Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized

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Spectroscopy and Photochemistry; General Theory

Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals Shan He, Xiao Luo, Xue Liu, YuLu Li, and Kaifeng Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02106 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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The Journal of Physical Chemistry Letters

Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals Shan He,† Xiao Luo,† Xue Liu,† Yulu Li,† and Kaifeng Wu†* †

State Key Laboratory of Molecular Reaction Dynamics and Dynamics Research Center

for Energy and Environmental Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China

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Upconversion (UC) is a photon management technique that can find important applications in life sciences (bio-imaging, phototherapy, etc.)1,2 and solar energy conversion.3-5 UC can in principle be realized in any materials via nonlinear processes or two-photon absorption, which, however, requires the use of coherent, high-intensity (often pulsed) light sources. In contrast, UC using rare earth metal (REM) cations or molecular triplet-triplet-annihilation (TTA) can operate under incoherent, continuouswave (cw) illuminations.6 Considering the narrow and weak light absorption of REM cations, molecular TTA-UC is more suited for broad-band solar energy harvesting. A typical TTA-UC system comprises sensitizers and emitters (or called annihilators).5 Molecular sensitizers, usually heavy-metal-containing organometallic complexes, absorb the incident low-energy photons, followed by an efficient intersystem crossing (ISC) to generate their triplets. These triplets then pass their energy to emitter molecules via triplet energy transfer (TET). When two emitter triplets encounter, TTA occurs, generating a singlet excited state which emits a high-energy photon. Using this principle of design, many TTA-UC systems across various spectral windows, such as near IR-to-visible, visible-to-visible and visible-to-UV, have been demonstrated.7-13 Among these systems, visible-to-UV UC is particularly suited for enhancing the efficiency of photocatalytic systems where wide-band-gap semiconductors such as TiO2 are used.14-16 Visible-to-UV upconversion may find important applications in organic photoredox catalysis, as the upconverted, highly-energetic excited state should have very strong reduction and/or oxidation powers for challenging bond activation or formation reactions.17,18 Yet the efficiency of visible-to-UV UC remains relatively low compared to IR-to-visible or visible-to-visible systems. For example, Castellano et al. reported a UC quantum yield 3 ACS Paragon Plus Environment


(A’UC), defined as the absolute UC photon emission yield multiplied by a factor of 2, of 0.58% by using biacetyl and 2,5-diphenyloxazole (PPO) as sensitizers and emitters, respectively.8 Kimizuka et al. improved the A’UC to ~1.0% by using Ir(C6)2(acac) (C6 = coumarin 6, acac = acetylacetone) as sensitizers, achieved by suppressing both reabsorption losses of UV photons and energy transfer from singlet excited states of emitters to sensitizers.19 The low efficiency is in part due to the lack of suitable sensitizers that can donate triplet energy efficiently to the UV emitters while absorbing efficiently in the visible, which is intrinsically linked to a large energy loss (on the order of ~0.5 eV or higher) in the ISC process of the molecular sensitizers.11 More recently, inorganic semiconductor nanocrystals (NCs) have emerged as alternative triplet sensitizers because they can enable efficient TET while minimizing the ISC energy loss.11,20-25 Tang et al. achieved a A’UC of ~5.2% by sensitizing PPO emitters using CdS/ZnS core/shell NCs.26 The shell coating eliminates some trap states on CdS surfaces, but on the other hand could act as a barrier for interfaical TET. In order to accelerate TET, relatively small NCs which absorb at ~405 nm were used, limiting the photon energy gain to 0.43 eV (405 nm to 355 nm).26 This issue may be overcome by lead halide perovskite NCs which feature a unique “defect tolerance” and can attain very high photoluminescence (PL) quantum yields (QYs) without the need of shell coating.27-29 However, the first example of using perovskite NCs as sensitizers reported a A’UC of 1.3% for green-to-blue UC.30 Very recently, our ultrafast spectroscopy studies have uncovered that the key to efficient TET from perovskite NCs to molecules is to use quantum-confined instead of bulk-like NCs,31,32 because quantum confinement enables a strong donor-acceptor wavefunction overlap required for Dexter-type TET.33 Here we 4 ACS Paragon Plus Environment

