Self-assembled buffer layer from conjugated diblock copolymers with ethyleneoxide side chains for high efficiency polymer solar cells

Article abstract of DOI:10.1039/c4tc01388c

In this article, we present a novel and promising approach to enhance the device performance and stability by the simple incorporation of all conjugated polythiophene diblock copolymers, poly(3-hexylthiophene)-b-poly(3-triethylene glycol thiophene) (P3HT-b-P3TEGT), into the active layer based on inverted device structures. During the spin-coating process, the triethylene glycol side chains of P3HT-b-P3TEGT would spontaneously migrate vertically towards the active layer surface and form a nanoscale self-assembled anode buffer layer, which simultaneously drives the orderly packing of donor polymer chains and vertical phase separation morphology, allowing electrons and holes to move more efficiently to the respective electrode. Moreover, the nanoscale self-assembled buffer layer can form interfacial modification and ohmic contact between the active layer and Ag (or MoO₃/Ag) electrode, reduce the contact resistance of the device, and increase the electrical conduction of the device, especially upon chelating lithium ions (Li⁺) to the triethylene glycol side chains of P3HT-b-P3TEGT. Combining the above advantages, the efficiency and stability of the polymer solar cells are enhanced. A remarkable improvement in the PCE with 7.3% (measured in air) is obtained for PBDTTT-C-T:PC₇₁BM devices. This journal is

Full text of DOI:10.1039/c4tc01388c

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Self-assembled buffer layer from conjugated
diblock copolymers with ethyleneoxide side chains
Cite this: J. Mater. Chem. C, 2014, 2,
for high efficiency polymer solar cells†
8054
a
a
a
ab
*
Yueqin Shi, Licheng Tan, Lie Chen and Yiwang Chen
In this article, we present a novel and promising approach to enhance the device performance and stability
by the simple incorporation of all conjugated polythiophene diblock copolymers, poly(3-hexylthiophene)-
b-poly(3-triethylene glycol thiophene) (P3HT-b-P3TEGT), into the active layer based on inverted device
structures. During the spin-coating process, the triethylene glycol side chains of P3HT-b-P3TEGT would
spontaneously migrate vertically towards the active layer surface and form a nanoscale self-assembled
anode buffer layer, which simultaneously drives the orderly packing of donor polymer chains and vertical
phase separation morphology, allowing electrons and holes to move more efficiently to the respective
electrode. Moreover, the nanoscale self-assembled buffer layer can form interfacial modification and
ohmic contact between the active layer and Ag (or MoO3/Ag) electrode, reduce the contact resistance
of the device, and increase the electrical conduction of the device, especially upon chelating lithium ions
(Li+) to the triethylene glycol side chains of P3HT-b-P3TEGT. Combining the above advantages, the
efficiency and stability of the polymer solar cells are enhanced. A remarkable improvement in the PCE
with 7.3% (measured in air) is obtained for PBDTTT-C-T:PC71BM devices.
Received 28th June 2014
Accepted 1st August 2014
DOI: 10.1039/c4tc01388c
www.rsc.org/MaterialsC
To enhance the PCE, the lower band gap copolymers instead
of P3HT and a higher LUMO energy acceptor material were
Introduction
considered. In addition, the morphology control of the active
layer is also processed to increase the efficiency such as thermal
annealing,22 solvent annealing,23 additives,24–28 and solvent
mixture.29,30 These techniques have been proven to be very
effective to obtain lateral nanoscale phase-separated structures
(the phase separation size is equal to the exciton diffusion
length). In particular, it should be noted that the nanoscale
vertical phase segregation morphology of the active layer is
equally critical for the enhancement of PCE. The vertical
components' distribution, i.e. the p-type copolymers donor are
rich near the high work function (WF) anode and n-type
acceptor materials are rich near the low WF metal cathode, can
form an efficient pathway for carrier transfer and allows elec-
trons and holes moving towards the cathode and anode more
efficiently.31,32
On the other hand, in order to achieve high-performance
BHJ PSCs, the interfacial engineering of OPVs is another
important approach. In BHJ PSCs, to decrease the energy
barriers for electron- and hole-extraction, the WFs of cathode
and anode should effectively match the acceptor's LUMO and
donor's highest occupied molecular orbital (HOMO), respec-
tively. The energy barriers between the electrodes and the active
layer could oen be reduced effectively by inserting buffer layers
at the cathode/anode. Over the past years, considerable efforts
have been expended in exploring a material that could act as an
efficient hole extraction layer for BHJ PSCs. The anode buffer
Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on
an active layer composed of a conjugated polymer as the donor
and fullerene derivative as the acceptor have received increasing
attention for their ease of fabrication, mechanical exibility,
light weight, and low-cost fabrication of large-area devices.
