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Sensitizing Singlet Fission with Perovskite Nanocrystals Haipeng Lu, Xihan Chen, John E Anthony, Justin Johnson, and Matthew C. Beard J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13562 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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Journal of the American Chemical Society

Sensitizing Singlet Fission with Perovskite Nanocrystals Haipeng Lu,1† Xihan Chen,1† John E. Anthony,2 Justin Johnson,1 and Matthew C. Beard*,1 1

Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States 2

Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States

†

These authors contributed equally to this work.

Corresponding Author Matthew C. Beard*: [email protected]

ABSTRACT The marriage of colloidal semiconductor nanocrystals and functional organic molecules has brought unique opportunities in emerging photonic and optoelectronic applications. Traditional semiconductor nanocrystals have been widely demonstrated to initiate efficient triplet energy transfer at the nanocrystal-acene interface. Herein, we report that unlike conventional semiconductor nanocrystals, lead halide perovskite nanocrystals promote an efficient Dexter-like singlet energy transfer to surface-anchored pentacene molecules rather than triplet energy transfer. Subsequently, molecular pentacene triplets are efficiently generated via singlet fission on the nanocrystal surface. Our demonstrated strategy not only unveils the obscure energy dynamics between perovskite nanocrystal and acenes, but also brings important perspectives of utilizing singlet fission throughout the solar spectrum.

Introduction Traditional single-junction solar energy conversion efficiencies are approaching their fundamental limits set by the Shockley-Queisser analysis. Next generation approaches need to enable technologies that exceed those efficiency limits. Singlet fission (SF), where two low-energy triplets can be produced from one high energy singlet state, has gained significant interest as a means to overcome the SQ limit in single-junction solar cells.1-4 Although tremendous effort and progress have been made in understanding the molecular mechanism that drives the SF process,511 successful implementation in solar energy conversion architectures remains a challenge. Pentacene, in particular, holds great promise due to its well-matched energy levels between the high energy singlet and lower energy triplet states: [E(S1) ≥ 2E(T1)]5 and SF in pentacene crystals is extremely efficient (occurring in less than 100 fs). However, successful implementation of SF in photovoltaic device architecture remains limited12-14 for two important reasons. (1) Pentacene suffers from an extremely limited wavelength range of its molar absorption coefficient especially

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in the 400 − 500 nm region,15-16 resulting in only a fraction of the available high-energy photons in the solar spectrum being absorbed into the SF absorber layer with the ability to generate a highyield of triplets. (2) The triplets must eventually be converted to mobile charge-carriers and there are competitive channels between the various photoinduced processes at the semiconductor/organic molecule interface that can reduce the overall SF yield.17-20 Thus, understanding and controlling interfacial energy flow is an essential component for better utilizing the SF process. To address the above-mentioned challenges, we couple pentacene molecules to a semiconductor nanocrystal (NC) photosensitizer. The NC inherently possesses strong and broadband absorption, which can sensitize the SF process. Recently, investigators have demonstrated efficient triplet energy transfer at such organic-inorganic NC interfaces that sensitizes the production of long-lived triplet states which can then be utilized for photon upconversion,21-23 photocatalysis,24 and enhanced light emission.25 For instance, Ehrler calculated that when SF molecules are coupled with inorganic quantum dots which can emit photons into an underlying solar cell, the photon multiplier has the potential to increase the limiting PCE by 4.2 % absolute.26 In that implementation the molecules absorb light producing singlet states that undergo SF and the triplets undergo energy transfer to the inorganic NCs. Here we are interested in when light is absorbed by the QD followed by singlet energy transfer to the attached ligands intiating SF and thereby producing multiple triplets per absorbed photon. Coupling SF molecules with a strong and complementary NC absorber would allow the development of novel solar energy conversion systems that can take advantage of the SF enhancing the photocurrent throughout the entire solar spectrum. To this end, we combine pentacene molecules with CsPbBr3 NCs. CsPbBr3 NCs exhibit exceptional light harvesting and emitting properties,27-28 thus they serve as an ideal candidate for singlet energy transfer. The electronic interaction, such as charge and/or energy transfer, between CsPbBr3 and functional organic chromophores remains largely unexplored. The lack of detailed studies stems from the relatively complicated surface chemistry29-31 and abnormal photophysical properties (such as the ‘bright’ ground state excitons32) of perovskite NCs compared to traditional quantum dots. Understanding these fundamental energy dynamics is thus extremely important for the implementation of perovskite NCs for advanced solar energy conversion concepts. Herein, we quantitatively exchange the as-prepared surface oleate ligands of CsPbBr3 NCs with triisopropylsilylethynyl pentacene carboxylic acid (TIPS-Pc) ligands via a 1:1 X-type ligand exchange reaction. The ligand exchanged NCs display characteristic absorption features from both CsPbBr3 and the attached pentacene molecules, with a completely quenched photoluminescence. Femtosecond transient absorption spectroscopy (TA) indicates an efficient singlet energy transfer from NC to pentacene, which is mediated by a Dexter-like electron exchange mechanism. The singlet energy transfer is initiated by an ultrafast hole transfer process followed by a slower electron transfer. The much faster Dexter-like energy transfer out competes slower Förster energy transfer, and subsequently, molecular triplets of the surface pentacene ligands are generated via


