Long-term accelerated current operation of white light-emitting diodes

Long-term accelerated current operation of white light-emitting diodes

ARTICLE IN PRESS Journal of Luminescence 114 (2005) 39–42 www.elsevier.com/locate/jlumin Long-term accelerated current operation of white light-emit...

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ARTICLE IN PRESS

Journal of Luminescence 114 (2005) 39–42 www.elsevier.com/locate/jlumin

Long-term accelerated current operation of white light-emitting diodes Takeshi Yanagisawa, Takeshi Kojima Energy Electronics Institute, National Institute of Advanced Industrial Science & Technology, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan Received 3 February 2004; received in revised form 6 July 2004; accepted 15 November 2004 Available online 4 January 2005

Abstract Long-term degradation tests regarding white light-emitting diodes based on InGaN were performed under accelerated current conditions, and the half-life of the light’s output was estimated. An estimated mean half-life of 1.5  104 h was obtained under the recommended 20-mA operating condition. The change in the emission spectrum was found to be slight, and the color quality was considered generally satisfactory over the long term. r 2004 Published by Elsevier B.V. PACS: 78.60.Fi Keywords: White light-emitting diodes; Accelerated current test; Degradation; Estimated lifetime

1. Introduction Significant progress has been made in the development of high-brightness light-emitting diodes (LEDs) with a blue–ultraviolet color light [1,2]. Consequently, white light-emitting diodes based on the excitation of a fluorescent substance have been put into practical use in energy-efficient lighting [3]. This application is expected to see wide use in the future since LEDs are believed to have a longer life than electric bulbs, fluorescent Corresponding author. Tel./fax: +81 29 861 5472.

E-mail address: [email protected] (T. Yanagisawa). 0022-2313/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jlumin.2004.11.010

lights, etc. Moreover, it is conceivable that LEDs of this type will be subject to further improvements in materials and structure, resulting in even higher levels of brightness. At present, data collection regarding their long-term stable performance is also being advanced [4–8]. However, little data have been produced regarding the reliability of the white LED based on a wideband gap semiconductor [9–11]. We report herein the results of an operations test under a high-humidity environment and current-accelerated conditions. Based on observation of the degradation of light output over time, the estimated half-life and the cause of the degradation were investigated.

ARTICLE IN PRESS T. Yanagisawa, T. Kojima / Journal of Luminescence 114 (2005) 39–42

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2. Experimental details The sample used in the test was a commercial white LED (2 cd) with a structure based on an epoxy lens/YAG fluorescent substance/InGaN LED chip. The accelerated current test was performed under the following environmental conditions: 40 1C, 90% RH, and three levels of operation current IT (recommended current; 20 mA): 36, 54, and 74 mA of operation for 5.7  103 h. Five samples were used. The light output P0, emission spectrum, I–V characteristics, and optical transmission of the epoxy resin lens were measured as the evaluation parameters. P0 was measured using a photo diode. Light transmission was measured through the optical fiber from the side of an epoxy lens using a white light and a spectroradiometer.

3. Results and discussion

P0 ¼ A  aðlog tÞ.

tL ¼ KI a T .

(2)

The value tL ¼ 1:5  104 h was estimated as the predicted life time while being operated at the recommended value of 20 mA. A much longer life is expected under milder environmental conditions through out the year. The junction temperature T presumed in the shift of the emission peak at 460 nm is 50–90 1C with the forward current IF: 36–74 mA, as shown in Table 1. T was estimated

1

1.E+05

0.5 Operation current IT: 36 mA 54 mA 74 mA 0

(1)

The line then bends with 1–2  103 h. Changes in the characteristics of the fluorescent substance, the LED chip, the optical transmission of the resin lens, etc. are considered to be the causes of the degradation of P0. Especially in this experiment, the cause of degradation can be considered to be the degradation of the emission intensity of the LED chip and the decrease in the fluorescence intensity accompanying it, and the performance degradation of the fluorescent material itself. Our results also suggest that the observed patterns are relatively complex in these changes. Fig. 2 shows the half-lifetime tL to reach 50% level of initial P0 estimated from an extrapolation of the degradation pattern. Although the test was carried out under the specified environmental conditions, a ¼ 1:5 and K ¼ 1:3  106 were calculated based on the approximated equation

