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

We investigated the carrier non-radiative relaxation process of InGaAs/GaAsP superlattice (SL) solar cells with different barrier thicknesses by combining piezoelectric photothermal (PPT) and surface photovoltage (SPV) measurements. The former technique detected heat generated by non-radiative relaxation and the latter detected the surface potential change induced by the carrier accumulation. Although the mechanisms of these measurements were different, corresponding signals of the transition between the quantum levels were observed in both spectra. The SPV signal intensities were independent of the barrier thicknesses. The carrier tunneling process functioned poorly under open-circuit conditions. Conversely, the PPT signal intensities increased with decreasing barrier thickness. We experimentally demonstrated that the non-radiative relaxation component increased as the barrier thicknesses decreased owing to the lattice relaxation at interfaces between the quantum well and barrier layers. We concluded that the non-radiative relaxation process in the SL structure must be directly evaluated to improve the solar cell performance.