Controlling the Optoelectronics of Poly(3-hexylthiophene-2,5-diyl) Dendritic Star Copolymers on a Polypropylenimine Core as Donor Materials for Organic Solar Cells

The development of novel copolymers as donor materials for organic solar cells and optimization of their properties are particularly interesting, but remain challenging. This research report presents the first study on the effect of copolymerization time on the synthesis of the copolymer poly-(propyleneimine) tetra-(<i>N</i>-methyl-2-pyrrolylmethylene amine)-<i>co</i>-poly-(3-hexylthiophene-2,5-diyl) (P3HT-PP) using the chemical oxidative polymerization method. Through thorough experiments and copolymer characterization, a preferred copolymerization time is proposed, ensuring that the properties of the synthesized copolymers are significantly improved. The structural, optical, morphological, thermal, and rheological properties of synthesized copolymers at different copolymerization times (24, 48, and 72 h) were investigated by using nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis, Raman spectroscopy, small-angle X-ray scattering, rheometry, X-ray diffraction, ultraviolet-visible spectroscopy, and photoluminescence spectroscopy. Additionally, the effects of copolymerization time on the LUMO/HOMO energy levels and charge-transfer processes were examined by using cyclic voltammetry and electrochemical impedance spectroscopy, respectively. It was revealed that the copolymer synthesized for 24 h (P3HT-PP24) has the best properties suitable for organic solar cell applications as a donor material as compared to copolymers synthesized for 48 h (P3HT-PP48) and 72 h (P3HT-PP72). The P3HT-PP24-based organic solar cell exhibited the best photovoltaic performance due to reduced photogenerated charge recombination and more efficient exciton separation.