Various types of multijunction solar cells have now been demonstrated with energy conversion efficiencies over 35%. Several prior art configurations are shown in FIGS. 1, 2, 3, and 4.
FIG. 1 shows GaAs light sensitive cells and GaSb IR sensitive cells mechanically-stacked in a triplet circuit in a voltage-matched configuration in a concentrator module with Fresnel lenses focusing the solar energy onto the cells. This is a cell configuration with 2 junctions in 2 different materials. A lens parquet 1 overlays a circuit 7 on a heat spreading back plate, on which top light sensitive cells 3 and bottom IR sensitive cells 5 are placed.
FIG. 2 shows a 2-junction monolithic InGaP/GaInAs cell mechanically-stacked on a separate GaSb cell. There are 3 junctions in 3 different materials. No circuit wiring is suggested for this ease. A prismatic cover with double AR coatings 9 cover the InGaP/GaInAs cell and a prismatic cover with single AR coating 15 covers the GaSb cell. Double AR coatings 11 and isolation 13 separate the two cells.
FIG. 3 shows a Cassegrain module with voltage-matched InGaP/GaAs 21 and GaSb 19 cells. This Cassegrainian PV module concept uses a dichroic hyperbolic secondary mirror to split the incoming light 17 spectrum into short and long wavelength bands to create two focal points. There are 3 junctions in 3 separate materials. The module is 33% efficient.
FIG. 4 shows a very popular InGaP/GaInAs/Ge monolithic 3-junction cell that has been developed for light-weight for satellite power systems with positive contact 23 and negative contact 25.
It is very desirable to integrate these high efficiency multijuction cell concepts with optics where the circuits are simple to fabricate and the resultant solar panels are inexpensive and produce maximum power with maximum energy conversion efficiency. However, problems with the prior art configurations have prevented such a product from being realized.
In the mechanically stacked configurations shown in FIGS. 1 & 2, the metal grids on the top and the bottom of the top cell produce shading losses for the IR bottom cell. Furthermore the top cell substrate can absorb substantial IR energy before it arrives at the IR cell for conversion. These reflection and absorption losses decrease the energy conversion efficiency. Furthermore, the grid on the back of the top cell adds complexity and cost.
Another problem for the configurations shown in FIGS. 1 and 3 as described in the prior art is a problem with high currents in the voltage-matched configuration. For example if in the FIG. 1 case, the GaAs cells each produce 10 Amps and the GaSb circuit produces 8 Amps, the voltage matched triplet will produce 38 Amps at 1 Volt.
There are also several problems with the InGaP/GaInAs/Ge monolithic cell shown in FIG. 4 when it is brought down from space for terrestrial applications. While it is light-weight for space, this is not important for terrestrial applications. There are really three problems that relate to the germanium cell.
The first problem with the germanium cell is that it produces excess current that can not be used. FIG. 5 shows the current available from the terrestrial spectrum as a function of the longest wavelength that a given semiconductor can receive. It follows from this curve that the most current available to an InGaP cell lattice matched to germanium will be about 17.5 mA/cm2 with a similar amount of current available to a lattice matched GaInAs cell. Meanwhile, the current available to the germanium cell is much larger at about 28 mA/cm2. Because of the monolithic series connection in this cell, the excess germanium cell current is wasted resulting in a lower than optimal conversion efficiency.
The second problem with the germanium cell is a lower than optimal voltage. Given the terrestrial spectrum, the bandgap of germanium is just too low. While it can theoretically absorb out to 1.9 microns, there are just no photons in the terrestrial spectrum beyond about 1.8 microns.
The third problem with germanium is that its open circuit voltage in practice is just not as good as it should be theoretically. FIG. 6 shows the open circuit voltage of materials having a variety of bandgaps, compared to the radiative limit. A Ge cell has a voltage of only 0.2 V whereas a GaSb cell (not shown) can produce 0.5 V.
Needs exist for improved simple solar concentrator panels with series connected cells with different compositions with all cells operating at their maximum power such that said panel operates at its maximum efficiency.