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Sequentially different AB diblock and ABA triblock copolymers as P3HT:PCBM interfacial compatibilizers for BHJ photovoltaics Hiroyuki Fujita, Tsuyoshi Michinobu, Seijiro Fukuta, Tomoyuki Koganezawa, and Tomoya Higashihara ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12437 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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ACS Applied Materials & Interfaces

Sequentially different AB diblock and ABA triblock copolymers as P3HT:PCBM interfacial compatibilizers for BHJ photovoltaics Hiroyuki Fujita,a Tsuyoshi Michinobu,a,* Seijiro Fukuta,b Tomoyuki Koganezawa,c and Tomoya Higashiharab,* a

Department of Organic and Polymeric Materials, Graduate School of Science and Engineering,

Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan b

Department of Polymer Science and Engineering, Graduate School of Science and Engineering,

Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan c

Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-

5198, Japan

KEYWORDS: Block copolymers, P3HT, organic photovoltaics, compatibilizer, GISAXS, GIWAXS

ACS Paragon Plus Environment

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ABSTRACT: The P3HT:PCBM bulk-heterojunction (BHJ) organic photovoltaic (OPV) cells using the AB diblock and ABA triblock copolymers (A = polystyrene derivative with donoracceptor units (PTCNE) and B = poly(3-hexylthiophene) (P3HT)) as compatibilizers were fabricated. Under the optimized blend ratio of the block copolymer, the power conversion efficiency (PCE) was enhanced. This PCE enhancement was clearly related to the increased short-circuit current (Jsc) and fill factor (FF). The incident photon to current efficiency (IPCE) measurement suggested that the P3HT crystallinity was improved upon addition of the block copolymers. The increased P3HT crystallinity was consistent with the increased photovoltaic parameters, such as Jsc, FF, and consequently the PCE. The surface energies of these block copolymers suggested their thermodynamically-stable location at the interface of P3HT:PCBM, showing the efficient compatibilizing performance, resulting in enlarging and fixing the interfacial area and suppressing the recombination of the generated carriers. Grazing incidence X-ray scattering (GIXS) results confirmed the superior compatibilizing performance of the ABA triblock copolymer when compared to the AB diblock copolymer by the fact that, after blending the ABA triblock copolymer in the P3HT:PCBM system, the enhanced crystallinity of matrix P3HT was observed in the excluded areas of the less aggregated PCBM domains, changing the P3HT crystalline domain orientation from “edge-on” to “isotropic”. This is, to the best of our knowledge, the first sequential effect (AB vs. ABA) of the block copolymers on the compatibilizing performances based on BHJ OPV device systems.


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INTRODUCTION OPVs have recently received much attention due to their lightweight, flexible, and low-cost large area productivity based on a roll-to-roll printing process.1-7 Toward the commercialization of OPVs, not only their PCEs, but also the device durability must be improved. It is well-known that the blend morphology of the active layer in a BHJ OPV device plays an important role in their performances; however, it has not yet been fully understood how to improve and maintain the ideal blend morphology in BHJ OPV devices, even for a basic P3HT:PCBM system. The establishment of controlling the nanostructural morphology in the P3HT:PCBM BHJ system is highly required because it would serve as the platform for extension to high-performance p-type polymer:PCBM BHJ OPV cells as well as fullerene-free all-polymer solar cells having more complex morphological issues. Generally, the macrophase separation of the P3HT and PCBM domains in a BHJ OPV device after aging causes a reduced interface area, resulting in the deactivation of generated excitons, and thereby suppressing the charge separation. Such an unwanted macrophase separation originated from the mismatched free surface energies of P3HT and PCBM as intrinsic parameters.8 Therefore, neither thermal/solvent annealing nor treatment with volatile additives, such as 1,8-diiodooctane (DIO) and chloronaphthalene (CN), could perfectly cancel the macrophase separation of a P3HT:PCBM blend. The ideal morphology has to be pursued by increasing the donor/acceptor interface on a 10-20 nm scale, which is comparable to the exciton diffusion length, and be maintained even after a long operating time. In order to meet this requirement, the addition of a nonvolatile compatibilizer to P3HT:PCBM would be effective, if it could accumulate at the


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P3HT:PCBM interface and reduce the contact energy between the P3HT and PCBM domains. In 2006, Fréchet et al. first employed a donor-acceptor AB diblock copolymer as a compatibilizer in P3HT:PCBM BHJ OPV devices not only to optimize the device morphology, but also to stabilize the device structure against destructive thermal phase segregation.9 Wudl and coworkers reported that the addition of 5 wt% P3HT-based AB diblock copolymers to the P3HT:PCBM BHJ system resulted in an improved performance (PCE = 3.5%), higher than that obtained from a pristine P3HT/PCBM device (PCE = 2.6%).10 Higashihara and coworkers then reported a significant enhancement of the PCE reaching 4.4% by employing an ABA triblock copolymer (A = poly(4vinyltriphenylamine) and B = P3HT) as a compatibilizer in the P3HT:PCBM BHJ system in 2010.11 After these primary reports, many papers focusing on the compatibilizing approach for a high efficiency P3HT:PCBM BHJ system have been reported.12-20 For instance, Xiao, Li and their coworkers reported the detailed morphological studies of the ternary blend system of the P3HT:PCBM:AB diblock copolymer (A = P3HT and B = polystyrene).12 In this report, they concluded that the AB diblock copolymer induced a favorable active layer morphology with interpenetrating nanoscale domains, and the enhanced P3HT crystallinity and orientation facilitate hole transport within the active layer. Brochon, Hadziioannou and their coworkers reported the compatibilizing effect of the AB diblock copolymer (A = P3HT and B = poly(4-vinylpyridine)) in the P3HT:PCBM BHJ system, showing a high PCE of 4.3% with an 8 wt% addition of the AB diblock copolymer.13 They studied the detailed blend morphology using grazing


