Fabrication of potentially modulated multi-quantum well solar cells

In order to realize superior photovoltaic performance as predicted for multiple quantum well (MQW) solar cells, one requires both a high material quality that ensures minimum non-radiative recombination losses, as well as an assignment of optimal quantum structure that provides efficient escape rates of photo-generated carriers out of QWs into the "base" region thereby keeping radiative recombination losses to a minimum. For the purpose of improved carrier collection from QWs, we propose and investigate potentially modulated (PM) MQW solar cells, for which QWs are modified from the conventional square-shaped to potentially modulated "step-like" structure. The measured external quantum efficiency (EQE) for a "3-step" InGaAs/GaAs PM-MQW solar cell has indicated that, (1) EQE improves in the wavelength region of 650 ∼ 850 nm, which can be attributed to an improved collection of carriers across the MQW region, and (2) EQE improves in the longer wavelength region (> 900 nm) owing to the fact that the carriers photo-excited in QWs can smoothly escape out of QWs. Together with dark-current characteristics data, the photovoltaic performance of PM-MQW solar cells can be estimated. The projected short-circuit current density for "3-step" InGaAs/GaAs PM-MQW solar cell is J_SC = 24.66 mA/cm², and AM1.5 conversion efficiency amounts to η = 18.27% at 1 sun.