These are fairly simple single-junction devices (diodes) that are limited by thermodynamic considerations to a maximum theoretical power conversion ef ciency of 31% under direct AM1.5 sun- light.2 Solar cells and modules are usually characterized accord- ing to the IEC norm3 under standard test conditions (STC), which correspond to 1 kW/m2 (100 mW/cm2) direct perpendicular irra- diance under a global AM 1.5 spectrum at 25°C cell temperature. This means that an ideal silicon solar cell operating under direct sunlight converts approximately 30% of the illuminating solar radiation into electrical power, although actual cells suffer from parasitic losses.