An all-small-molecule organic solar cell derived from naphthalimide for solution-processed high-efficiency nonfullerene acceptors

Article abstract of DOI:10.1039/c8tc05692g

Two small molecules BYG-1 and BYG-2 with fluorene donor and benzothiadiazole acceptor units connected to the terminal naphthamide group via ethyne linker were designed and synthesized. In this work we have discussed the effect of fluorine atoms connected with electron withdrawing benzothiadiazole unit to the fluorene core (BYG-1). In this study, we have fabricated solar cells with small-molecular donor and acceptor materials in the device architecture of bulk-heterojunction, using highly conjugated BYG-1 and BYG-2 as electron acceptors along with an appropriate small molecule donor (SMD). After improving the device architecture of the active layer using a suitable donor-to-acceptor weight ratio with solvent vapour annealing, we achieved power conversion efficiencies of 8.67% and 7.12% for BYG-1 and BYG-2, respectively. The superior photovoltaic performance of the fluorine-substituted BYG-1 can be attributed to its higher crystallinity, more balanced charge transport mobilities and efficient exciton dissociation.

Full text of DOI:10.1039/c8tc05692g

Journal of
Materials Chemistry C
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PAPER
An all-small-molecule organic solar cell derived
from naphthalimide for solution-processed
high-efficiency nonfullerene acceptors†
Cite this: J. Mater. Chem. C, 2019,
, 709
7
ab
a
c
c
B. Yadagiri, K. Narayanaswamy, Srikanth Revoju,
Bertil Eliasson,
d
ab
Ganesh D. Sharma * and Surya Prakash Singh
*
Two small molecules BYG-1 and BYG-2 with fluorene donor and benzothiadiazole acceptor units
connected to the terminal naphthamide group via ethyne linker were designed and synthesized. In this work
we have discussed the effect of fluorine atoms connected with electron withdrawing benzothiadiazole unit
to the fluorene core (BYG-1). In this study, we have fabricated solar cells with small-molecular donor and
acceptor materials in the device architecture of bulk-heterojunction, using highly conjugated BYG-1 and
BYG-2 as electron acceptors along with an appropriate small molecule donor (SMD). After improving the
device architecture of the active layer using a suitable donor-to-acceptor weight ratio with solvent vapour
annealing, we achieved power conversion efficiencies of 8.67% and 7.12% for BYG-1 and BYG-2,
respectively. The superior photovoltaic performance of the fluorine-substituted BYG-1 can be attributed
to its higher crystallinity, more balanced charge transport mobilities and efficient exciton dissociation.
Received 12th November 2018,
Accepted 11th December 2018
DOI: 10.1039/c8tc05692g
rsc.li/materials-c
2
4,25
been reported for OSCs based on NFAs,
surpassing the
Introduction
fullerene-based counterparts, representing immense future for
Organic solar cells (OSCs) have various fascinating advantages NFA-based OSCs.
such as being light weight, flexible and low-cost photovoltaic Small molecule donors (SMDs) have diverse advantages and
potential applications compared to their polymer counterparts,
1–5
devices fabricated through solution-processing on a large scale.
In OSC devices, active layer materials play a major role; fullerene such as known chemical structures, good photovoltaic perfor-
derivatives are used as electron acceptors whereas a variety of mance, less synthetic cost, and easy and reproducible purification
2
6–30
polymers and small-molecule organic materials are used as process.
electron donors with power conversion efficiencies (PCEs) of over investigation of NFAs has become a very hot topic in the research
Considering the above-mentioned advantages, the
6–14
11%.
Recently, a record PCE of above 14% has been reported area of OSCs, which may pave the way for potential large-area
for ternary active layer-based OSCs. However, due to some devices. Recently, OSCs based on SMD and SMA BHJ active layers
disadvantages of the fullerene derivatives, such as low absorption have shown remarkable PCEs in the range of 9–11%.
15
31,32
range, high cost of synthesis, no scope for tuning the energy levels
and high voltage loss, the PCE cannot be further improved.