During the past decades, extensive efforts have been focused on
the improvement of the power conversion efficiency (PCE) and
device stability for economically viable applications by molec-
ular structure engineering,1,2 morphology control,3–5 and device
optimization.6–10 To date, the highest reported PCE of PSCs has
approached 10%.11–17 The most common conjugated polymer
for donor and acceptor materials are poly(3-hexylthiophene)
(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester
(PC61BM).18-21 However, their performance is limited for the low
short circuit current (Jsc) and open-circuit voltage (Voc) origi-
nating from the relative wide band gap of P3HT donor and low
energy of the lowest unoccupied molecular orbital (LUMO) of
PC61BM.
aInstitute of Polymers/Department of Chemistry, Nanchang University, 999 Xuefu
Avenue, Nanchang 330031, China. E-mail: ywchen@ncu.edu.cn; Fax: +86 791
83969561; Tel: +86 791 83969562
bJiangxi Provincial Key Laboratory of New Energy Chemistry, Nanchang University,
999 Xuefu Avenue, Nanchang 330031, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc01388c
8054 | J. Mater. Chem. C, 2014, 2, 8054–8064
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layer should form an ohmic contact at the interface between the successfully (ESI†). Furthermore, the resulting copolymers were
active layer and anode electrode and possess higher WF and puried by sequential Soxhlet extraction using methanol,
higher hole mobility for hole transportation and collection.33 acetone, hexane and chloroform solvents. The weight ratios of
From these points of view, it is wise to select thiophene deriv- the two blocks were determined to be 8 : 1, 4 : 1 and 2 : 1
atives as a hole transport layer (HTL) to provide an ohmic (P3HT:P3TEGT) using 1H NMR spectroscopy; thus, we also
contact with the donor material.34,35 Traditionally, the buffer abbreviated these diblock copolymers (P3HT-b-P3TEGT) as
layer is fabricated into the device by an additional step. H8T1, H4T1 and H2T1, respectively, in the data, which were
However, such an additional process will complicate the device reported in our previous work.41 The gel permeation chroma-
fabrication processes, and consequently, increase the cost.
tography (GPC) using tetrahydrofuran (THF) as the eluent were
To achieve the nanoscale vertical phase segregation processed to estimate the number-average molecular weight
morphology of the active layer, as well as obtain the buffer layer (Mn) of the diblock copolymers. When the P3HT-b-P3TEGT or
without an additional spin-coating process, the self-assembled P3HT-b-P3TEGT:Li+ diblock copolymer was added to the
buffer layer is utilized in the BHJ PSCs. It had been reported that P3HT:PC61BM or PBDTTT-C-T:PC71BM active layers, it would
a fullerene derivative (F-PCBM) as a buffer layer between the spontaneously migrate to the surface and form the nanoscale
cathode and active layer could improve the performance of BHJ self-assembled buffer layer between the active layer and Ag or
PSC resulting from the spontaneous surface segregation of MoO3/Ag anode electrode, simultaneously inducing the orien-
uorocarbon chains onto the surface of the active layer.36 tation of the donor copolymer chains and vertical distribution
Moreover, the vertical phase separation and self-assembled of the active layer components (Scheme 1).
