The advantage of tandem solar cells is that they exploit sunlight better. A solar cell can only convert radiation with an energy level higher than the bandgap of the semiconductor used. If the radiation is weaker, no electricity is generated. If the radiation is higher in energy, the excess radiated energy is converted to heat and is lost. A double-layer solar cell like Empas perovskite CIGS cell can combine substances with differing bandgaps and thus convert a larger share of the irradiated solar energy to electricity.
More than 30% efficiency is possible
While very good single-layer polycrystalline solar cell may practically convert a maximum of 25% of the solar energy to electricity, tandem solar cells could increase this figure to beyond 30%. Thats according to Ayodhya Tiwari, head of the Thin Film and Photovoltaics laboratory. He does say, however, that a lot of research work is needed before that will be possible. What we have achieved now is just the beginning. We will have to overcome many obstacles before reaching this ambitious goal. To do this, we will need lots of interdisciplinary experience and a large number of combinatorial experiments until we have found a semi-transparent high-performance cell together with the right base cell, and technologies for electrical interconnections of these solar cells.
Stephan Bücheler, who coordinates the lab research in Tiwaris team, reminds us that the race for efficiency in solar cell research is certainly not just an academic show. When producing solar-powered electricity, only half of the costs are down to the solar module itself. The other half are incurred for the infrastructure: inverters, cables, carriers for the cells, engineering costs and installation. These ancillary costs are reduced when the solar cells become more efficient and can be built in smaller sizes as a result. This means that efficient solar cells are the key to low-cost renewable electricity.
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