Organic Solar Cells with Controlled Nanostructures Based on

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Organic Solar Cells with Controlled Nanostructures Based on Microphase Separation of Fullerene-attached Thiophene-Selenophene Heteroblock Copolymers Peihong Chen, Kyohei Nakano, Kaori Suzuki, Kazuhito Hashimoto, Tomoka Kikitsu, Daisuke Hashizume, Tomoyuki Koganezawa, and Keisuke Tajima ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14629 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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ACS Applied Materials & Interfaces

Organic Solar Cells with Controlled Nanostructures Based on Microphase Separation of Fullerene-attached Thiophene-Selenophene Heteroblock Copolymers Peihong Chen1,2, Kyohei Nakano1, Kaori Suzuki1, Kazuhito Hashimoto2, Tomoka Kikitsu1, Daisuke Hashizume1, Tomoyuki Koganezawa3 and Keisuke Tajima1,4* 1

RIKEN Center for Emergent Matter Science, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan,

2

Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1

Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, 3Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan and 4Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan *

Corresponding author

E-mail: [email protected]

Abstract Heteroblock copolymers consisting of poly(3-hexylthiophene) and fullerene-attached poly(3alkylselenophene) (T-b-Se-PCBP) were synthesized for organic photovoltaic applications by quasiliving catalyst transfer polycondensation and subsequent conversion reactions. Characterization of the polymers confirmed the formation of well-defined diblock structures with high loading of the fullerene at the side chain (~40 wt%). Heteroblock copolymer cast as a thin film showed a clear microphaseseparated nanostructure approximately 30 nm in repeating unit after thermal annealing, which is identical to the microphase-separated nanostructure of diblock copolymer consisting of poly(31 ACS Paragon Plus Environment


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hexylthiophene) and fullerene-attached poly(3-alkylthiophene) (T-b-T-PCBP). These heteroblock copolymers provide an ideal platform for investigating the effects of nanostructures and interfacial energetics on the performance of organic photovoltaic devices.

Keywords block copolymers; microphase separation; organic solar cells; energy cascade; selforganization; semiconducting polymers; thin films;

Introduction Recently developed organic photovoltaic (OPV) devices can achieve power conversion efficiencies (PCEs) of solar energy of over 11%.1-2 This high performance currently relies on an active layer structure, called a mixed bulk heterojunction (BHJ), which is a physical mixture of electron donor (D) and acceptor (A) materials. Both photogeneration and recombination of the charge pairs occur dominantly at the D/A interface in OPVs, and thus the interfacial energetic structure plays a critical role in determining the ultimate performance of the device3. The relationship between the interfacial structure and photovoltaic performance has been studied using a planar heterojunction structure as a model system, since BHJ structures are too complicated for systematic investigation.4 Recent studies involving experiments and model simulations have proposed that a gradient in the energy levels of the materials, a so-called “cascade” near the D/A interface, can simultaneously promote charge separation and suppress charge recombination. For example, Tan et al. reported a trilayer device with a cascade intermediate layer in which the gradient energy flow suppressed charge recombination.5 Izawa et al. reported that charge generation was promoted by cascade level alignment at the interfacial monolayers6, and that insertion of a monolayer with intermediate lowest unoccupied molecular orbital (LUMO) 2 ACS Paragon Plus Environment

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energy at the D/A interface results in the charge generation process becoming almost activation free. Recently, Nakano et al. elucidated the effects of the energy levels and charge cascade structures on OPV performance, and the functions of the intermixed layer in trilayer devices.7 Their results suggest that energy alignment at the D/A interface could simultaneously enhance both the photocurrents and the voltages of the OPV. Despite its importance for further improving OPV performance, control of the D/A interface in BHJ devices remains highly challenging. A third component with an intermediate energy could be added to the mixed BHJs to form a ternary blend but precise placement of this third component at the D/A interface would be difficult due to the lack of strong driving forces8-9. Honda et al. reported impressive success by utilizing a difference in surface energy to position a phthalocyanine-based sensitizer at the D/A interface10. The same group later changed the molecular structure of the sensitizer to make D and A amphiphilic, thereby significantly enhancing the loading of dyes possibly accumulated at the interfaces, and thus enhancing performance11. An alternative to ternary blends is the use of microphase separation in semiconducting block copolymers, as this dependable methodology allows more accurate positioning and orientation of the third component.12-14 We previously demonstrated the synthesis and phase separation behavior of semiconducting block copolymers comprising a poly(3-hexylthiophene) (P3HT) block and a functionalized poly(3-alkylthiophene) (P3AT) block covalently connected to a fullerene derivative through side chains15-17. Atomic force microscopy (AFM) and X-ray diffraction (XRD) analyses indicated microphase separation in the films into pure P3HT domains with high crystallinity, and fullerene-containing, disordered domains. This controlled structure could provide a foundation for modifying the energy structures at D/A interfaces in BHJs because the positions of the components 3 ACS Paragon Plus Environment


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can be more precisely controlled using molecular design. For example, we constructed an energetic cascade by using a fullerene-attached polymer block with a lower band gap.

