Complementary solvent additives tune the
orientation of polymer lamellae, reduce the sizes of
aggregated fullerene domains, and enhance the
performance of bulk heterojunction solar cells
†
Chih-Ming Liu,aYu-Wei Su,aJian-Ming Jiang,aHsiu-Cheng Chen,aShu-Wei Lin,a Chun-Jen Su,bU-Ser Jeng*bcand Kung-Hwa Wei*a
In this study we employed 1-chloronaphthalene (CN) and 1,8-diiodooctane (DIO) as binary additives exhibiting complementarily preferential solubility for processing the crystalline conjugated polymer poly [bis(dodecyl)thiophene-dodecyl-thieno[3,4-c]pyrrole-4,6-dione] (PBTC12TPD) and the fullerene [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) in chloroform. Using synchrotron grazing-incidence small-/wide-angle X-ray scattering and transmission electron microscopy to analyse the structure of the PBTC12TPD–PC71BM blendfilms, we found that the binary additives with different volume ratios in the processing solvent allow us to tune the relative population of face-on to edge-on PBTC12TPD lamellae and the size of PC71BM clusters in the blendfilms; the sizes of the fractal-like PC71BM clusters and the aggregated domains of PC71BM clusters increased and decreased, respectively, upon increasing the amount of DIO, whereas the relative ratio of face-on to edge-on PBTC12TPD lamellae increased upon increasing the amount of CN. When fabricating the photovoltaic devices, the short-circuit current density of the devices with the PBTC12TPD–PC71BM active layer having been processed with the binary additives is higher than that of the device incorporating an active layer processed without any additive. As a result, the power conversion efficiency of a device incorporating an active layer of PBTC12TPD–PC71BM (1 : 1.5, w/w) processed with binary additives of 0.5% DIO and 1% CN in chloroform increased to 6.8% from a value of 4.9%, a relative increase of 40%, for the corresponding device containing the same active layer but processed without any additive.
1.
Introduction
Solution-processed bulk heterojunction (BHJ) organic photo-voltaic devices based on composites of conjugated polymers and nanometer-sized fullerenes are attracting great attention because of their low cost, ease of fabrication, and the potential for application inexible devices.1–7Since the structure of the BHJ active layer signicantly inuences the device performance, the quest to develop an optimal active layer morphology has led to testing of various processing parameters, including anneal-ing situations,8 the solution casting conditions, different
solvents and solvent additives.9–13Moreover, the changes in the active layer morphology with time also need to be considered when designing polymer solar cells.14 A BHJ layer usually comprises a blend of donor (D), polymer, and acceptor (A), fullerene, materials that are organized into three phases: a nanometer-scale phase-separated polymer phase, an aggregated fullerene phase and a well inter-mixed polymer–fullerene phase. The volume fractions of each of these three phases will depend on the solvent used, the post processing conditions and the additives incorporated during their processing, if any, and will determine the optical and carrier transport properties of the active layer. These phases permit the absorption of a different part of the solar spectrum and play different important roles in the system; the well inter-mixed polymer–fullerene phase gives efficient exciton dissociation15,16 at the polymer–fullerene interfaces, and the phase-separated polymer and fullerene phase provide the transport pathways of holes and electrons to their respective electrodes. Our current understanding of the structures of these three phases in crystalline polymer–fullerene or amorphous polymer–fullerene17,18active layer morphologies, including their crystallization,19the extent of phase separation,
aDepartment of Materials Science and Engineering, National Chiao Tung University,
1001 Ta Hsueh Road, Hsinchu 30010, Taiwan. E-mail: khwei@mail.nctu.edu.tw; Tel: +886-3-5712121 ext. 31871
bNational Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Science-Based
Industrial Park, Hsinchu 30077, Taiwan. E-mail: usjeng@nsrrc.org.tw; Tel: +886-3-5780281 ext. 7108
cDepartment of Chemical Engineering, National Tsing Hua University, Hsinchu 30013,
Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ta04804k
Cite this:J. Mater. Chem. A, 2014, 2, 20760 Received 13th September 2014 Accepted 17th October 2014 DOI: 10.1039/c4ta04804k www.rsc.org/MaterialsA
Materials Chemistry A
PAPER
Published on 20 October 2014. Downloaded by National Chiao Tung University on 21/07/2015 10:55:24.
View Article Online
and solubility disparities20 between the polymer and the fullerene are qualitative at best; it arises essentially from the difficulty in distinguishing the nanometer-scale phase sepa-rated materials that both comprise mostly carbon atoms. Ideally, the electron donors and acceptors form two respective intertwined continuous phases, where the size of the domains will be approximately less than the diffusion length of the exciton in the polymer domain (ca. 10 nm). The photogenerated excitons must diffuse to the D–A interface in the active layer and dissociate prior to recombination to effectively convert photon energy to electrical energy. The development of a nanometer-scale morphology, which is necessary to manufacture a high-performance organic photovoltaics (OPV),21,22depends on the chemical composition of the D and A components, the nature of the host solvent and processing additives, and the post-pro-cessing treatment conditions. Moreover, the device perfor-mances also critically depend on the contact of the active layer with the electrode.23,24The incorporation of solvent additives, which are placed into the solutions from which the BHJ layers are fabricated, is performed extensively when fabricating high-performance BHJ organic solar cells.25–28Typically, the solvent additive will have a lower vapor pressure than that of the pro-cessing solvent and a better solubility for the fullerene than for the polymer, allowing an elongated drying time for the active layer. This in turn inuences the phase-separated morphology of the BHJ lm and, therefore, improves the device perfor-mance. The use of a solvent additive, such as 1,8-diiodooctane (DIO),29,301-chloronaphthalene (CN),31,32or a conjugated mole-cule33to control the morphology of the active layer is one of the simplest and most effective methods for optimizing the performance of a BHJ device.
In this study, we employed the binary additives DIO and CN, which have different relative solubilities for the polymer and fullerenes as well as different boiling points to tune the active layer morphology; one additive improved the dispersion of the fullerene in the amorphous polymer phase because of its rela-tively better fullerene solubility,11 while the other tuned the orientation of the polymer lamellae because of its relatively better solubility for the polymer. Notably, the mechanism behind the packing of polymers in a face-on orientation in thin lms has not been well understood.34,35Rather than covalently bondinguorine atoms or linear alkyl chains to the polymer,36,37 we incorporated binary additives into a processing solvent for inducing a greater face-on crystallite orientation relative to the substrate, thereby promoting hole transport toward the anode and inducing higher power conversion efficiency (PCE). Our approach provides a new means of tuning the face-on orienta-tion of polymer crystallites through variaorienta-tions in the nature and concentration of the binary additives.
