The highest conversion efficiency for solar cells has been obtained with the multijunctions approach. Through epitaxial growth the deposition of crystalline layers of compound semiconductors is possible whenever specific requirements on the lattice parameter are satisfied (Yamaguchi, 2002). Many layers of different semiconductors are stacked in order to create a structure where the first layers appear transparent at the light absorbed by the semiconductors at their bottom. This is obtained decreasing the band gaps of the compound semiconductors, from the frontal surface to the rear. The Germanium is often used as substrate material, both for its lattice parameter as well as for its band gap adapt for the bottom cell function. Unfortunately, some semiconductor compounds with suitable band gaps haven’t a lattice matching with the other materials useful for the stack; however, cells growth with lattice matched (LM) technique have achieved the 40% of efficiency under concentration. To further improve the performances of the cells, the metamorphic (MM) approach has been developed (King et al., 2007), delivering record cells efficiency higher than 41% under concentration; with this technique, consisting in the introduction of step-
graded buffer layers allowing for stress/strain relief to avoid the formation of dislocations in the layers growth, the flexibility in band gap selection is greatly improved, providing freedom from the constrain of same crystal lattice constant for all the stacked material in the monolithic structure as for LM. In fig. (11) a semplified MM multijunction cell structure from (King et al., 2007) and the distribution of irradiance absorbed for photovoltaic conversion by the three active materials are reported. To electrically connect the integrated sub-cells of different materials, tunnel junctions are formed.
These complex structures represent 3 solar cells series connected. So, the active element producing the lower current limits the current generation. The current produced by each layer depends on the light spectrum too, so spectral variations, as happen with different weathering conditions, can affect the performances of the cells (Muller, 2010).
Theoretically, a cell with 4 junctions can achieve an efficiency of 58% under an AM1.5 spectrum; with a combination of real and known materials, a terrestrial concentration cell with efficiency of 47% is possible. Until now, however, the most performing cells are 3-J solar cells; at the end, for energy production installations, a trade off between costs and performances in field must be carried out. Because of the detrimental effect of the spectral changes becomes more influent increasing the number of monolithically stacked junction, the convenience to use, in the future, 4-J solar cells instead of 3-junctions solar cells for in Sun installation must be demonstrated.
The cost of these devices is decreasing, but it is still in the order of 4€/cm2. To evaluate the cost contribution of the cells on the global system, let’s suppose a collected area of the concentrator of 400 cm2 and of a cell of 1cm2 (physical area of the cell, usually higher than the irradiated zone, because of, at least, the area for the pads for contact leads is necessary); with a nominal irradiation level of 850W/m2 and a module efficiency of 25% the cell
generates 8.5W, so the €/Wp contribution of the cell on the overall CPV system cost is of 4/8.5 = 0.47 €/W. It’s a significant voice of cost, but it can be reduced increasing the concentration level and with the specific cost reduction of the devices obtained with their volume production, as well as with their efficiency improvement.
New products based on III-V semiconductors are doing their first steps into the CPV market, moving from labs to pilot production lines. The approach of the strain balanced quantum well solar cells (SB-QWSC) (Barnham et al., 2002), appears of great technical interest for the efficiency improvement of multi-junctions solar cells as well as for the possibility to tail the cells on particular optical designs acting on the spectral properties of the light, like as dichroic concentrators (Martinelli et al., 2005).
In order to reduces the cost of these cells high research efforts have been invested, following different routes. From the manufacturing point of view, molecular organic chemical vapour deposition (MOCVD) equipments, industrially used for the epitaxial growth of the compound layers have been developed for high productivity. On the other side, different ways to reduce the cell cost replacing the Germanium or GaAs substrate with cheaper Silicon wafers (Archer et al., 2008) or using peeling-off techniques (Bauhuis, 2010) in order to use the same substrate for different growth have been investigated.