Figure 1. (a) Molecular structure of fullerene-attached thiophene-thiophene (T-b-T-PCBP) and thiophene-selenophene block copolymers (T-b-Se-PCBP) and (b) the corresponding energy level diagram. The HOMO and LUMO energy levels determined for the homopolymers by photoelectron spectroscopy and the optical band gaps, respectively, are taken from a previous report.18 In this work we synthesized fullerene-attached diblock copolymers based on P3HT and functionalized poly(3-alkylselenophene) (P3ASe) (Figure 1). P3ASe was selected to construct the exciton cascade energy structure because of its narrower bandgap compared with P3HT (Figure 1b)1819

and its similarity in terms of the structure and crystallinity in films20. Poly(3-hexylselenophene)

(P3HSe) has been used as a donor in OPVs in combination with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).21 In addition, the phase separation of rod-rod type block copolymers is significantly affected by the length and crystallinity of the blocks;22 consequently, the D/A ratio in OPVs requires optimization and thus it is critically important to finely control the block length and ratio between P3HT and P3ASe. Ni-catalyzed catalyst transfer polycondensation (CTP) exhibiting a quasi-living 4 ACS Paragon Plus Environment

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nature can satisfy these requirements23-24 and can afford thiophene-selenophene block copolymers with tunable thin film properties25-28. However, the synthesis of functionalized P3ASe and the formation of functionalized P3ASe nanostructures in thin films have not been reported. Microphase separation of the block copolymers can be induced by thermal annealing in thin film to provide a thermodynamically stable phase. The presence of an energy cascade and its impact on photophysical properties was examined in OPV devices by comparing these thiophene-selenophene block copolymer-based devices with those of the corresponding thiophene-thiophene block copolymer.

Synthesis Synthesis of selenophene monomers 3-Alkylselenophene monomer with a functionalized side chain was synthesized for the first time in this study following the route provided in Scheme 1. 3-Bromoselenophene (2) was synthesized by applying a tribromination-reduction method reported for thiophene29. Subsequently, the 3-position of the selenophene was coupled with 1-bromo-6-hexanol protected with a tetrahydropyranyl (THP) group by using a Ni catalyst to obtain the functionalized 3-alkylselenophene 3. THP was cleaved with ptoluenesulfonic acid to give the alcohol 4. In contrast to the corresponding thiophene monomer30, use of p-methoxylphenol as the protecting group resulted in the formation of polymerized products of selenophene in the cleavage step since the reactivity of selenophene is greater than that of thiophene. Subsequent SN2 reaction using CBr4/PPh3 converted the hydroxyl end group to the bromide (5). Similar to 3-alkylthiophene, 3-alkylselenophene showed high selectivity at the 2-position in the next bromination step with N-bromosuccinimide (NBS), to give the monobrominated selenophene (6). Subsequent iodination of 6 at the 5-position gave the target monomer (7) with adequate purity (see 5 ACS Paragon Plus Environment


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Figure S1 for 1H NMR and 13C NMR spectra).

Se

Br2, Et2O

Se

Br

Br

AcOH 1

Se

THF

AcOH, H2O

Br

BrMg(CH2)6OTHP Ni(dppe)Cl2

Se

Zn

2

Br

3

OTHP TsOH

Se

EtOH

CBr4, PPh3

NBS

Se

5

OH

Br

DMF

CH2Cl2 4

Se

I2, PhI(OAc)2

Se

Br

CHCl3 6

Br

I

7

Br

Br

Scheme 1. Synthetic route to the functionalized 3-alkylselenophene monomer.

Synthesis of fullerene-attached diblock copolymers The thiophene-thiophene (T-b-T-Br) and thiophene-selenophene (T-b-Se-Br) diblock copolymers were synthesized by a Ni-catalyzed CTP as shown in Scheme 2. The quasi-living nature of the CTP method31-33 allows excellent control of both the degree of polymerization and the block ratio in the synthesis of P3AT-based diblock copolymers,34-36 and has been extended to the polymerization of 3alkylselenophene18, 25. Instead of using Ni(dppp)Cl2 as the initiator, as in our previous reports17, 30, here we used an o-tolyl substituted Ni catalyst because of its high initiation efficiency, and because propagation from both ends of the polymer chain can be avoided37-38. We also found that the order of the polymerizations is important for synthesizing block copolymers with large differences in the degree of polymerization between the blocks (approximately 1 to 7 in this study). In our previous work on P3AT-based diblock copolymers, polymerization was conducted from the major block, and the lower