Specically, we varied the volume ratio of these two additives to inuence the drying time of polymer–fullerene lms as well as the relative solubility for the polymer and fullerene, thereby optimizing the morphology of the active layer and obtaining highly ordered lamellar sheets featuring a face-on orienta-tion.1,38–43We used synchrotron grazing-incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS)44–59to analyze the orientation of the crystalline lamellae of the polymer with
respect to the substrate10,12,60–62and the sizes of the fullerene aggregates.
We expect that the binary additives DIO and CN can reduce the size of the aggregated fullerene domains as well as tune the orientation of the polymer lamellae such that the exciton dissociation and the pathways for carrier transport are improved. Because the binary additives induce a decrease in the miscibility gap between the conjugated polymer and PC71BM,
the size of the aggregated fullerene domains is reduced. This indicates a larger volume fraction of intermixed polymer and fullerene domains, which can be inferred from the photo-luminescence (PL) and time-resolved (TR) PL spectroscopy study. As a consequence, we found that the PCE of a device incorporating poly[bi(dodecyl)thiophene-dodecyl-thieno[3,4-c]-pyrrole-4,6-dione] (PBTC12TPD)–[6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM) (1 : 1.5, w/w) as the active layer improved
from 4.9% when fabricated without any additive to 6.8% when processed with 0.5% DIO and 1% CN as additives in the pro-cessing solvent. Table 1 gives the solubility of PBTC12TPD and
PC71BM in CF, DIO and CN.
2.
Experimental section
2.1. Device fabrication
The synthesis of PBTC12TPD was reported elsewhere;63PC71BM
was purchased from Solenne BV; chloroform (CF), DIO, and CN were obtained from Sigma-Aldrich. The indium tin oxide (ITO), with a resistance 15U, was obtained from Merck. ITO-coated glass substrates were sequentially cleaned with detergent, water, acetone, and isopropyl alcohol (ultrasonication; 10 min each), and then dried in an oven for 1 h; the substrates were treated with UV ozone for 15 min prior to use. A thin layer (ca. 30
nm) of polyethylenedioxy-thiophene:polystyrenesulfonate
(PEDOT:PSS, Baytron PVP AI 4083) was spin-coated (4000 rpm) onto the ITO substrates. Aer baking at 140C for 20 min in air,
the substrates were transferred to a N2-lled glove box for
deposition of the active layers from PBTC12TPD–PC71BM
mixtures at a weight ratio of 1 : 1.5 (30 mg mL1) in CF solutions at 60C, with or without processing additives (DIO: 0, 0.5, or 1 vol%; CN: 0, 0.5, or 1 vol%). In the design of these experiments, three different additive concentrations (0, 0.5, 1 vol%) for each of the two additives resulted in nine combinations for analyzing the mixing effects of DIO and CN. Here, we denote all the combinations with DmCn, with the subscripts m and n
repre-senting the volume percentages of the additives of D and C,
Table 1 Solubility of PBTC12TPD and PC71BM in DIO, CN, and CF at room temperature, determined using ASTM E1148
Additive PBTC12TPD (mg mL1) PC71BM (mg mL1) DIO (b.p.¼ 333C) 1.4 21.7 CN (b.p.¼ 259C) 3.5 16.3 CFa(b.p.¼ 61C) 4.5 21.6 aFrom ref. 11.
respectively in the processing solvent; for example, D0.5C0.5
represents the device featuring an active layer comprising PBTC12TPD and PC71BM (1 : 1.5, w/w) that was processed with
CF containing 0.5 vol% DIO and 0.5 vol% CN. The boiling points of CF, DIO, and CN are 61 C, 333 C, and 259 C, respectively. The blend solutions with binary additives were spin-coated onto the PEDOT:PSS layers to form active layers having a thickness of approximately 300 nm. Devices were ready for measurements aer thermal deposition (pressure: ca. 1 106mbar) of a 20 nm thicklm of Ca and then a 100 nm thick Allm as the cathode.
2.2. Device characterization
Photovoltaic measurements of the devices were performed under simulated AM 1.5G irradiation (100 mW cm2) using a xenon lamp (Newport 66902, 150 W solar simulator). The J–V characteristics were evaluated using a Keithley 2400 source meter. The active layers were immersed in DI water for exfoli-ation and then transferred to a Cu foil for image recording through transmission electron microscopy (TEM, FEI Tecnai G2) operated at 120 keV. External quantum efficiencies (EQEs) were recorded using a spectral response measurement set-up (Optosolar SR150).
GIWAXS/GISAXS measurements were performed to investi-gate the structure of the crystalline PBTC12TPD and the size of
the aggregated PC71BM clusters, using the BL23A beamline
station of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan.64The BL23A beamline, equipped with a Pilatus 1M-F area detector for SAXS and aat-panel C9728DK-10 area detector for WAXS, allowed simultaneous GISAXS and GIWAXS measurements for correlated changes in crystalline structures and nanostructures in the same probing area of the thinlms of interest. With the sample surface dened in the x–y plane and the incident x-rays in the x–z plane, the scattering wave vector transfer q¼ (qx, qy, qz) can be decomposed into
three orthogonal components as follows: qx ¼ 2pl1
(cosb cos f cos a), qy¼ 2pl1(cosb sin f), and qz¼ 2pl1
(sina + sin b), where a and b stand for incident and exit angles, respectively, andf measures the scattering angle away from the y–z plane;54,56,59l is the wavelength of the X-rays. The 8 keV X-ray beam used was 0.2 mm diameter, with a sample incident angle of 0.2; the sample-to-detector distances were 5.0 and 0.25 m for the GISAXS and GIWAXS systems, respectively. The samples were prepared through spin-coating of PBTC12TPD–PC71BM
solutions onto 4 cm2Si substrates; thelm thickness was about the same for each sample (ca. 300 nm).
3.