In most of the non-fullerene-based OSCs, the electron
mobility of the acceptor is generally lower compared to the
1
6,17
3
3
In order to overcome these obstacles, during the last few years a hole mobility of the donor. This mismatched charge mobility
large amount of work has been dedicated towards the design leads to charge recombination and results in poor short-circuit
and development of non-fullerene small molecule acceptors current as well as poor fill factor. Therefore, in order to employ
1
8–23
(NFAs).
Remarkably high PCEs in the range of 11–13% have non-fullerene acceptors in the OSCs, it is necessary to design a
non-fullerene with high electron mobility. Small molecule non-
fullerene acceptors consisting of a strong electron-withdrawing
moiety like perylenediimide have high electron mobilities.
a
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical
Technology, Uppal road, Tarnaka, Hyderabad 500007, India.
E-mail: spsingh@iict.res.in
3
4
The incorporation of additional electron-withdrawing units like
cyano into the small molecule non-fullerene acceptor has also
b
Academy of Scientific and Innovative Research, CSIR-IICT, Hyderabad 500007,
India
3
5–38
been found to increase the electron mobility.
Furthermore,
c
Department of Chemistry, Umeå University, SE-90187 Umeå, Sweden
Department of Physics, The LNMIIT (Deemed University), Jamdoli, Jaipur,
Rajstan 302031, India. E-mail: gdsharma273@gmail.com
Electronic supplementary information (ESI) available. See DOI: 10.1039/
these strategies also lead to a lower LUMO energy level of the
d
3
9
acceptor. When such an acceptor is employed in BHJ OSCs
that have a donor with high-lying LUMO energy levels, it leads
to a reduction in the Voc value and also the PCE of the device
†
c8tc05692g
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Fig. 1 Chemical structures of the acceptors BYG-1, BYG-2 and donor SMD.
due to the large offset between LUMO of the donor and the with an asymmetrical D1–D2–A small molecule donor, which
4
0
acceptor, which causes great energy loss. Therefore, in order consists of a phenothiazine (D2) core and the terminal groups
4
5
to increase the PCE of the OSC devices, exploring new wide- of TPA (D1) and 1,3-indandione (A), denoted as SMD. The
bandgap non-fullerene molecules with appropriate redox potentials chemical structure of the present NFSMA along with the SMD
and high electron mobilities is also necessary.
are shown in Fig. 1. After the optimization of the BHJ active
We have designed, synthesized and characterized two small layer, we achieved overall PCEs of about 8.67% and 7.12%,
0
0
molecule non-fullerene acceptors 6,6 -((7,7 -(9,9-didecyl-9H-fluorene- respectively, for the BYG-1 and BYG-2 OSCs.
,7-diyl)bis(6-fluorobenzo[c][1,2,5]thiadiazole-7,4-diyl))bis(ethyne-
,1-diyl))bis(2-(2-ethylhexyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione)
2
2
0
0
(
BYG-1) and 6,6 -((7,7 -(9,9-didecyl-9H-fluorene-2,7-diyl)bis(benzo[c]- Results and discussion
[1,2,5]thiadiazole-7,4-diyl))bis(ethyne-2,1-diyl))bis(2-(2-ethylhexyl)-
Material synthesis and optoelectronic properties
1H-benzo[de]isoquinoline-1,3(2H)-dione) (BYG-2), see Fig. 1. The
The synthetic procedure of BYG-1 and BYG-2 non-fullerene
acceptors is described below (Scheme 1). The amide formation
of 4-bromo-1,8-naphthalic anhydride afforded intermediate 1
in a good yield. The Sonogashira coupling reaction between
intermediate 1 and 2-methylbut-3-yn-2-ol gave intermediate 2,
which after de-protection using NaOH resulted in intermediate
BYG-1 and BYG-2 compounds were molecularly engineered by
using the electron-donating fluorene unit as the central core,
connected on both the sides to the strong electron-withdrawing
2
-ethylhexyl naphthamide group via the strong electron acceptor
linking groups 5-fluorobenzo[c][1,2,5]thiadiazole and benzo[c]-
1,2,5]thiadiazole. The central fluorine core is a well-known
[
3
3
in a good yield. Intermediates 4 and 5 were prepared by using
and 4,7-dibromobenzo[c][1,2,5]thiadiazole, and 4,7-dibromo-
highly planar and electron-rich moiety, with a major advantage
