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The ability to coat porous media is critical for forming composite functional thin films. A major technical concern for accurately predicting this process is that the flow of the coating bead and the penetration process must be considered. These phenomena strongly influence each other. Therefore, both the flow into porous media and the coating-bead flow should be simultaneously treated. In this study, the target is a high-productivity coating system based on a roll-to-roll process using a slot die. Slot-die coating is a premetered, precision coating method. We investigated the coating of porous media to estimate the practical operability window and the penetration depth using two-dimensional numerical analysis. For this purpose, both the coating-bead pressure and the capillary pressure were considered as driving forces of penetration. Moreover, the curvature of the backup roll opposite the slot die was also taken into account to achieve an accurate estimation. We demonstrate that the penetration depth and operability window for defect-free coatings can be well estimated and that the results are consistent with experimental results.

We fabricated and characterized a two-junction GaInN-based solar cell using a tunnel junction fabricated by crystal growth. This solar cell has two active layers with a differing bandgap energy corresponding to blue or green light. We confirmed that the open-circuit voltage (VOC) in this solar cell was increased by the series connection using the tunnel junction. The short-circuit current density, VOC, fill factor, and energy conversion efficiency of this solar cell were 0.28 mA/cm2, 3.0 V, 0.5, and 0.41%, respectively, under an air mass filter of 1.5 G at 1-sun irradiation and room temperature.