6 ACS Paragon Plus Environment

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Ph2P ClMg

X

Ni

PPh2 Br

ClMg S Br

X

Br

X

NiL2Br

m

m

THF

Br

S

Br

H n

NaN3 n-Bu4NBr Toluene

Br

Toluene

X

S

S m

H n

N3

X = S : T-b-T-Br X = Se : T-b-Se-Br CuBr, PMDETA PCBP

X

X = S : T-b-T-N3 X = Se : T-b-Se-N3

H

m

n

N Y= N N

O

O O

O

PCBP Y X = S : T-b-T-PCBP X = Se : T-b-Se-PCBP

Scheme 2. Synthetic route to the fullerene-attached diblock copolymers.

initial concentration of the initiator lowered the monomer conversion rate, making control of the polymerization difficult17. In this work, the minor blocks of the functionalized P3AT or P3ASe were synthesized at the first step, followed by the elongation of the major block of P3HT, thus minimizing the effect of initiator dilution. Another advantage of the present sequence is easier purification of the resulting diblock copolymers; small amounts of functionalized polymers that are deactivated at the first stage of the polymerization can be removed by Soxhlet extraction using CH2Cl2 as solvent due to their relatively low molecular weight. The conversion rates of the monomers at the first and second steps are approximately 90%, as calculated from the 1H NMR data for both the selenophene and the thiophene monomers, and these conversion rates were unaffected by block length (see Figures S5-6 and Table S1).



Figure 2 shows the GPC profiles of the polymers at the end of each of the two polymerization steps. A clear peak shift to the higher molecular weight region was observed for both T-b-T-Br and T-b-SeBr after the second monomer was added to the system. The unimodal peak shape was maintained before and after this second addition, indicating the quasi-living nature of the polymerization. Although there is slight tailing in the traces after the first step, no peaks are observed in the region of the molecular weight of the first block at around 36 min, indicating that most of the first blocks were living during the reaction and subsequently transformed to the block copolymers upon the addition of the second monomer. The molecular weights, polydispersity indexes (PDI) in both steps, and the block ratios calculated from the 1H NMR spectra were determined after Soxhlet extraction and are summarized in Table 1. Both block copolymers showed high number-averaged molecular weights (Mn) over 30 kDa, with PDIs no larger than 1.2. 1H NMR (Figure S5-6) showed that the resulting block copolymers have a slightly lower ratio of the first block compared to the feed ratio of the monomers 1.0

(a) Normalized intensity (a.u.)

1.0 Normalized intensity (a.u.)

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0.5

0.0 20

(b)

0.5

0.0 25

30

35

40

45

20

25

Retention time (min)

30

35

40

45


Figure 2. GPC profiles of (a) T-b-T-Br and (b) T-b-Se-Br before the addition of the second monomer (dashed lines) and after the consumption of the second monomer (solid lines). The GPC samples were prepared without Soxhlet extraction.


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(m:n = 1:5.5). Matrix assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDITOF-MS) was also performed to confirm the average degree of polymerization since overestimation of the molecular weight by GPC with polystyrene standards is known for the conjugated polymers due to their rigidity of the backbone.39-40 MALDI-TOF-MS profiles of the first block (Figure S7) show the highest peak of 3767 (m = 15) and 4177 (m = 14) for T-b-T-PCBP and T-b-Se-PCBP, respectively. The average molecular weight of the P3HT second blocks can then be estimated by the block ratios calculated from 1H NMR (Table 1), which is 15400 (n = 93) and 15600 (n = 94) for T-b-T-PCBP and T-b-Se-PCBP, respectively. Therefore the Mn of the diblock copolymers determined by GPC may be overestimated by the factor of about 1.6-1.8. Table 1. Summary of the molecular weights and block ratios for the diblock copolymers. Mn (PDI) a

T-b-T-Br T-b-Se-Br a

First block

Total

4200 (1.21) 5000 (1.51)

36000 (1.14) 32000 (1.20)

Block ratio (m:n) b 1:6.2

T-b-T-N3

1:6.7

T-b-Se-N3

Mn (PDI) a

Block ratio (m:n) b

36000 (1.21) 32000 (1.19)

1:6.7 1:6.7

Expected fullerene content (wt%)c 42 41

Measured by GPC; PDI = Mw/Mn. b Calculated from the -CH2- group adjacent to the aromatic

ring in the 1H NMR spectra. c Assuming quantitative attachment of PCBP with the -N3 group.

