Influence of Triplet Diffusion on Lead Halide Perovskite-Sensitized

Jun 21, 2019 - The emerging field of lead halide perovskite-sensitized triplet–triplet annihilation (TTA) in rubrene shows great promise in upconver...
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Cite This: J. Phys. Chem. Lett. 2019, 10, 3806−3811

Influence of Triplet Diffusion on Lead Halide Perovskite-Sensitized Solid-State Upconversion Sarah Wieghold, Alexander S. Bieber, Zachary A. VanOrman, and Lea Nienhaus* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States

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S Supporting Information *

ABSTRACT: The emerging field of lead halide perovskite-sensitized triplet−triplet annihilation (TTA) in rubrene shows great promise in upconversion applications. By rapidly transferring single charge carriers instead of bound triplet states, perovskites enable a high triplet population in rubrene, yielding low Ith values. In this contribution, we investigate the role of the triplet population on the upconverted emission. Interestingly, two independent rates of TTA can be observed, as well as a sharp drop in the visible emission intensity over several seconds. This effect can be attributed to the triplet-density-based diffusion length: (i) at low triplet populations slow diffusion-mediated TTA yields singlets far from the interface and (ii) higher triplet populations lead to rapid TTA close to the perovskite/rubrene interface. Because of the proximity of the strongly absorbing perovskite, the singlet states created closer to the interface undergo stronger back-transfer to the perovskite film and therefore appear to exhibit a lower photoluminescence quantum yield.

U

pconversion (UC) is a promising approach to overcoming the Shockley−Queisser1 limit in single-junction photovoltaics (PVs)2−5 by converting low-energy photons to high-energy photons. In particular, triplet−triplet annihilationbased (TTA)-UC shows great potential as the energy is stored in long-lived spin-triplet states.6−9 Hence, TTA-UC can become efficient at low excitation densities and the approach of near-infrared (NIR)-to-visible UC provides an avenue applicable under subsolar irradiance.7,10 In sensitized TTA-UC systems, the sensitizer is optically excited followed by a spin-allowed Dexter-type energy transfer into the annihilator’s triplet state.8,9,11 In addition to established hybrid inorganic−organic TTA-UC systems using quantum confined materials as sensitizers, for example, lead sulfide (PbS),12−18 cadmium selenide (CdSe),12,19−21 or cesium lead halide perovskites,22−25 recently a newly developing field of bulk lead-halide perovskite-sensitized solid-state UC has been reported.26,27 While previous solidstate TTA-UC systems have been limited by the lack of longrange exciton diffusion in for example PbS nanocrystal arrays,14,15,28,29 the emerging field of bulk-perovskite-sensitized UC bears the advantage of using free carriers which are injected into the triplet state of the annihilator rubrene. This new approach foregoes the requirement of efficient singlet-totriplet conversion in the sensitizer,26 enabling TTA-UC in rubrene at subsolar incident fluxes.27 A schematic of the bilayer UC device is shown in Figure 1a: a 100 nm methylammonium formamidinium lead iodide (MAFA) perovskite thin film is used as the sensitizer; the annihilator layer consists of solution-cast rubrene30 doped with 1% dibenzotetraphenylperiflanthene (DBP).13 Rubrene was chosen because of the favorable band alignment with the underlying MAFA film allowing for hole extraction.31−36 © 2019 American Chemical Society

