【作者】 王静；

【导师】 王亚非；

【作者基本信息】 电子科技大学 ， 光学工程， 2015， 博士

【摘要】 纳米胶质量子点(Colloidal quantum dot,CQD)材料是目前国际研究的焦点,可用于各种光电器件中,如光电探测器、光电二极管、太阳能电池等。其中利用这种材料制成的第三代太阳能电池,具有成本低、可实现大面积生产、宽太阳能光谱吸收等优点,为太阳能电池产业带来了新希望。然而,相比于块体半导体,量子点材料的光电特性及很多实验现象尚未被深入理解,这制约着其推广应用以及对器件性能的提升。精确的激子寿命测量是解释材料光电实验现象的第一步。虽然光致发光(Photoluminescence,PL)技术是测量激子寿命的主要手段,但其较低的测量精度,使得研究人员们还无法建立关于量子点中激子输运等动态机制的精确物理模型。光载流子辐射测量技术(Photo-carrier radiometry,PCR)作为一种频域PL调制技术,具有全光、无损、非接触、快速等优点,非常适用于检测柔软的易受损的CQD材料。PCR的频率调制特性使其对CQD中激子的动态弛豫过程进行研究时显示出高的信噪比和精度,尤其是PCR信号的相位成分。但PCR技术欠缺与CQD材料相匹配的模型与方法。本文基于传统PCR技术发展了CQD-PCR模型,完成了CQD中激子输运性质的实验研究和理论分析。第一,发展了适用于量子点材料的CQD-PCR理论模型,使得PCR技术对半导体材料电参数的测量应用领域从三维块体材料扩展到了零维量子点材料。(1)建立了三种PCR频域信号分析模型,用以反演CQD中的激子寿命,包括:根据任意的量子点尺寸分布建立的连续单独Voigt型高斯分布模型、简化的多重弛豫方式叠加的离散ad hoc模型和经过严格数学推导得出的变分反演模型。(2)发展了激子弛豫速率方程模型。该模型同时包含了激子的辐射复合和非辐射复合通道,并且分析了PbS量子点材料的最低激子能级分裂以及最低激子能级与缺陷能级进行能量交换时会有多个声子参与的情况。第二,搭建了PCR实验系统,通过对两种经过不同表面修饰法得到的PbS CQD薄膜(非耦合样品和耦合样品)的PCR实验研究,验证了量子点材料的CQD-PCR理论模型,为CQD材料中激子输运等动态特性的研究开辟了一条新的途径。(1)通过CQD-PCR频率扫描实验结果验证了三种PCR频域信号分析模型的正确性,反演重建得到了PbS量子点薄膜材料在各温度下的激子寿命分布谱线和精确的有效寿命。在数据处理过程中发现,Voigt型只适用于非耦合样品,ad hoc型和变分反演法对两种样品都适用,但通过对比,变分反演法计算得到的激子寿命谱线可靠性最高,并且能够分辨出同时发生的不同激子弛豫机制,这在相关领域的研究中尚属首例。研究发现具有不同量子点间距的薄膜材料,能够产生不同类型的寿命分布谱线,体现出不同的激子输运性质,这对我们深入理解CQD材料中的激子弛豫机制具有重要意义。量子点间距离较长的薄膜(非耦合样品)中,光生激子被限制在本地复合,而量子点间距离较短的薄膜(耦合样品)中,光生激子能够通过最邻近跳跃(Nearest neighbour hopping,NNH)和福斯特共振能量转移(F?rster resonance energy transfer,FRET)等方式进入邻近的量子点中,再复合回到基态。PCR方法测得的激子寿命精度高于时间分辨PL方法。(2)分析随温度变化的激子寿命分布,验证了速率方程模型的正确性。分析中分别提取出了两种样品的辐射复合寿命τR和非辐射复合寿命τNR,并且计算得到了PbS量子点的最低激子能级的分裂能量差。所得结果进一步验证了两种样品中具有不同的激子弛豫方式,也符合材料制备过程中采取了不同表面修饰方法这一情况。(3)进行了三种PCR辅助实验研究,实验结果与PCR频率扫描实验结果相互补充,具有良好的一致性,进一步证明PCR技术是研究量子点材料光电特性的有效手段。PCR强度扫描实验说明本文所用PbS量子点薄膜样品对入射激光的响应是线性的;根据Arrhenius公式对PCR温度扫描实验结果进行分析,得到了两种样品的非辐射复合中心的激活能;PCR时间扫描实验结果证实了两种样品中所含缺陷能级的状态。第三,首次成功利用PCR技术对CQD太阳能电池开展了初步的实验研究。本工作中测试了PbS CQD点太阳能电池的伏安特性曲线,并对其开展了PCR频率扫描实验,重建了太阳能电池的光吸收层中的激子寿命分布谱线。发现电池中的激子寿命稍长于单纯薄膜中的激子寿命。综上所述,本文利用PCR技术成功开展了对量子点材料性能的研究。建立了三种适用于量子点材料的PCR频域信号分析模型,分别重建了两种样品的激子寿命分布谱线,反演计算得到了激子有效寿命,提高了量子点激子寿命测量的精度,深入理解了量子点中的激子弛豫途径,发展了量子点中的PCR方法。在纳米材料和器件席卷光电子、微电子、材料科学、生物医学等领域的趋势下,CQD-PCR方法必将发展成为一种理论更完备,测量更精确的技术。更多还原

