Effective harvesting of photons for improvement of solar energy conversion by graded bandgap multilayer solar cells

Various ways to harvest photons in the solar spectrum and heat energy from the surroundings have been considered by utilizing solid state physics principles. A graded bandgap multi-layer solar cell has been designed to absorb UV, visible and infrared radiation, and to combine impact-ionization and impurity photovoltaic (PV) effect to enhance photo-generation of charge carriers and the collection. The above theoretical concepts were experimentally tested using the well researched material system, GaAs/AlxGa(1-x)As. The highest reported V-oc similar to 1175 mV together with the highest possible fill factors (FFs) in the mid 80 %'s, have been achieved for the initial devices. The overall efficiency achieved using only two growths is similar to 20 %. As expected from the initial design, the impurity PV effect is experimentally observed producing Voc values in the range of 650-900 mV under complete darkness for the GaAs/AlxGa(1-x)As based devices. Both the theoretical concepts and experimental results are presented in this paper.