One important point should be made at the outset. Researchers use various sophisticated techniques to achieve ‘ record ’ efficiencies and can select their best cells for independent testing and accreditation. But PV companies engaged in large - scale production have an additional set of priorities: simple, reliable, and rapid manufacturing processes; high yield coupled with minimal use of expensive materials, all aimed at lower costs. Manufacturers are certainly interested in the commercial advantages of high cell effi ciency and over the years have incorporated many design advances coming out of research laboratories, but cost must always be a big consideration and there are often significant time lags.
Figure 2.14 summarises the main factors determining the effi ciency of a typical, commercial, crystalline silicon solar cell operated at or near its maximum power point. On the left the incident solar power is denoted by 100%. Successive losses, shaded in blue, reduce the available power to around 15 – 20% at the cell ’ s output terminals – its rated efficiency value. We will now discuss each loss category in turn.
Quantum Theory
We emphasised the fundamental limitations imposed by quantum theory in the previous section. They represent the biggest loss of effi ciency in a solar cell based on a single p – njunction. One way of reducing the problem is to stack together two or more junctions with different bandgaps, creating a tandem cell.A well - known example, which has been exploited commercially for many years, is based upon amorphous rather than crystalline silicon.
Optical Losses
Optical losses affect the incoming sunlight, preventing absorption by the semiconductor material and production of electron – hole pairs. The small section of solar cell shown in Figure 2.15 illustrates three main categories of optical loss: blocking of the light by the top contact (1); reflection from the top surface (2); and reflection from the back contact without subsequent absorption (3).
Shadowing by the top contact can obviously be minimized by making the total contact area as small as possible. This area comprises not only the metallic contact fingers shown in the figure (and previously in Figure 2.8 ) but also wider strips known as busbars that join many fingers together and conduct current away from the cell. Clearly a well - spaced grid of very fine fingers and narrow busbars helps reduce optical loss, but the disadvantage is increased electrical resistance. As always, practical design involves compromise.
The photo in Figure 2.16 shows the top surface of a monocrystalline silicon cell, surrounded by its neighbours in a PV module. This example has a very simple grid geometry, consisting of 49 fine vertical fingers and two horizontal busbars, giving a shadowing loss of about 11%. The fingers have constant width; a more efficient design would taper them to account for the increasing current each carries as it nears a busbar. The busbars are slightly tapered towards the low - current end; it would be better to taper them along their length as they pick up current from more and more fingers. Ideally the cross - sections of fi ngers and busbars should be roughly proportional, at each point, to the current carried. To illustrate this a small section of a more efficient finger - busbar design is shown in part (b) of the figure.
The metallization pattern of fingers and busbars, as well as having its own inherent resistance to current flow, introduces contact resistance at the semiconductor interface. This may be reduced by heavy doping of the top layer of semiconductor material, at the risk of forming a significant dead region at the surface that reduces the collection effi ciency of blue photons. Conventional top contacts are made from very thin metallic strips formed using a screen - printing process. A metallic paste is squeezed through a mask, or screen, depositing the desired contact pattern which is then fired. The shading loss, typically between 8 and 12%, represents a significant drain on cell efficiency. A major design improvement, pioneered in the 1990s at the University of New South Wales uses laser - formed grooves to define a metallisation pattern with narrower but deeper fingers just below the cell’s surface. Such buried contact solar cellsoffer valuable gains in efficiency compared with normal screen - printed designs.
The second category of optical loss illustrated in Figure 2.15 is reflection from the cell’s top surface. Two main design refi nements are commonly employed. The first is to apply a transparent dielectric anti reflection coating (ARC ) to the top surface, illustrated by Figure 2.17 . If the coating is made a quarter - wavelength thick, the light wave refl ected from the ARC/silicon interface is 180 ° out of phase with that refl ected from the top surface and when the two combine the resulting interference effects produce cancellation. This condition is met when:
where dis the thickness and nthe refractive index of the coating material, and λis the wavelength (interestingly, we are temporarily considering light as a wave rather than a stream of particles). Clearly, exact cancellation can only occur at one value of λ, normally chosen to coincide with the peak photon flux at about 0.65 μm. The antireflection performance falls off to either side of this value. For optimum performance the refractive index of the ARC material should be intermediate between that of the materials on either side, usually silicon and either air or glass.
The second design refi nement involves texturisingthe top surface so that light is reflected in a fairly random fashion and has a better chance of entering the cell. Almost any roughening is helpful, but the crystalline structure of silicon offers a special opportunity because careful surface etching can be used to create a pattern of minute raised pyramids, illustrated in Figure 2.18 . Light reflected from the inclined pyramidal faces is quite likely to strike adjacent pyramids and enter the cell.
The third type of optical loss is refl ection of light from the back of the cell, without subsequent absorption. This may be reduced by an uneven back surface that reflects the light in random directions, trapping some of it in the cell by total internal refl ection. The technique is referred to as light trapping and is very important in crystalline silicon cells because silicon is a relatively poor light absorber, especially of longer - wavelength (red) light. It is illustrated in Figure 2.19. It is difficult to put precise fi gures on the effi ciency losses caused by these various optical effects. However a cell that includes carefully designed metallisation, ARC, texturisation, and light trapping can give major improvements compared with the basic structure first illustrated in Figure 2.8.
To be Continued
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