Three-dimensional molecular donors combined with polymeric acceptors for high performance fullerene-free organic photovoltaic devices

Article abstract of DOI:10.1039/c5ta06639e

Non-fullerene acceptor based organic photovoltaic devices (OPVs) reported so far are inferior to those derived from fullerenes. This increases the speculation on whether donors need to be tailored for advancing non-fullerene OPVs. We explored herein two direct arylation-derived diketopyrrolopyrrole (DPP)-based three-dimensional (3D) donors that can deliver respectable power conversion efficiencies (PCEs) of 4.64% and 4.02% with polymeric acceptor N2200 blends, surpassing those obtained from PC₇₁BM (3.56% and 3.22%, respectively). It is found that these 3D-shaped molecular donors can yield improved photo-to-current conversion and balanced charge transport when blending with the linear N2200 polymer. This finding suggests that engineering molecular geometry can be a promising approach for developing high-performance materials.

Full text of DOI:10.1039/c5ta06639e

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Journal of Materials Chemistry A
ARTICLE
Three-dimensional molecular donors combined with polymeric
acceptors for high performance fullerene-free organic photovoltaics
Received 00th January 20xx,
Accepted 00th January 20xx
Shi-Yong Liu,abc Jae Woong Jung,a Chang-Zhi Li,ab Jiang Huang,ad Jianyuan Zhang,a Hongzheng
Chen*b and Alex K.-Y. Jen*ab
DOI: 10.1039/x0xx00000x
www.rsc.org/
The non-fullerene acceptors based organic photovoltaics (OPVs) reported so far are inferior to those derived from fullerenes.
This intrigues the speculation whether donors need to be tailored for advancing non-fullerene OPVs. We explored herein
two direct arylation-derived diketopyrrolopyrrole(DPP)-based three-dimensional (3D) donors that can deliver respectable
power conversion efficiencies (PCEs) of 4.64% and 4.02% with polymeric acceptor N2200 blends, surpassing those
obtained from PC71BM (3.56% and 3.22%, respectively). It is found that these 3D-shaped molecular donors can yield the
improved photo-to-current conversion and balanced charge transport when blending with linear N2200 polymer. This
finding suggests that the engineering molecular geometry can be a promising approach for developing high-performance
materials.
better performance in devices than the planar ones,4-23 which
coincides with the spherical feature of fullerenes facilitating multi-
directional charge-transport in OPVs.24 Among numerous non-
fullerene acceptors that have been developed so far, their device
performance are usually lower than those derived from fullerenes
using the same donors.3,4b,5-22 This intrigued the speculation whether
the donors previously developed for blending with fullerenes are
suitable for non-fullerene OPV. It inspires us to explore possible
strategies on making new donors for better pairing with efficient
electron acceptors.
1 Introduction
Organic electron donor (D) and acceptor (A) based bulk
heterojunction (BHJ) photovoltaics have attracted extensive
attentions in the past decade due to their light-weight, good
flexibility, and cost-effectiveness merits that can be potentially
realized through simple solution processing.1 Ideally, these electron
donors and acceptors should possess intensive and complementary
absorption for efficient light harvesting, and the ability to form nano-
structured interpenetrating networks of D-A phases for efficient
photon-charge generation and transport. Although fullerene
derivatives have been widely used as efficient electron acceptor for
the BHJ OPVs, they inherit certain drawbacks from pristine fullerene
such as weak visible light absorption, limited electronic tunability,
and high production cost. Therefore, vigorous efforts have been
devoted to developing efficient non-fullerene acceptors,2 leading to
encouraging performance for non-fullerene based OPVs.3, 4
In this paper, four DPP electron-donating units are installed onto
either a tetraphenyl-methane or a tetraphenyl-silane core via an atom
efficient direct arylation reaction to result in two novel 3D-shaped
donor molecules (named as C-DPP and Si-DPP, Fig. 1). These
donors are expected to transport hole carriers in multiple directions
in devices.24 To verify our design concept, two well-known electron
acceptors, polymer N220025,26 and PC71BM were chosen to pair with
3D-DPP donors (hereafter, 3D-DPPs refers to both C-DPP and Si-
DPP) for solution-processed BHJ OPV devices. The PCEs achieved
for C-DPP:N2200 and Si-DPP:N2200 devices are 4.64 and 4.02%,
respectively. In a parallel study, the performance of the devices
made from these two molecular donors with PC71BM, C-
DPP:PC71BM and Si-DPP:PC71BM have lower PCE with 3.56%
and 3.22%, respectively. The higher performance derived from the
3D-DPPs:N2200 devices are attributed to the improved incident
photon-to-current conversion efficiency (IPCE) from the
complementary and broader light absorption of 3D-DPPs:N2200
blends, and the more balanced charge mobility. The PCE of 4.64% is
a respectable value for the non-fullerene OPVs using molecular
donor and polymer acceptor.26a-c,27 The current system also
Usually, the development of non-fullerene based devices is
focused on pairing acceptors with well-established efficient donors.
