QUANTUM DOTS IN ENCLOSED ENVIRONMENT

Abstract

The invention provides a lighting device for providing light, the lighting device comprising a closed chamber with a light transmissive window and a light source configured to provide light source radiation into the chamber, wherein the chamber further encloses a wavelength converter configured to convert at least part of the light source radiation into wavelength converter light, wherein the light transmissive window is transmissive for the wavelength converter light, wherein the wavelength converter comprises luminescent quantum dots which upon excitation with at least part of the light source radiation generate at least part of the wavelength converter light, and wherein the closed chamber comprises a filling gas comprising one or more of helium gas, hydrogen gas, nitrogen gas or oxygen gas, the filling gas having a relative humidity at 19° C. of at least 5%.

Claims

1. A lighting device comprising (i) a closed chamber with a light transmissive window and (ii) a light source configured to provide light source radiation into the chamber, wherein the chamber further encloses a wavelength converter configured to convert at least part of the light source radiation into wavelength converter light, wherein the light transmissive window is transmissive for the wavelength converter light, wherein the wavelength converter comprises luminescent quantum dots which upon excitation with at least part of the light source radiation generate at least part of the wavelength converter light, and wherein the closed chamber comprises a filling gas comprising one or more of helium gas, or hydrogen gas, or nitrogen gas or oxygen gas, the filling gas having a relative humidity at 19° C. of at least 5%.

2. The lighting device according to claim 1, wherein the wavelength converter comprises a siloxane matrix wherein the luminescent quantum dots are embedded.

3. The lighting device according to claim 1, wherein the luminescent quantum dots comprise an inorganic coating.

4. The lighting device according to claim 1, wherein the filling gas comprises helium.

5. The lighting device according to claim 1, wherein at least 80% of the filling gas consists of He, the filling gas having a relative humidity at 19° C. of at least 5%, and wherein the chamber does not comprise liquid water at 19° C.

6. The lighting device according to claim 1, wherein at least 95% of the filling gas consists of He and O.sub.2, and wherein the gas comprises at most 25% oxygen.

7. The lighting device according to claim 1, wherein the closed chamber comprises a light bulb shaped light transmissive window.

8. The lighting device according to claim 1, wherein the light source is configured to provide blue light source radiation and wherein the wavelength converter configured to convert at least part of the light source radiation into wavelength converter light having one or more of a green component, ora yellow component, or an orange component or a red component.

9. The lighting device according to claim 1, wherein the light source comprises a solid state light source.

10. The lighting device according to claim 1, further comprising a heat sink in thermal contact with at least one of, the transmissive window, or the light source or the wavelength converter.

11. A process for production of a lighting device comprising a closed chamber with a light transmissive window and a light source configured to provide light source radiation into the chamber, wherein the chamber further encloses a wavelength converter configured to convert at least part of the light source radiation into wavelength converter light, wherein the light transmissive window is transmissive for the wavelength converter light, wherein the wavelength converter comprises luminescent quantum dots which upon excitation with at least part of the light source radiation generate at least part of the wavelength converter light, and wherein the closed chamber comprises a filling gas comprising one or more of helium gas, or hydrogen gas, or nitrogen gas or oxygen gas, the filling gas having a relative humidity at 19° C. of at least 1% , the process comprising assembling the chamber with the light transmissive window, the light source and the wavelength converter, wherein the filling gas and water are provided to the chamber, wherein the filling gas is obtained after a gas closure is provided to the chamber, and wherein the chamber further comprises a material that releases water during at least part of its lifetime.

12-15. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

[0067] FIG. 1a schematically depicts an embodiment of the quantum dot based luminescent material;

[0068] FIG. 1b schematically depicts an embodiment of the quantum dot based luminescent material;

[0069] FIG. 1c schematically depicts an embodiment of the wavelength converter;

[0070] FIGS. 2a-2e schematically depicts embodiments of a lighting device; and

[0071] FIG. 3 shows an experiment wherein the influence of water is tested.

[0072] The schematic drawings are not necessarily on scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0073] FIG. 1a schematically depicts a quantum dot based luminescent material. By way of example different types of QDs, indicated with reference 30, are depicted. The QD at the top left is a bare QD, without shell. The QD is indicated with C (core). The QD 30 at the right top is a core-shell particle, with C again indicating the core, and S indicating the shell. At the bottom, another example of a core-shell QD is schematically depicted, but a quantum dot in rod is used as example. Reference 36 indicates the outer layer, which is in the first example the core material at the external surface, and which is in the latter two embodiments the shell material at the external surface of the QD 30.

