The multijunction solar cells are those with multiple junction connecting n doped and p doped materials. A normal solar cell only has one p-n boundary which electrons are promoted across by the solar radiation and their return is movement of charge which is the electricity. The advantage of multijunction (also known as tandem) solar cells is that they can be designed so each junction is optimised for a different wavelength of light. This means that overall the the solar cell is more efficient over a greater range of wavelengths. Triple-junction solar cells, such as indium-gallium-phosphourous, gallium-arsenic, germanium (InGaP/GaAs/Ge) solar cells are found a particular use in space exploration.
However space is not a nice place, even for things that aren’t biological. Constant impacts from protons and electrons can critically damage these solar panels over time marked by a rapid decrease in performance. It can be difficult to retrieve radiation damaged parts for study and so detailed mechanism that leads to the breakdown of performance is still unknown. Overall an interesting effect of the radiation is that due to the multijunctions being placed in series, as the current decays the voltage across the components actually increases. Each junction is made of its own materials and so it is possible that each will react differently or at least at a different rate to the exposure damage. Researchers have recently been investigating whether a combination of current voltage (I-V) graphs and photoluminescence can be used to judge and predict the efficiency of these cells during exposure.
To test this ten triple junction cells of the kind mentioned above were created all demonstrating initial efficiencies of about 30%. Bombarding these solar cells with different electron fluences and recording both the I-V graphs and the decay of the photoluminescence in the InGaP and GaAs subcells. This allowed the electron dynamics of each subcell to be recorded and the results show that electron radiation weakens the recombination process in both InGaP and GaAs subcells. But it was also shown that the strong internal field of the InGaP results in this loss having minimal effect on the overall cell efficiency. Processes like this can be used to predict the decay of cells over time which will be incredibly useful for the planning of space missions.