Two-step photon absorption in InAs/GaAs quantum-dot superlattice solar cells

Abstract

We studied the two-step photon absorption (TSPA) process in InAs/GaAs quantum-dot superlattice (QDSL) solar cells. TSPA of subband-gap photons efficiently occurs when electrons are pumped from the valence band to the states above the inhomogeneously distributed fundamental states of QDSLs. The photoluminescence (PL)-excitation spectrum demonstrates an absorption edge attributed to the higher excited states of the QDSLs in between the InAs wetting layer states and the fundamental states of QDSLs. When the absorption edge of the excited state was resonantly excited, the superlinear excitation power dependence of the PL intensity demonstrated that the electron and hole created by the interband transition separately relax into QDSLs. Furthermore, timeresolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

Efficient two-step photocarrier generation in bias-controlled InAs/GaAs quantum dot superlattice intermediate-band solar cells

“…Here, it is of note that the second subbandgap absorption efficiently occurs when electrons are pumped into the ES of QDSLs in the first excitation step. Previously 14 , we reported a similar phenomenon and attributed it to the separation of carriers in the ES miniband owing to the internal electric field. The electron lifetime was extended by inhibiting electron-hole recombination, enhancing the second subbandgap absorption.

Figure 1EQE and ΔEQE spectra for QDSL-IBSC.

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Two-step photon up-conversion solar cells

“…In this wavelength region, the EQE signal also shows a gradual decrease with increasing wavelength because the optical absorption coefficient becomes small with increasing wavelength. In the near-IR wavelength region below the bandgap of GaAs, the EQE signal decreases drastically and shows a small structure at 912 nm that can be attributed to thermally excited carriers from the deep quantized states of the InAs wetting layer2140.…”

Understanding the electric field control of the electronic and optical properties of strongly-coupled multi-layered quantum dot molecules

“…Self-assembled quantum dot molecules (QDMs) made up of III-V materials are one of the most popular types of nanostructures used in a variety of devices for optoelectronic [1][2][3][4][5][6][7], photovoltaic [8][9][10][11][12], and quantum information technologies [13,14]. The QDMs typically grow in the form of vertical stacks due to the presence of strain, which stems from the lattice mismatch of the substrate and the QD material.…”

“…The physical insights provided are based on the state-of-the-art multi-million atom electronic structure and optical mode simulations, and deliver a reliable understanding of the complex physics of the QDMs, which involve subtle interplay between the applied electric fields and the strong interdot couplings. In order to answer the aforementioned outstanding questions, we apply electric fields along both parallel and anti-parallel directions to the growth direction of a QDM recently discussed in the experimental studies [8,9] and investigate its effects on the spatial confinements and symmetries of the electron and hole wave functions, band-gap wavelengths, ground state transition energies, and polarisation-resolved interband optical modes. Our results show that the application of a small electric field anti-parallel to the growth direction is sufficient balance the asymmetric effect of the underlying strain on the electron states.…”