Ternalization Approach for Tuning Light Absorption and Crystalline Structure of Diketopyrrolopyrrole-Based Polymer Using Bisthiadiazole Unit The global utilization of photovoltaics

Diketopyrrolopyrrole (DPP) and bisthiadiazole (BTDz)-based terpolymers were developed to obtain tunable optical properties and crystalline structures. Using dibromo-compounds of DPP and BTDz with distannylated thienylene vinylene (TV) moieties, high molecular weight polymers (44,100-99,200) with varied BTDz compositions (25, 50, or 75 mol%) were obtained. The introduction of BTDz generated a complementary light absorption band in the short-wavelength region ( ∼ 550 nm), while the DPP units created an intramolecular charge transfer band at ∼ 730 nm. As a result, terpolymers with a deep highest occupied molecular orbital energy of − 5.50 eV and narrow bandgap of < 1.5 eV were obtained. In addition, the crystal orientation of the DPP-based polymer was changed from edge-on to face-on by copolymerizing with only 25 mol% BTDz units.

The global utilization of photovoltaics (PVs) is growing at an accelerated pace. The total installed capacity, which was only 1.7 GW in 2005, reached 303 GW worldwide in 2016. 1 Accordingly, the cost of photovoltaic electricity has decreased dramatically and is now comparable to that of other renewable energy sources. To realize a sustainable social system, the widespread use of PV systems in the building walls, room interiors, and automobile roofs is desirable. Therefore, the demand for organic photovoltaics (OPVs), which is a thin-film, lightweight and highly flexible, is increasing more and more. [2][3][4] OPV is also expected as a potential energy source for a wide variety of sensors in internet of things (IoT) in the near future.
To harvest solar energy efficiently, the light absorption characteristics of the photoelectric conversion material are important factors. Because there is a theoretical limitation on the photoelectric conversion efficiency (PCE) of a single semiconductor, called the Shockley-Queisser limit, 5 an approach combining several semiconductors has been widely used for both inorganic and organic PVs. [6][7][8] Particularly in OPVs, a photoelectric conversion layer in which two to three kinds of organic semiconductors are mixed is used to take advantage of their solution processability. Except for examples such as fullerene derivatives 9 and ITIC, 10 electron or exciton transfer between mixed semiconductors is not guaranteed. In addition, it is difficult to predict the transfer efficiency because it can be influenced by the associated state of different molecules and the interface morphology. Random or semi-random copolymers that contain several different units in the polymer backbone can overcome this drawback. 11,12 Since different light absorbers are chemically bonded with continual conjugation, a wide wavelength of light can be absorbed by a single polymer film. Moreover, the third component gives additional freedom in the polymer chain packing and solubility. In addition, synergetic effects on the crystalline structure and charge carrier mobility can be achieved by finely tuning the composition. These features represent the unique and advantageous characteristics of polymeric semiconductors, compared with inorganic semiconductors or organic small molecules.
Diketopyrrolopyrrole (DPP)-based polymer is one of the best semiconducting polymers in terms of balanced performance, solubility, and synthetic facility. 13 Due to the high electron withdrawing properties of DPP, it has a deep LUMO energy level of (−3.5)-(−4.0) eV with an optical bandgap of 1.5-1.7 eV, which is optimal for harvesting sunlight. In addition, DPP-based polymers with optimized main and side chains show very z E-mail: thigashihara@yz.yamagata-u.ac.jp high charge carrier mobilities, even for solution-processed polycrystalline films. 14,15 However, the DPP-based polymer tends to form edge-on oriented crystals suitable for charge transfer in the in-plane direction, which is not preferable for OPVs requiring charge transport in the out-of-plane direction. Therefore, the modification of the side chains and main chain structures has been done to control the crystal orientation. [16][17][18] Based on the previous research, we attempted to expand the absorption spectrum and control the crystalline orientation of DPP-based polymer using a terpolymer approach. We previously developed a short-wavelength absorber using PBTDzTV, an alternating copolymer composed of a strongly electron-deficient unit, 2,2'-bis(1,3,4thiadiazole) (BTDz), and a weakly electron-rich unit, thienylene vinylene (TV). 19 Due to the strong electron withdrawing properties of BTDz, PBTDzTV has a deep HOMO energy level of −5.5 eV and a wide bandgap of 1.9 eV. In addition, due to enhanced intermolecular interactions, this material exhibits a highly crystalline nature with face-on orientation. Therefore, the hole mobility in the out-of-plane direction was 6.22 × 10 −2 cm 2 /Vs, which is high among polymer semiconductors. The OPVs using mixed films with PC 71 BM showed a high PCE exceeding 8%. Thus, in this work, DPP, BTDz, and TV are copolymerized to obtain a terpolymer, BTDz xx DPP yy where xx and yy are molar ratio (%) of BTDz and DPP respectively, that shows a complementary absorption spectrum and a face-on oriented crystalline structure.
