Optically invariant InGaN nanowire light-emitting diodes on flexible substrates under mechanical manipulation

Abstract

The integration of GaN-based light-emitting diodes (LEDs) onto flexible platforms provides opportunities for conformal lighting, wearable electronics, and bendable displays. While this technology may enhance the functionality of the light source, the development of flexible GaN LEDs suffers from performance degradation, when mechanical bending is applied during operation. A unique approach to eliminate the degradation employs dot-in-wire structures, using cylindrical light-emitting heterostructures that protrude above the flexible platform, separating the active light-emitting region from the bending substrate. Here, we demonstrate the optical enhancement of nanowire light emitters by changing the geometric orientation within a 1\u2009×\u20091\u2009mm2 array of nanowires on a flexible platform through bending of the substrate platform. The flexible structures were achieved by transferring GaN nanowire LEDs from sapphire substrates onto flexible polyethylene terephthalate (PET) using a “paste-and-cut” integration process. The I–V characteristics of the nanowire LEDs showed negligible change after integration onto the PET, with a turn-on voltage of 2.5\u2009V and a forward current of 400\u2009μA at 4\u2009V. A significant advantage for the nanowire devices on PET was demonstrated by tilting the LEDs through substrate bending that increased the electroluminescence (EL) intensity, while the I–V characteristics and the EL peak position remained constant. Through finite-element analysis and three-dimensional finite-difference time-domain modeling, it was determined that tilting the protruding devices changed the effective distance between the structures, enhancing their electromagnetic coupling to increase light output without affecting the electrical properties or peak emission wavelength of the LEDs.Titled nanowire LED is brighterNanowire LEDs are more robust than thought in their emission wavelengths, light output when put on flexible substrates, even under mechanical bending. A collaborative team led by Prof William S. Wong from University of Waterloo, Canada demonstrates vertical GaN nanowire LEDs on flexible PET substrates and optimize the light output by tuning the geometric orientation and substrate bending. A facile double-transfer method is developed to migrate the nanowires to a flexible PET substrate while keeping the devices in their original orientation and thus the light output insensitive to substrate bending. More interestingly, the light output can be greatly increased by tilting the GaN nanowires by a small angle from the normal plane, due to enhanced electromagnetic coupling. This work presents a viable way to add the flexibility to nanowire LED devices without compromising the performance.