Under 780 nm excitation, the device emits both NIR MAFA photoluminescence (PL) peaking at 780 nm and UC PL at ∼605 nm. The MAFA film is characterized by atomic force microscopy (SI Figure S1). Figure 1b highlights the absorbance spectrum (black), the steady-state PL spectrum under 405 nm excitation (blue), and the UC emission spectrum resulting from 780 nm excitation (orange) of the bilayer device. The MAFA absorption onset can be seen at ∼800 nm (optical bandgap of 1.55 eV)36 whereas the absorption features of rubrene appear at 430−530 nm,37 confirming the nonoxidized species of rubrene.38 Two unusual effects can be observed in MAFA−rubrene devices:26,27 (i) a slow reduction or reversible “photobleach” in the UC PL intensity on a time scale of multiple seconds (Figure 1c,d) and (ii) two rising components (regimes 1 and 2) in the UC PL dynamics followed by a slow decay (regime 3) (Figure 1e). Here, we seek to unravel the cause of these effects by investigating the mechanism of UC in MAFA− rubrene devices. Figure 1c shows the normalized PL intensity of the MAFA− rubrene UC device under pulsed 780 nm excitation. We observe a reduction in the UC PL quantum yield (PLQY) over the first ∼5 s after laser illumination, followed by a plateauing of the UC emission intensity. As this effect occurs repeatably, and reappears after several seconds of sample recovery in the dark (SI Figure S2), a photobleaching mechanism based on a physical decomposition of the MAFA film or rubrene can be ruled out. It is known that the long-lived rubrene triplets39 do Received: May 28, 2019 Accepted: June 21, 2019 Published: June 21, 2019 3806

DOI: 10.1021/acs.jpclett.9b01526 J. Phys. Chem. Lett. 2019, 10, 3806−3811

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observe that the UC PL plateaus at the same value under chopped and constant pulsed excitation. It has been reported that both the rate of diffusion-mediated TTA and the triplet lifetime are modulated by the existing triplet population.13,14 Hence, the UC dynamics are dependent on both the average excitation power and the repetition rate of the excitation source. However, as UC devices would be used under constant solar illumination, the UC properties under continuous wave (CW) operation are of great interest. To investigate the effect of CW excitation on the UC PLQY, we record the PL intensity of the MAFA−rubrene device under 780 nm CW excitation (Figure 2a) over 15 s. Because the MAFA PL peaks at the same wavelength as the excitation source, we record the PL spectrum only between 450 and 700 nm to avoid artifacts due to laser scatter. To ensure that the PL intensity changes at 700 nm are not simply due to a shift in the emission wavelength, but correctly reflect variations in the peak PL intensity, we also record the time-dependent MAFA PL spectra under 405 nm illumination (SI Figure S3). Figure 2b depicts the MAFA PL detected at 700 nm (black circles) and the UC PL at 605 nm (orange circles) over 15 s. Initially, both PL intensities drop. At later times (t > 1s), we observe an opposite trend in the PL intensities: the MAFA PL increases, while the UC PL decreases. The ratio of the PL intensity at 605 versus 700 nm (Figure 2c) is related to the UC efficiency (ηUC) of the device, defined as the number of absorbed photons that are converted to emissive singlet excitons in the annihilator.40 ηUC is the product of the energy-transfer efficiency from sensitizer to annihilator (ηET) and the TTA efficiency of the annihilator (ηTTA), where ηrub is the PLQY of the rubrene/DBP film.41

Figure 1. (a) Schematic of the bilayer upconversion device consisting of a 100 nm thick MAFA perovskite film and a solution-cast rubrene/ 1%DBP layer. The rubrene triplet state is populated by electron and hole transfer from the MAFA, and the device emits 605 nm light upon excitation at 780 nm. A 600 ± 40 nm band-pass (BP) filter is used to selectively investigate the UC PL, while an 800 nm long pass (LP) is used to investigate the NIR MAFA PL dynamics. (b) Absorbance spectrum of the bilayer device, showing the expected onset of MAFA absorption at 800 nm (black). PL of the bilayer device under 405 nm CW excitation (blue), highlighting the visible DBP emission at 605 nm, and the NIR MAFA emission at 780 nm. Visible UC PL is obtained under 780 nm CW excitation (orange). (c) Time dependence of the UC PL counts upon 780 nm pulsed excitation. (d) Time dependence of the UC PL counts under slow 20 Hz modulation of the pulsed laser by a mechanical chopper, resulting in a 25 ms “on time” and a 25 ms “off time”. (e) Dynamics of the UC PL (600 ± 40 nm) under 780 nm excitation, showing three different regimes: a fast-rising component (regime 1), a slow-rising component (regime 2), and a slow decay (regime 3) corresponding to the longlived triplet lifetime (τtriplet > 12 μs). The repetition rate of the pulsed laser is set to 31.25 kHz at an average incident power of 4 μW.

