Figure 1. Absolute EL measurement of the 3-junction solar cell.

(a). Schematic of the experimental setup for the absolute EL measurement of the solar cell. The top panel presents a photograph of the solar cell without and with injection current (15 mA/cm²). (b). Schematic structure of the GaInP/GaAs/Ge 3-junction solar cell. (c). Absolute EL spectra divided by injection-electron number (abs. EL efficiency) for various injection-current densities of the triple-junction solar cell. (d). The measured external EL quantum efficiencies (ext. EL. quan. efficiency) of the subcells under the operation of an LED (y_ext^LED) as a function of the injection-current density. Note that the y-axis of the EL spectra is the absolute photon number per second divided by the injection carrier number per second, i.e., the y-axis represents the external EL quantum efficiency (y_ext^LED) as a function of the photon energy. Consequently, the y_ext^LED of the subcells (Fig. 1d) can be obtained through the integrations of the EL peaks in the EL spectra.