Result and discussion
3.1. Molecular structures and characteristics
Fig. 1a presents the molecular structures of the materials used in this study: PBTC12TPD, PC71BM, DIO, and CN. Fig. 1b
displays the UV-Vis absorption spectra of spin-coated
PBTC12TPD–PC71BM thin lms, processed with various
amounts of the incorporated additives in the CF solvent for a similar thickness of approximately 300 nm. We attribute the
minor absorption peaks at 375 nm and the major broad peaks at 550 nm to PC71BM and PBTC12TPD, respectively. The
absorp-tion peak intensity at 550 nm for the PBTC12TPD–PC71BM blend
lms, processed with the 0.5 vol% of the CN additive, was larger than that of the blendlm processed without any additive; with 1 vol% CN, the peak intensity further increased slightly. With a distinct shoulder vibronic peak appearing at 640 nm, the spectrum of the PBTC12TPD–PC71BM thinlm processed in CF
incorporating both 0.5 vol% DIO and 1 vol% CN presented the highest absorption peak intensity among all of our testedlms. 3.2. Photovoltaic behavior
Fig. 2a presents the J–V curves of devices incorporating active layers of PBTC12TPD–PC71BM that were processed in CF
incor-porating various amounts of the additives. Table 2 presents the corresponding open-circuit voltages (Voc), values of Jsc, ll
factors (FFs), and PCEs of these devices. The values of Vocof
these devices were all in the range 0.90–0.91 V. The control device, D0C0, exhibited a value of Jscof 9.22 mA cm2, a value of
Vocof 0.90 V, an FF of 59% and a PCE of 4.9%. With increasing
CN content in the processing solutions, The Jscvalue increased
from 9.76 and 10.10 mA cm2, respectively for the devices D0C0.5and D0C1. Correspondingly, the PCE increased slightly
from 4.9% to 5.3% and to 5.5%. Fig. 2b displays the EQEs for the devices D0C0to D0.5C1. The integrated short-current
densi-ties (Jsc) deduced from our EQE spectra for the devices D0C0,
D0C0.5, D0C1, D0.5C0, D0.5C0.5, and D0.5C1were 8.9, 9.4, 9.8, 10.5,
11.3, and 12.2 mA cm2, respectively. These values (each obtained from the average of ten devices) differ from the
Fig. 1 (a) Molecular structures of PBTC12TPD, PC71BM, and the solvent additives DIO and CN. (b) Absorption spectra of PBTC12TPD–PC71BM spin-castedfilms processed in the absence and presence of DIO and CN additives at various volume ratios.
Fig. 2 (a) Current density–voltage curves of devices incorporating PBTC12TPD–PC71BM active layers processed in the presence of various volume ratios of the additives DIO and CN. (b) EQE curves of PBTC12TPD–PC71BM blendfilms that had been processed in CF with the incorporation of additives from D0C0to D0.5C1.
corresponding directly measured Jscby 4% at most, indicating
good accuracy in our measurements of device performance. Furthermore, Jscvalues deduced from the EQE spectra for the
devices D1C0, D1C0.5, D1C1 were respectively 8.9, 9.7 and 10.2
mA cm2, as shown in Fig. S1.†
The PCE of the device D0.5C0increased to 5.9% from 4.9% for
the control device D0C0, along with an increase in the value of Jsc
to 10.90 from 9.22 mA cm2. Fig. S2† shows that the PCE of the
device D1C0 decreased, however, to 4.2% upon further
increasing the concentration of DIO in the processing solvent; although DIO is a relatively better solvent for dispersing PC71BM clusters, it is a poorer solvent for PBTC12TPD when
compared with CN (Table 1).
A relatively high concentration of DIO in the processing solvent, such as in the case of device D1C0, resulted in a ll
factor (FF) of 50%, lower than that (59%) for the device featuring an active layer processed without any additives. The PCE increased to 5.3% for the device D0C0.5and to 6.2% for D0.5C0.5,
but decreased to 4.6% for D1C0.5. We observed the same trend
for the cases in which the processing solvent contained 1 vol% CN; increasing the content of DIO to 0.5 vol%, the PCE of D0.5C1
increased to 6.8% from 5.5% for D0C1, whereas the PCE
decreased to 4.8% for D1C1(i.e., when 1 vol% of DIO and CN
were both present in the processing solvent). Overall, the best performance device is D0.5C1with 6.8% PCE and a value of Jscof
12.57 mA cm2. Further increasing the CN content over 1.0 vol% in the processing solution, with 0.5 vol% DIO, did not improve PCE and Jsc values of the hence processed device,
implying a saturated CN effect. We have measured and obtained a lower PCE value of 6.2% for the D0.5C1.5device than that for
the D0.5C1 device, which might result from the unbalanced
effect of a high concentration of CN and a low concentration of DIO on the active layer during the processing. All the additive combination effects can be explained by the nature and the amount of DIO and CN: the boiling point of CN and DIO are 259C and 333C, respectively, indicating that CN had evapo-ratedrst aer CF (boiling point 61C). Evaporation of CN was then followed by the evaporation of DIO– thus this sequence resulted in the changes in polymer crystallinity and polymer lamellae orientationrst and then by the rening dispersion of
PC71BM aggregates. All devices processed with the binary
additives DIO and CN had been dried in a glove box for 12 h and then kept under vacuum at room-temperature for at least 5 h to completely remove the residual solvent additives in the active layer prior to evaporation of the back electrode.
3.3. Morphological and crystallinity study
To decipher the active layer morphology, we used simultaneous GISAXS and GIWAXS for quantitative analysis of the PC71BM
cluster size and the extent of PBTC12TPD crystallization and
lamellae orientation, respectively. The PBTC12TPD–PC71BM
blendlms prepared for these X-ray scattering experiments are denoted herein using the same nomenclature as that for device characterization. Fig. 3 displays the GISAXS proles extracted along the in-plane direction, qy, of the two-dimensional (2D)
GISAXS images of the PBTC12TPD–PC71BM blend lms
pro-cessed in CF solutions containing the additives. Fig. S4† shows that the intensity of the GISAXS proles for the blend lms
Table 2 Averaged (ten devices) photovoltaic characteristics of the devices incorporating PBTC12TPD–PC71BM active layers processed in the presence of various volume ratios of the additives DIO and CN
Device Additive concentration (vol%) Voc(V) Jsc(mA cm2) FF (%) PCE (%) DIO CN D0C0 0 0 0.90 0.01 9.22 0.11 59.1 0.2 4.9 0.13 D0C0.5 0 0.5 0.90 0.02 9.76 0.09 60.6 0.1 5.3 0.20 D0C1 0 1 0.91 0.01 10.10 0.15 59.3 0.2 5.5 0.11 D0.5C0 0.5 0 0.90 0.02 10.90 0.11 60.3 0.1 5.9 0.21 D0.5C0.5 0.5 0.5 0.90 0.01 11.60 0.09 59.4 0.2 6.2 0.14 D0.5C1 0.5 1 0.91 0.01 12.57 0.08 59.5 0.1 6.8 0.13 D1C0 1 0 0.90 0.02 9.30 0.14 50.5 0.2 4.2 0.20 D1C0.5 1 0.5 0.90 0.02 10.12 0.16 50.6 0.3 4.6 0.21 D1C1 1 1 0.90 0.01 10.61 0.12 50.5 0.2 4.8 0.15
Fig. 3 In-plane GISAXS profiles of spin-cast PBTC12TPD–PC71BM films, offset in intensity to show the feature shapes of the profiles that were originally largely overlapped (a) D0C0 D0C1, (b) D0.5C0 D0.5C1 and (c) D1C0 D1C1, fitted (solid curves) using a fractal-model comprising polydisperse spheres as shown in eqn (1).
increased with the weight ratio of PC71BM (0, 33, 60 to 70 wt%)
in thelms, suggesting strongly that the weak slope breaks in the GISAXS proles depend largely on PC71BM aggregation.