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ligands was achieved using a simple ligand exchange procedure (SI). According to previous studies on the surface chemistry of perovskite NCs, both carboxylate and alkylammonium ligands can bind to the NC surfaces,38,39 with the ligand binding being highly dynamic.40 This allows for the exchange of the native carboxylate and/or alkylammonium ligands for the carboxylate-functionalized naphthalene ligands (deprotonated NCA). Fig. 1b shows the PL spectra of NCs, NCs with NCA, NCs with PPO, and NCs with both NCA and PPO, all dispersed in hexane, excited with a 443 nm cw laser (58.8 W/cm2). The PL of NCs (peaked at ~461 nm) is negligibly quenched by PPO, due to the lack of an association between NCs and PPO. In contrast, because of a direct binding between NCs and NCA, the PL of NCs is strongly quenched by NCA; see Table S1 for details. However, the presence of either PPO or NCA only does not lead to any UC emission. A broad-band UC emission (from 340 to 443 nm) of PPO can be observed only when both PPO and NCA are present, consistent with the working principle shown in Scheme 1. The UC emission can be directly visualized by a cell phone camera with the aid of a bandpass filter to cut out the excitation light and the residual NC emission (Fig. 1b inset).



2

79.5 W/cm

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0.5 W/cm

b Normalized UC intensity

a UC Intensity (a.u.)

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0

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'UC,max = 10.2% -1

10

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Ith ~ 1.9 W/cm

-3

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340

0.1

360 380 400 Wavelength (nm)

1 10 2 Power density (W/cm )

Figure 2. (a) TTA-UC emission spectra with varying excitation power densities from 0.5 to 79.5 W/cm2. (b) Integrated TTA-UC emission intensities as a function of the excitation power density (purple squares) and the fits in the quadratic (red line) and linear (blue line) regimes. A’UC and Ith are labeled. In order to further confirm and characterize the observed TTA-UC phenomenon, we measured its excitation power dependence. As shown in Fig. 2a, the intensity of the UV emission band increases rapidly with the excitation power. The integrated emission intensity plotted in Fig. 2b shows that it increases first approximately quadratically and then linearly with the excitation power, which is a typical feature for TTA-UC emission.5 The quadratic regime is dominated by radiative and/or nonradiative decay of emitter triplets, whereas in the linear regime the number of sensitized emitter triplets is large enough to sustain a dominant TTA-UC mechanism. As such, the power density at the crossover point is usually defined as the UC threshold (Ith),41 which is ~1.9 W/cm2 for our system. This threshold is relatively high likely because of the use of PPO as annihilators. Indeed, the threshold was ~0.39 W/cm2 for the biacetyl sensitized PPO system reported by Castellano et al.8; and it was ~2.3 W/cm2 for the CdS/ZnS NC-NCA sensitized PPO


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system by Tang et al.26 Both are much higher than those of TTA-UC systems (as low as sub-mW/cm2) employing other annihilators.19 The UC quantum yield (A’UC), by tradition, is defined for the linearly power-dependent regime. The A’UC of our system, achieved by optimizing the choice of solvent and the concentrations of the PPO emitters and NC sensitizers (Fig. S2), is 10.2% at 58.8 W/cm2 (Table S1); see SI and Fig. S4 for measurement details. This yield represents an almost two-fold enhancement compared with the CdS/ZnS NC-sensitized system26 and is already on par with the highest green-to-blue A’UC sensitized with CdSe-based NCs42. Note that the yield is an “external” value whereas the “internal” yield should be even higher, because the UC photons are attenuated by the strong and continuous absorption of NC sensitizers in the UV that can induce reabsorption and/or energy transfer losses. Indeed, the optical density of the solution used in the TTA-UC is ~0.9 at the emission peak of 355 nm. The PL kinetics of photoexcited PPO (1PPO*) are similar in the presence and absence of CsPbBr3 NCs (Fig. S3), suggesting that the loss due to energy transfer from 1PPO*

to NCs is negligible and the reabsorption of UC photons by NCs is a major loss.

Also noteworthy is that the photon energy gain of our system is as high as 0.7 eV (443 nm to 355 nm), which is 0.27 eV higher than that achieved with the CdS/ZnS NC sensitized system (405 nm to 355 nm)26. The high A’UC achieved here is presumably enabled by efficient TET from NCs to NCA, as the rest steps are similar to those of previous systems.22 Indeed, the strong quenching of the PL of NCs by NCA implies efficient TET. In order to provide stronger evidence, we directly measured the TET dynamics using transient absorption (TA) and time9 ACS Paragon Plus Environment


resolved PL spectroscopy. 400 nm excitation pulses were used in these experiments to selectively excite the NCs and the pulse energies were maintained low enough to exclude

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the excitation of multiexcitons; see SI for experimental details.

