Detection of miniband formation in strain-balanced InGaAs/GaAsP quantum well solar cells by using a piezoelectric photothermal spectroscopy

Abstract

To investigate the effect of the miniband formation on the optical absorption spectrum, we adopted two non-destructive methodologies of piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies for strain-balanced InGaAs/GaAsP multiple quantum-well (MQW) and superlattice (SL) structures inserted GaAs p-i-n solar cells. Because the barrier widths of the SL sample were very thin, miniband formations caused by coupling the wave functions between adjacent wells were expected. From PR measurements, a critical energy corresponding to the inter-subband transition between first-order electron and hole subbands was estimated for MQW sample, whereas two critical energies corresponding to the mini-Brillouin-zone center (Γ) and edge (π) were obtained for SL sample. The miniband width was calculated to be 19 meV on the basis of the energy difference between Γ and π. This coincided with the value of 16 meV calculated using the simple Kronig–Penney potential models. The obtained PPT spectrum for the SL sample was decomposed into the excitonic absorption and inter-miniband transition components. The latter component was expressed using the arcsine-like signal rise corresponding to the Γ point in the mini-Brillouin zone that was enhanced by the Sommerfeld factor. The usefulness of the PPT methodology for investigating the inserted MQW and/or SL structure inserted solar cells is clearly demonstrated.

Effects of barrier thickness on carrier non-radiative relaxation process in InGaAs/GaAsP superlattice solar cells by piezoelectric photothermal and surface photovoltage spectroscopies

“…We attribute this disagreement to the overlap of the excitonic absorption and the inter-sub-band transition affected by the Sommerfeld factor, which corresponds to the Coulomb interaction of the exciton in the continuous states absorption edge. 29) In this case, the E g is located at a higher energy than that of the observed peak.…”

Effect of number of stack on the thermal escape and non-radiative and radiative recombinations of photoexcited carriers in strain-balanced InGaAs/GaAsP multiple quantum-well-inserted solar cells

“…To clarify the number of stack dependencies of the PPT and SPV signal intensities, the spectrum was decomposed using the same procedures as in our previous papers. [10][11][12][13] In this procedure, an excitonic absorption component was expressed by the convolution of the delta and Quasi-Voigt functions, whereas a subband transition was represented by the convolution of the unit step and Quasi-Voigt functions. The PPT and SPV spectra were then decomposed by curve-fitting method.…”

“…Further details regarding the PPT method can be found elsewhere. 12,13,15 For the SPV measurements, the same samples and experimental apparatus were used. An indium tin oxide (ITO) film, used as a transparent front electrode, was placed on the sample with a 0.5-mm-thick vacuum layer, creating a capacitive contact.…”

“…On the other hand, the PPT method is used to detect the heat generated by the non-radiative recombination of photoexcited carriers, and we have already developed a methodology to decompose the absorption spectrum of the MQW structure into the excitonic and subband transitions. [10][11][12][13] By investigating the temperature dependencies of these signal intensities, we calculated the activation energies of the thermal escape and recombination processes, and discussed the number of QW stack dependencies. The combination of these three non-destructive methodologies yields new insights for optimizing QW components, such as the well and barrier widths, barrier height, and number of stack, to further improve the cell performance.…”

Investigation of miniband formation and optical properties of strain-balanced InGaAs/GaAsP superlattice structure embedded in p–i–n GaAs solar cells

“…17) The PPT method is used to detect heat generated by the nonradiative recombination of photoexcited carriers. 18,19) When the barrier thickness was small such as 2.0 nm, miniband formation and the acceleration of carrier transport in the SL were confirmed by PR and PL analyses, respectively. On the other hand, the PPT signal intensities were found to still be large.…”