RSC_CC_C3CC41321G 3..5

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Published on 08 April 2013 on | doi:10.1039/C3CC41321G
Cite this: DOI: 10.1039/c3cc41321g
Received 20th February 2013,
Accepted 8th April 2013
DOI: 10.1039/c3cc41321g
Pan Zhang,a Chao Li,a Yaowen Li,*ab Xiaoming Yang,a Liwei Chen,b Bin Xu,c
Wenjing Tianc and Yingfeng Tu*a
was successfully synthesized, which could effectively induce poly(3hexylthiophene) (P3HT) to form highly ordered bulk heterojunction
structure without any external treatment. This ordered active layer
exhibits good photovoltaic performance.
Polymer solar cells (PSCs) with a bulk heterojunction (BHJ) active layer
have been widely investigated.1 Of the many candidate materials,
BHJ structures composed of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) have been extensively
investigated. The most outstanding advantage of this material pair is
that an ordered BHJ morphology with interpenetrating nanoscale
networks can be formed by external treatment methods, such as
thermal annealing, mixture of solvents, additives, and solvent
annealing, which is one key to improving BHJ device performance.
However, the external treatment like thermal annealing usually
possesses greater driving force for PCBM diffusion and aggregation,
which can induce the degradation of nanoscale BHJ morphology. On
the one hand, using the external treatment with annealing free
approaches like cooling the saturated P3HT solution,2 mixture of
solvents and additives it is difficult to obtain optimized morphologies
of blend films, because P3HT crystallization and phase separation of
the two components occur in a single step, and the two processes can,
therefore, interfere with each other.3 Therefore, a new approach
without external treatment which can achieve well controlled morphology is used to synthesize novel acceptors with controlled molecular
electronic structures and solid-state supramolecular structures.
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application,
Department of Polymer Science and Engineering, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou,
215123, P. R. China. E-mail: [email protected], [email protected];
Fax: +86 512 65882130; Tel: +86 512 65882130
i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO),
Chinese Academy of Sciences, Suzhou, 215123, P. R. China
Jilin University - State Key Laboratory of Supramolecular Structure and Materials,
Changchun, 130012, P. R. China
† Electronic supplementary information (ESI) available: Synthetic process and
characterizations of PCBB-C8; electrochemical properties; thermal properties;
SEM and AFM images and photovoltaic parameters. See DOI: 10.1039/c3cc41321g
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A fullerene dyad with a tri(octyloxy)benzene moiety
induced efficient nanoscale active layer for the
poly(3-hexylthiophene)-based bulk heterojunction
solar cell applications†
A bulk tri(octyloxy)benzene moiety grafted onto fullerene (PCBB-C8)
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It is known that the large free region of P3HT in P3HT:PCBM blend
films can be ‘‘frozen in’’ during spin-coating,4 because PCBM molecules
disperse between the polymer chains and serve as defect sites, thus
suppressing P3HT crystallization by destroying the stacking of polymer
chains. So the P3HT crystallization requiring a free region is generally
produced by the driving force obtained from external treatment.5,6 The
reported approaches: thermal annealing or solvent annealing can
provide the driving force to ‘‘heal’’ the disordered structure, with the
mobile P3HT chains self-organizing into ordered structure accompanied by free diffusion and aggregation of PCBM molecules.7–9 Therefore, a fullerene derivative with a bulk flexible substituent and
supramolecular structure, which can provide a C60-free region for
extending the P3HT chains and shortening the molecular relaxation
time, may facilitate P3HT crystallization in the absence of external
treatment. Furthermore, it is reported that P3HT crystallizes more easily
and rapidly than PCBM,10 implying that the rate of PCBM diffusion
and aggregation is a critical factor for determining the kinetic driving
force for the formation of ordered BHJ morphology. Thus, a weak
driving force for fullerene derivative diffusion and aggregation may
induce a better nanoscale BHJ morphology.
In this communication, we report design and synthesis of a
tri(octyloxy)benzene–fullerene dyad (PCBB-C8, Fig. 1a) and unambiguously demonstrate that it possesses weak diffusion and aggregation
ability, and it can induce P3HT to self-organize into P3HT crystallites
in the absence of external treatment. The as-cast P3HT:PCBB-C8
blend film based PSCs exhibit high Jsc, suggesting that this selforganized highly ordered blend film can facilitate charge separation
and transportation, which is a good perspective for the external
treatment free solar cell fabrication.