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SF. Our results not only unveil the energy flow dynamics between CsPbBr3 NCs and pentacene, but also demonstrate that SF can be effectively triggered through a sequential charge transfer. Results and Discussion Synthesis and ligand exchange. Oleate-capped CsPbBr3 NCs were first synthesized through a modified amine-free approach.33 Since common colloidal syntheses of CsPbBr3 NCs result in a complicated coupled dynamic binding motif involving both carboxylic and amine ligands,29 it is helpful to prepare the CsPbBr3 NCs with a relatively ‘simple’ and better explored surface chemistry for the subsequent surface modification (i.e., ligand exchange). The as-synthesized CsPbBr3 NCs crystallize in the orthorhombic phase,34 with an average diameter of 11.2 ± 1.6 nm (Supporting Information, Figure S1). The as-prepared oleate terminated NCs (OA−/CsPbBr3) exhibit a photoluminescence quantum yield (PLQY) of ~ 70%, suggesting high quality NCs that are well-passivated. Combined 1H NMR and FT-IR data (Figure S2 and S3) confirm that oleate ligands (OA−) decorate the NC surfaces. Similar to our previous studies,35-36 where cinnamic acids can readily replace OA− at the surface of PbS NCs through an 1:1 X-type proton mediated exchange, we find that excess TIPS-Pc ligands also drive the ligand exchange reaction to completion (Figure 1a), giving TIPS-Pc−/CsPbBr3 NCs. 1H NMR and FT-IR (Figure S2 and S3) data indicate that the vast majority of native oleate ligands (OA−) are efficiently removed from NC surfaces. The extent of ligand exchange can be controlled yielding NCs with varying amounts of TIPS-Pc bound to the surface. Purified TIPS-Pc−/CsPbBr3 NCs suspended in dichloromethane maintain reasonable colloidal stability. The UV-vis spectrum of TIPS-Pc−/CsPbBr3 NCs displays both CsPbBr3 excitonic absorption and sharp vibronic bands arising from surface acene molecules (Figure 1b). The acene bands exhibit a slight red-shift compared to solvated TIPS-Pc not attached to the NCs. The calculated absorption coefficient of TIPS-Pc at 400 − 500 nm range between 1 – 6 × 103 M−1 cm−1. In contrast, the absorption coefficient of CsPbBr3 NCs based on literature reports37 is ~ 106 M−1 cm−1 in the 400 − 500 nm spectral range, 3 orders of magnitude higher than that of TIPS-Pc. As such, we can selectively excite NCs at 400 nm (shaded region in Figure 1b) where we estimate > 97 % of incident photons are absorbed by the NC core. The energy diagram (Figure 1b) shows approximate energy levels for isolated CsPbBr3 NCs38 and TIPS-Pc20, and serves as a tentative qualitative guide for the expected photophysical processes. Based on the schematic energy diagram (Figure 1b), several photo-induced processes between CsPbBr3 NCs and TIPS-Pc are thermodynamically allowed and include hole transfer, electron transfer, and energy transfer for both singlets and triplets. Evidence that one or more of these processes occur can be found via the PL quenching in the TIPS-Pc−/CsPbBr3 (PLQY < 0.001 %, Figure 1c), in contrast to the high PLQY from the oleate-passivated NCs. Time-resolved PL (TRPL) measurements indicate that the PL lifetime of TIPS-Pc−/CsPbBr3 NCs is within the instrument response function (IRF, ~ 200 ps).


