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.
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