【作者】 王雷；

【导师】 孙洪波； 王海宇；

【作者基本信息】 吉林大学 ， 物理电子学， 2014， 博士

【摘要】 新型纳米光电材料在新能源等光电转换领域具有重要的应用前景。相比于传统的硅基太阳能电池和染料敏化系统，基于半导体量子点制备的光伏器件有着更大的摩尔消光系数，可调节的吸收光谱范围，以及理论上预期的更高的能量转换效率和更长的使用寿命。此外，荧光碳纳米材料，如碳纳米点（简称碳点）和石墨烯量子点等，由于具有储量丰富、毒性低、生物相容性高、化学惰性好及抗光致漂白强等特性，与高荧光量子效率的聚合物自组装纳米粒子一样，是近年出现的十分有前途的生物荧光探针，并具有替代传统有机光发射器件的潜力。但是，这些新型纳米光电材料目前各自都面临一些急待解决的问题，限制了基于这些新材料的光电器件的性能或者应用范围。本论文以超快光谱技术为主要研究手段，仔细研究这些新型纳米光电材料的光物理特性，揭示了其复合体系在光电转换领域里的工作机制，取得了以下创新研究成果：1.通过超快光谱技术手段，包括宽带飞秒瞬态吸收光谱、基于荧光上转换技术的飞秒时间分辨荧光及纳秒时间分辨的时间相关单光子计数技术，我们对石墨烯氧化物、石墨烯量子点和碳点的电子结构及其与发光态之间的关联进行了深入的研究。我们详细研究了石墨烯量子点中的发光机制，发现并阐明了表面类分子态对不同荧光发射的贡献。然后，我们在可作为合成石墨烯量子点的前驱体材料的石墨烯氧化物中首次发现了以前未曾直接观察到的量子限域的类石墨烯态，为阐述石墨烯氧化物的能级结构，判断所制备的石墨烯氧化物到底是“绝缘型”还是“半导体型”提供了可靠的光谱学依据。除了这些量子限域的类石墨烯态，我们还在石墨烯氧化物及其还原石墨烯氧化物中观察到了新颖的杂化态，这些杂化态是由围绕在类石墨烯态边缘的区域的碳原子由于具有的很高的sp3/sp2碳原子杂化例而形成。这很好的解释了之前在石墨烯量子点中观察到的固有的电子态。我们还进一步证明了在由电化学剥离法制备的碳点中也存在着这种杂化态的信号，由此说明这种杂化态是碳纳米材料中的共同存在的一种相互作用形式。随后，我们又通过对比多种碳点和石墨烯量子点的发光过程，我们进一步理解了荧光碳点和石墨烯量子点之间存在的共同的绿色荧光起源，解释了碳纳米骨架在这些荧光碳纳米材料中的作用。2.在利用纳米光电材料复合体系研发的新型光电转换器件领域，我们着重研究了敏化太阳能电池体系和有机/无机杂化太阳能电池体系。在前者，我们系统研究了染料敏化太阳能电池中的初始电子转移过程，发现了在敏化体系中普遍存在的有关电子注入过程的物理规律，提出了静态不均匀分布模型来系统解释敏化体系中电子注入动力学的复杂多指数衰减行为，并且这个模型可以推广到CdSe量子点敏化体系。对于后者，我们分了三步来研究。首先，我们通过时间分辨荧光技术探索了水溶液中混合聚合物纳米粒子中的超快能量传递过程，阐明了聚合物材料中能量转移距离显著缩短的根本原因。然后，利用飞秒时间分辨瞬态吸收光谱，我们研究了CdTe/CdS核壳结构量子点中的能级排列随CdS壳层厚度变化的规律，直观地从瞬态光谱上观测其超快电荷转移的演化过程。最后，我们通过超快光谱的手段系统揭示了由水相CdTe纳米晶和水相PPV基聚合物构成的杂化太阳能电池体系中的电荷分离和传输过程。我们的研究表明在这类杂化太阳能电池中，水相聚合物相对于生长后的纳米晶来说其作用大大减弱了。这一方面是因为水相聚合物自身在传导电荷方面存在不足，另一方面我们发现由于生长的纳米晶具有了稳态光谱中没有体现出来的自发形成的CdTe/CdS核壳结构，这使得纳米晶之间电荷的自发分离和传输更为便利。在CdS壳层的帮助下形成的纳米晶网络很好的承担了电荷分离和传输的任务，这也为开发基于水相半导体纳米晶的新型太阳能电池打开了新的窗口。更多还原