Subsequently, a two-phase SN2 reaction with NaN3 was carried out to convert the bromide at the side chain to the azide using a previously reported methodology.15 Complete conversion from -Br to N3 was confirmed by 1H NMR (Figure S8), and the block ratios were calculated as 1:6.7 for both block copolymers. The properties of the block copolymers T-b-T-N3 and T-b-Se-N3 are summarized in Table 1. 9 ACS Paragon Plus Environment


In the final step, the fullerene group was attached onto the block copolymers by a Huisgen-type click reaction catalyzed by CuBr. Use of an excess amount of the alkyne-containing fullerene (PCBP in Scheme 2) and a strictly oxygen-free environment enabled quantitative conversion of the azide groups of the side chains to the triazole to afford T-b-T-PCBP and T-b-Se-PCBP. The conversion of the azides was monitored using the FT-IR spectra of the polymer films prepared by drop-casting of the reaction mixtures, and the -N3 vibration peak at 2100 cm−1 disappeared completely after the reaction (Figure S9). Remaining PCBP was removed by preparative GPC and the purity of the block copolymers was confirmed by analytical GPC (Figure 3). Absorption by the fullerene can be monitored at 250 nm; no peak is observed at 44 min in the block copolymers, indicating complete removal of unreacted PCBP. The peaks of the polymers shifted to longer elution time (i.e., lower molecular weight) after the

Normalized intensity (a.u.)

(a) Normalized intensity (a.u.)

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1.0

0.5

0.0 20

(b) 1.0

0.5

0.0 25

30

35

40

45

50

20

25


30

35

40

45

50


Figure 3. GPC profiles of (a) T-b-T-N3 (dashed line) and T-b-T-PCBP (solid line), and (b) T-b-Se-N3 (dashed line) and T-b-Se-PCBP (solid line). The detection wavelengths are 250 nm for the fullereneattached block copolymers and 500 nm for the azide-functionalized block copolymers. The trace for PCBP (dotted lines, detection wavelength: 250 nm) is also presented to show the complete removal of PCBP. 10 ACS Paragon Plus Environment

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reaction with PCBP, despite the fact that fullerene attachment significantly increased the molecular weight. This anomalous change in GPC profile was previously observed for several fullerene-attached polymers15, 17, 41-42 and was attributed to the change in the exclusion volume of the polymers due to intrachain aggregation of the fullerene groups. GPC traces of the block copolymers detected at 500 nm and at 250 nm are very similar (Figure S10), suggesting that the PCBP was introduced in the block copolymer uniformly (i.e. little dependence of the introduction rate on the molecular weight) because the polymer and the fullerene are main absorber at 500 nm and 250 nm, respectively. The molecular weights of the copolymers are summarized in Table 2.

Table 2. Summary of the molecular weights and fullerene content of the fullerene-attached block copolymers.

a

Mn (PDI) a

Block ratio (m:n) b

Fullerene content c (wt%)

T-b-T-PCBP

24000 (1.74)

1:6.7

41

T-b-Se-PCBP

22000 (1.44)

1:6.7

40

Measured by GPC; PDI = Mw/Mn. b Calculated from the -CH2- group adjacent to the aromatic

ring in the 1H NMR spectra. c Calculated from the peak intensity in the UV-vis absorption spectra. See Supporting Information for the details.



Characterizations of Fullerene-attached Block Copolymers Photophysical properties of T-b-T-PCBP and T-b-Se-PCBP in solution Figure 4a shows the UV-vis absorption spectra of the fullerene-attached block copolymers together with the P3HT:PCBM physical mixture in a 10 μg/mL CHCl3 solution. They are basically the superposition of the absorption from P3HT and PCBM. The fullerene contents in the copolymer are evaluated by UV-vis absorption spectra in solution and presented in Table 2 (see Supporting Information for the details). These values are close to the expected ones assuming the quantitative attachment of PCBP with the -N3 group (Table 1) and also to the typical optimized ratios of mixed P3HT:PCBM bulk heterojunction devices (38 wt% ~ 44 wt%).43 Figure 4b shows the fluorescence spectra of T-b-T-PCBP and T-b-Se-PCBP at 0.5 μg/mL in CHCl3. A low concentration was used to

2.5 1.0

(a)

(b)

2.0 0.8 Intensity (a.u.)

Normalized absorbance (a.u.)

