Development of Benzobisoxazole-Based Novel Conjugated Polymers for Organic Thin-Film Transistors

Benzo[1,2-d:4,5-d′]bis(oxazole) (BBO) is a heterocyclic aromatic ring composed of one benzene ring and two oxazole rings, which has unique advantages on the facile synthesis without any column chromatography purification, high solubility on the common organic solvents and planar fused aromatic ring structure. However, BBO conjugated building block has rarely been used to develop conjugated polymers for organic thin film transistors (OTFTs). Three BBO-based monomers, BBO without π-spacer, BBO with non-alkylated thiophene π-spacer and BBO with alkylated thiophene π-spacer, were newly synthesized and they were copolymerized with a strong electron-donating cyclopentadithiophene conjugated building block to give three p-type BBO-based polymers. The polymer containing non-alkylated thiophene π-spacer showed the highest hole mobility of 2.2 × 10−2 cm2 V−1 s−1, which was 100 times higher than the other polymers. From the 2D grazing incidence X-ray diffraction data and simulated polymeric structures, we found that the intercalation of alkyl side chains on the polymer backbones was crucial to determine the intermolecular ordering in the film states, and the introduction of non-alkylated thiophene π-spacer to polymer backbone was the most effective to promote the intercalation of alkyl side chains in the film states and hole mobility in the devices.

Benzo [1,2-d:4,5-d ]bis(oxazole) (BBO) [30,31] is a heterocyclic aromatic ring composed of one benzene ring and two oxazole rings, which has unique advantages on the facile Scheme 1. Synthetic route for PBC1, PBC2, and PBC3. containing non-alkylated thiophene π-spacer (PBC2) showed strong crystallinity in the film states whereas the BBO-based polymer having alkylated thiophene π-spacer (PBC3) did not show crystallinity in the film state. As a result, the PBC2 polymer showed 100 times higher hole mobility (2.2 × 10 −2 cm 2 ·V −1 ·s −1 ) than the PBC3 polymer (5.0 × 10 −4 cm 2 ·V −1 ·s −1 ). In the case of BBO-based polymer without π-spacer (PBC1), it showed a moderate hole mobility of 9.2 × 10 −4 cm 2 ·V −1 ·s −1 . We found that the intercalation of alkyl side chains on the polymer backbones was important to increase the molecular ordering in the film states and hole mobility in the devices, and the introduction of non-alkylated thiophene π-spacer to polymer backbone was the best to promote the intercalation of alkyl side chains and polymer crystallinity by increasing vacant cites between BBO and CPDT building blocks.  3 (50.5 mg, 5.5 × 10 −2 mmol), p-(o-tolyl) 3 (56.0 mg, 1.8 × 10 −1 mmol) were dissolve in toluene (28.0 mL). The solution was then degassed by bubbling nitrogen through it for 30 min and then added the solution in an N 2 atmosphere flask containing 2-(tributylstannyl)thiophene (1.5 g, 4.05 mmol). The reaction mixture was heated to 120 • C overnight and then it was then extracted with dichloromethane and water. The crude product was purified by filtration on silica gel and then recrystallized to afford compound 4 as a luminous solid (0.600 g, 55.0%). 1 (30 mL) and chloroform (10 mL), and then N-bromosuccinimide (NBS) (0.247 g, 1.390 mmol) was added to the reaction mixture. After it was stirred at room temperature overnight, it was extracted with dichloromethane and water. After removing the solvent, the crude product was purified by filtration on the silica gel. The final monomer 5 was obtained from the recrystallization as a yellow solid. (0.417 g, 80.54%) 1 3 (86 mg, 0.2818 mmol) were dissolved in toluene (33 mL). The solution was then degassed by bubbling nitrogen through it for 30 min, and then trimethyl (4-octylthiophen-2-yl)stannane (2.4291 g, 6.7634 mmol) was added to the reaction mixture. After refluxing at 120 • C overnight, the reaction mixture was extracted with dichloromethane and water, and then dried over MgSO 4 . The resulting crude product was purified by column chromatography (dichloromethane: hexanes = 1:1 v/v%) to give compound 6 as a yellow solid (1.819 g, 83.48%). 1   PBC3 was synthesized by the identical procedure to PBC2. Compound 7 (300 mg, 0.3222 mmol), compound 8 (234.7 mg, 0.3222 mmol), Pd (PPh 3 ) 4 (18.6 mg, 1.61 × 10 −2 mmol), and toluene (6.4 mL) were used for the polymerization. Yield: 277.0 mg (73.2%).