Materials with non-planar and/or 3D architectures in general exhibit
a. Department of Materials Science and Engineering, University of Washington, Box
352120, Seattle, Washington 98195, USA.E‐mail: ajen@u.washington.edu
b. Department of Polymer Science & Engineering, Zhejiang University, Hangzhou,
310027, P. R. China. E‐mail: hzchen@zju.edu.cn
c. Department of Pharmacy & Chemistry, Taizhou University, Taizhou, 317000, P. R.
China
d. School of Optoelectronic Information, University of Electronic Science and
Technology of China, Chengdu 610054, P. R. China
†
Electronic Supplementary Information (ESI) available: The detailed synthetic
routes of all precursors, 1H NMR spectra of precursors, 1H and 13C NMR, and
MALDI‐TOF MS spectra of the target 3D‐DPPs, and other spectra mentioned in the
main text. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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represents one of the most efficient BHJs composed of organic Differential scanning calorimetry (DSC) measurement was used to
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DOI: 10.1039/C5TA06639E
semiconductors obtained via direct arylation.28, 29
gain insight of intermolecular interactions and crystallinity for C-
DPP and Si-DPP. The DSC scanning from 5 to 280 °C did not find
any melting or recrystallization peak (Fig. S6), suggesting both 3D-
DPPs are non-crystalline in nature, which is similar to the 3D tetra-
PDI previously reported.22
2 Results and discussion
2.1 Synthesis, characterization and theoretical calculation
Fig. 1 depicts the synthetic route of C-DPP and Si-DPP, and the
chemical structure of polymer N2200. An effective strategy to access
multi-DPPs29 was used to install four DPP units onto tetraphenyl-
silane and tetraphenyl-methane cores via a ligand-free palladium-
catalyzed direct arylation reaction to afford C-DPP and Si-DPP with
excellent yields (90% and 94%, respectively). The n-type polymer
N2200 employed in this work (Mn= 49.2kDa, Mw= 89.5 kDa and
Density functional theory (DFT) calculation was carried out to
study the molecular geometries of 3D-DPPs. The DFT-optimized
geometries show that four DPP units of 3D-DPPs are tetrahedrally
orientated with radii of 2.02 and 2.05 nm respectively (Fig. 1),
owing to the sp3 hybrid of C and Si atom centers. Such a tetrahedral
architecture should help suppress the tight packing of molecules.30
Structurally, the four DPP arms are connected via sp3 C and Si
centers. Although the DPP arms have less molecular orbital (MO)
overlap between each others, they are close to each other in space at
PDI=1.8) was synthesized by using
a
modified reported
procedure.25a The target 3D-DPPs have been fully characterized by atomic scale. We speculate that the charges on 3D-DPPs would
1H and 13C NMR, matrix-assisted laser desorption ionization time of
In this way, 3D distribution and transport of charges will lead to
flight (MALDI-TOF) mass spectroscopy (MS) (Fig. S1-S5, see
delocalize throughout the DPP arms, and can hop among each other.
statistically better D-A charge separation and collection.
Electronic Supplementary Information (ESI)) and elemental analysis.
