Sang Woo Seo

Enhancement of efficiency and long-term stability in graphene/Si-quantum-dot heterojunction photodetectors by employing bis(trifluoromethanesulfonyl)-amide as a dopant for graphene

We for the first time employ bis(trifluoromethanesulfonyl)-amide as a dopant for graphene to enhance the efficiency and the stability of graphene/Si quantum dot (SQDs)-embedded SiO2 (SQDs:SiO2) multilayer (ML) heterojunction photodetectors (PDs). With increasing the doping concentration (nD) to 30 mM, the sheet resistance of the doped-graphene transparent conductive electrode (TCE) sharply decreases to ∼155 ohm sq−1 with only 1% reduction in its transmittance at 550 nm, whilst the work function gradually increases to ∼4.95 eV, indicating p-type doping, useful for the graphene/SQDs:SiO2 MLs interface. The DC conductivity/optical conductivity ratio saturates to ∼75 at nD = 20 mM, much larger than the minimum industry standard (= 35) for the optoelectronic applications of TCEs. The PDs optimized at nD = 20 mM exhibit 0.413 A W−1 responsivity (R), 92 dB linear dynamic range, 1.09 × 1010 cm Hz1/2 W−1 detectivity, and 81.33% external quantum efficiency at a peak wavelength of 630 nm, and the loss of R is almost negligible while the PDs are kept for 700 h in air. These characteristics are comparable to those of commercially-available Si PDs and better than those of previously-reported graphene/Si PDs.

Enhancement of efficiency in graphene/porous silicon solar cells by co-doping graphene with gold nanoparticles and bis(trifluoromethanesulfonyl)-amide

Porous silicon (PSi) is an attractive building block for photonic devices, such as solar cells and photodetectors, due to its high surface to volume ratio, low reflection and high optical gain. In this work, PSi is prepared based on metal-assisted chemical etching by varying the deposition time (td) of Ag or Au nanoparticles (NPs) for the etching of Si from 1 to 7 s, thereby controlling the porosity of PSi. The co-doping of graphene with Au NPs and bis(trifluoromethanesulfonyl)-amide [(CF3SO2)2NH] is employed for the first time to enhance the performance of graphene/PSi Schottky-junction solar cells. Co-doping is proven to be very effective for increasing the work function of graphene as well as its electrical conductivity, resulting in efficient separation and collection of photo-induced electron–hole pairs in solar cells. The co-doped graphene/PSi solar cells show a maximum power conversion efficiency (PCE) of 10.69% at td = 5 s, and lose only 5% of the original PCE value after 15 days in air. These results provide a new route for fabricating highly efficient and stable graphene/PSi junction solar cells.

Semitransparent Flexible Organic Solar Cells Employing Doped-Graphene Layers as Anode and Cathode Electrodes

Semitransparent flexible photovoltaic cells are advantageous for effective use of solar energy in many areas such as building-integrated solar-power generation and portable photovoltaic chargers. We report semitransparent and flexible organic solar cells (FOSCs) with high aperture, composed of doped graphene layers, ZnO, P3HT:PCBM, and PEDOT:PSS as anode/cathode transparent conductive electrodes (TCEs), electron transport layer, photoactive layer, and hole transport layer, respectively, fabricated based on simple solution processing. The FOSCs do not only harvest solar energy from ultraviolet-visible region but are also less sensitive to near-infrared photons, indicating semitransparency. For the anode/cathode TCEs, graphene is doped with bis(trifluoromethanesulfonyl)-amide or triethylene tetramine, respectively. Power conversion efficiency (PCE) of 3.12% is obtained from the fundamental FOSC structure, and the PCE is further enhanced to 4.23% by adding an Al reflective mirror on the top or bottom side of the FOSCs. The FOSCs also exhibit remarkable mechanical flexibilities through bending tests for various curvature radii.