Photocurrent Enhancement due to Spin-Exchange Carrier Multiplication in Films of Manganese-Doped 'Inverted' CdSe/HgSe Quantum Dots

TL;DR

This paper demonstrates that Mn-doped CdSe/HgSe quantum dots can convert hot exciton kinetic energy into multiple electron-hole pairs via spin-exchange interactions, enhancing photocurrent for optoelectronic applications.

Contribution

It introduces a novel carrier multiplication mechanism in Mn-doped inverted CdSe/HgSe quantum dots driven by rapid spin-exchange energy transfer.

Findings

Carrier multiplication occurs through hot exciton capture by Mn ions.

Both electrons and holes are confined in the QD shell, facilitating extraction.

The process enhances photocurrent in the quantum dots.

Abstract

Incorporation of manganese impurities into II-VI semiconductors results in a dramatic change in their properties due to strong exchange interactions between the Mn ion and the semiconductor host. In colloidal quantum dots (QDs), these interactions result in a rapid bidirectional energy transfer between the magnetic impurity and the QD intrinsic states, which is characterized by an extremely high energy transfer rate of more than ~5 eV/ps. This rate is higher than the rate of energy loss due to phonon emission (typically, ~1 eV/ps or less), so Mn-QD interactions could in principle be used to capture and utilize the kinetic energy of the hot carrier before it is lost to phonons. Here, we demonstrate that by using Mn-doped CdSe/HgSe core/shell QDs, we can efficiently convert the kinetic energy of a hot exciton into an additional electron-hole pair (exciton). This carrier multiplication process occurs through rapid capture of a hot exciton by a Mn ion, which then undergoes spin flip relaxation, producing two excitons near the QD band edge. Due to the inverted geometry of the CdSe/HgSe QDs, both electrons and holes resulting from carrier multiplication occupy the QD shell, allowing them to be easily extracted from the QD for potential use in electro-optical devices or chemical reactions.