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Figure 1 | Characterization of surface ligand exchange process. a, Ligand exchange reaction of the assynthesized oleate-passivated OA−/CsPbBr3 NCs with TIPS-Pc ligand via a 1:1 X-type exchange mechanism. The quantitative ligand exchange is driven by excess pentacene ligands, giving TIPSPc−/CsPbBr3 NCs. b, Absorption spectra of OA−/CsPbBr3 (black line), TIPS-Pc−/CsPbBr3 NCs (red line), and neat TIPS-Pc (black dash line) solutions. OA−/CsPbBr3 NCs are dispersed in toluene, while TIPSPc−/CsPbBr3 NCs, and TIPS-Pc are both dispersed in dichloromethane. Inset illustrates the energy diagram between CsPbBr3 NCs and TIPS-Pc molecules. Green shaded region indicates that over 97% photons are absorbed by CsPbBr3 at 400 nm. c, PL spectra (excited at 480 nm) and time-resolved PL decay curves (inset, excited at 450 nm) of OA−/CsPbBr3 (black) and TIPS-Pc−/CsPbBr3 (red) NCs.

Triplet generation. To further probe the photophysical processes at the TIPS-Pc−/CsPbBr3 interface, we performed transient absorption (TA) spectroscopy.39-41 Both femtosecond and nanosecond TA spectra of a TIPS-Pc−/CsPbBr3 solution are collected in the visible and NIR regions using a pump excitation wavelength of 400 nm, for which the NC component is almost exclusively excited (vide supra). The average photoexcited excitons per NC (< N >) is ~ 0.04 (see calculation in SI), so no biexcitons are photoexcited during the experiment. The femtosecond TA spectra are dominated by the NC exciton bleach (XB, Figure 2a, red-color) resulting from the presence of electrons and holes in the NCs. We find that the XB feature is accompanied by two additional spectral components, compared to the oleate-capped NCs, in the visible region: (1) a broad photo-induced absorption (PIA) band centered at 450 − 550 nm (Figure 2a, blue-trace) and (2) two bleach peaks center of 602 and 655 nm (Figure 2a, red-trace), respectively. The XB peak


