Photophysical Study In Novel Optoelectronic Nanomaterials And Their Composite Systems

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.