[0074] FIG. 1b schematically depicts an embodiment of the luminescent material, but now the QDs 30 including the coating 45, especially an oxide coating, such as a silica coating. The thickness of the coating is indicated with reference dl. The thickness may especially be in the range of 1-50 nm. Especially, the coating 45 is available over the entire outer layer 36. Note however that a silica coating may be somewhat permeable. Note also that the outer layer 36 of the uncoated nanoparticle (i.e. not yet coated with the coating of the invention), is (in general) not an outer layer anymore after the coating process, as then an outer layer will be the outer layer of the coating 45. However, herein the term outer layer, especially indicated with reference 36, refers to the outer layer of the uncoated (core-shell) nanoparticle.

[0075] FIG. 1c schematically depicts a wavelength converter 300. Especially, the wavelength converter includes a body, such as schematically depicted here. The wavelength converter 300 comprises a matrix or matrix material 310, such as acrylate, wherein the quantum dots 30 may be embedded. By way of example, the QDs 30 include coating 45, such as a silica coating.

[0076] FIG. 2a schematically depicts an embodiment of a lighting device 100 comprising a closed chamber 200 with a light transmissive window 210 and a light source 10 configured to provide light source radiation 11 into the chamber 200. Here, by way of example the light source 10 is also enclosed in the chamber. The chamber 200 further encloses a wavelength converter 300 configured to convert at least part of the light source radiation 11 into wavelength converter light 301. The light transmissive window 210 is transmissive for the wavelength converter light 301. The wavelength converter 300 comprises luminescent quantum dots 30 (not depicted) (as luminescent material) which upon excitation with at least part of the light source radiation 11 generate at least part of said wavelength converter light 301. Further, the closed chamber 200 comprises a filling gas 40, for instance comprising one or more of He gas, H.sub.2 gas, N.sub.2 gas and O.sub.2 gas, and having a relative humidity at 19° C. of e.g. at least 5% but lower than 100%. Especially, at 19° C. the chamber does not include liquid water.

[0077] In this example, the wavelength converter 300 may be in physical contact of a light emitting surface of the light source 10, such as a (die of a) solid state light source.

[0078] The light source 10 is arranged on a support 205, such as a PCB. In this embodiment, the support provides part of the wall, which is indicated with reference 201. Another part of the wall 201 is provided by the light transmissive window 210. Reference 101 indicates the light generated by the lighting device 100 during operation. This lighting device at least comprises wavelength converter light 301 but may optionally also include light source radiation 11, especially when the light source 10 substantially provides light in the blue part of the spectrum. By way of example, the lighting device 100 further includes a heat sink 117. In the embodiment, the heat sink may be part of the support 205. However, the heat sink may also be arranged elsewhere. Further, the term “heat sink” may optionally also refer to a plurality if heat sinks

[0079] FIGS. 2b-2c schematically depict two further embodiments of the lighting device 100, with the latter having the light source 10 arranged external from the chamber. Note that in both embodiments the wavelength converter 300 is arranged at a non-zero distance from the light source 10, especially from its light emitting surface. The distance is indicated with reference d2 and may e.g. be in the range of 0.1-100 mm, such as 1-100 mm, like 2-20 mm. reference 211 in FIG. 2c refers to a radiation transmissive window. Note that optionally the entire wall 201 is radiation transmissive. Reference 240 refers to a material that releases water. The configuration of the water releasing material 240 in FIG. 2c as layer is only an example of the many options such material may be arranged.

[0080] FIGS. 2d-2e schematically depict how the lighting device may be assembled. For instance, an open chamber may be provided with walls 201 and including the wavelength converter 300. This may be arranged to the light sources 10, in this embodiment arranged on the support 205 (which may optionally also include a heat sink (see above)). This may lead to a closed chamber except for an optional opening for gas. Here, a gas stem or pump stem 206 is schematically depicted. The gas may be introduced and thereafter a closure may be provided to hermetically close the chamber. An embodiment of the closure, indicated with reference 207, may be a seal, such as schematically depicted in FIG. 2e. Thereafter, e.g. a cap 111, such as an Edison cap, may be provided to the closed chamber. The gas, i.e. the filling gas may e.g. be provided as the filling gas with the required humidity. However, also dry filling gas may be added and water (gas or liquid) may be added from another source, leading to the filling gas in the chamber 200 having the required relative humidity.

[0081] In a further example, red emitting quantum dots consisting of a CdSe core and a ZnS shell were silica coated using the reverse micelle method as adapted by Koole et al. (see above). They were incorporated into an optical quality silicone and dropcasted onto a glass plate. The silicone was cured at 150° C. for two hours. The optical properties of the quantum dot containing film were tested at 450 nm light of an intensity of 10 W/cm.sup.2 at a temperature of 100° C., detecting the intensity of the emitted light using an integrating sphere coupled to a spectrophotometer.

