A team of researchers of the Fraunhofer Institute for Solar Energy Research (ISE, Freiburg) and AMOLF (Amsterdam) have fabricated a multijunction solar cell with an efficiency of 36.1%, the highest efficiency ever reached for a solar cell based on silicon.
In addition, ideal solar cells are assumed, which have an external quantum efficiency (EQE) of unity and behave according to the one-diode model. As a rule of thumb between 70% and 80% of these theoretical efficiencies can be achieved in reality.
For multi-cell PV systems, the efficiency can be improved by reducing the losses due to thermalization and unabsorbed photons. The analysis shows that split-spectrum system should result in better efficiency when compared to multijunction and intermediate cells. Though bulky, it is easier to build.
Figure 3. Theoretical efficiency limit of (multijunction) solar cells as a function of the number of pn-junctions under the reference spectrum AM0 (1367 W/m 2) for space applications as well as under the reference spectrum AM1.5d (500×1000 W/m 2) for concentrator solar cells .
This great development in the efficiency is not matched if the cost of the device is considered. The highly efficient PVs (mainly multi-junction solar cells) are prohibitively expensive , . On the other hand, the efficiency of the most dominant technology in the market (i.e. Si) is 25% in the lab and less than 20% commercially.
Multi-junction solar cells have the highest efficiency among all the other traditional single-junction cells. The efficiency of single-junction photovoltaic cells can hardly meet all the needs due to their small absorption range of the incident light spectrum.