Characterization of the Synthesized Materials
The absorption spectrum of the active layer was measured using a JASCO V-730 UV/VIS spectrometer, 1 H nuclear magnetic resonance (NMR) spectra were recorded at 25 • C using a VNMRS 600 MHz spectrometer (California, USA), Cyclic voltammetry measurements were performed at a scan rate of 20 mV/s using a WonATech potentiostat/galvanostat/impedance analyzer ZIVE SP1 (1A) (Seoul, Republic of Korea), with a three-electrode cell and a 0.1 N Bu 4 NBF 4 solution in acetonitrile as the electrolyte, and the working electrode was coated with the polymer films by dipping them into their solutions in chloroform. Gel permeation chromatography measurements were recorded at 35 • C using an Agilent 1260 Infinity II GPC (California, USA).

Device Fabrication and Characterization
To investigate the electrical properties of the PBC1, PBC2, and PBC3 thin films, OTFTs were fabricated using the top contact/bottom gate configuration. Heavily n-doped Si wafers covered with thermally grown 300 nm thick SiO 2 dielectric layers were used as gate substrates; they were cleaned via piranha treatment, followed by repeated rinsing and sonication with deionized water. To further hydrophilize the as-cleaned SiO 2 surface, an ultraviolet (UV)/ozone treatment was performed for 20 min in a UV/ozone cleaner (PSD PRO-UV, Novascan Tech., Chicago, IL, USA). The substrates were then immersed in an octadecyltrichlorosilane (ODTS) solution (1 vol% in anhydrous toluene) for 1 h in ambient air. PBC1-3 polymer solutions (10 mg mL -1 in chloroform) were spin-coated at 2000 rpm for 45 s on the ODTS-treated Si/SiO 2 substrates. These solutions were prepared and coated onto the substrates in a glove box filled with N 2 gas (O 2 and H 2 O < 5 ppm). Some of the polymer films were thermally annealed for 10 min on a hot plate at various temperatures and cooled slowly to room temperature in a glove box. Next, a 100 nm-thick Au source and drain electrodes were deposited onto the active layers via thermal evaporation using a shadow mask (channel length (L) and width (W) were 100 and 300 µm, respectively).
The electrical properties of the TFTs were characterized using a Keithley 4200 semiconductor parameter analyzer in an N 2 -filled glove box. The field-effect mobility (µ) was extracted from the transfer curve in the saturation using the following equation: and V th denote the drain current, capacitance per unit area (300 nm: 10.0 nF cm -2 ), gate voltage, and threshold voltage, respectively. Twodimensional grazing incidence X-ray diffraction (2D-GIXD) measurements were performed at the 3C beamline of the Pohang Accelerator Laboratory (Pohang, Republic of Korea). A high-resolution synchrotron X-ray beam source (beam energy = 9.8 keV) with a grazing incidence angle of approximately 0.15 • was used to perform the 2D-GIXD experiments. To examine the surface morphologies of the polymer thin films, atomic force microscopy (AFM) analysis was performed using an atomic force microscope (XE-100, Park Systems, Suwon, Republic of Korea).
Three BBO-based novel conjugated polymers were synthesized by Still polycondensation of monomer 8 (CPDT) with monomer 3, 5, and 7, and the resulting polymers were named PBC1, PBC2, and PBC3, respectively. PBC1 is the simplest structure composed of CPDT and BBO building blocks, whereas PBC2 and PBC3 have π-extended polymeric backbone structures with additional thiophene π-spacers. To investigate the effect of the alkyl side chain on the π-spacer, both PBC2 with a non-alkylated thiophene π-spacer and PBC3 with an octylthiophene π-spacer were synthesized. The detailed synthetic routes of PBC1, PBC2, and PBC3 were shown in Scheme 1.
The molecular weights of the synthesized polymers were evaluated using gel permeation chromatography (GPC). The number average molecular weights of PBC1, PBC2, and PBC3 were measured to be 13,400 (Ð = 2.28), 13,500 (Ð = 2.29), and 8400 (Ð = 1.53), respectively, and their GPC spectra were recorded in Figures S9-S11. PBC3 showed slightly lower molecular weight than the other polymers, which is expected to be from the steric hindrance between alkylated monomer 7 and CPDT.

