Article pubs.acs.org/Macromolecules
Solar Cell Performance of Phenanthrodithiophene−Isoindigo Copolymers Depends on Their Thin-Film Structure and Molecular Weight Hiroki Mori,† Shuto Hara,‡ Shuhei Nishinaga,‡ and Yasushi Nishihara*,† †
Research Institute for Interdisciplinary Science and ‡Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan S Supporting Information *
ABSTRACT: Phenanthro[1,2-b:8,7-b′]dithiophene (PDT)− isoindigo (IID)-based polymers 12OD (L) and 12OD (H) with a different molecular weight were synthesized and characterized. By using further purified PDT and IID monomers, the high-molecular-weight polymer 12OD (H) with number-average molecular weight (Mn) over 50 kDa was obtained. Both 12OD (L) and 12OD (H) polymers have the same energy gap and highest occupied molecular orbital (HOMO) energy levels, indicating that the influence of molecular weight on their electronic structure is negligible, although 12OD (H) has stronger aggregation behavior than 12OD (L). 12OD (H)-based solar cells fabricated by using optimal solvent and additives showed an increased short-circuit current density (Jsc) with same open-circuit voltage (Voc) and fill factor (FF), resulting in a significantly improved power conversion efficiency (PCE) of up to 6.1%, which is approximately 70% higher than that of the 12OD (L)-based cell (3.5%). This result is due to the different molecular orientation caused by the higher molecular weight. Grazing incidence wide-angle Xray scattering (GIWAXS) analyses revealed that the blended film of 12OD (H)/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) formed a face-on orientation with a long-range ordered structure, while a low crystalline edge-on structure was observed in the blended film of 12OD (L)/PC61BM. Such high crystalline and favorable molecular orientation could promote light harvesting efficiency and hole transporting ability, resulting in high Jsc and thus an excellent PCE. From the detailed GIWAXS analysis, the orientation change of 12OD (H) was induced by the addition of PC61BM. This result suggests that the strong aggregation ability of 12OD (H) can drive crystallization and favors active interaction with PC61BM to form a face-on orientation. In this study, the increase in molecular weight can improve not only the thin-film structure such as crystallinity and phase separation structure but also the molecular orientation in the PDT-based polymer system.
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INTRODUCTION Semiconducting polymer/fullerene-based solar cells (PSCs) are one of the more promising candidates for next-generation and low-cost energy harvesting because their potential features include the production of large areas with a low-energy process, light weight, flexibility, and elasticity.1−5 Among these devices, donor−acceptor (D−A) polymers that combine electron-rich aromatic cores with electron-deficient ones have been widely used to develop high-performance materials for PSCs because typical D−A polymers have an extended absorption throughout the visible region, a deep highest occupied molecular orbital (HOMO) energy level, and strong intermolecular interactions owing to a structural charge distribution that produces electrostatic interactions.6−9 In addition, their electronic structure can readily be tuned by changing key donor and acceptor units such as HOMO and lowest unoccupied molecular orbital (LUMO) energy levels and their absorption profile.6−9 To date, numerous D−A polymers have been developed and their power conversion efficiency (PCE) has reached 10−12%.10−17 © XXXX American Chemical Society
The efficiency of PSCs strongly depends not only on the energy levels of D−A polymers but also on the blended morphology of p-type polymers with n-type fullerene derivatives. Since bulk-heterojunction (BHJ) solar cells consist of intermixed photoactive layers, with p-type D−A polymers and n-type soluble fullerenes, blended films should form a suitable phase separation structure with optimal domain size to achieve efficient carrier transport and charge dissociation.18,19 Furthermore, most D−A semiconducting polymers show anisotropic carrier transport through π-orbital overlap. Therefore, they should facilitate the optimal molecular orientation, i.e., face-on, for efficient carrier transport in a BHJ cell.10,11,13−16,20−22 In order to achieve the ideal blended morphology and thin-film structure, the optimization of various parameters such as side chains,10,13,16,23−32 molecular weight,33 chemical structure of backbone,34,35 and device fabrication Received: April 15, 2017 Revised: June 1, 2017
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DOI: 10.1021/acs.macromol.7b00778 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthesis of PDT−IID Polymers
