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Article Understanding Temporal Evolution of Electroluminescence Intensity in Lead Sulﬁde (PbS) Colloidal Quantum Dot Infrared Light-Emitting Diodes

Minkyoung Kim and Byoungnam Park * Department of Materials Science and Engineering Hongik University 72-1, Sangsu-dong, Mapo-gu, Seoul 04066, Korea; [email protected] * Correspondence: [email protected]

Received: 2 September 2020; Accepted: 20 October 2020; Published: 23 October 2020

Abstract: We, for the first time, report a temporal evolution of the electroluminescence (EL) intensity in lead sulfide (PbS) colloidal quantum dot (CQD) infrared light-emitting diodes. The EL intensity was varied during infrared light emission, and its origin is attributed to competition between the achievement of charge balance associated with interfacial charging at the PbS/ZnO CQD interface and the electric-field induced luminescence quenching. The effect of multi-carrier emission on the enhanced EL intensity is discussed relating to shifting in the wavelength at the peak EL intensity.

Keywords: PbS; infrared CQD LED; electroluminescence; luminescence quenching; interfacial charging

1. Introduction Colloidal quantum dots (CQDs) have been assembled into a variety of devices, including light-emitting diodes (LEDs), solar cells, and biomedical sensors [1–5]. Excellent spectral tunability induced by quantum confinement effect associated with adjustable optical band gaps, simple solution process, and highly efficient photoluminescence (PL) quantum yields have motivated extensive research towards quantum dot light-emitting diodes (QLEDs), emerging as a next-generation display technology which can compete with organic light emitting diodes [5–9]. Understanding of the underlying mechanism for efficient emission has been one of the leading research themes, revealing that suppression of electric-field induced electrostatic interactions, including quantum dot (QD) charging and electric-field assisted exciton dissociation is crucial in reducing the external quantum efficiency (EQE) roll-off due to non-radiative loss, enabling integration of highly efficient QLEDs [10–12]. The main issue in structuring highly efficient QLEDs is to confine electrons and holes to the QD emission layer, suppressing the non-radiative recombination process [13–15]. Charge injection in the QD emission layer can lead to an unbalanced charge state that causes the non-radiative Auger process. QDs with a high surface to volume ratio are favorable for the formation of trap sites which facilitates the non-radiative process, quenching luminescence. To reduce the surface states, core QDs were shelled by inorganic or organic layers, and a variety of surface ligand chemistries were applied for surface passivation [16–18]. To improve the EQE, researchers have carried out structural engineering of hetero-structured QDs with adjustment of the shell thickness and the composition of the core/shell structure [19,20]. To prevent the non-radiative Auger recombination process due to QD charging, an interfacial alloy layer was introduced between the core and the shell QDs, mitigating the steepness of the confinement potential [19]. Bae et al. demonstrated an enhanced EQE by the insertion of an intermediate layer

Appl. Sci. 2020, 10, 7440; doi:10.3390/app10217440 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7440 2 of 11

(CdSexS1 x) between CdSe QD layers, suppressing the Auger decay process [12]. The conduction band − edge was engineered for a higher energy level to reduce electron injection, improving charge balance within the QDs. Various strategies including QD core/shell alloying, the formation of giant QD, and the adjustment of QD stoichiometries have successfully led to significant improvement in the EQE [21–23]. Electric field-induced luminescence quenching arising from QD charging and spatial separation of electrons and holes has also been considered one of the primary sources of the low EQE in QD LEDs [10,11]. Therefore, understanding the mechanism of electric field-induced luminescence quenching is crucial in enhancing the EQE. Particularly, distinguishing between field-induced charging and field-induced luminescence quenching has remained elusive because the two effects occur simultaneously during the emission process, complicating the separation of the isolated effect on the EQE loss. Bozyigit et al. reported the fabrication of a test platform device in which multi-layers of QDs are sandwiched by SiO2 dielectric films [10]. This test structure enabled a fundamental study on the effect of the electric field on the EQE, excluding the effect of QD charging due to charge injection. They revealed that electric field-induced spatial separation of electrons and holes still occurs with the presence of the relaxed confinement potentials in the core/shell structures designed to extend either electrons or holes into the shell. Importantly, most of the charge carrier was localized over long time scales, revealing that the primary source of luminescence quenching is electric field-induced coupling to the surface states of QDs. Retaining the overlap of electron and hole wave functions at a reduced electric field is, therefore, a prerequisite in enhancing the EQE over structural engineering of the hetero-structured QDs. As mentioned above, many efforts have been made to understand the electric field-induced luminescence quenching as well as QD charging. Surface states coupled luminescence quenching is expected to be more severe at the interface with adjacent functional layers such as carrier transport and blocking layers. Therefore, interfacial charging related to the surface states and/or the energy level as well as the structural engineering within the QD emission layer can be a key parameter in determining the EQE of the QD LEDs. Compared with the huge success of QLEDs in the visible range, as discussed above, progress in the near-infrared (NIR) CQD LEDs is not so fast. NIR CQDs such as PbS and PbSe have attracted much attention due to their applications to biomedical imaging, telecommunications, and night vision [24,25]. Contrary to CQD LEDs in the visible range, luminescence efficiency in the NIR CQD LEDs lags in the presence of self-quenching in which efficient charge injection causes transport-assisted trapping of mobile carriers within QDs [25,26]. Various strategies to suppress self-quenching in CQDs, including organic ligand capping, insertion into the polymer or hybrid perovskite matrix, and shelling of the surface have led to enhanced EQE of up to ~7.9% [26,27]. Most of all, unlike CQD LEDs in the visible range, extensive studies of interfacial charging and electric field-induced luminescence quenching associated with PbS CQDs have not been conducted. Here, we report, for the first time, the temporal evolution of the electroluminescence (EL) intensity during light emission in a PbS CQD LED testbed device. We correlated the temporal evolution of the EL intensity to electric-field-induced interfacial charging between the emission and transport layers, and luminescence quenching. To articulate the temporal evolution, we investigated the spectral shift and the magnitude of the current simultaneously. Surprisingly, to the best of our knowledge, the effect of temporal charging over long time scales during light emission at the emission/transport layer interface on the EL intensity has not been reported in NIR CQD LEDs, despite the fact that the time scale associated with electric-field induced charging can extend up to tens of seconds depending on the charge injection into QDs and carrier trapping process on the QD surface.