of a low-lying HOMO energy level. The attachment of two n-decyl
chains at the C-9 position of the fluorine unit improves the
solubility and aggregation behaviour of the calamitic-type
5-fluorobenzo[c][1,2,5]thiadiazole substrates in the Sonogashira cou-
pling reaction, respectively. Alkylation of 2,7-dibromo-9H-fluorene
resulted in intermediate 6 and the Miyaura borylation reaction of 6
4
1
NFAs. This is also helpful for high charge carrier ability and
enhances the open-circuit voltage (Voc) of the devices. In addition
to the electron-withdrawing properties of the benzo[c][1,2,5]-
thiadiazoles in BYG-1 and BYG-2, which lowers the LUMO energy
levels, these hetero-aromatic groups extend the conjugated
systems and increase the planarity of compounds, which results
0
yielded 2,2 -(9,9-didecyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (intermediate 7). Finally, Suzuki coupling
of intermediate 7 with 4 and 5 yielded the small molecules
BYG-2 and BYG-1, respectively. Column chromatography using
silica gel was applied for the purification of both the molecules
1
13
4
1,42
and they were characterized by H-NMR, C-NMR and mass
spectrometry. Details of these analyses are reported in the
supporting information. The synthesis and structural charac-
in stronger light absorption and better charge transport.
Among the designs of n-type molecules for organic solar
cell applications, 1,8-naphthalimide (NAI) is a gifted candidate.
The NAI molecules generally own a LUMO energy level that
can match well with small molecular donors and some of the
4
5
terization of SMD was reported in our earlier study. BYG-1
and BYG-2 are highly soluble in dichloromethane (DCM), ethyl
4
3,44
acetate (EtOAc), chloroform (CHCl ), dimethylsulfoxide (DMSO)
3
polymer donors.
Further, the presence of NAI groups in the
and dimethylformamide (DMF).
terminal positions of the designed molecules can influence
the molecular self-assembly tendencies. Due to the bulkiness of
the NAI moiety, intermolecular interactions can be weakened,
UV-visible absorption
which can be used for the control of aggregation. In addition, UV-vis absorption spectra of the BYG molecules were obtained
the solubility of molecules with NAI groups can notably in dilute chloroform solution and as well as in thin films. The
increase by introducing an alkyl chain at the N-position of absorption spectra of BYG-1 and BYG-2 showed similar absorp-
the NAI moiety. In the present study, we have explored these tions in dilute solution (Fig. 2), where both molecules exhibited
NFSMAs for their application in all small molecule OSCs along a broad absorption range in the region from 300 nm to 500 nm
710 | J. Mater. Chem. C, 2019, 7, 709--717
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Scheme 1 Synthetic procedure of BYG-1 and BYG-2 non-fullerene acceptors.
with strong absorptions at 443 nm and 458 nm due to intra- wavelength 500–750 nm with an optical band gap of 1.74 eV,
molecular charge transfer (ICT) transitions and weak absorp- which is similar for BYG-1 and BYG-2.
tions at 375 nm and 378 nm due to p–p transitions, respectively.
Herein, both BYG molecules have similar molecular struc-
The molecular molar extinction coefficients (e) of BYG-1 and ture and show similar absorption spectra. The main difference
4
ꢁ1
ꢁ1
BYG-2 (Fig. 2b) were determined to be 8.11 ꢀ 10 M cm and between BYG-1 and BYG-2 is that the incorporation of F atom
4
ꢁ1
ꢁ1
7
.97 ꢀ 10 M cm , respectively. The absorption spectrum of in the benzo[c][1,2,5]thiadiazole (BT) group increases the inter-
BYG-1 and BYG-2 in thin films extends up to 456 nm and action and enhances the push–pull effect among the donor and
70 nm, respectively (Fig. 2a). This suggests that they are acceptor units due to the strong electron-withdrawing effect of
effectively aggregated in the thin film state due to the existence F atom, which increases the molar extinction coefficient, and
of strong intermolecular p–p interactions derived from the hence, enhances the light harvesting capacity and Jsc
4
.