process18,19 is required. Among these parameters, the most important issue is the control of polymer molecular weight. In most cases, high number-average molecular weight (Mn) polymer-based devices showed significantly higher PCEs than did cells based on low-Mn polymers.33 One reason for these increased PCEs is their improved blended morphology. Since typical high-Mn polymers have better miscibility between two components such as D−A polymer and [6,6]-phenyl-C61butyric acid methyl ester (PC61BM), they form a well-separated and optimal interpenetrating network.33,36−38 These structures can allow efficient charge separation and carrier transport, leading to high short-circuit current density (Jsc) and fill factor (FF). In another scenario, when the solubility of polymers decreased, they tended to aggregate more as the molecular weight of polymers increased, which can enhance the crystallization into a microphase separation structure, resulting in improved PCE.33,39−41 On the other hand, it has been reported that the molecular weight of polymers also strongly affects their molecular ordering in thin films, while many studies have focused on morphology. For instance, Higashihara and co-workers reported the molecular weight dependence of 2,2′-bis(1,3,4thiadiazole) (BTDz)-based polymers.42 That higher molecular weight polymer also formed a well-separated interpenetrating network. In addition, the molecular orientation of BTDz polymers was drastically changed from bimodal to a solely faceon arrangement as the number-average molecular weight (Mn) increased from 5.2 to 26.6 kDa. These changes resulted in a 20fold enhancement in PCE from 0.36 to 8.04%. Intriguingly, thiazolo[5,4-d]thiazole (TzTz)-based copolymers reported by Osaka and co-workers showed no dependence of their blend morphology on molecular weight.43 In contrast, their molecular orientation in blended films was also changed from bimodal to the favorable face-on orientation by increasing the molecular weight from 13 to 33 kDa, leading to higher Jsc and FF. However, to the best of our knowledge, such an orientation change due to increased molecular weight has received very little study.42−45 Thus, the elucidation of molecular weight’s effect on the molecular orientation of polymers is a key to gaining new insight into the structure−property relationship. It should be noted that excessively high-Mn polymers gave reduced efficiency owing to several problems such as lower domain purity, 44 larger phase separation, 40 and lower crystallinity.43 Therefore, the optimal molecular weight of each polymer is quite different because it depends strongly on the molecular structure of the polymer backbone, which affects
the intrinsic viscosity, solubility, and crystallinity. The control of molecular weight is thus an effective way to develop highperformance D−A polymers for PSCs. Recently, we have developed phenanthro[1,2-b:8,7-b′]dithiophene (PDT)46−51−isoindigo (IID) copolymers.52,53 By optimization of the solubility of side chains, n-dodecyl- and 2octyldodecyl-substituted PDT polymer, poly[phenanthro[1,2b:8,7-b′]dithiophen-2,9-diyl-alt-(E)-1,1′-bis(2-octyldodecyl)bis(4-dodecylthiophen-2-yl)-(3,3′-biindolinylidene)-2,2′-dione6,6′-diyl] (12OD, Scheme 1) formed a dense π-stacking structure with short π-stacking distance of 3.5−3.6 Å, and a fabricated field-effect transistor showed good hole transport ability with mobility of up to 0.16 cm2 V−1 s−1. In addition, its deep HOMO energy level and the formation of optimal blend morphology led to good solar cell performance, with PCE of up to 5.1% in an optimized inverted solar cell with phenyl-C71butyric acid methyl ester (PC71BM) as an n-type semiconductor. However, 12OD/PCBM blended film exhibited a low crystalline nature, predominantly in an edge-on manner, which may limit its solar cell performance. One possible reason for its low crystallinity may be due to its relatively low molecular weight (∼26.8 kDa). Polymerization by Migita−Kosugi−Stille coupling is a very powerful way to prepare high molecular weight and high performance D−A polymers.8,54−58 Many types of D−A polymers have been synthesized, but it is difficult to synthesize high molecular weight polymers because the purity, stability, and stoichiometric ratio of monomers strongly affect the polymerization results.56 Thus, the synthesis of high-Mn polymers is one of the challenging issues in the creation of high-performance D−A polymers. Herein, we report the synthesis, characterization, and solar cell application of highMn polymer 12OD (H) synthesized by using specially purified monomers. The dependence of their thin-film structure and photovoltaic properties on their molecular weight was also investigated.
