**6. Photogenerated carriers of III–V NWs**

For solar cells, one of the key needs is to realize efficiency that keeping a huge optical thickness to facilitate high light absorption and a tiny low electrical thickness to facilitate high photogenerated carrier assortment at the contacts. The gathering of high photogenerated carriers depends powerfully on the diffusion length of minority *Solar Energy Conversion Efficiency, Growth Mechanism and Design of III–V Nanowire-Based… DOI: http://dx.doi.org/10.5772/intechopen.105985*

carriers, which decline quickly with the rise in density of defect [22]. Generated carriers are going to be wasted when they are quite one diffusion length far away from the space charge region [92]. The diffusion length, Ld, of electrons or holes in a semiconductor is defined by the mean distance the relevant charge moves within the semiconductor. It is influenced by the mean distance the relevant charge moves within the semiconductor and recombination/extraction from the semiconductor. Diffusion is the movement of charge carriers directed by a concentration gradient. The diffusion coefficient (D) and additionally the equivalent term among the presence of a field, mobility (μ), are associated with one another by the relation [92]:

$$\mathbf{D} = \frac{\mu k T}{\mathbf{q}} \tag{1}$$

and

$$L\_d = \left(D\tau\right)^{1/2} \tag{2}$$

where τ is the charge lifetime.

When the cell is not operational at open-circuit voltage, that is, the charge is extracted, and then the lifespan can clearly be less due to the removal of the charge extracted. This is no longer an intrinsic property of the absorbing semiconductor itself, however, depends on the interfaces that exist between the semiconductor and charge extraction phases. The lifespan of charge refers to the minority charge carrier lifespan for semiconductors that are obviously either n-type or p-type. Differentiation into majority and minority carrier lifetimes is not obvious for an intrinsic semiconductor, such as the intrinsic semiconductor in a p-i-n cell [93].

In a conventional thin-film device, the gathering path of the generated carriers is parallel to the solar photon traveling path. Thus, thick enough absorption materials are in high demand on the quality of the crystal, in order that the carriers can easily undergo without any substantial recombination. The morphological anisotropy of nanowires provides the advantage of decoupling the optical and electrical thickness of PV cells by using the co-axial contact structure [91]. It can absorb sunlight along the entire nanowire, while the generated carriers are frequently separated within the radial direction. The radial distance that carriers need to travel (in the 100 s nm range) is generally much lower than, or similar to the minority carrier diffusion length. So far, the orthogonally severed sunshine and carrier separation paths can cause low bulk recombination, and hence high effectiveness. Also, the NWs have a high surface-to-volume ratio, which offers a large junction area that will further enhance the charge separation effectiveness.

The study showed that the influence of adjusting the diffusion length under radial junction may be a smaller amount than in planar junction, that is the utmost efficiency of both radial p-n junction geometry and planar geometry can increase with increasing diffusion length, but the planar geometry increases more [94, 95]. The difference in the performance between the planar and radial structures for III–V semiconductors with a high carrier diffusion length is, not as clear as that for Si [94]. However, NWs have a large surface-to-volume ratio and hence, a large density of surface state [93, 96]. All these merits allow using lower-purity, less expensive materials with low minority carrier diffusion lengths to make high-efficiency solar cells. Consequently, the use of the NW structure can enormously decrease the device cost. Due to these advantages, NWs are promising high-efficiency and less expensive solar cells and have the potential to revolutionize solar power harvesting technology.
