Controllable Preparation Of Graphene Quantum Dots And Its Applications In Optoelectronic Devices

As a kind of zero-dimensional carbon nanomaterials, graphene quantum dots(GQDs) have several advantageous characteristics conferred by graphene. Moreover, GQDs possess unique optical and electrical transportation properties arising from their strong quantum confinement and edge effects. As a result, GQDs have intrigued intensive interest due to their promising applications in optoelectronic devices. Owing to the tunable band-gap and optical absorptivity, GQDs may have great applications in photodetectors and light-emitting diodes. And the ability of efficient hot-carrier harvesting and multiple carrier generation may make the possibility of exceeding traditional Shockley-Queisser limit of 32% of silicon based solar cells. However, there are also some chllenges in preparing large scale GQDs and widely applying GQDs in optoelectronic devices. Thus, the deeper research of GQDs and its applications is in high demand.In this thesis, we demonstrated the synthesis of GQDs through a hydrothermal method for high-performance deep ultraviolet(DUV) photodetectors and silicon-based solar cells. The main works include:1. The controllable solution-processing preparation of GQDs via a hydrothermal method. Briefly, graphene oxide(GO) sheets were obtained via improved Hummers’ method, and then graphene sheets(GSs) were prepared via thermal deoxidization of the GO sheets. After oxidizing the resulting GSs in a mixed solution of H2SO4 and HNO3, purified oxidized GSs were obtained. Eventually, smaller size GSs were obtained by hydrothermal deoxidization of GO sheets, and GQDs were obtained by hydrothermal cutting of GSs. The morphology and optical characterizations reveal that the size of GQDs is controllable, and the obtained GQDs are highly crystallized with a narrow size distribution. Moreover, the band-gap and luminescence properties show strong size dependence.2. GQD-based DUV photodetectors were constructed. Due to the tunable large band-gap of GQDs, the solution-processing devices were capable of detecting DUV light as short as 254 nm. By studying the energy band diagrams of devices and the mechanism of charge transport in GQDs, we had a preliminary understanding on the work mechanism of the GQD-based photodetectors. Meanwhile, the impacts of asymmetric electrode structures and the size of GQDs on devices performance were further studied. Finally, with the aid of an asymmetric electrode structure, 2-6 nm size GQD-based devices show the highest performance. An on/off ratio of ~6000 under 254 nm illumination was achieved. The devices also exhibit excellent stability and reproducibility with a fast response speed due to the high chemical/physical stability of GQDs. Given the solution-processing capability of the devices and extraordinary properties of GQDs, the use of GQDs will open up unique opportunities for future high-performance, low-cost DUV photodetectors.3. A novel type of solar cell based on GQDs/Si heterojunction was constructed. Due to the unique electron transportation properties and band structure of GQDs, the photo-generated electron-hole pairs could be effectively separated. On the other hand, the relatively large barrier for electrons would effectively suppress carrier recombination. Eventually, by optimizing the sizes of GQDs and the thickness of GQD layer, a power conversion efficiency(PCE) as high as 12.35 % under 1.5 G irradiation was obtained. Moreover, the devices exhibit good stability due to the high chemical/physical stability of GQDs. This work demonstrated our GQDs/Si heterojunction devices may have great potential applications in future high-performance and low-cost photovoltaics.