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at 602 nm is partially cancelled by excited state absorption and thus is more difficult to detect than the 655 nm peak. The broad PIA band overlaps with the XB, but the PIA decays on a much longer timescale (Figure 2c). The peak energy of the PIA is in close agreement with the reported triplet absorption (T1−Tn)42-45 of TIPS-Pc. The two peaks at 602 and 655 nm are in-line with the vibronic bands of TIPS-Pc and are indicative of a ground state bleach of the S0−S1 transition (Figure 2d). These spectroscopic features are clearly resolved in the representative 3 ns spectrum (Figure 2b, full TA spectra are in Figure S4), with the red-coloring represents bleach signals and the bluecoloring is for PIA features. The XB decays within 100 ns, while both the PIA and pentacene singlet bleach signals last for over 10 μs, confirming that these two additional components can be associated with the presence of TIPS-Pc triplet states. Additionally, TA spectra in the NIR region show two PIA bands centered around 850 and 966 nm (blue, Figure 2 a, c bottom), both of which match the previously observed triplet absorption bands of TIPS-Pc in the NIR2. Notably, the PIA peak at 966 nm appears to evolve separately from the broad PIA peak around 876 nm (Figure 2a, bottom). The PIA bands in the NIR also decay within several microseconds. The combination of positive triplet species and negative bleach species, along with their microsecond decay lifetime, clearly suggest that triplet states of TIPS-Pc are effectively produced via the direct excitation of CsPbBr3 NCs. One mechanism for the observed triplets is through triplet energy transfer, where the triplet states of the acene molecules are populated by the sensitizer (NCs in this case). Such a triplet energy transfer mechanism has been previously observed in acene-modified metal-chalcogenide NCs24 and successfully utilized for photon up-conversion.23 For instance, CdSe NCs with a similar bandgap (2.46 eV) undergo effective triplet energy transfer to 9-anthracenecarboxylic acid (ACA) or 1-pyrenecarboxylic acid (PCA), generating molecular triplets.24 As such, it is plausible that TIPS-Pc triplets are generated through a similar triplet energy transfer mechanism involving CsPbBr3 NCs. However, when we exchanged the native oleate ligands with TIPS-ACA or PCA ligands, where triplet energy transfer is thermodynamically allowed (but singlet energy transfer is not), no triplet states are observed (Figure S5). Therefore, the triplet formation in TIPS-Pc after excitation of CsPbBr3 NCs must proceed through a different mechanism. To further gain insight into the triplet generation mechanism at the perovskite-acene interface, below we conduct a detailed kinetic analysis of all components.


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Figure 2 | Transient absorption (TA) spectra of TIPS-Pc−/CsPbBr3 NCs pumped at 400 nm. Pseudocolor image of femtosecond (a) and nanosecond (c) TA spectra in visible and NIR regions. Red and blue color in pseudocolor images represent the magnitude of the reduced (bleach) and increased (photoinduced absorption, PIA) absorbance upon photoexcitation, respectively. Major spectra features are indicated by red and blue arrows which represent NCs exciton bleach (513 nm, red), TIPS-Pc ground state bleach (602 and 655 nm, red), TIPS-Pc cation PIA (876 nm, blue), and TIPS-Pc triplets PIA (450 − 550 nm, 966 nm, blue). b, Transient spectra taken at 3 ns. Likewise, here the red part of spectrum indicates bleach (negative), while the blue part of spectrum shows the PIA (positive). d, Static absorption spectrum of TIPS-Pc−/CsPbBr3 NCs overlapped with the TA spectra showing NCs exciton (513 nm) and TIPS-Pc singlets (602 nm and 655 nm).

Triplet formation kinetics. We first compare the decay kinetic of XB between oleate-terminated and TIPS-Pc-terminated CsPbBr3 NCs. Exchanged NCs (Figure 3a, red-trace) display a much faster XB decay, with ~33% of its amplitude decaying within 10 ps (exponential time-constant of 4.8 ± 0.2 ps). In contrast, oleate-terminated NCs show a negligible XB decay within 20 ps (Figure S4 shows the full TA spectra). Concurrently, we find that the rise of the TIPS-Pc singlet bleach (655 nm) can be described with a biexponential function, with time constants of 4.8 ± 0.7 and 360 ± 160 ps, respectively. The initial decay of the NC XB bleach and the initial rise of the TIPS-Pc bleach coincide with each other, suggesting that the initial ultrafast XB decay leads to the rise of the pentacene singlet bleach signals, i.e., the initial excitation leaves the NCs and arrives at the pentacene ligands. Furthermore, we observe a PIA in the NIR (~ 876 nm, Figure 3b) whose peak energy closely matches the reported NIR TIPS-Pc cation absorption.20 Kinetic fitting of the NIR PIA gives a single-exponential rise-time of 7.2 ± 0.5 ps (Figure 3c) in close agreement with the


