IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 6, MARCH 15, 2007 399

Phosphor Thermometry in WhiteLight-Emitting Diodes

Pranciskus Vitta, Paulius Pobedinskas, and Arturas Zukauskas

Abstract—A method for the in situ measurement of phosphortemperature in high-power white light-emitting diodes (LEDs) wasdemonstrated. The method is based on the dependence of the flu-orescence decay time in inorganic phosphors on temperature. Thedecay time was estimated using the frequency-domain technique,which relies on the measurement of the phase shift of the sinu-soidal waveform of fluorescence relative to that of the photoex-citation, obtained by driving the LED at high-frequency current.LEDs containing a single phosphor (“cool-white”) and a phosphorblend (“warm-white”) were examined. By comparison of the phos-phor temperature with that of the LED junction and metal mount,the character of the spatial profile of temperature within the LEDpackages was revealed.

Index Terms—Fluorescence lifetime, frequency domainmeasurements, light-emitting diodes (LEDs), phosphorthermometry, thermal management, white light.

PHOSPHOR-CONVERSION white light-emitting diodes(LEDs) are already widely used in niche lighting ap-

plications, such as decorative, architectural, automotive, andmedical lighting [1], [2]. Further penetration of LEDs into thegeneral-lighting market is anticipated through the developmentof high-power devices. One of the issues of the developmentand use of high-power LEDs is thermal management, which isof crucial importance for maintaining stable output characteris-tics and optimal aging. To this end, methods for monitoring ofthe thermal regime of high-power LEDs are of high interest.

In colored LEDs, which comprise a semiconductor diemounted on a heat-conductive pad and covered with a translu-cent cap for protection and light extraction, practical monitoringof the thermal regime can be provided through measurements ofjunction/chip temperature, by investigation of the peak positionand short-wavelength slope of the electroluminescence spectra,temperature dependence of Raman scattering by phonons,measurements of the forward voltage, and thermal imaging[3]–[5].

White LEDs with a partial or complete conversion in phos-phors of the initial blue or near UV radiation, respectively, aremore complex devices, where micrometer-sized phosphor parti-cles can be characterized by a unique thermal regime due to thethermal contact with the semiconductor chip and encapsulantas well as due to heat generation in the photon down-conver-sion process. Besides, the thermal regime is conditioned by thegeometrical arrangement of the phosphor within a white LED

Manuscript received July 13, 2006; revised December 6, 2006. This workwas supported by the European Commission supported SELITEC Center underContract G5MA-CT-2002-04047.

The authors are with the Institute of Materials Science and Applied Research,Vilnius University, LT-10222 Vilnius, Lithuania.

Digital Object Identifier 10.1109/LPT.2007.891595

depending on the device design [1], [6], [7]. An increase in phos-phor temperature can invoke thermal quenching, which resultsin a reduced output, a shift of the color point, and a change ofthe color rendering properties of the white LED. An even morecomplex situation occurs when blends of phosphors with dif-ferent thermal properties are used. In addition, thermal degra-dation of the encapsulant in white LEDs [8] is probably dueprimarily to the contact with the phosphor, which is commonlydeposited over the semiconductor die. Data on phosphor tem-perature can provide information not only on the thermal con-dition of the phosphor itself but also on the temperature gradi-ents along the device package. Thermal effects in white-LEDphosphor particles were investigated using numerical simula-tions [6]. However, experimental methods for measurement ofphosphor temperature within white LEDs have been not devel-oped so far.

Meanwhile for monitoring of the thermal regime of a phos-phor, one can exploit the dependence of its fluorescence decaytime on temperature. Such phosphor thermometry is a widelyused technique for remote and contactless temperature mea-surements [9]. Common activators in white-LED phosphors areCe , Eu , and Mn [10], which have fluorescence lifetimesin the range of – s, – s, and – s,respectively [11]. Phosphors based on these activators exhibitthermal quenching [9], [12], accompanied by a decrease in thefluorescence decay time, which under appropriate calibrationcan be used for the evaluation of the phosphor temperature.

A straightforward way for in situ measurement of fluores-cence decay time in the phosphor of a white LED is to usepulsed or harmonically modulated photoexcitation from the na-tive semiconductor chip and to examine the fluorescence re-sponse of the phosphor in the time or frequency domain, re-spectively [13]. The frequency-domain method is more attrac-tive, since the operating regime of an LED under investigationis close to continuous-wave one. In this case, the driving cur-rent of the semiconductor chip is modulated sinusoidally at ahigh frequency providing a sinusoidal waveform of the phos-phor photoexcitation. However, because of a finite fluorescencedecay time, the resulting sinusoidal waveform of the phosphoremission is phase-shifted in respect of the excitation one. Corre-spondingly, the fluorescence decay time can be extracted fromthe measured phase shift between the spectral components orig-inating from the semiconductor chip and phosphor, respectively.

