A domestic research team has developed a groundbreaking "ink" material that can render the organic semiconductor circuits used in lightweight, flexible displays and wearable electronics far more finely and precisely than before. The technology is drawing attention as a key advance that could accelerate the commercialization of next-generation electronics by significantly simplifying the production process for large-area flexible devices.
A team led by Professor Kim Bong-soo of the Department of Chemistry at Ulsan National Institute of Science and Technology (UNIST), in collaboration with a team led by Professor Cho Jeong-ho of Yonsei University, announced Tuesday that they have developed a new photo-crosslinker called "Diazo-6Bx," which shows excellent miscibility with organic semiconductor polymers while efficiently linking polymer chains.
A photo-crosslinker is an essential material for processing organic semiconductors in liquid form. When semiconductor ink containing this material is applied to a substrate and exposed to ultraviolet (UV) light in the shape of a circuit, only the light-exposed areas harden and become fixed. The material is used in "direct photo-patterning" technology, in which a solvent then washes away only the areas that were not exposed to light, leaving a precise circuit behind.
However, the conventional and widely used photo-crosslinker (6Bx) based on perfluorophenyl azide had a critical drawback: its high fluorine (F) content made it difficult to mix with hydrocarbon-based organic semiconductors. As a result, the crosslinker clumped together, causing circuit edges to erode or patterns to become uneven. In addition, the bonds (N-H) left behind after the UV reaction acted as "traps," defects that impede the flow of charge inside the semiconductor and degrade device performance.
The joint research team solved these problems by completely removing the fluorine atoms from the existing crosslinker and designing a new six-branched photo-crosslinker, "Diazo-6Bx," in which the reactive functional group was changed from an azide to a diazo ester group.
The newly developed material is evenly distributed within the ink, and when exposed to UV light, it converts into a highly reactive carbene that penetrates deep between the carbon-hydrogen (C-H) bonds of the polymer, firmly weaving the chains into a three-dimensional net structure. Because it generates only electrically harmless bonds, it can also prevent the trap phenomenon that impedes charge flow.
The results of the team's actual patterning of straight-line circuits using this material were successful. The error deviating from the designed line width dropped sharply from 10.3 micrometers to 2.5 micrometers, roughly one-quarter of the previous level. The angle of the circuit edges, which had been bluntly eroded, steepened from 67.9 degrees to 87 degrees, close to vertical, making the boundaries much sharper.
Device stability also far surpassed that of existing materials. Even under harsh conditions in which a constant voltage was applied to the transistor for 4,000 seconds, the current reduction and the change in threshold voltage were markedly smaller than before.
Notably, by applying this technology, the team succeeded in continuously patterning p-type and n-type polymer semiconductors using the same solvent, without the inconvenience of having to use different solvents. Based on this, they fully implemented an organic thin-film transistor (OTFT) array composed of 84 transistors on a substrate, as well as complementary logic circuits (NOT, NAND, and NOR gates).
"The photo-crosslinker we developed this time is a material that can make organic semiconductor circuits finer and more uniform while also maximizing the operational stability of devices," Professor Kim Bong-soo of UNIST said. "Because p-type and n-type semiconductors can be continuously patterned with the same solvent, it greatly reduces the burden of solvent selection and process design, and will contribute to simplifying the production process for large-area flexible displays and wearable electronics."
Meanwhile, the research was carried out with support from the Samsung Future Technology Fostering Program, the Ministry of Science and ICT, and the National Research Foundation of Korea (NRF). The findings were published in ACS Nano, a globally authoritative academic journal in the nanotechnology field, on the 23rd of last month.