opt
conjugated backbone structure. The optical band gaps (E
of BYG-1 and BYG-2 as thin films were determined to be 2.49 eV
g
)
Cyclic voltammetry
and 2.45 eV, respectively. It is likely that the broader absorption The frontier molecular orbital HOMO and LUMO values of BYG-1
of BYG-2 compared to BYG-1 can result in more efficient and BYG-2 were calculated by cyclic voltammetry in anhydrous
exciton generation and higher short-circuit current density DCM by using 0.1 M tetrabutyl ammonium perchlorate (TBAF)
( JSC), and lead to a better photovoltaic performance in BHJ as the supporting electrolyte. The cyclic voltammograms were
+
OSCs. The absorption spectra of SMD in the thin film is also consistent using a ferrocene/ferrocinium (Fc/Fc ) redox couple.
shown in Fig. 2a, and exhibits a strong absorption band in the Both BYG-1 and BYG-2 showed multiple reduction waves with
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Fig. 2 (a) UV-visible absorption spectra of compounds BYG-1, BYG-2 and SMD in solution and in the thin film. (b) Molar extinction coefficient plots of
BYG-1 and BYG-2.
for SMD/BYG-1 and SMD/BYG-1 are 0.61 eV and 0.56 eV,
respectively. This is sufficient for efficient hole transfer from
the BYG-1 or BYG-2 to SMD. The LUMO offset in SMD/BYG-1
and SMD/BYG-2 is about 0.12 eV, which is lower than the
4
6
generally accepted empirical threshold value of 0.3 eV. The
optical and electrochemical properties of BYG-1, BYG-2 and SMD
indicate that BYG-1 and BYG-2 can be suitable as acceptors along
with SMD as donor for the all small molecule OSCs.
Photoluminescence (PL) spectra
In order to get high PCE for the OSCs, high values of both Jsc
and Voc are required, and it is necessary to minimize the LUMO
offset among the donors and acceptors used in the BHJ active
layer of OSCs and maintain a well-organized charge transfer from
the donor to the acceptor. In order to get the details about the
exciton separation and charge transfer in the BHJ active layer, PL
spectra of the pristine donor, pristine acceptor and the blend film
were studied. Fig. 4 demonstrates the PL spectra of SMD, BYG-1
and their blend films. From the properties of the absorption
spectra of SMD and BYG-1 in thin film, both molecules, BYG-1
and SMD, were selected and excited at 450 nm and 580 nm,
respectively, in pure form as well as in blend. The PL intensity
of BYG-1 was notably quenched when compared to that of
SMD:BYG-1 blend at 450 nm excitation, indicating favourable
hole transfer from BYG-1 to SMD. Importantly, PL emission
of SMD was also quenched at 580 nm during the addition of
BYG-1, which can be ascribed to the well-organized electron
transfer from SMD to BYG-1. A Similar PL spectrum was also
observed for SMD:BYG-2 thin film. An effective PL quenching
behaviour was observed in these active layers indicating that
the appropriate nano-scale phase separation of the blend film
Fig. 3 Cyclic voltammograms of BYG-1 and BYG-2 recorded in dichloro-
methane with 0.1 M TBAP.
similar structure within the potential window of the solvent.
The CV plots of BYG-1 and BYG-2 are represented in Fig. 3 and
the relevant data are presented in Table 1. The HOMO energy
levels of BYG-1 and BYG-2 were determined to be ꢁ5.89 eV and
ꢁ
ꢁ
5.84 eV, respectively, calculated using the equation EHOMO =
e[Eox + 4.80 ꢁE(Fc/Fc+)]. The LUMO energy levels of BYG-1 and
BYG-2 were determined to be ꢁ3.40 eV and ꢁ3.39 eV, respectively,
from the equation ELUMO = EHOMO + Eo–o. The reason for the lower
HOMO and LUMO energy levels for BYG-1 is evidently due to the
high electronegativity of the F atom in the benzodithiazole ring.
The LUMO energy level of both BYG-1 and BYG-2 are quite
high compared to that of PC71BM, and this should be beneficial
for achieving high Voc values for the corresponding OSCs.