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1.5

1.0

0.5

0.0

0.6

0.4

0.2

300

400

500

600

0.0 500

Wavelength (nm)

600

700

800

Wavelength (nm)

Figure 4. (a) UV-vis absorption spectra normalized by the absorbance at 450 nm and (b) fluorescence spectra of T-b-T-PCBP (blue lines), T-b-Se-PCBP (red lines) and a P3HT:PCBM physical mixture (1.5:1 by weight, black lines) in CHCl3 solution. The dashed line in (b) is for a pure P3HT solution. The fluorescence spectra were obtained by excitation at 450 nm and were normalized by the absorbance of the solutions at 450 nm. 12 ACS Paragon Plus Environment

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decrease intermolecular aggregation. The spectra of the P3HT homopolymer and the P3HT:PCBM physical mixture are also shown for comparison. Blending P3HT with PCBM did not affect the fluorescence intensity of P3HT, due to the large intermolecular distance between P3HT and PCBM in solution, resulting in negligible electron transfer. On the other hand, significant fluorescence quenching was observed in both the T-b-T-PCBP and T-b-Se-PCBP polymers (58% and 56%, respectively, calculated from the peak intensity), probably caused by electron transfer from the covalently connected fullerene to the main chain. T-b-Se-Br showed 51% fluorescence quenching at the same concentration (Figure S13) and this quenching is likely attributed to energy transfer from the polythiophene block to the polyselenophene block, resulting in formation of the non-fluorescent triplet state of polyselenophene44. Fluorescence quenching in the absence of fullerene strongly supports an exciton energy cascade from polythiophene and polyselenophene, achieved through their covalent connection.

Thermal properties The presence of crystalline domains in the solid state polymers was confirmed by differential scanning calorimetry (DSC) (Figure S14). Both the T-b-T-PCBP and T-b-Se-PCBP copolymers showed a melting peak (Tm) at 233 °C and a crystallization peak (Tc) at 180 °C, and both the melting and crystallization peaks can be attributed to the thermal behavior of the predominant P3HT blocks. These temperatures are lower and the peaks are significantly broader than those of pure P3HT with a molecular weight comparable to the block copolymers (Figure S14a, Tm and Tc are 241 °C and 207 °C, respectively) and are likely due to the covalently attached fullerene disturbing the packing of the crystalline P3HT block. The Tm of P3HSe homopolymer with a comparable molecular weight was 145 °C, whereas no peak was observed for the melting of P3ASe block in the T-b-Se-PCBP copolymer 13 ACS Paragon Plus Environment


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(Figure S14b), suggesting that P3ASe blocks connected to a fullerene have low crystallinity due to aggregation of the fullerene, or due to their low molecular weight.

Thin Film Properties of the Fullerene-attached Block Copolymers Two-dimensional grazing-incidence wide-angle X-ray scattering (2D GIWAXS) analysis 2D GIWAXS was performed to investigate the crystalline structure of the block copolymer films. Figure 5 shows the diffraction patterns of the copolymer films on silicon substrates after thermal annealing at two temperatures. All the films showed basically similar patterns that are typically observed for P3HT:PCBM mixture films (see Figure S15 for this pattern). The diffraction peaks at low

Figure 5. 2D GIWAXS patterns of (a-c) T-b-Se-PCBP and (d-f) T-b-T-PCBP films: (a and d) ascast, (b and e) after annealing at 180 °C, and (c and f) after annealing at 250 °C. 14 ACS Paragon Plus Environment

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2θ angles (~3.5°) are due to the lamellar structure of P3HT with higher intensity in the out-of-plane direction as compared to the in-plane direction, and those at higher angles (~15.0°) are from π-π stacking. The broad ring diffraction centered at 12.7° (4.52 Å) can be assigned to the halo caused by PCBM aggregation.45 Figure S16 shows the line profiles for T-b-Se-PCBP films along the in-plane and out-of-plane directions, and are reproduced from the original images. The d-spacings calculated from the diffraction peaks are summarized in Table 3. The diffraction intensity from both the lamellar structure and π-π stacking was rather low in the as-cast film, suggesting that a large fraction of the polymer chains remained amorphous. Thermal annealing significantly increased the diffraction intensity and sharpened the peaks, without significantly affecting their position. The highest diffraction intensity was achieved after annealing at 250 °C, suggesting that the higher chain mobility in the molten phase promoted crystallization of the P3HT block. π-π stacking peaks also intensified in the in-plane direction after thermal annealing at 250 °C (Figures 5b and c), with a slight shift to a wider angle (Table 3). These change indicate that the π-π stacking distance decreases and the polymer adopts a more edge-on orientation, consistent with the behavior of other semicrystalline conjugated polymers gradually cooled from the molten phase.46 T-b-T-PCBP films showed a similar tendency towards crystallinity upon annealing at the same temperatures (Figures 5d-f). These results indicated that

Table 3. Summary of the diffraction peak positions in the 2D GIWAXS patterns. T-b-T-PCBP

T-b-Se-PCBP

As-cast

180 °C

250°C As-cast 180 °C

Lamellar (100) (Å)

16.0

16.2

16.2

16.2

π-stacking (Å)

3.85

3.82

3.79

3.84

P3HT:PCBM

250°C

As-cast

16.5

16.5

15.7

3.84

3.82

3.85



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thermal annealing at 250 °C induces the development of crystalline structures of P3HT with an edgeon orientation. An increase in the fraction of P3HT lamellar structure in the film was an important indicator of a higher degree of phase separation, as we discuss below.

