**1. Introduction**

The introduction of *π*-conjuageted polymers to the fast-growing solar cell technology has brought forth new features of flexibility and solution processabiltiy in the so-called organic solar cells (OSCs) devices. OSCs have shown a leap in efficiency in the last five years to above 18% in single-junction [1], and over 17% in multi-junction [2], devices. This tremendous improvement is mainly attributed to the introduction of new donor polymers and non-fullerene acceptors that can harvest the broad solar spectrum with appropriate energetics in the active layer. A vast number of polymers have been synthesized and characterized to enrich the structure-property relation recipe for efficient OSCs. Alternating donor–acceptor (D–A) copolymers are commonly synthesized to obtain low-band-gap polymers with good solar harvest that extends to the near-infra-red region. Some important parameters that need to be addressed during the synthesis of the D–A copolymers include the HOMO level, which should be lower-lying, with the LUMO lying in the region for efficient exciton dissociation.

The synthesis of low-band-gap polymers using a simple and economically viable technique has gained the attention of researchers due to its potential for industrialization of the OSC technology. The direct arylation polycondensation method (DAP) has notably shortened the synthesis route compared to the commonly used Suzuki and Stille crosscoupling techniques. In this reaction, the unsubstituted arylene monomers are directly

**Citation:** Tegegne, N.A.; Abdissa, Z.; Mammo, W. Photophysical, Thermal and Structural Properties of Thiophene and Benzodithiophene-Based Copolymers Synthesized by Direct Arylation Polycondensation Method. *Polymers* **2021**, *13*, 1151. https://doi.org/10.3390/ polym13071151

Academic Editor: Edina Rusen

Received: 28 February 2021 Accepted: 22 March 2021 Published: 4 April 2021

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coupled with dihaloarylene units with the help of transition metals—usually palladium acetate. The by-product at each of the synthesis steps is, therefore, drastically reduced, making the DAP method more environmentally friendly and economically viable. In addition, the DAP method made the synthesis of polymers that were difficult to prepare with the traditional Suzuki and Stille cross-coupling techniques possible [3]. The mass production of OSCs will need an enormous amount of donor and acceptor organic materials. An easy and fast synthesis method such as DAP is undoubtedly important in the realization of the commercialization of OSC technology. Despite its many interesting merits, the method has a drawback: its selectivity between C–H bonds for some monomers is poor. Consequently, the method is more commonly used to synthesize homopolymers like poly-3-hexylthiophene (P3HT) and benzodithiophene (BDT) [4–6]. However, D–A copolymers synthesized by DAP method are limited. Recently, our group and others have reported the successful synthesis of D–A copolymers using the DAP method [7–10].

The choice of D and A units in the synthesis of D–A copolymers determines their electrical, optical and structural properties, which that will subsequently determine the performance of the OSC that the materials are used for. Isoindigo is a widely used acceptor unit due to its good electron-withdrawing properties. The fused ring benzo[1,2-b:4,5 b ]dithiophene (BDTs) and thiophene units are the most commonly used donor units in the synthesis of D–A copolymers that produced high power conversion efficeny (PCE) of over 11% in OSCs [11]. These polymers are usually contain aloxy or alkyl side chains to enhance their solubility. A basic understanding of the structure–property relationships of the polymers is vital for the optimization of their structure for higher PCE of OSCs. The optical properties, intra/inter-chain interactions in the polymers, and their morphologies in the films are some of the parameters that play important roles in the photogeneration of charges in OSCs. In addition, their thermal properties determine their lifetimes in harsh environment conditions, and also their application in flexible OSC devices, which are normally processed at temperatures below 150 °C.

In this work, we report the synthesis of three D–A copolymers based on an isoindigo acceptor unit. The two copolymers have alkyl-substituted BDT (**PBDTI-OD** and **PBDTI-DT**) donor units, while in the third copolymer (**PTI-DT**), an alkyl-substituted thiophene unit is used. The structures of the BDT-based copolymers were systematically tailored by increasing the side chain length in the isoindigo units from 2-octyldodecyl (OD) in **PBDTI-OD** to 2-decyltetradecyl (DT) in **PBDTI-DT**, while keeping the backbone structure the same. The third copolymer, **PTI-DT**, was synthesized to study the effect of electron-donating groups by changing the donor units from BDT to thiophene. The structural differences in the three copolymers allowed us to investigate the effect of backbone structure on their photo-physics, thermal and structural properties. We found two relaxation channels in the BDT-based copolymers, while a one-channel relaxation was found in **PTI-DT**. The copolymers showed excellent thermal stability with decomposition temperatures (T*<sup>D</sup>* = 5% mass loss) above 380 °C. The X-ray diffraction patterns of the copolymers suggested that a better structural ordering was found in the thiophene-based copolymer.