The HOMO and LUMO energy levels of SMD are ꢁ5.28 eV
and ꢁ3.26 eV, respectively. Hence, the HOMO energy offsets
4
7–49
morphology is favourable for efficient exciton dissociation.
The efficient exciton dissociation and charge transfer in these
blends explained the advantage of effective photocurrent
generation for the resulting OSCs.
Table 1 Optical and electrochemical parameters BYG-1 and BYG-2
l
max,solution (nm)
ꢁ1
l
max,film
E
(eV)
0–0
HOMO
(eV)
LUMO
(eV)
ꢁ
1
Charge carrier mobility
Material
(e, M cm
)
(nm)
The electron mobilities of pristine BYG-1 and BYG-2 were calcu-
lated by the space charge limited current (SCLC) measurement
BYG-1
BYG-2
443 (81 103)
458 (79 737)
456
470
2.49
2.45
ꢁ5.89
ꢁ5.84
ꢁ3.40
ꢁ3.39
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Fig. 4 (a) PL spectra of BYG-1 and BYG-2 in DCM solution. (b) PL spectra of BYG-1 and SMD:BYG-1 blend in a thin film at different wavelengths of light.
with device structures ITO/Al/BYG-1 and BYG-2/Al. The BYG-1 SCLC measurement, and responsible for the improvement of Jsc
ꢁ4
2
ꢁ1 ꢁ1
showed higher electron mobility (3.64 ꢀ 10 cm V
s
) than and FF of the corresponding OSCs.
). The high electron
mobility of BYG-1 may be related to the fact that the incorporation
ꢁ
4
2
ꢁ1 ꢁ1
that of BYG-2 (2.18 ꢀ 10 cm
V
s
Photovoltaic properties
of F atom in the BT improves the intermolecular interaction The performance of the OSCs was based on BYG-1 and BYG-2 as
through C–FꢂꢂꢂS and C–FꢂꢂꢂH interactions. The enhanced electron electron acceptors along with SMD as the donor using the
mobility of BYG-1 benefits the balanced charge transport and conventional configuration of ITO/PEDOT:PSS/SMD:BYG-1 or
reduces the probability of charge recombination during their BYG-2/PFN/Al. SMD was selected as a donor due to its com-
transport in the active layer towards the electrode when acting plementary absorption spectra to that of both BYG-1 and BYG-2
as an electron acceptor in the blend.
and suitable HOMO and LUMO energy levels of ꢁ5.28 eV and
3.26 eV, respectively. Initially, we optimized the donor and
acceptor weight ratio using chloroform solution and observed
ꢁ
XRD patterns of the BYG-1 and BYG-2 thin films
To understand the difference in molecular ordering and crystal- that the optimized weight ratio of 1 : 2 showed the best PCEs
ꢁ
2
linity of pristine BYG-1 and BYG-2, we recorded the X-ray of 3.12% ( Jsc = 7.34 mA cm , Voc = 1.12 eV and FF = 0.38)
ꢁ
2
diffraction patterns, shown in Fig. 5. Both the SMAs showed a and 2.73% ( Jsc = 7.03 mA cm , Voc = 1.08 eV and FF = 0.36) for
100) diffraction peak at 2y = 4.72 corresponding to the lamellar BYG-1- and BYG-2-based OSCs, respectively. The fabrication of
(
stacking distance of 1.72 nm. Moreover, the (100) diffraction OSCs and their characterization are reported in the supplementary
peak of BYG-1 was significantly stronger than that of BYG-2. information. For the implementation of these device PCEs, we
The (010) diffraction peaks were found at 2y = 23.02 and 22.64 performed SVA treatment of the active layer (in THF for 60 s) to
for BYG-1 and BYG-2, respectively, which can be attributed to improve the phase separation and morphology. The J–V character-
the p–p stacking. The p–p stacking distance extracted from the istics of the SVA-based devices are shown in Fig. 6a and their
(010) diffraction peak was 0.37 nm and 0.40 nm for BYG-1 and photovoltaic values are displayed in Table 2. The optimized devices
BYG-2, respectively. The stronger lamellar and p–p stacking presented overall PCEs of 8.67% and 7.12%. It was observed that
properties and compact intermolecular packing of BYG-1 indi- the Voc of these devices was fairly high (more than 1.0 V), which
cated that BYG-1 had a higher crystallinity than BYG-2, which is might be due to the higher values of LUMO energy levels of both
50–53
beneficial for high electron mobility, as confirmed from the BYG-1 and BYG-2.
