Spin-Exchange Carrier Multiplication in Manganese-Doped Inverted Core/Shell CdSe/HgSe Quantum Dots

Carrier multiplication (CM) is a process in which a single absorbed photon generates multiple electron-hole pairs (excitons). Typically, CM occurs through a Coulombic collision of an energetic hot carrier with a valence-band electron, causing the latter to move into the conduction band. A competing energy-relaxation pathway is fast phonon-assisted cooling, which is the major factor limiting the CM yield. In this study, we show that this limitation can be overcome by utilizing not direct but spin-exchange (SE) Coulomb interactions in Mn-doped core/shell CdSe/HgSe/ZnS quantum dots (QDs). Our goal is to enhance the SE-CM process by leveraging the strong exchange coupling within the Mn-HgSe system, facilitating multiexciton generation in the QDs. We also implement an 'inverted' core/shell structure by incorporating a lower bandgap material in the shell region, making both electrons and holes electrically accessible. In this QD design, SE-CM occurs via two steps: (1) SE-energy transfer from a hot exciton generated in the CdSe core to an interfacial Mn ion, followed by (2) energy- and spin-conserving relaxation of the excited Mn ion to produce two excitons (bright and dark) in the HgSe shell. Due to extremely short SE time scales, both SE steps occur without considerable interference from phonon emission, leading to high SE-CM efficiencies. Transient absorption (TA) measurements with spectrally tunable pump pulses reveal a sharp onset of SE-CM near the energy of the Mn spin-flip transition (E_Mn = 2.1 eV). The measured quantum efficiency (QE) of photon-to-exciton conversion exhibits a step-like growth and reaches 164%. Importantly, due to an inverted architecture of our QDs, SE-CM also leads to a considerable enhancement of a photocurrent in close-packed QD films. The QEs obtained from photocurrent measurements align closely with those derived from TA studies. Our findings highlight the significant potential of SE-CM for applications in advanced photoconversion systems.