C2H5
C4H9
S
S
O
C2H5
N
N
O
C4H9
N
N
C2H5
C4H9
O
O
S
C2H5
S
C2H5
C4H9
O
Br
Br
Pd(OAc)2, PivOH
K2CO3, DMAc
C2H5
N
X
S
S
X
110oC, 24h, -HBr
Yields 90 and 94%
+
H
N
C2H5
C4H9
O
C2H5
S
S
Br
= C or Si
C4H9
Br
O
C2H5
N
TBPM
TBPS
O
C4H9
Ph-DPP
X
N
N
C2H5
C4H9
O
N
O
S
S
C2H5
C4H9
C10H21
C8H17
O
N
O
S
S
n
O
N
O
C8H17
C10H21
N2200
Si-DPP
C-DPP
Fig. 1 Direct arylation synthesis of C-DPP and Si-DPP, and their corresponding DFT optimized geometries (ethyl-hexyl chains
replaced by methyl groups) shown by space filling models, and the structure of N2200.
2 | J. Name., 2012, 00, 1‐3
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2.2 Optical and electrochemical properties
Besides complementary light absorption of the blends, suitable
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DOI: 10.1039/C5TA06639E
energetic levels with sufficient frontier orbital energy (FOE) gap
Fig. 2a shows the normalized individual UV-vis absorption spectra
of the donors (C-DPP and Si-DPP), and the acceptor polymer
N2200 from spin-coated films. C-DPP and Si-DPP films exhibit
similar absorption spectra, with two peaks at 586 and 640 nm (C-
DPP) and 582 and 640 nm (Si-DPP). These peaks are red-shifted
compared to their solution absorption spectra (Fig. S7). In order to
maximize light harvesting of BHJs, low bandgap acceptor, N2200
with strong absorption peaks around 400 and 704 nm was chosen to
complement the absorption of C-DPP and Si-DPP donors (Fig. 1a
and 2b). Consequently, the blend films composed of 3D-DPPs and
N2200 exhibit a strong absorption band ranged from 550 to 750 nm.
In contrast to 3D-DPPs:N2200, the 3D-DPPs:PC71BM blends show
mainly the 3D-DPPs absorption features with a narrower band from
586 to 640 nm, along with the intense absorption less than 350 nm
arising from PC71BM(Fig. S8, see ESI).
between donor and acceptor is also important in driving photon-
generated charge dissociation and thus determining the performance
of BHJ OPVs. The electrochemical properties of C-DPP, Si-DPP,
N2200, and PC71BM were evaluated by cyclic voltammetry (CV) in
CH2Cl2 solution. The lowest unoccupied molecular orbital (LUMO)
and the highest occupied molecular orbital (HOMO) are estimated
from the E1/2 values in solution, using the value of -5.1 eV for
Fc/Fc+.31 Fig. 2c shows the FOEs of the donors and acceptors studied
in this work. The bandgap of C-DPP, Si-DPP, N2200, and PC71BM
are 2.01, 2.04, 1.61 and 2.03 eV, respectively. The LUMO of N2200
(-4.11 eV) is close to that of PC71BM (-4.02 eV), indicating N2200
may have comparable electron affinity with PC71BM.
1.0
C-DPP
Si-DPP
N2200
C-DPP:N2200
Si-DPP:N2200
400 nm
586-640 nm
1.0
0.8
0.6
0.4
0.2
0.0
a)
b)
0.8
0.6
0.4
0.2
0.0
704 nm
400
500
600
700
800
900
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
c)
d)
2
‐3.41 ‐3.35
0
-2
-4
-6
-8
C-DPP:N2200
Si-DPP:N2200
C-DPP:PC71BM
Si-DPP:PC71BM
‐4.02
‐4.11
‐5.36
‐5.45
‐5.72
‐6.05
-0.2
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 2 a) UV-vis spectra of neat films of 3D-DPPs and N2200; b) UV-vis spectra of 3D-DPPs:N2200 BHJ films; c) FOE diagram of 3D-
DPPs, N2200 and PC71BM; d) J-V curves of the optimal solar cells of 3D-DPPs:N2200 and 3D-DPPs: PC71BM.
