Local Order Is Not Enough: Synthetically Accessible Polymers Reveal the Role of Side-chain Branching Point in Organic Solar Cell and Transistor Performance

Abstract

[Abstract]: Side-chain engineering provides a powerful strategy for controlling the microstructure and electronic properties of conjugated polymers, but improved crystalline order does not necessarily translate into enhanced device performance. To investigate this, a homologous series of donor polymers, FO6-T, 3-FO6-T, 4-FO6-T, and 5-FO10-T, in which the branching point is moved progressively away from the backbone, were synthesized using a two-step protocol. Thin-film microstructure was analyzed by grazing-incidence wide-angle X-ray scattering and atomic force microscopy, and optoelectronic properties by optical spectroscopy and electrochemical measurements. Shifting the branching point further from the backbone improved local crystal quality, evidenced by increased coherence lengths and reduced paracrystalline disorder, but resulted in a progressive deterioration in organic solar cell and thin-film transistor performance. This counterintuitive result is explained by the interplay between overall crystallinity and polymer orientation. FO6-T combines the highest overall crystallinity with a predominantly face-on orientation and outperforms the other polymers in both devices. The latter exhibit either insufficient overall crystallinity or unfavorable edge-on orientation, despite superior local crystal order. These findings demonstrate that local crystal quality alone does not ensure high device performance and highlight the branching point as a critical parameter for balancing crystallinity, orientation, and electronic coupling.