Solar Cells Based on Single PN Junctions - Nanophysics of Solar and Renewable Energy - page 130

6
Solar Cells Based on Single PN Junctions

6.1
Single-Junction Cells

The semiconductor properties described in Chapter 3 underlie the behavior of solar
cells. Most relevant aspects are the formation and properties of PN junctions. In
discussing leading types of single-junction solar cells, we start with Section 3.7,
Equations 3.65 - 3.70, and Figures 3.16 and 3.17. Figure 3.17 shows the I - V curve
under illumination of a silicon solar cell. We begin with silicon solar cells, which have
evolved over time toward goals of improved ef ciency and toward lower fabrication
cost.
Figure 6.1 sketches the bands of a Si N þ P solar cell, built to be illuminated from the
left, a cell that might be similar in its characteristic to that shown in Figure 3.17. The
nomenclature and basic processes that go on in such a conventional single-crystal Si
device were shown in Figure 3.16c. The device shown in Figure 6.1a in the dark, but
Figure 6.1b shows the expected pattern of decaying intensity. It is important to
understand that the absorption process strongly depends on the wavelength of the
light, which is related to the photon energy through the relation E hc /l. The

hatched regions on right and left indicate metallic contacts to connect to a load, and
details are omitted, such as how the left contact is made optimally transparent to
incoming light.
In practice, the front contact is applied as narrow fingers of metal, to allow light to
enter. The front surface may be textured to reduce re ection and promote internal
trapping of light as discussed below. In addition, the front surface may be coated with
an antire ection coating (AR or ARC) typically with refractive index close to 2.
Materials used on silicon include MgF 2 , ZnS, Si 3 N 4 ,Ta 2 O 5 , and TiO 2 . The rear
contact, here shown as a uniform metal layer, is sometimes replaced by an array of
low-resistance contacts, with locally diffused P þ regions. In this case, the larger
portion of the rear surface will be oxidized to formSiO 2 , quartz. The Si/SiO 2 interface
reflects light back into the silicon and also, especially if passivated with hydrogen,
reduces undesired recombination to a level less than would occur at a metal contact.
The desired outcome for photogenerated electrons and holes is to be collected in the
opposite terminals of the device and to drive an external current. Recombination

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