Effects of Nanoscale Morphology on Molecular Doping in Conjugated Polymers

Conjugated polymers are inherently nanostructured, with a mixture of crystalline and amorphous domains, investigating the underlying mechanism of doping benefits from nanoscale real-space imaging. Here, we studied the molecular doping on P(g₃2T-TT) in solution-dripping doping with varying film-casting condition and dopants. Firstly, we perform spectroelectrochemistry to calibrate the optical bleaching to known polaron levels, and calculate the doping efficiency of molecular doping. We further perform functional atomic force microscopy (scanning Kelvin probe and conductive atomic force microscopy) methods to visualize the dopant aggregation and heterogeneity of local extent of doping on CPs. We show that the doping efficiency and dopant aggregation are both correlated with the ability of the dopant/solvent solution to swell the conjugated polymer, with combinations that swell resulting in more efficient doping and smoother films with less aggregation. We use these methods to provide a link between optical measurements of doping and local work function variations for films that have been molecularly and electrochemically doped. These techniques provide spatial mapping of topology with work function and time-resolved doping behavior of high resolution (sub-micron scale). Additionally, we investigated further dopant aggregations upon modifying the solvent of the dopant solution, side-chain and crystallinity of CPs; it reveals that swelling of dopant solution, and crystallinity both govern the aggregation of dopant on CPs, thereby limiting the doping efficiency of the CPs. This work will provide significant insight to understand molecular or electrochemical doping of conducting polymers at the microscopic level.