Quantum Well Solar Cell Using Ultrathin Germanium Nanoabsorber

Abstract

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 embedded in optical resonant nanocavity, using cost-effective, industrial-compatible and low-temperature production processes. Due to a drastic reduction of a-Ge:H QW thickness from 20 nm down below 2 nm, the quantum size effects are manifested, inducing a significant modulation of the energy bandgap from 0.98 eV up to 1.56 eV. In single QW a-Ge:H solar cell, due to QC effects, the band gap widening and the upward shift of conduction band edge reduce the band offset at the a-Ge:H /a-Si:H heterojunction, leading to considerable tuning of the photovoltaic characteristics, while maintaining a comparable power conversion level. The decrease in the photo generation current density (Jsc) due to the reduction of nanoabsorber thickness from 20 nm down below 2 nm is compensated by a major gain up to a factor of two in open-circuit voltage (Voc) exceeding 700 mV and a considerable enhancement of the fill factor (FF) from 45 to 65 %. Moreover, due to the reduction of nanoabsorber thickness, high transmittance above 65% through the n-i-p multilayers without back reflector is achieved. 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.