Highly Efficient Spin-Exchange Carrier Multiplication in Mn-Doped Colloidal Quantum Dots

During impact ionization or inverse Auger recombination, a single hot carrier relaxes to a lower-lying state within the same band, which is accompanied by the excitation of a valence band electron across the energy gap (<i>E</i>_g). As this leads to generation of an additional electron-hole (e-h) pair, this process is often referred to as carrier multiplication (CM).¹ In principle, CM could improve the performance of a variety of optoelectronic, photovoltaic and photocatalytic devices.^2,3 However, practically realized CM efficiencies are still not sufficiently high to achieve an appreciable boost in device performance.<br/>Recently, it was demonstrated that the rate of Auger-type energy transfer could be dramatically enhanced by employing spin-exchange interactions enacted by introducing magnetic impurities (Mn) into colloidal quantum dots (QDs).⁴ Extremely fast rates of spin-exchange processes allow for ‘uphill’ Auger-type energy transfer with an energy-gain rate that greatly exceeds the intraband cooling rate.^4,5 In ref. 4, this effect was exploited to realize highly efficient photoemission due to ejection of a hot electron. A highly favorable energy gain/loss-rate ratio could also enable new schemes for capturing kinetic energy of hot, unrelaxed carriers to instigate a highly efficient CM process.<br/>In the present study, we exploit strong Mn-doping-induced enhancement in the energy gain/loss ratio for achieving low-threshold, high-yield CM due to inverse spin-exchange Auger recombination. For this purpose, we develop 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 two types of spin-exchange processes involving the PbSe core. In one, the difference between the energy of the excited Mn ion (<i>hv</i>_Mn) and the band-edge PbSe core exciton (<i>hv</i>_PbSe) is released in the form of a near-infrared photon whose energy (<i>hv</i>_SE) closely correlates with <i>hv</i>_Mn – <i>hv</i>_PbSe when<i> hv</i>_PbSe is tuned by changing QD dimensions. In the second process, the energy surplus given by <i>hv</i>_Mn – <i>hv</i>_PbSe relaxes via a CM-like spin-exchange process which leads to generation of two core excitons. The corresponding quantum efficiency measured at 2.6<i>E</i>_g is ~140%, implying that the e-h pair creation energy is less than 1.5<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 practical 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. Singh, R., Liu, W., Lim, J., Robel, I. & Klimov, V. I. <i>Nat. Nanotech</i>. <b>14</b>, 1035-1041, (2019).<br/>5. Livache, C., Kim, W.D., Jin, H., Kozlov, O.V., Fedin, I. & Klimov, V. I. <i>et al</i>. <i>submitted</i> (2021).