*2.2. Absorption and Photoluminescence Spectroscopy*

The absorption spectra of **PBDTI-OD**, **PBDTI-DT** and **PTI-DT** in both solutions and spin-coated films were recorded using a Perkin Elmer Lambda 19, UV/Vis/NIR spectrophotometer. The thin films were spin-coated from a CF solution of concentration 10 mg/mL at a spin speed of 1000 rpm and the films were annealed at 85 °C for 10 min. Similarity, their photoluminescence (PL) spectra were recorded both in solutions and spin-coated films using JY Horiba, 'Fluoromax'-4 spectrofluorometer. The absorption and PL spectra in solutions were taken in CF, o-DCB, and cyclohexane (Chex) with polarity indices of 4.1, 2.7 and 0.2, respectively. The PL spectra of the copolymers in solutions and films were taken at excitations matching their longer wavelength electronic transition. The solutions for PL measurement absorb only 0.05 at the maxima to avoid self absorption.

#### *2.3. X-ray Diffraction*

The X-ray diffraction (XRD) patterns of the copolymers were recorded for for 2*θ* values from 2 to 40° in drop-casted films on a glass substrates using Shimadzu Scientific Instruments X-ray diffractometer 7000S with a copper target, Cu-K*α*-radiation with

40 kV × 40 mA power. The interlayer spacing between the planes (*d*) was calculated using Bragg's Equation (1).

$$n\lambda = 2d\sin(\theta),\tag{1}$$

where *n*, *λ* and *θ* are order of diffraction (1), wavelength of the X-ray (0.154 nm) and angle of diffraction, respectively.

#### *2.4. Thermogravimetric Analysis*

The thermal properties of the copolymers were investigated using DTG-60H thermogravimetric analyser in platinum crucibles under nitrogen atmosphere flowing at a rate of 50 mL/min.

#### *2.5. Electrochemistry*

Square wave voltammetry (SWV) was done to measure the frontier molecular orbitals of the copolyers in a three-electrode setup. Platinium electrodes were used both as working and counter electrodes, while Ag/AgCl was used as a reference electrode, which was calibrated against ferocene/ferrocynium ion (Fc/Fc+). The redox potential of Fc/Fc<sup>+</sup> was found to be 0.44 eV versus the reference electrode, taking its energy level to be 4.8 eV below the vacumm. The copolymer films were drop-casted on the Pt disc working electrode, and a 0.1 M solution of tetrabutylammonium hexafluorophosphate (*Bu*4*NPF*6) in anyhdrous acetonitrile served as a supporting electrolyte. The higherst occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were determined from the onsets of oxidation and reduction potentials, respectively, using Equation (2) [14].

$$E\_{\rm HOMO/LIMO} = e(E\_{ox/red} + 4.4)eV \tag{2}$$

#### **3. Results and Discussion**
