Low Illumination Light (LIL) Solar Cells: Indoor and Monochromatic Light Harvesting

Low and indoor light energy harvesting is needed to meet the demands of zero net energy (ZNE) building, Internet of Things (IoT), and indirect energy conversion isotope battery (IDEC iBAT) systems. Characterizing photovoltaic (PV) solar cells under low intensity and narrow light spectrum conditions has not been clearly examined. PV operating values under 1 sun illumination do not scale linearly under low intensity and monochromatic light conditions (efficiency drops from 30% to 3% at 1 Wopt/cm2). However, limited energy conversion efficiencies and metrics can be improved by choosing a PV whose band gap matches the light source. By quantifying losses on single-junction semiconductors, we can determine the theoretical maximum efficiency of a PV material for different light sources. We measure single-junction solar cells parameters under 3 different white light (indoor light) and near monochromatic light spectrum sources with light intensities ranging from 0.5 to 100 Wopt/cm2. Measurements show that indium gallium phosphide (InGaP) has the highest surface power density and conversion efficiency (29% under 1 Wopt/cm2 at 523 nm for 470-nm, 523-nm, and white light). The results will aid the US Army s decision on selecting the best PV type for the IDEC iBAT and other energy harvesting systems. The original document contains color images.