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incidence wide-angle X-ray scattering (GIWAXS) experiments and concluded that the enhancement of the PCE was ascribed to the formation of a well-optimized nanoscale structure after adding the compatibilizer that allowed for a more efficient exciton dissociation and charge transport due to the lower charge recombination and/or trapping probability. Although the relationship between the compatibilizing effect of the AB diblock copolymers and their morphology has become clear to some extent, the comparison of the sequentially different AB diblock and ABA triblock copolymers has never been reported. We now report the utility of the AB diblock and ABA triblock copolymers (A = polystyrene derivative with donor-acceptor units (PTCNE) and B = poly(3hexylthiophene) (P3HT), see Scheme 1) as compatibilizers in the classical P3HT:PCBM BHJ OPV devices for the first time. The relationship between the OPV performance and detailed morphology evaluated by GIWAXS and grazing incidence small-angle X-ray scattering (GISAXS) results is discussed, along with comparing the AB diblock and ABA triblock copolymers.

EXPERIMENTAL SECTION Materials The block copolymers, P3HT-b-PTCNE (AB diblock copolymer, Mn = 42,300, Đ = 1.15, P3HT:PTCNE = 24:76, w:w) and PTCNE-b-P3HT-b-PTCNE (ABA triblock copolymer, Mn = 56,300, Đ = 1.07, P3HT:PTCNE = 16:84, w:w), were synthesized by the combined synthetic techniques of Kumada Catalyst-Transfer Polymerization (KCTP),


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living anionic polymerization, and click postfunctionalization, as illustrated in Scheme 1.21 Commercial P3HT (Mn = 25,000 g/mol, Đ = ~2, regioregularity = ~98.5%) and PCBM (nanom spectra E100H) were obtained from Rieke-Metal and Frontier Carbon, respectively. The anhydrous o-dichlorobenzene was purchased from Aldrich. Insert Scheme 1 here.

Instrumentation UV-vis absorption spectra were recorded on a JASCO V-670 spectrophotometer. Fluorescence spectra were measured on a JASCO FP6500 spectrophotometer. Atomic force microscopy (AFM) images were taken by a Seiko Instruments SPA-400 with a stiff cantilever of Seiko Instruments DF-20.

OPV fabrication and characterization Bulk-heterojunction type photovoltaic cells were prepared on a patterned ITO-coated glass substrate by a wet process. The patterned ITO substrates (Luminescence Technology Corp., 5 Ω sq-1) were cleaned sequentially with aqueous detergent solution, distilled water, acetone, and isopropanol for 15 min each in an ultrasonication bath, subsequently dried on a hot plate at 80 oC for 10 min. The ITO was exposed to UV-ozone (Filgen, UV253E) for 10 min. After an aqueous PEDOT:PSS (poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate)) solution (H.C.Starck, P VP.AI 4083) was passed through a 0.45 µm filter, they were spin-coated on the ITO at 3500 rpm for 70 s and dried at 140 oC for 20 min under air, then the samples were transferred to the glove box. A solution of the P3HT (8.5 mg) and PCBM (6.8 mg) in o-dichlorobenzene (0.5 mL) was


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stirred overnight. After the solution was passed through a 0.2 µm filter, it was spin-coated on the PEDOT:PSS layer at 450 rpm for 60 s, followed by annealing at 150 oC for 10 min. Subsequently, the cathode consisting of Ca (30 nm) and Al (100 nm) was deposited through a shadow mask by thermal evaporation under high vacuum to produce four solar cell circuits per substrate, each with an active area of 4 mm2. The devices were encapsulated with UV-curable sealant (Addison Clear Wave AC A1438) and glass plate. Then the devices were moved out from the glove box. The current density-voltage (J-V) curves of solar cell devices were measured using a source meter (Keithley 2400) with a solar simulator (Peccell, PEC-L11) under the illumination of AM 1.5G at 100 mM cm-2. A lamp was calibrated using a standard Si cell (Peccell, PECSI01). The incident photon to current efficiencies (IPCEs) at short-circuit conditions of solar cell devices were measured using an IPCE measuring equipment (Peccell, PEC-S2024). The light intensity at each wavelength was calibrated using a standard Si photodiode (Hamamatsu Photonics, S1337-1010BQ).

Contact angle measurements and surface energy calculation The polymer films for contact angle measurements were prepared by spin-coating 10 g L-1 solutions in chlorobenzene on silicon wafers. Static contact angles of the polymer films were measured by the sessile drop technique using deionized water and glycerol as probe liquids with a Surface & Electro-Optics Phoenix 150/300 Contact Angle Analyzer. The surface energies were calculated based on the Owens-Wendt method.22

GIXS experiments


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The polymer samples for GIXS experiments were prepared by spin-casting onto Si wafers from o-dichlorobenzene solutions followed by annealing at 150 °C for 10 min. GISAXS patterns were obtained at beamline BL46XU of SPring-8, Japan. The monochromated energy of the X-ray source was 10.314 keV (λ = 0.12022 nm) and the incidence angle αi was 0.15°. GISAXS patterns were recorded with a 2D image detector (Pilatus 2M) with the sample-to-detector distances of 2981 mm. GIWAXS measurements were conducted at the beamline BL46XU of SPring-8, Japan. The sample was irradiated at a fixed incident angle αi on the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.398 keV (X-ray wavelength λ = 0.10002 nm), and the GIWAXS patterns were recorded with a 2D image detector (Pilatus 300K) with the sample-todetector distances of 174.1 mm. The scattering vectors qy and qz for GIXS are defined in Equation (1).23 (qy, qz) = (2 π (sin ψ cos αf) / λ, 2 π (sin αf + sin αi) / λ)

(1)

where ψ is out-of-plane angle, αf is exit angle.