Table 1 Photovoltaic Parameters of the Optimal Cells based on 3D-DPPs:N2200 and 3D-DPPs:PC71BM under the illumination of AM 1.5G,
100 mW cm-2
d
μ
μ
μ
μ
D:A
VOC
[V]
JSC
FF
PCE
[%]c
Cal. JSC
/
h
e
h e
[mA cm-2]
[mA cm-2]
[10-4 cm2 V-1s-1] e)
3.58
2.87
0.89
0.11
0.33
1.25
2.34
20
C-DPP:N2200a
Si-DPP:N2200a
0.87±0.01
0.86±0.01
8.59±0.18
8.14±0.22
7.69±0.14
7.12±0.14
0.62±0.01 4.64(4.45)
0.54±0.03 4.02(3.93)
0.50±0.01 3.56(3.41)
0.49±0.01 3.22(3.12)
8.71
2.12
2.20
1.65
8.21
C-DPP:PC71BMb 0.89±0.01
Si-DPP:PC71BMb 0.89±0.01
7.74
5
7.19
aThe D:A ratio is 1:1 (w/w). bThe D:A ratio is 1:1.5 (w/w). cThe best and average (in brackets, over 20 devices) PCEs. dCalculated from
IPCE; average values from 12 devices. eHole and electron mobilities by SCLC method.
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and 7.19 mA cm-2, respectively (Table 1), wDhOicIh: 1a0r.e10c3o9n/Csi5sTtAen0t66w3i9tEh
the measured JSC values. Interestingly, the IPCEs of 3D-
2.3 BHJ device performances
Inverted devices with the configuration of indium tin oxide DPPs:N2200 blends extend from 350 nm to 800 nm, which match
(ITO)/ZnO/3D-DPPs:N2200/MoO3/Ag were subsequently fabricated well with their UV-vis absorption spectra (Fig. 3b). It indicates that
to investigate the performance of corresponding BHJ OPVs. The light absorbed by both 3D-DPPs and N2200 are effectively
devices based on Si-DPP:N2200 (1:1) BHJ are first optimized with converted into current in solar cells (Fig. S11), suggesting excitons
three sets of solvents: chloroform (CF), chlorobenzene (CB), and the generated from donor and acceptor domain can be dissociated
mixed CF/CB (1:1, v/v). The PCEs of Si-DPP:N2200 (1:1) based effectively at the D-A junction.
devices processed by CF, CB and CF/CB were 1.03, 1.73 and 3.76%,
The IPCE spectra of 3D-DPPs:PC71BM BHJs are ranging from
350-700 nm, which are slightly lower and narrower than those of
3D-DPPs:N2200. Aside from the weak absorption of PC71BM (Fig.
S8, S12), the amorphous 3D-DPPs may intimately mix with PC71BM
to prevent the formation of suitable nanoscale D-A phase separation.
As shown by the DFT calculations, 3D-DPPs have a radius of more
than 2.0 nm (Fig. 1). The spherical PCBM with a diameter of ~ 0.8
nm34 would be embraced by 3D-DPPs, leading to intimate D-A
mixing (Fig. S13a). Such an intimate D-A mixing will enhance
quenching of PL emission (ESI, Fig. S14), however, it will adversely
affect charge separation,36 and decrease the IPCE of 3D-
DPPs:PC71BM BHJs. Unlike fullerene-based BHJs, the blend of
crystalline N22003b,25 with amorphous 3D-DPPs allows easier
nanoscale phase separation (Fig. S13b), facilitating better charge
respectively (Fig. S9 and Table S1). Note that the PCE of 3.76% in
the Si-DPP:N2200 based device was accomplished without applying
any post-treatments or additives. It is close to the highest PCE (3.9%)
reported for molecular donor/polymer acceptor device using di-
DPP:N2200 BHJ with 1-chloronaphthalene (CN) as additive.26a By
adding 3% (v/v) CN, the PCE of Si-DPP:N2200 is further improved
to 4.02%. Under the similar processing condition, C-DPP:N2200
exhibits the highest PCE of 4.64% (with open circuit voltage, VOC of
0.86 V; short current, JSC of 8.72 mA cm-2, and fill factor, FF of
0.62). Fig. 2d shows the current density-voltage (J-V) characteristic
curves of the optimal cells under the simulated AM 1.5 G irradiation
with intensity of 100 mWcm-2, and Table 1 summarizes the
photovoltaic parameters.