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EXPERIMENTAL SECTION
General. All the reactions were carried out under an argon atmosphere. Dehydrated toluene was purchased from Kanto Chemicals Co., Ltd. Polymerizations were performed with a Biotage initiator microwave reactor. Molecular weights of polymers were determined by gel-permeation chromatography (GPC) with a TOSOH HLC-8321GPC/HT and TSKgel GMHHR-H HT using a polystyrene standard and o-dichlorobenzene (o-DCB) as the eluent at 140 °C. B
DOI: 10.1021/acs.macromol.7b00778 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
under high vacuum (∼6 × 10−5 Pa) through a shadow mask. The active area of all devices was 0.16 cm2. Characterization of Solar Cell Devices. The characteristics of the solar cell devices were measured through a 4 × 4 mm photomask, with a Keithley 2401 semiconductor analyzer, using a Xe lamp (Bunkokeiki OTENTO-SAN III type G2) as the light source, under AM 1.5 G simulated solar irradiation at 100 mW cm−2 at room temperature under a nitrogen atmosphere. The light intensity was determined by a calibrated standard silicon solar cell (Bunkokeiki, BS520BK). Fabrication and Characterization of Hole-Only Devices. Hole-only devices were fabricated as follows. ITO substrates were washed and treated with UV-ozone in the same manner as described above. Then ITO substrates were spin-coated with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios P VP AI 4083) through a 0.45 μm PVDF syringe filter at 5000 rpm for 30 s and dried at 120 °C for 10 min in air. After being dried, the substrates were immediately transferred into a nitrogen-filled glovebox. Thin films of an active layer were deposited by spin-coating at 400 rpm for 30 s and 1000 rpm for 5 s from a solution containing the polymer samples (5 mg/mL) with appropriate amounts of PC61BM in anhydrous chlorobenzene (CB). The thickness of all the active layers was 250 nm. After the thin films were dried, MoO3 (6 nm) and Al (80 nm) layers with an active cathode area of 0.16 cm2 were deposited under high vacuum (∼6 × 10−5 Pa) through a shadow mask. Current density−voltage (J−V) characteristics of the fabricated devices were measured using a Keithley 2401 source meter in the dark. Voltage sweeps were performed in the range of 0−6 V, and hole mobilities were estimated from the J−V curve of the Mott−Gurney space-charge limited current (SCLC) law:61
2,9-Bis(trimethylstannyl)phenanthro[1,2-b:8,7-b′]dithiophene (1)50 and (E)-1,1′-bis(2-octyldodecyl)-6,6′-bis(5-bromothiophen-2-yl)[3,3′-biindolinylidene]-2,2′-dione (2)52 were synthesized according to the reported procedures. All other chemicals were used without further purification unless otherwise indicated. Synthesis of High-Molecular-Weight Polymer 12OD (H). Further purified monomers 1 (30.8 mg, 0.05 mmol) and 2 (74.1 mg, 0.05 mmol), Pd(PPh3)4 (1.3 mg, 1 μmol), and toluene (2.5 mL) were added to a reaction vessel, which was sealed and refilled with argon and then heated at 180 °C for 40 min in a microwave reactor. After being cooled to room temperature, the reaction mixture was poured into 100 mL of methanol containing 5 mL of concentrated hydrochloric acid and stirred for 2 h. The precipitate was then subjected to sequential Soxhlet extraction with methanol, hexane, and chloroform to remove low-molecular-weight fractions. The residue was extracted with chlorobenzene, and concentrated solution was poured into 100 mL of methanol. The precipitates obtained were collected by filtration and dried under vacuum to afford the polymer 12OD (H) (23.3 mg, 29%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 50.7 kDa, Mw = 97.3 kDa, and PDI = 1.92. Synthesis of Low-Molecular-Weight Polymer 12OD (L). Equal amounts of monomers 1, 2, and Pd catalyst in toluene were subjected to the polymerization at 140 °C for 30 min, and sequential Soxhlet extraction with methanol, hexane, and then chloroform was employed to obtain the polymer termed 12OD (L) (65.6 mg, 81%) as a metallic purple solid. GPC (o-DCB, 140 °C): Mn = 29.0 kDa, Mw = 49.9 kDa, PDI = 1.72. Instrumentation. UV−vis absorption spectra were measured using a Shimadzu UV-2450 UV−vis spectrometer. Cyclic voltammograms (CVs) were recorded on electrochemical analyzer CHI-600B in acetonitrile containing tetrabutylammonium hexafluorophosphate (TBAP, 0.1 M) as supporting electrolyte at a scan rate of 100 mV/ s. A Pt electrode (surface area: A = 0.071 cm2, BAS), an Ag/Ag+ (Ag wire in 0.01 M AgNO3/0.1 M TBAP/CH3CN), and a Pt wire electrode were used as working, reference, and counter electrodes, respectively. Samples of the polymer films were prepared by dropcasting on a working electrode from their chloroform solutions. All the potentials were calibrated with the standard ferrocene/ferrocenium redox couple (Fc/Fc+: E1/2 = +0.07 V measured under identical conditions). Dynamic force-mode atomic force microscopy was carried out using an SPA 400-DFM (SII Nano Technologies). Grazing incidence wide-angle X-ray scattering (GIWAXS) analyses were carried out at SPring-8 on beamline BL46XU. The samples were irradiated at a fixed angle on the order of 0.12° through a Huber diffractometer with an X-ray energy of 12.39 keV (λ = 1 Å), and the GIWAXS patterns were recorded on a two-dimensional (2D) image detector (Pilatus 300K). Films of the polymers with PC61BM were fabricated by spin-coating on a zinc oxide (ZnO) treated indium tin oxide (ITO) substrate. The thickness of the active layer was measured with an Alpha-Step IQ surface profiler (KLA Tencor). Fabrication of Inverted Bulk-Heterojunction Solar Cells. The inverted bulk-heterojunction solar cells were fabricated as follows. ZnO precursor solution was prepared by hydrolysis of Zn(OAc)2.59 The ITO substrates (ITO, Geomatec Co. Ltd., thickness = 150 nm, sheet resistance
