crystals Article Novel Method for Estimating Phosphor Conversion Efficiency of Light-Emitting Diodes Chih-Hao Lin 1 , Che-Hsuan Huang 1, Yung-Min Pai 1 , Chung-Fu Lee 1, Chien-Chung Lin 2, Chia-Wei Sun 1, Cheng-Huan Chen 1, Chin-Wei Sher 1,3,* and Hao-Chung Kuo 1,* 1 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] (C.-H.L.); [email protected] (C.-H.H.); [email protected] (Y.-M.P.); [email protected] (C.-F.L.); [email protected] (C.-W.S.); [email protected] (C.-H.C.) 2 Institute of Photonic System, National Chiao Tung University, Tainan 711, Taiwan; [email protected] 3 Fok Ying Tung Research Institute, Hong Kong University of Science and Technology, Hong Kong 511458, China * Correspondence: [email protected] (C.-W.S.); [email protected] (H.-C.K.) Received: 23 October 2018; Accepted: 26 November 2018; Published: 27 November 2018 Abstract: This study presents a novel method for estimating the phosphor conversion efficiency of white light-emitting diodes (WLEDs) with different ratios of phosphors. Numerous attempts have been made for predicting the phosphor conversion efficiency of WLEDs using Monte Carlo ray tracing and the Mie scattering theory. However, because efficiency depends on the phosphor concentration, obtaining a tight match between this model and the experimental results remains a major challenge. An accurate prediction depends on various parameters, including particle size, morphology, and packaging process criteria. Therefore, we developed an efficient model that can successfully correlate the total absorption ratio to the phosphor concentration using a simple equation for estimating the spectra and lumen output. The novel and efficient method proposed here can accelerate WLED development by reducing costs and saving fabrication time. Keywords: light emitting diode; electro-optical devices 1. Introduction Recently, light-emitting diodes (LEDs) have gained considerable attention because of their long-lifetime, high-efficiency, and energy-saving properties which make them good candidates for solid-state lighting [1–3]. White LEDs (WLEDs) through epitaxy using GaN-based chips have demonstrated great improvements in efficiency [4–8]. The basic concept underlying WLED fabrication involves coating blue LEDs (BLEDs) with a yellow light-emitting color-conversion element, such as phosphors or organic dyes. This has become the most common approach for white light generation [9]. To achieve different color gamut, color rendering index (CRI), light quality, and efficiency specifications for various applications, many novel phosphors that increase the efficiency or color performance of LEDs have been developed [10–13]. Among the several phosphors for LED applications, yttrium aluminum garnet (YAG)-, silicate-, and nitride-based phosphors are used most widely in WLEDs because of their well developed fabrication processes and outstanding features. Several studies have focused on optimizing the package design of LEDs to improve their efficiency and color quality. The parameters of the LED packaging depend on the type of phosphor and dopant concentration used. These properties considerably affect the LEDs’ efficiency, correlated color temperature (CCT), and CRI [14–17]. For practical applications, simulating the spectrum or chromatic performance of LEDs Crystals 2018, 8, 442; doi:10.3390/cryst8120442 www.mdpi.com/journal/crystals Crystals 2018, 8, x FOR PEER REVIEW 2 of 8 have investigated models based on Monte Carlo ray tracing combined with the Mie scattering theory [18–23]. However, accurate prediction results have been difficult to achieve, regardless of the parametersCrystals used,2018, 8, 442including the absorption coefficient, particle size distribution, phosphor2 of density, 9 scattering model, and phosphor morphology. Numerous attempts to develop an accurate simulation platformunder have various provided design conditionssuboptimal is necessary.results, thereby Many researchers indicating have that investigated accurate models simulation based onrequires substantialMonte effort Carlo and ray tracing experience combined because with the the Mie micros scatteringcope theory properties [18–23]. of However, phosphor accurate are unavoidable prediction for accurateresults simulation have been [24–28]. difficult toDe achieve,spite these regardless efforts, of the predicting parameters th used,e spectrum including precisely the absorption when the packagecoefficient, design particleor type size of distribution,phosphor phosphorchanges density,in real scatteringcases remains model, anddifficult. phosphor Different morphology. phosphors have differentNumerous crystal attempts phases, to develop particle an accurate size simulationdistributions, platform