2. Materials and Methods PbS CQDs were synthesized by the hot injection method [28]; 1.45 mL of oleic acid (Sigma Aldrich, 90%) and 18.5 mL of 1-octadecene (ODE, Sigma Aldrich, 90%) were added to 0.45 g of lead (II) oxide (Sigma Aldrich, 99.999%) in a three-neck flask until the solute (PbO) is completely dissolved and Appl. Sci. 2020, 10, 7440 3 of 11 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 11 becomesand becomes transparent. transparent. Temperature Temperature is increased is increased to the target to the reaction target temperaturereaction temperature followed byfollowed injection by µ ofinjection a hexamethyldisilathiane of a hexamethyldisilathiane (TMS, Sigma (TMS, Aldrich)-ODE Sigma Aldrich)-ODE solution containing solution 110 containingL of TMS 110 into μL 15of mLTMS ofinto ODE. 15 ThemL three-neckof ODE. The flask three-neck is immersed flask intois immers ice watered into immediately ice water immediately after the reaction afteris the completed, reaction is followedcompleted, by afollowed centrifugation by a centrifugation process. For process. CQD washing, For CQD we wash centrifugeding, we centrifuged the synthesized the synthesized solution, separatingsolution, unreactedseparating material.unreacted The material. supernatant The wassupernatant transferred was to transfe new tubesrred andto new centrifuged tubes and at 9000centrifuged rpm for 10at min.9000 Therpm precipitatefor 10 min. is The re-dispersed precipitate in is hexane. re-dispersed We repeated in hexane. the washing We repeated process the severalwashing times process and theseveral sediments times wereand the re-dispersed sediments in were chlorobenzene re-dispersed (10 in mL) chlorobenzene to obtain a PbS(10 mL) CQD to (~40obtain mg /amL). PbS CQD (~40 mg/mL). ToTo fabricate fabricate PbS PbS CQD CQD LEDs LEDs as as structured structured in in Figure Figure1a, 1a, PEDOT:PSS PEDOT:PSS (70 (70 nm, nm, M124 M124 HTL HTL Solar Solar fromfrom Ossila) Ossila) was was spin-coated spin-coated onto onto the the patterned patterned indium indium tin tin oxide oxide (ITO, (ITO, 70 70Ω Ω□) substrate) substrate at at 6000 6000 rpm rpm forfor 60 60 s. s. PbS PbS CQD CQD solution solution in in chlorobenzene chlorobenzene (40 (40 mg mg/mL)/mL) was was spin-coated spin-coated onto onto the the PEDOT:PSS PEDOT:PSS film film atat 1000 1000 rpm rpm for for 30 30 s. s. As As an an electron electron transport transport layer, layer, ZnO ZnO CQDs CQDs were were synthesized. synthesized. Zinc Zinc acetate acetate dehydratedehydrate (5 (5 mmol) mmol) in in Dimethyl Dimethyl sulfoxide sulfoxide (DMSO) (DMSO) was was stirred stirred for for 10 10 min min at at room room temperature. temperature. Simultaneously,Simultaneously, 5 5 mmol mmol of of a tetramethylammoniuma tetramethylammonium hydroxide hydroxide pentahydrate pentahydrate (TMAH) (TMAH) (0.968 (0.968 g) g) in in ethanolethanol was was prepared prepared followed followed by by being being added added to to the the zinc zinc acetate acetate solution. solution. After After the the injection injection process, process, thethe mixed mixed solution solution was was cooled cooled in in air. air. For For washing washing process, process, we we added added acetone acetone (35 (35 mL) mL) into into the the solution solution (10(10 mL), mL), followed followed by by centrifugation centrifugation at at 9000 9000 rpm rpm for for 10 10 min. min. After After centrifugation, centrifugation, the the solvent solvent was was removedremoved and and ZnO ZnO CQDs CQDs were were dried dried for for 1 min.1 min. We We added added ethanol ethanol (10 (10 mL) mL) into into the the conical conical tube tube containingcontaining ZnO ZnO CQDs CQDs for for another another centrifugation. centrifugation. After After washing, washing, ZnO ZnO solution solution was was spin-coated spin-coated at at 10001000 rpm rpm for for 60 60 s, producings, producing a thickness a thickness of ~60 of ~60 nm followednm followed by thermal by thermal evaporation evaporation of aluminum of aluminum (Al) electrodes(Al) electrodes [5]. Using [5]. photoelectron Using photoelectron spectroscopy spectr in airoscopy (PESA) in andair optical(PESA) absorption and optical spectroscopy, absorption thespectroscopy, lowest unoccupied the lowest molecular unoccupied orbital molecular (LUMO) and orbital the highest(LUMO) occupied and the molecularhighest occupied orbital (HOMO)molecular energyorbital levels (HOMO) were energy calculated. levels were calculated.