Here we report on a method for in situ measurement ofphosphor temperature in commercial high-power white LEDsusing phosphor thermometry. Fluorescence decay time wasestimated using the frequency-domain technique based ondirect high-frequency modulation of the LED driving currentand measuring of the phase shift of the resulting phosphoremission in respect of that of the semiconductor chip.

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400 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 6, MARCH 15, 2007

Two types of Philips Lumileds Lighting high-powerwhite LEDs were investigated. A high-operation-tempera-ture “cool-white” LXK2-PW14 LED, which contains singlestandard YAG : Ce phosphor, was studied in a wide range ofthe mount temperature (up to 480 K). To demonstrate the ther-mometry in a phosphor blend, a “warm-white” LXHL-BW03LED, which contains YAG : Ce and a red phosphor, was ex-amined in the temperature range up to 420 K. The LEDs weredriven by dc and RF power using a bias tee (Picosecond 5547).To vary the junction temperature, the dc driving current wasvaried from 0 to 870 and 475 mA, respectively. The RF modu-lation in the range from 50 kHz to 20 MHz was provided by ahigh-power digital signal generator (Aeroflex IFR 2023A). Theoptical signal from an LED under investigation was filtered bya grating monochromator (JY HD10) with a spectral bandpassof 8 nm, detected by a subnanosecond-risetime photomultiplier(Hamamatsu H6780-01) and split into the ac and dc compo-nents using another bias tee. The resulting ac electrical signalwas amplified by a wideband amplifier (Hamamatsu C5594).The ac and dc components of the signal were fed into an RFlock-in amplifier (Stanford Research Systems SR844) for thephase and modulation depth measurements of the sinusoidalwaveform [14]. The temperature of the metal mount of theLED was measured by a thermocouple gauge with theaccuracy of 1 K, whereas the temperature of the junction wasestimated from the high-energy slope of the electrolumines-cence spectra. The emission spectra were recorded using adouble monochromator (JY HRD 1) and a photomultiplier(Hamamatsu R1463 P) operating in the photon-counting mode.The ambient temperature was maintained at about 295 K.

For calibration purposes, the LEDs were disassembled andthe silicone film containing phosphor particles was removedfrom the chip. The film was mounted on a brass finger and fas-tened by a brass plate with a pinhole. A resistive heater was usedto heat the finger and the temperature of the finger was mea-sured by means of the thermocouple gauge. A high-power blueLED (Lumileds LXHL-LR5C) was used for external photoexci-tation of the phosphor film. For the modulation of the LED andmeasurement of the phase of the fluorescence waveform, the ar-rangement described above was used, except that to determinethe phase of the photoexcitation waveform, the blue emissionreflected from the film was examined.

Fig. 1 shows the emission spectra of the white LEDs inves-tigated at different driving powers. The “cool-white” LED ex-hibits two peaks at about 2.15 and 2.75 eV (577 and 450 nm)originating from the InGaN chip and YAG : Ce , respectively[Fig. 1(a)]. The “warm-white” LED has an additional peak atabout 1.92 eV (645 nm) due to the red phosphor [Fig. 1(b)]. Inboth cases, the far short-wavelength slope of the spectra is sen-sitive to the driving power and reflects the junction temperature

. To extract the value of the junction temperature, the shapeof the slope was approximated by a Boltzmann-like form

(1)

where is the photon energy, and is the correction con-stant, which accounts for the spectral distortions due to inho-mogeneous broadening of the spectra caused by uneven indiumdistribution and quantum-well width fluctuations in the activeInGaN layers, reabsorption in phosphor, background emissionfrom the cladding and barrier layers and fluorescence of the

Fig. 1. Emission spectra of high-power (a) “cool-white” and (b) “warm-white”LEDs at different driving powers (indicated). Dashed lines show the exponentialapproximation of the high-energy slopes that were used for the estimation of thejunction temperature. The gray bars show the spectral windows that were usedin the fluorescence lifetime measurements.

Fig. 2. Phase shift (open points) and demodulation factor (filled points) as func-tions of frequency for phosphor films of the (a) “cool-white” and (b) “warm-white” LEDs measured at some temperatures (indicated). The lines show theleast-square fits to the (a) single-exponent and (b) two-exponent fluorescenceresponse functions with the lifetimes as indicated.

phosphor (assumed to weakly depend on temperature), etc. Fora small correction constant , the accuracy of suchan approach is about . In our experiment, thecorrection constant was estimated from the zero-current limit,when , and typically had a value in the range of 10 K,i.e., much below .

