Chen et al. identified six classes of nanowire arrays based on the orientation of the p-n junction, as shown in Figure 2.10 [120]. Types I and II have axial and radial p- n junctions, respectively. In axial junction nanowire, the p-n junction is present axially along the length of the nanowire (type I). While in radial nanowires, the p-n junction is formed radially outwards around the axis (type II). In type III nanowires, opposite doping types are used in nanowires with respect to the substrate. Type IV represents the combination where nanowires stand top on a substrate with a p–n junction. The last type (type V) is also a radial configuration, but in this case, the NWs together with the substrate serves as a deposition template on which radial junction shell structures are deposited
Figure 2.10 Different nanowire junction configurations with respect to the substrate. (a) Type I (b) Type II (c) Type III (d) Type IV (e) Type V. Blue, green and gray bars represent p-type, n-type, and inactive part (conductor), respectively [120].
Among these five configurations of nanowire junctions, only the first three are commonly used for solar cell applications. These three types of NW SCs are illustrated in figure 2.11, along with their specific advantages. Radial junction nanowire arrays have the most advantages of nanowires, including the added benefit of orthogonal charge collection and separation, which is absent in axial junction NW SCs. Substrate junctions further lack the ability to be removed from the substrate to be tested as single-nanowire solar cells.
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Figure 2.11 Nanowire solar cell classification [83].
Despite The recent surge in popularity of nanowire solar cells, they are unlikely to exceed the efficiency limits of planar PV devices even in their best configuration.
Their merit lies instead in their ability to relax the conditions needed to approach those limits, paving the way to utilizing low-cost, previously disregarded materials and processing possibilities, ultimately leading to cost reduction compared to wafer- based films. The main advantages of nanowire solar cells are:
Absorption losses in solar cells are caused by two separate phenomena:
reflection and transmission. Several studies have shown that nanowires can minimize both of these events, resulting in enhanced absorption. In optically thick nanowires, where reflections losses dominate the extinction process, nanowires exhibit their property to reduce reflection [115], [116]. When the absorber layer is too thin, transmission losses become significant. In such cases, the light trapping properties of nanowires result in reduced transmission losses. Nanowires can act as strong scattering centers and show diameter- dependent optical resonance [67], [123]. Apart from scattering, resonance [124], and increased effective optical cross-section benefits obtained from single nanowires of random nanowire arrays, ordered arrays have the added advantage of diffraction effects, collective resonances [125], and changes in
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the optical density of states. Finally, the transfer of resonant energy from dyes or quantum dots near the semiconductor surface can improve absorption in nanowires. All of these properties significantly increase absorption in the nanowires [125].
Heat loss or carrier relaxation due to thermal energy is the second-largest loss mechanism in every photovoltaic device, and typically between 30% and 40%
wastage of the incident solar energy is represented by it [28]. This loss can be reduced by using optimum bandgap semiconductors. However, the miscibility gap prohibits the synthesis of uniform single-crystalline films at arbitrary compositions. Nanowires have the ability to close this miscibility gap due to enhanced strain relaxation, allowing for material synthesis with optimum bandgaps [126]. Moreover, if the nanowire size is less than or approximately equal to the Bohr radius of the material, quantum confinement can be achieved, which ultimately reduces the carrier relaxation losses [127].
It has been shown that through quantum confinement, tapered nanowires can exhibit can separate electron-hole pairs without any dopants [127]. Moreover, variations in bandgap caused by strain throughout the nanowire length can also be used to separate charges in a similar way [128]. Thus, nanowires unleash the chances of making solar cells from undoped materials and thereby eliminating various losses associated with doping [4].
Nanowire solar cells show efficient carrier collection due to band conduction.
It is shown that for Dye-sensitized solar cells, carrier collection efficiency is significantly higher for nanowire cases than the nanoparticle case where diffusion transport mechanism is responsible for charge separation, which is limited by trap density [129]. Moreover, radial junction NW SCs show decreased sensitivity to bulk defects [123].
The consequences of the above-discussed nanowire advantages result in solar cell cost reduction in a variety of ways. The amount of material needed for
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nanowires is substantially less than planar cells due to increased absorption by light trapping. The material contenders to be used as absorbers layer are also increased as a result of band gap tuning, new charge separation mechanisms, or by the relaxation of purification requirement of materials stemming from enhanced charge-separation. This results in the freedom to explore new cheaper materials in solar cells. Furthermore, significant cost savings in the fabrication of high-performance multijunction solar cells by eliminating the need for an expensive lattice-matched substrate can be brought about by nanowires.
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