Figure 1. (a) Schematics of PbS infrared light-emitting diodes (LED) structure and the energy band diagram.Figure 1. (b )(a transmission) Schematics electron of PbS infrared microscopy light-emitting (TEM) image diodes of the (LED) lead sulfide structure (PbS) and colloidal the energy quantum band dotdiagram. (CQDs) ( synthesized.b) transmission (c) Taucelectron plot microscopy of a PbS film. (TEM) An energy image bandof the gap lead of 0.9sulfide eV was (PbS) calculated. colloidal (dquantum) A Plot of dot emission (CQDs) yield synthesized. as a function (c) Tauc photon plot energyof a PbS for film. a PbS An CQD energy film. band A highestgap of 0.9 occupied eV was molecularcalculated. orbital (d) A (HOMO) Plot of emission energy level yield of as 5.1 aeV function was calculated. photon energy for a PbS CQD film. A highest occupied molecular orbital (HOMO) energy level of 5.1 eV was calculated. The EL intensity of a PbS NIR QLED device was monitored at a driving voltage applied to the ITOThe side. EL intensity With a photo-spectrometerof a PbS NIR QLED device (Hamamatsu was monitored TG-cooled at a series) driving and voltage a semiconductor applied to the parameterITO side. analyzerWith a photo-spectrometer (Keithley 2400), the (Hamamatsu EL intensity TG-cooled and the series) electrical and currenta semiconductor were plotted parameter as a functionanalyzer of (Keithley time, respectively, 2400), the duringEL intensity light and emission the electr at aical particular current voltage were plotted over the as a turn-on function voltage. of time, respectively, during light emission at a particular voltage over the turn-on voltage. Interfacial Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 11 Appl. Sci. 2020, 10, 7440 4 of 11 charging/discharging experiments were carried out by applying positive and negative voltages to the ITO side. Interfacial charging/discharging experiments were carried out by applying positive and negative voltages3. Results to theand ITO Discussion side.

3. ResultsThe size and of Discussion the synthesized PbS CQDs was estimated to be ~3 nm from the transmission electron microscopy (TEM) image, as seen in Figure 1b. An energy band gap of 0.9 eV and a highest occupied The size of the synthesized PbS CQDs was estimated to be ~3 nm from the transmission electron molecular orbital (HOMO) energy level of 5.1 eV were determined from the optical absorption and microscopy (TEM) image, as seen in Figure1b. An energy band gap of 0.9 eV and a highest occupied PESA characterizations, respectively, as seen in Figure 1c,d, which is consistent with the size molecular orbital (HOMO) energy level of 5.1 eV were determined from the optical absorption and determined from the TEM image. During PESA measurements to measure the HOMO level of the PESA characterizations, respectively, as seen in Figure1c,d, which is consistent with the size determined PbS CQDs, the photocurrent was recorded in the photon energy ranging between 4.2 and 6.2 eV. We from the TEM image. During PESA measurements to measure the HOMO level of the PbS CQDs, plotted the emission yield as a function of the incident photon energy to estimate the HOMO level, the photocurrent was recorded in the photon energy ranging between 4.2 and 6.2 eV. We plotted the which is determined from the incident photon energy at which the effective emission is negligible. emission yield as a function of the incident photon energy to estimate the HOMO level, which is To prevent charging, we deposited PbS CQDs films on ITO-coated glass substrates. determined from the incident photon energy at which the effective emission is negligible. To prevent The temporal evolution of the EL intensity and current during light emission was charging, we deposited PbS CQDs films on ITO-coated glass substrates. simultaneously investigated through sampling measurements in which a constant voltage (1.5 V) The temporal evolution of the EL intensity and current during light emission was simultaneously over the turn-on voltage is applied. Figure 2a shows plots of the EL intensity as a function of the investigated through sampling measurements in which a constant voltage (1.5 V) over the turn-on emission wavelength. Importantly, the EL intensity significantly increased during electrical biasing voltage is applied. Figure2a shows plots of the EL intensity as a function of the emission wavelength. at the entire emission wavelength between 1100 and 1600 nm, while the current decreased, as seen in Importantly, the EL intensity significantly increased during electrical biasing at the entire emission Figure 2b. We also observed that the abrupt increase in the EL intensity immediately after application wavelength between 1100 and 1600 nm, while the current decreased, as seen in Figure2b. We also of the electric field occurred with a significant decrease in the current in Figure 2b, which is addressed observed that the abrupt increase in the EL intensity immediately after application of the electric field later. occurred with a significant decrease in the current in Figure2b, which is addressed later.

