Quasi-Fermi level splitting in InAs quantum-dot solar cells from photoluminescence measurements

Abstract

In intermediate band solar cells (IBSCs), voltage preservation is a key issue to overcome efficiency limit in singlejunction solar cells. To achieve this, quasi-Fermi level splitting of respective transitions should be investigated because equivalent circuit model of an IBSC is series-parallel connected diodes. In this study, we have quantitatively investigated quasi-Fermi level splitting, Δμ in InAs quantum dot solar cells (QDSCs) by performing absolute intensity calibrated photoluminescence (PL) spectroscopy. Multi-stacked InAs/GaAs QDs were fabricated in the i-region of a GaAs p-i-n single-junction solar cell. QD ground states and GaAs band edge emissions were observed simultaneously by using a near-infrared sensitive CCD spectrometer. Excitation density dependence and temperature dependence were investigated in detail to clarify photo-carrier kinetics in QDSCs and tackle the voltage preservation issue on IBSCs. At room temperature, nonlinear increase in PL intensity was clearly observed at high excitation density above 1000 suns. Absolute PL spectra was analyzed at respective transitions by using generalized Plank’s law. As the result of detail analysis, increase in Δμ was confirmed at high excitation density and at room temperature, which suggested voltage recovering via photo-filling effect. It would be desirable to implement voltage preservation in IBSCs.