GaAs-AlGaAs heterojunction solar cells with increased open-circuit voltage

In order to reduce the dark current we studied the incorporation of AlGaAs layers at the junction of GaAs solar cells. According to Shockley´s diode model, the dark current is reduced when the junction is made of wide bandgap material. However the GaAs-AlGaAs interfaces form a potential barrier for the generated carriers, which degrades the collection efficiency and sets a limit to the Al concentration. A set of homojunction cells, GaAs and Al_0.1Ga_0.9As, and heterojunction cells, with Al concentration of 5, 10 and 15%, as well as a cell with quantum wells have been fabricated. For the characterisation, we measured the dark current, I-V curve under illumination, temperature dependence of the short-circuit current, spectral response and the emission spectra. Compared to the GaAs cell the heterojunction cells reveal a higher open-circuit voltage due to the reduced dark current. The 5% heterojunction cell showed only a small degradation of the short-circuit current and relative to the GaAs cell the efficiency was enhanced by 7%. In case of the MQW cell and the 10 and 15% heterojunction cells the reduction of the short-circuit current was more extreme and the advantage of the higher open-circuit voltage was by far compensated, resulting in a reduced efficiency compared to the GaAs cell