Figure 2. (a) Plots of electroluminescence (EL) spectra as a function of time during light emission under applicationFigure 2. of(a) 1.5 Plots V to of the electroluminescence indium tin oxide (ITO) (EL) side. spectra (b) Plots as aof function current andof time EL intensityduring light as a functionemission ofunder time. application of 1.5 V to the indium tin oxide (ITO) side. (b) Plots of current and EL intensity as a function of time. Simultaneous change in the magnitude of the current and the EL intensity provides a hint of the originSimultaneous of the rise change in the in EL the intensity magnitude during of the emission. current and We the hypothesize EL intensity that provides charge a balance hint of inthe theorigin emission of the layer rise canin the be attainedEL intensity by electrical during bias-inducedemission. We modificationhypothesize ofthat the charge interfacial balance electrical in the contacts,emission altering layer can the be magnitude attained ofby the electrical injection bias-induced current across modification the interface of betweenthe interfacial PbS and electrical ZnO CQDcontacts, layers. altering It is well-known the magnitude that non-radiative of the injection Auger current recombination across the caninterface be mitigated between by PbS suppressing and ZnO QDCQD charging, layers. achieving It is well-known charge balance that innon-radiative the emission layerAuger [10 recombination,19]. can be mitigated by suppressingTo address QD the charging, origin of achieving the current charge variation balance during in lightthe emission emission, layer we carried [10,19]. out experiments in whichTo the address ITO/PEDOT:PSS the origin/ PbSof the/ZnO current/Al device variation is charged during andlight discharged emission, we by carried applying out positive experiments and negativein which voltages the ITO/PEDOT:PSS/PbS/ZnO/Al to the ITO side, respectively, device for tens is of charged seconds, and followed discharged by measurement by applying of positive the EL intensityand negative at a driving voltages voltage to the ofITO 10 side, V. Prior respectively, to the charging for tens/discharging of seconds, experiments,followed by measurement we confirmed of that,the atEL a intensity high voltage at a over driving 10 V, voltage the EL intensityof 10 V. Prior increased to the with charging/discharging voltage and time (data experiments, not shown), we indicatingconfirmed that that, electrical at a high biasing voltage at over such 10 a V, high the voltage EL intensity for the increased charging with/discharging voltage and experiments time (data hasnot not shown), damaged indicating the device. that electrical In Figure 3biasinga, the PbSat such QLED a high device voltage was charged for the bycharging/discharging applying a high experiments has not damaged the device. In Figure 3a, the PbS QLED device was charged by Appl. Sci. 2020, 10, 7440 5 of 11 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 11 positiveapplying (10 a V)high or positive negative (10 voltage V) or (negative10 V) for voltage 2 min. (− Notably,10 V) for the2 min. measured Notably, EL the intensity measured under EL − applicationintensity under of 1.5 application V, after applying of 1.5 a V, negative after applying voltage, a was negative far lower voltage, than thatwas after far lower applying than a that positive after applying a positive voltage, indicating that the effect of electrical pre-biasing on emission is voltage, indicating that the effect of electrical pre-biasing on emission is significant. It is noted that significant. It is noted that reproducible results were obtained after applying smaller voltages (5 V reproducible results were obtained after applying smaller voltages (5 V and 5 V) for charging and and −5 V) for charging and discharging, respectively. − discharging, respectively.