ηUC = ηET ·ηTTA ·ηrub

(1)

An initial ratio reduction of the PL intensity at 605 versus 700 nm followed by a plateau at 0.23 can be seen over 15 s. This indicates a time-dependent change in ηUC and therefore a change in either ηET or ηTTA over time. To investigate the role of the triplet population, we again modulate the 780 nm CW excitation source at 20 Hz. Figure 2d shows the timedependent evolution of the PL spectrum. In Figure 2e the intensities at 605 nm (orange) and at 700 nm (black) are plotted, corresponding to the UC and MAFA PL, respectively. We see a similar trend in the behavior: an initial dip in the PL intensity of both traces, followed by an anticorrelated increase in the MAFA PL and a decrease in the UC PL. The ratio of the UC PL versus MAFA PL is plotted in Figure 2f. A similar trend is observed as in the constant CW excitation: an initial reduction in the ratio, followed by a plateau. Here, the plateau is at a higher value of 0.3, indicating an overall higher UC efficiency of the device under the modulated illumination. To infer the role of energy transfer to the triplet state and the TTA efficiencies on the overall UC efficiency, we employ time-resolved PL spectroscopy (TRPL). Changes in the rate or efficiency of energy transfer to the rubrene triplet state will manifest as changes in the quenching dynamics of the sensitizer (MAFA) PL. Changes in ηTTA or the TTA process itself will change the UC PL dynamics. We have established that modulating the triplet population by allowing it to fully relax to the ground state increases the UC PLQY. As a result, we can extract the responsible part for the UC PLQY changes by comparing the PL dynamics under pulsed and pulsed illumination modulated by the chopper. Figure 3a shows the MAFA PL decay dynamics under pulsed (black) and chopped

not fully decay between subsequent laser pulses, which causes a buildup of triplet excitons. It appears surprising that an increase of triplet excitons may result in a decrease of the UC PLQY, as this should increase the rate of diffusion-mediated TTA. To investigate this unusual observation, we modify the built-up triplet population by allowing the system to fully relax to the ground state. To achieve this, an additional slow modulation of the incident pulsed laser is introduced by means of a mechanical chopper wheel (20 Hz). The 25 ms “off time” is sufficiently long to allow both the full relaxation of all triplets to the ground state and any trapped carriers to equilibrate within the perovskite film. Figure 1d shows the normalized PL intensity of the MAFA−rubrene UC device under chopped excitation. Unsurprisingly, because of the duty cycle of 50%, the initial peak PL intensity is roughly halved. However, we 3807

DOI: 10.1021/acs.jpclett.9b01526 J. Phys. Chem. Lett. 2019, 10, 3806−3811

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Figure 2. (a) Time-dependent evolution of the MAFA−rubrene/1% DBP emission spectrum obtained under 780 nm CW excitation. The MAFA emission is observed at 700 nm, and the rubrene/1% DBP emission is obtained at 605 nm. (b) PL intensity of the NIR MAFA PL at 700 nm (black) and the UC PL at 605 nm (orange) as a function of time. (c) Ratio of the emission obtained at 605 and 700 nm, indicative of a change in the UC efficiency as a function of time. (d) Time-dependent PL spectra of the MAFA−rubrene/1% DBP bilayer device under 20 Hz chopped 780 nm excitation. (e) PL intensity tracking of the NIR MAFA PL at 700 nm (black) and the UC PL at 605 nm (orange). (f) Ratio of the emission detected at 605 and 700 nm under chopped excitation. The laser power is set to an average CW power of 37 mW.