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decay of XB. Therefore, we attribute the first-step of the photoinduced process to a ~ 5 ps hole transfer from the NCs to TIPS-Pc molecules. We also find that the hole transfer time is strongly dependent on the concentration of surface TIPS-Pc ligands, with more acene molecules per NC resulting in a faster hole transfer time (Figure S6). This effect may be related to the onset of intermolecular coupling between the ligands at higher concentrations that causes delocalization and stabilizes the charge-transfer state,46 although more information is needed to confirm this mechanism. A similar photoinduced hole transfer process has been previously reported from CsPbBr3 NC to phenothiazine (~ 50 ps)47 and to 1-aminopyrene (~ 120 ps)48, although in those cases no direct evidence of the oxidized organic cations were reported. Since the decay time of the singlet bleach is observed to be several microseconds, we hypothesize that the second-rise of the bleach (τ ~ 360 ps) is associated with triplet formation. During the SF process an additional ground state of a neighboring TIPS-Pc molecule is consumed as the singlet proceeds to two triplets residing on two adjacent and electronically coupled TIPS-Pc molecules. This results in an additional singlet bleach.49 We find that the second rise of the TIPS-Pc singlet bleach is coupled with the rise of another PIA in the NIR centered at 966 nm. As shown in Figure 3b, the PIA peak assigned to pentacene cations is clearly evolving into two separate peaks centered at 850 and 966 nm. These two PIA peaks are consistent with the reported absorption bands within the triplet manifold,2 and they decay on the microsecond (Figure S7) timescale. Although both peaks overlap with the pentacene cation absorption, we can extract the rise time for the triplets by examining kinetics at 966 nm where the triplet absorption dominates (Figure 3c). Bi-exponential fitting for the triplet PIA gives time constants of 3.5 ± 0.2 and 580 ± 43 ps, respectively. Combined with the formation of the ground state bleach, we conclude that pentacene triplets are formed sequentially after the ultrafast hole transfer with a time constant of 400 − 600 ps, which is likely associate with the subsequent electron transfer and SF. These photo-sensitized triplets dominate the ns TA spectra after 100 ns and decay on the 10 − 14 μs timescale as determined from both triplet PIA and singlet bleach kinetics (Figure 3d).


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Figure 3. TA spectra and kinetics of TIPS-Pc−/CsPbBr3 NCs pumped at 400 nm. a, Kinetics of transient response for CsPbBr3 with oleate ligand (OA) (black) and CsPbBr3 with TIPS-pentacene ligand (TIPS-Pc) (red) NCs probed at exciton bleach center of ~ 513 nm. Faster decay dynamics is observed for TIPS-Pc ligand. Yellow curve shows the TA kinetics of TIPS-Pc−/CsPbBr3 NCs at pentacene singlet bleach center of ~ 655 nm. The ultrafast rise (within 20 ps) dynamics of pentacene bleach matches the ultrafast decay of CsPbBr3 exciton. b, TA spectra of TIPS-Pc−/CsPbBr3 NCs probed from 800 nm to 1400 nm at 126 ps and 2.8 ns. Light blue and dark red dash lines indicate pentacene cation (876 nm) and triplet (966 nm) species, respectively. c, TA kinetics of pentacene cation (light blue) and triplets (dark red). Two-step rise of triplets can be clearly seen in the kinetics. d, TA spectra of TIPS-Pc−/CsPbBr3 NCs probed from 450 nm to 930 nm at different time delay (5.5 ns, 87 ns, and 14 μs). The spectra are dominated by the exciton bleach (513 nm) at early time (< 10 ns), followed by the PIA after 100 ns. Inset of Figure 3d shows the TA kinetics of pentacene singlet states (655 nm, yellow) and triplet states (545 nm, purple), both showing a triplet decay lifetime of around 10−14 μs.

