The efficiency of conventional photovoltaic cells is about 15%. This means that only 15% of the sunlight is converted in electricity. One of the reasons these cells have such a low efficiency is that they only absorb a narrow band of optical wavelengths. This is because these panels have single-junction cells what makes them relatively inexpensive.
Now they have developed a multi-junction photovoltaic cell with 44.7% efficiency. This multi-junction cells consist of several layer. Each of these layers are tuned to a particular wavelength of light. This makes them more efficient but they’re much more expensive. That’s why we won’t see them on the rooftops any time soon. But maybe in a few years they could replace the single-junction panels.
As we are going to use more efficient photovoltaic cells in the future, the importance of cleaning these panels will thereby become much clearer.
You can find the entire text here
solarcleanenergy 
I don’t find it clear in the text out of which materials these cells are made, probably because I don’t know a lot about solar panels. Are they also made out of germanium?
The key is not out of what material it is made, but that it has a lot higher efficiency. This because of the multiple layers.
I assume they cost more then 3 times the normal solar panels?
I also think those cells are difficult to make ? Or not ?
They will probably be more expensive because their more difficult to make.