illumination (gray) at an incident power of 4 μW. As it is known that perovskites have power-dependent lifetimes and the peak power is not as relevant as the average incident power,42 we also show the PL dynamics under pulsed excitation at half the incident power (2 μW, purple). All MAFA PL dynamics show very similar behavior, and therefore, we conclude that neither the energy-transfer efficiency ηET nor the related transfer rate is responsible for the time-dependent change in the UC PLQY. The UC PL dynamics on the other hand (Figure 3b,c) paint a very different picture. Again, we highlight the three cases: (i) illumination at a pulsed power of 4 μW (black), (ii) 2 μW average incident power of the pulsed laser (purple), and (iii) modulated pulsed illumination with a duty cycle of 50% (gray) at an incident power of 4 μW (“on time”). In contrast to the MAFA PL (Figure 2a) which stems from direct optical excitation of the MAFA perovskite, the UC PL emission relies on the creation of triplet excitons and reflects the population of the triplet state over time. At the time of the first laser pulse, the population of triplets is zero and increases because of triplet sensitization. Hence, the rise time reflects the combined rate of charge transfer from the MAFA perovskite to the triplet state of rubrene, triplet diffusion prior to TTA, Förster resonance energy transfer (FRET) from rubrene to DBP, and the respective emission rate. The slowest of these steps will be rate-limiting and dictate the overall observed rise time. Commonly, this will be the rate of triplet diffusion in solidstate UC devices. The slow decay reflects a lower bound of the long-lived triplet state (τtriplet > 12 μs), limited by the resolution of our TRPL setup. Figure 3b shows the longterm dynamics highlighting both the rise of the triplet population and the long-lived decay, while Figure 3c shows a zoomed-in view of the early time PL dynamics. Clearly visible is a change in the UC PL dynamics based on the incident excitation: (i) At 4 μW, two distinguishable rise times can be observed, corresponding to a fast and slow rising component, followed by a slow decay. (ii) At 2 μW, two rising regimes are

found. However, the initial fast rise is slower and the magnitude of the fast component is less, which indicates a power dependency of the fast-rising regime 1. (iii) Under chopped excitation, a small amount of a fast early rise time can be observed; the main component, however, corresponds to the slow rise dictated by triplet diffusion through rubrene. The UC PL dynamics under chopped excitation are well fit by a simple exponential rise τ2, followed by a monoexponential decay τ1: Ich(t ) ∝ (e−t / τ1 − e−t / τ2). Because of the duty cycle of 50% of the chopped excitation, the average incident power is 2 μW. However, during the “on time” the incident power is 4 μW. As there are differences between all three conditions, we conclude that neither the average power nor the peak power is the underlying cause. Rather, we observe an increase in the rate and magnitude of the fast-rising regime 1 if the triplet population builds up over time (SI Figure S4). This is further supported by the faster rise in regime 1 at a higher incident power. We observe that the decay traces can be deconvolved into two components, one of which corresponds to the dynamics extracted in the case of the mechanically chopped excitation Ich(t), with an additional component corresponding to a quadratic exponential rise τ4 and decay τ3: Ipulsed(t ) ∝ (e−t / τ3 − e−t / τ4)2 + Ich(t ) (Table 1). Because TTA is a process involving two triplet states, bimolecular recombination dynamics are expected.43 In the case of slow triplet diffusion, this quadratic dependency is often not observed, as the rate-limiting step is the long diffusion time. If diffusion is fast, the rate-limiting step will be triplet sensitization. Here, bimolecular recombination dynamics are expected, and the rise time will amount to an upper bound of the triplet sensitization time. As a result, we assert that the two rising regimes in the triplet dynamics are due to two distinct processes: (1) rapid UC close to the interface and (2) slow diffusion-mediated TTA. 3808

