Sudan University of Science and Technology College of Graduate Studies Department of Electrical Engineering
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ph (by equation 1), and higher the value of V oc . Similarly, lower Is0 can also cause higher Voc. Since Is0 is the reverse saturation current for the pn junction, it is given by I s0 = 𝑛 𝑖 2 e ( 𝐷 𝑒 𝐿 𝑒 𝑁 𝐴 + 𝐷 ℎ 𝐿 ℎ 𝑁 𝐷 ) …………………………………. 3.7 Figure 3.8 (a) pn junction solar cell under illumination with an external load. The equivalent circuit (b) without and (c) with an external load. Figure 3.9 (a) A solar cell connected to an external load (b) Equivalent circuit, with a constant current source, a forward biased pn junction and the external load. 30 Figure 3.10 I−V characteristics of Si pn junction solar cell under dark conditions and under illumination with light of increasing intensity Solar cell materials and e ffi ciency: Conventional solar cells are made of Si single crystal and have an e ffi ciency of around 22-24%, while polycrystalline Si cells have an e ffi ciency of 18%. A schematic representation of such a cell is shown in figure 3.7. The e ffi ciency of the solar cell depends on the band gap of the material and this is shown in figure3.12. Polycrystalline solar cells are cheaper to manufacture but have a lower e ffi ciency since the microstructure introduces defects in the material that can trap carriers. Amorphous solar cells have an even lower e ffi ciency but can be grown directly on glass substrates by techniques like sputtering so that the overall cost of manufacturing is lowered. There are also design improvements in the solar cell that can enhance the e ffi ciency. PERL (passivated emitter rear locally di ff used) cells, shown in figure 3.13, have an e ffi ciency of 24% due to the inverted pyramid structure etched on the surface that enhances absorption. 31 Figure 3.11 I-V curve for a solar cell with maximum power indicated by the shaded area Figure 3.12 Solar cell e ffi ciency as a function of band gap of the semiconductor material Figure 3.13 Si solar cell with an inverted pyramid structure to enhance absorption of the incoming radiation 32 Typical solar cells are made of the same material so that the pn junction is a homojunction. Some solar cell materials and their e ffi ciencies are summarized in table 3.2. A comprehensive state of current research in di ff erent solar cell technologies and their e ffi ciency is available in figure 3.14. Heterojunction solar cells are also possible and they have the advantage of minimizing absorption in regions other than the depletion region, but overall cost increases because of the use of di ff erent materials and the tight processing conditions needed to produce defect free interfaces. A schematic of such a cell based on GaAs/AlGaAs is shown in figure 3.15. The shorter wavelengths are absorbed by the AlGaAs layers while the longer wavelengths, with higher penetration depths, are absorbed by the GaAs layer. This leads to an overall e ffi ciency of around 25%, see table 3.2. It is also possible to have a homojunction solar cell but with a passivating layer of another material at the surface to reduce defects. This is shown in figure 3.16. The surface passivating layer removes the dangling bonds and minimizes carrier trapping. The passivation layer is a thin layer of a higher band gap material to minimize absorption. Similarly, amorphous semiconductor materials like Si and Ge also have a passivating layer of H, a-Si:H or a-Ge:H, to reduce dangling bonds. Another way of improving solar cell e ffi ciency is to have more than one cell in tandem. These are called tandem solar cells and a schematic is shown in figure 3.16. These consist of two pn junction solar cells, with the first one having a higher band gap than the second. Thus, the shorter wavelengths can be absorbed in cell 1, see figure 16, while the longer wavelengths are absorbed in cell 2. The advantage is that a larger portion of the solar radiation is used so that tandem cells have high e ffi ciency, see table1, but it also adds a layer of complexity in growth and increases cost. Tandem cells can also be made using amorphous Si:H and Ge:H. These are cheaper to make and more e ffi cient than individual amorphous solar cell devices. 33 F igur e 3 .14 : E ffi cienc y o f v a rious r ese a rc h sol a r c ell 34 Figure 3.15 (a) GaAs/AlGaAs based heterojunction solar cell. (b) Energy band alignment across the junction Figure 3.16 Schematic of a GaAs based homojunction solar cell with a surface passivating layer to minimize surface recombination Figure 3.17 Tandem solar cells 35 Table 3.2 some common solar cell materials and their characteristics. Adapted from Principles of Electronic Materials - S.O. Kasap |