Objective The third generation of lighting source, light emitting diode (LED), has the advantages of energy savings and environmental protection, no startup delay, low energy consumption, and long life. As a result, it is widely used in traffic signal display, street lighting, car lighting, home lighting, high-power stadium lighting, liquid crystal display backlight, full-color display, and other fields. Currently, the white LED which is composed of blue LED chips and cerium-doped yttrium aluminum garnet (Ce∶Y3Al5O12, abbreviated as Ce∶YAG) yellow phosphor conversion materials is mainly used for white light illumination and display. Part of the blue light emitted by the blue LED is absorbed by the cerium ion in the yttrium aluminum garnet and converted into yellow light, which is mixed with the remaining blue light to form white light. As the power of the blue LED chips increases, the requirements for the luminescence performance and thermal stability of yellow phosphor conversion materials also increase. Yellow phosphor conversion materials are mainly phosphor powder, phosphor glass, phosphor crystal, and phosphor ceramics. Researchers at home and abroad have conducted several in-depth studies on the luminescence properties of phosphor conversion materials. However, there are only a few reports on the effect of excitation conditions and environment on the luminescence performance of blue LED chips and then on the luminescence of white LED chips.
Methods The changing trend of blue LED chip electroluminescence (EL) spectra is studied based on two excitation modes—constant temperature and constant current. Ce∶YAG phosphor ceramic has been made and processed to a suitable size and encapsulated on the blue chip to form a white LED. The white LED is fixed inside the integrating sphere’s bracket to perform a constant temperature and variable current excitation test. The ambient temperature of the test was 15 ℃, and a pulse driving current of 150--1000 mA was applied. The photoluminescence (PL) spectra and the photoelectric data of the white LED with constant temperature and variable currents were measured using the HAAS-2000 high-precision rapid spectroradiometer. The white LED was fixed on the temperature control bracket inside the integrating sphere to perform the excitation test with constant current and variable temperatures. Pulse currents of 500 and 1000 mA were selected, and a variable temperature environment of 15--145 ℃ was applied to the temperature control bracket. The PL spectra and the optoelectronic data of the white LED under the constant current and variable temperature conditions were measured using the HAAS-2000 high-precision rapid spectroradiometer. The results have a reference value for the blue LED control and white LED encapsulation.
Results and Discussions The EL spectra of the blue LED chip and the PL spectra of the white LED (Ce∶YAG phosphor ceramic and blue LED chip package) are studied with two modes of constant temperature and constant current. The results show that the emission peak of the blue LED chip generates a blue shift with the increase of excitation current at a certain temperature (Fig. 1). Under the condition of constant excitation current, with the increase of temperature, the emission peak of the blue LED chip appears red shift (Fig. 3). Under constant temperature conditions, the emission peak of white LED does not change with the increase of excitation current. However, the relative intensity of the emission peak decreases with the increase of current (Fig. 6). When the excitation current of the white LED is a fixed value, the emission peak of the phosphor ceramics is redshifted with the increase of ambient temperature. The relative intensity of the emission peak increases with the increase of ambient temperature (Fig. 8). With the increase of the excitation current, the color temperature of white LED increased. When the ambient temperature is less than 100 ℃, the color temperature of white LED does not change significantly, while when the temperature exceeds 100 ℃, the color temperature increases significantly (Fig. 9).
Conclusions Under constant temperature conditions, the emission peak of a white LED does not change as the excitation current increases; whereas, the relative intensity decreases. When the excitation current of a white LED remains constant, the emission peak of phosphor ceramics redshifts as the ambient temperature increases. As a result, the relative intensity of the emission peak increases with the increase of the ambient temperature. Therefore, the working conditions and the environment of white LED lighting affect the optical parameters of the white lighting products. For the application and the quality management of lighting products, it is necessary to strictly match the absorption and emission wavelengths of phosphor conversion materials and blue light LED chips, as well as pay attention to the influence of working environment temperature on product performance.