RESULTS AND DISCUSSION Polymer Design We have synthesized a coil-rod-coil triblock copolymer bearing electroactive triphenylamine units in coil segments, namely poly(4-vinyltriphenylamine)-b-P3HT-bpoly(4-vinyltriphenylamine)

(PTPA-b-P3HT-b-PTPA).11

In

addition

to

the

compatibilizing effect, PTPA-b-P3HT-b-PTPA was optoelectrically active and possessed p-type semiconducting features. Thus, the device composed of P3HT:PCBM blended


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with PTPA-b-P3HT-b-PTPA showed a high PCE of 4.4%, which is the highest value among the solar cell devices using block copolymers as compatibilizers to date.11 However, the light absorption range of PTPA-b-P3HT-b-PTPA is limited. Also, the shallow lowest unoccupied molecular orbital (LUMO) level (-3.11 eV) generally hinders the efficient energy transfer from the P3HT to PTPA segments. To expand the absorption range into the visible-near infrared and lower the LUMO level, intramolecular chargetransfer (CT) interactions are often used. Since aromatic amines are a strong electrondonating group, the direct substitution by electron-accepting moieties would result in a new low-energy CT band due to the elevation of the highest occupied molecular orbital (HOMO) level and reduction of the LUMO level. Very recently, we have succeeded in modifying the energy levels of P3HT-b-PTCNE (AB diblock copolymer) and PTCNEb-P3HT-b-PTCNE (ABA triblock copolymer)21 based on the formation of donoracceptor structures during the last stage of the polymer functionalization by high-yielding [2+2] cycloaddition-retroelectrocyclization reactions between electron-rich alkynes and strong acceptor molecules, such as tetracyanoethylene (TCNE) (Scheme 1).24 The developed block copolymers showed an expanded absorption, suitable energy levels for fast energy transfer between both segments, and a well-defined microphase separation.21 All these features reasonably indicated that they are an interesting class of new P3HTbased block copolymers for advanced studies of various organic devices, especially for the application as an interfacial compatibilizer for P3HT:PCBM photovoltaic cells.

Optoelectronic Properties


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The energy diagrams of the block copolymers are illustrated in Figure 1. The HOMO levels were calculated from the Eox,onset values based on the assumption of Fc/Fc+ = -4.80 eV. The LUMO levels were calculated from the HOMO levels and the optical band gaps. The LUMO levels of P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE, originating from the 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) moieties, are lower than that of P3HT (the HOMO and LUMO levels of -5.17 and -3.27 eV, respectively25), but higher than that of PCBM. This decrease in the LUMO levels suggested that the energy transfer from the P3HT to PTCNE segments in the block copolymers would be facilitated. This was confirmed by fluorescence measurements (Figure 2). The thin films of the P3HT derivative, i.e., P3HT-Br (Scheme 1) and block copolymers were prepared on a quartz plate. The P3HT-Br film displayed a clear emission band ranging from 600 to 750 nm when excited at 523 nm, which is the absorption band of the P3HT unit. While the emission bands of the precursor polymers, P3HT-b-PDHPS and PDHPS-b-P3HT-bPDHPS (Scheme 1), were still visible, the postfunctionalized polymers P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE showed no emission band because of the efficient energy transfer from the P3HT to PTCNE segments. These results suggested that a smooth energy transfer from P3HT to PCBM would occur through the interfacially located P3HT-b-PTCNE or PTCNE-b-P3HT-b-PTCNE in the P3HT:PCBM blend.

Insert Figures 1 and 2 here.

OPV Characteristics


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The photovoltaic performances of the devices composed of P3HT:PCBM blended with block copolymers were evaluated based on the current density-voltage (J-V) curves under AM 1.5G light illumination at 100 mV cm-2. The typical device structure was ITO/PEDOT:PSS (40 nm)/P3HT:PCBM (1:0.8 w/w) blended with a block copolymer (80 nm)/Ca (30 nm)/Al (100 nm). Figure 3 and Figure 4 show the J-V curves. The corresponding photovoltaic parameters, Jsc, open-circuit voltage (Voc), FF, and PCE of the devices composed of P3HT:PCBM blended with block copolymers, are summarized in Table 1. The addition of the block copolymer to the P3HT:PCBM devices strongly affected the device performances. It was found that there was a positive correlation between the PCE of the device and the block copolymer amount at least up to 0.5 wt% (for P3HT-b-PTCNE) and 1.0 wt% (for PTCNE-b-P3HT-b-PTCNE). The PCE of the pristine P3HT:PCBM system was 2.71%, whereas it was slightly increased to 2.81% when 0.5 wt% of P3HT-b-PTCNE was added. Better results were observed for PTCNEb-P3HT-b-PTCNE. The PCE of 2.89% was achieved in the presence of 1.0 wt% of PTCNE-b-P3HT-b-PTCNE. A careful observation of each photovoltaic parameter revealed that the Jsc and FF mainly accounted for the enhancement of the PCE (Figure 4). When 0.5 wt% of P3HT-bPTCNE was added to the P3HT:PCBM system, the Jsc increased from 8.06 to 8.14 mA cm-2. This value further increased as more P3HT-b-PTCNE was added, and finally reached 8.23 mA cm-2 upon a 1.0 wt% addition. The FF also increased from 0.565 to 0.596 at the maximum. In contrast, the Voc value slightly decreased from 0.595 V (0wt%) to 0.432 V (9.0wt%) after adding P3HT-b-PTCNE, presumably due to the lower film quality, as compared with the pristine P3HT:PCBM system. On the other hand, such a


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decrease in the Voc value is not found using PTCNE-b-P3HT-b-PTCNE at all. In addition, when 1.5 wt% PTCNE-b-P3HT-b-PTCNE was added, the Jsc gradually increased and reached 8.22 mA cm-2, and the increase in the FF was clearly observed when compared to P3HT-b-PTCNE. Although there is no clear explanation for the drop of Jsc value after adding 0.5 wt% addition of PTCNE-b-P3HT-b-PTCNE, it should be mentioned that this result was reproducible. We speculate that the Jsc values might be affected by thinner film thickness than other devices. Consequently, the PCE of the devices reached the maximum value of 2.89% at the optimum PTCNE-b-P3HT-bPTCNE blend ratio of 1.0 wt%, which corresponded to a 6.6% enhancement when compared to the reference pristine P3HT:PCBM device.

