Novel NIR-absorbing benzotrithiophene-based copolymers for organic photovoltaics

Three benzotrithiophene (BTT)-based copolymers were designed and synthesized by engineering alkyl chains on BTT and incorporating them into DPP or DFBT as acceptors. These polymers were fully characterized and the devices fabricated from these polymers were investigated for the correlation of polymer structures with photovoltaic performances. The molecular weight (Mn) and PDI of P1–P3 were measured as being in the range of 28.4–51.4 kDa and the range of 1.3–1.6, respectively. The thermal stability of the polymers as decomposition temperatures at 5% weight loss was found to be between 354 and 377 °C. The absorption spectra of P1, P2, and P3 as a film or in solution covered a broad range of wavelengths, extending to the NIR region. The absorption spectrum of P1 as a film displays the most red-shifted absorption maximum (λ_max) at 799 nm, whereas P2 and P3 showed maxima at 716 nm and 653 nm, respectively. The HOMO levels of P1, P2, and P3 were found to be −5.25, −5.16, and −5.23 eV, respectively, through electrochemical parameters. After being optimized by polymer concentration, additive concentration, and film thickness, the average PCEs of the devices fabricated from the polymer blends were measured as 3.12% for P1, 0.13% for P2, and 0.70% for P3. AFM study revealed that addition of DIO to the blend film of P1 brought a dramatic change in its morphology, from a big granular structure to a smooth and nano-scale phase separation, leading to greatly improving the PCE. However, DIO addition to P2 and P3 did not have a positive effect on the blend film morphologies, which still remained rough and aggregated, leading to inferior PCEs. These AFM results proved the close correlations between the polymer blend morphologies and the corresponding device performances.