Revealing the Origin of Polarity Inversion in Polymer Semiconductors

- A key insight into next-generation flexible electronics and thermoelectric devices

▲ (From left to right) Prof. Boseok Kang (Sungkyunkwan University), 

Hoimin Kim (Ph.D. candidate), Prof. Yun-Hi Kim (Gyeongsang National University), 

Landep Ayuningtias (Ph.D. candidate), and Prof. Han-Sol Lee (Gachon University)

A research team led by Prof. Boseok Kang at Sungkyunkwan University has uncovered the origin of polarity inversion—a long-standing phenomenon in polymer semiconductors that occurs only in certain materials—attracting significant attention.

The National Research Foundation of Korea (NRF) announced that the team, in collaboration with Prof. Yun-Hi Kim (Gyeongsang National University) and Prof. Han-Sol Lee (Gachon University), has elucidated the mechanism behind polarity inversion in polymer semiconductors.

This work was supported by the Ministry of Science and ICT (MSIT) of Korea and the NRF, and was published online on February 15 in the journal Advanced Functional Materials.

Polymer semiconductors are considered key materials for next-generation electronics due to their lightweight, flexibility, and solution processability, enabling low-cost fabrication via printing or coating techniques.

It has been reported that increasing the doping level in polymer semiconductors can induce polarity inversion, where charge transport switches from p-type to n-type. This phenomenon enables both p-type and n-type behavior within a single material, simplifying device structures and improving manufacturing efficiency.

However, polarity inversion has been observed only in a limited number of polymers, and the fundamental reason why it occurs in some systems but not others—despite similar doping conditions—remains unclear.

To address this, the research team systematically compared polymer semiconductors with different molecular structures and investigated the conditions required for polarity inversion. They found that polarity inversion occurs only when the amount of dopant absorbed into the polymer film exceeds a critical threshold.

Beyond this level, dopant-derived anions interact strongly with the polymer, altering charge transport behavior and inducing a transition from p-type to n-type conduction. In contrast, when dopant uptake is insufficient, polarity inversion does not occur.

These results reveal that polarity inversion is not determined solely by the doping process itself, but by the polymer’s molecular structure, which governs dopant uptake and polymer–dopant interactions.

This study provides a systematic explanation for why polarity inversion appears only in certain polymers and offers important design guidelines for enabling controllable polarity switching or stable n-type behavior in polymer semiconductors.

The researchers note that further studies are needed to explore a broader range of dopant systems and practical device conditions. Prof. Boseok Kang commented, “The current device performance is still at an early stage, and further improvements will require optimization of both molecular design and device architecture.”

▲ Schematic illustration of p-type to n-type polarity inversion

 in polymer semiconductors as a function of dopant uptake capability