Insert Figures 3 and 4 here. Insert Table 1 here.

On the other hand, when the blend ratio of the block copolymer was further increased, the PCE dramatically decreased despite the fact that the morphology was maintained even when 5.0 wt% of P3HT-b-PTCNE was blended (Figure 5). Therefore, this PCE decay was probably caused by the increasing amount of the insulating main chain of the PTCNE units at the interface between P3HT and PCBM.

Insert Figure 5 here.


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To understand why the PCE increased when the block copolymers were blended, the external quantum efficiencies, namely, the incident photon to current efficiencies (IPCEs), of the devices based on P3HT:PCBM blended with or without 1.0 wt% of P3HT-b-PTCNE or PTCNE-b-P3HT-b-PTCNE under monochromatic illumination were estimated (Figure 6(a)). When the P3HT-b-PTCNE or PTCNE-b-P3HT-b-PTCNE was blended with the P3HT:PCBM system, the IPCE values at around 355 and 500-620 nm increased. These bands were ascribed to the absorption of PCBM and crystalline P3HT, respectively. The enhanced IPCEs were probably due to the increased interfacial area of the P3HT:PCBM layer, leading to the increase in the generated carrier amount. It was also speculated that the crystalline P3HT domain was extended by adding a suitable amount of block copolymers. This was supported by the ratio of the IPCE value at 545 nm versus that at 490 nm (Figure 6(b)). This ratio qualitatively represents the extent of the P3HT crystallinity. It was clearly shown that the P3HT crystallinity was improved by the addition of 1.0 wt% of the block copolymers. The triblock copolymer PTCNE-bP3HT-b-PTCNE displayed a better result than the diblock copolymer P3HT-b-PTCNE. The increased P3HT crystallinity was consistent with the increased photovoltaic parameters, such as Jsc, FF, and consequently, the PCE of the P3HT:PCBM devices blended with the block copolymers.


Contact angle measurements and surface energy calculation


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The accumulation of block copolymers at the interface between the P3HT and PCBM domains could be supported by their surface energies, which were determined by contact angle measurements (Table 2). The surface energies were calculated based on the OwensWendt method.22 The precursor block copolymers, P3HT-b-PDHPS/PDHPS-b-P3HT-bPDHPS, and the postfunctionalized polymers, P3HT-b-PTCNE/PTCNE-b-P3HT-bPTCNE, have the surface energies of 19.4, 19.9, 27.926, and 27.726 mN m-1, respectively. These values are intermediate between the reported values of P3HT (16.8 mN m-1) and PCBM (30.1 mN m-1).8 Furthermore, the PTCNE homopolymer21 showed the surface energy of 26.9 mN m-1, which is relatively close to the value of PCBM, leading to the favourable interactions with PCBM. Therefore, it is reasonable to consider that the PTCNE segments of P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE preferentially adhere to the surface of the PCBM domains. This specific interaction will definitely promote the other intermolecular interaction between the P3HT segment of the block copolymer compatibilizers and the surface of P3HT domains, leading to the long range crystallinity of P3HT in the blend system.12

Insert Table 2 here.

P3HT crystalline structures and blend morphology In order to gain insight into the detailed P3HT crystalline structures as well as morphology of the P3HT:PCBM:block copolymer ternary blend systems, GIWAXS and GISAXS experiments were performed. Figure 7 shows the 2D GIWAXS patterns of


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P3HT:PCBM (1:0.8, w:w), P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend films.


There are clear arc diffraction patterns for P3HT(100), P3HT(200), and P3HT(300) in the qz axis (out-of-plane) as well as that for P3HT(010) in the qy axis (in-plane), corresponding to the “edge-on” orientation of the P3HT crystalline structures in the P3HT:PCBM blend film (Figure 7(a)). In case of the P3HT:PCBM:P3HT-b-PTCNE ternary blend film, there is no significant difference from the pristine P3HT:PCBM blend film in terms of the P3HT crystalline structure and orientation (Figure 7(b)). In contrast, the

P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE

blend

film

displayed

ring-shaped

diffraction patterns for the P3HT crystalline structures, indicating that the P3HT crystalline orientation was transformed from “edge-on” to “isotropic” (Figure 7(c)). In addition, the intensity of the broad diffraction ring patterns for the PCBM aggregates (q ~13 nm-1) decreased after adding PTCNE-b-P3HT-b-PTCNE to the P3HT:PCBM system. Figure 8 depicts the out-of-plane/in-plane GIWAXS profiles extracted along the qy/qz directions from the 2D GIWAXS images in Figure 7, respectively. Obviously, the P3HT(010) peak newly appeared in the out-of-plane profile after adding the compatibilizer of PTCNE-b-P3HT-b-PTCNE to the P3HT:PCBM system, again supporting the transformation from the “edge-on” to “isotropic” orientation. The spacing values and correlation lengths of the P3HT crystalline structures determined by the


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GIWAXS profiles are summarized in Table 3. It was found that there were almost no changes in the d-spacing values for the P3HT crystalline structures; however, the correlation length (L) of the P3HT crystalline phase determined by the P3HT(100) diffraction peak significantly increased from 17.5 nm to 28.7 nm by adding PTCNE-bP3HT-b-PTCNE, thus indicating the increase in the matrix P3HT crystallinity. We speculate that the microphase separation is improved by PTCNE-b-P3HT-b-PTCNE with migrating PCBM toward the PTCNE block and thereby increasing the crystallinity of the matrix P3HT and P3HT block of PTCNE-b-P3HT-b-PTCNE, as also proposed by Sun et al..12 The obtained GIWAXS results are quite consistent with the OPV characteristics, in which the best PCE was obtained with the 1.0 wt% additive of PTCNE-b-P3HT-b-PTCNE, and with the results of the IPCEs showing the improved photo-conversion, especially in the P3HT absorption wavelength region.

