Yoon-Ho Nam

Highly active and durable carbon nitride fibers as metal-free bifunctional oxygen electrodes for flexible Zn–air batteries

The design of flexible, highly energetic, and durable bifunctional oxygen electrocatalysts is indispensable for rechargeable metal-air batteries. Herein we present a simple approach for the development of carbon nitride fibers co-doped with phosphorus and sulfur, grown in situ on carbon cloth (PS-CNFs) as a flexible electrode material, and demonstrate its outstanding bifunctional catalytic activities toward ORR and OER compared to those of precious metal-based Pt/C and IrO2 on account of the dual action of P and S, numerous active sites, high surface area, and enhanced charge transfer. Furthermore, we demonstrate the flexibility, suitability, and durability of PS-CNFs as air electrodes for primary and rechargeable Zn-air batteries. Primary Zn-air batteries using this electrode showed high peak power density (231 mW cm-2), specific capacity (698 mA h g-1; analogous energy density of 785 W h kg-1), open circuit potential (1.49 V), and outstanding durability of more than 240 h of operation followed by mechanical recharging. Significantly, three-electrode rechargeable Zn-air batteries revealed a superior charge-discharge voltage polarization of ∼0.82 V at 20 mA cm-2, exceptional reversibility, and continuous charge-discharge cycling stability during 600 cycles. This work provides a pioneering strategy for designing flexible and stretchable metal-free bifunctional catalysts as gas diffusion layers for future portable and wearable renewable energy conversion and storage devices.

Ultrathin Al2O3 interface achieving an 11.46% efficiency in planar n-Si/PEDOT:PSS hybrid solar cells

Hybrid organic-inorganic photovoltaic devices consisting of poly(3,4-etyhlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and n-type silicon have recently been investigated for their cost-efficiency and ease of fabrication. We demonstrate that the insertion of an ultrathin Al2O3 layer between n-Si and PEDOT:PSS significantly improves photovoltaic performance in comparison to the conventional interfacial oxide employing SiO2. A power-conversion efficiency of 11.46% was recorded at the optimal Al2O3 thickness of 2.3 nm. This result was achieved based upon increased built-in potential and improved charge collection via the electron blocking effect of Al2O3. In addition, the hydrophilicity enhanced by Al2O3 improved the coating uniformity of the PEDOT:PSS layer, resulting in a further reduction in surface recombination.

Mie resonance-mediated antireflection effects of Si nanocone arrays fabricated on 8-in. wafers using a nanoimprint technique

AbstractWe fabricated 8-in. Si nanocone (NC) arrays using a nanoimprint technique and investigated their optical characteristics. The NC arrays exhibited remarkable antireflection effects; the optical reflectance was less than 10% in the visible wavelength range. The photoluminescence intensity of the NC arrays was an order of magnitude larger than that of a planar wafer. Optical simulations and analyses suggested that the Mie resonance reduced effective refractive index, and multiple scattering in the NCs enabled the drastic decrease in reflection. PACS: 88.40.H-; 88.40.jp; 81.07.Gf

Toward a planar black silicon technology for 50 μm-thin crystalline silicon solar cells

Auger and surface recombinations are major drawbacks that deteriorate a photon-to-electron conversion efficiencies in nanostructured (NS) Si solar cells. As an alternative to conventional frontside nanostructuring, we report how backside nanostructuring is beneficial for carrier collection during photovoltaic operation that utilizes a 50-μm-thin wafer. Ultrathin (4.3-nm-thin) zinc oxide was also effective for providing passivated tunneling contacts at the nanostructured backsides, which led to the enhancement of 24% in power conversion efficiency.

A novel wrap-around metal contact optimized for radial p–n junction Si wire solar cells

We demonstrate wrap-around top metal contacts for radial junction silicon wire solar cells. A thin Ag sheet surrounds the silicon wires close to the planar bottom contacts. A fill factor of 65.4% was obtained, which is higher than that of conventional top contacts made using transparent conductive oxides and metal grids.

Hydroxyl functionalization improves the surface passivation of nanostructured silicon solar cells degraded by epitaxial regrowth

Metal-assisted chemical etching is useful and cost-efficient for nanostructuring the surface of crystalline silicon solar cells. We have found that the nanoscale epitaxy of silicon occurs, upon subsequent annealing, at the Al2O3/Si interface amorphized by metal-assisted etching. Since this epitaxial growth penetrates into the pre-formed Al2O3 film, the bonding nature at the newly formed interfaces (by the regrown epitaxy) is deteriorated, resulting in a poor performance of Al2O3 passivation. Compared to the conventional hydrogen (H–) passivation, hydroxyl functionalization by oxygen plasma treatment was more effective as the wafer became thinner. For ultrathin (∼50 μm) wafers, ∼30% depression in surface recombination velocity led to the improvement of ∼15.6% in the short circuit current. The effectiveness of hydroxyl passivation validated by ultrathin wafers would be beneficial for further reducing the wafer cost of nanostructured silicon solar cells.

Nanotexturing and Post-Etching for Diamond Wire Sawn Multicrystalline Silicon Solar Cell

The effects of nanotexturing and post-etching on the reflection and quantum efficiency properties of diamond wire sawn (DWS) multicrystalline silicon (mc-Si) solar cell have been investigated. The chemical solutions, which are acidic etching solution (HF-), metal assisted chemical etching (MAC etch) solutions (-HF-DI, HF--DI) and post-etching solution (diluted KOH at ), were used for micro- and nano-texturing at the surface of diamond wire sawn (DWS) mc-Si wafer. Experiments were performed with various post-etching time conditions in order to determine the optimized etching condition for solar cell. The reflectance of mc-Si wafer texturing with acidic etching solution showed a very high reflectance value of about 30% (w/o anti-reflection coating), which indicates the insufficient light absorption for solar cell. The formation of nano-texture on the surface of mc-Si contributed to the enhancement of light absorption. Also, post-etching time condition of 240 s was found adequate to the nano-texturing of mc-Si due to its high external quantum efficiency of about 30% at short wavelengths and high short circuit current density () of .

Planar n-Si/PEDOT:PSS hybrid heterojunction solar cells utilizing functionalized carbon nanoparticles synthesized via simple pyrolysis route.

Herein, we present a facile and simple strategy for in situ synthesis of functionalized carbon nanoparticles (CNPs) via direct pyrolysis of ethylenediaminetetraacetic acid (EDTA) on silicon surface. The CNPs were incorporated in hybrid planar n-Si and poly(3,4-etyhlenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solar cells to improve device performance. We demonstrate that the CNPs-incorporated devices showed increased electrical conductivity (reduced series resistance) and minority carrier lifetime (better charge carrier collection) than those of the cells without CNPs due to the existence of electrically conductive sp 2-hybridized carbon at the heterojunction interfaces. With an optimal concentration of CNPs, the hybrid solar cells exhibited power conversion efficiency up to 11.95%, with an open-circuit voltage of 614 mV, short-circuit current density of 26.34 mA cm-2, and fill factor of 73.93%. These results indicate that our approach is promising for the development of highly efficient organic-inorganic hybrid solar cells.

12.7% efficient 50-μm-thick crystalline silicon solar cells using back-side nanostructures passivated by ultrathin zinc oxide layer

Back-side nanostructured Si solar cells using thin wafers exhibit higher carrier collection than front-side counterpart. Ultrathin ZnO layer was adopted for passivated tunneling contacts, which led to the enhancement of 23% in cell efficiency.