Hence, these GISAXS proles are dominated by the aggregation behaviour of PC71BM. Moreover, Fig. 3a–c show the GISAXS
proles for the blend lms exhibit substantial changes when processed with varying DIO contents; in contrast, relatively minor changes in the GISAXS proles for the blend lms are displayed when processed with varying CN contents. These results are consistent with the preferential solubility of the additives: DIO has a better solubility for PC71BM than CN and
thus presents a more signicant effect on PC71BM dispersion
than does CN in the active layers.
We determined the PC71BM cluster sizes in these binary
additives processed PBTTPD–PC71BM blend lms, DmCn, by
tting the GISAXS, I(qy), proles using a fractal model
comprising polydisperse spheres9,43 that were dened by the following four equations.
I(q) ¼ A < P(q) > S(q) (1)
The scattering intensity, I(q), is determined by the size-averaged form factor P(q) and structure factor S(q), with both of them being functions of scattering vector q, and a scaling parameter A. Since the present GISAXS proles were measured in relative intensity scales, we have combined all the intensity related factors, such as the scattering contrast and volume fraction, into a single scaling parameter A in the model. The form factor P(q) is proportional to the polydisperse sphere of radius R, as determined in eqn (2).65
P(q) f |3j1(qR)/(qR)|2 (2)
Where j1is therst order spherical Bessel function. The Schultz
size-distribution function, f(r), is dened by eqn (3). f ðrÞ ¼ z þ 1 Ra zþ1 Rzexp z þ 1 Ra R 1 Gðz þ 1Þ; z . 1 (3) Where f(r) is a function of the mean radius Ra, width parameter
z, and polydispersity p¼ (z + 1)1/2. The fractal structure factor, S(q), is dened in eqn (4). SðqÞ ¼ 1 þ 1 ðqRaÞD DGðD 1Þsin½ðD 1Þtan1ðqxÞ h 1 þ ðqxÞ2iðD1Þ=2 (4)
S(q) describes a fractal structure comprising primary PC71BM
aggregates of a mean radius Ra(adapted from the average size of
the polydisperse sphere model in our case as an approxima-tion), with the fractal dimension d,66 and correlation length x.44,67
Table 3 lists atted averaged size of PC71BM fractal cluster of
6.0 nm with a fractal dimension of 2.9 that represents densely packed aggregated PC71BM clusters for the PBTC12TPD–PC71BM
lm processed without any additive, indicating that PC71BM
clusters can easily form large aggregated domains (a fractal
dimension of 3 represents highly dense aggregated PC71BM
cluster).
The PC71BM fractal cluster size increased to 8.0 and 8.1 nm
with fractal dimension of 2.7 and 2.5 for the D0C0.5lm and
D0C1lm, respectively, suggesting that the incorporation of the
additive CN in the processing solvent induced larger PC71BM
cluster sizes but with lower fullerene packing density. These larger but less densely packed PC71BM clusters can be explained
by the speculation that the PC71BM clusters might have been
intercalated by a few PBTC12TPD polymer chains.
When the amount of CN increased whilexing the amount of DIO to be 0.5 vol%, the PC71BM cluster sizes increased to 7.6,
11.0 and 11.2 nm with the corresponding fractal dimension of 2.5, 2.5 and 2.4 for D0.5C0, D0.5C0.5 and D0.5C1 lms,
respec-tively, indicating relatively loosely packed PC71BM cluster as
compared to those cases without DIO (as a result of better intermixing between polymer and fullerene).
The PC71BM cluster size increased to 13.2, 13.5 and 14.0 nm
with the corresponding fractal dimension of 3.0, 2.9 and 2.7 for D1C0, D1C0.5 and D1C1 lms, respectively. Possibly, the
DIO-overcharged binary-additive results in an increasingly larger solubility difference between the PC71BM and the polymer
duringlm drying, leading to larger and less compact PC71BM
clusters.
Based on the normalized integrated intensity of (100) peak in Table 3, the relative population of the face-on polymer lamellae increased with the amount of CN at axed amount of DIO with the largest increase in the relative population of the face-on polymer lamellae taking place in the case of 0.5 vol% DIO. This can be explained by the fact that DIO is a relatively poorer solvent than is CN for PBTC12TPD and will lead to diminishing
polymer crystallinity when the concentration of DIO increases to 1 vol%.
Fig. 4a and b present 2D GIWAXS patterns of the PBTC12
-TPD–PC71BMlms processed with or without the binary
addi-tives (D0.5C1 and D0C0 lms); we recorded these patterns to
correlate the ordering structures of the polymers, correspond-ing to the edge- and face-on orientation packcorrespond-ing of the PBTC12TPD lamellae, relative to the substrate. Fig. 4c–h
pres-ents GIWAXS proles recorded along the out-of-plane (Fig. 4c–e) and in-plane (Fig. 4f–h) directions of the PBTC12TPD–PC71BM
lms processed with and without the additives. Fig. 4c displays the prole of the D0C0lm, with strong lamellar peaks (100),
(200), and (300) located at qz ¼ 0.22, 0.44, and 0.66 ˚A1,
respectively, due to alkyl stacking. This is representative of the scattering resulting from the edge-on PBTC12TPD lamellae
having an out-of-plane orientation relative to the substrate plane. In addition, an amorphous halo appeared at a value of qz
of 1.41 ˚A1, corresponding to the distance between two neigh-bouring fullerene particles, hence, indicating a short-range ordering of PC71BM. Therst-order, (100), alkyl stacking peak
position was located at a value of qzof 0.22 ˚A1, corresponding
to a lamellar packing distance of ca. 2.86 nm for the D0C0lm;
all other DmCn lms, lms processed with different binary
additive concentration, show similar crystalline peaks but with different peak intensities and widths. Fig. 4f–h showing the (100) peak for face-on lamellae in the GIWAXS proles change
with the variation of CN content in the processing solvent. To quantitatively determine the CN additive effects, we calculated the integrated intensities of the (100) peaks of I(qz) and I(qy) for
the edge-on and face-on PBTC12TPD lamellae, respectively, from
the GIWAXS proles; from the (100) peak widths, the corre-sponding approximately crystal dimensions (or correlation lengths) were also deduced based on the Scherrer equation for comparison.10,44,56,68 We note that the presented GIWAXS proles along the out-of-plane direction of the lm in Fig. 4c–h are extracted directly from the 2D GIWAXS patterns before pole-gure corrections.69 Nevertheless, the GIWAXS 2D patterns corrected for polegures have contained missing wedges along the qz(Fig. 4a and b), as detailed in a previous report.69Hence,
the presented GIWAXS proles along qz (obtained from the
original 2D GIWAXS patterns) represent only approximately the diffraction intensity from upright edge-on lamellae. To remedy this approximation, we have carried out X-ray diffraction (XRD) measurements on a D0C0lm for a diffraction I(qz) prole from
upright edge-on crystalline lamellae.69Fig. S5† shows the XRD prole is consistent with the GIWAXS prole, particularly in the low-qz region for the (100) peak. Nevertheless, the (100) peak
width from the GIWAXS prole is about 25% larger than that from the XRD prole (0.046 vs. 0.037 A1). We therefore
systemically increased all the crystal domain sizes extracted from the GIWAXS (100) peak width by 25% for the edge-on polymer lamellae. Table 3 summarizes the results; we note that the size correction would not alter the relative trend of crystal-lization behavior for these DmCnlms.
