Nanoscale engineering: optimizing electron-hole kinetics of quantum dot solar cells

Abstract

We report the substantial increase in power efficiency in InAs/GaAs quantum dot (QD) solar cells due to n-doping of the inter-dot space in p⁺-δ-n⁺ structures and investigate the physical mechanisms that provide this significant improvement. We have compared the GaAs reference cell to undoped, n-doped and p-doped QD solar cell structures and found that the short circuit current, J_SC, of the undoped QD solar cell is comparable to that of the GaAs reference cell. On the other hand, while p-doping deteriorates the device performance, n-doping significantly increases J_SC without degradation of the open circuit voltage, V_OC. The photovoltaic device, n-doped to provide approximately six electrons per dot, demonstrates 60% increase in J_SC, from 15.07 mA/cm² to 24.30 mA/cm². Strong increase in the photoresponse and J_SC of the IR portion of the solar spectrum has been observed for the n-doped structures. From the photoluminescence data, the electron capture noticeably dominates over hole capture leading to an accumulation of electrons in the dots. We have observed that QDs with built-in charge (Q-BIC) enhances harvesting of IR energy, suppresses the fast electron capture process, and stabilizes the open circuit voltage. All of these factors lead to a significant improvement of the cell efficiency.