Screen printable graphite-graphene hybrid ink for flexible electronics: Correlating particle size and binary solvent system on conductivity and print quality

The large-scale production of flexible electronics requires cost-effective, high-precision printing processes and optimized conductive inks. Screen printing with carbon-based formulations offers uniformity, scalability, and low production costs; however, achieving the required balance between rheology, conductivity, and mechanical stability remains challenging. This work presents a formulation strategy for graphite-graphene hybrid ink designed for screen printing on PET substrates. We systematically investigate the effects of graphite particle size, graphene content, and a binary solvent system on rheological behavior, print fidelity, and electrical performance. The optimized ink-comprising 200-mesh graphite, 2 wt% graphene, and a 47:13 wt% mixture of cyclohexanone and ethyl acetate - exhibits pronounced shear-thinning (viscosity drop from 318.72 Pa.s to 8.16 Pa.s between 0.1 and 500 s-1), high thixotropic recovery (99.11%), strong adhesion (ASTM D3359, 5B), and a low resistivity of (1.19 ± 0.10) × 10-3 Ω.m at 14.20 ± 2 µm thickness. These features enable sharp pattern definition and mechanical robustness, with only 1.6% resistivity change after 300 bending cycles. The results highlight the synergistic role of particle size control, nanosheet incorporation, and solvent engineering in advancing carbon-based inks for scalable, high-performance flexible electronics.