Figure 3. Voltage polarity (+10 V and 10 V) dependent EL intensity change. EL intensity in (a) was Figure 3. Voltage polarity (+10 V and− −10 V) dependent EL intensity change. EL intensity in (a) was measured after application of +10 V and 10 V.The diode current in (b) was measured right immediately measured after application of +10 V− and −10 V. The diode current in (b) was measured right after application of +10 V and 10 V as a function of time under application of 1.5 V. immediately after application− of +10 V and −10 V as a function of time under application of 1.5 V. A significant difference in the current over an order of magnitude was observed depending on the A significant difference in the current over an order of magnitude was observed depending on polarity of the electrical pre-biasing (10 V and 10 V for 2 min), as seen in Figure3b. This suggests that the polarity of the electrical pre-biasing (10 V− and −10 V for 2 min), as seen in Figure 3b. This suggests the charge injection energy level between the transport (PEDOT:PSS or ZnO) and the emission (PbS) that the charge injection energy level between the transport (PEDOT:PSS or ZnO) and the emission layer was altered, depending on the polarity of the electrical pre-biasing. Indeed, a subtle change at the (PbS) layer was altered, depending on the polarity of the electrical pre-biasing. Indeed, a subtle interfacial energy level by electrical bias stress causes an exponential difference in the magnitude of the change at the interfacial energy level by electrical bias stress causes an exponential difference in the current, assuming the electric-field enhanced thermionic emission model [29,30]. An interfacial energy magnitude of the current, assuming the electric-field enhanced thermionic emission model [29,30]. level-dependent diode current was confirmed in the I-V curve of the ITO/PEDOT:PSS/PbS/ZnO/Al CQD An interfacial energy level-dependent diode current was confirmed in the I-V curve of the LED below the turn-on voltage in Figure4 in which the forward and reverse bias current was clearly ITO/PEDOT:PSS/PbS/ZnO/Al CQD LED below the turn-on voltage in Figure 4 in which the forward observed. The formation of the forward and reverse bias regions suggests that the test structure we used and reverse bias current was clearly observed. The formation of the forward and reverse bias regions consists of energetically well-defined interfaces for asymmetric carrier transfer. In the test structure suggests that the test structure we used consists of energetically well-defined interfaces for with distinct electron and hole energy barriers, under the application of a positive or negative voltage asymmetric carrier transfer. In the test structure with distinct electron and hole energy barriers, under to the ITO side, for example, carriers are selectively accumulated and depleted at the PEDOT:PSS/PbS the application of a positive or negative voltage to the ITO side, for example, carriers are selectively and/or PbS/ZnO interfaces, modifying the interfacial energy level. accumulated and depleted at the PEDOT:PSS/PbS and/or PbS/ZnO interfaces, modifying the During the charging process in Figure3b at a high voltage (10 V), electrons are injected through Al interfacial energy level. and passivate the surface trap sites on the ZnO and PbS CQDs. The surface of ZnO CQDs are depleted of electrons due to adsorption of oxygen molecules as well as a high density of surface traps [31–34]. A high injection of electrons fills and passivates the surface traps on the ZnO CQDs close to the PbS/ZnO interface. According to Weaver et al., indeed, reductive passivation of surface traps through electron injection over the trap energy level of ZnSe CQDs led to large photoluminescence electro-brightening in the ZnSe CQDs, implicating interfacial carrier charging in determining luminescence intensity [35]. On the other hand, PbS CQDs are very well-known to form dynamic traps through which electrons are depleted, as will be discussed later. In contrast to the charging process, the discharging process under application of 10 V depletes the charged electrons at the PbS/ZnO interface. Therefore, the subsequent − bias at 1.5 V, after the charging process, causes a far higher current in comparison with that after the discharging process, as shown in Figure3b. It is important to note that the diode current increases with time at a higher voltage (10 V) while the current decreases at a lower voltage (5 V, Data not shown). At a high electric field, injected electrons fill the trap states fast and begin to passivate the interface Appl. Sci. 2020, 10, 7440 6 of 11 while, at a low electric field, injected carriers are still in the process of being trapped. On the basis of the framework, the current decrease during the increase in the EL intensity at a low voltage (1.5 V) implicatesAppl. Sci. 2020 electron, 10, x FOR trapping PEER REVIEW at the PbS /ZnO interface, achieving charge balance. 6 of 11

