*3.1. Optical and Electrochemical Properties of PBDTI-OD, PBDTI-DT and PTI-DT*

The absorption spectra of **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** were taken both in CF solutions and as thin films, as shown in Figure 1a,b. The spectra of **PBDTI-OD** and **PBDTI-DT** in CF solutions are characterized by one dominant transition peaking at 631 and 628 nm, respectively, and two peaks below 450 nm. On the other hand, the absorption spectrum of **PTI-DT** is quite different from the BDT-based copolymers where a dominant one transition in solution that peaks at 615 nm and a modest peak at 450 nm are observed. The two-energy-band profile exhibited in **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** is common in push–pull copolymers. The low energy band is due to the *S*<sup>0</sup> → *S*<sup>1</sup> transition, usually with intra-molecular charge transfer (ICT) characteristics due to the charge transfer between the donor and acceptor moieties in the copolymers. On the other hand, the high energy band is due to the *π* − *π*<sup>∗</sup> transition, which is common in the *π*-conjugated polymers due to the alternating single and double bonds in their backbone [15]. The presumably assigned ICT state transition in **PBDTI-OD** and **PBDTI-DT** is red-shifted by more than 11 nm with respect to **PTI-DT**, suggesting a better electron-donating property of the BDT moiety [16]. This could also be due to the long alkyl side chains on the thiophene and isoindigo units of **PTI-DT** that can sterically intract, leading to a twisting in the backbone of the copolymer. Computaion analusis of a similar BDT-based copolymer showed that the copolymers are likely to adopt a planar structure [16] which facilitates ICT in **PBDTI-OD** and **PBDTI-DT** chains.

**Figure 1.** Absorption spectra of **PBDTI-OD** (black), **PBDTI-DT** (red) and **PTI-DT** (green) in (**a**) CF solutions and (**b**) thin films.

The absorption spectra of thin films of **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** were found to be red-shifted by 30, 25 and 40 nm from their solution spectra, respectively. The red-shift in absorbance upon solidification shows a better interchain interaction in the films due to *π* − *π* stacking, which is beneficial for charge transport when the copolymers are used in OSCs. The larger red-shift in absorbance of **PTI-DT** upon solidification indicates that a different intra- and intermolecular interaction from the DBT-based copolymers exists due to the thiophene donor unit. One of the reasons for this could be that **PTI-DT** is twisted in the absence of strong intermolecular interaction in dilute solution. However, the stronger inter-molecular force in the solid films pushes the copolymer into a planar conformation, resulting in a siginicant red-shift in absorption onset [17,18]. Similarly, the smaller absorption onset shift in the films of **PBDTI-DT** compared to **PBDTI-OD** shows the more planar structure of **PBDTI-OD** due to the shorter side chain on the isoindigo unit [19]. The optical band gaps (E*g*) of the three copolymers were calculated from the onsets of absorption spectra of the films and were found to be 1.53, 1.54 and 1.56 eV for **PBDTI-OD**, **PBDTI-DT**, and **PTI-DT**, respectively. The copolymers have a similar bandgap, that is suitable to harvest substantial solar irradiation in the high solar flux region [20]. The backbone structures of the copolymers have negligible effect on their band gaps. The optical properties of the copolymers are summarized in Table 2.



<sup>1</sup> Abs peak shift from solution to film, <sup>2</sup> *Eg* = 1200/*λonset*.

The PL spectra of the thin films were recorded by exciting at their absorption maxima in the longer wavelength region and were found to be Stokes-shifted by 179, 189 and 185 nm in **PBDTI-OD**, **PBDTI-DT** and **PTI-DT**, respectively (see Figure 2). Since an efficeint overalp between the absorption and emission spectra of the copolymers is needed for Förster resonance energy transfer (FRET), the large Stokes shift in the copolymers will inhibit interchain excitation energy transfer [21]. However, the large Stokes shift also shows that multiple processes have taken place to stabilize the first excited state. Since ICT formation in solid films is not efficient, the emission from the first excited state (*S*<sup>0</sup> ← *S*1) is ascribed to tightly bound intrachain exciton relaxation. The strong *π* − *π* interaction upon solidfication of the copolymers enables an efficient interchain charge transfer interaction that dominates over intrachain charge transfer (i.e., ICT population). Our work on three similar copolymers with two and three thiophene donor units, coupled with an isoindgo acceptor unit, showed similar results [7,8].