Half-life time, h

Normalized light output Po

Fig. 1 shows the degradation pattern (average value) of the normalized output P0 over time. The quantity of degradation is dispersed between the samples. Nevertheless, degradation increases depending on the operation current IT. Moreover, as for this degradation pattern with time, two modes were observed in the test of all IT conditions. The latter mode appears after 1–2  103 h from the start of operation. As for the degradation pattern with LED time, various approximate formulas have been reported based on different materials

and structures [7,9]. In the present sample, the degradation pattern is able to roughly approximate a straight line using the following equation:

10

100

1000 10000 Operation time, h

100000

Fig. 1. Degradation patterns of normalized light output (P0) over time.

1.E+04

1.E+03 10

100 Operation Current IT, mA

Fig. 2. Operation current IF dependences of the half-life (tL) of light output.

ARTICLE IN PRESS T. Yanagisawa, T. Kojima / Journal of Luminescence 114 (2005) 39–42 Table 1 Relation between forward current IF, estimated junction temperature T and l; based on the forward current IF and temperature dependences of a 460 nm peak wavelength l IF (mA) T (1C) Peak l (nm)

20 43 461.13

36 51 461.36

54 64 461.68

74 86 462.11

3000

2000

substance. The shift next to the long wavelength of the peak was only approximately 2 nm, and was stable. It also appeared that the color quality was generally satisfactory, although the intensity of the spectrum for the 550-nm peak decreased relatively. Fig. 4 shows an example of the change of the current–voltage characteristics (IF–VF, IR–VR) both before the test and after the test at the operation current IT: 74 mA. The figure shows change in the region of high forward current and the reverse current value. Fig. 5 shows the changes with time of the forward voltage VF (normalized value) at the forward current I F ¼ 20 mA: The reverse current

1000 t=5700h 0 400

Initial

1.00E-01

600 Wavelength, nm

800 t=5700h

1.00E-05

1.00E-07

1.00E-09

1.00E-11

2

2.5

3 3.5 Voltage VF, VR, V

4

4.5

Fig. 4. Change of the current–voltage characteristics at an operation current IT: 74 mA.

Normalised VF (IF=20mA)

based on the forward current IF and temperature dependences of the peak wavelength by measuring the emission from the LED sample attached to the temperature chamber using an optical fiber and spectrometer. Based on the measured results, the activation energy Ea of a reaction that caused degradation, 0.3–0.5 eV, was calculated from the plot of ln tL1/T. Here, Ea is conceived as the primary synthesis of the reaction of both fluorescence and the LED chip. Fig. 3 shows the emission spectrum before the test and during the test, with the change representing the synthesis of 460-nm emission peaks from an LED element and the 550-nm emission peaks from excited fluorescent material. When the pattern of both peak intensities is compared over time, although each intensity decreases, the decline in intensity in the 550-nm region is quite remarkable after progress of 1–2  103 h. This change corresponds to the pattern seen in Fig. 1. That is, the former mode primarily suggests the influence of a base LED, while the latter suggests the influence of multiplication with the characteristic change in the base LED and the fluorescent

Current, IF, IR, A

1.00E-03

Fig. 3. Changes with time in the emission spectrum at an operation current IT: 54 mA.

0.E+00

1.3

1.2 1.1

1 10

Operationcurrent IT: 36mA 54mA 74mA

100 1000 Operation time, h

-3.E-09 -6.E-09

IR (VR=-4V)

Light intensity, arb

Initial

41

-9.E-09 10000

Fig. 5. Change patterns with time of VF in I F ¼ 20 mA and IR in VR ¼ 4 V.

