Carrier dynamics and recombination in silicon doped InAs/GaAs quantum dot solar cells with AlAs cap layers

Abstract

The effects of doping InAs quantum dots (QDs) with Si on charge carrier dynamics and recombination in the InAs/GaAs QD solar cells (QDSCs) with AlAs cap layers was investigated. Non-radiative and radiative recombination paths in the doped cells were identified by changes in emission intensity, long-wavelength photovoltage (PV) as well as time-resolved PV and photoluminescence (PL) measurements. We find that the reduction of long-wavelength PV and PL with n-doping is due to the electron population of the QD ground states (GSs) and shrinkage of the depletion layer. The time constants, derived from the time-resolved PV, grow non-monotonically with increasing of the doping density in the QDs due to redistribution of electrostatic potential in the intrinsic region of p-i-n diode and electron population of EL2 defect states of GaAs barriers. We also find that the GS emission from the InAs QDs decreases with n-doping. The results show that PL traces depend on carrier dynamic in the top QD layers populated partially with electrons from ionized impurities, whereas PV transients were found to be strongly dependent on recombination via QD and defect states located outside the depletion layer. We conclude that the non-radiative recombination of photogenerated electrons and holes via defects is suppressed due to the spatial separation by the local electric fields in and around doped AlAs/InAs QDs, as the potential profile of the intrinsic region is modulated spatially by built-in charges. The interpretation of experimental data suggests limiting mechanisms in the InAs/GaAs QDSCs operation and sheds light on possible approaches for their further improvement.