Fast and Selective Photochemical Doping of Spiro‐OMeTAD

ABSTRACT

Oxidizing a redox‐active molecule rapidly and selectively to a targeted oxidation state, without residual dopant ions, is a central challenge in molecular doping. Spiro‐OMeTAD exemplifies it: the standard LiTFSI/O ₂ protocol is slow (>12 h), leaves hygroscopic Li ⁺ aggregates in the film, and yields an uncontrolled +1/+2/+3 mixture. Here we report a biphasic photochemical doping platform, the crown ether/acid‐assisted doping system (CEADS), in which a chlorobenzene solution of spiro‐OMeTAD/LiTFSI is layered above an aqueous phase of concentrated HCl and 12‐crown‐4 (12C4) and irradiated with broadband UV. Doping completes in 60 s, yielding a > 10 ⁴ ‐fold conductivity enhancement (σ = 1.18 × 10 ⁻² S cm ⁻¹ , ≈60‐fold over LiTFSI/O ₂ ) and removing >98% of Li ⁺ from the organic phase. Three sequentially optimized parameters control the spiro‐OMeTAD oxidation manifold: high H ⁺ quenches over‐oxidation, Cl ⁻ provides secondary kinetic modulation at intermediate acid, and 4 equiv 12C4 enforces charge‐balanced phase transfer. Temperature‐dependent conductivity yields an Arrhenius activation energy E _a = 0.19 eV ( R ² = 0.99) over 193–298 K, consistent with reduced Li‐related deep traps. CEADS produces lithium‐free, cleanly +1/+2‐oxidized hole‐transport films on a minute timescale.