The effect of tapered shape on electrical properties of heterojunction silicon nanowire (SiNW) solar cells was simulated with a two-dimensional quantum device simulator. When the quantum effect was taken in account, opencircuit voltage (V<sub>oc</sub>) and fill factor (FF) of heterojunction SiNW solar cells were drastically improved from 390 to 862 mV and from 0.662 to 0.795, respectively. This is due to the bandgap widening and the enhancement of electric field in the intrinsic SiNW. When a top side diameter (d<sub>1</sub>) of the SiNW was set at 2 nm and a bottom side diameter (d<sub>2</sub>) was varied from 2 to 6 nm, short-circuit current density (J<sub>sc</sub>) was drastically increased from 6.96 to 30.8 mA/cm<sup>2</sup>. The main reason is the absorption enhancement due to a tapered shape with a graded refractive index. Ultimately, conversion efficiency was monotonically increased with increasing d<sub>2</sub> in the range from 2 to 6 nm. The quantum size effect and the tapered shape can enhance conversion efficiency of heterojunction SiNW solar cells.
Polycrystalline silicon nanowires (poly-SiNWs) films were successfully prepared by using metal assisted chemical etching of polycrystalline silicon (poly-Si) films. The poly-Si films were prepared by solid-phase crystallization of amorphous silicon (a-Si) deposited by different deposition techniques on different substrates. In the case of the electron beam evaporated a-Si on a quartz substrate, the formation of poly-SiNWs was not observed and the structure was found to be porous silicon. On the other hand, poly-SiNWs successfully formed from poly-Si on a silicon substrate. We also found that deposition techniques for a-Si films affect the formation of poly-SiNWs.
We demonstrate the minority carrier lifetime measurements of polycrystalline silicon nanowires (poly-SiNW) films passivated with aluminum oxide (Al<sub>2</sub>O<sub>3</sub>) deposited by atomic layer deposition (ALD). The poly-SiNW films were prepared by metal-assisted chemical etching of poly-Si films. The poly-Si films were prepared by solid phase crystallization of a-Si films deposited by radio-frequency sputtering on aluminum induced crystallized poly-Si template. The deposition of an ALD-Al<sub>2</sub>O<sub>3</sub> passivation layer and subsequent annealing enabled us to measure effective minority carrier lifetime of the poly-SiNW films. The effective lifetime was found to be 5.76 μs. This result indicates that ALDAl<sub>2</sub>O<sub>3</sub> is beneficial to surface passivation of poly-SiNW films.