Optical and Electrochemical Properties
The UV-Vis absorption spectra of PBC1, PBC2, and PBC3 were measured in dilute solution and film states. As shown in Figure 1a,b and Table 1, the maximum absorption peaks of PBC1, PBC2, and PBC3 appeared at 564 (606), 544, and 504 nm, respectively, in the solution, and 575 (617), 560 (592) and 530 nm, respectively, in film states. The absorption peaks of PBC1 were the most red-shifted in both solution and film states. This indicates that intramolecular charge transport interaction between CPDT (donor unit) and BBO (acceptor unit) was the most effective without π-spacers. In the comparison of PBC2 and PBC3, the absorption peak of PBC3 containing an alkylated π-spacer was more blue-shifted than that of PBC2 having a non-alkylated π-spacer. Moreover, the PBC3 polymer did not show shoulder absorption in the solution or films. This indicates that the alkyl side chains on the thiophene π-spacer induced a more twisted backbone structure of PBC3 and interrupted molecular ordering of PBC3 in the film states. The absorption peaks of the polymers in film states were more red-shifted than those in the solution, and the degree of red-shift in the absorption maximum peak of PBC1, PBC2, and PBC3 (solution → film) was 11, 47, and 23 nm, respectively. This indicates that all the polymers make better intermolecular interaction in the film states, and notably, PBC2 having a non-alkylated thiophene π-spacer makes the strongest intermolecular interaction and ordering in the film states. which is beneficial for hole transport in the devices. The energy level diagram and CV data were summarized in Figure 1d and Table 1, respectively.

OTFT Properties and Analysis
The top contact/bottom gate OTFTs were fabricated to study the hole-transporting characteristics of the synthesized polymers. The transfer and output characteristics of the OTFTs are shown in Figures 2 and S12, respectively. All the polymers showed typical ptype charge transporting characteristics, and the maximum hole mobility of PBC1, PBC2, and PBC3 in the as-cast films was 1.3 × 10 −4 , 1.5 × 10 −2 and 4.7 × 10 −4 cm 2 /Vs, respectively. Notably, PBC2 showed 100 times higher hole mobility than those PBC1 and PBC3. Since The energy levels of the polymers were measured from cyclic voltammetry (CV) and their cyclic voltammogram were shown in Figure 1c. The energy level of the ferrocene/ferrocenium (Fc/Fc + ) reference electrode was assumed as −4.8 eV to vacuum, and its half-wave potential (E 1/2 ) was measured to be 0.15 V. The highest occupied molecular orbital (HOMO) energy level was estimated from the oxidation onset potential (E ox ) [37,38]. The E ox of PBC1, PBC2, and PBC3 were 0.493, 0.367, and 0.369 V, respectively, which corresponds to the HOMO energy level (E HOMO ) of −5.12, −5.00, and −5.00 eV, respectively. The lowest unoccupied molecular orbital (LUMO) energy level of PBC1, PBC2, and PBC3 was estimated from the reduction onset potential (E re ), and their values were −1.108, −1.160, and −1.139 V, respectively, which correspond to LUMO energy level (E LUMO CV ) of −3.52, −3.47 and −3.49 eV, respectively. The LUMO energy level (E LUMO Opt ) was also calculated from the E HOMO and optical bandgap of the polymers, and the E LUMO Opt of PBC1, PBC2, and PBC3 was −3.26, −3.10, and −3.05 eV, respectively. The values of E LUMO CV and E LUMO Opt are quite different due to the different measuring methods, but their trends are similar to each other. In a comparison of PBC2 and PBC3, their conjugated backbone structures are identical, thus there is no significant difference in the energy levels (−5.0 eV). On the other hand, PBC1 has a deeper HOMO energy level (−5.12 eV) than those PBC2 and PBC3. This indicates that the introduction of an electron-donating thiophene π-spacer to the polymer chain effectively upshifts the HOMO energy level of the polymer, which is beneficial for hole transport in the devices. The energy level diagram and CV data were summarized in Figure 1d and Table 1, respectively.

