无镉半导体量子点的光致发光性质及其白光发光二极管应用研究

	无镉半导体量子点的光致发光性质及其白光发光二极管应用研究
	袁曦
学位类型	博士
导师	赵家龙
	2014-07
学位授予单位	中国科学院大学
学位专业	凝聚态物理
摘要	胶体量子点（QD）是可溶液加工的半导体纳米晶，具有尺寸可调谐的光电子性质，被广泛地应用在发光二极管（LED）、光伏电池和生物荧光标记等领域。量子点合成技术经过三十多年的发展，人们已经可以合成各种高质量的纳米材料，其光致发光效率可以达到90%以上。此外，经过科研人员的努力，基于量子点的QD-LED性能得到了很大的改善，其外量子效率（EQE）从不到0.01%提高到18%，已经接近于有机发光二极管（OLED）的外量子效率。但是这些量子点中大多含有Cd等重金属元素，严重影响我们的生存环境及量子点器件的商品化转换。因此，关于低毒性的，不含重金属的半导体量子点的研究被给予了广泛的关注，如I-III-VI族和过渡金属掺杂量子点等。这些不含重金属的量子点的发光可通过尺寸与组分在可见和近红外区范围内调谐，其发光效率可达70-85%，适用于照明及显示等领域。然而，基于无镉量子点的发光二极管的性能远低于基于CdSe量子点的性能指标。因此，研究无镉量子点与发光二极管常用电荷传输材料间的相互作用及提高量子点自身的稳定性，对于优化无镉量子点发光器件性能具有重要的意义。本论文主要研究了无镉CuInS2-ZnS合金（ZnCuInS）量子点与有机电荷传输材料之间的能量传递过程，无镉Mn掺杂量子点的发光热稳定性和无镉Cu掺杂量子点在白光LED中的应用，获得如下创新性的研究结果： 1、通过稳态和时间分辨光谱研究了无机/有机混合薄膜中从有机电荷传输材料到ZnCuInS量子点的能量传递过程。量子点发光强度的增加与有机电荷传输材料荧光寿命的变短说明发生了从供体有机电荷传输材料到受体量子点的能量传递过程。我们发现能量传递效率明显随着ZnCuInS量子点中Zn含量的减少和量子点尺寸的增加而迅速增大。这一结果与我们计算出的Forster半径随量子点尺寸的增加和Zn含量的减少从3 nm增加到5 nm的结果相吻合。此外，在小尺寸ZnCuInS量子点与电荷传输材料的混合体系中，观察到了量子点电荷分离相关的荧光猝灭现象。并基于能级取向，很好地解释了有机电荷传输材料与ZnCuInS量子点之间的能量传递和电荷分离过程。 2、通过测量Mn:ZnS、Mn:ZnSe及Mn:ZnSeS量子点从80 K到500 K的变温发光强度及变温荧光寿命，研究了量子点发光的热稳定性。发现掺杂量子点基质的能级结构和壳层厚度严重影响了Mn2+离子发光的热稳定性。在500 K温度下，Mn:ZnS的发光几乎没有被热猝灭，荧光量子效率仍然可以达到~50 %。Mn:ZnSe及Mn:ZnSeS也表现出了很好的热稳定性，其热猝灭温度均超过了200 ºC，明显优于CdSe量子点。在加热-降温循环实验中发现，厚壳层的Mn掺杂量子点的发光是可逆的，但薄壳层的Mn掺杂量子点的发光只能部分恢复。此外，从理论上对Mn掺杂量子点的发光热稳定性机理进行了讨论。 3、制备了基于具有超大斯托克斯位移的Cu:ZnInS/ZnS量子点的高效暖白光LED。通过非注入一锅法合成了发光颜色组分可调的Cu:ZnInS/ZnS量子点，其发光可从绿光逐渐调到深红光。将红、绿光Cu:ZnInS量子点与蓝光GaN基LED相结合制备出的白光LED，其显色指数可高达96，流明效率可达70-78 lm/W，色温可在3800到5760 K之间调节。通过时间分辨光谱的测量，发现在Cu:ZnInS量子点白光LED中，Cu:ZnInS量子点间的能量传递基本可以忽略，这与Cu:ZnInS量子点的吸收谱和发射谱之间的光谱交叠很小的结果相吻合。实验结果表明无镉Cu:ZnInS量子点适合应用于固态照明。
其他摘要	Colloidal quantum dots (QDs) are solution-processed nanoscale crystals of semiconducting materials. The QDs with unique size-dependent optical properties have been widely applied in light-emitting diodes (LEDs), photovoltaic cells, and biological labels. After three decades’ development of synthesis technology for QDs, the photoluminescence (PL) quantum yield (QY) of QDs has reached 90%. Moreover, the external quantum efficiency of QD-LEDs has increased to 18% from less than 0.01%, which approaches to that of organic light emitting diodes (OLEDs). However, the intrinsic toxicity of elements such as cadmium potentially hinders their ultimate research transformation and commercialization. Zinc chalcogenide doped with transition metal ions and I-III-VI based semiconductor nanomaterials have markedly low toxicity and large ensemble Stokes shift, avoiding the self-absorption process. These properties make them ideal for different optical applications. These cadmium-free QDs exhibit size- and composition- tunable emission from the visible to NIR region and the PL QY up to 70~85%, which make them relevant to applications in solid state lighting and full color displays. Despite all, the performances of LEDs based on cadmium-free QDs are much lower than those based on CdSe QDs. The energy transfer between QDs and charge transporting materials and the QD emission thermal stability are very important to optimize the performances of QD-LEDs. Therefore, we study the energy transfer between charge transporting materials and