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
- 손동익; 권병욱; 양정도; 박동희; 서원선; 이현복; 이연진; 이창렬; 최원국
- 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
- NANO RESEARCH
- 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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