Monolithic tandem solar cell based on GaAlAs-GaAs system

Low and medium concentrations (10 to 50 suns) are a suitable way to improve the efficiency of solar cells without the expensive two-axis concentrators. The open circuit voltage, V_oc, rapidly increases for the lower concentrations, while a good value of the fill factor is achieved when a large ratio of the dark current at V_oc is from the diffusion current, i.e., in the 30-100-suns range for the AlGaAs and GaAs solar cells. For tandem solar cells, a 40% efficiency can be theoretically exceeded under moderate sunlight concentration. Some improvements to a linear three-terminal solar cell built on a GaAs substrate are described. The first junction is formed by two GaAs layers, Sn and Ge doped, grown by liquid phase epitaxy (LPE) on a p⁺Zn-doped substrate. Then a thick Sn-doped Al_0.32Ga_0.68As (about 10 μm) is grown. Finally, a thin graded AlGaAs window layer is deposited by isothermal contact between the solid and the Be-doped melt corresponding to Al_0.85Ga_0.15As solid. During the contact, Be diffuses into the Al_0.32Ga_0.68As layer to form the upper junction of the device. The total efficiency of the device is 26.6% at 40 AM1.5D