Table 3 lists the normalized integrated intensity, qz-100and
qy-100, of (100) peak I(qz) and I(qy) of the edge- and face-on
PBTC12TPD lamellae for all the DmCn lms. For comparison
convenience, we have normalized the qz-100and qy-100values for
all samples by the qz-100of the D0C0lm. Table 3 shows that
when the CN concentration increased from 0 to 1% while the volume concentration of DIO was kept at 0.5%, similar PC71BM
dispersions (d 2.4–2.5 and 2Ra 7.6–11.2 nm) occurred in the
blend lms, but the normalized integrated intensity of (100) peaks of the face-on lamellae increase more than 25% (48 vs. 38) relative to that of the D0C0lm. As a result of better fullerene
dispersions (less fullerene aggregation) and higher population of face-on PBTC12TPD lamellae in the case of binary additives,
Table 3 Structural parameters of PBTC12TPD–PC71BMfilms processed with and without additives
Notation 2Raa(nm) db qz-100c(edge-on lamellae) (%) Ld(nm) (edge-on) qy-100c(face-on lamellae) (%) Ld(nm) (face-on)
D0C0lm 6.0 0.55 2.9 0.019 100 16 38 15 D0C0.5lm 8.0 0.62 2.7 0.027 87 23 38 22 D0C1lm 8.1 0.58 2.5 0.025 77 23 41 22 D0.5C0lm 7.6 0.47 2.5 0.028 85 23 45 21 D0.5C0.5lm 11.0 0.53 2.5 0.036 95 21 46 20 D0.5C1lm 11.2 0.56 2.4 0.029 88 20 48 22 D1C0lm 13.2 0.57 3.0 0.014 62 24 30 23 D1C0.5lm 13.5 0.63 2.9 0.023 69 21 34 20 D1C1lm 14.0 0.61 2.7 0.038 70 23 36 21 a2R
a: average size of PC71BM clusters.bd: fractal dimension.cNormalized integrated intensity of (100) peak for qz-100and qy-100.dL: edge-on (or face-on) lamellar size of the conjugated polymer.
Fig. 4 Polefigures of the 2D GIWAXS patterns of (a) D0C0and (b) D0.5C1. Corresponding out-of-plane GIWAXS profiles of (c) D0C0to D0C1, (d) D0.5C0to D0.5C1and (e) D1C0to D1C1and in-plane GIWAXS profiles of (f) D0C0to D0C1, (g) D0.5C0to D0.5C1and (h) D1C0to D1C1 from the spin-cast PBTC12TPD–PC71BMfilms.
as compared to the case without any additive, the PCE values are enhanced to 5.9, 6.2, and 6.8% for the D0.5C0, D0.5C0.5, and
D0.5C1 devices, respectively, from 4.9% for the D0C0 device.
Table 3 also reveals that increasing the concentration of DIO from 0.5 to 1 vol% led to signicant decreases in the population of both the edge- and face-on polymer lamellae, implying that a content of DIO of 1 vol% was too high and could interfere with the polymer PBTC12TPD crystallization in the processing
solvent (CF). Table 3 also lists the calculated dimensions of PBTC12TPD (100) edge- and face-on lamellar crystal using the
Scherrer equation.68The introduction of additives (DIO, CN, or DIO + CN) enhanced the edge- and face-on lamellae sizes to 24 nm and 23 nm from 16 and 15 nm, respectively, equating to 50 and 53% increases, respectively, relative to those in the pristine lm. Hence, using DIO and CN as binary additives, we can simultaneously reduce the size of PC71BM aggregate domains
and tune the relative orientation of the PBTC12TPD lamellae.
Fig. 5 summarizes the results of our GISAXS and GIWAXS studies. The results suggest that lamellar crystallites having an edge-on orientation might partially reorient to a face-on arrangement upon increasing the volume percentages of DIO and CN. We suggest that the binary additives might affect the polarity between the substrate surface and the linear alkyl chains of PBTC12TPD, thereby changing the polymer's preferred
orientation into a face-on arrangement, particularly when the additive concentrations are high. Through the use of the binary additives, we not only can modulate a large PC71BM fractal
structure network (domain) that comprises many closely packed PC71BM clusters to smaller aggregated PC71BM domains that
consist of more appropriated PC71BM cluster size (11 nm) of a
lower fractal dimension, but also can induce an increase in the population of the face-on polymer lamellae at the expense of the edge-on polymer lamellae, resulting in a more isotropic orien-tation as compared to the case of without any additive. A combination of more appropriated PC71BM cluster size and
higher population of the face-on polymer lamellae can lead to superior pathways for carrier transport. Thus, Fig. 5 shows the PCE value was enhanced to 6.8% for the D0.5C1device with an
active layer comprising an appropriated PC71BM cluster size
with a fractal dimension of 2.4, and the face-on polymer lamellae intensity of 48 from 4.9% for the D0C0device, with an
active layer comprising a large PC71BM fractal structure network
with a fractal dimension of 2.9 and the face-on polymer lamellae intensity of 38.