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pulses, triplets are able to diffuse far from the interface prior to finding a “partner” to undergo TTA (Figure 3f). As the number of triplets is increased, the time required for two triplets to collide via diffusion is decreased, with the lower limit being no diffusion occurring. Consequently, a fraction of TTA will occur more rapidly and closer to the interface (Figure 3g). However, as both processes have the same underlying mechanism, it is unexpected to anticipate a difference in ηTTA. To tie into the change in the observed UC PL intensity, we hypothesize that it is not the inherent TTA efficiency that is being reduced, rather only the observed ηTTA is reduced because of parasitic back-transfer of the created emissive singlet states. Back-transfer of singlet excitons has been shown to be detrimental in solid-state UC devices.27,28 The schematics in Figure 3f,g highlight the differences in rapid TTA close to the interface and slow-diffusion-mediated TTA. The limit of the rapid TTA process is interface-mediated TTA. Emissive singlets created at the interface will rapidly undergo back-transfer by weak far-field reabsorption but also highly efficient near-field processes such as FRET.44 Triplet diffusion lengths above a typical FRET radius of ∼10 nm result in singlet excitons that are able to be only weakly far-field reabsorbed, and emission created by this pathway will exhibit a higher observed UC PLQY. In conclusion, we have investigated the UC emission properties of MAFA−rubrene bilayer devices. Modulating the triplet population by means of a mechanical chopper wheel results in an increase in the UC PLQY. A decrease in the UC efficiency is observed in the first ∼5 s after illumination, and two distinct rise times are observed in the UC PL dynamics. TTA close to the interface results in strong parasitic backtransfer of the emissive singlet states created and reduces the observed UC efficiency. This result indicates that there is a trade-off in the triplet sensitization rate, the triplet diffusion lengths, and the achieved UC efficiency and highlights the importance of the measurement conditions on reported UC efficiencies. Previous reports have shown that the underlying power-dependent recombination kinetics of the perovskite sensitizer also influences the UC emission.27 As a result, UC PLQYs in perovskite-sensitized UC devices are highly sensitive to both the time after illumination the measurement is taken and the incident power.

Figure 3. (a) NIR MAFA PL dynamics under pulsed excitation at 31.25 kHz at an average incident power of 4 μW (black), 2 μW (purple), as well as under 20 Hz chopped illumination (gray, 4 μW incident power, 50% chopper duty cycle, 25 ms “on time”, 2 μW average power). (b) UC PL dynamics (600 ± 40 nm) under the same excitation conditions. (c) Zoomed-in view of the early time dynamics of the time-resolved UC PL, highlighting the two rise times observed. (d) Schematic of the triplet population modulation by the chopped excitation source. (e) Schematic of the triplet population under continuous pulsed excitation. (f) Cartoon highlighting the long triplet diffusion lengths at low triplet concentrations, followed by TTA far from the MAFA/rubrene interface. (g) Cartoon of the two regimes occurring at high triplet populations: rapid interface-mediated TTA, which is subject to strong parasitic back-transfer of the emissive singlet states, and slow diffusion-mediated TTA.

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EXPERIMENTAL METHODS Device Fabrication. The bilayer devices were prepared as detailed previously.26,27 Briefly, the perovskite sensitizer films were prepared with PbI2 (1.2 M, 99.99% Sigma-Aldrich) and MAI (1.2 M, Dyenamo), in a 1:1 molar ratio, and PbI2 (1.2 M, 99.99% Sigma-Aldrich) and FAI (1.2 M, Dyenamo), in a 1:1 molar ratio, both in anhydrous DMF:DMSO 9:1 (v:v, SigmaAldrich). 2-fold dilution with DMF:DMSO 9:1 (v:v) yielded the final precursor solution. The perovskite film was fabricated by a two-step program: 1000 rpm for 10 s and 4000 rpm for 30 s. After 10s of the second step, 100 μL of anhydrous chlorobenzene (Sigma-Aldrich) was dropped onto the substrate. The films were annealed at 100 °C for 10 min in a nitrogen-filled glovebox. Rubrene (99.99%) and DBP (98% HPLC) were purchased from Sigma-Aldrich and used as received. A 10 mg/mL rubrene solution was prepared in anhydrous toluene (SigmaAldrich) and doped with DBP at a 1% ratio. The rubrene/DBP solution was deposited by spin-coating onto the previously prepared perovskite layer at 6000 rpm for 20 s. The

Table 1. UC PL Dynamics under Pulsed and Chopped Pulsed Excitation (e−t / τ3 − e−t / τ4)2

e−t / τ1 − e−t / τ2

ICh I4μW I2μW

τ1

τ2

τ3

τ4

12 μs 12 μs 12 μs

3 μs 3 μs 3 μs

7.2 μs 9.8 μs

0.09 μs 0.21 μs

To create a link between the hypothesis of rapid TTA-UC close to the interface, slow diffusion-mediated TTA, and the change in the UC PLQY, we refer to the schematics in Figure 3d,e. Under modulated illumination (Figure 3d), the triplet population increases until the excitation source is turned off. The triplets will decay with their inherent rate until all triplets have relaxed to the ground state. Steady-state or fast pulsed excitation results in an increase of the triplet population until it becomes saturated (Figure 3e). In the case of a low triplet population, i.e. all triplets relax to the ground state between 3809

