| 摘要: | 白光發光二極體(White light-emitting diode, WLED)已被廣泛地研究與應用於照明及顯示器，而在WLED中，由於量子點具有波長可調性、放射波長窄及高量子效率等優點，也被廣泛的應用為新穎下轉換材料。常見製備WLED的方法是使用不同顏色的量子點混合成白光，但因此製程相當複雜耗時且又有再吸收效應。因此，在本研究我們透過一個簡便的熱注射方式來製備具有雙放射波長的雙色CdSe量子點並解決再吸收效應，量子點依照不同硒粉前驅物之濃度分別命名為GR1.5、GR1.1與GR0.75，並使用雙色量子點及其與Y3Al5O12:Ce3+ 混合(YAG:Ce)的兩種方式製備WLED。
本研究分為三個部分，第一部分為雙色量子點的製備以及物性的探討，結果顯示透過控制成核成長速率，能夠製備出具有不同放射強度之綠色與紅色雙色量子點，且該綠色量子點對紅色量子點的激發面積比(Agex/Arex)隨著硒粉前驅物之濃度降低而上升，因此可以降低綠色量子點之能量被紅色量子點吸收之再吸收效應，此外硒之氧化程度直接影響整體之量子效率，實驗結果顯示GR0.75在所有雙色量子點中具有最低程度的硒之氧化以及最高的量子效率與最高放射面積比，並在時效4個月後的穩定性測試結果顯示其具有最佳的穩定性。
第二部分探討不同的雙色量子點以藍光晶片激發形成白光之元件特性，結果顯示GR0.75最能夠改善再吸收效應並成功製備WLED，且透過不同比例的UV膠與GR0.75混合，WLED的光色可從暖白調整至冷白光，穩定性也能夠從40分鐘提升至18小時，此外當UV膠在GR0.75中的重量為30 %時，該元件之色度座標為(0.35, 0.34)且應用於顯示器的背光源時，顯示該元件可在CIE1931色彩空間分別涵蓋89 %之NTSC色域與126 %之sRGB色域。
第三部分探討不同的雙色量子點添加於黃色螢光粉(Y3Al5O12:Ce3+)時，對於元件本身的特性之影響。結果顯示隨著雙色量子點的增加，WLED的光色可以從暖白調整到冷白光，當添加3 wt. %的GR1.5量子點時，發光效率可以從73提升至88 lm/W，增加至50 wt. %時，演色性能從66提升至83，其中當添加30 wt. %時可以被應用於固態照明，其元件特性為色度座標(0.33, 0.34)，色溫5482 K，演色性75，發光效率65 lm/W。顯示這些雙色量子點對於黃色螢光粉的發光效率與演色性有顯著的影響。此外這些WLED經過長時間測試後顯示添加GR1.5量子點所製備的WLED元件不僅能夠保持初始的光色及演色性，還可以提升整體效率。
由以上結果可知藉由控制成核成長速率可製備具有雙色的量子點，當應用在WLED，GR0.75為最具有應用潛力之雙色奈米螢光粉，具有簡化製程與減少再吸收效應之優點，而GR1.5最能夠改善黃色螢光粉的發光效率與演色性，顯示出雙色量子點能成功輔助螢光粉的光學特性。
;White light emitting diodes (WLEDs) have been investigated comprehensively due to their broad applications in lighting and display and the application of quantum dots (QDs) as novel down-converting materials in WLEDs has been also attracted lots of attentions because of their controllable emission wavelength, narrow band emission and high quantum yields (QYs) of QDs. The common method to prepare the WLEDs is mixing multi-colors of QDs, which is complex, time-consuming and has strong reabsorption. Therefore, in this study, we have demonstrated a facile hot-injection method to synthesize direct bi-color CdSe QDs (named as GRx, x = 1.5, 1.1 and 0.75, x value is associated with TOP-Se concentration) with dual-wavelengths against the reabsorption effect, and the GRx QDs have been applied to fabricate the QDs- and Y3Al5O12:Ce3+ (YAG:Ce)-based WLEDs.
This study includes three major parts. The first part focuses on the preparation and characterization of bi-color QDs. The results show that the GRx QDs consist of different emission intensities between green and red QDs through control of nucleation and growth rates. The excitation area ratio of green to red (Agex/Arex) of QDs can be significantly increased as the x value decreases, meaning that the energy of the green QDs absorbed by the red QD is reduced to improve the reabsorption effect. However, the QY of these GRx QDs is mainly affected by the degree of Se oxidation. The GR0.75 QDs have the lowest degree of Se oxidation, the highest QY value, the highest excitation area ratio, and the best stability after aging in hexane for 4 months.
The second part focuses on the application of different GRx QDs in blue LED. The results show that the GR0.75 QDs can strongly against the reabsorption effect and generate the white light successfully. When the weight ratio of UV resin in GR0.75 QDs increases from 0 to 45 wt. %, the color can be easily adjusted from warm to cold white light, and the operation time can be increased from 40 min to 18 hr. However, when the UV resin content is 30 wt. %, the Commission international de I’Eclairage (CIE) located at (0.35, 0.34) can be applied as backlight source, providing 89 and 126 % color gamut in NTSC and sRGB standards, respectively.
The third part focuses on the performance of YAG:Ce-based WLED when the GRx QDs have been added to YAG:Ce phosphor as additional components. The color of YAG:Ce-based WLEDs can be changed from warm to cold white light as the contents of GRx QDs increase. The best improvement in luminous efficacy is the addition of 3 wt. % of the GR1.5 QDs and the efficacy can be improved from 73 to 88 lm/W. On the other hand, the CRI can be promoted from 66 to 83 when the GR1.5 QDs content is 50 wt. %. Besides, the optimal condition to be applied in solid state lighting is mixing with 30 wt. % GR1.5 QDs. The CIE, CCT, CRI and luminous efficacy of device properties is (0.33, 0.34), 5482 K, 75 and 65 lm/W, respectively. The results show that these GRx QDs have a significant effect on luminous efficacy and color rendering index (CRI) of YAG:Ce phosphor. In addition, the long-term test shows that the addition of GR1.5 QDs into WLED can not only maintain the initial lighting color and CRI, but also improve the overall efficacy.
Based on the above results, the bi-color GRx QDs can be prepared by controlling the nucleation and growth rates. In QDs-WLED applications, the GR0.75 QDs exhibit the potential as bi-color nanophosphor to simplify the preparation procedure and decrease the reabsorption effect. In addition, the GRx QDs can be used as additional components to increase the luminous efficacy and CRI, especially GR1.5 QDs, compared to the original YAG:Ce-based WLED. |