We also qualitatively determined the degree of molecular-scale intermixing between the polymer and fullerene with photoluminescence (PL) and time-resolved PL (TRPL) spectra. Fig. 6 shows the PL and TRPL spectra of the PBTC12TPD–
PC71BMlms, and the inset shows that the intensity of the PL
peak at 833 nm in the case of D0.5C1lm (processed with 0.5
vol% DIO and 1 vol% CN) is quenched, as compared to that of D0C0lm (processed without any additive). This indicated that
the extent of charge recombination is reduced, and thus more excitons are available for dissociation. Moreover, Fig. 6 displays a faster decay of the transient PL peak for the D0.5C1lm than
that for the D0C0lm, revealing a slightly shorter exciton life
time for D0.5C1lm than for the D0C0lm (sc¼ 0.27 ns vs. sc¼
0.32 ns). A shorter exciton life time suggests possibly a better molecule-scale intermixing between the fullerene and polymer, as facilitated by the additives used.15,16The PL and TRPL results are consistent with the GISAXS result that the binary additives enhanced more homogeneous dispersion of the PC71BM in the
polymer.
Fig. 7 presents the top-view TEM images of the D0C0and
D0.5C1lms, respectively. For the D0C0lm processed without
additives, Fig. 7a reveals bright and dark regions representing the conjugated polymer- and fullerene-rich domains, respec-tively, arising from the large difference in electron scattering density between the fullerene (1.5 g cm3) and the polymer (1.1 g cm1); large PC71BM aggregated domains (average diameter
size: ca. 150 nm; mean spacing: ca. 210 nm) were present. In Fig. 7b, we observe that the PC71BM aggregated domains in the
Fig. 5 Normalized integrated intensity of (100) peak of the edge- and face-on lamellae and PC71BM cluster size in the PBTC12TPD–PC71BM films processed with additives, ranging from D0C0to D1C1.
Fig. 6 Time-resolved photoluminescence (TRPL) spectra, detected with an 833 nm laser for the D0C0and D0.5C1films. The wavelength 833 nm was chosen premeditatedly owing to a PBTC12 TPD-domi-nated PL (absorption) intensity (inset). The PBTC12TPD-dominated exciton lifetimes (sc) are comparable (0.32 nsvs. 0.27 ns). The inset is the corresponding PL spectra excited with a 550 nm laser.
D0.5C1lm had decreased dramatically (to ca. 40 nm). Based on
our GISAXStting results and TEM images, we noted that for the case without any additive, D0C0, the large aggregated
PC71BM domains (150 nm) comprise individual PC71BM
mole-cule (cluster) that are densely packed (fractal dimension 2.9). The size of aggregated PC71BM domains become much smaller
and the PC71BM clusters become more porous (2.4) in the case
of D0.5C1. When the DIO concentration increased to a critical
concentration of 1 vol% in the binary additives, the PC71BM
clusters become highly packed.
Fig. S3† shows the TEM images for other DmCn lms. The
lm morphology is relatively more homogenous without large domain boundary when compared to the D0C0lm; these may
correspond to the larger fractal network (larger correlation length; cf. Table S1†) formed by the better dispersed (lower fractal dimension) PCBM aggregates, revealed from GISAXS.
Fig. 8 presents the images of the PBTC12TPD–PC71BM
morphologies that are consistent with the structural infor-mation we obtained from the GISAXS, GIWAXS, and TEM analyses. We found that the optimal ratio of the binary additives enhanced the population of polymer face-on lamellae, at the expense of slightly depressed population of the edge-on lamellae, while modulating PC71BM clusters for
forming lower dimension fractal structure (dz 2.4) with an appropriate primary aggregate size of 11 nm. We note that the lower fractal dimension also suggests a better intermixed PC71BM and polymer in the matrix (acting as the third phase),
as also illustrated in the images. The possible role of the third phase that comprised well-mixed PC71BM and polymer has
been discussed extensively.23,24 This well-mixed phase asso-ciates implicitly with the packing of PC71BM and the
crys-tallinity of the polymer that were determined from GISAXS and GIWAXS, respectively, due to conservation of the total
volume. Better miscibility between PC71BM and polymer
results in a larger matrix phase, presumably corresponding to
smaller domains of PC71BM aggregates and polymer
crystallites.
4.
Conclusions
CN and DIO have relative preferential solubility for PBTC12TPD and PC71BM, respectively, and therefore can tune
the orientation of polymer lamellae and the degree of
dispersion of PC71BM in PBTC12TPD–PC71BM lms,
prepared from CF solutions containing these additives. We examined the resulting morphologies of theselms through GISAXS/GIWAXS and TEM characterizations and concluded that DIO can effectively modulate the size of the fractal-like PC71BM clusters and their aggregated domains while CN can
induce higher population of face-on polymer lamellae under proper binary additive concentrations. Correspond-ingly, an improvement in device performance with a PCE of 6.8% could be achieved with a device incorporating a PBTC12TPD–PC71BM thinlm deposited with 0.5 vol% DIO
and 1 vol% CN as additives in the processing solvent. Presumably, this combination of additives provided a more balanced solubility between the polymer and the fullerene, resulting in an optimized degree of phase separation in the active layer, as compared to the case without any additive. In conclusion, the binary additives approach leads to a better active layer morphology that combines the suitable polymer lamellae orientation and better fullerenes dispersion, resulting in an increase in the generated photocurrent because of more efficient exciton dissociation and better carrier transport pathway. Additionally, it leads to enhanced PCE as compared to the case without additive.
Fig. 7 TEM images of PBTC12TPD–PC71BMfilms prepared (a) in the absence of additives and (b) in the presence of 0.5 vol% DIO and 1 vol% CN in the processing solvent.
Fig. 8 Schematic representation of polymer lamellae with edge-on and face-on orientations and fractal-like PC71BM clusters in spin-cast PBTC12TPD–PC71BMfilms that were processed: (a) in the absence of additives and (b) in the presence of a mixture of DIO and CN. This figure is not proportional to the real scale.
Acknowledgements
We thank the National Science Council, Taiwan, fornancial support (NSC 102-3113-P-009-002).
Notes and references
1 G. Li, R. Zhu and Y. Yang, Nat. Photonics, 2012, 6, 153–161. 2 J. A. Bartelt, J. D. Douglas, W. R. Mateker, A. E. Labban,
C. J. Tassone, M. F. Toney, J. M. J. Fr´echet, P. M. Beaujuge and M. D. McGehee, Adv. Energy Mater., 2014, 1301733. 3 J. M. Jiang, M. C. Yuan, K. Dinakaran, A. Hariharan and
K. H. Wei, J. Mater. Chem. A, 2013, 1, 4415–4422.
4 J. Peet, A. J. Heeger and C. G. Bazan, Acc. Chem. Res., 2009, 42, 1700–1708.
5 B. C. Thompson and J. M. J. Fr´echet, Angew. Chem. Int. Ed., 2008, 120, 62–82.
6 Y. W. Su, S. C. Lan and K. H. Wei, Mater. Today, 2012, 15, 554–562.
7 J. M. Jiang, P. Raghunath, H. K. Lin, Y. C. Lin, M. C. Lin and K. H. Wei, Macromolecules, 2014, 47, 7070–7080.
8 M. Pfaff, P. Muller, P. Bockstaller, E. Muller, J. Subbiah, W. W. H. Wong, M. F. G. Klein, S. R. Puniredd, W. Pisula, A. Colsmann, D. Gerthsen and D. J. Jones, ACS Appl. Mater. Interfaces, 2013, 5, 11554–11562.
