Limits...
Reduced graphene oxide enwrapped phosphors for long-term thermally stable phosphor converted white light emitting diodes

View Article: PubMed Central - PubMed

ABSTRACT

The long-term instability of the presently available best commercial phosphor-converted light-emitting diodes (pcLEDs) is the most serious obstacle for the realization of low-cost and energy-saving lighting applications. Emission from pcLEDs starts to degrade after approximately 200 h of operation because of thermal degradation of the phosphors. We propose a new strategy to overcome this thermal degradation problem of phosphors by wrapping the phosphor particles with reduced graphene oxide (rGO). Through the rGO wrapping, we have succeeded in controlling the thermal degradation of phosphors and improving the stability of fabricated pcLEDs. We have fabricated pcLEDs with long-term stability that maintain nearly 98% of their initial luminescence emission intensity even after 800 h of continuous operation at 85 °C and 85% relative humidity. The pcLEDs fabricated using SrBaSi2O2N2:Eu2+ phosphor particles wrapped with reduced graphene oxide are thermally stable because of enhanced heat dissipation that prevents the ionization of Eu2+ to Eu3+. We believe that this technique can be applied to other rare-earth doped phosphors for the realization of highly efficient and stable white LEDs.

No MeSH data available.


Related in: MedlinePlus

(a) EL spectra of the pcLEDs fabricated using fresh, GO 1200 and GO 1350 phosphors. The inset shows the corresponding CIE coordinates. (b) Variation of EL intensities with time of pcLEDs fabricated using fresh and GO 1200 phosphors as well as commercial YAG:Ce3+ and silicate phosphor and operated at 85 °C and 85% rel. humidity. The long term reliability test under stress conditions shows superior thermal stability of rGO wrapped phosphors compared to fresh and commercial phosphors. (c) The Raman G peak shift as a function of the incident laser power. (d) Schematic illustration showing the heat dissipation mechanism in pcLEDs fabricated using unwrapped and rGO-wrapped phosphor. Because of the high thermal conductivity of rGO, thermal oxidation of Eu2+ activators to Eu3+ is considerably reduced, which gives rise to long term reliability of pcLEDs fabricated using rGO-wrapped phosphors.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC5037423&req=5

f4: (a) EL spectra of the pcLEDs fabricated using fresh, GO 1200 and GO 1350 phosphors. The inset shows the corresponding CIE coordinates. (b) Variation of EL intensities with time of pcLEDs fabricated using fresh and GO 1200 phosphors as well as commercial YAG:Ce3+ and silicate phosphor and operated at 85 °C and 85% rel. humidity. The long term reliability test under stress conditions shows superior thermal stability of rGO wrapped phosphors compared to fresh and commercial phosphors. (c) The Raman G peak shift as a function of the incident laser power. (d) Schematic illustration showing the heat dissipation mechanism in pcLEDs fabricated using unwrapped and rGO-wrapped phosphor. Because of the high thermal conductivity of rGO, thermal oxidation of Eu2+ activators to Eu3+ is considerably reduced, which gives rise to long term reliability of pcLEDs fabricated using rGO-wrapped phosphors.

Mentions: Since the GO 700 phosphor exhibits very low PL emission intensity, only fresh phosphor, GO 1200, and GO 1350 phosphors were used for fabricating pcLEDs. The electroluminescent (EL) spectra from the pcLEDs fabricated from fresh phosphor and from phosphors GO 1200 and GO 1350 are shown in Fig. 4(a). The corresponding commission internationale d’Eclaraige (CIE) coordinates are also shown in the inset of Fig. 4(a). A minor shift in the coordinates is observed, presumably due to the rGO wrapping. If there is a large shift in the CIE coordinates, the quality of the emitted white light from the pcLEDs will be poor2. It is worth mentioning that in our rGO wrapped phosphors, shift in the CIE coordinates is trivial and therefore rGO wrapping does not affect the quality of emitted light. The relative EL intensity from the phosphor (yellow region of the spectrum) decreases for the LEDs fabricated using GO 1200 and GO 1350 phosphors presumably because of the rGO coating on the phosphor particles. The optical parameters of the fabricated LEDs are summarized in Table 1. The luminescence efficacy of the device fabricated using fresh phosphor was 117 lm/W while that of the device fabricated using GO 1200 was 98 lm/W. The efficiency drop corresponds to the reduced emission intensity because of absorption of emitted light in the rGO wrapping. However, the color rendering index (CRI) values are relatively high for GO 1200 and GO 1350. Among all the fabricated pcLEDs, the one made with GO 1200 phosphor exhibited the highest CRI Ra of 70 (Table 1).


