# Photovoltaic Solar Cells: A Review
## 2 Solar Cells
### Design Considerations  
The design of a PV cell is defined by the cell structure [37]. Solar cells can be fabricated in different designs. First, a single-junction solar cell, which consists of a highly doped emitter layer and a lightly doped base layer. For a single-junction PV cell, GaAs is best suited to achieve the highest efficiency, nearly 30%, due to its bandgap (1.42 eV) being near to the ideal bandgap and high absorption coefficient, which offers substantial benefit in the design and fabrication of highly efficient PV devices [38; 39]. Highly efficient n-on-p single-junction GaAs thin-film solar cells have been fabricated by [40]. A metal combination of AuBe/Pt/Au was employed as a new p-type ohmic contact. The PCE of the fabricated single-junction GaAs thin-film solar cells reached 22.08% under Air Mass (AM) 1.5 global illumination. Another experiment by [41] demonstrated a PV GaAs device based on single junction thin film under the non-concentrated light (1 sun illumination), achieving a high efficiency of 27.6%.  
Second, multijunction PV cells are the most effective solar cells to date. GaAs-based multijunction PV cells achieved the highest efficiency of 42.3%, as it is possible to grow three or more junctions for one cell. The novel designs of Si and GaAs wafer-based double-heterojunction solar cells were demonstrated by [42; 43]. The cells were comprised of five layers, a cathode metal layer, three layers of semiconductor materials of III-V, II-VI, and IV families, and a layer of anode metal. The device structures have been optimized for the analysis of the PCE of the Si and GaAs solar cells considering high defect densities at and near each heterojunction. The PCEs obtained were almost 38.9%.  
Furthermore, p-i-n structures are developed by inserting an undoped or intrinsic layer (i) between very highly doped (p) and (n) regions. p-i-n structures increase the carrier's lifespan and density in the undoped region. The intermediate layer has a very low concentration of usually n-type in the order of 10\({}^{13}\) cm\({}^{-3}\). The p-i-n structure offers  
Figure 7: (I-V) and (P-V) characteristics of a solar cell under illumination.  
carriers confinement in the intermediate undoped layer, providing a higher output current. In the forward bias, both types of carriers are injected into the intrinsic layer, leading to an increase in the carriers' density and consequently the output current. The direct and flexible bandgap of the p-i-n double heterostructures offers a unique opportunity to develop highly efficient solar cells [44, 45, 46]. An experimental work by [47] focused on developing a p-i-n PV model with the compound materials AlGaInP/AlGaAs. The cell recorded the highest conversion efficiency of 51.50% under the AM 1.5 standard.  
The performance of solar cells is affected by design considerations as well as the properties of the employed materials. Designers can select a small subset of the most promising materials to be employed for solar cells using, e.g., CES EduPack software [48]. Furthermore, doping and alloying affect the properties of the employed materials. For example, the density of donor atoms \({}^{\prime}\)N\({}_{d}\)' and acceptor atoms \({}^{\prime}\)N\({}_{a}\)' affects the width of the depletion region. Moreover, adding some elements by alloying affects the physical properties of the employed materials [49].  
Thus, it is important to consider all the main parameters affecting the efficiency of the PV cell to obtain an ideal cell design [50, 51, 52].