The optimized 3D-DPPs:PC71BM devices were employed to separation and transport.
compare with 3D-DPPs:N2200 devices. Unlike the 3D-DPPs:N2200,
the desired solvent and effective additive for 3D-DPPs:PC71BM
a)
were CF and 1,8-diiodooctane (DIO, 3% v/v), respectively. The
highest PCEs obtained for Si-DPP:PC71BM and C-DPP:PC71BM are
3.22% and 3.56%, respectively (Fig. 2d and Table 1), which are
albeit lower than the N2200-based cells. The inferior PCEs of 3D-
DPPs:PC71BM are mainly ascribed to their lower JSC and FF.
Although a large variety of star-shaped molecular donors have been
developed for OPVs,32-34 the highest PCE for PCBM-based OPVs
using these donors remain around 5%.34a,b These molecular donor
may not be able to form optimal nanoscale D-A phase separated
morphology with PC71BM (ESI, Fig. S13a). However, the PCE
(4.64%) achieved in our study without using fullerene is very close
to the highest PCE of star-shaped donor:PC71BM BHJs reported so
far,34a,b implying polymer acceptors have the potential to be further
explored for pairing with the star-shaped or 3D-shaped molecular
donors.
25000
20000
15000
10000
C-DPP
Si-DPP
N2200
C-DPP:N2200
Si-DPP:N2200
650
700
750
800
850
Wavelength (nm)
50
b)
40
30
20
10
0
2.4 Photoluminescence (PL) and IPCE measurements
To better understand their photophysical properties, PL spectra of
neat films (3D-DPPs, N2200, PC71BM) and their corresponding D-A
blends, in conjunction with the device IPCE spectra were studied.
The PL of 1:1 blends show effective and mutual quenching of
individual 3D-DPPs and N2200 emission, suggesting efficient
charge transfer between3D-DPPs and N2200 (Fig. 3). However, the
emission of C-DPP can be quenched more effectively by N2200
than that of Si-DPP, which may account for the higher Jsc observed
C-DPP:N2200
Si-DPP:N2200
C-DPP:PC71BM
Si-DPP:PC71BM
400
500
600
700
800
Wavelength (nm)
from C-DPP:N2200 BHJ OPVs (Fig. 3a and Table 1). Fig. 3b Fig. 3 a) Steady PL spectra of 3D-DPPs, N2200 neat films, and 3D-
displays the corresponding IPCE spectra of solar cells based on C- DPPs:N2200 BHJ films (1:1, w/w) (excited at 650 nm) b) IPCE
DPP:N2200,
Si-DPP:N2200,
C-DPP:PC71BM,
and
Si- spectra of 3D-DPPs:N2200 and 3D-DPPs:PC71BM BHJs.
DPP:PC71BM. The calculated JSCs from IPCE are 8.71, 8.21, 7.74,
4 | J. Name., 2012, 00, 1‐3
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2.5 BHJ morphology and charge carrier mobility
3 Conclusions
View Article Online
DOI: 10.1039/C5TA06639E
Atomic force microscopy (AFM) and space-charge-limited-current
(SCLC) measurements were further employed to investigate surface
morphology and charge mobility of the active layers. AFM scans
were carried out for films spin-coated on ITO substrates. AFM
images of the active layers based on 3D-DPPs:N2200 and 3D-
DPPs:PC71BM are shown in Fig. S15. As compared to Si-DPP-
based BHJs, C-DPP-based BHJs generally exhibit smoother surface
(Fig. S15). For the SCLC tests, all devices were fabricated by using
the identical procedure for solar cell preparation. The J-V curves for
the hole-only and electron-only devices are shown in Fig. S16. The
third-last and second-last columns in Table 1 list the corresponding
values. The hole mobility (μh) of C-DPP:N2200, Si-DPP:N2200, C-
DPP:PC71BM and Si-DPP:PC71BM BHJs are 3.58, 2.12, 2.20, and
1.65 (×10-4cm2 V-1 s-1), respectively, and the corresponding electron
mobility (μe) are 2.87, 0.89, 0.11, and 0.33 (×10-4cm2 V-1 s-1),
respectively. Among the four devices, C-DPP:N2200 exhibits the
highest and the most balanced charge mobility with the μh/μe ratio of
1.25 (Table 1), compared with the more imbalanced μh/μe ratios of
2.3 for Si-DPP:N2200, 20.0 for C-DPP:PC71BM, and 5.0 for Si-
DPP:PC71BM. This may explain the higher FF (0.62) obtained for
C-DPP:N2200 based device.