buffer layer via the spontaneous migration of PEG to the top
To investigate the vertical surface segregation of P3HT-b-
could also be obtained for efficient and air-stable PSCs by P3TEGT during the spin-coating process, the X-ray photoelec-
blending the poly(ethylene glycol) (PEG) molecules or fullerene- tron spectroscopy (XPS) proles are shown in Fig. 1. All the lms
end-capped PEG (PEG-C60) into the photoactive layer.33,36–39
were prepared on ZnO coated-ITO glass substrates by spin-
In addition, compared with conventional BHJ PSCs, inverted- coating the P3HT:PC61BM solutions blended with various
type devices exhibited better long-term ambient stability by concentrations of H4T1, pure Li+ and 0.6 mg mLꢀ1 H4T1
avoiding the need for corrosive and hygroscopic hole-transporting chelated with Li+ ion (H4T1:Li+) with a xed concentration of
poly(3,4-ethylenedioxylenethiophene):poly(styrenesulphonic acid) P3HT at 20 mg mLꢀ1. It was observed that the peak of O 1s at
(PEDOT:PSS) and low WF metal cathode, both of which were 531.8 eV became stronger with the gradual increase of the H4T1
harmful towards the device lifetime.
amount and even showed a slight red-shi of H4T1:Li+, which
Here, we demonstrated an interesting and promising way to could belong to the chelation of the triethylene glycol side
improve the performance and air stability of PSCs based on chains with Li+.42–45 Moreover, the Li 1s spectra showed that the
inverted device structures ITO/ZnO/active layer/Ag (or MoO3/Ag) intensity of the peaks at 49.3 eV and 63.3 eV, resulting from the
by introducing poly(3-hexylthiophene)-b-poly(3-triethylene Li+ salt, became stronger with the addition of Li+ salt into the
glycol thiophene) (P3HT-b-P3TEGT), an all diblock copolymer P3HT:PC61BM solution, especially for the P3HT:PC61
-
containing polythiophene derivative segments whose side chain BM:H4T1:Li+ lm. The O/C atomic ratios calculated from the
was triethylene glycol, into the P3HT:PC61BM or PBDTTT-C- copolymer blend compositions in solution and measured from
T:PC71BM active layers. During the spin-coating and solvent the XPS results of the lms surface were plotted as a function of
evaporation processes, P3HT-b-P3TEGT would spontaneously the H4T1 concentration. From the results, the O/C ratios of the
self-segregate onto the lm and form nanoscale self-assembled lms' surface measured from the XPS results were much higher
buffer layers due to the vertical migration of triethylene glycol than the calculated ratio from the copolymer blend composi-
side chains to the active layers' surface. The chain orientation of tions for all the concentrations, revealing the vertical migration
P3HT-b-P3TEGT at the surface would also force the donor of H4T1 or H4T1:Li+ to the lm surface during the spin-coating
copolymer chains in the active layer to adopt the same orien- process. When the concentration of H4T1 increased to ꢁ0.6 mg
tation due to the high crystallinity of copolymer donors and mLꢀ1, the O/C atomic ratio (measured by the XPS) attained the
simultaneously induce the vertical phase segregation saturation point, which was consistent with the results of the
morphology of the active layer. Furthermore, the presence of water contact angles in Fig. S1 (ESI†).46 On the other hand,
triethylene glycol side chains between the active layer and the H8T1 and H2T1 diblock copolymers also demonstrated
anode electrode (Ag or MoO3/Ag) would form an amphiphilic the saturation point at concentrations of ꢁ1.2 mg mLꢀ1 and
interface, decreasing the energy barriers for hole collection and ꢁ0.3 mg mLꢀ1, respectively. The saturation behavior was also
forming an ohmic contact, especially upon chelating the observed in our previous work on self-organized HTLs based on
lithium ion (Li+) to the triethylene glycol side chains of P3HT-b- uorinated side chains diblock copolymers.10 The active layer
P3TEGT (P3HT-b-P3TEGT:Li+), leading to further enhancement surface was almost covered by the triethylene glycol side chains
of the device performance and stability.