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1 ps 35 ps 280 ps 2 ns 7ns

NC TA NC PL NC-NCA TA NC-NCA PL

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3

NCA* TA fits

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Figure 3. (a) TA spectra of the NC-NCA sample at indicated time delays following the excitation by a 400 nm pulse. (b) TA kinetics of the NC-NCA sample probed at the XB centre (~450 nm; light blue triangles) and at the 3NCA* (~439 nm; red open circles) and PL kinetics probed at the spectral peak (~461 nm; dark blue diamonds). XB and PL kinetics of free NCs (light green triangles and dark green squares, respctively) are also shown for comparison. The gray solid lines are their exponential fits. Fig. 3a shows the TA spectra of the NC-NCA sample at indicated delays following the pump pulse. The spectral features are dominantly contributed by NCs, including an exciton bleach (XB) feature peaked at ~450 nm and an absorptive feature peaked at ~430 nm. The XB feature is contributed by photogenerated excitons (electrons and holes) in the NCs43,44 and the absorptive feature may arise from forbidden transitions activated by the excitons strongly-confined in the small NCs45. Specifically, when the NC is excited with an electron-hole pair (an exciton), the band edge transition (~450 nm) should be partially bleached due to state-filling. In addition, because of the symmetry-breaking effect of the exciton in the strongly-confined NCs, the optical selection rule is modified; 10 ACS Paragon Plus Environment

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as such, one can observe forbidden transitions with energy located between those of the band edge exciton (~450 nm) and the next higher-energy exciton (~410 nm; Fig. 1a).45 Compared with free NCs (Fig. S5), the decay of NC TA features is accelerated for the NC-NCA sample. At very long delay time, positive TA features corresponding to the T1 Tn absorption of NCA triplets (3NCA*)31 can be detected (Fig. S6). The formation kinetics of the 3NCA* is complementary to the decay kinetics of the exciton bleach (XB) feature of NCs (Fig. 3b), directly evidencing TET from photoexcited NCs to NCA. Moreover, time-resolved PL kinetics monitored at the NC PL peak is also consistent with the XB decay kinetics, confirming the robustness of the kinetics measurements. By fitting the TA and PL kinetics in Fig. 3b, we find a single-exponential lifetime of 5.93±0.07 ns for free NCs and an averaged lifetime of 1.02±0.03 ns for the NC-NCA sample. From these time constants, we can calculate an averaged TET time of 1.23±0.05 ns and a TET yield of ~82.8%; see SI and Table S2 for details. The TET yield is consistent with the PL quenching efficiency of the NC-NCA sample relative to free NCs calculted from Fig. 1b (~84.1%). This ultrafast and efficient TET is made possible by both the high PL QY and the strong quantum confinement effect of our CsPbBr3 NCs. The high PL QY implies suppressed carrier trapping which may compete with interfacial TET. Quantum confinement results in wavefunction “leakage” to NC surfaces, promoting the wavefunction overlap between NC donors and NCA acceptors for Dexter-type TET (Scheme 1).32 We note that, during the preparation of this work, Kimizuka et al. reported a visible-toUV TTA-UC work using a similar system design (perovskite NCs + NCA +PPO).46 In 11 ACS Paragon Plus Environment


that work, in order to sensitize NCA ligands, they used alloyed CsPbClxBr3-x NCs such that the exciton energy is higher than 3NCA*. The sizes of these NCs were ~9.4 nm, which means that they were essentially bulk-like. A key distinction for the work here is that, rather than tuning the NC compositions, we tune the NC sizes into the quantumconfinement regime, which not only satisfies the energetics requirement but also enables a strong donor-acceptor wavefunction overlap for efficient TET.31,32 As such, the PL quenching efficiency of the bulk-like CsPbClxBr3-x NCs by NCA (or the TET efficiency) was only 32%, whereas in our case this efficiency reaches 84%. In fact, because the system design is very similar in both works except for the NC part, the UC quantum yield should roughly scale with the TET efficiency. This is indeed the case. Our UC yield is ~2.55-fold higher than that work (10.2% versus 4%), which is consistent with TET efficiency ratio (2.625; 84% versus 32%). Such a comparison also validates the accuracy of our measurements. At last, we comment on the potential applications of the current visible-to-UV TTA-UC system. As mentioned in the introduction, the most straightforward application is to enhance the efficiencies of photocatalytic systems based on wide-band-gap semiconductors such as TiO2 under solar illumination. The TTA-UC system using biacetyl sensitized PPO has already been applied to such an application.16 Considering the low UC quantum yield of the system (0.58%), our newly developed sensitized system should significantly boost the performance of such photocatalytic reactions. This type of application, however, will be ultimately limited by the reabsorption of UC photons by NCs lowering the UC yield. It is thus highly desirable if the reabsorption loss can be either overcome or bypassed in a working system. This can be achieved in the second 12 ACS Paragon Plus Environment

