Research

[POSTECH-SKKU Joint research team Discovers the Molecular Mechanism Behind Polarity Switching in Conjugated Polymer Semiconductors]

A South Korean research team has, for the first time, uncovered the molecular-level mechanism by which trace amounts of impurities—known as dopants—can reverse charge polarity in organic polymer semiconductors.

A  joint research team led by Professor Kilwon Cho, PhD candidates Eunsol Ok and Sein Chung from the Department of Chemical Engineering at POSTECH, and Professor Boseok Kang from the Department of Nano Engineering at Sungkyunkwan University (SKKU), has revealed at the molecular level how adjusting the concentration of a single dopant enables polymer semiconductors^*1 to switch between positive (p-type) and negative (n-type) conduction^*2. Their findings were recently published in the highly-ranked materials science journal, Advanced Materials.

Semiconductors are core materials that regulate current flow in modern electronic devices. While traditional silicon-based semiconductors offer excellent performance, their rigidity limits their use in emerging applications such as stretchable displays, wearable electronics, and electronic skin. In contrast, organic polymer semiconductors are lightweight and mechanically flexible, making them promising candidates for next-generation electronics.

However, a major challenge has been the limited availability of stable n-type organic semiconductors. Most conjugated polymers naturally exhibit p-type behavior, while existing n-type counterparts often suffer from poor ambient stability. To enable practical applications, a strategy is needed that allows both p-type and n-type functionalities within a single polymer system.

The research team addressed this issue through a phenomenon known as polarity switching. When a typically p-type polymer is doped with a sufficiently high concentration of a p-type dopant such as gold(III) chloride (AuCl₃), the dominant charge carriers shift from holes to electrons. This concentration-dependent polarity reversal allows a single polymer to exhibit both p-type and n-type characteristics—eliminating the need for separate materials or complex multilayer device architectures.

To uncover the underlying mechanism, the team analyzed polymer films doped with AuCl3. They found that the oxidation states of gold and chloride ions evolve during doping, leading to a substitutional chlorination reaction with the polymer chains. This chemical reaction induces structural reordering of the polymer backbone, realigning the molecular structure and reorganizing charge transport pathways, ultimately driving the polarity switching.

Based on this mechanism, the researchers fabricated a p–n organic homojunction diode using a single polymer doped at two different concentrations. The device exhibited a rectification ratio tens of thousands of times greater than conventional single-material organic diodes, highlighting its potential for high-performance, flexible electronic devices with simplified architectures. 

Professor Kilwon Cho and Boseok Kang explained, “Our study is the first to identify the precise chemical and structural mechanism behind polarity switching in polymer semiconductors. This discovery paves the way to precisely control the electrical properties of organic semiconductors, making future electronic devices simpler, more stable, and more efficient.”

The research was supported by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT through several national programs, including the Basic Research Program, Nano & Material Technology Development, Future Technology Laboratories, and the National Core Materials Research Group.

DOI: 10.1002/adma.202505945

1) Polymer Semiconductor: A type of plastic-like material with semiconducting properties. These materials are used in organic electronic devices such as OLEDs, wearable sensors, and energy storage systems.

2) p-type and n-type Semiconductors: Semiconductor classifications based on charge carrier type—p-type conducts through positive "holes," while n-type conducts via negative electrons. Both are essential to forming circuits in electronic devices.