The electrical transport properties of nanocrystalline polycrystalline semiconductors often vary with grain size, but this correlation does not prove that size is the sole controlling factor. Grain size reduction is usually accompanied by increased grain boundary density, altering defect occupancy, depletion layer width, and cross-boundary barriers. This paper, based on a grain boundary barrier model, employs literature review and comparative analysis to synthesize experimental results from zero-dimensional quantum dot solids, one-dimensional polycrystalline silicon nanowires, and two-dimensional polycrystalline thin films, analyzing the different effects of geometric scale and grain boundary electrical activity on carrier transport. The results show that average grain size primarily affects the frequency of carrier encounters with the interface and the selectable conductive paths; the energy level, density, occupancy, and atomic structure of grain boundary states determine the degree of obstruction at a single interface and may simultaneously reduce carrier concentration and mobility. Grain boundary orientation, applied bias voltage, and illumination further alter local barriers and recombination behavior. Therefore, material evaluation should combine grain size statistics with local potential, atomic structure, and recombination kinetics, and employ grain boundary engineering techniques such as defect passivation, segregation control, or growth optimization based on the specific system.

Working fluid selection is critical to the efficiency of Ocean Thermal Energy Conversion (OTEC) systems, where the inherently small temperature difference between warm surface and cold deep seawater limits achievable efficiency. Solar-boosted OTEC (SOTEC) addresses this where solar thermal energy raises the warm seawater source temperature. However, established working fluid selection criteria — dry/wet fluid classification, boiling temperature, latent heat of vaporization, and critical pressure — have only been validated above SOTEC-relevant heat source temperatures (50–60°C). This study evaluates these criteria at the pertinent temperature and identifies suitable working fluids for SOTEC deployment in Singapore across a three-stage thermodynamic screening methodology: (1) design-point efficiency comparison of 21 pure fluids at evaporator/condenser boundary temperatures (333.15K/283.15K), tested through Spearman rank correlation and Mann-Whitney U tests; (2) seasonal analysis of top-performing fluids across Singapore's monthly daytime and nighttime seawater temperatures; and (3) crossover analysis across a 10-50K solar boost sweep. Latent heat of vaporization was the only criterion convincingly corroborated at SOTEC-scale temperatures ( r s = +0.760, p < 0.001), with critical pressure showing moderate correlation ( r s = +0.545, p = 0.011). However, wet/dry classification and boiling temperature showed no significant relationship. Ammonia performed best throughout all conditions due to its shallow saturation curve slope, which raises its turbine enthalpy recovery, combined with ammonia's large latent heat of vaporization, which dilutes the relative cost of pump work. In the absence of solar-boosting, the efficiency spread among candidates significantly narrowed, emphasizing the significance of working fluid selection in solar boosting contexts. Additionally, solar boosting was found to raise ammonia's efficiency by 2.68-3.16 times, with the efficiency margin with its closest competitor widening.