Cesium lead iodide (CsPbI3) with a bandgap of ~1.7 eV is an attractive material for use as a wide-gap perovskite in tandem perovskite solar cells due to its single halide component, which is capable of inhibiting halide segregation. However, phase transition into a photo inactive δ-CsPbI3 at room temperature significantly hinders performance and stability. Thus, maintaining the photo-active phase is a key challenge because it determines the reliability of the tandem device. The dimethylammonium (DMA)-facilitated CsPbI3, widely used to fabricate CsPbI3, exhibits different phase transition behaviors than pure CsPbI3. Here, we experimentally investigated the phase behavior of DMA-facilitated CsPbI3 when exposed to external factors, such as heat and moisture. In DMA-facilitated CsPbI3 films, the phase transition involving degradation was observed to begin at a temperature of 150 °C and a relative humidity of 65 %, which is presumed to be related to the sublimation of DMA. Forming a closed system to inhibit the sublimation of DMA significantly improved the phase transition under the same conditions. These results indicate that management of DMA is a crucial factor in maintaining the photo-active phase and implies that when employing DMA designs are necessary to ensure phase stability in DMA-facilitated CsPbI3 devices.

A thin film thermoelectric generator that consisted of 5 p/n pairs was fabricated with 1 μm-thick n-type In3Sb1Te2 and p-type Ge2Sb2Te5 deposited via radio frequency magnetron sputtering. First, 1 μm-thick GST and IST thin films were deposited at 250 oC and room temperature, respectively, via radio-frequency sputtering; these films were annealed from 250 to 450 oC via rapid thermal annealing. The optimal power factor was found at an annealing temperature of 400 oC for 10 min. To demonstrate thermoelectric generation, we measured the output voltage and estimated the maximum power of the n-IST/ p-GST generator by imposing a temperature difference between the hot and cold junctions. The maximum output voltage and the estimated maximum power of the 1 μm-thick n-IST/p-GST TE generators are approximately 17.1 mV and 5.1 nW at ΔT = 12K, respectively.