ARTICLE IN PRESS T. Yanagisawa, T. Kojima / Journal of Luminescence 114 (2005) 39–42

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IR in VR ¼ 4 V was calculated based on the current–voltage characteristics in order to compare the details of each examination condition. These findings represent the characteristics of the LED chip based on 460-nm emissions. IR increased from the test’s initiation with time, with the rate of increase being dependent on the operation current IT conditions. After 5.7  103 h, IR increased to approximately six times the initial value under I T ¼ 74 mA conditions, and to three times the value under the I T ¼ 36 mA condition. This increase in VF indicates the increase in the series resistance of the diode chip. Although it increased 10–17% after the tests, it has no clear dependence on IT conditions. The junction capacitance increased 5–10% after the tests, and although the slope of the straight line approximating the 1/C3 plot did not change, it shifted as shown in Fig. 6. The measurement conditions included an applied bias voltage of 1–4 V, a measurement voltage of 50 mV, and a frequency of 500 kHz. This change suggests that the space charge density changed almost uniformly in the bulk near the depletion layer. When the change in junction capacitance is taken into consideration in accordance with the increase in IR, it appears that this causes a defect inside a semiconductor. The light transmittance of the epoxy resin measured on the side of an LED’s lens fluctuated to a maximum of +4% at 520 nm, and a

2.5

1/C3, 105. nF-3

Initial 2 t=5700h 1.5

1 -6

-4

-2

0

2

Bias voltage,V

Fig. 6. Changes in the applied bias voltage dependency of junction capacitance at an operation current IT: 54 mA.

maximum of 6% in the range of 460–500 nm after 5.7  103 h. A change of this magnitude scarcely influences the degradation of P0. It is also thought that the characteristics of transmission are not changed by the high humidity of the test environment.

4. Conclusion The operation test of the fluorescent material excited by the white LED was performed under high humidity and accelerated current conditions. The degradation mode of P0 was investigated with the passage of time, and 1.5  104 h was estimated as the mean half-life at the recommended operation current. Based on observation of the emission spectrum, the color quality appeared to be generally satisfactory over the long term. The changes in the parameters of I–V characteristics and junction capacitance suggested the generation of a defect inside the InGaN semiconductor. The light transmittance of the epoxy resin lens was less than 6% under environmental conditions of 40 1C and 90% RH. References [1] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [2] K. Tadatomo, H. Okagawa, T. Tsunekawa, Y. Imada, M. Kato, T. Taguchi, Jpn. J. Appl. Phys. 40 (2001) L583. [3] P. Schlotter, J. Baur, C.H. Hielscher, M. Kunzer, H. Obloh, R. Schmidt, J. Schneider, Mater. Sci. Eng. B 59 (1999) 390. [4] T. Egawa, H. Ishikawa, T. Jimbo, M. Umeno, Appl. Phys. Lett. 69 (1996) 830. [5] T. Yanagisawa, Microelec. Reliab. 37 (1997) 1239. [6] D.L. Barton, M. Osinski, P. Perlin, C.J. Helms, N.H. Berg, Proceedings of the SPIE—International Society for Optical Engineering 3279 San Jose, CA, January 1998, p. 17. [7] T. Shirakawa, Mater. Sci. Eng. B 91–92 (2002) 470. [8] G. Meneghesso, S. Levada, E. Zanoni, G. Scamarcio, G. Mura, S. Podda, M. Vanzi, S. Du, I. Eliashevich, Microelec. Reliab. 43 (2003) 1737. [9] T. Yanagisawa, Microelec. Reliab. 38 (1998) 1627. [10] G. Meneghesso, S. Levada, E. Zanoni, G. Scamarcio, G. Mura, S. Podda, M. Vanzi, S. Du, I. Eliashevich, Microelec. Reliab. 43 (2003) 1737. [11] N. Narendran, Y. Gu, J.P. Freyssinier, H. Yu, L. Deng, J. Cryst. Growth 268 (2004) 449.