Heterosubstrate Illumination Effects in III–V Solar Cells

Reference cell-based current versus voltage (
I–V
) 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 subbandgap 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
I–V
and electroluminescence measurements and modeling that reproduces experimental results. This phenomenon has implications for device characterization under nonstandard 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.