In our study to utilize microphase-separated structure for photovoltaic device, a diblock copolymer based on poly(butyltriphenylamine) as a hole transporting segment was designed and synthesized by means of palladium C–N coupling polymerization. Previously, using a poly(ethylene oxide) terminal modifier, we have successfully prepared the diblock copolymer poly(butyltriphenylamine)-b-poly(ethylene oxide) (PTPA-b-PEO) [40]. In this article, 2-bromopropionate derivative (1) as an initiator for atom transfer radical polymerization (ATRP) with bromophenyl moiety was synthesized in two step reactions in order to introduce vinylpolymer at one terminal of PTPA (Scheme 1). Taking its good compatibility with PCBM into account, PS was selected as the other segment. The PS terminal modifier (PS-Br) was synthesized by ATRP initiated from 1 using CuBr/CuBr₂/PEDMTA catalytic system as reported elsewhere [42]. Two kinds of PS-Br with different number average molecular weights (M_n = 2,300 and 4,300) were prepared.

C–N coupling polymerization of self-condensing monomer 2 was carried out in the presence of PS-Br as a terminal modifier to afford the diblock copolymers consisting of PTPA and PS segments (Scheme 2). Using different PS-Br or changing the amount of PS-Br, three kinds of PTPA-b-PS with different weight ratios of PS were prepared as shown in Table 1. After the polymerization, the crude polymers were reprecipitated in acetone to remove remaining unreacted PS-Br. All the polymers showed M_n around 5,000 and broaden PDI. Although the molecular weights of diblock copolymers are relatively low compared to that of PTPA homopolymer, which reaches up to 14,000, the gel permeation chromatography (GPC) profiles of PTPA-b-PSs were monomodal with a slight shift toward high molecular weight from that of PS-Br (Figure 1). Therefore, we assumed that the formation of PTPA-b-PS succeeded without contamination of PS homopolymer. The obtained diblock copolymers showed good solubility in common solvents such as chloroform and tetrahydrofuran.

The structures of the diblock copolymers were characterized by ¹H NMR spectra. Figure 2 shows the representative ¹H NMR spectrum of B2 with those of homopolymers. In addition to the signals assignable to PTPA backbone, new signals derived from PS segment appeared at aromatic and aliphatic regions. Furthermore, the signals assignable to methylene units at the junction point were observed at around 3.6 ppm (Figure 2(c), inset), which shifted from those of PS-Br at around 4.0 ppm (Figure 2(a)). These results clearly indicate that the PS block was successfully introduced at a terminal of PTPA. The weight ratio of PS was calculated by comparing the integral ratio of meta protons of styrene ring (assigned as h) at 6.6 ppm with methylene protons of butyl group at 2.6 ppm (assigned as c). The weight ratios of PS segment were determined as listed in Table 1, which ranged from 5.3 to 39%.

The thermal properties of PTPA-b-PS were measured by DSC. Figure 3 shows the DSC curve of B3 in the second heating scan. The glass transition temperatures (T_g) were observed at 70 and 180 °C corresponding to those of PS and PTPA hopopolymers, respectively. Only one T_g derived from PTPA segment was observed for PTPA-b-PS with low PS content, B1 and B2.

The hole mobility was measured for the block copolymer films by means of time of flight (TOF) method. As a charge generation layer, Ti-phthalocyanine was used. The drift mobilities of PTPA and PTPA-b-PS (B2) at various electric fields are shown in Figure 4. The hole mobility was almost constant and independent of the electric field for the homopolymer, whereas slightly negative slope was obtained for the block copolymer. This negative electric field dependence of the hole mobility was also observed for an amorphous triphenylamine compound [43]. The block copolymer film showed higher hole mobility than the homopolymer film, which increased up to 1.3 × 10⁻⁴ cm²/Vs. This finding indicates that the introduction of PS segment does not disturb the charge transportation in the PTPA domain, furthermore, and that a slight increase of the hole mobility was achieved by the presence of PS segment. The inert second block is assumed to attribute to the formation of the defect-free film. Unfortunately, the hole mobility of the block copolymer film with higher PS content (B3) could not be measured because of cracking all over the film.