OTFT Properties and Analysis
The top contact/bottom gate OTFTs were fabricated to study the hole-transporting characteristics of the synthesized polymers. The transfer and output characteristics of the OTFTs are shown in Figure 2 and Figure S12, respectively. All the polymers showed typical p-type charge transporting characteristics, and the maximum hole mobility of PBC1, PBC2, and PBC3 in the as-cast films was 1.3 × 10 −4 , 1.5 × 10 −2 and 4.7 × 10 −4 cm 2 /Vs, respectively. Notably, PBC2 showed 100 times higher hole mobility than those PBC1 and PBC3. Since hole mobility can be changed depending on the surface roughness, morphology, and temperature [39][40][41], the charge-transporting properties of the polymers were further optimized by the post-thermal annealing process from the as-cast films. The hole mobility of PBC1 was significantly increased to 9.2 × 10 −4 cm 2 /Vs via thermal annealing at 200 • C, which indicates that the molecular ordering was highly improved after thermal annealing. On the other hand, the hole mobility of PBC3 was not increased at all under thermal treatment, which implies weak crystallinity of PBC3. In the case of PBC2, the hole mobility was increased up to 2.2 × 10 −2 cm 2 /Vs and the on/off ratio reached 10 5 via thermal annealing at 100 • C. The hole-transporting properties of PBC2 were superior to those of the other polymers, which strongly implies that PBC2 has the most preferred molecular ordering for charge transport in OTFTs. After annealing at 150 • C, lower mobility was observed compared to the as-cast condition. As seen in Figure S13, there was a significant change in surface morphology. At the as-cast film and 100 • C, a granular structure was well-formed, but it was greatly reduced and numerous holes began to appear due to the decomposition at 150 • C. OTFT properties of the polymers are summarized in Table 2. at 100 °C. The hole-transporting properties of PBC2 were superior to those of the other polymers, which strongly implies that PBC2 has the most preferred molecular ordering for charge transport in OTFTs. After annealing at 150 °C, lower mobility was observed compared to the as-cast condition. As seen in Figure S13, there was a significant change in surface morphology. At the as-cast film and 100 °C, a granular structure was wellformed, but it was greatly reduced and numerous holes began to appear due to the decomposition at 150 °C.  To understand the difference in the charge transporting characteristics of the synthesized polymers, 2D grazing incidence wide-angle X-ray diffraction (2D-GIXD) of the polymer films was investigated before and after thermal annealing (Figure 3 and Figure S14-S16) [42]. Figure 3 shows 2D-GIXD images and the corresponding line-cut spectra along the in-plane (q xy ) and out-of-plane (q z ) directions. As shown in Figure 3a-c, it is clearly observed that only PBC2 showed strong molecular ordering and orientation in the film state, whereas PBC1 and PBC3 polymers showed featureless ordering behaviors in the film state.
In detail, as shown in Figure 3b,e,h,k, the lamellar ordering reflection (100, 200) peaks of PBC2 clearly appeared at 0.33 and 0.67 Å −1 along the q xy axis, respectively, which corresponds to d-spacing of 19 Å in the lamellar ordering. The π-π stacking (010) peak was also observed at 1.69 Å −1 along the q z axis, indicating π-π stacking d-spacing of 3.7 Å. After thermal annealing at 100 • C, the intensity of lamellar ordering (100, 200) peaks are increased, and (010) peak was shifted to 1.73 Å −1 (π-π stacking d-spacing: 3.6 Å). This indicates that the lamellar structure of PBC2 is more developed, and π-π stacking interaction among polymer backbones becomes stronger after thermal annealing. This high crystallinity and strong molecular ordering could result in the superior hole mobility of PBC2.
As shown in Figure 3a,d,g,j, the molecular ordering of PBC1 is relatively weak, but it showed the mixed molecular orientation in both face-on and edge-on modes. (100) peaks appeared at 0.35 Å −1 and 0.34 Å −1 along the q xy and q z axis, respectively, and the lamellar d-spacing was 18 Å. (010) peaks were observed at approximately 1.5 Å −1 , indicating ππ stacking d-spacing of 4.2 Å. PBC1 film showed relatively large π-π stacking distance, and weak intermolecular ordering compared to those of PBC2 film. However, similar to PBC2, the lamellar and π-π stacking ordering of PBC1 was effectively enhanced by thermal annealing. As shown in Figure 3g,j, (100) peaks are shifted to~0.37 Å −1 along the q xy and q z axis after annealing at 200 • C, and the new ordering patterns appeared at 0.54 and 1.66 Å −1 along the q xy axis. The thermal annealing on PBC1 films effectively improved molecular ordering and orientation, which is in good agreement with the enhanced hole mobility of PBC1 via thermal treatment. (π-π stacking), respectively. In addition, the thermal annealing process rather reduced the molecular ordering of PBC3 film. This means that PBC3 has the weakest intermolecular interactions and low crystallinity, resulting in the lowest hole mobilities among the three polymers.  The corresponding line-cut profiles of (g,j) PBC1, (h,k) PBC2, and (i,l) PBC3 films along the in-plane (q xy ) axes (g-i) and out-of-plane (q z ) and (j-l).
