Monocrystalline Silicon - Refining The Design II

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Recombination Losses

It occurs when light - generated electrons and holes, instead of being swept across the p – n junction and collected, meet up and are annihilated. The wastage of charge carriers adversely affects both the voltage and current output from the cell, reducing its efficiency.

Some recombination takes place as electrons and holes wander around in the body of the cell ( bulk recombination), but most occurs at impurities or defects in the crystal structure near the cell ’ s surfaces, edges, and metal contacts, as illustrated in Figure 2.20 . The basic reason is that such sites allow extra energy levels within the otherwise forbidden energy gap. Electrons are now able to recombine with holes by giving up energy in stages, relaxing to intermediate energy levels before finally falling back to the valence band. In effect they are provided with stepping stones to facilitate the quantum leaps necessary for recombination.


What can be done to reduce recombination? Three important techniques may be briefl y mentioned here. The fi rst involves processing the cell to create a back surface fi eld (BSF ). Although the details are subtle, the tendency of red photons to recombine at the back of the cell may be reduced  by including a heavily doped aluminium region which also acts as the back contact. Next, it is possible to reduce recombination at the external surfaces by chemical treatment with a thin layer of passivating oxide. And  finally, regions adjacent to the top contacts may be heavily doped to create ‘minority carrier mirrors’ that dissuade holes in the n- type top layer from approaching the contacts and recombining with precious free electrons.

Resistance Losses

The final efficiency loss is due to electrical resistance. We previously noted that a solar cell is best thought of as a current generator. As with other current generators, it is desirable to minimise resistance in series with the output terminals and maximise any shunt resistance that appears in parallel with the current source. Figure 2.21 shows two equivalent circuits for a solar cell modified to include a series resistance R1 in part (a) and a shunt resistance R2 in part (b). Ideally, R1 would be zero and R2 infinite, but needless to say, we cannot expect these values in practice.

The physical interpretation of R1 is straightforward. It represents the resistance to current flow offered by the busbars, fingers, contacts and the cell ’ s bulk semiconductor material. A well - designed cell keeps R1 as small as possible. R2 is more obscure, relating to the non ideal nature of the p – n junction and impurities near the cell ’ s edges that tend to provide a short - circuit path around the junction. In practical designs both resistors cause losses, but it is simpler to appreciate their effects if we treat them separately.

The black I – V characteristic in part (a) is for R1 = 0, the ideal case, which we refer to as the reference cell. The red characteristic is for a cell with a finite value of R1. Let us fi rst consider the open - circuit condition, I= 0. In this case there is no current through R1 and no voltage drop across it, so the open - circuit voltage Voc must be the same as for the reference cell. We conclude that series resistance due to a cell ’ s busbars, fingers, contacts and  semiconductor material has no effect on the open - circuit voltage. However, full circuit analysis shows that it causes a small reduction in short - circuit current and a considerable loss of fi ll factor, as indicated.

Part (b) of the figure shows the effects of shunt resistance and it is helpful to consider the short - circuit condition, V= 0. In this case there is no voltage across R2 and no current through it, so the short - circuit current Isc must be the same as for the reference cell. We conclude that finite shunt resistance due to imperfections in and around the cell ’ s   p – n junction has no effect on the short - circuit current. However, it has a minor effect on the open - circuit voltage and a considerable one on the fi ll factor. To conclude, a practical cell with both series and shunt resistance losses is expected to suffer small reductions in both Voc and Isc; but the most serious effect is generally degradation of fill factor.

We have now covered the main categories of efficiency loss in crystalline silicon solar cells. The techniques for counteracting them have been conceived and enhanced over many years in R&D laboratories around the world, leading to continuous improvements in cell and module efficiencies. Of course, the degree to which they are employed in a commercial product depends upon the manufacturer ’ s expertise and judgement; the number and complexity of processing steps have a big impact on cost and there is inevitably a trade - off between cost and performance.

Monocrystalline Silicon - Refining The Design

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Solar cell designers are constantly striving to improve conversion efficiencies, and have used their ingenuity over many years to refine crystalline silicon cells beyond the basic scheme. Some of the constraints on effi ciency are caused by fundamentals of light and quantum theory, others by the properties of semiconductor materials or the problems of practical design.

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