You typically connect multiple solar cells together to make solar modules. This is primarily a good thing because we can increase the voltage that the solar cell can produce, and this can make it easier for us to use the power the solar cell is generating in some electronics or even if you want to transmit the power over long distances. A typical solar module will have several solar cells connected in series. So you can see in this case, we have one, two, three, four, five, six, seven, eight solar cells connected in one long series. But what happens when we connect multiple solar cells together, and could there be any drawbacks? First we have a parallel connection of solar cells. So the module I showed you before was serially connected. In this case we're looking at a parallel connections. When we have solar cells connected in parallel, the voltage will be the same for every solar cell in the connection. However, the current will be the sum of the current from each solar cell. And you can see the I-V curve can be drawn here for one cell, two cells in parallel and three cells in parallel and so on. For a series connection, it's slightly different. Here, the voltage will be the sum of each solar cell, while the current will be shared for all of them. And you can see we get these I-V curves. So for one cell, two cells, and three cells. It's of course important to consider what happens in a module when not all the solar cells are illuminated equally. And we can look at this in this case. So here we have a single I-V curve from a single solar cell and if we have three of them as is drawn here, we get the I-V curve for an unshaded module. However, the problem is if we shadow one of them, what happens? We can see what happens here, we get the shaded cell will have this I-V curve. And this is even a partial shading. So the problem is, since the current needs to be the same for every solar cell or the current runs through each solar cell, we draw down the current for the entire module and in a series connection we see that the resulting I-V curve is extremely limited. So we go from an unshaded module with three solar cells to a module with two solar cells that are illuminated and one that's partially shaded. This is completely detrimental to the efficiency to the power output of the module. In a parallel connection, this situation is not nearly as bad. So we can see we have one solar cell will have this I-V curve and a module of three will have this I-V curve where we sum up the currents. When we shadow one of them, we only lose the amount of current that's produced by that solar cell. So this is not nearly as bad as the other situation. Of course, we need to look at what can we do to rectify the situation. So as you just saw in the quiz, the answer to what we can do is to insert bypass diodes and the reason bypass diodes works is that we can circumvent the problem of a shaded solar cell by having the current run through the bypass diode, instead of the solar cell. This means we end up with an I-V curve in a module with one shaded solar cell looking like this. So it's not nearly as detrimental as we had before where we had the shaded module. So if you want to make efficient modules where have a small problem with partial shading, we need to insert bypass diodes for each cell. However, this is typically not what is seen because first of all, it's expensive to include bypass diodes for each cell. Secondly, there's a problem that will end up having heating of the bypass diode. And if we include these in the elimination then we might end up having problems. So typically we see bypass diodes for each string of solar cells in a module.