Synthesis, characterization, and photovoltaic performance of the polymers based on thiophene-2,5-bis((2-ethylhexyl)oxy) benzene-thiophene

• Introducing TBT unit as D unit for D–A conjugated polymers.
• The relationship between the chemical structure and the aggregation structure.
• The effects of different A units on the morphology of the photoactive layers.

Three novel conjugated copolymers based on thiophene-2,5-bis((2-ethylhexyl)oxy)benzene-thiophene (TBT) as electron-donating units, either isoindigo or both isoindigo and diketopyrrolopyrrole (DPP) as electron-withdrawing units have been designed and synthesized by Stille-coupling reaction. All the polymers exhibit high thermal stability, broad absorption in the range of 300–800 nm, and the low-lying energy level of highest occupied molecular orbits (HOMO) (−5.47 to −5.19 eV). After introduced with additional hexylthiophenes and further introduced with DPP units, the polymers PTBT-HTID and PTBT-HTID-DPP show smaller lamellar distance and π–π stacking distance, and the morphology of the corresponding photoactive layers possess more appropriate microphase separation and smaller domain size, which lead to high short circuit current densities (Jsc) and power conversion efficiency (PCE). The polymer photovoltaic devices based on PTBT-HTID-DPP/PC61BM exhibit a high Jsc value of 11.13 mA cm−2, a fill factor (FF) of 0.57, and the PCE of 4.2%.

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