**5. Absorption depth and diffusion length for III–V NWs**

Based on the axis of charge carrier separation, an axial and a radial junction device are the two broadly classifying NW solar cells. The charge carrier separation happens along the length of the nanowire and the radial axis, in axial junction, and radial junction solar cells, respectively. **Figure 9a** and **b** display sunlight absorption and charge carrier separation in both axial and radial junctions NW solar cells correspondingly. In a solar cell, the minimum length needed to attain ample absorption is characterized by absorption depth. The absorption depth explains how deeply light penetrates the NW semiconductor or every type of solar cell device before being absorbed. At the same time, diffusion length describes the maximum length that the minority charge carrier can travel before making recombination non-radiatively [91]. For solar cells, in order for them to efficiently operate, the diffusion length should be higher than the absorption depth, as schematically shown in **Figure 9**. Radial junction is preferable for the fabrication of large-efficiency devices by connecting the light absorption and charge carrier separation axes. In a radial junction PV cell, sunlight absorption is along the main axis of the NW, while the charge carrier separation takes place within the radial direction, which is in nm-scale thickness. In other words, to realize the optimum performance of the NW photovoltaic cell in a radial junction photovoltaic cell, both charge carrier separation, and light absorption can separately be optimized.

Yao et al. [69] have carried out an optical simulation to predict the optimized axial junction and radial NW array for maximum light absorption and have compared the merits and demerits of these NWs. They have also synthesized GaAs NWs

**Figure 9.**

*Nanowires solar cells schematic representation of (a) an axial p-n junction, and (b) a radial p-n junction. α denotes the absorption coefficient of the active material and Ln and Lp denote the electrons and holes diffusion lengths, respectively [91].*

solar cells with an axial p-i-n junction by selective area growth method, which is compatible with MOCVD technique, and they observed that low filling ratio NWs are highly absorbed. They have also studied the effect of the diameter and revealed that thicker NWs are favorable because of the high surface recombination velocity on the bare GaAs NW surface. They identified that by decreasing junction depth to around 100 nm and maintaining diameter at 320 nm, able to achieve efficiencies as high as 7.58%. Their results demonstrated that GaAs NWs are good candidates for high-efficiency and low-cost solar energy conversion and open up great opportunities for the next generation photovoltaic based on multi-junction devices composed of lattice-mismatched material systems.