Surface morphology analysis by atomic force microscopy (AFM) Film morphology and the effect of thermal annealing on microphase separation were investigated by tapping mode AFM. Figure 6 shows height images of T-b-T-PCBP and T-b-Se-PCBP films before and after annealing at 180 °C and 250 °C; the corresponding phase images are shown in Figure S17. Changing the minor block from P3AT to P3ASe induced no significant difference in the nanopatterns.

Figure 6. Tapping mode AFM height images of (a-c) T-b-T-PCBP and (d-f) T-b-Se-PCBP films: (a and d) as-cast, (b and e) annealed at 180 °C, and (c and f) annealed at 250 °C. Scale bars: 200 nm. 16 ACS Paragon Plus Environment

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As-cast films showed clear spherical aggregation structures 30-40 nm in diameter in the topography images (Figures 6a and d). Annealing at 180 °C somewhat obscured the spherical structures (Figures 6b and e) and this was accompanied by a decrease in the root-mean-square of the surface roughness (RRMS), from 2.5 nm to 2.0 nm. A large change in the morphology was observed after annealing the films at 250 °C: a more connected fibrous morphology was formed, with fiber widths of around 30 nm, and the film was smoother (RRMS of 1.3 nm) (Figures 6c and f). Such drastic changes in morphology after heat treatment above the melting temperature is indicative of large scale microphase separation of the block copolymer, driven by the rearrangement and repacking of the crystalline P3HT block. This is in contrast to the P3HT:PCBM mixture, in which treatment above the melting point induces large scale immediate crystallization of PCBM.30 The formation of spherical structures in the as-cast films might be due to micelle formation in the solution. The high concentration and strong aggregation tendency of the fullerene-containing block could lead to the formation of micelle cores via intermolecular aggregation in the stock solution prior to spin-coating. This would disrupt the packing of P3HT block in the shell domains, in agreement with the 2D GIWAXS patterns, showing that the as-cast films have the lowest crystallinity. Unlike previously reported observations17, annealing at 180 °C did not induce a lamellar pattern, even in the case of T-b-T-PCBP. This discrepancy may be attributed to both the higher molecular weight of the block copolymers and their higher fullerene content in the present study, which limited the motion of the main chain. Nevertheless, at 180 °C, the P3HT major block was less confined and thus stacking of the P3HT chains between the micelles became possible, leading to the lower contrast observed between aggregates in Figures 6b and e. In contrast, cooling from the molten state at 250 °C allowed the block copolymer to assemble into nanofibers with an average thickness of 30 nm. P3HT is known to pack 17 ACS Paragon Plus Environment


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into nanowires, in which the main-chains orientate perpendicular to the nanowire normal.47 In Figures 6c and f, the observed fiber width of about 30 nm coincides with the extended head-to-end length of a P3HT chain with a degree of polymerization of about 90 (molecular weight around 15 kDa by MALDITOF-MS), which is approximately the length of the block copolymers used in this study. Furthermore, the change in the diffraction intensity observed by X-ray diffraction analysis (Figure S16) indicates that the fibrous crystalline domains of pure P3HT develop along with microphase separation by annealing the block copolymer films over their Tm.

Transmission electron microscopy (TEM) analysis TEM analysis was performed to investigate the microphase-separated structures in the bulk of the thin films. Figure 7 shows bright field TEM images of T-b-T-PCBP and T-b-Se-PCBP films after annealing at 180 °C or 250 °C. The bright regions can be assigned to P3HT-rich domains, and the dark regions to fullerene-containing domains owing to the higher electron scattering density of the fullerene group48. Consistent with the AFM observations, T-b-T-PCBP and T-b-Se-PCBP showed very similar morphologies and changes after thermal annealing. In the as-cast films, periodic spherical dark domains around 20 nm in diameter were observed and were separated from each other by brighter domains (Figures 7a and d), and the shapes and the sizes of the structures were similar to the surface structures observed by AFM. This phase pattern indicated aggregation of the fullerene-containing blocks and formation of micelle-like structures in the bulk of the as-cast film. Annealing at 180 °C improved both the connectivity of the fullerene domains and the uniformity of the film, and the contrast between the bright and dark domains was greatly enhanced (Figures 7b and e), suggesting demixing of the crystalline P3HT and fullerene-containing blocks. Annealing at 250 °C afforded the most 18 ACS Paragon Plus Environment

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Figure 7. TEM images of (a-c) T-b-T-PCBP and (d-f) T-b-Se-PCBP films: (a and d) as-cast, (b and e) annealed at 180 °C, and (c and f) annealed at 250 °C. Scale bars: 100 nm.

