Size Regulation And Surface Functionalization Of Graphene Quantum Dots And Their Applications In Organic Solar Cells

Graphene quantum dots(GQDs)assume unique physical/chemical properties,such as ultral high carrirer mobility,tunable bad-gap,and so on,and hold great promising for the applications in the field of organic solar cells.It is well known that the active layer whithin the organic solar cell mainly dominates its photovoltaic performance.It is generally considered that increasing the exciton separation efficiency and carrier transport rate of the active layer material is the most effective way to improve the device performance.The photoelectronic properties of GQDs can be effectively regulated by the modulation of their lateral size and the functionality modification of their surface,which make GQDs available in the active layer of organic solar cells.At the same time,further exploration of the mechanism of GQDs affecting the performance of organic solar cells is also conducive to the deep understanding of processes and mechanisms of exciton generation,diffusion and separation in organic solar cells.Therefore,based on the previous research results,the effect mechanism of GQDs in P3HT/PCBM/GQDs-based ternary hybrid organic solar cell is studied firstly in this paper.Then GQDs with controlled lateral sizes are acquired through gel column chromatography.The surface functionalties of GQDs are further modified by redctution and even certain chemical reactions.Finally,the organic solar cells containg different GQDs are assembled,and their photovoltaic performances are studied.The main reaserch works and as-acquired results are as follows:1.The effects of GQDs prepared by photo-Fenton reaction as electron acceptors on the performance of binary(ITO/Zn O/P3HT:GQDs/Mo O₃/Ag)and ternary(ITO/Zn O/P3HT:PCBM:GQDs/Mo O₃/Ag)organic solar cells are systematically investigated.The highest power conversion efficiencies(PCEs)of 0.25%and 4.13%are achieved for the two types of devices,respectively.The improvement mechanisms of GQDs on the performance of different types of devices are systematically studied by comparing the film morphology of the active layer and performance of the two types of devices.It is found that the introduced GQDs play a dual role of the electron acceptor and charge transport channel in ternary organic solar cells,promoting the processes of exciton separation and charge transport,thereby improving the device performance.2.To get insight into the effects of the GQDs lateral dimension on the performance of organic solar cells,the GQDs with wide size distribution prepared by photo-Fenton reaction are separated into eight kinds of GQDs with different sizes through gel column chromatography.Among them,three kinds of GQDs,namely GQDs-blue,GQDs-green,GQDs-orange,with average sizes of 5.6,11.2,17.6 nm,respectively,are introduced into the active layer of organic solar cells,and binary(ITO/Zn O/P3HT:GQDs/Mo O₃/Ag)and ternary(ITO/Zn O/P3HT:PCBM:GQDs/Mo O₃/Ag)organic solar cells based on the three types of GQDs are fabricated.The effect of GQDs size on the performances of ternary organic solar cells is studied.It is found the size modulating on GQDs can adjust their energy level for matching with the electron band structures of donor/acceptor in the active layer,and thus the performance of organic solar cells can be effectively improved.3.The surface functionality of GQDs prepared by photo-Fenton reaction are modified through certain chemical reactions,imcluding the reductions of the oxygen containing groups.First,the GQDs are chemically modified with hexylamine,oleylamine,and aniline.The ternary organic solar cells(Ag/Mo O₃/P3HT:PCBM:GQDs/Zn O/ITO)are fabricated using the as-modified GQDs as electron acceptor,and the highest PCE(3.87%)is obtained from the device containing hexamine-modified GQDs.In addition,the oxygen containing groups on GQDs are reduced in different ways.It is illustrated that the highest fluorescence quantum yield(14.8%)can be obtained from hydrothermally reduced GQDs.Ternary organic solar cells(Ag/Mo O₃/P3HT:PCBM:GQDs/Zn O/ITO)are fabricated using the as-reduced GQDs as electron acceptor,and its PCE can reach to 4.12%.4.Organic solar cells(ITO/Zn O:GQDs/P3HT:PCBM/Mo O₃/Ag;ITO/GO/P3HT:PCBM/Mo O₃/Ag)are fabricated using the as-reduced GQDs and graphene oxide(GO)as electron transport layer materials respectively,and it is found that the PCE(3.15%)obtained from the device containing hydrothermally reduced GQDs is higher than that of the control group(3.01%),while the PCE(2.53%)obtained from the device containing GO is lower than that of the control group.