and have morphologies, provided suboptimal which results,are hard to thereby indicating that accurate simulation requires substantial effort and experience because the measured and define specifications for. The phosphor particle size distribution and morphology are microscope properties of phosphor are unavoidable for accurate simulation [24–28]. Despite these the mainefforts, factors predicting affecting the spectrum the accuracy precisely of whena simula the packagetion model. design However, or type of phosphor even when changes all the in real essential informationcases remains from the difficult. emitting Different materials phosphors is meas have differentured to crystalpredict phases, the performance particle size distributions, of WLEDs, the simulationand morphologies, remains restricted which are to harda narrow to measured range, and su definech as specificationssingle-package for. Thegeometry, phosphor as particle well as the fixed physicalsize distribution properties and morphology of phosphor. are theTherefore, main factors this affecting paper pres the accuracyents a novel of a simulation method model. based on a wavelengthHowever, conversion even when concept all the essential instead information of a traditional from the complex emitting simulation materials is measured in order toto predict increase the consistencythe performance between ofpredicted WLEDs, the results simulation and remainsexperime restrictedntal data. to a narrowThe proposed range, such wavelength as single-package conversion geometry, as well as the fixed physical properties of phosphor. Therefore, this paper presents a novel method is more suitable than microcosmic concepts involving complicated scattering cases, which method based on a wavelength conversion concept instead of a traditional complex simulation in lead toorder unpredictable to increase simulation the consistency results. between predicted results and experimental data. The proposed Awavelength novel method conversion based method on the is moredown-conversion suitable than microcosmic luminescence concepts efficiency involving (DCLE) complicated concept is employedscattering to predict cases, which the leadperformance to unpredictable of the simulation LED, including results. the pumping source and emitted photons forA novelany methodpackage based design. on the The down-conversion major difference luminescence between efficiency this model (DCLE) and concept traditional is simulationsemployed is that to predict it is easier the performance to measure of thethe LED, parameters including required the pumping when source using and out emitted method, photons and it is more straightforwardfor any package design. to correlate The major the difference total absorpti betweenon this ratio model to and the traditional phosphor simulations concentration is that it in real is easier to measure the parameters required when using out method, and it is more straightforward applications. We believe that this method will accelerate the development of WLED applications to correlate the total absorption ratio to the phosphor concentration in real applications. We believe becausethat of thisits simplicity, method will acceleratehigh accuracy, the development and lack ofof WLEDrigorous applications restrictions. because of its simplicity, high accuracy, and lack of rigorous restrictions. 2. DCLE Concept for Phosphor Conversion System 2. DCLE Concept for Phosphor Conversion System Figure 1 is the schematic of a WLED used for the novel model in this study. A dispensing Figure1 is the schematic of a WLED used for the novel model in this study. A dispensing package package was used as the template. In general, the white light emitted by a WLED comprises blue was used as the template. In general, the white light emitted by a WLED comprises blue photons and photonsfluorescent and fluorescent yellow rays yellow emitted rays from emitted the phosphor. from the Here, phosphor. the white lightHere, was the divided white into light two was parts divided into twoto analyzeparts to the analyze wavelength the conversionwavelength between conv theersion BLEDs between and the the phosphors. BLEDs Theand algorithm the phosphors. for the The algorithmphosphor for conversionthe phosphor system conversion was developed system by analyzing was thedeveloped wavelength by conversion analyzing between the thewavelength two conversionparts inbetween an emission the spectrumtwo parts of in white an emission light. spectrum of white light. Figure 1. Schematic of a WLED structure. Figure 1. Schematic of a WLED structure. 3. Phosphor Conversion Model Based on the DCLE Concept 3. Phosphor Conversion Model Based on the DCLE Concept To develop an optical