**9. Conclusion and outlook**

In order to convert solar energy into electrical energy, harvesting solar energy is required and solar photovoltaic is the most promising device for this purpose. Despite the excess of sunlight reaching the earth's planet, the current percentage of solar energy is much smaller than both renewable and nonrenewable energies. This is due to low energy density, low efficiency, and relatively high-cost materials compared to other types of energy technologies. Therefore, novel materials that can enormously harvest sunlight are important and they are the current issues attracting research interest. Due to its unique properties from bulk materials, III–V NWs can be used as high-performance solar cells because of their attractive advantages, such as unique optical and electrical properties, direct band, and fewer solar light reflections. In constructing novel solar cells with high-efficiency and low-cost, the distinctive structure and advanced properties of NWs provide more freedom. Today's solar cell market is dominated by the thin film of Si, which has the lowest efficiency, but low cost. By combining the advantages of III–V NWs and Si by growing III–V NWs on Si substrate tandem solar cells, enormously improved performance of the solar cells can be achieved. By controlling the III–V NW morphology and its geometry with optimum diameter, period, and length it is possible to get high-efficiency solar cell materials. Furthermore, III–V NW solar cells can be designed as tandem solar cells, axial and radial tandem solar cells, multiterminal solar cells, inorganic nanowire/organic hybrid solar cells, branched solar cells, and flexible solar cells.

Future works can be focused on the optimum design that can overcome all the limitations of III–V NW solar cells in order to achieve high-performance and low-cost III–V NW-based solar cells. Thus, one of the best aspects of III–V NWs commercialization with high power conversion efficiency may be achieved by designing it in a way that it can absorb solar light enormously with reduction of materials used and low-cost substrates. According to our understanding, all the designs of III–V NWs that are mentioned in this review are beneficial for future commercialization; however, it is good to identify the one that is more attractive than the others by conducting research on each design. All types of design have their own advantages in case of reducing materials used for the fabrications of solar cells and cost reductions. So far, it is good if this area will be researched more, especially on the architecture of III–V NWs due to its infinite advantages. Despite the challenges of achieving high efficiency in these NWs, they are the hope of the next-generation solar cells due to their flexibility for designing it even in a multi-junction of different NWs, which can absorb the different wavelengths of solar light for harvesting huge solar light. Furthermore, to advance III–V NW-based solar cells toward possible commercialization the power conversion efficiency should be increased, for which the tandem architecture is highly interesting by growing on Si substrate, which is cost-effective.
