, the film of PTB7-Th:Tr(Hex)6-3IC demonstrates obvious phase
separation. It is well recognized that excessive phase separation is
harmful to efficient exciton dissociation and charge transport. On the
contrary, the blend films of Tr(Dec)6-3IC and Tr(Hex)6-3BR show
homogeneous morphology lacking noteworthy phase separation,
agreeing well with the AFM images. Combined, the low electron mo-
bility, the recombination, and the unfavorable morphology explain the
low performance of truxene-based solar cells.
The charge transport properties of the blend films were investigated
by fabricating single-carrier devices with a device structure of ITO/
PEDOT:PSS/PTB7-Th:acceptor/MoO3/Ag and Al/PTB7-Th:acceptor/
Ca/Al for hole only devices and electron only devices, respectively. The
hole and electron mobilities were acquired by fitting the J–V with
space-charge-limited current (SCLC) model. The J–V curves of the de-
vices are shown in Fig. 5. The hole mobilities (μh) are estimated to be
1 × 10−3 to 6 × 10−3 cm2 V−1 −1, which are comparable with the
s
value that obtained from PTB7-Th:fullerene devices [46]. In contrast,
the electron mobilities (μe) of the blend films of PTB7-Th:truxene-based
acceptor are measured to be 1.5 × 10−5 to 2.0 × 10−6 cm2 V−1 s−1
,
which are two or three orders of magnitude lower than that of PTB7-
Th:fullerene film [47]. The low electron mobility and highly im-
balanced μh/μe seriously restrict the charge transport and result in bi-
molecular recombination, which in turn led to low FF and Jsc [8,48].
4. Conclusion
In summary, three truxene-based acceptors with an aim of ex-
tending dimensionality were synthesized for application in OSCs. The
Fig. 4. J–V characteristics curves (A) and EQE spectra (B) of
OSCs based on PTB7-Th and truxene-based acceptors.
48

K. Lin et al.
OrganicElectronics52(2018)42–50
Table 2
Photovoltaic parameters of OSCs based on PTB7-Th and truxene-based acceptors under AM 1.5G illumination at 100 mW cm−2
.
Acceptor
Voc (V)
Jsc (mA cm−2
)
Jcal (mA cm−2
a
)
FF (%)
PCE (%)
PCEmax (%)
Tr(Hex)6-3IC
Tr(Dec)6-3IC
Tr(Hex)6-3BR
0.88
0.74
1.02
0.01
0.01
0.01
3.71
2.55
5.92
0.08
0.10
0.12
3.68
2.50
5.85
38.1
27.9
33.4
0.5
0.2
0.3
1.23
0.53
2.01
0.05
0.02
0.09
1.28
0.55
2.10
a
Calculated from EQE intetrations.
Fig. 5. J1/2–V characteristics of the blend films of PTB7-
Th:truxene-based acceptor in (A) hole-only devices and
(B) electron-only devices.
Fig.
6. AFM
topographic
images
(5μm × 5 μm) of the blend films of PTB7-
Th:truxene-based acceptor: (A) PTB7-Th:Tr
(Hex)6-3IC, (B) PTB7-Th:Tr(Dec)6-3IC, and
(C) PTB7-Th:Tr(Hex)6-3BR.
Fig. 7. TEM images of the blend films of PTB7-
Th:truxene-based acceptor: (A) PTB7-Th:Tr
(Hex)6-3IC, (B) PTB7-Th:Tr(Dec)6-3IC, and (C)
PTB7-Th:Tr(Hex)6-3BR.
new acceptors differ in the length of side chain (hexyl vs. decyl), π-
bridge (thiophene vs. benzothiadiazole), and terminal acceptor unit ((2-
(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile vs. 3-ethylrhoda-
nine). The acceptors were evaluated in terms of thermal properties,
optical properties, energy levels, and device performance in solar cells.
The photovoltaic properties of the truxene-based acceptors were stu-
died by combining with a narrow band-gap donor PTB7-Th. All these
three acceptors afforded moderate photovoltaic performance. The rea-
sons were revealed to be low electron mobility, imbalanced transport of
hole and electron, serious charge recombination, and unfavorable
morphology in blends, which consequently led to low FF and Jsc.
Nevertheless, this is just a small step on exploiting the potential of
truxene-based molecules for use as non-fullerene acceptors in organic
solar cells. New molecules with rational design based on truxene are in
progress in our lab, which are expected to achieve higher photovoltaic
properties in the near future.
Acknowledgments
The research was financially supported by the Startup Fund for
Distinguished Scholars of South China University of Technology. This
work was also supported by the Ministry of Science and Technology
(No. 2014CB643501), the Natural Science Foundation of China (Nos.
21520102006 and 91633301) and the Foundation of Guangzhou
Science and Technology Project (201707020019).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.
doi.org/10.1016/j.orgel.2017.10.009.
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