Organic semiconductors (OSCs) continue to move from the laboratory to industrial applications. They are lightweight, flexible, and much less expensive than their inorganic counterparts. By chemical manipulation, their properties can be optimized for a chosen application. Presently, they are used in devices like organic light-emitting diodes (OLEDs) in high-resolution displays, organic radio-frequency identification tags, organic photovoltaic cells, and printed batteries. They can even lead to biocompatible and biodegradable electronics. However, OSCs have one drawback that needs to be resolved—charge carrier transport is not efficient, and extra charge carriers must be provided by means of “electronic-doping.”
Recent work reported in Nature Materials (https://doi.org/10.1038/s41563-021-00980-x), by an international group of researchers, introduced an easy and clean way of OSC doping, which works with a large number of OSCs and is inexpensive.
The key innovation is the use of an adduct—a single reaction product resulting from the addition of two or more distinct molecules—instead of the molecular and metal complex-based dopants currently used. Here, the adduct-forming agent is a mixture of dimethyl sulfoxide with hydrobromic acid (HBr) in a ~1:2 molar ratio. When this agent is added to a solution of an organic hole-transporting material (HTM), doping occurs through a reaction that yields an oxidized HTM molecule, a Br anion, that stabilizes the positive charge on the HTM molecule, and all other reaction products leave during film formation.
This p-type doping process, inferred from in situ attenuated total reflection Fourier transform infrared spectroscopy, increased the conductivity of six different HTM materials by 2–3 orders of magnitude. Furthermore, the doped HTM films were thermally stable up to 100°C, which opens the door for typical industrial processes. The chemicals used to form the adduct-dopant are inexpensive and the process is clean because the byproducts do not remain on the organic thin film.
For more complex device architectures, the process can even be tweaked to obtain graded (asymmetric) doping in a device stack, or selective doping on one side only of a charge-conducting layer. The applicability of the new p-doping scheme is shown for a variety of organic HTMs, ranging from small molecules to polymers. In addition to the solution-based p-doping process, the method can be used in vapor-phase processing, providing new opportunities for advanced fabrication processes.
With three different types of optoelectronic devices—organic thin-film transistors, perovskite solar cells, and OLEDs—the researchers showed the beneficial effect of their doping approach for HTM layers.
Pabitra K. Nayak of the University of Oxford and Tata Institute of Fundamental Research, a corresponding author of the article, tells MRS Bulletin, “Our method can be extended to other adduct systems with different sulfoxide-containing molecules in combination with different activators. This work opens up a new area of research and will also stimulate industrial activity.”
According to Norbert Koch from the Integrative Research Institute for the Sciences at Humboldt-Universität zu Berlin (who was not involved in the research), this work represents an important milestone in the field of OSC electronic doping.
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Balasubramanian, N. Adduct-based p-doping of organic semiconductors increases conductivity. MRS Bulletin (2021). https://doi.org/10.1557/s43577-021-00178-1
- Composition & Microstructure Chemical Element Br
- Composition & Microstructure Chemical Element H
- Composition & Microstructure Material Type organic
- Composition & Microstructure Material Type perovskites
- Performance Functionality dopant
- Performance Functionality semiconducting
- Properties Transport electronic structure