#### **2. Materials and Methods**

#### *2.1. Materials and Synthesis*

Three copolymers based on an isoindigo acceptor were designed and synthesized using a direct heteroarylation polymerization (DAP) method for the first time. The two copolymers were synthesized by the copolymerization of a 4,8-bis(decyl)benzo[1,2-b;4,5 b ]dithiophene (BDT) (**1**) donor unit and an isoindigo acceptor unit. N-Alkylation of the isoindigo moiety was achieved with 2-octyldecylbromide and 2-decyltetradecylbromide to obtain compounds **2** and **3** which were used to synthesize **PBDTI-OD** and **PBDTI-DT**, respectively, as shown in Scheme 1. In the synthesis of **PBDTI-OD**/**PBDTI-D**, a 1:1 ratio of BDT (**1**) (0.5 mmol) and isoindigo (**2/3**) (0.5 mmol) was copolymerized using palladium acetate, (0.01 mmol), pivalic acid (0.15 mmol) and potassium carbonate (1.3 mmol) as the catalyst, acidic additive and base, respectively. The polymerization reaction took place

in an inert atmosphere at a temperature of 100 °C using anhydrous dimethylacetamide (DMAc) for **PBDTI-DT** and a mixture of DMAc and toluene as the reaction solvents for **PBDTI-OD**. The heating was stopped after 7 and 19 h for **PBDTI-OD** and **PBDTI-DT**, respectively, and the solutions were left to cool down to room temperature. The copolymers were precipitated in methanol and filtered. The crude copolymers were dissolved in chloroform (CF) and washed with aqueous solution of ethylene-diamine tatraacetic acid (EDTA) (pH = 8) to remove trace metals, followed by 0.1 N HCl and distilled water. **PBDTI-OD** was Soxhlet-extracted with methanol, hexane, diethylether and chloroform while **PBDTI-DT** was extracted with methanol, acetone, ethylacetate, hexane, diethylether and chloroform. The CF extract of **PBDTI-OD** was concentrated and precipitated in acetone and filtered. After drying the polymer at 40 °C in a vacuum oven for 24 h, the solid obtained was further stirred in hexane overnight and filtered to obtain 0.36 g blue-black copolymer (56%). Similarly, the CF extract of **PBDTI-DT** was concentrated precipitated in methanol, filtered, dried in an oven at 40 °C for 24 h to afford 0.32 g dark brown solid (43%).

**Scheme 1.** Syntheses scheme of **PBDTI-OD** and **PBDTI-DT**.

**PTI-DT** was synthesized by copolymerizing 3-octylthiophene (**4**, 0.5 mmol) with isoindigo (**3**), as shown in Scheme 2. The polymerization was done in anhydrous DMAc for 3.5 h. Following the same procedure, the crude polymer was extracted. **PTI-DT** was Soxhlet-extracted in methanol, acetone, ethyl acetate and CF, which afforded 0.37 g (66%) dark-brown solid after drying in an oven at 40 °C overnight. Note here that the long side chains attached to both the donor and acceptor units of the copolymers increased their solubilities in chlorinated solvents like CF and 1,2-dichlorobenzene (oDCB). Comparing the three copolymers, **PBDTI-OD** was found to be less soluble in these chlorinated solvents like CF and oDCB. On the other hand, introduction of the long side chains might create steric hindrance that might twist the backbones of the copolymers. However, BDT has a planar structure that is expected to make the backbones of **PBDTI-OD** and **PBDTI-DT** less twisted and have a longer conjugation length than the thiophene-based copolymer, **PTI-DT** [12,13].

**Scheme 2.** Synthesis scheme of **PTI-DT**.

The molecular weights of the copolymers weree determined by size exclusion chromatography was done using a Waters Alliance GPVC2000 instrument with a refractive index detector. The columns used were Waters Stragel® HMW 6EX2 and Waters Stragel® HT 6EX1 and the measurements were taken in 1,2 4-trichlorobenzene at a temperature of 135 °C. The relative molecular mass was measured against polystyrene standards. The average molecular weights (M*w*) of **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** were found to be 53,461, 71,234 and 23,574 g/mol, respectively. The high M*w*s of the copolymers are due to the long alkyl side chains appended both in the donor and acceptor units. The calculated poly-dispersity indices (PDI = M*w*/M*n*) of the three copolymers show that **PTI-DT** has a more uniform molecular weight distribution. The molecular weight and yield data are summarized in Table 1.


**Table 1.** Molecular weight and yield data of **PBDTI-OD**, **PBDTI-DT** and **PTI-DT**.