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(2) Trupke, T.; Green, M. A.; Würfel, P. Improving Solar Cell Efficiencies by Up-Conversion of Sub-Band-Gap Light. J. Appl. Phys. 2002, 92, 4117−4122. (3) Meng, F.-L.; Wu, J.-J.; Zhao, E.-F.; Zheng, Y.-Z.; Huang, M.-L.; Dai, L.-M.; Tao, X.; Chen, J.-F. High-Efficiency near-Infrared Enabled Planar Perovskite Solar Cells by Embedding Upconversion Nanocrystals. Nanoscale 2017, 9, 18535−18545. (4) van Sark, W. G.; de Wild, J.; Rath, J. K.; Meijerink, A.; Schropp, R. E. Upconversion in Solar Cells. Nanoscale Res. Lett. 2013, 8, 81. (5) Khnayzer, R. S.; Blumhoff, J.; Harrington, J. A.; Haefele, A.; Deng, F.; Castellano, F. N. Upconversion-Powered Photoelectrochemistry. Chem. Commun. 2012, 48, 209−211. (6) Zhao, J.; Ji, S.; Guo, H. Triplet−Triplet Annihilation Based Upconversion: From Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields. RSC Adv. 2011, 1, 937−950. (7) Singh-Rachford, T. N.; Castellano, F. N. Photon Upconversion Based on Sensitized Triplet−Triplet Annihilation. Coord. Chem. Rev. 2010, 254, 2560−2573. (8) Monguzzi, A.; Mezyk, J.; Scotognella, F.; Tubino, R.; Meinardi, F. Upconversion-Induced Fluorescence in Multicomponent Systems: Steady-State Excitation Power Threshold. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 195112. (9) Haefele, A.; Blumhoff, J.; Khnayzer, R. S.; Castellano, F. N. Getting to the (Square) Root of the Problem: How to Make Noncoherent Pumped Upconversion Linear. J. Phys. Chem. Lett. 2012, 3, 299−303. (10) Mahboub, M.; Huang, Z.; Tang, M. L. Efficient Infrared-toVisible Upconversion with Subsolar Irradiance. Nano Lett. 2016, 16, 7169−7175. (11) Dexter, D. L. A. Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21, 836−850. (12) Huang, Z.; Tang, M. L. Designing Transmitter Ligands That Mediate Energy Transfer between Semiconductor Nanocrystals and Molecules. J. Am. Chem. Soc. 2017, 139, 9412−9418. (13) Wu, M.; Congreve, D. N.; Wilson, M. W. B.; Jean, J.; Geva, N.; Welborn, M.; Van Voorhis, T.; Bulović, V.; Bawendi, M. G.; Baldo, M. A. Solid-State Infrared-to-Visible Upconversion Sensitized by Colloidal Nanocrystals. Nat. Photonics 2016, 10, 31−34. (14) Nienhaus, L.; Wu, M.; Geva, N.; Shepherd, J. J.; Wilson, M. W. B.; Bulović, V.; Van Voorhis, T.; Baldo, M. A.; Bawendi, M. G. Speed Limit for Triplet-Exciton Transfer in Solid-State PbS NanocrystalSensitized Photon Upconversion. ACS Nano 2017, 11, 7848−7857. (15) Nienhaus, L.; Wu, M.; Bulovic, V.; Baldo, M. A.; Bawendi, M. G. Using Lead Chalcogenide Nanocrystals as Spin Mixers: A Perspective on Near-Infrared-to-Visible Upconversion. Dalton Trans 2018, 47, 8509−8516. (16) Garakyaraghi, S.; Mongin, C.; Granger, D. B.; Anthony, J. E.; Castellano, F. N. Delayed Molecular Triplet Generation from Energized Lead Sulfide Quantum Dots. J. Phys. Chem. Lett. 2017, 8, 1458−1463. (17) Bender, J. A.; Raulerson, E. K.; Li, X.; Goldzak, T.; Xia, P.; Van Voorhis, T.; Tang, M. L.; Roberts, S. T. Surface States Mediate Triplet Energy Transfer in Nanocrystal−Acene Composite Systems. J. Am. Chem. Soc. 2018, 140, 7543−7553. (18) Huang, Z.; Lee Tang, M. Semiconductor Nanocrystal Light Absorbers for Photon Upconversion. J. Phys. Chem. Lett. 2018, 9, 6198−6206. (19) Huang, Z.; Li, X.; Mahboub, M.; Hanson, K. M.; Nichols, V. M.; Le, H.; Tang, M. L.; Bardeen, C. J. Hybrid Molecule−Nanocrystal Photon Upconversion Across the Visible and Near-Infrared. Nano Lett. 2015, 15, 5552−5557. (20) Huang, Z.; Li, X.; Yip, B. D.; Rubalcava, J. M.; Bardeen, C. J.; Tang, M. L. Nanocrystal Size and Quantum Yield in the Upconversion of Green to Violet Light with CdSe and Anthracene Derivatives. Chem. Mater. 2015, 27, 7503−7507. (21) Underwood, D. F.; Kippeny, T.; Rosenthal, S. J. Ultrafast Carrier Dynamics in CdSe Nanocrystals Determined by Femtosecond Fluorescence Upconversion Spectroscopy. J. Phys. Chem. B 2001, 105, 436−443.