**Figure 2.** Absorption (solid line) and PL (broken line) spectra of **PBDTI-OD** (black), **PBDTI-DT** (red) and **PTI-DT** (green).

The HOMO and LUMO energy levels of the copolymers were determined using SWV from the onsets of their oxidation and reduction potentials, as shown in Figure 3. The HOMO and LUMO energy levels of the copolymers are summarized in Table 2. The difference in the HOMO levels between the copolymers with the same donor unit (**PBDTI-OD** and **PBDTI-DT**) was 0.2 eV, whereas the difference is only 0.1 eV between **PBDTI-DT** and **PTI-DT** (with the same acceptor unit). The highest LUMO level was found in **PBDTI-OD**, followed by **PBDTI-OD** and **PTI-DT**. The *π*-electron density in the copolymers is not expected to be significantly affected by the alkyl side chains [22]. The reason for the difference in HOMO levels between **PBDTI-OD** and **PBDTI-DT** should, therefore, be due to the effect of the length of the alkyl side chains on the reorganization of the copolymers in the drop-casted films [8,19]. This is expected due to the relatively lower solubility of **PBDTI-OD**.

**Figure 3.** Square wave voltammograms of **PBDTI-OD** (black), **PBDTI-DT** (red) and **PTI-DT** (dark yellow).

The difference in the energetics of **PBDTI-DT** and **PTI-DT** is significantly dependent on the electron-donating strength of BDT vesus thiophene units, respectively. The stronger electron-donating property of BDT would enhance the ICT in **PBDTI-DT**, which will stabilize its LUMO level compared to **PTI-DT**, in which the donor unit is a thiophene [23]. Note here that the reorganization of the copolymers in the casted films also plays a role. The medium-lying HOMO levels of the copolymers are beneficial for attaining high opencircuit voltage when they are used in OSCs as donor materials, since *VOC* <sup>=</sup> *<sup>e</sup>*(*EAcceptor LUMO* − *EDonor HOMO*), where *e* is the elementary charge [24]. The LUMO offset of the copolymers with a fullerene acceptor should be around 0.3 eV for efficient exciton disscociation at the donor/acceptor interface [24,25]. **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** have LUMO offsets of 0.1, 0.2 and 0.3 eV, respectively with respect to the commonly used fullerene acceptor, PC71BM (LUMO = 4.0 eV). The LUMO offsets in the BDT-based copolymers are below the recommended value, which is detrimental to exciton dissociation in fullerene-based OSCs, while efficient exciton dissociation is expected in **PTI-DT**-based OSCs. Hence, **PBDTI-OD** and **PBDTI-DT** might work better when blended with non-fullerene acceptors.

#### *3.2. Photophysical Properties of the Copolymers*

To further understand the photophysics of the copolymers, their absorption and PL spectra were recorded in CF, oDCB and Chex solution in a concentration range of 125–5.7 μg/mL. The absorbances of all the copolymers in the three solvents showed negligible shift with concentration gradient, which indicates the absence of aggregation in the solutions. The absorption spectra of the copolymers in the three solvents of different polarity are almost similar, with a slight difference in the apperance of a shoulder when the poor solvent (Chex) was used. However, the low-energy absorption peaks of the copolymers remained the same with increasing solvent polarity, as shown in Figure 4a–c (solid line). This confirms that the ground states (S0) of the copolymers are non-polar. The PL spectra of **PBDTI-OD** showed a red-shift of 18 nm as the solvent polarity increased from 2.7 (oDCB) to 4.2 (CF). The Stokes shift of **PBDTI-OD** in Chex is higher than both oDCB and CF, despite its low polarity index of 0.2. Similarly, a 29 nm red-shift in PL of **PBDTI-DT** is found with increasing solvent polairty from 2.7 (oDCB) to 4.2 (CF). An ever

larger Stokes shift in was observed when Chex was used as a solvent. The PL spectra of **PTI-DT** showed negligible shifts in the three solvents.