defined phase separation, with higher connectivity of the bright and dark domains (Figures 7c and f): the boundaries between the bright and dark regions become clearer and the contrast further improved, indicating a high degree of demixing between the crystalline P3HT and amorphous fullerenecontaining blocks. Spatial information was extracted by fast Fourier transformation (FFT) of the TEM images (Figure S18). As-cast films showed a broad peak centered at 6 nm and another peak at 26 nm. The former peak could be attributed to the intermixing parts, appearing in the TEM image as gray areas between the dark spots. This peak was very weak in the physical mixture of P3HT and PCBM48-49, suggesting that covalent connection of the donor and acceptor produced a more intermixed region. The peak at 26 nm could be attributed to the average size of the fullerene aggregates. Annealing at 180 °C significantly 19 ACS Paragon Plus Environment


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decreased the intensity of the peak due to the intermixing parts whereas the position of the 26 nm peak remained essentially unchanged after annealing, indicating that the fullerene aggregates did not rearrange at this temperature. After annealing at 250 °C, the peak at 6 nm shifted to 10 nm and the peak at 26 nm broadened and shifted to around 30 nm. These changes could be attributed to the crystallization and assembly of the P3HT blocks during cooling from the molten state, as well as rearrangement of the fullerene-rich domains. The distance of ~30 nm likely no longer reflects the size of the spherical aggregations, but rather the repeat distance in the fibrous periodic structure of the bright and dark regions. Similar information on the structural changes caused by annealing was obtained by grazing-incidence small-angle X-ray scattering (GISAXS) measurements on the annealed films (Figure S19); the results showed scattering peak maxima in the in-plane direction at 26.3 nm and 28.9 nm for T-b-T-PCBP as a cast film and after annealing at 250 °C, respectively, and at 27.1 nm and 27.5 nm for T-b-Se-PCBP as a cast film and after annealing at 250 °C, respectively.

Photophysical properties of the films Figure 8 shows the UV-vis absorption spectra of the films on a quartz substrate after annealing at 150 °C for the physical mixture and at 250 °C for the block copolymers (see SI for details). For all samples, the polymer absorption maximum red-shifted from 450 nm for the solutions to 515 nm for the films. An absorption shoulder at around 610 nm is evident in the spectra of both P3HT:PCBM and the copolymer films, and has been assigned to absorption by the crystalline domain of P3HT.50 Taking the spectroscopic data together, the highly crystalline P3HT domain grows in the block copolymer films due to phase separation of the P3HT major block and the fullerene-attached P3AT or P3ASe block. Fluorescence spectra normalized by the absorbance at the excitation wavelength (450 nm) are 20 ACS Paragon Plus Environment

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shown in Figure 8b. Significant fluorescence quenching was evident in both copolymer films compared with the pure P3HT film after annealing, with quenching efficiencies of 87% for T-b-TPCBP and 89% for T-b-Se-PCBP. These quenching efficiencies are slightly lower than that of the P3HT:PCBM physical mixture film (94%). The less efficient quenching in the block copolymers could be due to the larger phase separation (around 30 nm by TEM) compared with that of the P3HT:PCBM physical mixture films (typically around 10 nm51) and higher phase purity in the P3HT domains, resulting in a lower probability of singlet exciton migration to the D/A interface.

1.5

(a)

1.0 Normalized intensity (a.u.)

Normalized absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0

0.5

0.0

300

400

500

600

700

800

(b)

0.5

0.0

600

Wavelength (nm)

700

800

Wavelength (nm)

Figure 8. (a) UV-vis absorption spectra normalized by the absorbance at 515 nm and (b) fluorescence spectra of T-b-T-PCBP (blue lines), T-b-Se-PCBP (red lines) and P3HT:PCBM physical mixture (1.5:1 by weight, black lines) as films. The dashed line in (b) is a pure P3HT film. P3HT:PCBM and the block copolymer films were annealed at 150 °C and 250 °C, respectively. The fluorescence spectra were taken using an excitation wavelength of 450 nm and were normalized by the absorbance of the films at 450 nm.



Photovoltaic performance Organic solar cells were fabricated using T-b-T-PCBP and T-b-Se-PCBP films as the active layers and the device had the structure ITO/ZnO/active layer/MoO3/Ag. Figure 9a shows the current densityvoltage (J-V) curves obtained during irradiation with AM1.5 simulated solar light (100 mW/cm2). Table 4 summarizes the device performance obtained using T-b-T-PCBP and T-b-Se-PCBP annealed at 180 or 250 °C. Note that devices fabricated from the as-cast films showed negligible photovoltaic performance for both polymers, possibly due to poor connection of the fullerene domains with the copolymer domains, as suggested by the TEM and AFM images. For both block copolymers, annealing at 250 °C induced a slight decrease in JSC and an increase in VOC and fill factor compared to those annealed at 180 °C. T-b-Se-PCBP performed slightly better than T-b-T-PCBP due to the higher JSC after annealing at 250 °C. 0.5

