Nanocrystalline YDC thin films with grain-sizes ranging from 38 to 93-nm were prepared using pulsed laser deposition followed by thermal annealing. Ionic conductivity decreased up to four orders of magnitude as the grain size increased. Using energy-dispersive X-ray spectroscopy, we showed that the counter-intuitive reduction in conductivity with grain-size is likely due to dopant and impurity segregation near grain-boundaries. Spectroscopic evidence suggests that the blocking effect due to defect segregation is more dominant on ionic conductivity than the grainsizes.

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In recent years, there has been growing consideration of renewable energy especially photovoltaic devices. A silicon (Si) based solar cell is the most popularly and frequently considered among the photovoltaic devices, but its bulk thickness issue lowers the performance and hinders widespread application due to the material cost. Also, this thick nature causes difference in length between minority carrier diffusion and sufficient light absorption. To mitigate the issues there have been many recent studies on Si photovoltaic devices adopting nanostructuring strategies to enhance the performance. Therefore, we report two different approaches on recent nanostructuring techniques for photovoltaic devices; bottom-up and top-down processes, which are composed of vapor-liquid-solid, solution-liquid-solid, reactive ion etching with Langmuir Blodgett and metal assisted chemical etching. Those fabrication processes enable the fabrication of nanostructures with a highly ordered and alignment structures leading to enhance the light absorption and have an appropriate thickness of Si substrate regressing Auger recombination. The fabricated nanowire and nanocone array structures outperform existing results with light absorption exceeding 90%.

Grain boundaries are known to block ionic conduction across grain boundaries in oxide ion conductors due to adjacent space charge layers. Since the positively charged grain boundary core is intensified with a high local concentration of defects such as oxygen vacancies, uniform distribution of a dopant may mitigate the formation of space charge layers and enhance the ionic conductivity. To investigate the dopant segregation effect on the space charge layer and ionic conductivity, we provided thermal energy to nanocrystalline gadolinia-doped ceria (GDC) thin film by post-annealing at different temperatures of 700 • C, 900 • C, and 1100 • C. STEM-EELS analysis demonstrates strong dopant segregation and a higher Ce 3+ content near the grain boundary than in the bulk after post-annealing. The concurrent segregation of dopants and Ce 3+ ions implies that once thermal treatment is applied to nanocrystalline GDC thin films, complete space charge layers are formed while the non-thermally treated GDC film with a uniform distribution of dopants has less of a space charge effect and exhibits superior ionic conductivity.Nanocrystalline materials have attracted a great deal of attention for applications in various energy conversion and storage systems including rechargeable lithium ion batteries, oxygen/ozone gas sensors, oxygen storage systems, and solid oxide fuel cells (SOFCs). 1-5 Compared to macro-or micro-scale materials, nanocrystalline materials possess extraordinary electrical or electrochemical properties (i.e., ionic conduction or surface exchange reactions). One major cause of the unusual properties is related to the fine grain size (<100 nm), which corresponds to an extremely high grain boundary density. For instance, nanocrystalline calcium oxide-stabilized zirconia (0.14 um grain size) showed a 15 times higher specific grain boundary conductivity than microcrystalline materials (>4 um grain size). 6 In terms of the surface kinetics, a nanocrystalline interlayer (∼65 nm grain size) applied to the interface between the cathode and electrolyte of a SOFC exhibited a 5-6 times lower electrode interface resistance than a microcrystalline interlayer (∼6 um grain size). 7 Therefore, understanding the grain boundary properties is important to appropriately utilize nanocrystalline materials for energy conversion devices due to their exceptional features compared to bulk materials.Recently, oxide ion conductors (e.g., gadolinia-, samaria-, or yttriadoped ceria (GDC, SDC, or YDC)) usually with a polycrystalline structure, have been widely studied as electrolyte materials for lowtemperature SOFCs (LT-SOFCs) since they exhibit higher ionic conductivity and surface exchange rate than the most commonly used electrolyte, yttria-stabilized zirconia (YSZ), especially in the low operating temperature regime (<500 • C). 7-9 In terms of the ionic conductivity, polycrystalline acceptor-doped ceria has shown grain boundary blocking of ionic conduction originating from the space charge effect. 10-14 A high local concentration of oxygen...

Scandia-stabilized zirconia (ScSZ) is employed as a cathodic functional layer onto yttria-stabilized zirconia based fuel cell systems for low-temperature solid oxide fuel cells.

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