Measurement of built-in electrical potential in III–V solar cells by scanning Kelvin probe microscopy

We report on direct measurements of the built-in electrical potential in III–V semiconductor-based solar cell devices by using scanning Kelvin probe microscopy. Potential profiles on cross sections of the devices were measured quantitatively and spatially resolved in open and short circuits, under and without illuminations, with selective photon energies matching band gaps of the junctions. The measurements provide valuable information about the electrical properties of the devices, and are useful for understanding the performance of solar cells. On a ${\mathrm{GaInP}}_2$ -single junction cell, two potential features were measured and were assigned to the $\mathrm{p-n}$ junction and the potential barrier at the interface between the ${\mathrm{GaInP}}_2$ base layer and the GaAs substrate. The potential on the $\mathrm{p-n}$ junction is photoactive, and that on the ${\mathrm{GaInP}}_2/\mathrm{GaAs}$ interface is photoinactive. On a ${\mathrm{GaInP}}_2/\mathrm{GaAs}$ tandem cell, two potential features were measured near the top and the bottom $\mathrm{p-n}$ junctions. When the sample was illuminated by light with photon energies larger than the band gap of the top junction, in the condition of short circuit, a potential peak appeared on the boundary between the bottom cell and tunnel junction due to charge accumulations on the bottom junction.