The thin polymer films blended with fullerene derivative PCBM, a well known electron transporting material, were fabricated on the glass substrate, and the surface morphology of the films was studied by AFM measurement after annealed at 170 °C for 1 h. The weight ratio of polymer:PCBM was fixed as 1:1 for all the blend films. Diblock copolymer PTPA-b-PS (B3) itself exhibits microphase-separated structure, as shown in Figure 5(a), with the domain size of 10 nm. As a control, PTPA with M_n of 13,000 was blended with PCBM, and the surface morphology of the homopolymer blend film was examined (Figure 5(b)). Slight contrast can be observed at the surface of homopolymer blend film, however, where no clear phase separation occurred. For the diblock copolymer blends, three types of PTPA-b-PS (B1 to B3) were used to investigate the effect of PS content on the surface morphology of the blend films. When the weight fraction of PS was low (5.3 wt%), the surface became completely homogeneous as shown in Figure 5(c). Increasing the PS content, the surface morphology gradually changed to give a phase-separated structure with the domain size of around 20 nm at 39 wt% of PS content (Figure 5(d,e)). Because the PTPA segment has good compatibility with PCBM, blends films using PTPA homopolymer or B1 showed the homogeneous surface. On the other hand, PS is more compatible component with PCBM than PTPA, which separated PTPA (rich) domain and PS with PCBM (rich) domain at higher PS contents. The black area in the AFM phase image indicates soft domain, therefore, assignable to PS (rich) domain.

In order to investigate the availability of PTPA-b-PS as a promoter, three-component blend films consisting of PTPA-b-PS (B3), PTPA, and PCBM were prepared. The compositions of the blends were 15:35:50 and 5:45:50 in the weight ratio, whereas the overall PS contents were 12 and 4 wt%, respectively. The surface of the blend films was observed by AFM before and after annealing at 170 °C for 1 h (Figure 6). When the weight fraction of B3 was 15 wt% (Figure 6(a,b)), the microphase-separated structure similar to B3/PCBM blend was observed in spite of low overall PS content. The microphase separation already exists at the surface of as-made film, and the average domain size of 20 nm was maintained after annealing process. On the other hand, no microphase separation formed in as-made film using 5 wt% of B3 (Figure 6(c)). After annealing, the phase separation was observed, but the structure was slightly distorted (Figure 6(d)).

The photovoltaic devices based on the polymer blend systems, PTPA and/or PTPA-b-PS blended with PCBM in 1:1 weight ratio, were fabricated to evaluate the I-V characteristics as shown in Figure 7. The devices were constructed as the following structure: ITO/PEDOT:PSS (30 nm)/polymer blend (70 nm)/LiF (0.5 nm)/Al (100 nm). The results using the various polymers are summarized in Table 2. Homopolymer (PTPA) blend film showed no photovoltaic characteristic. When the PTPA-b-PSs with different PS contents were used, only the device based on B2 (PS content: 13 wt%) exhibited photovoltaic characteristics. However, the open circuit voltage (Voc) and the fill factor were low. Because the blend film of B1 as well as PTPA showed homogeneous morphology in AFM observation, where the charge recombination may frequently occur in the donor-acceptor mixture within a molecular size, a photocurrent was not obtained from these devices. On the other hand, the device based on B3 also showed no photocurrent due to the high PS contents, which caused the insulation for charge transportation. The three-component blend consisting of PTPA, PTPA-b-PS (B3), and PCBM were utilized for the active layer of the device. The device performances were improved by using three-component blends compared with block copolymer blend using B2. By adding B3 to the blend of PTPA and PCBM, the morphology in the active layer changed to the microphase-separated structure, as revealed in AFM images, without the content of PS increased. Because PTPA backbone can absorb the light only below 400 nm, the efficiency was quite low for all the devices. Chemical modification of PTPA backbone for the improvement of absorption range is under investigation.

Tetrahydrofuran (THF) was used as distilled over sodium and benzophenone. Toluene was distilled over calcium hydride and stored under nitrogen. Styrene was used as freshly distilled over calcium hydride under reduced pressure. 2-(4-Bromophenoxy)ethanol was prepared from 4-bromophenol and 2-chloroethanol according to the reference [44]. 4-(4′-Bromophenyl)-4″-butyldiphenylamine (2) was prepared from 4,4′-dibromobiphenyl and 4-butylaniline according to the previous report [40]. The other reagents were used as received.

To a flask equipped with a stopcock were added 2-(4-bromophenoxy)ethanol (0.438 g, 2.01 mmol), pyridine (1.20 mL), and THF (15.0 mL) at 0 °C under nitrogen. After 2-bromopropionyl bromide (0.492 g, 2.30 mmol) was added dropwise, the mixture was stirred for 30 min at 0 °C and for 24 h at room temperature. THF was removed by rotary evaporator, and the residue was extracted by chloroform and water. The organic layer was dried with magnesium sulfate and concentrated by rotary evaporator. The crude product was purified by column chromatography eluted with hexane:ethyl acetate = 1:1 to give yellow liquid. The yield was 0.480 g (68.2%).