We also constructed the devices based on
PC71BM as acceptor under identical conditions for comparison,
and the data are presented in Table 2. Although, the Jsc and FF
values of OSC based on PC BM are comparable to that for the
7
1
non-fullerene acceptors, the difference in the values of Voc is
quite significant, due to the different LUMO energy levels of
non-fullerene acceptors and PC71BM. This indicates that our
design for the new wide-bandgap acceptor with a higher value
of LUMO energy level is effective to improve the Voc
.
In order to know the different Jsc values of the OSCs using
BYG-1 and BYG-2 as acceptors, the incident photon to current
conversion efficiency (IPCE) responses of the OSCs are displayed
in Fig. 6b. Both the OSCs exhibit a broad response ranging from
350 nm to 750 nm, and it resembles the optical absorption
spectra of the blend active layers (as shown in Fig. S3, ESI†)
Fig. 5 XRD patterns of the BYG-1 and BYG-2 thin films.
suggesting that both the donor and acceptors employed in the
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Fig. 6 (a) Characteristic J–V curves of the optimised SMD:BYG-1- and SMD:BYG-2-based BHJ photovoltaic devices measured under AM 1.5G
illumination at 1 sun intensity. (b) IPCE spectra of BYG-1 and BYG-2 molecules.
Table 2 Photovoltaic parameters of OSCs based on BYG-1 and BYG-2 as
acceptors
ꢁ2
Acceptor
J
sc (mA cm
)
Voc (V) FF
PCE (%)
Eloss (eV)
a
BYG1 (as cast) 7.34
BYG2 (as cast) 7.03
1.12
1.08
1.08
1.02
0.82
0.38 3.12 (3.05)a
0.36 2.73 (2.67)a
BYG1 (SVA)
BYG2 (SVA)
PC71BM
12.95
11.84
12.06
0.62 8.67 (8.61)a 0.66
0.59 7.12 (7.05) 0.72
0.63 6.23
0.92
a
Average of 8 devices.
active layer are responsible for the photocurrent generation.
The maximum values of EQE for the BYG-1- and BYG-2-based
devices are about 68.4% and 63.5%, respectively. The EQE
response in the 350–500 nm wavelength regions corresponds
to the exciton dissociation generated in BYG-1 or BYG-2 with
the electron hole transfer from the HOMO of BYG-1 or BYG-2 to
that of SMD. Moreover, the EQE response in the wavelength
Fig. 7 Photocurrent density variation with effective voltage plots of BYG-1
and BYG-2 molecules.
To better understand the charge transport we measured the
range of 550–750 nm corresponds to the exciton generation in electron and hole mobilities in the optimized active layers
the SMD and electron transfer from the LUMO level of SMD using the dark J–V characteristics and fitting these with the
to that of BYG-1 or BYG-2. The Jsc values estimated from the SCLC model (as shown in Fig. 8 only for electron-only devices).
ꢁ
2
integration of EQE response are about 12.84 mA cm and
ꢁ2
1
1.76 mA cm for BYG-1 and BYG-2, respectively, which are in
good agreement with the values observed in the J–V character-
istics under illumination.
We have performed the variation of photocurrent density
(
Jph, i.e. Jph = J ꢁ J , where J and J are the current densities in
L
D
L
D
the presence of light and in dark, respectively) with effective
voltage (Veff, defined as Veff = V is the voltage
when Jph is zero and Vapp is the applied voltage) (Fig. 7). In both
the OSCs, the Jph, after linear variation with Veff at less voltages,
reached saturation quickly (saturation photocurrent ( Jphsat)) at
o
ꢁ Vapp, where V
o
0.73 V and 0.96 V for BYG-1 and BYG-2, respectively, indicating
that all the photogenerated excitons were dissociated into free
charge carriers and subsequently collected at the electrodes
5
4
very efficiently. The overall exciton dissociation efficiency and
charge collection efficiency (Pdiss) are estimated from the ratio
of Jphsat/Jph under short-circuit environment and the values are
Fig. 8 Dark current–voltage characteristics of electron-only devices
using optimized BYG-1BYG-1:SMD and BYG-2:SMD active layers. Straight
lines are SCLC fitting.
