Figure 10
(a) Approximate curve of actual solar radiation energy on the earth’s surface from 200 nm to 1200 nm and (b) absorption efficiency of micro-nano hybrid structures and micro structures obtained for different processes. Based on the actual solar radiation energy at the earth’s surface31, an approximate curve is illustrated in (a), which consists of three line segments, to simplify the absorption efficiency of the six micro-nano hybrid black silicon samples and single micro structures samples. As shown in (b), the wet-pit samples exhibit the lowest absorption efficiency among No. 1~No. 6 samples, and the wet-tip samples exhibit the highest absorption efficiency among the nine samples. Tips for both wet and dry etching methods show larger absorption efficiencies than pits. The efficiencies of the tip samples exceed the efficiencies of the pit samples by almost 10% for the wet etching method; however, for the dry etching method, the tip and pit samples are close in efficiency. In addition, the tip samples obtained using the wet etching method exhibit a slightly higher efficiency than those obtained using the dry etching method, whereas the flat sample obtained via the wet etching method has a slightly lower efficiency than that of the dry etching method. From the (b), silicon surface with no treatment showed lowest among all samples since it has neither micro nor nano structures that contribute to light absorption. The surfaces with pits indicated more 2% efficiency than those with tips since pits may have continuous ring side to capture lights. That is to say, surfaces with pits have more side surface area than those with tips, increasing the probability of reflection. That showed the efficiency of the samples with hybrid structures, single nano structures, single micro structures and no structures, descending from first one to last one.