9 C. M. Liu, C. M. Chen, Y. W. Su, S. M. Wang and K. H. Wei, Org. Electron., 2013, 14, 2476–2483.
10 M. S. Su, C. Y. Kuo, M. C. Yuan, U. Jeng, C. J. Su and K. H. Wei, Adv. Mater., 2011, 23, 3315–3319.
11 C. M. Liu, M. S. Su, J. M. Jiang, Y. W. Su, C. J. Su, C. Y. Chen, C. S. Tsao and K. H. Wei, ACS Appl. Mater. Interfaces, 2013, 5, 5413–5422.
12 Y. Gu, C. Wang and T. P. Russell, Adv. Energy Mater., 2012, 2, 683–690.
13 M. Kim, J. H. Kim, H. H. Choi, J. H. Park, S. B. Jo, M. Sim, J. S. Kim, H. Jinnai, Y. D. Park and K. Cho, Adv. Energy Mater., 2014, 4, 1300612.
14 C. J. Schaffer, C. M. Palumbiny, M. A. Niedermeier, C. Jendrzejewski, G. Santoro, S. V. Roth and P. M¨ uller-Buschbaum, Adv. Mater., 2013, 25, 6760–6764.
15 E. T. Hoke, K. Vandewal, J. A. Bartelt, W. R. Mateker, J. D. Douglas, R. Noriega, K. R. Graham, J. M. J. Fr´echet, A. Salleo and M. D. McGehee, Adv. Energy Mater., 2013, 3, 220–230.
16 G. J. Hedley, A. J. Ward, A. Alekseev, C. T. Howells, E. R. Martins, L. A. Serrano, G. Cooke, A. Ruseckas and I. D. W. Samuel, Nat. Commun., 2013, 4, 2867.
17 Z. M. Beiley, E. T. Hoke, R. Noriega, J. Dacu˜na, G. F. Burkhard, J. A. Bartelt, A. Salleo, M. F. Toney and M. D. McGehee, Adv. Energy Mater., 2011, 1, 954–962. 18 T. Wang, A. J. Pearson, A. D. F. Dunbar, P. A. Staniec,
D. C. Watters, H. Yi, A. J. Ryan, R. A. L. Jones, A. Iraqi and D. G. Lidzey, Adv. Funct. Mater., 2012, 22, 1399–1408. 19 M. Y. Chiu, U. Jeng, C. H. Su, K. S. Liang and K. H. Wei, Adv.
Mater., 2008, 20, 2573–2578.
20 P. Kohn, Z. Rong, K. H. Scherer, A. Sepe, M. Sommer,
P. M¨uller-Buschbaum, R. H. Friend, U. Steiner and
S. H¨uttner, Macromolecules, 2013, 46, 4002–4013.
21 S. B. Darling and F. You, RSC Adv., 2013, 3, 17633–17648. 22 W. Chen, M. P. Nikiforov and S. B. Darling, Energy Environ.
Sci., 2012, 5, 8045–8074.
23 G. Kaune, E. Metwalli, R. Meier, V. Korstgens, K. Schlage, S. Couet, R. Rohlsberger, S. V. Roth and P. Muller-Buschbaum, ACS Appl. Mater. Interfaces, 2011, 3, 1055–1062. 24 S. Yu, G. Santoro, K. Sarkar, B. Dicke, P. Wessels, S. Bommel, R. D¨ohrmann, J. Perlich, M. Kuhlmann, E. Metwalli, J. F. H. Risch, M. Schwartzkopf, M. Drescher, P. M¨ uller-Buschbaum and S. V. Roth, J. Phys. Chem. Lett., 2013, 4, 3170–3175.
25 X. Guo, N. Zhou, S. J. Lou, J. Smith, D. B. Tice, J. W. Hennek, R. P. Ortiz, J. T. L. Navarrete, S. Li, J. Strzalka, L. X. Chen, R. P. H. Chang, A. Facchetti and T. J. Marks, Nat. Photonics, 2013, 7, 825–833.
26 M. T. Dang and J. D. Wuest, Chem. Soc. Rev., 2013, 42, 9105– 9126.
27 F. Liu, C. Wang, J. K. Baral, L. Zhang, J. J. Watkins, A. L. Briseno and T. P. Russell, J. Am. Chem. Soc., 2013, 135, 19248–19259.
28 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497–500. 29 C. Piliego, T. W. Holcombe, J. D. Douglas, C. H. Woo, P. M. Beaujuge and J. M. J. Fr´echet, J. Am. Chem. Soc., 2010, 132, 7595–7597.
30 J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim, K. Lee, G. C. Bazan and A. J. Heeger, J. Am. Chem. Soc., 2008, 130, 3619–3623.
31 C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T. Q. Nguyen and G. C. Bazan, Adv. Mater., 2010, 22, E63–E66.
32 C. H. Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and J. M. J. Fr´echet, J. Am. Chem. Soc., 2010, 132, 15547–15549. 33 P. Cheng, L. Ye, X. Zhao, J. Hou, Y. Li and X. Zhan, Energy
Environ. Sci., 2014, 7, 1351–1356.
34 J. Jo, J. R. Pouliot, D. Wynands, S. D. Collins, J. Y. Kim, T. L. Nguyen, H. Y. Woo, Y. Sun, M. Leclerc and A. J. Heeger, Adv. Mater., 2013, 25, 4783–4788.
35 B. R. A¨ıch, J. Lu, S. Beaupr´e, M. Leclerc and Y. Tao, Org. Electron., 2012, 13, 1736–1741.
36 I. Osaka, T. Kakara, N. Takemura, T. Koganezawa and K. Takimiya, J. Am. Chem. Soc., 2013, 135, 8834–8837. 37 A. C. Stuart, J. R. Tumbleston, H. Zhou, W. Li, S. Liu, H. Ade
and W. You, J. Am. Chem. Soc., 2013, 135, 1806–1815. 38 M. S. Chen, J. R. Niskala, D. A. Unruh, C. K. Chu, O. P. Lee
and J. M. J. Fr´echet, Chem. Mater., 2013, 25, 4088–4096. 39 L. A. Perez, P. Zalar, L. Ying, K. Schmidt, M. F. Toney,
T. Q. Nguyen, G. C. Bazan and E. J. Kramer,
Macromolecules, 2014, 47, 1403–1410.
