Voltage-Gated Dedoping of n-Doped Poly(benzodifurandione) as an Interfacial Protective Mechanism in Electrochromic Devices

Abstract

Electrochromic polymers (ECPs) offer promising applications in dynamic optical devices, but their operational stability is often compromised under overpotential stress. Here, we introduce a transparent, highly conductive interlayer of n-doped poly(benzodifurandione) (n-PBDF) between indium tin oxide and a representative 3,4-propylenedioxythiophene based ECP-Blue (ECP-B). In its doped state, n-PBDF is highly conductive, lowering interfacial resistance and supporting rapid charge injection. Upon de-doping near +0.8 V, it transitions into a resistive barrier that restricts excessive charge flow. Charge density measurements further highlight this protective role as single-layer ECP-B accumulates ∼1.49 mC cm−2 more charge than the bilayer at +1.5 V, reflecting uncontrolled overoxidation, whereas the n-PBDF/ECP layer suppresses it. Long-term cycling confirms that ECP-B loses redox activity under stress, while the bilayer retains the ECP-B onset with a broadened n-PBDF feature. Electrochemical impedance spectroscopy validates this voltage-gated mechanism, showing low resistance at operational bias, a sharp rise (∼60%) during n-PBDF de-doping, and a resistive-to-capacitive transition above +1.2 V. Thin-films spectroelectrochemistry indicates that the bilayer maintains optical contrast comparable to ECP-B across the operating window and, under overpotential, limits additional loss, preserving ∼50% at +1.5 V.