upconversion devices were sealed with a coverslip using a 2part epoxy (Devcon) under nitrogen atmosphere prior to removal from the glovebox. Steady-State Optical Spectroscopy. Absorption spectra were measured on a UV−vis spectrometer (Shimadzu UV-2450). PL spectra were measured using an OceanOptics spectrometer (HR2000+ES) with a CW excitation wavelength of 405 nm (PicoQuant LDH-D-C-405) and 780 nm (PicoQuant LDH-DC-780). The integration time was set to 100 ms, which corresponds to roughly 2 full “on/off” cycles under chopped illumination. Time-Resolved Photoluminescence Spectroscopy. Time-resolved photoluminescence lifetimes were obtained by time-correlated single-photon counting (TCSPC). The samples were excited by a picosecond pulsed diode laser (PicoQuant LDH-D-C780) at a repetition rate of 31.25 kHz. The incident excitation power was determined by a silicon power meter (ThorLabs PM100-D). To obtain the NIR MAFA PL lifetimes, laser scatter was removed by an 800 long-pass filter (ThorLabs). To detect the UC PL dynamics, the NIR MAFA PL and excess laser scatter were removed by a 600 bandpass filter (ThorLabs). In all cases, the resulting emission was focused onto a single-photon counting avalanche photodiode (Micro Photon Devices) using reflective optics. A HydraHarp 400 (PicoQuant) was used to record the photon arrival times. The T2 mode of the HydraHarp was applied to investigate the long-term PL intensity over 15 s, and the resulting photon arrival times were histogrammed. To modulate the triplet population by modulating the excitation laser, we add an additional mechanical chopper (ThorLabs) to the existing (time-resolved) PL setup. The chopper is set to 20 Hz and a duty cycle of 50%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.9b01526. Additional experimental details, AFM image of the perovskite film (Figure S1), time-dependence of the UC PL dynamics under chopped pulsed illumination (Figure S2), NIR MAFA PL under 405 nm excitation (Figure S3), and time-dependence of the UC PL over 25 h (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sarah Wieghold: 0000-0001-6169-3961 Lea Nienhaus: 0000-0003-1412-412X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge Florida State University startup funds. REFERENCES

(1) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510− 519. 3810

DOI: 10.1021/acs.jpclett.9b01526 J. Phys. Chem. Lett. 2019, 10, 3806−3811

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DOI: 10.1021/acs.jpclett.9b01526 J. Phys. Chem. Lett. 2019, 10, 3806−3811