0.95 and 0.91 for the OSCs based on the optimized SMD:BYG-1
and SMD:BYG-2 active layers, respectively.
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Fig. 9 TEM images of BYG-1:SMD and BYG-2:SMD in optimized active layers. Scale bar is 200 nm.
The hole mobility in both the blends was about 9.93 ꢀ
ꢁ
5
2
ꢁ1 ꢁ1
1
10
0
cm
V
s
, but the electron mobility was 2.87 ꢀ
ꢁ4 2 ꢁ1 ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
cm V
s
and 1.9 ꢀ 10 cm V
s , for BYG-1:SMD
and BYG-2:SMD, respectively. The higher value of electron
mobility for BYG-1:SMD leads to an increase in both Jsc
and FF, thereby resulting in higher PCE of the OSC based on
BYG-1:SMD.
Active layer morphology
To study the phase separation and morphology of the active
layers, we carried out transmission electron microscopy (TEM)
measurements, as shown in Fig. 9, for the optimized active
Fig. 10 Optimized structures of BYG-1 (a) and BYG-2 (b) molecules.
layers. The TEM images of the as-cast active layers are shown in using the conductor-like polarizable continuum model (CPCM).
Fig. S4 (ESI†), which show poor phase separation that hinders The optimized geometric structures of BYG-1 and BYG-2 mole-
charge transport. However, when the active layers were sub- cules are shown in Fig. 10.
jected to SVA treatment, they showed much better phase
separation, which is beneficial for charge transport. As can be
TDDFT for BYG-1 and BYG-2
seen from Fig. 9, the BYG-1:SMD active layer showed more We used the time-dependent density functional theory (TDDFT)
phase separation (15–20 nm) than the BYG-1:SMD (12–15 nm) method to study the correlation of the experimental absorption
counterpart. Phase separation is useful for charge transport, results with the theoretical absorption results of both the
which leads to the enhancement of the overall PCE of the OSCs. molecules using similar exchange correlation functional and
In OSCs, one of the important factors is the voltage energy loss basis set in Gaussian 09 program. The geometric parameters
(
E
loss), which is defined as Eloss = E
g
ꢁ Voc, where E
g
represents the of both the molecules in the singlet state reproduced the
5
0
optical band gap of the donor or the acceptor. The values of Eloss experimental values. We performed several excitations to cover
for optimized OSCs based on BYG-1, BYG-2 and PC BM are the visible and ultraviolet regions of the dyes from 300 nm to
71
0.66 eV, 0.72 eV and 0.92 eV, respectively. The lower value of Eloss 600 nm. Herein, we have mainly focused on the visible
for the non-fullerene acceptor than that for PC71BM may be region to observe the charge transfer property. The theoretical
attributed to the higher values of Voc for our devices, leading to absorption spectra of both molecules were well aligned with
the higher values of PCE for our non-fullerene OSCs.
the experimental absorption spectra, as shown in Fig. 11. The
frontier molecular orbital energies calculated at the B3LYP/6-
Theoretical study
311g(d,p) level of theory compared with the experimental values
The theoretical studies of BYG-1 and BYG-2 molecules were are shown in Table 3, and also, the major allowed transitions in
conducted using the B3LYP exchange correlation functional the range of 290–450 nm of BYG-1 and BYG-2 were calculated by
with 6-311g(d,p) as the basis set in Gaussian 09 program. To using B3LYP/6-311g(d,p) level of theory in DCM solvent, which
know the solvation effect from DCM on both the molecules, we is briefly explained in the supporting information (Fig. S1, S2
performed the self-consistent reaction field (SCRF) calculation and Tables S1, S2, ESI†).
