Investigation of Quantum Size Effects on the Optical Absorption in Ultrathin Single Quantum Well Solar Cell Embedded as a Nanophotonic Resonator

Subwavelength optical nanocavity using semiconductor nanostructures is a key nanophotonic approach for various optoelectronic devices. In such low-dimensional systems, size-dependent changes can arise due to quantum confinement (QC). In this article, the implications of quantum-size effects on the light harvesting in ultrathin single quantum well solar cell (SQWSC) are analyzed. This device is based on silicon (barrier)/germanium (QW) nanostructures integrated as a deep-subwavelength nanophotonic resonator. Compared with the state-of-the-art, the novelty of this article consists of the investigation of the synergy between photonic and electronic confinements at both functional materials and device levels. It is shown that QC effects enhance the optical absorption efficiency in both low-dimensional germanium single layers and QW structures. However, this is affected by the barrier heights and the interface states. In the SQWSC devices, the changes of the photocurrent output, the resonance condition and the absorption edge as a function of QW thickness are explained based on the optical field and the local absorption distributions. Shorter nanocavity lengths with thinner QW nanoabsorbers result in blue-shifted resonance wavelength, suitable maximum optical field intensity in the visible wavelengths range as well as promoted photonic confinement. This enables an enhancement of the optical absorption efficiency relative to thick counterparts. The nonlinearity in the thickness-related photocurrent implies a drastic reduction in QW thickness while preserving a high photocurrent level. The presented considerations in SQWC could be extended as design rules for the optimization of the photocurrent in derived multiple quantum well systems and for relevant color-neutral semitransparent devices.