40 J. Guo, Y. Liang, J. Szarko, B. Lee, H. J. Son, B. S. Rolczynski, L. Yu and L. X. Chen, J. Phys. Chem. B, 2009, 114, 742–748. 41 M. R. Hammond, R. J. Kline, A. A. Herzing, L. J. Richter,
D. S. Germack, H. W. Ro, C. L. Soles, D. A. Fischer, T. Xu, L. Yu, M. F. Toney and D. M. DeLongchamp, ACS Nano, 2011, 5, 8248–8257.
42 K. Schmidt, C. J. Tassone, J. R. Niskala, A. T. Yiu, O. P. Lee, T. M. Weiss, C. Wang, J. M. J. Fr´echet, P. M. Beaujuge and M. F. Toney, Adv. Mater., 2014, 26, 300–305.
43 W. Ma, J. R. Tumbleston, L. Ye, C. Wang, J. H. Hou and H. Ade, Adv. Mater., 2014, 26, 4234–4241.
44 W. R. Wu, U. Jeng, C. J. Su, K. H. Wei, M. S. Su, M. Y. Chiu, C. Y. Chen, W. B. Su, C. H. Su and A. C. Su, ACS Nano, 2011, 5, 6233–6243.
45 B. Schmidt-Hansberg, M. Sanyal, M. F. G. Klein, M. Pfaff, N. Schnabel, S. Jaiser, A. Vorobiev, E. M¨uller, A. Colsmann, P. Scharfer, D. Gerthsen, U. Lemmer, E. Barrena and W. Schabel, ACS Nano, 2011, 5, 8579–8590.
46 A. J. Pearson, T. Wang, A. D. F. Dunbar, H. Yi, D. C. Watters, D. M. Coles, P. A. Staniec, A. Iraqi, R. A. L. Jones and D. G. Lidzey, Adv. Funct. Mater., 2014, 24, 659–667.
47 K. W. Chou, B. Yan, R. Li, E. Q. Li, K. Zhao, D. H. Anjum, S. Alvarez, R. Gassaway, A. Biocca, S. T. Thoroddsen, A. Hexemer and A. Amassian, Adv. Mater., 2013, 25, 1923– 1929.
48 S. J. Lou, J. M. Szarko, T. Xu, L. Yu, T. J. Marks and L. X. Chen, J. Am. Chem. Soc., 2011, 133, 20661–20663. 49 J. T. Rogers, K. Schmidt, M. F. Toney, G. C. Bazan and
E. J. Kramer, J. Am. Chem. Soc., 2012, 134, 2884–2887. 50 S. Guo, E. M. Herzig, A. Naumann, G. Tainter, J. Perlich and
P. Muller-Buschbaum, J. Phys. Chem. B, 2014, 118, 344–350. 51 C. S. Tsao and H. L. Chen, Macromolecules, 2004, 37, 8984–
8991.
52 H. C. Liao, C. C. Ho, C. Y. Chang, M. H. Jao, S. B. Darling and W. F. Su, Mater. Today, 2013, 16, 326–336.
53 P. M¨uller-Buschbaum, Anal. Bioanal. Chem., 2003, 376, 3–10. 54 H. C. Liao, C. S. Tsao, T. H. Lin, C. M. Chuang, C. Y. Chen, U. Jeng, C. H. Su, Y. F. Chen and W. F. Su, J. Am. Chem. Soc., 2011, 133, 13064–13073.
55 M. Y. Chiu, U. Jeng, M. S. Su and K. H. Wei, Macromolecules, 2010, 43, 428–432.
56 F. Liu, Y. Gu, X. Shen, S. Ferdous, H. W. Wang and T. P. Russell, Prog. Polym. Sci., 2013, 38, 1990–2052. 57 J. Perlich, J. Rubeck, S. Botta, R. Gehrke, S. V. Roth,
M. A. Ruderer, S. M. Prams, M. Rawolle, Q. Zhong, V. K¨orstgens and P. M¨uller-Buschbaum, Rev. Sci. Instrum., 2010, 81, 105105.
58 C. H. Hsu, U. Jeng, H. Y. Lee, C. M. Huang, K. S. Liang, D. Windover, T. M. Lu and C. Jin, Thin Solid Films, 2005, 472, 323–327.
59 C. M. Palumbiny, C. Heller, C. J. Schaffer, V. K¨orstgens, G. Santoro, S. V. Roth and P. M¨uller-Buschbaum, J. Phys. Chem. C, 2014, 118, 13598–13606.
60 F. Liu, Y. Gu, C. Wang, W. Zhao, D. Chen, A. L. Briseno and T. P. Russell, Adv. Mater., 2012, 24, 3947–3951.
61 J. W. Jung, F. Liu, T. P. Russell and W. H. Jo, Chem. Commun., 2013, 49, 8495–8497.
62 J. T. Rogers, K. Schmidt, M. F. Toney, E. J. Kramer and G. C. Bazan, Adv. Mater., 2011, 23, 2284–2288.
63 M. C. Yuan, M. Y. Chiu, S. P. Liu, C. M. Chen and K. H. Wei, Macromolecules, 2010, 43, 6936–6938.
64 U. Jeng, C. H. Su, C. J. Su, K. F. Liao, W. T. Chuang, Y. H. Lai, J. W. Chang, Y. J. Chen, Y. S. Huang, M. T. Lee, K. L. Yu, J. M. Lin, D. G. Liu, C. F. Chang, C. Y. Liu, C. H. Chang and K. S. Liang, J. Appl. Crystallogr., 2010, 43, 110–121. 65 U. Jeng, Y. S. Sun, H. Y. Lee, C. H. Hsu, K. S. Liang, S. W. Yeh
and K. H. Wei, Macromolecules, 2004, 37, 4617–4622. 66 U. Jeng, W. J. Liu, T. L. Lin, L. Y. Wang and L. Y. Chiang,
Fullerene Sci. Technol., 1999, 7, 599–606.
67 J. M. Lin, T. L. Lin, U. Jeng, Z. H. Huang and Y. S. Huang, So Matter, 2009, 5, 3913–3919.
68 C. R. Singh, G. Gupta, R. Lohwasser, S. Engmann, J. Balko, M. Thelakkat, T. Thurn-Albrecht and H. Hoppe, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 943–951.
69 J. L. Baker, L. H. Jimison, S. Mannsfeld, S. Volkman, S. Yin, V. Subramanian, A. Salleo, A. P. Alivisatos and M. F. Toney, Langmuir, 2010, 26, 9146–9151.