Insert Figure 8 and Table 3 here.

The specific interface area (Sv, total interface area per unit volume between pure PCBM clusters and others) could be evaluated by following the previous study of the P3HT:PCBM blend morphology using GISAXS.27 Figure 9 shows the in-plane GISAXS profiles extracted along the qy direction from the 2D GISAXS images (Figure S1) of the pristine P3HT, P3HT:PCBM (1:0.8, w:w), P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) films. The main GISAXS intensities for the blend films (Figures 9(b), 9(c), and 9(d)) are attributed to the PCBM clusters compared to the flat GISAXS profile of the pristine P3HT film


16

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(Figure 9(a)), similar to the previous study by Tsao, Su, and their coworkers.27 The structural parameters determined by Porod’s law and model-fitting methods from the blend films are summarized in Table 4. Comparing the P3HT:PCBM (1:0.8, w:w) and P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend systems did not reveal any noticeable changes in all the parameters, i. e., η (volume fraction of PCBM cluster), R (the mean PCBM cluster radius), ξ (the correlation length of the PCBM-dispersed P3HT phase), and Sv. In sharp contrast, the Sv value significantly increased from 1.34 x 10-1 nm-1 to 2.15 x 10-1 nm-1 after adding PTCNE-b-P3HT-b-PTCNE. These GISAXS results also support the superior compatibilizing ability of the ABA triblock copolymer over that of the AB diblock copolymer, resulting in the achievement of the best PCE in OPV characteristics when using the ABA triblock copolymer.

Insert Figure 9 and Table 4 here.

CONCLUSIONS The AB diblock copolymer, P3HT-b-PTCNE, and ABA triblock copolymer, PTCNE-b-P3HT-b-PTCNE, showed an expanded absorption compared to the previous P3HT-based block copolymer compatibilizers,11 and suitable energy levels for the smooth energy transfer from P3HT to PCBM via block copolymers in the P3HT:PCBM blend films. The P3HT:PCBM BHJ OPVs using the block copolymers as interfacial compatibilizers were fabricated and their device performances at different blend ratios were tested to evaluate the compatibilizing effect in detail. By optimizing the blend ratio


17


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Page 18 of 40

of the block copolymers, the PCE increased by 6.6% when using PTCNE-b-P3HT-bPTCNE, compared to the reference pristine P3HT:PCBM device. The IPCE measurement suggested that the P3HT crystallinity was improved by adding the block copolymers. The accumulation of block copolymers at the interface between the P3HT and PCBM domains could be expected by determining the surface energies of these block copolymers. The more significant increase in the matrix P3HT crystallinity after adding PTCNE-b-P3HT-b-PTCNE rather than P3HT-b-PTCNE was confirmed by GIWAXS experiments, being consistent with the improved PCE and IPCE of the BHJ OPV devices with blend films. The GISAXS results further supported the superior compatibilizing ability of PTCNE-b-P3HT-b-PTCNE versus that of P3HT-b-PTCNE. To the best of our knowledge, this is the first investigation of the sequentially different AB diblock and ABA triblock copolymers as P3HT:PCBM interfacial compatibilizers for BHJ photovoltaics.

AUTHOR INFORMATION: Corresponding Author *Tsuyoshi Michinobu ([email protected]). *Tomoya Higashihara ([email protected])

ACKNOWLEDGMENTS:


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This work is supported by the Japan Science and Technology Agency (JST), PRESTO program (JY 220176) and by Japan Society for the Promotion of Science (JSPS) (KAKENHI: Proposal No. 26620172 (H.T.) and 15KT0140 (T.M.)). GIWAXS and GISAXS experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014A1530 and 2015B1633), respectively. We thank Prof. Itaru Osaka (RIKEN) for partly operating the GIWAXS experiments. S.F. thanks the Innovative Flex Course for Frontier Organic Material Systems and Graduate School of Science and Engineering, Yamagata University for his financial support.

Supporting Information. 2D GISAXS patterns of the blend films and the overlaid diffraction intensity curves in Figure 9(b)-(d).

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Triblock

Copolymer

Containing

Poly(3-hexylthiophene)

and

Poly(4-

vinyltriphenylamine) Segments. Macromolecules, 2010, 43, 6085-6091. (12)

Sun, Z.; Xiao, K.; Keum, J. K.; Yu, X.; Hong, K.; Browning, J.; Ivanov, I. N.; Chen, J.; Alonzo, J.; Li, D.; Sumpter, B. G.; Payzant, E. A.; Rouleau, C. M.; Geohegan, D. B. PS-b-P3HT Copolymers as P3HT/PCBM Interfacial Compatibilizers for High Efficiency Photovoltaics. Adv. Mater. 2011, 23, 5529-5535.

(13)

Renaud, C.; Mougnier, S. J.; Pavlopoulou, E.; Brochon, C.; Fleury, G.; Deribew, D.; Portale, G.; Cloutet, E.; Chambon, S.; Vignau, L.; Hadziioannou, G. Block Copolymer as a Nanostructuring Agent for High-Efficiency and Annealing-Free Bulk Heterojunction Organic Solar Cells. Adv. Mater. 2012, 24, 2196-2201.