【Abstract】 Colloidal quantum dots(CQD) materials which can be applied on photodetector, photodiode, solar cells and so on, have been the world research focus in recent years. The third generation solar cells based on those materials promote the solar cell industry due to its merits of low-costs, broadband solar spectra absorption and roll-to-roll fabrication. However, the optical-electronic properties and many experiments phenomena of CQD are still opaque which restricts the application. Measuring accurate exciton lifetime is the first step for understanding the optical-electronic properties of CQDs. Photoluminescence(PL) is the common method for exciton lifetime measurements, but its low testing accuracy impedes the formation of accurate physical models for dynamic mechanism of exciton transport in CQDs. Photo-carrier radiometry(PCR), as a frequency domain PL modulation technology is all-optical, non-destructive, non-contact and fast which is applicable for soft and vulnerable CQD testing. Moreover, the frequency modulation property of PCR makes the tests of CQDs with high signal to noise ratio and accuracy, especially the PCR phase signal. But there lacks of matched models and method of PCR for CQDs materials. In this work, a CQD-PCR model was developed based on conventional PCR method and exciton transportation properties in CQDs were tested and analyzed.Firstly, a CQD-PCR theoretical model was developed, this promote the application area of PCR for semiconductor electrical parameters measurements from 3D bulk materials to 0D CQD materials.(1) Three CQD-PCR models describing lifetime distribution have been developed to analyze PCR frequency scan signals: the first one based on Gaussion CQD size distribution can yield homogeneous and inhomogeneous Voigt lifetime broadening. The second one is based on direct ad hoc fits to PCR amplitude and phase data, reconstructing a superposition of multiple independent lifetime line spectra. The last one is a rigorous variational method developed to invert simultaneous integral equations in the lifetime distribution function(spectrum) and involves similar fits to the PCR in-phase and quadrature signal channels to reconstruct the lifetime spectrum.(2) A PCR theory with a rate equation model taking into account the lowest 1S-1S exciton state splitting and multi-phonon-assisted carrier trapping to states outside a PbS QD was also developed.Secondly, a PCR experimental system was build up. The correctness and applicability of the CQD-PCR models were verified by the PCR frequency scan experiments of two PbS CQD thin films which were handled with different surface passivation methods(un-coupled and coupled sample), that means a new technological approach was found for exciton transportation properties measurements in CQDs.(1) More accurate exciton mean-lifetime has been found and lifetime distribution spectra at various temperatures in the PbS thin films have been reconstructed. During the research process, the Voigt model is only applicable for un-coupled sample, while the ad hoc model and the variational model are applicable for both samples. The lifetime distribution spectra reconstructed from the variational model with highest reliability could distinguish simultaneous multiple exciton decay mechanisms, this is the first case in related research fields. We found that, samples with different inter-dot spacing show different lifetime distribution spectra and exciton transport properties, which has great significance for understanding the exciton relaxation mechanisms in CQDs. For non-interacting(uncoupled) CQDs LO phonon-exciton interactions limit the radiative lifetime and luminescence-photon-flux throughout the entire 100 – 300 K temperature range. For interacting(coupled) CQDs, variational and ad hoc lifetime line spectra exhibit similar mechanisms to uncoupled CQDs with the important addition of direct exciton-to-exciton transitions in the form of efficient transport(hopping, tunneling or FRET) at all temperatures which result in de-excitation dynamics dominated by radiative emission channels. PCR method is more accurate than PL.(2) The correctness of the rate equation model was verified by analyzing the temperature-dependence of exciton lifetime distribution. The radiative τR and non-radiative lifetimes τNR of the two samples were extracted, and the lowest 1S-1S splitting energy were calculated.(3) Three supplementary PCR experiments were also carried out, which show coincident results with PCR frequency scan results. PCR intensity scan results show that the PCR signals from PbS CQD thin films are linear under laser exposure. Activation energy of the non-radiative recombination center was found from PCR temperature scan data. PCR time scan results show that the two samples have different trap states.Thirdly, preliminary PCR experiments were carried out on PbS CQD solar cell for the first time. The current-voltage curves were tested at various temperatures. PCR frequency scans were done and exciton lifetime distribution spectra of the PbS CQDs which is the light absorption layer in the solar cell were reconstructed. It was found that the exciton lifetime in the solar cell is longer than in the thin films.In summary, CQD-PCR method and experiments were finished for CQD properties research. Three CQD-PCR frequency scan signal analyzing models were deveploded. Based on this, exciton lifetime distribution spectra for the two PbS CQD thin film samples were reconstructed and accurate exciton lifetime values were calculated. In this work, the precision of exaction lifetime measurements were improved and the exciton decay pathways in CQDs were deeply researched. Under the trend of nano-materails and nano-devices wild application in optoelectronics, microelectronics, material science and biomedicine field, CQD-PCR method will be a more maturity and accurate testing technology.更多还原