Reduced graphene oxide enwrapped phosphors for long-term thermally stable phosphor converted white light emitting diodes
(a) EL spectra of the pcLEDs fabricated using fresh, GO 1200 and GO 1350 phosphors. The inset shows the corresponding CIE coordinates. (b) Variation of EL intensities with time of pcLEDs fabricated using fresh and GO 1200 phosphors as well as commercial YAG:Ce3+ and silicate phosphor and operated at 85 °C and 85% rel. humidity. The long term reliability test under stress conditions shows superior thermal stability of rGO wrapped phosphors compared to fresh and commercial phosphors. (c) The Raman G peak shift as a function of the incident laser power. (d) Schematic illustration showing the heat dissipation mechanism in pcLEDs fabricated using unwrapped and rGO-wrapped phosphor. Because of the high thermal conductivity of rGO, thermal oxidation of Eu2+ activators to Eu3+ is considerably reduced, which gives rise to long term reliability of pcLEDs fabricated using rGO-wrapped phosphors.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037423&req=5

f4: (a) EL spectra of the pcLEDs fabricated using fresh, GO 1200 and GO 1350 phosphors. The inset shows the corresponding CIE coordinates. (b) Variation of EL intensities with time of pcLEDs fabricated using fresh and GO 1200 phosphors as well as commercial YAG:Ce3+ and silicate phosphor and operated at 85 °C and 85% rel. humidity. The long term reliability test under stress conditions shows superior thermal stability of rGO wrapped phosphors compared to fresh and commercial phosphors. (c) The Raman G peak shift as a function of the incident laser power. (d) Schematic illustration showing the heat dissipation mechanism in pcLEDs fabricated using unwrapped and rGO-wrapped phosphor. Because of the high thermal conductivity of rGO, thermal oxidation of Eu2+ activators to Eu3+ is considerably reduced, which gives rise to long term reliability of pcLEDs fabricated using rGO-wrapped phosphors.
Mentions: Since the GO 700 phosphor exhibits very low PL emission intensity, only fresh phosphor, GO 1200, and GO 1350 phosphors were used for fabricating pcLEDs. The electroluminescent (EL) spectra from the pcLEDs fabricated from fresh phosphor and from phosphors GO 1200 and GO 1350 are shown in Fig. 4(a). The corresponding commission internationale d’Eclaraige (CIE) coordinates are also shown in the inset of Fig. 4(a). A minor shift in the coordinates is observed, presumably due to the rGO wrapping. If there is a large shift in the CIE coordinates, the quality of the emitted white light from the pcLEDs will be poor2. It is worth mentioning that in our rGO wrapped phosphors, shift in the CIE coordinates is trivial and therefore rGO wrapping does not affect the quality of emitted light. The relative EL intensity from the phosphor (yellow region of the spectrum) decreases for the LEDs fabricated using GO 1200 and GO 1350 phosphors presumably because of the rGO coating on the phosphor particles. The optical parameters of the fabricated LEDs are summarized in Table 1. The luminescence efficacy of the device fabricated using fresh phosphor was 117 lm/W while that of the device fabricated using GO 1200 was 98 lm/W. The efficiency drop corresponds to the reduced emission intensity because of absorption of emitted light in the rGO wrapping. However, the color rendering index (CRI) values are relatively high for GO 1200 and GO 1350. Among all the fabricated pcLEDs, the one made with GO 1200 phosphor exhibited the highest CRI Ra of 70 (Table 1).

View Article: PubMed Central - PubMed

ABSTRACT

The long-term instability of the presently available best commercial phosphor-converted light-emitting diodes (pcLEDs) is the most serious obstacle for the realization of low-cost and energy-saving lighting applications. Emission from pcLEDs starts to degrade after approximately 200 h of operation because of thermal degradation of the phosphors. We propose a new strategy to overcome this thermal degradation problem of phosphors by wrapping the phosphor particles with reduced graphene oxide (rGO). Through the rGO wrapping, we have succeeded in controlling the thermal degradation of phosphors and improving the stability of fabricated pcLEDs. We have fabricated pcLEDs with long-term stability that maintain nearly 98% of their initial luminescence emission intensity even after 800 h of continuous operation at 85 °C and 85% relative humidity. The pcLEDs fabricated using SrBaSi2O2N2:Eu2+ phosphor particles wrapped with reduced graphene oxide are thermally stable because of enhanced heat dissipation that prevents the ionization of Eu2+ to Eu3+. We believe that this technique can be applied to other rare-earth doped phosphors for the realization of highly efficient and stable white LEDs.

No MeSH data available.


Related in: MedlinePlus