Maximum efficiencies of indoor photovoltaic devices

Photovoltaic (PV) converters on the centimeter scale are considered to be the most promising energy supplier for energy-autarkic microsystems in indoor applications, i.e., to power wireless sensor nodes in automated buildings. We provide the first systematic discussion of the optimum material and efficiency limits of indoor photovoltaics. The limiting efficiency η of an ideal photovoltaic converter for various indoor radiation sources and two calculation models ranges between 46% for a fluorescent tube and a bandgap E_g = 1.95 eV and 67% for a sodium discharge lamp and E_g = 2.10 eV. The optimal bandgap for typical narrow-band artificial light sources is 1.90-2.00 eV. For E_g = 2.25 eV, i.e., matched to the photon energy of a monochromatic source emitting at 555 nm, η calculated from the detailed balance model reaches 72.98%. The performance when irradiated by indoor Planckian radiators, such as incandescent bulbs and halogen tubes, is always less than under any solar irradiance. The experimental efficiencies of 11 sample groups with a maximum efficiency of 16%, which were measured with a radiometric indoor characterization setup especially developed for this study, support the theoretically predicted changes in efficiency when compared with efficiencies under the conditions of IEC 60904-3.