The chemical structure of PCBB-C8 is shown in Fig. 1a, and the
detailed synthetic procedure and characterizations of compounds are
described in ESI† (Scheme S1 and Fig. S1–S4). PCBB-C8 possesses
good solubility in common organic solvents due to the long
tri(octyloxy)benzene substituent. Differential scanning calorimetry
(DSC) study shows no obvious endothermic peaks and phase transition, which indicates that PCBB-C8 is amorphous (Fig. S6,
ESI†). The energy levels of PCBB-C8 and PCBM for comparison
were investigated using cyclic voltammetry (CV). Similar lowest
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Fig. 1 (a) Chemical structure of PCBB-C8; (b) UV-vis absorption spectra of PCBM
and PCBB-C8 in chloroform solution with different concentrations; (c) normalized
UV-vis absorption spectra of P3HT:PCBM and P3HT:PCBB-C8 blend films with and
without thermal annealing; (d) photographs of spin-coated P3HT:PCBM and
P3HT:PCBB-C8 blend films before and after thermal annealing.
unoccupied molecular orbital (LUMO) energy levels of PCBB-C8
and PCBM ( 3.80 eV and 3.83 eV) are attributed to similar
fullerene skeletons. Meanwhile, the two fullerene derivatives
showed exactly the same UV-vis absorption spectra in shapes and
intensity for the same dilute concentration in chloroform (Fig. 1b).
All these results indicate that the tri(octyloxy)benzene substituent
does not affect the electronic structure of the molecule.
To test for self-organizing properties of P3HT and PCBB-C8 in
the solid state, we investigated the UV-vis absorption spectra of
P3HT:fullerene derivative blend films (Fig. 1c). Generally, the thermally
annealed P3HT:PCBM blend film shows much more flat and redshifted peaks at around 500 nm and 554 nm (P3HT p–p* transitions),
and a clearer vibronic shoulder at 604 nm (P3HT inter-chain interaction), respectively, compared with that of blend films before thermal
annealing. All of these features are the typical absorption peaks for the
self-organized P3HT crystallites, which are induced by the thermal
annealing process.5,11 Interestingly, the P3HT:PCBB-C8 blend film
without the thermal annealing process exhibits a shape nearly identical
to that of the thermally annealed P3HT:PCBM blend film, with an even
larger red shift at around 500 nm, and higher vibronic shoulder
intensity at 554 nm and 604 nm. Even after thermal annealing, the
absorption spectrum of the P3HT:PCBB-C8 blend film shows no
obvious difference. All these suggested that the P3HT:PCBB-C8 blend
film has already formed highly crystalline P3HT before thermal
annealing.12 This result can be further confirmed by the violet color
emission of P3HT:PCBB-C8 blend films, which is even more violet than
that of thermally annealed P3HT:PCBM blend films (Fig. 1d).
To gain an understanding of the self-organized microstructures
and confirm the crystallization of P3HT:PCBB-C8 blend films, out-ofplane X-ray diffraction (XRD) was used to determine the stacking
mode. As shown in Fig. 2a, the P3HT:PCBM blend film without
thermal annealing shows a broad and weak diffraction peak at q =
0.39 Å. It indicates that P3HT possesses low crystallinity or an
amorphous phase, which is caused by the highly dispersed PCBM
among the P3HT chains and restricted free regions for P3HT
crystallization.9,13–15 For the P3HT:PCBM blend film with thermal
annealing and the P3HT:PCBB-C8 blend film without thermal
annealing, sharp diffraction peaks at q = 0.39 Å corresponding to
Chem. Commun.
Fig. 2 (a) Left: the out-of-plane XRD spectra of P3HT:PCBM and P3HT:PCBB-C8
blend films without and with thermal annealing, right: the P3HT crystallite
structure; (b) the speculative self-organized process of P3HT:PCBB-C8 blend films
by the solution spin-coating method.
the (100) orientation for the P3HT lamellar organized crystallites13 are
observed. The slightly lower intensity and the same angle of diffraction peaks at q = 0.39 Å of the P3HT:PCBB-C8 blend film suggest
slightly decreased crystallinity of P3HT and the same lamellar
d-spacing between them. Analogous intensity and angle of diffraction
peaks of the P3HT:PCBB-C8 blend film without and with thermal
annealing further confirm that the crystallization of P3HT is mainly
determined by the inducement of PCBB-C8 during the spin-coating
process. Overall, the tri(octyloxy)benzene bulk substituent acts as a
driving force for thermal annealing to induce the crystallization of
P3HT. As illustrated in Fig. 2a (right), the calculated d-spacing value of
crystallite arrangement of interdigitated P3HT side-chains from X-ray
diffraction agrees well with that reported in the literature.14
Transmission electron microscopy (TEM) was used to further
investigate their microstructure and nanoscale morphology. After
thermal annealing the P3HT:PCBM blend film shows obvious changes
in the long fibrillar P3HT crystallites (in the range of 40–65 nm) than in
the state before annealing (Fig. 3a and b), which are broadly distributed
in the image with an optimum morphology for high-performance
BHJ PSCs.10 In spite of this, the thermally grown PCBM clusters as
illustrated using in situ scanning electron microscopy (SEM)
(Fig. S8a–d, ESI†) are unable to participate in the well ordered BHJ
morphology, and may cause defects in the electronic properties.15 In
this study, the P3HT:PCBB-C8 blend film without thermal annealing
has relatively smaller P3HT crystal size (in the range of 20–35 nm)
and good phase separation in the TEM image (Fig. 3c). No
obvious PCBB-C8 aggregation (Fig. S8e–h, ESI†) and slightly
Fig. 3 TEM images obtained by the blend films spin-coated with chlorobenzene in
10 mg mL 1 (a) P3HT:PCBM (w/w, 1 : 1) without thermal annealing; (b) P3HT:PCBM
(w/w, 1 : 1) with thermal annealing at 150 1C for 10 min; (c) P3HT:PCBB-C8 (w/w, 1 : 1)
without thermal annealing.