Triplet formation mechanism and yield. Given that excited pentacene singlets undergo rapid SF, along with the observation of triplet formation after ultrafast hole transfer, we propose the following triplet formation mechanism (Figure 4). Upon photoexcitation, a 5 ps hole transfer occurs from CsPbBr3 NCs to surface pentacene molecules. The hole transfer is then followed by electron transfer to generate excited pentacene singlet states. As such, the energy transfer from perovskite NCs to pentacene is through a rapid Dexter-like energy transfer rather than a resonant


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Förster energy transfer mechanism.50 Excited surface pentacene molecules subsequently undergo efficient SF on the surface of the NCs. We find that molecular triplets are only generated in CsPbBr3 NCs when there is a high coverage of TIPS-Pc, whereas no triplets are observed for partial exchanged NCs (Figure S6). This concentration-dependent triplet yield supports our proposed SF mechanism rather than direct triplet energy transfer from NCs, as previously reported by Mase51, because effective SF requires a close intermolecular distance between pentacene molecules.2, 52 The time scale of electron transfer and subsequent SF is estimated to be 400 − 600 ps (vide supra). Since we only observe a small population of singlets in the NIR (center ~ 1350 nm) in fullyexchanged NCs as compared to partial-exchanged NCs (Figure S8), we assign the electron transfer as the rate-limiting step with a time constant of 400 − 600 ps, whereas the SF occurs quickly (< 100 ps) after the electron transfer. In the case of partial-exchanged NCs, a singlet state population appears following the electron transfer, yet decays concurrently with the pentacene cation species even though no SF is observed (Figure S8). It is likely that the transferred electron residing in the singlet state (S1) of TIPS-Pc can transfer back to the NC conduction band on a timescale similar to the forward electron transfer due to the near resonance of these two states (Figure 1b). This quasi-equilibrium leads to the coordinated decay of cation and singlet exciton features that is disrupted when SF becomes fast in fully exchanged samples. The quasi-equilibrium allows the electron transfer to sample multiple TIPS-Pc molecules so that in the fully exchanged QDs the electron transfer can always find the molecule that had previously been oxidized resulting in energy transfer and subsequent SF. Lastly, we seek to estimate the triplet formation quantum yield of the sensitization process. Based on the number of triplets and absorbed photons, the triplet yield ΦT is determined to be 113% (detailed calculation can be found in SI). Using the literature-reported absorption coefficients for TIPS-Pc bleach and triplets,43 we calculate the quantum yield of SF, ΦSF to be 145 % (SI). The quantum yield of SF can be further confirmed by comparing the rise amplitude of pentacene bleach states (Figure 3a). If we assume that the initial rise of singlets is due to hole transfer and the secondary rise is ascribed to SF from the localized singlets,49 then the SF quantum yield can be estimated as 144 %, which is in excellent agreement with the results calculated based on the absorption coefficients. Since the triplet formation is mediated through a singlet energy transfer and the subsequent SF (ΦT = ΦEnT × ΦSF), the quantum yield of the singlet energy transfer ΦEnT is estimated to be 79 %. It is well-known that SF shows a strong dependence on the intermolecular distance in solution because of the bimolecular excimer mechanism2. For instance, Friend and co-workers find that in order to achieve over 100 % SF yield in solution, the pentacene intermolecular distance needs to be less than 8 nm, where the molecular concentration should be on the order of 10−2 M.2 However, our absorbance data shows that the concentration of TIPS-Pc in solution is only ~10−4 M. Therefore, we hypothesize that the NC surface provides a unique cooperative platform for pentacene molecules, forming self-assembled pentacene monolayers. Based on the absorption spectra and extinction coefficients, ~ 427 TIPS-Pc molecules are bound on the surface of each NC. We estimate the surface pentacene intermolecular distance to be as close as ~ 1 nm (SI). By such a self-assembly approach, the intermolecular distance is significantly reduced, favoring SF on the