It was difficult to remove low molecular weight of oligomer, because PDPPTV fully dissolves even in hexane. Thus, the polymer was used for characterization without further purification. Measurements.-Molecular weights (M n , M w ) and dispersities (Ɖs) were measured by size exclusion chromatography (SEC) on a Jasco GULLIVER 1500 equipped with a pump, an absorbance detector (UV, λ = 254 nm), and three polystyrene gel columns based on a conventional calibration curve using polystyrene standards. Tetrahydrofuran (40 • C) was used as a carrier solvent at a flow rate of 1.0 mL min −1 . 1 H and 13 C nuclear magnetic resonance (NMR) spectra were recorded on JEOL JNM-ECX400 in chloroform-d calibrated to tetramethylsilane as an internal standard (δH 0.00). Thermal analy-sis was performed on a Seiko EXSTAR 6000 TG/DTA 6300 thermal analyzer at a heating rate of 10 • C min −1 for thermogravimetric analysis (TGA). A TA Instruments Q-100 connected to a cooling system at a heating rate of 10 • C min −1 for differential scanning calorimetry (DSC). UV-vis absorption spectra were recorded using a JASCO V-630BIO UV-vis spectrophotometer. Cyclic voltammetry (CV) experiments were performed on a BAS electrochemical analyzer (model 660C) in acetonitrile solutions with 0.1 M tetrabutylammonium perchlorate as a supporting electrolyte. A three-electrode cell was used with platinum electrodes as both the counter and working electrodes.

Results and Discussion
According to Scheme 1, the Stille-coupling copolymerization of BTDz, DPP and TV was conducted. The feed ratio of TV/BTDz/DPP was varied from 1.0/0.75/0.25, 1.0/0.50/0.50, 1.0/0.25/0.75, and 1.0/0/1.0. After the removal of impurities and the low molecular weight fraction by Soxhlet extraction, the target polymers were obtained with number-averaged molecular weights (M n s) of 44,000-99,000 and dispersities (Ɖs) of 1.84-5.85, which is summarized in Table I. The structure was confirmed by 1 H NMR spectroscopy and elemental analysis. All the polymers showed sufficient solubility in chlorinated solvents, and polymers with high DPP content partially dissolved even in hexane.
The thermal properties, listed in Table II and Figure 1, were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the samples have similar 5 wt% weight loss temperatures (T d5% ) of ∼400 • C, which indicates sufficient thermal   stability to withstand the metal deposition conditions during device fabrication. PDPPTV showed no transition peaks from 50 to 250 • C, implying its amorphous nature, which is supported by the grazing incidence wide-angle X-ray scattering (GIWAXS) measurement described below. On the other hand, BTDz 50 DPP 50 and BTDz 25 DPP 75 showed broad but clear melting/crystallizing peaks at 178/197 • C and 204/217 • C, respectively. The difference between PDPPTV and the terpolymers indicates that the crystallinity is enhanced by the BTDz units. BTDz 75 DPP 25 does not exhibit a phase transition from 25 to 280 • C, but an exothermic peak was observed at ∼240 • C in the first heating process. Thus, BTDz 75 DPP 25 also has a crystalline nature,  though its crystallization rate seems to be slow compared to the time scale of the DSC measurement. The light absorption properties were evaluated by UV-Vis-Near infrared (NIR) absorption spectroscopy. Figure 2 displays the UV-Vis-NIR spectra of sample films on glass substrates. The sample films were prepared by spin-coating at 400 rpm for 60 s from 25 mg/mL solutions in chlorobenzene (CB), followed by drying at room temperature (15 • C) for 1 h. All the terpolymers showed an absorption peak or shoulder at ∼730 nm, corresponding to intramolecular charge transfer (ICT) between DPP and TV. In addition to these peaks, complementary absorption bands were observed at ∼550 nm. Because these peaks are similar to those observed in the PBTDzTV absorption spectrum, they are attributed to π-π * transitions resulting from the BTDz-TV structure. The π-π * transition peaks of BTDz 75 DPP 25 and  BTDz 50 TV 50 have a shoulder in the long-wavelength region (∼600 nm), indicating aggregated states. The optical bandgaps calculated from the absorption onsets range from 1.34 to 1.52 eV and decrease according to the DPP content. Thus, the ternalization of the polymer with BTDz successfully expanded the absorption band, tuning the optical bandgap of PDPPTV.