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Lai, Y. C.; Higashihara, T.; Hsu, J. C.; Ueda, M.; Chen, W. C. Enhancement of Power Conversion Efficiency and Long-term Stability of P3HT/PCBM Solar Cells Using C60 Derivatives with Thiophene Units as Surfactants. Sol. Energy Mater. Sol. Cells 2012, 97, 164-170.

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Chen, J.; Yu, X.; Hong, K.; Messman, J. M.; Pickel, D. L.; Xiao, K.; Dadmun, M. D.; Mays, J. W.; Rondinone, A. J.; Sumpterad, B. G.; Kilbey II, S. M. Ternary Behavior and

Systematic

Nanoscale

Manipulation

of

Domain

Structures

in

P3HT/PCBM/P3HT-b-PEO Films. J. Mater. Chem., 2012, 22, 13013-13022. (16)

Yun, M. H.; Kim, J.; Yang, C.; Kim, J. Y. A Simultaneous Achievement of High Performance and Extended Thermal Stability of Bulk-heterojunction Polymer Solar Cells Using a Polythiophene–fullerene Block Copolymer. Sol. Energy Mater. Sol. Cells 2012, 104, 7-12.


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Rattanathamwat, N.; Wootthikanokkhan, J.; Nimitsiriwat, N.; Thanachayanont, C.; Asawapirom, U. Feasibility for Enhancing Power Conversion Efficiency of P3HT /C60 Polymer Solar Cell by Adding Donor-acceptor Block Copolymer as a Compatibilizer. Adv. Mater. Res. 2013, 747, 313-316.

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Rattanathamwat, N.; Wootthikanokkhan, J.; Nimitsiriwat, N.; Thanachayanont, C.; Asawapirom,

U.;

Keawprajak,

A.

Poly(3-hexyl

thiophene)-b-Fullerene

Functionalized Polystyrene Copolymers (P3HT-b-PSFu) as Compatibilizer in P3HT /Phenyl-C61-butyric Acid Methyl Ester (PCBM) Solar Cells. Int. J. Polym. Mater. Polym. Biomater. 2014, 63, 476-485. (19)

Yuan, K.; Chena, L.; Chen, Y. Photovoltaic Performance Enhancement of P3HT/PCBM Solar Cells Driven by Incorporation of Conjugated Liquid Crystalline Rod-coil Block Copolymers. J. Mater. Chem. C 2014, 2, 3835-3845.

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Raïssi, M.; Erothu, H.; Ibarboure, E.; Cramail, H.; Vignau, L.; Cloutet, E.; Hiorns, R. C. Fullerene-capped Copolymers for Bulk Heterojunctions: Device Stability and Efficiency Improvements. J. Mater. Chem. A 2015, 3, 18207-18221.

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Fujita, H.; Michinobu, T.; Tokita, M.; Ueda, M.; Higashihara, T. Synthesis and Postfunctionalization of Rod–Coil Diblock and Coil–Rod–Coil Triblock Copolymers Composed

of

Poly(3-hexylthiophene)

and

Poly(4-(4’-N,N-

dihexylaminophenylethynyl)styrene) Segments. Macromolecules 2012, 45, 96439656. (22)

Owens, D. K.; Wendt, R. C. Estimation of The Surface Free Energy of Polymers. J. Appl. Polym. Sci. 1969, 13, 1741-1747.


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(23)

Muller-Buschbaum, P. The Active Layer Morphology of Organic Solar Cells Probed with Grazing Incidence Scattering Techniques. Adv. Mater. 2014, 26, 7692-7709.

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Michinobu, T. Adapting Semiconducting Polymer Doping Techniques to Create New Types of Click Postfunctionalization. Chem. Soc. Rev. 2011, 40, 2306-2316.

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Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Influence of the Ethylhexyl SideChain Content on the Open-Circuit Voltage in rr-Poly(3-hexylthiophene-co-3-(2ethylhexyl)thiophene) Copolymers. Macromolecules 2012, 45, 3740-3748.

(26)

Note: In the contact angle measurements, we used water and glycerol droplets which are hydrophilic solvents. Due to the block sequence of P3HT and PTCNE segments, the latter segment might selectively migrate at the surface of the block copolymer films to minimize the contact energy with such hydrophilic solvents. As a result, block copolymers surface energy may tend to show almost the same surface energy as PTCNE homopolymer. The slightly higher surface energy values for block copolymers than the PTCNE homopolymer may be within the experimental errors.

(27)

Liao, H. C.; Tsao, C. S.; Lin, T. H.; Chuang, C. M.; Chen, C. Y.; Jeng, U. S.; Su, C. H.; Chen, Y. F.; Su, W. F. Quantitative Nanoorganized Structural Evolution for a High Efficiency Bulk Heterojunction Polymer Solar Cell. J. Am. Chem. Soc. 2011, 133, 13064-13073.

