a b s t r a c tThe collision of single water droplets with a hot Inconel 625 alloy surface was investigated by a two-directional flash photography technique using two digital still cameras and three flash units. The experiments were conducted under the following conditions: the pre-impact diameters of the droplets ranged from 0.53 to 0.60 mm, the impact velocities ranged from 1.7 m/s to 4.1 m/s, and the solid surface temperatures ranged from 170°C to 500°C. When a droplet impacted onto the solid at a temperature of 170°C, weak boiling was observed at the liquid/solid interface. At temperatures of 200 or 300°C, numerous vapor bubbles were formed. Numerous secondary droplets then jetted upward from the deforming droplet due to the blowout of the vapor bubbles into the atmosphere. No secondary droplets were observed for a surface temperature of 500°C at the low-impact Weber numbers ($30) associated with the impact inertia of the droplets. Experiments using 2.5-mm-diameter droplets were also conducted. The dimensionless collision behaviors of large and small droplets were compared under the same Weber number conditions. At temperatures of less than or equal to 300°C, the blowout of vapor bubbles occurred at early stages for a large droplet. At a surface temperature of 500°C, the two dimensionless deformation behaviors of the droplets were very similar to each other.

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We have revealed that the insertion of a Cu(In,Ga)3Se5 layer at the CdS/Cu(In,Ga)Se2 interface is effective for improving the efficiency of Cu(In,Ga)Se2 solar cells fabricated at low temperatures and with single-graded band profiles. Although the double-graded profile is usually adopted for Cu(In,Ga)Se2 solar cells grown at around 600 °C, the single-graded profile is more suitable for low-temperature-grown solar cells owing to the higher controllability of band profiling. We also analyzed the role of the Cu(In,Ga)3Se5 layer by transmission electron microscopy energy-dispersive X-ray spectrometry (TEM–EDX) and scanning electron microscopy electron-beam-induced current (SEM-EBIC), and found that Cd diffuses to the Cu(In,Ga)3Se5 layer, resulting in a shift of the pn-junction electrical interface from the highly defective structural interface. Finally, by applying this new band profile, VOC and FF increased, resulting in an efficiency improvement from 13.4 to 14.5% at a substrate temperature of 450 °C.

Alkali treatment effects on Cu(In,Ga)Se2 (CIGS) solar cells deposited on polyimide-coated soda lime glass (PI-coated SLG) were investigated. CIGS on PI-coated SLG shows Na diffusion from the substrate, which should be controlled to obtain high efficiencies. Further incorporation of Na was achieved by enhancing diffusion from the substrate or by external incorporation using post-deposition treatment (PDT) methods. Both methods lead to a high efficiency of approximately 15%. Moreover, aside from Na, K was also incorporated by KF-PDT, resulting in efficiency improvement from 12% for an untreated CIGS to more than 18% at the maximum substrate temperature of 450 °C, which is comparable to CIGS deposited at higher temperatures using the same equipment. It was also found that the alkali concentration of CIGS deposited on PI-coated SLG shows almost the same behavior as that of a film deposited on a rigid glass, suggesting that the deposition technique for CIGS on the rigid glass can be applied to flexible substrates.

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