Quantum Well Solar Cell Using Ultrathin Germanium Nanoabsorber

Quantum-confining nanostructures are a key approach for efficient solar energy conversion in advanced designs of photovoltaic devices. In this study, we report the first demonstration of quantum confinement (QC) effects in single quantum well (QW) solar cells based on ultrathin hydrogenated amorphous germanium (a-Ge:H) nanoabsorber, using cost-effective, industrial-compatible and low-temperature production processes. The drastic reduction of a-Ge:H thickness in single QW solar cell, from 20 nm down below 2 nm, results in QC-tunable optoelectronic properties and photovoltaic characteristics, while maintaining a comparable power conversion level. For the overall efficiency, the decrease in the photo generation current density (J_sc) due to the reduction of nanoabsorber thickness is compensated by a major gain up to a factor of two in open-circuit voltage (V_oc) exceeding 700 mV and a considerable enhancement of the fill factor (FF) from 45 to 65 %. The successful demonstration of ultrathin a-Ge:H QW solar cells underlines the promising potential of bandgap engineering and multiple quantum confining nanostructures in our device technology with high relevance for semi-transparent power-generating systems, especially in window-integrated PV or in greenhouses, when combined with appropriate transparent conductive electrodes.