To a Schlenk tube were added 1 (0.10 g, 0.285 mmol), styrene (3.30 mL, 28.5 mmol), copper (I) bromide (41.7 mg, 0.290 mmol), copper (II) bromide (6.4 mg, 0.0285 mmol), and N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA, 0.0593 mL, 0.284 mmol), and anisole (2.00 mL) under nitrogen. The mixture was subjected to freeze-thaw cycles to eliminate the air contamination, and stirred for 3 h at 95 °C. After small amount of THF, the solution was poured in methanol. The precipitate was filtered, and purified by the reprecipitation in methanol twice to give white powder. The yield was 1.66 g (54.1%).

To a two-necked flask equipped with a stopcock and a condenser were added 2 (0.60 g, 1.57 mmol), sodium t-butoxide (0.152 g, 1.58 mmol), and THF (3.00 mL) under nitrogen. The solution of PS-Br (0.367 g, 0.0857 mmol), palladium (II) acetate (7.1 mg, 0.0176 mmol), and tri-t-butylphosphine (30.0 mL) in THF (2.00 mL) was prepared in another flask equipped with a stopcock under nitrogen, stirred for 30 min at room temperature, and added to the solution of 2 by a syringe in one portion. The mixture was stirred for 24 h at reflux temperature. The solution of diphenylamine (0.0267 g, 0.157 mmol) in THF (0.6 mL) was added to the solution, and additional stirring for 1 h was performed. The resulting solution was poured into methanol, and the precipitate was filtered. The polymer was reprecipitated in acetone. After the filtration and drying, pale greenish polymer was obtained. The yield was 0.568 g, 41.6%).

Aluminum (150 nm) was vacuum-deposited on a well-cleaned glass slide as an anode. Thin charge generating layer was laminated on aluminum by spin-coating from chloroform solution of titanium phthalocyanine at a rate of 2,500 rpm for 30 s. Sequentially, the polymer layer was deposited by bar-coating from 20 wt% of 1,1,2,2-tetrachloroethane solution. The thickness of the polymer layer was in a range of 9 to 10 μm. After the polymer layer was dried in vacuum, the semi-transparent gold layer was sputtered on the polymer layer as a cathode. The pulsed light was illuminated on the device to generate charges in the phthalocyanine layer by a xenon lamp, and the photocurrent was recorded on an oscilloscope.

Prior to preparation of devices, a glass slide with indium tin oxide (ITO) patterns was washed by an alkaline cleaner under sonication and rinsed with deionized water. The substrate was subsequently washed by 2-propanol under sonication, rinsed with clean 2-propanol, and dried with nitrogen. PEDOT:PSS with 30 nm of thickness was spin-coated on the substrate at 2500 rpm for 30 s from the dispersion in water filtered by 0.2 μm of membrane filter, followed by annealing at 200 °C for 1 h. Polymer blend layer was laminated on PEDOT:PSS by spin-coating at 1,000 rpm for 30 s from 1,1,2-trichloroethane solution (10 mg/mL) filtered by 0.2 μm of membrane filter, and annealed at 170 °C for 1 h under nitrogen atmosphere. As a cathode, lithium fluoride with 0.5 nm of thickness followed by aluminum with 100 nm of thickness was vacuum-deposited on the polymer layer at a rate of 0.1 Å/s and a rate of 4.5 Å/s using tantalum and tungsten boats, respectively. A typical size of the photo-active area was 4 mm². The photocurrent-voltage characteristic was measured upon the exposure of the light by a xenon lump with 100 mW/cm².

¹H and ¹³C NMR spectra were obtained on a JEOL ALPHA500 instrument at 500 and 125 MHz, respectively. Deutrated chloroform was used as a solvent with tetramethylsilane as an internal standard. Number- and weight-average molecular weights (M_n and M_w) were determined by gel permeation chromatography (GPC) analysis with a JASCO RI-2031 detector eluted with chloroform at a flow rate of 1.0 mL min⁻¹ and calibrated by standard polystyrene samples. Differential scanning calorimetry (DSC) analyses were performed on a Rigaku DSC-8230 under a nitrogen atmosphere at heating and cooling rates of 10 °C/min. Atomic force microscopy (AFM) measurements were performed on a JEOL JSPM-4200 system in tapping mode (phase and topographic modes) with an MPP-11100-10 silicon probe (resonant frequency: 300 kHz, force constant: 40 N/m). All thin films of polymers were spin-cast onto glass slide by a MIKASA 1H-D7 spin coater from 1,1,2,2-tetrachloroethane solutions at 1,500 rpm for 30 s.