of P3HT-b-P3TEGT diblock copolymers completely above the
saturation point. In addition, the approximate thicknesses of
the
P3HT:PC61BM,
P3HT:PC61BM:P3HT-b-P3TEGT
and
Results and discussion
P3HT:PC61BM:P3HT-b-P3TEGT:Li+ lms calculated from the
The P3HT-b-P3TEGT diblock copolymers were synthesized AFM analysis were 100 nm, as shown in Fig. S2,† revealing that
following the procedure of McCullough and co-workers40 the buffer layer segregated on the surface of the active layer was
This journal is © The Royal Society of Chemistry 2014
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Scheme 1 Schematic illustration of (a) inverted bulk heterojunction polymer solar cells device based on the vertical segregation of P3HT-b-
P3TEGT chelated with Li+ on the active layer surface, (b) the morphology of active layer:Li+ and active layer:P3HT-b-P3TEGT:Li+, (c) the chemical
structures of donor and acceptor materials in the active layer used in this study.
very thin, possibly a monolayer or having a thickness of several more orderly and highly crystalline than the packed P3HT
layers.
chains induced by the orientation of the surface segregation of
We optimized the electrical and optical characterizations of H4T1. To further validate the ordering and crystalline of thio-
the P3HT:PC61BM:H4T1 active layer by changing the concen- phene chains, the X-ray diffraction (XRD) was monitored
tration of H4T1 in 1,2-dichlorobenzene (DCB) solution using (Fig. 2). The XRD results revealed that the introduction of H4T1
the UV-vis absorption and X-ray diffraction (XRD) measure- or H4T1:Li+ into the P3HT:PC61BM system could improve the
ments. The normalized absorption spectra of the polymer lms crystalline and ordered structures of P3HT in the active layer.
spin-coated from the DCB solution (Fig. S3†) were similar and The pristine P3HT:PC61BM lm showed a diffraction peak at 2q
ꢂ
˚
revealed that the absorption band of thiophene chains z 5.4 , corresponding to a d-spacing value of 16.3 A of P3HT.
appeared at about 510, 560 and 610 nm and that of PC61BM at Aer the introduction of H4T1 or H4T1:Li+ into the
330 nm. Compared with the pristine P3HT:PC61BM lm, the P3HT:PC61BM solution, all the d-spacing peaks were shied
˚
absorption peaks of thiophene chains in P3HT:PC61BM:H4T1 slightly, which was expanded by ꢁ1.0 A. It was more interesting
lms was somewhat red-shied by approximately 6 nm, and that a new diffraction peak appeared at 2q z 6.5ꢂ (d-spacing of
˚
simultaneously, a stronger absorption was observed at 610 nm, ꢁ13.58 A) in the ternary blends, which was probably a result of
while the absorption peak of PC61BM displayed no shi. This the self-assembly of thiophene chains of copolymers in the
demonstrated that the active layer was organized into super- active layer. Moreover, the results also indicated that the P3HT
molecular assemblies and exhibited increasing planarity, even polymer chains might have a slight preference in the face-on or
8056 | J. Mater. Chem. C, 2014, 2, 8054–8064
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(three-dimensional images in Fig. 3). In addition, AFM images
were also examined to elucidate the effect of H4T1 or H4T1:Li+
on the phase separation of P3HT:PC61BM thin lms. The
surface morphology of pristine P3HT:PC61BM demonstrated a
ne phase separation and bicontinuous network image with 2.3
nm root-mean-square (RMS) roughness. Considering the pris-
tine H4T1 lms showed a 2.84 nm RMS, it could be inferred that
increasing the H4T1 concentration would lead to aggregation of
the triethylene glycol side chains on the surface of the active
layer and the lms exhibited a rougher surface. However, add-
ing H4T1 attened the lm surface and made it uniform with
the RMS values of 0.62, 0.68 and 1.62 nm for H4T1 concentra-
tions of 0.2, 0.6 and 1.0 mg mLꢀ1, respectively. On the other
hand, we also utilized AFM measurements to investigate
the effect of H4T1:Li+ on the orientation and distribution of
P3HT:PC61BM active layer components and surface
morphology47 shown in Fig. 4. The added Li+ ions could chelate
with the triethylene glycol side chains of P3HT-b-P3TEGT and
drive the polymer chains to self-assemble into the nanowire
morphology, according to our previous work in ref. 41. A well-
ordered lamellar structure of H4T1:Li+ covering P3HT:PC61BM
active layer surface was found at concentrations of 0.06 mg
mLꢀ1 and 0.18 mg mLꢀ1 of Li+ salt, resulting from the Li+ ions
driving the diblock copolymer chains into nanowires on the
surface of the active layer.48 Furthermore, uniform lm
morphology with 1.88 and 1.59 nm RMS induced by the vertical
surface segregation of triethylene glycol side chains of poly-
thiophenes was observed during spin-coating. However, upon
further increasing the concentration of H4T1:Li+, large aggre-
gations gradually appeared on the surface with the RMS values
of 2.78 and 2.92 nm, and even the well-ordered lamellar struc-
tures disappeared.
