Letter pubs.acs.org/JPCL
Electrical Detection of Quantum Dot Hot Electrons Generated via a Mn2+-Enhanced Auger Process Charles J. Barrows, Jeffrey D. Rinehart,† Hirokazu Nagaoka,‡ Dane W. deQuilettes, Michael Salvador,§ Jennifer I. L. Chen,∥ David S. Ginger, and Daniel R. Gamelin* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States S Supporting Information *
ABSTRACT: An all-solid-state quantum-dot-based photon-to-current conversion device is demonstrated that selectively detects the generation of hot electrons. Photoexcitation of Mn2+-doped CdS quantum dots embedded in the device is followed by efficient picosecond energy transfer to Mn2+ with a long-lived (millisecond) excited-state lifetime. Electrons injected into the QDs under applied bias then capture this energy via Auger deexcitation, generating hot electrons that possess sufficient energy to escape over a ZnS blocking layer, thereby producing current. This electrically detected hot-electron generation is correlated with a quench in the steady-state Mn2+ luminescence and the introduction of a new nonradiative excited-state decay process, consistent with electrondopant Auger cross-relaxation. The device’s efficiency at detecting hot-electron generation provides a model platform for the study of hot-electron ionization relevant to the development of novel photodetectors and alternative energy-conversion devices.
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photocatalysis17 and to detect transient QD hot-electron surface trapping18 and hot-carrier transfer3,19 spectroscopically. Auger ionization of QDs is also well-known.20,21 In metals, extraction of hot electrons generated by plasmonic excitation has been demonstrated,22 and various plasmonic hot-electron devices have been described.23−26 During the course of this investigation, hot-electron detection was reported from similar Mn2+-doped CdS/ZnS core/shell QDs deposited on transparent conducting electrodes and placed in a photoelectrochemical cell.27 Harvesting hot carriers generated by photoexcited QDs in solid-state devices remains largely unexplored. Figure 1A summarizes the electron-Mn2+ Auger process investigated here: QD photoexcitation creates an exciton that rapidly (ps) localizes at a Mn2+,28 generating a long-lived (∼ms) Mn2+ 4T1 d−d excited state. This excited state can decay radiatively (Figure 1A, bottom left),29 but in the presence of an excess conduction-band electron can also decay via an electronMn2+ Auger-type cross-relaxation4 to yield a hot electron and the ground-state Mn2+ (Figure 1A, bottom right). Although this Auger process is intrinsically fast (ps), it is manifested experimentally as a new voltage-dependent μs Mn2+ PL nonradiative decay component because of slow electron diffusion through the QD solid.4 Electrons are able to sample a large volume of the Mn2+-doped CdS QD film on the time scale of the Mn2+ PL, making electron-Mn2+ Auger crossrelaxation extremely efficient.4 In spectroelectrochemical measurements, Mn2+ cw PL quenching and accelerated PL
evices that convert light into current have broad applications from sensors to solar energy conversion, and can also serve as powerful tools for investigating fundamental material properties. Semiconductor devices harnessing nonthermalized “hot” charge carriers are presently targeted because they could eventually enable solar efficiencies exceeding the Shockley-Queisser limit.1−3 Here, we explore a fundamentally new photon-to-current conversion device based on hot electrons. Photoexcitation of Mn2+-doped CdS quantum dots (QDs) deposits energy in long-lived Mn2+ d−d excited states. Electrons injected into the QD layer harvest this energy via highly efficient electron-Mn2+ Auger cross-relaxation.4 The resulting hot electrons possess sufficient energy to escape over a ZnS blocking layer, producing current. Photocurrents with Mn2+-doped QDs are over an order of magnitude greater than in undoped-QD control devices and show clear rectification. These devices offer a unique electrical probe of QD Auger and hot-electron processes. Auger processes are deleterious in many contexts, for example causing QD darkening,5−8 LED efficiency droop,9 and multiexciton recombination.10,11 Using spectroelectrochemistry, we recently demonstrated efficient photoluminescence (PL) quenching by conduction-band electrons in Mn2+doped CdS QDs via an unusual electron-Mn2+ Auger crossrelaxation process.4 Here, we seek to harness this efficient Auger process to generate hot electrons within a solid-state device. Hot-carrier extraction from semiconductors is challenging, mainly due to rapid thermalization via coupling to lattice vibrations or other charge carriers.1,12−14 Carrier cooling within semiconductors typically occurs in femtoseconds1,15 but may be slowed to picoseconds in QDs.2,16 Recent studies have shown that it is possible to use hot electrons generated in QDs for © XXXX American Chemical Society
Received: September 28, 2016 Accepted: December 5, 2016 Published: December 5, 2016 126
DOI: 10.1021/acs.jpclett.6b02219 J. Phys. Chem. Lett. 2017, 8, 126−130
Letter
The Journal of Physical Chemistry Letters
Figure 2. (A) PL spectra of a multilayer device incorporating d = 5.0 nm Cd0.9Mn0.1S QDs, measured under increasing forward bias (0 to +15 V) and 0.12 μW/cm2 illumination at 405 nm. The Cd0.9Mn0.1S QD absorption onset occurs at ∼443 nm (see Supporting Information). (B) Responsivity (left axis) and PL quenching (right axis) for the same device, plotted versus bias from −15 to +15 V. The responsivity reaches a maximum of 56.9 A/W at +15 V. (C) PL spectra of a control device using undoped CdS QDs (d = 5.4 nm), measured under forward bias (0 to +15 V) using 9.56 μW/cm2 illumination at 405 nm. The CdS QD absorption onset occurs at ∼453 nm (see Supporting Information). (D) Responsivity (left axis) and PL quenching (right axis) from −15 to +15 V in the control device. The responsivity reaches a maximum of only 2.8 A/W at +15 V. The dark currents of the control and Mn2+-doped devices are similar (see Supporting Information), indicating similar device quality. The PL intensities are linearly dependent on excitation power under these experimental conditions.
Figure 1. (A) Energy pathways in photoexcited Mn2+-doped CdS QDs. Photoexcitation creates an exciton within the CdS QD that rapidly localizes at a Mn2+ ion (ps, dashed red line) to generate the Mn2+ 4T1 excited state. In the absence of a charge carrier, this 4T1 excited state decays back to the Mn2+ 6A1 ground state (∼ms). In the presence of a conduction-band electron (black circle), electron-Mn2+ Auger cross-relaxation occurs (ps), generating a hot electron possessing ∼2.1 eV excess energy (the energy of the 4T1 state) and Mn2+ in its 6A1 ground state. (B) Representative cross-sectional SEM image of a prototype multilayer device incorporating Cd1−xMnxS QDs. (C) Schematic illustration of the above device, integrating an Al contact, a QD active layer, a ZnS electron-blocking layer, and an ITO collection electrode. Upon electron injection through the Al electrode, hot electrons are generated in photoexcited Cd1−xMnxS QDs via electron-Mn2+ Auger cross-relaxation, extracted over the ZnS blocking layer, and collected at the ITO contact. The CdS/ZnS conductionband offset is ∼1.0 eV, and the excess energy transferred to the CdS conduction-band electron from Mn2+ is 2.1 eV, placing the initial Auger-excited electron well above the ZnS conduction-band edge.
PL intensity has decreased by 75%. The Mn2+ PL is only slightly sensitive to negative bias, however, decreasing by