FIGURE CAPTIONS: Figure 1 Schematic energy diagrams of block copolymers, P3HT and PCBM. The data for P3HT and PCBM were taken from the reference.25


23


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Page 24 of 40

Figure 2. Fluorescence spectra of (a) P3HT-Br, P3HT-b-PDHPS, and P3HT-b-PTCNE and (b) P3HT-Br, PDHPS-b-P3HT-b-PDHPS, and PTCNE-b-P3HT-b-PTCNE in thin film states after thermal annealing at 160 oC for 1 h, excited at 523 nm. Figure 3 J-V curves of the P3HT:PCBM (1:0.8, w/w) devices blended with the 0 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% of (a) P3HT-b-PTCNE and (b) PTCNE-b-P3HT-b-PTCNE under AM 1.5G light illumination at 100 mV cm-2. Figure 4. OPV characteristics of the P3HT:PCBM (1:0.8, w/w) devices blended with the 0 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% of P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE: (a) Jsc, (b) Voc, (c) FF, and (d) PCE. Figure 5. AFM phase images of the P3HT:PCBM (1:0.8, w/w) films blended with the (a) 0 wt%, (b) 1.0 wt%, and (c) 5.0 wt% of P3HT-b-PTCNE, (d) 1.0 wt% and (e) 5.0 wt% of PTCNE-bP3HT-b-PTCNE cast from o-dichlorobenzene on quartz plates after thermal annealing (150 oC, 10 min). Figure 6. (a) IPCE curves and (b) IPCE value ratio at 545 nm and 490 nm of the devices based on P3HT:PCBM blended with or without 1 wt% of P3HT-b-PTCNE or PTCNE-b-P3HT-bPTCNE. Figure 7. 2D GIWAXS patterns of (a) P3HT:PCBM (1:0.8, w:w) blend film annealed at 150 oC for 10 min, (b) P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend film annealed at 150 o

C for 10 min, and (c) P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend film

annealed at 150 oC for 10 min.


24

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Figure 8. (a) Out-of-plane GIWAXS profiles extracted along qz direction from the 2D GIWAXS images in Figure 7, and (b) in-plane GIWAXS profiles extracted along qy direction from the 2D GIWAXS images in Figure 7. Black, red, and blue lines correspond to P3HT:PCBM (1:0.8, w:w), P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and P3HT:PCBM:PTCNE-b-P3HTb-PTCNE (1:0.8:0.01, w:w:w) blend films, respectively. Figure 9. In-plane GISAXS profiles extracted along qy direction from the 2D GISAXS images in Figure S1. (a) pristine P3HT, (b) P3HT:PCBM (1:0.8, w:w), (c) P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and (d) P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) films. The blue line/circles and orange lines correspond to the experimentally obtained diffractions and model fitting curves (dispersed hard sphere model27), respectively. See Figure S2 for the overlaid diffraction intensity curves in Figure 9(b)-(d).

SCHEME CAPTION: Scheme 1. Synthesis of P3HT-b-PTCNE (AB diblock copolymer) and PTCNE-b-P3HTb-PTCNE (ABA triblock copolymer).21


25


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-3.27 eV

Page 26 of 40

-3.43 eV -3.79 eV -4.2 eV

-5.17 eV

-5.12 eV -5.50 eV

P3HT

-6.0 eV

P3HT-b-PTCNE PCBM PTCNE-b-P3HT-b-PTCNE

Figure 1. Schematic energy diagrams of block copolymers, P3HT and PCBM. The data for P3HT and PCBM were taken from the reference.25


26

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(a)

600

(b) P3HT-Br P3HT-b-PDHPS P3HT-b-PTCNE

650

Intensity (a.u.)

Intensity (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


700

750

600

Wavelength (nm)

P3HT-Br PDHPS-b-P3HT-b-PDHPS PTCNE-b-P3HT-b-PTCNE

650

700

750

Wavelength (nm)

Figure 2. Fluorescence spectra of (a) P3HT-Br, P3HT-b-PDHPS, and P3HT-b-PTCNE and (b) P3HT-Br, PDHPS-b-P3HT-b-PDHPS, and PTCNE-b-P3HT-b-PTCNE in thin film states after thermal annealing at 160 oC for 1 h, excited at 523 nm.


27

(a)

(b) 0wt% 0.5wt% 1.0wt% 1.5wt%

10

0

-10

Current density (mA cm-2)

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

Current density (mA cm-2)


Page 28 of 40

0wt% 0.5wt% 1.0wt% 1.5wt%

10

0

-10

0

0.2

0.4

0.6

0.8

0

Voltage (V)

0.2

0.4

0.6

0.8

Voltage (V)

Figure 3. J-V curves of the P3HT:PCBM (1:0.8, w/w) devices blended with the 0 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% of (a) P3HT-b-PTCNE and (b) PTCNE-b-P3HT-b-PTCNE under AM 1.5G light illumination at 100 mV cm-2.


28

Page 29 of 40

(a)

(b) 0.65

VOC (V)

-2

(mA cm )

8.5

7.5

0.60

0.55

P3HT-b-PTCNE PTCNE-b-P3HT-b-PTCNE

J

SC

8.0

0.0

0.0

0.5

1.0

P3HT-b-PTCNE PTCNE-b-P3HT-b-PTCNE 0.0

1.5

0.0

Surfactant (wt%) (c) 0.65

1.0

1.5

(d) 3.2 PCE (%)

0.60

0.55

2.8

2.4


0.5

Surfactant (wt%)

FF

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


0.0

0.5

1.0

1.5


0.0

Surfactant (wt%)

0.5

1.0

1.5

Surfactant (wt%)

Figure 4. OPV characteristics of the P3HT:PCBM (1:0.8, w/w) devices blended with the 0 wt%, 0.5 wt%, 1.0 wt%, and 1.5 wt% of P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE: (a) Jsc, (b) Voc, (c) FF, and (d) PCE.


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Page 30 of 40

Figure 5. AFM phase images of the P3HT:PCBM (1:0.8, w/w) films blended with the (a) 0 wt%, (b) 1.0 wt%, and (c) 5.0 wt% of P3HT-b-PTCNE, (d) 1.0 wt% and (e) 5.0 wt% of PTCNE-bP3HT-b-PTCNE cast from o-dichlorobenzene on quartz plates after thermal annealing (150 oC, 10 min).