The electrochemical properties of the sample films were obtained from cyclic voltammetry. The oxidation peaks in the positive scans are depicted in Figure 3a. The HOMO energy levels evaluated from the oxidation peak edges and LUMO energy levels, calculated from the HOMO levels and optical bandgaps, are shown in Figure 3b. The sample with a higher BTDz content shows a deeper HOMO energy level (∼-5.5 eV) and a deep LUMO energy level (∼-4.0 eV) due to DPP. Thus, the obtained energy levels in BTDz 75 DPP 25 and BTDz 50 DPP 50 are ideal for OPV applications. 4 Grazing incident wide-angle X-ray scattering (GIWAXS) was performed on the thin film samples to analyze the crystal structures. Figure 4 displays the two-dimensional GIWAXS images, the onedimensional profiles corresponding to the in-plane and out-of-plane directions and the azimuth profile of (100) scattering. The PDPPTV  1.87 nm). The azimuth profile clearly suggests that the (100) scattering is observed in the out-of-plane direction, indicating the edge-on orientation of the crystallites, as (100) is attributed to their lamellar spacing. When the BTDz unit is incorporated into PDPPTV, the crystalline structure is dramatically changed. All the terpolymers show a semi-crystalline nature with (100) spots in the in-plane direction and (010) spots in the out-of-plane direction, indicating face-on orientation. The volume fractions of the face-on oriented area in the crystalline region were quantified by the integral ratio between the peaks at 0-60 deg and 60-90 deg in the azimuth profiles ( Figure 5a, The photovoltaic performance was evaluated using the conventional device architecture, ITO/PEDOT:PSS/active layer/Ca/Al, where the composition of the active layer was 1:1.5 (wt/wt) for polymer: [6,6]-phenyl C 71 butyric acid methyl ester (PC 71 BM). The solvent for the active layer was a mixture of chloroform, odichlorobenzene and 1-chloronaphthalene (3:1:0.12, vol/vol), where chloroform and 1-chloronaphthalene were used to suppress PC 71 BM aggregation in the DPP-containing polymers. 20,21 Figure 6 and Table IV show the J-V curves and characteristic OPV parameters. The PDPPTV-based device showed an open-circuit voltage (V OC ) of 0.62 V, which was improved to 0.74 V by increasing the BTDz content. This trend follows that of the HOMO energy levels of the polymers. The short-circuit current density (J SC = 5.65 mA/cm 2 ) and fill factor (FF = 0.645) were determined in the PDPPTV-based device and found to decrease in the BTDz 25 DPP 75 and BTDz 50 DPP 50 -based devices. The limited photocurrent can be explained by the energy level and

Conclusions
Based on a ternalization strategy, we copolymerized BTDz, DPP and TV to realize an expanded absorption spectrum and desired crystalline orientation. The obtained terpolymers showed two absorption peaks, attributed to BTDz and DPP units, resulting in an expanded absorption band as intended. The HOMO/LUMO energy levels were also tuned to be −5.5/−4.0 eV, respectively, which reflected the characteristics of both the BTDz and DPP units. In addition, the introduction of BTDz enhanced the crystalline nature of the polymer thin films and changed the crystalline orientation from edge-on to face-on. Therefore, the absorption spectrum, energy levels and crystalline structure were successfully tuned for OPV applications. Unfortunately, the photocurrent is limited by the aggregation of PC 71 BM. Further optimization of the fabrication process is ongoing.