[Authors] Professor Jaesang Lee, Postdoctoral Researcher. Donghyun Ko, and Undergraduate Chanyong Jeong, SNU ECE Quantitatively identified the percolation mechanism in organic devices by controlling charge flow via guest concentration. Expected to contribute to performance optimization of organic electronic devices such as OLEDs and solar cells. Published in Nano Letters, a top-tier international journal.

Professor Jaesang Lee’s team (Dr. Donghyun Ko, postdoctoral researcher, Chanyong Jeong, undergraduate) quantitatively elucidated the percolative charge-transport mechanism occurring within the host–guest structure of organic semiconductor devices and proposed a new theoretical model to explain it. The study was published on June 26, 2025, in Nano Letters, a leading materials science journal issued by the American Chemical Society. (Paper title: “Percolative Charge Transport in Organic Semiconductor Devices with Host–Guest Layers”)

In organic electronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs), the active layer is typically based on a host–guest molecular blend structure. It is known that the role of guest molecules is not limited to simple emitters or traps; they can also affect charge transport. The study focused in particular on the “percolation” phenomenon, in which, once the guest concentration exceeds a critical threshold, a network forms between guest molecules that allows current to flow.

To this end, the team designed a special unipolar device structure (a hole-only device) that forces current to flow exclusively through the guest molecules, and experimentally measured charge-transport characteristics independently as the guest concentration varied. As a result, they defined the percolation critical concentration as the point at which charge transfer via direct guest–guest hopping accounts for roughly 1% or more of the total device current, and demonstrated that the device’s electrical response changes abruptly at this point.

They also revealed that the deeper the trap depth of the guest molecules, the lower this percolation threshold becomes—because charges tend to reside on the guest rather than the host. Based on these characteristics, the team established a quantitative model that captures the factors governing charge transport and showed that it agrees with the experimental results.

This study is among the first to quantitatively isolate and analyze the complex charge-transport pathways in multicomponent organic devices, providing concrete design criteria for optimizing efficiency by tuning the concentration and energy levels of guest molecules. In particular, by demonstrating the universality of the model at both room and low temperatures and across various host materials, the work highlights its potential as a design guideline for next-generation organic devices such as high-efficiency OLEDs and OPVs.

The research was supported by Samsung Display, the Korea Institute for Advancement of Technology (KIAT), and the National Research Foundation of Korea (NRF).
Image of driving voltage variation and percolation-current activation against guest content in host–guest devices

Source:https://ece.snu.ac.kr/ece/news?md=v&bbsidx=56652
Translated by: Dohyung Kim, English Editor of the Department of Electrical and Computer Engineering, kimdohyung@snu.ac.kr