[0082] A stream of dry nitrogen was flown over the sample for one hour, slight photobrightening occurred in this time frame. Subsequently the flow was switched to humidified nitrogen which led to an increase in the photoluminescence with about a factor 2. Switching back to dry nitrogen, 90 minutes later, showed a strong decrease in photoluminescence. This result demonstrates that these silica coated quantum dots need water for optimal luminescence. These data are depicted in FIG. 3, with on the x-axis time in seconds and on the y-axis the integrated intensity in arbitrary units. The dotted line (N) at intensity 1 indicates the normalized transmitted laser intensity, and the curve (S) indicates the normalized corrected photoluminescence.

[0083] In a second embodiment, silica coated QDs (peak maximum of ˜610 nm at room temperature) were mixed into commercial silicone. YAG:Ce powder was added to the QD-silicone mixture, and this blend was dispensed into LED packages, after which the phosphor-silicone blend was cured for 2 hours at 150C. The concentration of QDs and YAG:Ce material was tuned in order to achieve a color temperature of 2700 K-3000 K (close to or on the black body line), and high CRI (80, 85, 90, or higher).

[0084] In a third embodiment, LEDs as described in the second embodiment are placed on metal core (MC) PCB's by solder attach, and mounted inside a glass bulb in a process similar to that used to build conventional incandescent light bulbs. The glass bulb allows for hermetic sealing, and prior to sealing the atmosphere within the bulb can be adjusted. Electrical connection to the LED is still possible by metal wires through the glass (as is also done for conventional glass bulbs). Each glass bulb contains 1 LED, and various bulbs were sealed at 950 mbar pressure of air. The relative humidity of the air with which the bulb was filled was varied by using a well-controlled mixture of dry (10 ppmV) and water-saturated air, making use of mass flow controllers. In this way, bulbs were filled with relative humidity's (RH) (at room temperature) of 0% (actually 0.05-0.25%), 1%, 10%, and 80%. The gas content of a few test bulbs was analyzed which confirmed the control over humidity within the sealed glass bulb (see further also below the data in the table).

[0085] The LEDs within sealed glass bulbs with various humidity levels were tested on stability, by measuring the light output and spectra of the lamp at fixed time intervals. The spectra were recorded prior to sealing/filling, after sealing/filling, and subsequently the LEDs were driven continuously at I.sub.F=150 mA (V.sub.F=˜6 V). It was found that the QDs are at an average temperature of approximately 85° C. under these drive conditions. At fixed intervals the LEDs were switched off to measure the light output and spectra off line, there-after they were remounted and switched on again at the same drive current setting.

[0086] Using the 1960 CIE color diagram, u′ is the appropriate parameter to follow the QD emission over time because the QDs emit at around 610-620 nm. A shift in u′ larger than 0.007 over the LED lifespan is generally considered to be not acceptable. Upon sealing (so without turning on/off the LED), it is observed that the LEDs enclosed under dry conditions (0%, and 1% RH) show a significant drop in u′ (i.e. loss in QD emission). The LED sealed under 10% RH shows a moderate drop in u′, and the LED in 80% RH shows an increase in u′, similar to the LED that was not sealed (i.e. ambient conditions). A control LED without QDs which was also sealed at 80% RH did not show any changes upon sealing. Next, when the LEDs are driven at 150 mA, a significant further drop is observed for the LEDs under dry conditions (0%, 1% RH), and the 10% RH LED shows a further moderate drop. The 80% RH and open LED show a further increase in u′, albeit small. After the 50 h data point, it is observed that the 0%, 1%, and 10% RH LEDs recover (albeit partly) from the initial drop, until 500 h, after which it stabilizes and decays after 1000 h and further. The LEDs at 80% RH and open condition show fairly stable behavior from 50 h and further. The reference LED without QDs at 80% RH shows no significant changes, which pinpoints that the observed effects are QD related.

[0087] The data show that 0% is not wanted and 1% is less desirable, 80% is the same as open, and that in the order of about 5-10% RH is a critical filling value for these lamps. In general, the lower value may be 5% RH but this may depend upon the lamp type and pressure. Hence, the value of at least 1% is chosen, even more especially at least 5%, such as at least 10%.

[0088] The above examples show that silica coated QDs require a controlled amount of water in their environment for optimal performance. Under dry conditions (0%, 1%, and 10% to a certain extent) a significant initial drop and recovery in QD emission is observed which is not desired in view of constant light output, CRI, and CCT over time. At 80% RH these effects are not observed. Therefore it is disclosed here that in case QD-LEDs are sealed, a controlled amount of water should be enclosed, preferably above 10%, and below 100%. The upper limit of 80-90% is in view of water condensation that could occur at lower temperature that may result in unwanted side-effects on the electronics (eg shorts), or an undesired visual appearance of droplets.

