Taketo Aihara

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

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 s...

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

Three non-destructive methodologies, namely, surface photovoltage (SPV), photoluminescence, and piezoelectric photothermal (PPT) spectroscopies, were adopted to detect the thermal carrier escape from quantum well (QW) and radiative and non-radiative carrier recombinations, respectively, in strain-balanced InGaAs/GaAsP multiple-quantum-well (MQW)-inserted GaAs p-i-n solar cell structure samples. Although the optical absorbance signal intensity was proportional to the number of QW stack, the signal intensities of the SPV and PPT methods decreased at high number of stack. To explain the temperature dependency of these signal intensities, we proposed a model that considers the three carrier dynamics: the thermal escape from the QW, and the non-radiative and radiative carrier recombinations within the QW. From the fitting procedures, it was estimated that the activation energies of the thermal escape ΔEbarr and non-radiative recombination ΔENR were 68 and 29 meV, respectively, for a 30-stacked MQW sample. The ...

Non-radiative carrier recombination mechanism in the InGaAs/GaAsP strain-balanced quantum well solar cells with different number of stacks by using a piezoelectric photothermal method

To optimize the multiple quantum well (QW) structure of the strain-balanced InGaAs/GaAsP inserted into GaAs p-i-n solar cell, carrier escaping process from QW, carrier radiative and non-radiative recombination processes in QW were investigated by using surface photovoltage (SPV), photoluminescence (PL) and piezoelectric photothermal (PPT) spectroscopies, respectively. Distinctive peaks at 1.19 eV were observed for all spectra below the bandgap of GaAs substrate (1.42 eV) and concluded that the peak was arisen from the excitonic transitions associated between the 1st order subbband in QWs. Although the optical absorption intensity of this transition was proportional to the number of QW stacks, SPV and PPT signals showed saturation above the QW stacks of 20. Band diagram calculation showed that an entire region of 10-stacked QWs was located in the flat band potential area, whereas a part of 20-stacked QWs was placed in an internal electric field. It was then suggested that the potential barrier height of 20-stacked QWs is small than that of 10-stacked QW.