As shown in Figure 3c,f,i,l, PBC3 showed quite weak (100) and (010) peaks at 0.32 Å −1 and 1.48 Å −1 , respectively, which correspond to d-spacing of 19.6 Å (lamellar) and 4.2 Å (π-π stacking), respectively. In addition, the thermal annealing process rather reduced the molecular ordering of PBC3 film. This means that PBC3 has the weakest intermolecular interactions and low crystallinity, resulting in the lowest hole mobilities among the three polymers.
To understand the relationship between the polymer structure and hole-transporting properties, two repeating units of PBC1, PBC2, and PBC3 were simulated by densityfunctional theory (DFT) (B3LYP/6-31G). As shown in Figure 4, we found that intercalation between the alkyl side chains is present in all polymer films because the length of alkyl side chains from the polymer backbone is approximately 13 Å, and the lamellar d-spacing is 18~19 Å. In the case of PBC1, there are vacancies between alkyl side chains on the polymer backbone and thus the intercalation of alkyl side chains happens to increase the molecular ordering of PBC1. The addition of non-alkylated thiophene π-spacers to the polymer backbones increases these vacancies between CPDT and BBO repeating units, allowing for more efficient intercalation of alkyl side chains and improving intermolecular ordering and crystallinity of PBC2. However, the addition of alkylated thiophene π-spacers to the polymer backbones rather reduces these vacancy sites between CPDT and BBO repeating units, preventing efficient intercalation of alkyl side chains and giving PBC3 low crystallinity. Thus, the molecular ordering and hole mobility was increased in the order of PBC3 < PBC1 < PBC2. The introduction of a non-alkylated π-spacer unit between CPDT and BBO building blocks was crucial to improving the hole mobility of the polymers. The annealing effect was also related to the ordering of alky side chains. In the case of PBC1 and PBC2, thermal annealing makes more optimal intercalation of alkyl side chains, which increases the molecular ordering and hole mobility in the film states. However, in the case of PBC3, thermal energy during the annealing process rather increases the steric repulsion between the alkyl side chains due to the inefficient intercalation of alkyl side chains on the polymer backbones. Thus, only the hole mobilities of PBC1 and PBC2 devices were enhanced via thermal annealing.
Polymers 2023, 15, 1156 11 of 15 To understand the relationship between the polymer structure and hole-transporting properties, two repeating units of PBC1, PBC2, and PBC3 were simulated by densityfunctional theory (DFT) (B3LYP/6-31G). As shown in Figure 4, we found that intercalation between the alkyl side chains is present in all polymer films because the length of alkyl side chains from the polymer backbone is approximately 13 Å, and the lamellar d-spacing is 18~19 Å. In the case of PBC1, there are vacancies between alkyl side chains on the polymer backbone and thus the intercalation of alkyl side chains happens to increase the molecular ordering of PBC1. The addition of non-alkylated thiophene π-spacers to the polymer backbones increases these vacancies between CPDT and BBO repeating units, allowing for more efficient intercalation of alkyl side chains and improving intermolecular ordering and crystallinity of PBC2. However, the addition of alkylated thiophene πspacers to the polymer backbones rather reduces these vacancy sites between CPDT and BBO repeating units, preventing efficient intercalation of alkyl side chains and giving PBC3 low crystallinity. Thus, the molecular ordering and hole mobility was increased in the order of PBC3 < PBC1 < PBC2. The introduction of a non-alkylated π-spacer unit between CPDT and BBO building blocks was crucial to improving the hole mobility of the polymers. The annealing effect was also related to the ordering of alky side chains. In the case of PBC1 and PBC2, thermal annealing makes more optimal intercalation of alkyl side chains, which increases the molecular ordering and hole mobility in the film states. However, in the case of PBC3, thermal energy during the annealing process rather increases the steric repulsion between the alkyl side chains due to the inefficient intercalation of alkyl side chains on the polymer backbones. Thus, only the hole mobilities of PBC1 and PBC2 devices were enhanced via thermal annealing.  