Charge Separation and Ultraviolet Photovoltaic Conversion of ZnO Quantum Dots Conjugated with Graphene Nanoshells

Charge Separation and Ultraviolet Photovoltaic Conversion of ZnO Quantum Dots Conjugated with Graphene Nanoshells
ZnO-graphene; quantum dots; Ultraviolet photovoltaic; photo convenrsion effiiency; Graphene nanoshell; ZnO-graphene quasi core-shell quantum dot (QD); ultraviolet (UV); photovoltaic cell; photoinduced charge separation; quenching
Issue Date
VOL 5, NO 10, 739-753
ZnO–graphene quasi core–shell quantum dot (QD) structures in which the inner ZnO QDs are covered with graphene nanoshells have been synthesized via a simple solution process method. The outer graphene nanoshells were identified as a single graphene layer using high resolution transmission electron microscopy (HR-TEM). Zn–O–C (graphene) chemical bonds between the inner ZnO QDs and the oxygen-containing functional groups introduced into the graphene layer are believed to be important in the formation of the consolidated quasi core–shell QD structure. A multilayer structure organic ultraviolet (UV) photovoltaic (PV) device was fabricated using ZnO–graphene core–shell QDs as the absorption layer. A quenching behavior as large as 71% near the UV emission peak for the ZnO–graphene core–shell QDs was observed in the photoluminescence. Density of state (DOS) calculations for the graphene using density functional theory (DFT) revealed that the static quenching can be attributed to a faster charge separation via the direct electron transfer from the conduction band (CB) of the ZnO QDs to the induced lowest unoccupied molecular orbitals (LUMO) of the graphene nanoshell resulting from the Zn–O–C bonding. This charge separation mechanism was confirmed experimentally using time-correlated single photon counting (TCSPC) measurements. The calculated average lifetime of 0.13 ns and 0.165 ns of the 375 and 383 nm UV emissions, respectively, for the ZnO–graphene core–shell QDs were approximately 10 times faster than those of 1.86 ns and 1.83 nm for the reference ZnO QDs; this is indicative of the existence of an additional high efficiency relaxation channel. The observed saturation current density (Jsc), open circuit voltage (Voc), fill factor (FF), and power conversion efficiency (η) were 196.4 μA/㎠, 0.99 V, 0.24, and 2.33%, respectively.
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