Figure 4. PbS infrared LED I-V characteristic curve. Figure 4. PbS infrared LED I-V characteristic curve. In our previous studies, we investigated interfacial charging of PbS CQDs, finding that the time scale of carrierDuring trapping the charging associated process with in surfaceFigure 3b traps at a corresponds high voltage to (10 tens V), of electrons seconds which are injected is consistent through withAl that and of passivate the current the and surface EL intensity trap sites change on the in ZnO Figure and2b [PbS31, 32CQDs.]. The The long surface time scale of ZnO over CQDs tens of are secondsdepleted and of the electrons significant due current to adsorption change over of oxygen an order mo oflecules magnitude as well in as Figure a high3a density in combination of surface with traps the[31–34]. charging A/discharging high injection experiments of electrons in Figurefills and3b pass allowivates us to the infer surface that the traps increase on the in ZnO the EL CQDs intensity close to duringthe PbS/ZnO light emission interface. is ascribed According to the to interfacial Weaver et trapping al., indeed, of carriers reductive at the passivation PbS/ZnO interface. of surface traps throughIndeed, electron suppression injection of the over injection the trap current energy due le tovel interfacial of ZnSe charge CQDs trapping led to large should photoluminescence be considered in articulatingelectro-brightening the origin in of the the ELZnSe intensity CQDs, increase implicating during interfacial emission. carrier The net charging injection currentin determining in the disorderedluminescence material intensity systems [35]. embedding On the other CQDs hand, is determined PbS CQDs by are the very diff erencewell-known between to theform injected dynamic currenttrapsacross through interface which andelectrons the surface are dep recombinationleted, as willcurrent be discussed [36–39 ].later. Many In studiescontrast have to the reported charging thatprocess, the electrical the discharging contact resistance process at under the interface application involving of −10 CQDs V depletes is governed the charged by the chargeelectrons carrier at the mobilityPbS/ZnO as wellinterface. as the Therefore, interfacial the energy subsequent level [40 bias,41]. at PbS 1.5 CQDsV, after possess the charging a high densityprocess, of causes surface a far chargehigher trapping current states in comparison originating with from that unpassivated after the di cationicscharging lead process, sitesand as shown oxide speciesin Figure such 3b. as It is important to note that the diode current increases with time at a higher voltage (10 V) while the PbSO3 and PbSO4 (electron traps), and anionic sulfur sites (hole traps) [42–46]. currentIt is important decreases to at emphasize a lower voltage that Auger (5 V, recombinationData not shown). as wellAt a ashigh CQD electric charging field, is injected mitigated electrons due to thefill the reduction trap states in the fast net and injection begin to current. passivate According the interface to Baewhile, et al.,at a the low onset electric of thefield, EQE injected roll-o carriersff in theare CQDs still within the reduced process Auger of being recombination trapped. On ratethe basis starts of at the higher framework, currents the with current increased decrease emission during effitheciency increase [12,19 in]. Theythe EL demonstrated intensity at a thatlow thevoltage EQE (1.5 roll-o V)ff implicatesin the CdSe electron/CdS structure trapping is at not the a ffPbS/ZnOected byinterface, electric field-induced achieving charge luminescence balance. quenching due to spatial separation between electrons and hole. InIn other our words,previous the studies, optimum we electricinvestigated field atinterfacial which the charging degree of PbS charge CQDs, balance finding is maximized that the time exists,scale and of acarrier higher trapping value of associated EQE is obtained, with surface as the degreetraps corresponds of CQD charging to tens is of reduced. seconds Indeed, which is chargeconsistent balance with state that was of improved the current by increasingand EL intensity the interfacial change energy in Figure barrier 2b [31,32]. through The the introductionlong time scale over tens of seconds and the significant current change over an order of magnitude in Figure 3a in of a thin outer shell (Zn0.5Cd0.5S) between the CdS (shell) and ZnO. Suppression of electron injection intocombination the emission with layer the by thecharging/discharging outer shell exhibited experiments far improved in roll-oFigureff behavior.3b allow us to infer that the increaseTo elucidate in the theEL originintensity of theduring EL intensitylight emission increase, is ascribed we traced to the interfacial emission spectra trapping in of Figure carriers5a. at Thethe change PbS/ZnO in the interface. EL intensity peak wavelength is plotted as a function of time under the application of a voltageIndeed, in Figure suppression5b. The wavelengthof the injection at the current emission due peak to was,interfacial importantly, charge blue-shiftedtrapping should from be 1450considered to 1350 nm in inarticulating 2 min, ruling the outorigin the of possibility the EL inte of electric-field-inducednsity increase during quantum-confinedemission. The net injection Stark effectcurrent [47]. in The the observeddisordered blue-shift material couldsystems be embedding a signature CQDs of multi-carrier is determined emission, by the difference contributing between to anthe increase injected inthe current EL intensity across interface [12]. In aand previous the surf study,ace recombination for example, the current core/alloy [36–39]. layer Many/shell studies QD have reported that the electrical contact resistance at the interface involving CQDs is governed by the charge carrier mobility as well as the interfacial energy level [40,41]. PbS CQDs possess a high density of surface charge trapping states originating from unpassivated cationic lead sites and oxide species such as PbSO3 and PbSO4 (electron traps), and anionic sulfur sites (hole traps) [42–46]. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 11

It is important to emphasize that Auger recombination as well as CQD charging is mitigated due to the reduction in the net injection current. According to Bae et al., the onset of the EQE roll-off in the CQDs with reduced Auger recombination rate starts at higher currents with increased emission efficiency [12,19]. They demonstrated that the EQE roll-off in the CdSe/CdS structure is not affected by electric field-induced luminescence quenching due to spatial separation between electrons and hole. In other words, the optimum electric field at which the degree of charge balance is maximized exists, and a higher value of EQE is obtained, as the degree of CQD charging is reduced. Indeed, charge balance state was improved by increasing the interfacial energy barrier through the introduction of a thin outer shell (Zn0.5Cd0.5S) between the CdS (shell) and ZnO. Suppression of electron injection into the emission layer by the outer shell exhibited far improved roll-off behavior. To elucidate the origin of the EL intensity increase, we traced the emission spectra in Figure 5a. The change in the EL intensity peak wavelength is plotted as a function of time under the application of a voltage in Figure 5b. The wavelength at the emission peak was, importantly, blue-shifted from 1450 to 1350 nm in 2 min, ruling out the possibility of electric-field-induced quantum-confined Stark Appl.effect Sci. 2020[47]., 10The, 7440 observed blue-shift could be a signature of multi-carrier emission, contributing7 ofto 11 an increase in the EL intensity [12]. In a previous study, for example, the core/alloy layer/shell QD device showed a blue-shift of the EL spectra, which is translated into the contribution arising from multi- devicecarrier showed emission a blue-shift increased of [12,19 the EL]. spectra,In the CdSe/CdS which is translated QDs, a fraction into the contributionof electrons arisingand holes from are multi-carrierseparated in emission the core/shell increased structure. [12,19 ].In Incase the that CdSe multi-carrier/CdS QDs, aemission fraction take of electronss place in and the holes structure, are separatedspatial distribution in the core /ofshell carriers structure. causes In repulsion, case that increasing multi-carrier the emission energy of takes the multi-carrier place in the system. structure, We spatialeliminate distribution the Joule-heating of carriers effect causes as repulsion,a possible increasingorigin because the energythe EL ofintensity the multi-carrier increased system.despite a Weblue-shift eliminate [48–50]. the Joule-heating eﬀect as a possible origin because the EL intensity increased despite a blue-shift [48–50].

