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/sub 0.1/Ga/sub 0.9/As, 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.