Researchers at Korea's Kumoh National Institute of Technology and Yeungnam University have developed a flexible GaN light-emitting diode (LED) that integrates chemical vapor deposition (CVD) graphene as a transparent current-spreading (TCS) layer on a carbon-supported polyethylene terephthalate (PET) substrate. The device, fabricated using a 2-inch wafer-scale laser lift-off (LLO) process, addresses key limitations of conventional rigid inorganic optoelectronics while maintaining high performance.
The introduction of graphene as the TCS layer plays a central role in improving device efficiency. Compared with a reference flexible GaN LED without graphene, the graphene-integrated device exhibits a 35% increase in electroluminescence intensity at an injection current of 50 mA. Photoluminescence (PL) measurements further confirm enhanced optical performance: the graphene-based LED shows increased PL peak intensity along with a blue shift in peak wavelength, indicating higher output power. In contrast, the reference device exhibits reduced PL intensity and a red shift, consistent with lower emission efficiency.
This performance enhancement is attributed to graphene’s exceptional electrical conductivity, optical transparency, and carrier transport properties. As a two-dimensional carbon lattice, graphene enables efficient lateral current spreading across the device surface, reducing current crowding and improving radiative recombination within the InGaN/GaN multiple quantum wells (MQWs). This leads to more uniform carrier injection and higher light output.
The study also systematically investigates strain-dependent behavior through the piezophototronic effect, which arises in non-centrosymmetric materials such as GaN. Under mechanical deformation, polarization charges are generated due to displacement between positive and negative ion centers in the crystal lattice. These charges modulate the local energy band structure at junction interfaces, directly influencing carrier transport and recombination rates.
In the graphene-integrated flexible LED, strain-induced polarization effects interact with the anisotropic properties of the InGaN/GaN MQWs, resulting in measurable changes in optical output. The modulation of energy bands under strain alters recombination dynamics, allowing the device’s emission characteristics to be tuned mechanically. This coupling of mechanical, electrical, and optical properties highlights the importance of the piezophototronic effect in next-generation flexible optoelectronics.
By combining laser lift-off processed GaN thin films with a graphene TCS layer on a mechanically robust carbon-supported PET substrate, the work demonstrates a viable pathway toward high-performance, flexible inorganic LEDs. The results provide important insight into the design of bendable optoelectronic devices that require both mechanical durability and high luminous efficiency, particularly for applications in wearable displays, integrated photonic systems, and advanced sensing platforms.