Figure 5. (a) Plots of EL intensity as a function of time wavelength for a PbS NIR QLED. (An applied Figure 5. (a) Plots of EL intensity as a function of time wavelength for a PbS NIR QLED. (An applied voltage was 1.5 V during light emission.) (b) A plot of peak wavelength as a function of time. voltage was 1.5 V during light emission.) (b) A plot of peak wavelength as a function of time. The presence of competing mechanisms decreased the EL intensity in 2 min, as seen in Figure6Thea. Suppressionpresence of competing of the blue-shift mechanisms occurred decreased with a the decrease EL intensity in the in EL 2 intensity,min, as seen as seen in Figure in Figure6a. Suppression6b, indicating of thatthe electricblue-shift field-induced occurred with luminescence a decrease quenchingin the EL intensity, mechanisms as seen were in activated, Figure 6b, involvingindicating spatial that electric separation field-induced of the electrons luminescence and holes, quenching QD charging, mechanisms and carrier were coupling activated, to involving surface states.spatial In separation this regime, of wethe speculateelectrons and that holes, trapping QD ofch thearging, injected and carrier carriers coupling into the to localized surface surfacestates. In states,this regime, i.e., carrier we speculate coupling that to surfacetrapping states, of the hasinject aed significant carriers into effect the on localized the decrease surface in states, the EL i.e., intensity.carrier coupling Importantly, to surface PbS CQDs states, have has been a signific reportedant toeffect form on dynamic the decrease traps underin the highEL intensity. electric field,Importantly, originating PbS from CQDs field-induced have been ionization reported of to capping form moleculesdynamic traps and nanounder morphology high electric change field, oforiginating the CQD surface from field-induced [43]. In our ionization previous studyof capping [31], indeed,molecules we and observed nano morphology a time constant change of tensof the ofCQD seconds surface in the [43]. time-domain In our previous measurements study [31], forindeed, pentacene we observed/PbS CQD a time bilayer constant field-e offf tensect transistor of seconds devices,in the allowingtime-domain usto measurements connect the EL for decay pentacene/PbS under high CQD electric bilayer field field-effect to dynamic transistor trap formation. devices, Fromallowing a far us greater to connect time constantthe EL decay than under that with high static electric electronic field to trapsdynamic of fstrap to ms,formation. we argued From that a far dynamicgreater trapstime constant originating than from that impurities with static associated electronic with traps hydroxyl of fs to ms, and we hydrogen argued arethat present dynamic on traps the PbSoriginating CQD surface. from Itimpurities has been acceptedassociated that with injection hydroxyl of charge and hydrogen carriers intoare PbSpresent CQDs on activatesthe PbS CQD the reactions (O +2H O+4e 4OH- and O +4H++4e 2H O) associated with dynamic traps, causing a 2 2 −↔ 2 −↔ 2 reduction of the mobile carrier density by depleting carriers through production of hydroxyl and hydrogen ions [51–54]. As mentioned before, indeed, the time scale of the current decay in the EL measurement is consistent with a long charge retention time on the order of minutes associated with dynamic trap formation, enabling us to infer that luminescence quenching is caused by the presence of the dynamic traps on the PbS CQD surface. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 11

surface. It has been accepted that injection of charge carriers into PbS CQDs activates the reactions (O2+2H2O+4e−↔4OH- and O2+4H++4e−↔2H2O) associated with dynamic traps, causing a reduction of the mobile carrier density by depleting carriers through production of hydroxyl and hydrogen ions [51–54]. As mentioned before, indeed, the time scale of the current decay in the EL measurement is consistent with a long charge retention time on the order of minutes associated with dynamic trap formation, enabling us to infer that luminescence quenching is caused by the presence of the dynamic Appl.traps Sci. 2020on the, 10, PbS 7440 CQD surface. 8 of 11

