Quantum dot solar cells: Effective conversion of IR radiation due to inter-dot n-doping

We report a 50% increase in power efficiency for our novel InAs/GaAs quantum dot with built-in charge (Q-BIC) solar cells. We found that n-doping the inter-dot space of a quantum dot solar cell (QDoSC) increases the short circuit current density from 15.07 mA/cm² in undoped QDoSC to 24.30 mA/cm² in the device doped to provide approximately six electrons per dot. To identify the physical mechanisms that provide this significant improvement, we investigate the photovoltaic response and its spectral characteristics in GaAs reference cell, undoped, n-doped, and p-doped QDoSCs. We found that the photovoltaic efficiency of the undoped QDoSC is almost the same as that of the reference cell. The efficiency monotonically improves with increasing n-doping, while the p-doping deteriorates the photovoltaic conversion. Studies of the photoluminescence of p- and n-doping show that the photoelectron capture into QDs is substantially faster than the hole capture, which leads to an accumulation of electrons in QDs. The built-in-dot electron charge enhances electron inter-subband QD transitions, suppresses the fast electron capture processes, and together with charged donors, forms the potential profile which precludes degradation of the open circuit voltage. All of these factors lead to the enhanced harvesting of IR energy and to a radical improvement of the QDoSC efficiency. Even higher efficiencies are anticipated for higher n-doping levels.