Reference cell based current vs voltage (IV) measurements assume that the effect of an illumination spectrum on a solar cell's performance can be fully captured by the multiplication of the spectrum with the device's spectral response and subsequent integration. This is based on a fundamental understanding that sub-bandgap light will be minimally absorbed, if at all, in the active layers of the solar cell and therefore not contribute to the power generation. In this work we show a novel phenomenon in which illumination of the substrate is required for good performance in solar cells with III-V active layers and germanium substrates, despite negligible contribution to the short circuit current or open circuit voltage, or increase of generated power beyond that expected from the III-V junction. Discovered in the course of characterizing cells for low-light conditions, we confirm the observation with additional IV and electroluminescence measurements and modeling that reproduces experimental results. This phenomenon has implications for device characterization under non-standard light sources, the development of solar cells for conditions lacking long-wavelength light such as indoor photovoltaics under light-emitting diode illumination, and the prediction of device performance under spectra that differ from the test conditions.
Keywords: Characterization of PV; Circuit analysis; III-V and Concentrator PV; Photovoltaic cells.