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Fig. 4 (a) Current–voltage characteristics of photovoltaic cells based on
P3HT:PCBM and P3HT:PCBB-C8; (b) the external quantum efficiency (EQE) spectra
of devices based on P3HT:PCBM and P3HT:PCBB-C8. Device fabrication conditions: ablend film without the thermal annealing process; bblend film with
thermal annealing at 150 1C for 10 min.
varied roughnesses in Atomic Force Microscopy (AFM) (Fig. S9e–h,
ESI†) during the thermal annealing process indicate that the selforganized P3HT crystallites were induced by the free region (bulk
tri(octyloxy)benzene moiety) of PCBB-C8 during the spin-coating
process. Considering the previous results and the mechanism of
P3HT crystallization, the speculative self-organized process of the
P3HT:PCBB-C8 blend film by the solution spin-coating method is
illustrated in Fig. 2b, and several conclusions can be derived. First,
the bulk tri(octyloxy)benzene moiety of PCBB-C8 can decrease
crystallization, diffusion and aggregation of PCBB-C8. Second, due
to the flexible trioctyloxyl chains and weakly ordered aggregation of
PCBB-C8, PCBB-C8 provides the C60-free region and shortens P3HT
relaxation time for P3HT crystallization during the spin-coating
process. The weak PCBB-C8 aggregation may be deemed as supramolecular C60 nano/micro-architectures using the intermolecular
forces introduced by C60 (p–p) and octyloxyl chain interactions (van
der Waals, vdW).16 Third, this free region is smaller than that
provided by the thermal annealing P3HT:PCBM blend film, which
may minimize the self-organized P3HT crystallized scale.
We tested how this self-organized P3HT:PCBB-C8 blend film
induced by PCBB-C8 impacted the properties of photovoltaic
devices. The BHJ PSCs were fabricated under typical P3HT:PCBM
device fabrication conditions.11 Fig. 4a shows the current density–
voltage (J–V) characteristics of the devices, and the corresponding photovoltaic parameters are given in Table S1 (ESI†).
The P3HT:PCBM based device exhibits significantly improved
photovoltaic performance after thermal annealing due to the
nanoscaled P3HT crystallization and good phase separation17
which have also been demonstrated by our previous results. It
is interesting to find that Jsc of the P3HT:PCBB-C8 blend film
without the thermal annealing process based device is significantly
improved from 1.99 mA cm 2 to 8.70 mA cm 2 compared with the
P3HT:PCBM based device under the same device fabrication conditions, and is even similar to that of P3HT:PCBM with the thermal
annealing process. Therefore, these results firmly identify our
speculation that substituting PCBB-C8 for PCBM as the acceptor
material can induce P3HT to self-organize with good phase separation, higher nanoscale crystallites and improved microstructure,
which can facilitate the charge separation and transportation.
However, it possesses deteriorated FF compared with that of the
thermally annealed P3HT:PCBM device. Even after the thermal
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annealing process (Fig. S10 and Table S1, ESI†), the FF cannot be
still improved. The deteriorated FF may be influenced by the
unbalanced (holes vs. electrons) and decreasing charge transport
and thus a higher exciton recombination rate, which is determined
by the relatively higher series resistance (Rs) and lower shunt
resistance (Rsh) of the devices (Table S1, ESI†). PCBB-C8 with a
nonconducting long tri(octyloxy)benzene moiety given a low content of C60 in the P3HT:PCBB-C8 blend film may lead to the
relatively low electron mobility of the blend film. External quantum
efficiency (EQE) spectra of the above devices are shown in Fig. 4b.
Although the P3HT:PCBB-C8 based device shows a Jsc value similar
to that of the P3HT:PCBM based device (with thermal annealing),
the photon harvesting ability increasing at around 510 nm is
attributed to the contribution of better self-organized shorter
nanoscale P3HT crystallites and interpenetrating microstructure of higher interface area induced by PCBB-C8 during the
spin-coating process.
In summary, we have successfully synthesized bulk moiety
grafted fullerene PCBB-C8. It can provide a free region for P3HT
crystallization and induce the two components to form good phase
separation without any external treatment. The self-organized
highly ordered P3HT:PCBB-C8 blend film gives an efficient charge
separation and transportation, therefore leading to high Jsc of P3HT
based BHJ PSCs. Further investigations into the use of these high
C60 content fullerenes as acceptor materials and the external
treatment free technology for large area flexible PSC fabrication
are currently ongoing.
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