Band Filling, Electrochemical Reaction, and Re-Entrant Insulating Behavior in Electrolyte-Gated BBL Polymer...

Area of Science:

• Materials Science
• Electrochemistry
• Polymer Science

Background:

• Electrochemical doping is crucial for tuning polymer semiconductor properties.
• Understanding charge transport mechanisms in doped polymers is essential for device applications.

Purpose of the Study:

• To investigate the electrochemical doping process of n-type poly(benzimidazobenzophenanthroline) (BBL) using ionic liquids.
• To correlate charge accumulation, redox events, and conductivity changes in BBL films.
• To elucidate the relationship between doping stoichiometry and film structure.

Main Methods:

• Electrochemical doping of BBL films with ionic liquids.
• Simultaneous measurement of drain current-gate voltage (ID-VG) and gate current-gate voltage (IG-VG).
• Integration of IG-VG curves to determine charge accumulation.
• Ex situ grazing incidence wide-angle X-ray scattering (GIWAXS) for structural analysis.

Main Results:

• A conductivity peak was observed during electrochemical doping, linked to electron accumulation in the BBL LUMO.
• Three distinct redox events were identified, corresponding to different polymer salt stoichiometries (2:1 and 1:1).
• Salt formation led to conductivity collapse, with the 1:1 phase being insulating.
• GIWAXS showed initial contraction and maintained lamellar order upon doping.

Conclusions:

• Electrochemical doping of BBL involves a complex interplay between band filling and electrochemical reactions.
• Non-monotonic conductivity behavior as a function of charge density is observed, potentially general for polymer semiconductors.
• The study provides insights into the fundamental charge transport mechanisms in electrochemically doped BBL.