Efficient Electrical Doping of Organic Semiconductors Via an Orthogonal Liquid‐Liquid Contact

Abstract

Doping is an effective strategy to tune the electrical properties of organic semiconductors. Traditional solution‐processed doping methods, including “host‐dopant mixing‐doping” and “post‐fabrication doping” methods, present challenges for their use in applications in optoelectronic devices. This work reports about a novel method to prepare electrically doped films, the authors call orthogonal liquid‐liquid‐contact (OLLC) doping. In OLLC doping, dopant and polymer semiconductors are dissolved in water and an organic solvent, respectively, and electrical doping occurs during film formation at the orthogonal liquid‐liquid (aqueous‐organic) interface. A large free volume of polymer and dopant in their solutions enables diffusion for effective doping. Thanks to the high surface tension of water, nanometer‐thick polymer films form spontaneously on the aqueous surface and simultaneously get doped. The doped thin polymer films on the aqueous surface can be easily transferred to devices to facilitate hole collection/injection in organic photovoltaics and light‐emitting diodes with solution‐processed top electrodes.

Water Transfer Printing of Multilayered Near‐Infrared Organic Photodetectors

“…Because the surface tension of organic solvent is lower than that of water, the Marangoni force created by the surface tension difference drives the droplet of organic solution to spread on the water surface rapidly. [ 26–28 ] The organic film was formed in a few seconds as the organic solvent evaporates. [ 29 ] Subsequently, we stamped the substrate onto the top of the organic film, and the film was then transferred to the target substrate.…”

The future of solution processing toward organic semiconductor devices: a substrate and integration perspective

“…The film will be successfully transferred to the target substrate and can be used for the subsequent device fabrication. 134 3.1 Surface self-assembly and confined-assembly of organic small molecules Self-assembly is a process of spontaneous assembly under intermolecular interaction force. Now with development, it has become one of the most effective methods for preparing OSC thin films.…”

“…The film will be successfully transferred to the target substrate and can be used for the subsequent device fabrication. 134…”

Increasing Volume in Conjugated Polymers to Facilitate Electrical Doping with Phosphomolybdic Acid

“…25,26 It is worth noting though that PMA dissolved in water has recently been shown to enable electrical p-type doping of conjugated polymers, but such doping takes place during film formation, at the orthogonal liquid−liquid (aqueous−organic) interface, and as such it differs significantly from the postprocessing, sequential doping technique discussed here. 27 To address the first issue indicated above, we assess the stability of the PMA Keggin structure throughout the doping process. Therefore, 31 P nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy experiments are carried out.…”

“…Second, while p-type doping has been demonstrated using PMA dissolved in nitromethane and acetonitrile, only these two solvents enabled p-type doping to take place efficiently following the postprocessing, immersion method. In other words, the application of this method to organic semiconducting films remains solvent-dependent and the reason behind this phenomenon is poorly understood. , It is worth noting though that PMA dissolved in water has recently been shown to enable electrical p-type doping of conjugated polymers, but such doping takes place during film formation, at the orthogonal liquid–liquid (aqueous–organic) interface, and as such it differs significantly from the postprocessing, sequential doping technique discussed here …”