[0089] During sealing of glass bulbs using the conventional process in a production line, the melting of the stem into the bulb and the actual sealing of the bulb is done consecutively, on one and the same line.

[0090] In an embodiment, one may add silica powders within the LED bulb (e.g. for making a “frosted” LED bulb) that adsorb/absorb excess of water to avoid condensation of water at eg the LED (in view of shorts). This could also allow for higher than 100% RH (at RT) water enclosure if desired. At the same time, the silica may act as “getter” for water, so effectively take away water from the QDs. In that case, higher (initial) loading with water may be needed. In summary, when silica powder is added to the bulb, the (initial) optimal water concentration may be beyond the 10%-80% RH at RT. Silica powder, or other powder used, to make a bulb “frosted” may take up water. This will reduce the RH and hence affect the QD quantum efficiency. This would require to include more water than anticipated, because the silica will take up (significant amounts of) water and the RH will drop. The final RH in the bulb after the moisture level in the silica has equilibrated should still be >10% RH. Silica powder and/or other powders like titania, may be provided as coating at the internal surface of at least part of the wall(s) of the chamber, especially the light transmissive part, to provide a frosted appearance.

[0091] A further example was executed with other LEDs and supports (see table below). Substantially the same type of LEDs and QD-YAG:Ce phosphor mixture were used, and again the LEDs were enclosed in substantially the same type of glass bulbs under various RH (at room temperature): 0%, 1%, 10% and 80%. For reference, one glass bulb containing a QD-LED was not sealed (“open”) , and one LED without QDs was sealed under 80% humidity (“ref LED”).The operation temperatures were between 80-120° C. The same tests were performed with different components, and the same trend was found. Below, one of the series of test data is provided. This table indicates delta u′ as function of time (in hours) for LEDs enclosed in a glass bulb under various relative humidities at room temperature:

TABLE-US-00001 Time (h) filling −50 0 41 200 500 1000 2000 3000 ref 80% RH 0 0 −0.0004 −0.0005 −0.0007 −0.0007 −1E−04 −0.0007 LED 1 open 0 0.0007 0.0033 0.0029 0.0032 0.0026 −0.0019 −0.0068 2 open 0 0.0015 0.0031 0.0026 0.0032 0.0017 −0.0031 −0.0079 3  0% RH 0 −0.0119 −0.0211 −0.0117 −0.0083 −0.008 −0.0111 −0.015 4  0% RH 0 −0.0114 −0.0192 −0.0131 −0.0073 −0.0056 −0.0058 −0.0076 5  0% RH 0 −0.0117 −0.0196 −0.0106 −0.0043 −0.0043 −0.0065 −0.0101 6  1% RH 0 −0.0113 −0.0216 −0.0103 −0.0029 −0.0041 −0.013 −0.0218 7  1% RH 0 −0.0075 −0.0177 −0.0097 −0.0055 −0.0058 −0.0081 −0.011 8 10% RH 0 −0.001 −0.0028 0.0003 0.0026 0.0012 −0.0036 −0.0091 9 10% RH 0 −0.0035 −0.0113 −0.0055 −0.0019 −0.0024 −0.006 −0.0102 10 80% RH 0 0.0026 0.0047 0.0037 0.0038 0.0028 −0.0024 −0.0082 11 80% RH 0 0.0028 0.004 0.0033 0.0037 0.0026 −0.0029 −0.0086

[0092] The measurement at −50 h is a measurement before filling and sealing; i.e. a measurement in ambient air. Filling and sealing (melting pump stem) is done at 0 h, where after the 0 h measurement (and the other measurements) are done.

[0093] In a further example, red emitting quantum dots consisting of a CdSe core and a ZnS shell were silica coated using the reverse micelle method as adapted by Koole et al. (see above). They were incorporated into an optical quality silicone and dropcasted onto a glass plate. The silicone was cured at 150° C. for two hours. The optical properties of the quantum dot containing film were tested at 450 nm light of an intensity of 10 W/cm.sup.2 at a temperature of 100° C., detecting the intensity of the emitted light using an integrating sphere coupled to a spectrophotometer.

[0094] All Relative Humidities mentioned in the document are relative humidities at room temperature (19° C.). For example, 80% RH at 19° C. equals 1.77 vol % H.sub.2O.

[0095] Karl Fischer experiments, as known in the art, were used to measure relative humidities of gasses in light bulbs. Fight bulbs filled with water/gas mixtures were analyzed using a specific method for the analysis of water. The bulb is positioned in a cracker purged with dry nitrogen. The nitrogen purge gas is fed into a water detector based on a Karl-Fisher titration. After several blank runs (each lasting 15 minutes) the bulb is cracked and the water released is swept into the water detector for analysis.