Topographic images of the as-cast and thermally annealed PBC1, PBC2, and PBC3 films were measured by atomic force microscopy (AFM) (Figure 5). At as-cast films, PBC1, PBC2, and PBC3 films showed the highest root-mean-square surface roughness (R q ) of 0.31, 1.13, and 1.55 nm, respectively. PBC1 showed highly uniform surface morphology, whereas PBC2 showed highly ordered nanomorphology. In other words, PBC2 films seem to have sizable crystallinity, whereas PBC1 films have weak molecular ordering and crystallinity [43,44]. In the case of PBC3, it showed relatively poor surface images. After thermal annealing, the R q values of PBC1, PBC2, and PBC3 were slightly increased to 0.42, 1.19, and 2.42 nm, respectively, but there was no significant difference. Topographic images of the as-cast and thermally annealed PBC1, PBC2, and PBC3 films were measured by atomic force microscopy (AFM) (Figure 5). At as-cast films, PBC1, PBC2, and PBC3 films showed the highest root-mean-square surface roughness (Rq) of 0.31, 1.13, and 1.55 nm, respectively. PBC1 showed highly uniform surface morphology, whereas PBC2 showed highly ordered nanomorphology. In other words, PBC2 films seem to have sizable crystallinity, whereas PBC1 films have weak molecular ordering and crystallinity [43,44]. In the case of PBC3, it showed relatively poor surface images. After thermal annealing, the Rq values of PBC1, PBC2, and PBC3 were slightly increased to 0.42, 1.19, and 2.42 nm, respectively, but there was no significant difference.

Conclusions
We synthesized three novel BBO-based conjugated polymers to study holetransporting characteristics in OTFTs. PBC1 is the simplest structure composed of CPDT and BBO building blocks, whereas PBC2 and PBC3 have π-extended polymeric backbone structures with additional thiophene π-spacers. To investigate the effect of the alkyl side chain on the π-spacer, both PBC2 with a non-alkylated thiophene π-spacer and PBC3 with an octylthiophene π-spacer were synthesized. The top contact/bottom gate OTFTs were fabricated to study hole transporting characteristics of PBC polymers, and PBC2 devices showed the highest hole mobility of 2.2 × 10 −2 cm 2 V −1 s −1 , which was 100 times higher than the PBC1 and PBC3 devices. PBC1 showed weak molecular ordering in the film states, thus the introduction of a π-spacer unit between CPDT and BBO was important to control the molecular ordering of the polymers. PBC2 showed the strongest molecular ordering and crystallinity, whereas PBC3 almost lose its crystallinity. From the 2D-GIXD and DFT calculation, we found that the non-alkylated thiophene π-spacer provides empty spaces between CPDT and BBO repeating units. Thus, the alkyl side chains on the conjugated backbone can be easily intercalated with each other, which increased the intermolecular ordering and hole mobility. On the other hand, the alkylated thiophene π-spacers fill the empty space between CPDT and BBO repeating units, which prevents intercalation of

Conclusions
We synthesized three novel BBO-based conjugated polymers to study hole-transporting characteristics in OTFTs. PBC1 is the simplest structure composed of CPDT and BBO building blocks, whereas PBC2 and PBC3 have π-extended polymeric backbone structures with additional thiophene π-spacers. To investigate the effect of the alkyl side chain on the π-spacer, both PBC2 with a non-alkylated thiophene π-spacer and PBC3 with an octylthiophene π-spacer were synthesized. The top contact/bottom gate OTFTs were fabricated to study hole transporting characteristics of PBC polymers, and PBC2 devices showed the highest hole mobility of 2.2 × 10 −2 cm 2 V −1 s −1 , which was 100 times higher than the PBC1 and PBC3 devices. PBC1 showed weak molecular ordering in the film states, thus the introduction of a π-spacer unit between CPDT and BBO was important to control the molecular ordering of the polymers. PBC2 showed the strongest molecular ordering and crystallinity, whereas PBC3 almost lose its crystallinity. From the 2D-GIXD and DFT calculation, we found that the non-alkylated thiophene π-spacer provides empty spaces between CPDT and BBO repeating units. Thus, the alkyl side chains on the conjugated backbone can be easily intercalated with each other, which increased the intermolecular ordering and hole mobility. On the other hand, the alkylated thiophene π-spacers fill the empty space between CPDT and BBO repeating units, which prevents intercalation of alkyl side chains on the conjugated backbones and decreases the intermolecular ordering and hole mobility. BBO conjugated building block has great potential to utilize in OTFTs and the selection of conjugated π-spacer in BBO-based polymers is important to improve OTFT performances.