Light Absorption Mechanism of c-Si/a-Si Half-Coaxial Nanowire Arrays for Nanostructured Heterojunction Photovoltaics

We theoretically studied the assembly of horizontal single crystalline-silicon (c-Si)/amorphous-silicon (a-Si) core/shell nanowires (NWs) into c-Si/a-Si half-coaxial NW arrays (NWAs), which can be realized in large size for c-Si/a-Si heterojunction solar cells. Through the finite-difference time-domain simulations, we investigated the absorption mechanism of the half-coaxial scheme. The single building block of half-coaxial NWAs owns strong leaky mode resonances, which are the key for NWs to exceed the planar absorption limit. These resonances can be well preserved in the NWAs, leading to an excellent absorption enhancement. We further carefully studied the influences of various structural factors, i.e., the light interaction effect of periodic arrays, leaky mode resonances in single blocks, and the effect of indium tin oxide coatings. The optimized half-coaxial NWAs are capable of absorbing most of the incident light with only 10-μm thick c-Si substrate. Thus, the half-coaxial proposal can significantly cut the required c-Si wafer thickness in heterojunction solar cells, which loosens the restriction on material quality of the c-Si substrate. This half-coaxial NWAs structure may serve as a new way to improve the efficiency and reduce the cost of silicon heterojunction solar cells.