cadmium-free CuInS2-based QDs, the thermal stability of cadmium-free Mn-doped QDs, and the applications of cadmium-free Cu-doped QDs in white LEDs. The original works are organized as follows: (1) The energy transfer processes from organic charge transporting materials (CTMs) to CuInS2-ZnS alloyed (ZCIS) QDs with different emission wavelength were studied by steady-state and time-resolved PL spectroscopy. The change in the PL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearly demonstrated an efficient energy transfer process in the ZCIS/CTM blend films. It was found that the efficiency of Forster resonance energy transfer significantly increases with increasing the particle size and decreasing the Zn content in the QDs, which is well consistent with the estimated Forster radii (R0) varying from 3 nm to 5 nm. In addition, the PL quenching of the QDs related to the charge separation process was also observed in some of the samples. The energy transfer and charge separation processes in the films were well explained based on the band alignment between the ZCIS QDs and CTMs. (2) The thermal stability of luminescence is important for application of QDs in light-emitting devices. The temperature-dependent PL intensities and decay times of Mn-doped ZnS, ZnSe, and ZnSeS alloyed core/shell QD films were studied in the temperature range from 80 to 500 K by steady-state and time-resolved PL spectroscopy. It was found that the thermal stability of Mn-doped QD emissions was significantly dependent on the shell thickness and the host bandgap, which was higher than that of workhorse CdSe QDs. Nearly no PL quenching took place in Mn:ZnS QDs with a thick ZnS shell, which kept a high PL QY of ~50% even at 500 K. And the thermally stable PL was also observed in highly luminescent Mn:ZnSe and Mn:ZnSeS QDs with the quenching temperature over 200 ºC. Further, the stability of Mn-doped QDs with different shell thickness at high temperature was also examined through heating-cooling cycling experiments. The PL quenching in the thick shell-coated Mn-doped QDs was almost totally recovered. The PL quenching mechanisms of the Mn2+ ion emissions were discussed. (3) The efficient white LEDs based on Cu:ZnInS/ZnS core/shell QDs with super large Stokes shifts were fabricated. The composition-controllable Cu:ZnInS/ZnS QDs with tunable emission from deep red to green were prepared by a one-pot noninjection synthetic approach. The high performance Cu:ZnInS QD-white LEDs with colour rendering index up to 96, luminous efficiency of 70-78 lm/W, and colour temperature of 3800-5760 K were successfully fabricated by integration of red and green Cu-doped QDs. Negligible energy transfer between Cu-doped QDs was clearly found by measuring the PL lifetimes of the QDs, consistent with the small spectral overlap between QD emission and absorption. The experimental results indicated low toxic Cu:ZnInS/ZnS QDs could be suitable for solid state lighting.
语种	中文
文献类型	学位论文
条目标识符	http://ir.ciomp.ac.cn/handle/181722/41497
专题	中科院长春光机所知识产出
推荐引用方式 GB/T 7714	袁曦. 无镉半导体量子点的光致发光性质及其白光发光二极管应用研究[D]. 中国科学院大学,2014.

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