Preparation And Photovoltaic Applied Research Of Graphene Quantum Dots

As a new type of zero-dimensional carbon nano-materials, due to their strongquantum confinement effect, edge/size effect and the accompanying unique ability ofhot carrier injection and up-conversion luminescence, graphene quantum dots (GQDs)have a broad application prospect in the field of optoelectronics. The solar cells thatused GQDs in it have the potential exceeding the Shockley-Queisser limit in solarenergy utilization. But the large-scale controllable preparation of GQDs still has notbeen solved effectively, and the research of their band gap and luminous mechanism isalso in the heated discussion. So it didn’t have much research of GQDs in the solarenergy application field.In this thesis, we have mainly studied the preparation of graphene quantum dotsby hydrothermal method and its application in the field of silicon-based solar cells.Firstly, we have prepared GQDs by hydrothermal route and the product has beencharacterized in detail. Hydrothermal method is a facile synthetic route that commonlyused for the preparation of GQDs. It uses micrometer-size graphene oxide (GO) sheetsas the starting material, by ultrasonication and oxidization in acid condition andhydrothermal deoxidation under weakly alkaline conditions. The product’s sizedistribution is relatively narrow and can be adjusted through different ultrasonic time.The GQDs’ band gap and luminescence properties show strong size dependence. Onthe other hand, the GQDs prepared by hydrothermal route have a lot ofoxygen-containing functional groups and thus have a higher reactivity. It offers thepotential for the following surface functionalization.Secondly, Fabrication of c-Si/GQDs heterojunction solar cells could beaccomplished via a facile solution process. Because GQDs have a large band gap and EC-LUMO offset, it can effectively suppress the carrier recombination at the interface.The insertion of GQDs can avoid the direct contact of Au with Si, suppressing theformation of thin silicide layer. The GQDs layer can not only act as the hole transportlayer but also serve as an electron blocking layer for reducing the carrierrecombination. This should lead to a lower saturation current density and hence alarger VOCfor the device. The efficient carrier separation and transportation at theGQDs/Si interface are suggested to be responsible for the high efficiency of theheterojunction solar cells. We also studied the effect of different silicon’s surfacemodification and different thickness/size distribution of GQDs on the device’sperformance. We analyzed the energy band structure and have a preliminaryunderstanding on the device’s mechanism. This kind of simple and effective method toconstruct the heterojunction devices offers the potential for the future research ofGQDs’heterojunction devices.Thirdly, the research based on the highly transparent conductive polymer PEDOT:PSS of silicon-based hybrid heterojunction devices has made great progress. To date,the power conversion efficiency generally can reach more than10%for this kind ofhybrid cells. However, PEDOT:PSS can only act as the hole transport layer in thedevice. After mixed with GQDs, it has become a highly conductive composite film thatcombined the excellent properties together. This film cannot only act as hole transportlayer, but also serves as electron blocking layer. At the same time, the film’sconductivity is very high and the carrier’s separation and transportation is also veryfast and efficient. The VOCand JSCof hybrid heterojunction device based on thisstructure are relatively high and the PCE can reach12.55%. It provides a newopportunity for the future’s research and develop low cost and high efficiency solarcell.