Fig. 2 shows some phase and modulation depth dependencesversus frequency for “cool-white” [Fig. 2(a)] and “warm-white”[Fig. 2(b)] phosphor films measured at some temperatures inthe calibration experiment. Fluorescence from the “cool-white”and “warm-white” films was sampled at 600 and 635 nm, re-spectively. In both cases with increasing frequency, the phaseshift between the excitation and fluorescence waveforms in-creases, whereas the modulation depth decreases. To extract thefluroescece lifetime, the experimental dependences of phase anddemodulation factor were fit to those calculated for multiexpo-nent fluorescence response functions [15].

The fit of the experimental results shown in Fig. 2 to the cal-culated ones revealed that emission of the phosphor film of the“cool-white” LED exhibits a single-exponent decay [Fig. 2(a)],whereas the emission of the “warm-white” phosphor film has atwo-exponent decay [Fig. 2(b)].

Fig. 3 shows the obtained calibration results. In the “cool-white” phosphor, the single fluorescence lifetime was measuredwith a high accuracy ( 0.4%) and equaled about 63 ns up to380 K with further decrease to about 53 ns at 485 K [Fig. 3(a)].The obtained values are typical of YAG : Ce phosphor [12].This calibration data allows for the estimation of the phosphortemperature in the range above 380 K within an error of about5 K.

Of two fluorescence lifetimes in the “warm-white” phosphorblend, one is close to that observed in YAG : Ce [not shown

VITTA et al.: PHOSPHOR THERMOMETRY IN WHITE LEDs 401

Fig. 3. Fluorescence lifetime versus temperature calibration dependences forthe YAG : Ce phosphor of the (a) “cool-white” LED and the red phosphor ofthe (b) “warm-white” LED. The uncertainty due to the least-square fit of thephase and demodulation factor frequency dependences (Fig. 2) are shown bythe size of (a) point and (b) error bars.

Fig. 4. Temperatures of the metal mount (squares), junction (circles), and phos-phor (triangles) as functions of driving power in (a) “cool-white” and (b) “warm-white” high-power LEDs. The lines are guides for the eye.

in Fig. 3(b)], whereas the second one monotonically decreaseswith temperature. The room-temperature value of this lifetimeis 720 ns and it drops to 350 ns at 352 K. Despite a reduced accu-racy in the estimation of the red-phosphor fluorescence lifetimedue to an uncertainty of the decomposition of the two-exponentdecay ( 4%–8%), the steeper calibration dependence allows forestimation of the phosphor temperature within the same 5 Kerror in the entire range of temperatures (from 290 K to 350 K).

The obtained calibration dependences of fluorescence life-time on temperature were used for the measurement of phosphortemperature inside operating white LEDs. Now, the phase of thephosphor emission was determined relative to the intrinsic bluecomponent of the LED (at 450 nm). To extract the phosphortemperature at various LED driving powers, the phase shiftswere collated with the calibration data (Fig. 3). Fig. 4 showsthe cumulative results on the metal mount, junction, and phos-phor temperatures as functions of driving power. One can seein Fig. 4 that the junction temperature of the LEDs, even withthe experimental error taken into account, exceeds the mounttemperature by less than 20 K per 1 W of the driving power,what is in line with the thermal resistance presented in the man-ufacturer datasheets. In both LEDs, the phosphor temperature isclearly seen to be lower than that of the junction. In particular inthe “warm-white” LED, the phosphor temperature is even lowerthan that of the mount. This indicates that phosphor particlesare out of thermal equilibrium with the chip and dissipate heatto the colder encapsulant. Consequently, this means that in theinvestigated white phosphor-conversion LEDs the temperatureis peaked at the junction, although some modeling results pre-

dict that the temperature can have highest values in the phos-phor layer [6]. Note that maintaining of the phosphor temper-ature below that of the chip is favorable in terms of a reducedthermal quenching of the phosphor and more stable output andchromaticity.

In conclusion, we employed the dependence of phosphor flu-orescence lifetime on temperature for the in situ characterizationof the phosphor thermal condition in high-power white LEDs,that were directly modulated by high-frequency driving current.Phosphor temperatures within the typical error of about 5 Kwere measured as a function of driving power in LEDs con-taining a single phosphor and a phosphor blend. The phosphortemperature turned out to be lower than that of the LED junction,indicating on a temperature gradient inside the LED packages.

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