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NC surface. However, the SF yield is still lower than expected based on such a small intermolecular distance. The lower SF yield can be a result of several factors. (1) The binding between CsPbBr3 NC and TIPS-Pc can be relatively dynamic and may not be effectively coupled with other TIPS-Pc molecules. (2) The electron and hole transfer steps are likely not highly coherent and coordinated, thus the secondary electron transfer step does not necessarily occur at the same TIPS-Pc that underwent the hole-transfer. Fortunately, the electron can easily transfer back to the NC and therefore sample many TIPS-Pc surface molecules. When it finds the oxidized molecule, it undergoes irreversible SF. This dynamic is likely interpreted by defects states and degrade the ultimate SF yield. (3) Finally, as compared to the conventional engineering to selfassemble pentacene molecules with the appropriate solid-state order that optimizes the SF process, the NC surface may not provide a perfect platform for assembling precisely-coupled TIPS-Pc molecules. For instance, it has been shown that a balanced intermolecular coupling is essential for high SF efficiency.53 An ideal intermolecular coupling should be strong enough to initiate the fast and efficient first step, yet weak enough to maintain the independent triplet behavior. Molecular engineering of the TIPS-Pc molecules should allow for more ideal packing on the NC surface to further enhanced the SF yield.

Figure 4. Illustrative mechanism (Jablonski diagram) of triplet sensitization from CsPbBr3 NCs. Surface TIPS-Pc molecules are sensitized by CsPbBr3 NCs through a Dexter-like electron exchange mechanism. First, excitation of NCs induces an ultrafast hole transfer to TIPS-Pc (5 ps), which is followed by an electron transfer process (400 − 600 ps), generating an excited singlet state of TIPS-Pc. Upon the completion of energy transfer, singlet fission is then triggered in less than 100 ps. The lifetime of the triplet is around 10 − 14 μs.

In contrast to the mechanism observed here, Hasobe and co-workers54 observe an initial singlet energy transfer from CdSe/ZnS NCs to surface pentacene dimers which is proposed to follow a Förster mechanism. Since photoexcited excitons in CdSe NCs can also undergo efficient triplet energy transfer to surface acenes,24 the decoupling between these competitive photoinduced processes can be challenging. Dexter-like energy transfer from the lowest NC exciton state, which is of mixed singlet/triplet character, is the only relevant mechanism to produce a triplet on the organic ligand, which would circumvent the SF process. We attribute the low efficiency of direct triplet energy transfer from CsPbBr3 NCs to surface-anchored organics to the poor wavefunction overlap between these two phases and the ‘bright’ triplet character from perovskite NCs. Förster


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energy transfer in our system is also inhibited by the low amplitude of the overlap integral between singlet emission of the NC and ground state absorption of the ligand (Figure S9). We see no evidence of the presence of direct energy transfer, and we rationalize this conclusion by the expected slow rate constants (< 20 ns−1) as compared to hole transfer.

Conclusion Our results demonstrate that molecular triplets can be effectively generated in solution by singlet energy transfer and subsequent SF. The singlet energy transfer is mediated through a Dexter-like electron exchange mechanism at the CsPbBr3/pentacene interface, with an initial ultrafast hole transfer process followed by electron transfer from NC to TIPS-Pc. Excited TIPS-Pc singlets can then undergo efficient SF on the NC surface due to the sufficient intermolecular coupling provided by dense surface binding. Our strategy can improve the SF efficiency based on the following: first, the high absorption coefficient of NCs can significantly increase the light absorption efficiency in the solar spectrum; second, compared to the commonly-observed sensitization from NCs via triplet energy transfer, our strategy can produce molecular triplets with a 200 % quantum yield based on the excitation of the NCs if the quantum yields of singlet energy transfer and SF are both unity. Lead halide perovskite NCs appear to be an ideal system for such an application as they can effectively decouple the photoinduced singlet and triplet energy transfer with SF molecules. As such, our approach provides a new paradigm for constructing relevant optoelectronic devices and solar energy conversion systems.

