Quantum confinement-tunable solar cell based on ultrathin amorphous germanium

Abstract Nanoscale semiconductors involve distinct fundamental phenomena and novel properties emerge due to dimensional restriction of the charge carriers’ motion, known as quantum confinement (QC). In this study, the first investigation of quantum size effects in single quantum well (QW) solar cells based on ultrathin hydrogenated amorphous germanium (a-Ge:H) nanoabsorber is reported, using cost-effective, industrial-compatible and low-temperature production processes. The confinement in the growth direction due to the thickness reduction of the a-Ge:H absorber layer from 20 nm down below 2 nm, results in tunable optoelectronic properties and photovoltaics (PV) characteristics, while maintaining a comparable power conversion level. A major gain by a factor of two in open circuit-voltage is demonstrated, exceeding 700 mV with reducing the a-Ge:H QW thickness by an order of magnitude. Furthermore, the band gap widening yields a considerable enhancement of the fill factor from 45 to 65% due to the reduction of the conduction band offset at a-Ge:H (QW)/a-Si:H (barrier) heterojunction interface. The successful demonstration of a-Ge:H QW cells indicates the promising potential for multiple QWs implementation as nanoabsorber material in solar cells.