Fig. 1 (a) O 1s and (b) Li 1s regions of the X-ray photoelectron spec-
troscopy (XPS) profiles of the surface of P3HT:PC61BM (PP) films with
different H4T1 concentrations, pure Li+ ion, H4T1 and H4T1 (0.6 mg
mLꢀ1):Li+, (c) O/C atomic ratio of the P3HT:PC61BM:H4T1 (PP:H4T1)
films and the PP:H4T1 (0.6 mg mLꢀ1):Li+ film surfaces measured by
XPS and the O/C atomic ratio of PP:H4T1 solutions calculated from the
polymer chemical compositions as
concentration.
a function of the H4T1
We fabricated the inverted devices based on P3HT:PC61
-
BM:Li+ and P3HT:PC61BM:H4T1:Li+ active layers to investigate
the effect of Li+ doping on the device performance. The effects
of different Li+ concentrations in P3HT:PC61BM and
P3HT:PC61BM:H4T1 solutions on the device performance were
studied and the device parameters are plotted in Fig. 5 as
functions of the Li+ concentration. The J–V curves of these
devices are shown in Fig. S5† and the correlated parameters are
summarized in Table S1.† All the parameters of the devices
based on P3HT:PC61BM:Li+ dropped sharply with an increase in
the Li+ concentration. However, the parameters of the devices
based on P3HT:PC61BM:H4T1:Li+ rst rose and then decreased
gradually with the increase of the Li+ concentration. Compared
with the P3HT:PC61BM:Li+ device, the better performance and
stability of the device based on P3HT:PC61BM:H4T1:Li+ was
mainly due to the chelation of the triethylene glycol side chains
of H4T1 with Li+ ions, which could drive the H4T1 diblock
copolymer chains into nanowires and to migrate onto the
surface of the active layer. On the contrary, the addition of Li+
into P3HT:PC61BM might not form an efficient anode buffer
layer and resulted in a dramatic decrease of the PCE. The results
indicated that the photovoltaic properties of BHJ PSCs could be
easily improved by introducing P3HT-b-P3TEGT:Li+ into the
active layer system.10 On the other hand, the dark J–V charac-
teristics of devices based on active layer P3HT:PC61BM,
Fig. 2 XRD patterns of P3HT:PC61BM (PP) blended with different
concentrations of H4T1 and 0.6 mg mLꢀ1 H4T1 chelated with Li+ films.
the co-existence of the face-on and other tiled chain orienta-
tions4 facilitated by the driving force from the vertical surface
separation of P3HT-b-P3TEGT or P3HT-b-P3TEGT:Li+.
The morphology of the active layer was critical to determine
the performance of BHJ PSCs. With a large interfacial area
and bicontinuous vertical distribution of the active layer
components, the charge separation from the donor copolymer
to acceptor fullerene derivative could be enhanced and
charge carriers collection from the active layer to the respective
electrode also could be maximized. We used atomic force
microscopy (AFM) in the tapping mode to measure the surface
morphology of P3HT:PC61BM (1 : 1, w/w) with various
H4T1 concentrations (height images in Fig. S4†) and H4T1:Li+
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 8054–8064 | 8057