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

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, time-resolved PL measurements demonstrated that the electron lifetime is extended by thereby inhibiting recombination with holes, enhancing the second subband-gap absorption.

Figure 1

EQE spectra measured at 20 and 300 K, (a) entire spectra, and (b) magnified plots in the subband-gap region. The horizontal axis is the difference between the photon energy and the GaAs band edge ($E_{\mathrm{g}}=1.52$ eV at 20 K and 1.42 eV at 300 K).

Figure 2

(a) $\mathrm{\Delta }\mathrm{EQE}$ spectrum measured at 9 K. The IR light source used here was a pulsed laser light with a photon energy of 0.30 eV and repetition rate of 200 kHz. The excitation intensity was $6.5\times {10}^{18}$ photons/(${\mathrm{cm}}^2$ s). (b) PL and PLE spectra measured at 9 K. PLE spectrum indicates the PL peak intensity of the fundamental transitions of QDSLs. Several absorption edges are indicated by arrows in the PLE spectrum. (c) Comparison of $\mathrm{\Delta }\mathrm{EQE}$ spectra obtained for InAs/GaAs(4 nm) QDSL and uncoupled InAs/GaAs(30 nm) stacked QDs at 9 K.

Figure 3

Integrated PL intensity from the fundamental transitions as a function of the excitation power. The excitation wavelengths, 940, 850, and 780 nm, were chosen to excite the absorption edge at 950 nm, the InAs WL states, and GaAs barrier, respectively. Here, the excitation power density was carefully chosen by taking into account the relative absorption coefficient estimated from the PLE spectrum. Dotted lines are fitting of the power dependence.

Figure 4

(a) Time-resolved PL intensity from the fundamental transitions. The excitation wavelengths, 900 and 800 nm, were chosen to excite the absorption edge at 950 nm and GaAs barrier, respectively. PL decay curves at different electric fields were measured at the 900-nm excitation. Solid lines indicate best fitting curves, where the coefficients of determination, R${}^2$ values, were 0.936, 0.943, 0.974, and 0.968 for 7 kV/cm at the 800-nm excitation, and 7, 20, and 30 kV/cm at the 900-nm excitation, respectively. (b) Analyzed decay times as a function of the detection-time delay in the stretched exponential decay profile. (c) Excitation power density dependence of the decay time. The excitation power densities were chosen by taking into account the relative absorption coefficient estimated from the PLE spectrum.