Figure 6. (a) Plots of EL intensity and current as a function of time during emission. An applied Figure 6. (a) Plots of EL intensity and current as a function of time during emission. An applied voltage is 1.5 V. The inset shows the current decay plots to calculate the stretched exponential fitting voltage is 1.5 V. The inset shows the current decay plots to calculate the stretched exponential fitting parameters (τ = 161 s and β = 1.3). The black and red-dashed curves represent fit and experimental data,parameters respectively. (τ = (161b) A s plotand ofβ = peak 1.3). wavelength The black and as a red-dashed function of timecurves during represent emission. fit and experimental data, respectively. (b) A plot of peak wavelength as a function of time during emission. In addition to electric field-induced luminescence quenching due to dynamic trap formation, the electricalIn addition bias-stress to electric effect field-induced is thought to luminescence decrease the ELquenching intensity. due Contribution to dynamic oftrap the formation, bias stress the effelectricalect to luminescence bias-stress effect quenching is thought is evidenced to decrease from the theEL intensity. fact that theContribution current decay of the in bias Figure stress6a effect is β well-matchedto luminescence with quenching a stretched is exponential evidencedfunction, from theI (fat)ct= thatIoexp the( ( tcurrent/τ) ), which decay has in been Figure extensively 6a is well- − β usedmatched to explain with carrier a stretched trapping-induced exponential bias function, stress eIff(tect) = asIoexp seen(−( int/τ the) ), insetwhich of Figurehas been6a. extensively In the relation, used Io isto theexplain pre-exponential carrier trapping-induced current, τ and biasβ are stress the time effect constant as seen and in the dispersion inset of parameter,Figure 6a. In respectively. the relation, A relativelyIo is the pre-exponential long time constant current, (τ) ofτ and 161 β s isare consistent the time constant with that and of charge dispersion injection-induced parameter, respectively. dynamic trapA relatively formation. long Importantly, time constant the current(τ) of 161 decay s is consistent during the with increase that of in charge the EL injection-induced intensity in Figure dynamic2b is nottrap well formation. fitted to the Importantly, stretched exponential the current function, decay during clarifying the increase that two in di thefferent EL intensity mechanisms in Figure compete 2b is tonot determine well fitted the ELto intensity.the stretched It is notedexponential that the functi contributionon, clarifying of the that Joule-heating two different effect mechanisms on the EL intensitycompete is to not determine substantial the because EL intensity. blue-shift It is was noted suppressed. that the contribution of the Joule-heating effect on theIn EL elaborating intensity is onnot the substantial spectral because shift during blue-shift EL intensity was suppressed. variation, transition in the emission zone shouldIn elaborating be considered. on the spectral Temporal shift blue-shift during EL until inte 2nsity min variation, can be interpreted transition in that the electron-hole emission zone recombinationshould be considered. zone moved Temporal from the ZnOblue-shift/PbS interface until 2 withmin an can energy be interpreted difference of that 0.8 eVelectron-hole between therecombination LUMO of the ZnOzone (moved4.3 eV) from and the HOMOZnO/PbS of interfac the PbSe CQDwith (an5.1 energy eV) to difference the PbS CQD of 0.8 with eV abetween band − − gapthe of LUMO 0.9 eV. Toof the elucidate ZnO ( the−4.3 competing eV) and the mechanisms, HOMO of the more PbS controlled CQD (−5.1 experiments eV) to the inPbS combination CQD with a withband transient gap of absorption 0.9 eV. To and elucidate luminescence the competin measurementsg mechanisms, are required. more controlled experiments in combination with transient absorption and luminescence measurements are required. 4. Conclusions 4. Conclusions We report, for the first time, that the temporal evolution of the EL intensity in a PbS CQD NIR LEDWe is report, a result for of the competition first time, betweenthat the temporal change in evolution the charge of injection the EL intensity/extraction in barriera PbS CQD due toNIR interfacialLED is a charging result of that competition leads to charge between balance change and in electric the charge field-induced injection/extraction luminescence barrier quenching. due to Theinterfacial increasing charging EL intensity that leads is thought to charge to bebalance initiated and by electric the achievement field-induced of luminescence charge carrier quenching. balance, deactivatingThe increasing non-radiative EL intensity Auger is thought recombination. to be initia Theted following by the achievement EL intensity of decrease charge iscarrier ascribed balance, to electricdeactivating field induced-QD non-radiative charging Auger coupledrecombination. with surface The following states and EL biasintensity stress decrease effect based is ascribed on the to suppressionelectric field of theinduced-QD spectral blue charging shift and coupled the current with decaysurface due states to carrier and bias trapping. stress Oureffect findings based raiseon the a criticalsuppression issue ofof the electricspectralfield-induced blue shift and dynamicthe current trap decay formation due to associatedcarrier trapping. with interfacial Our findings or QD raise surface charging, providing insights into designing functional interfaces to enhance device stability as well as the EQE in the NIR CQD QLEDs.

Author Contributions: Conceptualization, B.P. and M.K.; methodology, B.P. and M.K.; M.K.; data curation, B.P.; writing—original draft preparation, B.P.; writing—review and editing, M.K.; visualization, B.P.; supervision. All authors have read and agreed to the published version of the manuscript. Appl. Sci. 2020, 10, 7440 9 of 11

Funding: This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1A6A1A03031833, NRF-2019R1F1A1060042 and NRF-2020R1A2C1007258). This work was also supported by the 2020 Hongik Faculty Research Support Fund. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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