Byoung Wook Kwon

Charge separation and ultraviolet photovoltaic conversion of ZnO quantum dots conjugated with graphene nanoshells

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/cm2, 0.99 V, 0.24, and 2.33%, respectively. In this study, it was found that the UV power conversion efficiency of ZnO QDs could be significantly improved by invoking a fast photoinduced charge separation and the subsequent transport of carriers to the collecting electrodes through conjugation with highly conductive graphene nanoshell acceptors to the ZnO QDs donor.Graphical abstract

High efficiency ultraviolet photovoltaic cells based on ZnO–C60 core–shell QDs with organic–inorganic multilayer structure

We report on the successful conjugation of C60 molecules on the surface of ZnO quantum dots (QDs) and their application in multi-layer structured ultraviolet (UV) photovoltaic (PV) devices. In situgrowth of C60 on the surface of ZnO QDs with a core–shell structure was realized via a mild solution-process method, which resulted in an improvement in photo-induced charge separation and transport of carriers to the collecting electrodes in a fabricated device. The conjugation of the C60 with ZnO QDs leads to a PL quenching of about 99.8%, which can be attributed to an efficient transfer of photo-induced electrons from the ZnO QDs to the C60 through a Zn–O–C chemical bonding. UV PV cells with ZnO–C60 core–shell QDs as an active layer and fabricated through an all layer simple chemical method exhibit high power conversion efficiency as much as about 3.02%.

Synthesis of p-type GaN nanowires

GaN has been utilized in optoelectronics for two decades. However, p-type doping still remains crucial for realization of high performance GaN optoelectronics. Though Mg has been used as a p-dopant, its efficiency is low due to the formation of Mg-H complexes and/or structural defects in the course of doping. As a potential alternative p-type dopant, Cu has been recognized as an acceptor impurity for GaN. Herein, we report the fabrication of Cu-doped GaN nanowires (Cu:GaN NWs) and their p-type characteristics. The NWs were grown vertically via a vapor-liquid-solid (VLS) mechanism using a Au/Ni catalyst. Electrical characterization using a nanowire-field effect transistor (NW-FET) showed that the NWs exhibited n-type characteristics. However, with further annealing, the NWs showed p-type characteristics. A homo-junction structure (consisting of annealed Cu:GaN NW/n-type GaN thin film) exhibited p-n junction characteristics. A hybrid organic light emitting diode (OLED) employing the annealed Cu:GaN NWs as a hole injection layer (HIL) also demonstrated current injected luminescence. These results suggest that Cu can be used as a p-type dopant for GaN NWs.

Hybrid polymer-quantum dot based single active layer structured multi-functional device: Organic bistable device, LED and Photovoltaic cells

We demonstrate the hybrid polymer-quantum dot based multi-functional device (Organic bistable devices, Light-emitting diode, and Photovoltaic cell) with a single active-layer structure consisting of CdSe/ZnS semiconductor quantum-dots (QDs) dispersed in a poly N-vinylcarbazole (PVK) and 1,3,5-tirs-(N-phenylbenzimidazol-2-yl) benzene (TPBi) fabricated on indium-tin-oxide (ITO)/glass substrate by using a simple spin coating technique. The multi-functionality of the device as Organic bistable device (OBD), Light Emitting Diode (LED), and Photovoltaic cell can be successfully achieved by adding an electron transport layer (ETL) TPBi to OBD for attaining the functions of LED and Photovoltaic cell in which the lowest unoccupied molecular orbital (LUMO) level of TPBi is positioned at the energy level between the conduction band of CdSe/ZnS and LiF/Al electrode (band-gap engineering).