(a)

(b) External Quantum Efficiency

0

Current density (mA/cm2)

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-2

-4

0.4

0.3

0.2

0.1

-6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.0 300

400

Voltage (V)

500

600

700

800

Wavelength (nm)

Figure 9. (a) Current density-voltage (J-V) curves during irradiation with AM1.5 simulated solar light (100 mW/cm2) and (b) external quantum efficiency plots for the photovoltaic devices based on T-b-T-PCBP films annealed at 180 °C (blue lines) and at 250 °C (red lines), and T-b-Se-PCBP films annealed at 180 °C (green lines) and at 250 °C (magenta lines). 22 ACS Paragon Plus Environment

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Table 4. Summary of the performances of organic photovoltaic devices based on the block copolymers. JSC (mA/cm2) T-b-T-PCBP

T-b-Se-PCBP

VOC (V)

FF

PCE (%)

180°C

5.22 (0.08)

0.44 (0.01)

0.52 (0.00)

1.18 (0.03)

250°C

4.81 (0.05)

0.51 (0.01)

0.57 (0.01)

1.38 (0.06)

180°C

5.25 (0.01)

0.46 (0.00)

0.51 (0.01)

1.23 (0.03)

250°C

5.02 (0.06)

0.51 (0.00)

0.59 (0.01)

1.52 (0.03)

*Standard deviations are in the parentheses. The decrease in JSC after annealing at 250 °C could be related to the demixing of P3HT and the fullerene-attached domains, which decreases the effective molecular interfacial area between the fullerene and the donor, as suggested by the TEM images. On the other hand, the increase in VOC after annealing at 250 °C could be due to either a higher energy of charge transfer state at the interface or a slower change in recombination kinetics, or a combination of these two factors. The increase in VOC could also originate from the demixing of the domains, but more detailed analysis of the charge recombination dynamics is required to elucidate the origin of these changes. External quantum efficiency (EQE) plots show that both block copolymers exhibit a blue-shift in their spectra and a decreased contribution from the absorption shoulder at 610 nm after annealing at 250 °C (Figure 9b). This spectral change is opposite to the changes in the absorption spectra shown in Figure 8a, where the absorptions were red-shifted and the absorption shoulder at 610 nm became more pronounced after annealing. The observed changes in the EQE plots could be attributed to the decrease in charge generation from the excitons generated in the crystalline domains of P3HT. The well-defined microphase-separated structures would likely reduce the number of excitons reaching the P3HT/fullerene interface, given that the size of the P3HT domains estimated from AFM and TEM (~30 nm) is larger than the exciton diffusion length reported for crystalline P3HT (~9 nm).52 23 ACS Paragon Plus Environment


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Under the current conditions used to evaluate OPV performance, there was little difference in the performance of OPVs based on T-b-T-PCBP or T-b-Se-PCBP. There may be a slight enhancement of JSC in OPVs containing annealed T-b-Se-PCBP, due to enhanced exciton collection expected from the exciton energy cascade structure at the interface of the nanostructure. However, the other factors besides the energy cascade structure cannot be excluded at this stage such as the subtle difference in the morphology that cannot be detected by the measurements or the electronic transport properties between P3HT and P3HSe. More detailed analysis, such as transient absorption spectroscopy, is necessary to elucidate the effect of the energy structure on the initial stage of charge generation.

Conclusion We have synthesized new fullerene-attached thiophene-selenophene diblock copolymers. The analogous chemical structures of P3AT and P3ASe resulted in thin films of the T-b-T-PCBP and T-bSe-PCBP block copolymers showing identical film structures, including crystallinity, molecular orientation and microphase separation, after thermal annealing. Photovoltaic devices based on these copolymers showed that demixing of the blocks accompanied by microphase separation can affect exciton collection and charge recombination efficiency. This study provides a foundation to investigate the relationship between interfacial energy structures and device performance in controlled BHJ structures.

Supporting information. Experimental, synthetic scheme, 1H and 13C NMR, MALDI-TOF-MS, FTIR, UV-vis, fluorescence, DSC, GIWAXS, AFM phase images, FFT of TEM images and GISAXS supplied as Supporting Information.


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Acknowledgements This study is supported in part by the New Energy and Industrial Technology Development Organization (NEDO), Japan. GIWAXS and GISAXS experiments were performed at beamline BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal 2015A1952, 2015A1696 and 2015B1904). Mass spectrometry was carried out by Dr. Takemichi Nakamura at Molecular Structure Characterization Unit, CSRS, RIKEN. We thank Dr. Jianming Huang for the materials characterizations.

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