【Abstract】 Novel optoelectronic nanomaterials have great application potential in the photo electronconversion fields, especially for green energy fields. In contrast to traditional silicon-basedsolar cells and dye-sensitized systems, photovoltaic devices on the basis of semiconductorquantum dots possess larger molar extinction coefficient, tunable light-harvesting range, higherpower conversion efficiency in theory and longer working life. In addition, due to theabundance of raw material in nature, low toxicity, excellent biocompatibility, and superiority inchemical inertness and resistance to photobleaching, fluorescent carbon nanomaterials, such ascarbon nanodots and graphene quantum dots, as well as self-assembly polymer nanoparticleswith high photoluminescence quantum yield, have recently emerged as promising fluorescentbiological probes and candidates for the prospective substitution for traditional organiclight-emitting devices. However, there are still a lot of technical problems that need solving inthese novel optoelectronic nanomaterials, which limit their device performances or applicationrange. This thesis detailedly studies on the photophysical properties in these nanomaterials byultrafast spectroscopy, and unravels the work mechanisms of their composite systems in thefield of photovoltaic devices. The main researches are listed as follows:1. By the combined usage of various ultrafast spectroscopy techniques, including broadbandfemtosecond transient absorption spectroscopy, femtosecond time-resolved fluorescencedynamics measured by a fluorescence upconversion technique, as well as a nanosecondtime-correlated single-photon counting technique, we have deeply investigated the electronicstructure which could be related to the emission states in graphene oxide, graphene quantumdots and carbon nanodots. We have studied the photoluminescence mechanism in detail ingraphene quantum dots, and unraveled the contributions of molecule-like states to differentfluorescent emissions. Then, we have discovered quantum-confined graphene-like states in theprecursor of graphene quantum dots graphene oxide for the first time. This provided reliablespectroscopic evidences for unraveling the energy structure of graphene oxide, and estimating that the as-prepared graphene oxide was either “insulation” type or “semiconductor-like” type.Moreover, we have observed the novel hybrid states in graphene oxide and reduced grapheneoxide, which were originated from the regions with high sp3/sp2carbon atom ratio surroundingthe graphene-like states. It explained the previously observed intrinsic state in graphenequantum dots very well. We also demonstrated these hybrid states existed in electrochemicallyfabricated carbon dots. This indicated that the hybrid state was a common interaction mode inthese carbon nanomaterials. Following these results, in comparison with the excited-stateprocesses among carbon nanodots and graphene quantum dots, we have further understood thecommon origin of green luminescence, which could be explained by the role of carbonbackbone in these fluorescent carbon nanomaterials.2. In the fields of photo-electron conversion where these optoelectronic nanomaterials andtheir composite systems are used, we focus on the photophysical studies in sensitized andorganic/inorganic hybrid photovoltaic systems. For the former, we have systemically studiedthe initial nanointerfacial electron transfer processes in various dye-sensitized solar cells, andfound that there was a universal physical behavior which controlled the electron injectiondynamics. We have proposed a static inhomogeneous electronic coupling model to explain thecomplex multi-exponential decay for the electron injection in sensitized systems. Furthermore,we found that this model can be extended to CdSe quantum dot-sensitized films. For theinvestigation on the latter hybrid photovoltaic systems, we have prepared three steps. At first,by time-resolved fluorescence techniques, we have investigated the ultrafast energy transferprocesses in water-solution polymer-blend dots. These results explained the essential reason forthe energy transfer distance “shortening” in polymer materials. Then, as the CdS shell thicknessincreased in CdTe/CdS core-shell quantum dots, we have studied the band-structure-typechange by femtosecond time-resolved transient absorption spectroscopy. In that way, we havedirectly observed the transient spectral evolution of ultrafast charge transfer in these core-shellquantum dots. At last, by ultrafast spectroscopy techniques, we have unraveled the chargeseparation and transport mechanism in hybrid solar cells consisting of water-solution CdTenanocrystals and poly(p-phenylenevinylene)(PPV)-based aqueous polymers. This workindicated that the effect of aqueous polymers was weakened to a great extent in comparisonwith grown nanocrystals in these aqueous-processed hybrid solar cells. On one hand, this was due to the shortage of charge transport in aqueous polymers themselves. On the other hand, wehave found that the grown CdTe nanocrystals were partly capped CdS shells, andspontaneously formed a CdTe/CdS core-shell structure, which was “invisible” in steady-statespectroscopy. These core/shell nanocrystals facilitated the charge separation and transport in thevicinity of nanocrystals. With the help of CdS shell, these grown nanocrystals formed effectivecharge transport networks, and played a dominant role in the charge separation and carriertransport. As a result, these findings provided a new window for highly effective water-solutionsemiconductor nanocrystal based solar cells.更多还原