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Journal of Materials Chemistry C
Fig. 11 Normalized plots of experimental and simulated absorption spectra of BYG-1 (a) and BYG-2 (b) molecules.
Table 3 The comparison of frontier molecular orbital energies (theoretical
and experimental values)of both the molecules
2 L. Zuo, S. Zhang, M. Shi, H. Li and H. Chen, Mater. Chem.
Front., 2017, 1, 304.
3
4
P. Cheng and X. Zhan, Chem. Soc. Rev., 2016, 45, 2544.
L. Y. Lu, T. Y. Zheng, Q. H. Wu, A. M. Schneider, D. L. Zhao
and L. P. Yu, Chem. Rev., 2015, 115, 12666.
H. Yao, L. Ye, H. Zhang, S. Li, S. Zhang and J. Hou, Chem.
Rev., 2016, 116, 7397.
Experimental
Theoretical
a
b
c
Compound
E
HOMO
E
LUMO
E
0–0
E
HOMO
E
LUMO
HLG
BYG-1
BYG-2
ꢁ5.89
ꢁ5.84
ꢁ3.40
ꢁ3.39
2.49
2.45
ꢁ5.91
ꢁ5.89
ꢁ3.37
ꢁ3.28
2.54
2.61
5
6
7
8
9
a
b
Obtained by adding 4.8 to Eoxd
values. HOMO–LUMO gap.
.
Obtained by adding 4.8 to LUMO
J.-D. Chen, C. Cui, Y.-Q. Li, L. Zhou, Q.-D. Ou, C. Li, Y. Li and
J.-X. Tang, Adv. Mater., 2015, 27, 1035.
c
Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang,
T. P. Russell and Y. Cao, Nat. Photonics, 2015, 9, 174.
Y. Liu, J. Zhao, Z. Li, C. Mu, W. Ma, H. Hu, K. Jiang, H. Lin,
H. Ade and H. Yan, Nat. Commun., 2014, 5, 5293.
J. Huang, J. H. Carpenter, C.-Z. Li, J.-S. Yu, H. Ade and A. K.-Y.
Jen, Adv. Mater., 2016, 28, 967.
Conclusion
Two wide-bandgap small molecule non-fullerene acceptors BYG-1
and BYG-2 were synthesized and their photophysical properties
were investigated. The optical and electrochemical properties
indicated that both BYG-1 and BYG-2 were suitable as acceptors
along with an appropriate donor with complementary absorption
spectra and similar HOMO and LUMO energy levels. We used a
small molecule donor (SMD) along with these non-fullerene
acceptors for the construction of the all small OSCs devices. Upon
optimization of the active layer phase separation, we achieved
overall PCEs of 8.67% and 7.12% for BYG-1 and BYG-2 acceptors,
respectively, which were higher than that for the PC71BM acceptor.
The increase in the PCEs for the non-fullerene acceptors may be
attributed to their higher LUMO energies and low voltage losses.
1
1
1
1
1
0 J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma and
H. Yan, Nat. Energy, 2016, 1, 15027.
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3 T. Kumari, S. M. Lee, S.-H. Kang, S. Chen and C. Yang,
Energy Environ. Sci., 2017, 10, 258.
4 J. Wan, X. Xu, G. Zhang, Y. Li, K. Feng and Q. Peng, Energy
Environ. Sci., 2017, 10, 1739.
1
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Conflicts of interest
The authors declare no competing financial interest.
1
1
8 G. Nicola, W. Andrew, M. Maximilian, D. Baran, I. McCulloch
and C. J. Brabec, Adv. Energy Mater., 2018, 8, 1703298.
9 F. Liu, T. Hou, X. Xu, L. Sun, J. Zhou, X. Zhao and S. Zhang,
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Acknowledgements
BY would like to thank University Grants Commission (UGC),
Government of India for providing the junior and senior 20 J. Hou, O. Ingan ¨a s, R. H. Friend and F. Gao, Nat. Mater.,
research fellowship. SPS thanks DST for Indo-Poland project
2018, 17, 119–128.
DST/INT/POL/P-26/2016.
21 P. Cheng, G. Li, X. Zhan and Y. Yang, Nat. Photonics, 2018,
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