**Figure 4.** Absorption (solid line) and PL (broken line) of (**a**) **PBDTI-OT**, (**b**) **PBDTI-DT** and (**c**) **PTI-DT** in Chex (black), oDCB (red) and CF (dark yellow) solutions.

The red-shift in PL of **PBDTI-OD** and **PBDTI-DT** with increasing solvent polarity while their absorption remained the same confirms the higher dipole moments of their first excited states (S1). This is consistent with our assumption that the longer wavelength region transitions have ICT characterers. Hence, their electronic structure changes from the nonpolar *<sup>D</sup>* <sup>−</sup> *<sup>A</sup>* to the dipolar *<sup>D</sup>*<sup>+</sup> <sup>=</sup> *<sup>A</sup>*<sup>−</sup> configuration [15]. The larger Stokes shift observed in Chex solutions of **PBDTI-OD** and **PBDTI-DT** shows the population of a different state than ICT. Consequently, the electrons in the first excited state of the copolymers have two channels; one is the ICT state and the other is the tightly bound excitonic state. The latter is populated when there is a significant interchain interaction either in films or solutions. A poor solvent like Chex is expected to form aggregates that induce interchain interaction in the solutions, resulting in red-shifted PL. Thus, the emission in the non-polar solvent could be explained by the tightly bound interchain exciton model instead of the commonly used ICT model [26,27]. However, the Uv-Vis absorbances of **PBDTI-OD** and **PBDTI-DT** taken in solutions of similar concentration, as the PL measurements confirmed the absence of aggregation. Therefore, the aggregation effect observed in the Chex solutions should be a self-aggregation, causing the copolymer chains to form tight coils. As a result, the excited electrons could be transferred to the other part of the chain acting as interchain excitons [7,28]. Consequently, the PL spectra were red-shifted despite the low polarity of the solvent.

The photophysics of **PTI-DT** is quite different in that both its ground and exited states are not influenced by the polarities of the solvents. The similar Stokes shift, both in polar (CF and oDCB) and the non-polar (Chex) solvents, shows that self-aggregation of the copolymer chain exists in all solvents. **PTI-DT** showed a large absorption onset shift as well as a peak shift, while the BDT-based copolymers showed no peak shift upon solidification (see Table 2). This can be attributed to the twisting of the thiophene-based copolymer in dilute solution in the absence of a strong intermolecular interaction. The twisting in the backbone of **PTI-DT** favours the self-aggregation effect in any solvent. The ICT population from the first excited state of **PTI-DT** is dominated by the excitonic state, making the emitting state insensitive to polarity of the environment.

In summary, the BDT-based copolymers have a bi-relaxation channel that can be modulated with the solvents, while **PTI-DT** has a quasi-one-relaxation channel. The population of the ICT and tightly bound excitonic states were found in the BDT-based copolymers, while only the latter was found in **PTI-DT**. The results confirm the importance of donor units in the synthesis of D–A copolymers for efficient ICT processes.

### *3.3. Thermal Properties of PBDTI-OT, PBDTI-DT and PTI-DT*

The thermal properties of **PBDTI-OT**, **PBDTI-DT** and **PTI-DT** were studied using TGA, as shown in Figure 5. The three copolymers have agood thermal stabilities, with decomposition temperatures ( T*<sup>D</sup>* = 5% weight loss) above 380 °C. The thermal degradation in these materials follows two steps: one around 380 °C and the other above 550 °C. The first degradation accounts for the breaking up of the side chains. The shorter side chain in the isoindigo unit of **PBDTI-OT** slightly increased its thermal stability compared to **PBDTI-DT**, despite its lower molecular weight. The second thermal degradation is due to the decomposition of the backbone structures of the copolymers. The slight increase in the degradation temperature in **PBDTI-OT** also shows the strong bonding in its backbone structure compared to **PBDTI-DT** and **PTI-DT**. In general, the thermal stabilities of the copolymers make them applicable to flexible OSCs, which are normally processed at temeratures below 150 °C.