In summary, two novel DPP-based 3D molecular donors were
designed and facilely synthesized via an atom efficient direct C-H
arylation reaction. The 3D-DPPs:N2200 BHJ non-fullerene based
devices exhibit better performance than those derived from the
fullerene-based ones, showing a respectable PCE of 4.64% for the
3D-DPPs:N2200, which is ~30% higher than that derived from the
3D-DPPs:PC71BM. The superior performance is attributed to more
efficient IPCE generated from the complementary light absorption of
3D-DPPs and N2200, and the high FFs obtained from the balanced
charge mobilities. In contrast to the well-developed BHJs that
consist of non-planar or 3D acceptors and polymer donors,4-23 the
current work represents the first BHJ OPV based on 3D donors and
polymer acceptors. This preliminary yet promising result
demonstrates the strategy of blending 3D-donors with linear polymer
acceptors can be effective for achieving high-performance OPVs.
4 Experimental
4.1 Materials and methods
Unless otherwise specified, chemicals and solvents were purchased
from Aldrich. All 1H and 13C NMR spectra were obtained in
chloroform-d, with Bruker 300, and Bruker Avance DRX-499. 13C
NMR (126 MHz) spectra were measured with a proton-decoupling
pulse program. Chemical shifts for 1H and 13C NMR were referenced
to residual signals from CDCl3 (1H NMR δ =7.26 ppm and 13C NMR
δ =77.23 ppm). Matrix-assisted laser desorption-ionization time of
flight mass spectrometry (MALDI-TOF MS) was performed on a
Bruker Autoflex II. Samples were prepared by diluting the molecules
in CH2Cl2 using 2,5-dihydroxybenzoicacid as the matrixes.
Elemental analyses were conducted on a Flash EA 1112 elemental
analyzer. Theoretical calculations based on density functional
methods have been performed for 3D-DPPswith Gaussian09
program. Becke’s three-parameter gradient-corrected functional
(B3LYP) with 6-31G(d,p) basis was used to optimize the geometry.
Although electron mobility of 3D-DPPs:N2200 BHJs (~10-4 cm2
V-1 s-1) is one order of magnitude higher than those of 3D-
DPPs:PC71BM BHJs (~10-5 cm2 V-1 s-1), the hole mobility of all four
blends are very similar(~10-4 cm2 V-1 s-1) (Table 1). This suggests
that the hole-transporting channels generated from 3D-DPPs are
quite insensitive to mixing with either N2200 or PC71BM, resulting
in similar level of hole mobility. This coincides well with the
findings of other 3D-shaped conjugated structures reported in the
literature that also form uniform, amorphous thin films with
isotropic properties.30 On the other hand, the electron transport is
more sensitive to electron acceptor used with polymer N2200
forming better nanoscale continuous networks than that of molecular
fullerene when blended with 3D-DPPs.
2.6 General applicability
DSC was measured on a WCT-2 thermal balance. UV−vis
spectra were recorded with a Jasco V-670. PL spectra were
measured on Fluoroskan Ascent FL. CV was done on a CHI 660C
electrochemical workstation with Pt disk, Pt plate, and standard 10
calomel electrode (SCE) as working electrode, counter electrode,
Usually, efficient OPV devices often consist of spherical shaped
fullerenes as acceptor and linear polymer as donor, and the best
performing non-fullerene acceptors are also non-planar or 3D-
shaped.4-23 The same rationale could also be applied here, with a BHJ
composed of a 3D donor and a linear polymer acceptor with one
of the donor or acceptor4-23 be non-planar or 3D-shaped instead of
both 3D-shaped, to achieve high efficiency in devices.
and reference electrode, respectively, in
1tetrabutylammoniumhexafluorophosphate
a
0.1 molL-
CH2Cl2
(Bu4NPF6)
solution. The current-voltage (J-V) curves were measured with
Keithley 2400 measurement source units at room temperature in air.
