Spin-Exchange Carrier Multiplication in Manganese-Doped Colloidal Quantum Dots

During carrier multiplication (CM) a high-energy, ‘hot’ electron or hole relaxes within the same band by creating new electron-hole (e-h) pairs.¹ This occurs via one or more impact ionization events whereby a pre-existing valence-band electron is excited to the conduction band after an Auger-type collision with a high-energy carrier. In principle, CM could improve the performance of a variety of optoelectronic, photovoltaic (PV) and photocatalytic devices because due to this process, the quantum efficiency of photon to e-h pair conversion (<i>Q</i>eh) becomes greater than one.^2,3 However, practically realized CM efficiencies are still not sufficiently high to achieve an appreciable boost in device performance, suffering from the high CM threshold (<i>hv</i>_th) and the high e-h pair creation energy (<i>e</i>_eh).<br/>The use of engineered colloidal quantum dots (QDs) allows one to reduce <i>hv</i>_th down to the energy-conservation-defined value of 2<i>E</i>_g.⁴ However, <i>e</i>_eh is not appreciably decreased compared to bulk solids. To reduce <i>e</i>_eh, one needs to increase the energy-gain rate associated with Coulomb collisions versus the phonon-related energy-loss rate. Here, to accomplish this objective, we exploit not ‘direct’ but ultrafast ‘spin-exchange’ Coulomb interactions in magnetically doped QDs.^5,6 For this purpose, we synthesize Mn-doped core/shell PbSe/CdSe QDs wherein the dopants exhibit strong spin-exchange coupling to both CdSe and PbSe QD components. By applying transient photoluminescence measurements, we observe a highly efficient excitation transfer from the light-harvesting CdSe shell to the Mn dopants, which is followed by a CM-like spin-exchange process which leads to generation of two core excitons. Due to spin conservation, the biexciton produced via relaxation of the excited Mn ion is a combination of a dark and a bright exciton (spins 1 and 0, respectively) in two different L-valleys of PbSe. The corresponding quantum efficiency measured at 2.5<i>E</i>_g is ~140%, almost three-fold enhancement versus undoped QDs, implying that the e-h pair creation energy is less than 1.25<i>E</i>_g. This is near the fundamental one-bandgap limit and is also considerably smaller (a factor of &gt;2.5) than for the reference undoped QDs. These results suggest that the use of spin-exchange interactions represents a viable approach for realizing highly efficient CM-enhanced solar-photoconversion schemes.<br/>1. Klimov, V. I. <i>Ann. Rev. Condens. Matter Phys</i>. <b>5</b>, 285-316 (2014).<br/>2. Semonin, O. E. <i>et al</i>. <i>Science</i>, <b>334</b>, 1530-1533 (2011).<br/>3. Yan, Y. et al. <i>Nat. Energy</i> <b>2</b>, 17052 (2017).<br/>4. Cirloganu, C. M. et al. <i>Nat. Commun</i>. <b>5</b>, 4148 (2014).<br/>5. Singh, R., Liu, W., Lim, J., Robel, I. & Klimov, V. I. <i>Nat. Nanotech</i>. <b>14</b>, 1035-1041, (2019).<br/>6. Livache, C, <i>et al</i>. <i>Nat. Photon.</i> <b>16</b>, 433-440 (2022).