30

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Figure 6. (a) IPCE curves and (b) IPCE value ratio at 545 nm and 490 nm of the devices based on P3HT:PCBM blended with or without 1 wt% of P3HT-b-PTCNE or PTCNE-b-P3HT-bPTCNE.


31


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Figure 7. 2D GIWAXS patterns of (a) P3HT:PCBM (1:0.8, w:w) blend film annealed at 150 oC for 10 min, (b) P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend film annealed at 150 o

C for 10 min, and (c) P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) blend film

annealed at 150 oC for 10 min.


32

Page 33 of 40

P3HT(100)

(b)

P3HT:PCBM (1:0.8) th4 P3HT:PCBM: P3HT-b-PTCNE th5 (1:0.8:0.01) P3HT:PCBM: th7 PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01)

P3HT (100)

Intensity [a.u.]

(a) Intensity [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


P3HT(200) P3HT(300)

P3HT (300)

P3HT (010)

0

5

10 15 qz [nm-1]

20

Amorphous halo PC61BM PC61BM P3HT P3HT (200) (010)

25

0

5

10 15 qy [nm-1]

20

25

Figure 8. (a) Out-of-plane GIWAXS profiles extracted along qz direction from the 2D GIWAXS images in Figure 7, and (b) in-plane GIWAXS profiles extracted along qy direction from the 2D GIWAXS images in Figure 7. Black, red, and blue lines correspond to P3HT:PCBM (1:0.8, w:w), P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and P3HT:PCBM:PTCNE-b-P3HTb-PTCNE (1:0.8:0.01, w:w:w) blend films, respectively.


33


1.E+07

(a)

(b)

(c)

(d)

1.E+06 Intensity (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

Page 34 of 40

1.E+05 1.E+04 1.E+03 1.E+02 0.01

0.1 qy (nm-1)

1

0.01

0.1 qy (nm-1)

1 0.01

0.1 qy (nm-1)

1 0.01

0.1 qy (nm-1)

1

Figure 9. In-plane GISAXS profiles extracted along qy direction from the 2D GISAXS images in Figure S1. (a) pristine P3HT, (b) P3HT:PCBM (1:0.8, w:w), (c) P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01, w:w:w), and (d) P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01, w:w:w) films. The blue line/circles and orange lines correspond to the experimentally obtained diffractions and model fitting curves (dispersed hard sphere model27), respectively. See Figure S2 for the overlaid diffraction intensity curves in Figure 9(b)-(d).


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Scheme 1. Synthesis of P3HT-b-PTCNE (AB diblock copolymer) and PTCNE-b-P3HTb-PTCNE (ABA triblock copolymer).21


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Table 1. OPV characteristics of the P3HT:PCBM (1:0.8, w/w) devices blended with the different ratio of block copolymers P3HT-b-PTCNE and PTCNE-b-P3HT-b-PTCNE compatibilizer

none

P3HT-b-PTCNE

Voc (V)

FF

PCEmax

content

Jsc

(wt%)

(mA cm-2)

0.0

8.06

0.595

0.565

2.71 [2.69]

0.5

8.14

0.579

0.596

2.81 [2.79]

1.0

8.23

0.572

0.583

2.75 [2.69]

1.5

7.86

0.549

0.548

2.36 [2.29b]

3.0

6.69

0.512

0.476

1.63 [1.59]

5.0

7.19

0.516

0.407

1.51 [1.41]

9.0

4.05

0.432

0.365

0.64 [0.62]

0.5

7.61

0.587

0.599

2.68 [2.63]

1.0

8.07

0.589

0.608

2.89 [2.86]

1.5

8.22

0.584

0.580

2.78 [2.76]

[PCE]ava (%)

PTCNE -b-P3HT-bPTCNE a

Average PCE of 3 circuits. b Average PCE of 2 circuits.


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Table 2. Contact angle of water and glycerol on polymer films for determination of surface energies film

a

water

glycerol

surface energy

(deg)

(deg)

(mN m-1)

P3HTa

100.2

90.3

16.8

P3HT-b-PDHPS

97.7

86.5

19.4

PDHPS-b-P3HT-b-PDHPS

94.5

84.4

19.9

PTCNE

86.8

74.4

26.9

P3HT-b-PTCNE

95.1

78.9

27.9

PTCNE-b-P3HT-b-PTCNE

90.6

76.0

27.7

PCBMa

75.0

68.1

30.1

The data were taken from the reference.8


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Table 3. Spacing values and correlation lengths of P3HT crystalline structures determined by GIWAXS profiles. filmb

dP3HT(100)

d P3HT(010)

L

(nm)

(nm)

(nm)a

P3HT:PCBM (1:08)

1.77

0.383

17.5

P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01)

1.77

0.382

17.8

P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01)

1.77

0.383

28.7

a

The correlation length (L) of the P3HT crystalline phase is determined by the P3HT(100) diffraction peak. b Annealed at 150 oC for 10 min.


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Table 4. The structural parameters determined by the Porod’s law and model-fitting methods from the blend films a

ηb

Rc

ξd

Sve x 10-1

(%)

(nm)

(nm)

(nm-1)

P3HT:PCBM (1:08)

20

12.5

30

1.34

P3HT:PCBM:P3HT-b-PTCNE (1:0.8:0.01)

18

12.5

30

1.18

P3HT:PCBM:PTCNE-b-P3HT-b-PTCNE (1:0.8:0.01)

22

12.5

32

2.15

film

a

The reference27 was followed for determining all the parameters. b The volume fraction of PCBM cluster. c The mean PCBM cluster radius. d The correlation length of the PCBM-dispersed P3HT phase. e The total interface area per unit volume between pure PCBM clusters and others.


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Table of Contents Graphic

Compatibilizing Effect in P3HT:PCBM:Block Copolymer OPV


40

Sequentially Different AB Diblock and ABA Triblock ... - ACS Publications

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