Zn–bipyridine structure design for constructing a robust transparent polyimide as the substrate for flexible photodetectors

Abstract

Microelectronic and optoelectronic applications increasingly require colorless polyimide (CPI) films with excellent optical, thermal, and mechanical properties. Considering that the thermal expansion coefficient (CTE) and mechanical strength of CPI as a substrate for flexible optoelectronic devices are insufficient, introducing metal ions into CPI with coordination sites is an effective approach to address this issue. Herein, a bipyridine-containing diamine, 3,3′-bis(trifluoromethyl)-[2,2′-bipyridine]-5,5′-diamine (TFPA), was synthesized as a coordination-enabled monomer, and subsequently copolymerized with 2,2′-bis(trifluoromethyl)benzidine (TFDB) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) to prepare a series of CPI films. Systematic investigations indicate that the electron-withdrawing pyridine nitrogen atoms and their interactions with polymer chains govern the optical, thermal, and mechanical properties of the CPI films, with optimal performance achieved at a TFPA content of 40%. Subsequently, varying amounts of Zn2+ were introduced into the 40% TFPA polymerization system to fabricate transparent PI films featuring Zn-N coordination structures. The optimized PI-4-Zn10 film exhibits a high glass transition temperature (Tg = 351 °C), low thermal expansion coefficient (CTE = 10.0 ppm/K), remarkable tensile modulus (E = 8.2 GPa) and elongation at break (ε = 17.9%), and excellent transmittance (T550 = 86.9%). Flexible photodetectors based on the PI-4-Zn10 deliver a responsivity of 2.51 mA W⁻1 and a detectivity of 3.74 × 109 Jones at 850 nm, together with stable performance under bending and enabling successful optical communication demonstrations. This work demonstrates a viable strategy for constructing high-performance polyimide substrates for flexible optoelectronic devices.