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application mentioned in the introduction, that is, organic photoredox catalysis. In this case, the upconverted, highly-energetic excited state (1PPO* in our case) can be directly use to initiate challenging chemical reactions via single electron transfer (SET), bypassing the need of photon emission and extraction. For example, the TTA-UC system using biacetyl sensitized PPO has been shown to be capable of activating stable aryl-Br bonds,17 because 1PPO* has a high energy of ~3.6 eV (or 350 kJ/mol). Again, we expect that such chemical reactions could be accelerated by employing our efficient visible-toUV system. In conclusion, we demonstrated a UV-to-visible TTA-UC system with a UC quantum yield of 10.2% and a photon energy gain of up to 0.7 eV. This system comprised CsPbBr3 perovskite NCs as the triplet sensitizers, NCA grafted on NC surfaces as the transmitters, and PPO as the emitters. The key to the success of this system is the use of highemissive, strongly-confined CsPbBr3 NCs that can efficiently transfer triplet energy to NCA transmitters. This work not only presents important progress in the quest of efficient visible-to-UV TTA-UC systems, but also provides a new paradigm for the use of perovskite NCs to sensitize TTA-UC in other spectral windows.

ASSOCIATED CONTENT Supporting Information. Figures S1-S6, Sample preparations, TTA-UC measurements, Time-resolved spectroscopy measurements, Kinetics fitting and Table S1and S2. AUTHOR INFORMATION



Corresponding Author * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge financial support from the Strategic Pilot Science and Technology Project of Chinese Academy of Sciences (XDA21010206), the Ministry of Science and Technology of China (2018YFA0208703), and the LiaoNing Revitalization Talents Program (XLYC1807154). REFERENCES (1) Askes, S. H. C.; Bonnet, S. Solving the oxygen sensitivity of sensitized photon upconversion in life science applications. Nat. Rev. Chem. 2018, 2, 437-452. (2) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Controlling upconversion nanocrystals for emerging applications. Nat. Nanotechnol. 2015, 10, 924. (3) van der Ende, B. M.; Aarts, L.; Meijerink, A. Lanthanide ions as spectral converters for solar cells. Phys. Chem. Chem. Phys. 2009, 11, 11081-11095. (4) Huang, X.; Han, S.; Huang, W.; Liu, X. Enhancing solar cell efficiency: the search for luminescent materials as spectral converters. Chem. Soc. Rev. 2013, 42, 173-201. (5) Schulze, T. F.; Schmidt, T. W. Photochemical upconversion: present status and prospects for its application to solar energy conversion. Energy & Environ. Sci. 2015, 8, 103-125. (6) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395-465. (7) Zhao, W.; Castellano, F. N. Upconverted Emission from Pyrene and Ditert-butylpyrene Using Ir(ppy)3 as Triplet Sensitizer. J. Phys. Chem. A 2006, 110, 1144011445. (8) Singh-Rachford, T. N.; Castellano, F. N. Low Power Visible-to-UV Upconversion. J. Phys. Chem. A 2009, 113, 5912-5917. (9) Singh-Rachford, T. N.; Castellano, F. N. Triplet Sensitized Red-to-Blue Photon Upconversion. J. Phys. Chem. Lett. 2010, 1, 195-200. (10) Amemori, S.; Sasaki, Y.; Yanai, N.; Kimizuka, N. Near-Infrared-toVisible Photon Upconversion Sensitized by a Metal Complex with Spin-Forbidden yet Strong S0–T1 Absorption. J. Am. Chem. Soc. 2016, 138, 8702-8705. 14 ACS Paragon Plus Environment

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Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized

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