The photocurrent was measured under a calibrated solar simulator
(Abet 300 W) at 100 mW cm-2.
To further assess the general applicability of 3D-donor/linear
polymer acceptor combination, fluoro-substituted N2200 (di-F-
N2200)3b was also used as another polymeric acceptor (ESI, Fig. S10)
to pair with the 3D-DPP donors. The preliminary device results
derived from the 3D-DPPs:di-F-N2200 BHJs also showed promising
PCEs of 4.41% (Si-DPP:di-F-N2200) and 4.60% (C-DPP:di-F-
N2200), respectively (data not shown), which supports the
hypothesis of the effectiveness of 3D-donor:polymer acceptor blends.
The EQE system uses a lock-in amplifier (Stanford Research
Systems SR830) to record the short circuit current under chopped
monochromatic light. AFM images were obtained by a Nano
ScopeIIIa (Digital instrument Inc.) operating in the tapping mode.
SCLC were tested in electron-only devices with a configuration of
ITO/ZnO/3D-DPPs:N2200 or PC71BM/Ca (20 nm)/Al (130 nm) and
hole-only devices with a configuration of ITO/PEDOT:PSS/3D-
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DPPs:N2200 or PC71BM/MoO3/Ag. The devices were prepared found, 2732.09; elemental analysis: calcd: C, 73.86V;ieHw ,Art6ic.l9e4O;nliNne,
DOI: 10.1039/C5TA06639E
using the above-mentioned procedure. In hole-only deices, 4.10%. Found: C, 73.74; H, 6.92; N, 4.11%.
PEDOT:PSS (Clevios-PVP AI 4083) was spin-coated on ITO
4.3 Device fabrication
substrates at 5000 rpm for 30 sec and then annealed at 140 ℃for 10
min. The mobilities were determined by fitting the dark current
Solar cell devices were fabricated with an inverted structure of
according to the following equation:
ITO/ZnO (30 nm)/active layer (90−100 nm)/MoO3 (8 nm)/Ag (120
nm). The ITO-coated glass was cleaned in an ultrasonic bath of
acetone and isopropanol, and treated in ultraviolet-ozone chamber
for 30 min. A ZnO precursor solution was spin-coated onto pre-
cleaned ITO glass substrates at 4000 rpm for 60 s and then annealed
Where J is the dark current density (mA cm-2), μ0 is the zero-field
mobility (cm2/V sec), ε0 is thepermittivity of free space (88.54*10-12
at 200 °C for 1 h in air to complete the thin layer of ZnO (ca. 30
nm).The ZnO precursor solution was prepared by dissolving zinc
acetate dehydrate C4H6O4Zn·2(H2O) (99.5%, Merck1 g) and
monoethanolamine (HOCH2CH2NH2, 98% Acros, 0.28 g) in2-
methoxyethanol (CH3OCH2CH2OH, Aldrich, 98%, 10 mL) under
stirring for 8 h for hydrolysis reaction and aging.The active layer
was spin-coated from the blend solution (30 mg/mL for 3D-
DPPs:N2200, and 15 mg/mL for 3D-DPP:PC71BM). After spin-
coating, the films were placed in a closed jar for slow drying of
solvent. Finally, MoO3 and Ag were thermally evaporated as anode
electrode at a pressure of < 10-7 torr. The active area of each device
was 3.14 mm2 defined by a shadow mask which was cut by laser
beam technique. All masked tests gave consistent results with
relative errors within 1%. The photovoltaic performance was
measured under nitrogen inside a glove box. The current density-
voltage (J-V) characteristics were measured with a Keithley2400
source-meter under AM 1.5 G (100 mWcm-2) illumination which
was accurately calibrated employing a standard Si photodiode
detector equipped with a KG-5 filter, which can be traced back to the
standard cell of the National Renewable Energy Laboratory (NREL).
