Optical performances degradation of InGaN/GaN MQW LEDs related to fluorescence shift of copolymer-based silicone coating

Abstract

Accelerated ageing tests (1500h/85°C/30mA) performed on packaged InGaN/GaN MQW LEDs have reported a fluorescence shift of silicone oil responsible for optical losses. Electrical and optical characteristics highlight a 65% loss of optical power. Through measurements of the copolymer silicone coating fluorescence emission spectra, we demonstrate that the polymer fluorescence (induced from the blue light emitted from the chip) enlarges the LED emission spectrum (7%) and shifts central wavelength (5 to 7 nm). To understand such a fluorescence shift, Attenuated Total Reflection, Nuclear Magnetic Resonance (NMR), mass spectrometry and Differential Scanning Calorimetry (DSC) have been performed. The copolymer molecular structure has been affected after ageing. Actually, NMR and Mass spectrometry evidences the disappearance of low molecular weight molecules and the presence of high molecular weight molecules after ageing. Such a mechanism is associated with the polymerization of the silicone oil after ageing. Indeed, DSC has confirmed that silicone oil polymerization process is activated by temperature. Finally, both polymerization of the silicone oil due to temperature and fluorescence shift activated by photothermal process have been identified as the main failure mechanisms responsible for optical power degradation. CONTEXT AND OBJECTIVESGaN-based LEDs are currently used in a wide range of applications such as solid-state lighting, backlighting or full-color displays. It is well-known that polymer-based packaging degradation impacts operational reliability of InGaN/GaN Multiple Quantum Well (MQW) LEDs thus limiting their performances. Up to date, the polymer degradation mechanisms are not fully understood. To analyze such limitation, ageing tests have been delimited from both operating conditions and environment. Temperature, current or thermal cycling are the most predominant environmental stresses for the previous quoted applications. This paper mainly focuses on active storage as it is the most destructive one. Thus, primary challenge is to work out failure mechanisms responsible for the optical power decrease when the device is submitted to active storage (1500h/T max /I nominal ). Among the different parts of the packaging, the co-polymer silicone coating is pointed out to be the most sensitive part of the studied GaN-based technology. *raphael.baillot@ims-bordeaux.fr; phone +33 540 002-470; fax +33 556 371-545; www.ims-bordeaux.fr