**Figure 5.** TGA measurement of **PBDTI-OT** (black), **PBDTI-DT** (red) and **PTI-DT** (green).

#### *3.4. Structural Properties of PBDTI-OT, PBDTI-DT and PTI-DT Films*

The structures of the copolymers were studied by powder XRD in drop-casted thin films, and the results are shown in Figure 6. The XRD traces of the three copolymers exhibit a broad peak (010) centered around 2*θ* = 22° due to the *π* − *π* stacking.

**Figure 6.** XRD traces **PBDTI-OT** (black), **PBDTI-DT** (red) and **PTI-DT** (dark yellow) in drop-casted films.

The calculated *π* − *π* stacking distances using Bragg's equation Equation (1) are 0.38, 0.41 and 0.41 nm in **PBDTI-OT**, **PBDTI-DT** and **PTI-DT** films, respectively, confirming the presence of strong intermolecular interaction in the copolymer backbones [29]. **PBDT-DT** and **PTI-DT** exhibit a (100) diffraction peak at 2*θ* = 3.82° and 3.60°, corresponding to lammelar spacings of 2.3 and 2.5 nm, respectively. The inter-lammelar spacing in **PTI-DT** (2.5 nm) is higher than the well-studied thiophene-based homopolymer, poly-3-hexylthiophne (P3HT) (d = 1.63) [30]. This is due to the longer side chains both in the thiophene and isoindigo units of **PTI-DT** compared to P3HT. Note here that the (100) plane diffraction peak is sharper in **PTI-DT**, indicating its slightly higher crystallinity over the BDT-based copolymer. On the other hand, the BDT-based copolymer with a shorter side chain on the isoindigo unit (**PBDT-OT**) is fully amorphous. The higher crystallity of thiophene-based copolymer is beneficial for improved charge transport.

#### **4. Conclusions**

Three copolymers with BDT (**PBDTI-OD** and **PBDTI-DT**) and thiophene (**PTI-DT**) donor units and isoindigo acceptor units were designed and synthesized using the DAP method. The molecular weights of the BDT-based copolymers were more than two-fold higher than that of **PTI-DT**. The copolymers absorb in a wide range from from 300 to above 760 nm in thin films, making them low-band-gap polymers with E*<sup>g</sup>* ≈ 1.5 eV. The photophysical properties of the copolymers were studied in both films and solutions. Solvent polarity-dependent spectroscopic studies on **PBDTI-OD** and **PBDTI-DT** showed that the ground state is not sensitive to polarity of the environment, while the excited state is. This led us to conclude that the ground states of the BDT-based copolymers are non-polar. The PL spectra of the BDT-based copolymers were red-shifted with increasing solvent polarity of chlorinated solvents, confirming the dipolar characteristics of their first excited states. Therefore, the first excited state has ICT characteristics. However, the Stokes shift in **PBDTI-OD** and **PBDTI-DT** in the non-polar solvent, Chex, was not blue-shifted, as

expected from its low polarity index. Self-aggregation of the copolymers in the poor solvent was found to favour the population of a tightly bound excitonic state, thereby inhibiting the generation of ICT state. The absorption and PL spectra of the thiophene-based copolymer were not influenced by solvent polarity. This led us to conclude that self-aggregation in the thiophene-based copolymer is induced despite the solvent polarity. Therefore, ICT state population in **PTI-DT** is not efficient. The copolymers showed excellent thermal stability, with a decomposition temperature above 380 °C. The XRD patterns of the three copolymers showed a halo from (010) plane due to *π* − *π* stacking. A second diffraction peak was observed in **PBDTI-DT** and **PTI-DT**, confirming a better crystallinity in these copolymers.

**Author Contributions:** Z.A. and W.M. synthesized the copolymer and wrote the synthesis part of the manuscript. N.A.T. characterized the polymers and part of wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by UNESCO and the international Development research Center, Ottawa, Canada, grant number 4500406703.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data will be available based on request.

**Acknowledgments:** This work was carried out with the aid of a grant from UNESCO and the international Development research Center, Ottawa, Canada. The views expressed herein do not necessarily represent those of UNESCO, IDRC or its Board of Governors. Z.A. and W.M. would like to acknowledge the financial support from the international Science Program (ISP), Uppsala, Sweden.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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