Charge Injection and Photooxidation of Single Conjugated Polymer Molecules

TLDR

Charge transfer and oxidative damage in organic electronic devices involve complex, coupled mechanisms. The study used a sandwich device (Au/TPD/MEH‑PPV:PMMA/SiO₂/ITO) with a SiO₂ charge‑blocking layer to suppress ITO injection. Fluorescence of isolated MEH‑PPV molecules showed bias‑dependent quenching from hole interactions, photobleaching from photooxidation defects, and a novel carrier‑induced repair, revealing a distinct photobleaching mechanism linked to charge transport and persistent photoconductivity.

Abstract

The complex, coupled mechanisms of charge transfer and oxidative damage in organic electronic devices (such as organic light-emitting diodes (OLED), solar cells, etc.) have been elucidated by a new technique that combines single-molecule spectroscopy with charge injection from a metal electrode. The experiments employed a sandwich device architecture (Au/TPD/MEH-PPV:PMMA/SiO2/ITO), essentially a modified OLED with a charge-blocking layer (SiO2) to suppress charge injection at the ITO electrode. The fluorescence (photoluminescence) of isolated MEH-PPV conjugated polymer molecules imbedded in the device was observed to exhibit diverse time- and electrical bias-dependent effects. These include: (i) fluorescence quenching due to interactions between MEH-PPV and holes in the TPD hole-transport layer; (ii) fluorescence quenching, or "photobleaching", due to chemical defects at MEH-PPV generated by photooxidation; and (iii) a novel process, reductive "repair" of the oxidative chemical defects by externally injected carriers. These results demonstrate a very different mechanism for photobleaching of organic conjugated materials than is generally assumed to operate and, furthermore, suggest an intimate relationship among photobleaching, charge transport, and persistent photoconductivity in organic materials.