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

Carrier multiplication (CM) is a process whereby a single absorbed photon generates multiple excitons. In a standard scenario, CM occurs due to a Coulombic collision of an energetic, <i>hot</i> carrier that promotes a near-valence-band electron to the conduction band. A competing energy-relaxation pathway is fast phonon-assisted cooling, which is the major factor limiting CM yields. Here, we show that this limitation can be overcome by employing not direct but <i>spin-exchange (SE) Coulomb interactions</i> in Mn-doped core/shell CdSe/HgSe/ZnS quantum dots (QDs). We aim to enhance the SE-CM process by exploiting a high exchange coupling in the Mn-HgSe system where the resulting excitons are generated in the QDs. Furthermore, we ‘invert’ a core/shell structure by placing a lower bandgap material in the shell region to make both an electron and a hole electrically accessible. In the developed QD structure, 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 create 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 indicate a sharp onset of SE-CM near the energy of the Mn spin-flip transition (<i>E</i>_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, SE-CM also leads to a considerable enhancement of a photocurrent in close-packed QD films. The QEs obtained from photocurrent measurements are in excellent agreement with those inferred from the TA studies. Our findings provide a considerable potential of SE-CM in advanced photoconversion systems.