Devices were measured with shadow masking. The AM 1.5 G
illumination was simulated by using an Oriel 300 W Solar Simulator.
The solar cell devices were fabricated and measured over 20 devices
for reliability of the device performance.
mA sec/V sec), εr is the relative permittivity of the material (3), V is
the effectie voltage (V=VApplied-VBuilt-in-Vseriesresistane), and L is the
thickness of the active layer.
4.2 Synthetic procedures
The synthetic proceduresforprecursors TBPM, TBPS and Ph-DPP
can be found in ESI.
Synthesis of C-DPP and Si-DPP: The starting Ph-DPP (140 mg,
0.23 mmol), TBPM or TBPS (0.05 mmol, 31.8 mg and 32.6 mg for
TBPM and TBPS, respectively), anhydrous K2CO3 (55 mg, 0.4
mmol), pivalic acid (PivOH, 6.1 mg, 0.06 mmol), Pd(OAc)2 (2.3 mg,
0.01 mmol) were stirred in anhydrous dimethylacetamide (DMAc, 5
mL) for 24 h at 105 oC under argon in a Schlenk tube. After cooling
to room temperature, the mixture was poured into a 250 mL NaCl
aqueous solution to remove salts and high boiling point solvent
(DMAc). The precipitate was extracted with CHCl3 (3×20 mL). The
combined organic layers was washed with distilled water and dried
over anhydrous Na2SO4. Removal of CHCl3 by a rotary evaporator
afforded the crude products, which were then purified by column
chromatography on silica gel using the mixture of CH2Cl2 and
hexane as eluent (6:1, v/v) and gave the products. Both C-DPP (139
mg, yield 90%) and Si-DPP (145 mg, yield 94%) as dark blue solids.
Acknowledgements
1
C-DPP: H NMR (300 MHz, CDCl3) δ 8.99 (dd, J = 7.1, 4.2 Hz,
The authors acknowledge the Asian Office of Aerospace R&D
(FA2386-11-1-4072), the Office of Naval Research (N00014-1-
0170), the National Natural Science Foundation of China (21244008,
21374075 and 51473142), and the China Scholarship Council
(201308330072) for financial support. The authors thank Dr. Jun-
Yong Wu for molecular geometry optimization.
8H), 7.67 (t, J = 8.0 Hz, 16H), 7.52 –7.31 (m, 28H), 4.09 (d, J = 7.4
Hz, 16H), 1.96 (s, 8H), 1.33 (d, J = 33.1 Hz, 64H), 0.98 – 0.82 (m,
48H); 13C NMR (126 MHz, CDCl3, δ): 161.75, 150.04, 148.79,
146.65, 140.00, 139.73, 136.91, 133.19, 131.57, 129.18, 129.03,
128.83, 126.15, 125.67, 124.71, 124.51, 108.60, 46.09, 39.15, 30.35,
28.47, 23.69, 23.17, 14.13, 10.61; MALDI-TOF MS m/z: [M]+ calcd
for C169H188N8O8S8, 2715.89; found, 2716.12; elemental analysis:
calcd: C, 74.74; H, 6.98; N, 4.13%. Found: C, 74.54; H, 7.00; N,
4.12%.
Notes and references
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View Article Online
DOI: 10.1039/C5TA06639E
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The graphical and textual abstract for
the Table of contents entry:
View Article Online
DOI: 10.1039/C5TA06639E
Three-dimensional molecular
donors combined with polymeric
acceptors for high performance
fullerene-free organic photovoltaics
Shi-Yong Liu, Jae Woong Jung, Chang-Zhi Li, Jiang
Huang, Jianyuan Zhang, Hongzheng Chen* and
Alex K.-Y. Jen*
Two novel diketopyrrolopyrrole-based 3D electron donors have been
synthesized via direct arylation. The fullerene-free organic
photovoltaics (OPVs) based on the 3D-molecular donor:polymer
acceptor showed a respectable power conversion efficiency of 4.64%,
which outperforms OPVs derived from PC71BM.
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