Methods General methods. All sample preparations were performed using standard air-free techniques on a Schlenk line under nitrogen atmosphere or in a nitrogen-fill glovebox. Materials. All chemicals were used as received unless otherwise indicated. Anhydrous toluene (99.5%), anhydrous methyl acetate (MeOAc, 99%), anhydrous dichloromethane (DCM, ≥ 99.8%), anhydrous chloroform-d (CDCl3, ≥ 99.8%) were obtained from Sigma Aldrich. TIPS-pentacene-COOH (TIPS-Pc) were synthesized according to previously reported procedures55. Oleate-capped CsPbBr3 NCs, OA−/CsPbBr3 synthesis. Oleate-capped OA−/CsPbBr3 NCs were synthesized based on a procedure adapted from the literature33. Briefly, 0.5 mmol of cesium acetate (96 mg) and 1 mmol of lead acetate (380 mg) were placed in a three-neck flask with 4 mL of oleic acid and 10 mL of 1-ODE. The mixture was kept under vacuum at 100 °C for 30 min to dissolve the salts, forming CsOA and PbOA. The flask was then switch to constant N2 flow and the temperature was reduced to 75 °C by adding 40 mL of anhydrous toluene. We found that it is essential to keep the precursor concentration sufficiently low in order to initiate the nucleation of CsPbBr3 NCs. 1 mmol of tetraoctylammonium bromide (TOABr, 0.5 g) was dissolved in 40 mL anhydrous toluene and was then rapidly injected into the reaction flask at 75 °C. The reaction was quenched after 5 seconds by removing the heat and the reaction was allowed to cool by air. The reaction flask was then switched to vacuum for 30 min to remove excess toluene before bringing into a N2-filled glovebox. The green CsPbBr3 NCs was purified by one round of precipitation/centrifugation/redissolution using anhydrous MeOAc and toluene. The as-prepared


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OA−/CsPbBr3 NCs were finally dispersed in 6 mL of anhydrous toluene. The NC concentration of stock solution is generally 5 − 9 μM. Complete ligand exchange of CsPbBr3 NCs with TIPS-Pc molecules. In a nitrogen-filled glovebox, 0.5 mL of OA−/CsPbBr3 NCs (~ 8 μM) stock solution was first mixed with 1.5 mL DCM. To the stirred solution of OA−/CsPbBr3, 0.5 mL of TIPS-Pc stock solution (6.4 mM, ~ 800 ligands per NC) was added. The mixed solution was allowed to stir for 30 min, followed by precipitation with MeOAc (3 mL) and redissolution in DCM. The exchange process was repeated for another round. The final exchanged TIPS-Pc−/CsPbBr3 NCs were redispersed in DCM. Partial ligand exchange of CsPbBr3 NCs with TIPS-Pc molecules. In a nitrogen-filled glovebox, OA−/CsPbBr3 NCs in DCM solution were prepared in the same manner as the complete ligand exchange procedure. To the stirred solution of OA−/CsPbBr3, 50 μL, 100 μL, and 300 μL of TIPS-Pc stock solution (corresponding to 80 (TIPS-Pc1), 160 (TIPS-Pc2), and 500 (TIPS-Pc3) ligands per NC, respectively) was added. The mixed solution was then allowed to stir for 30 min, followed by precipitation with MeOAc (3 mL) and redissolution in DCM. Steady-state experiments. Static absorption spectra were measured with a Cary 50 UV-vis spectrophotometer. Steady-state PL spectra were recorded with a Horiba Jobin Yvon Model Fluoro Max4. A monochromatized Xe lamp was used as the excitation source. For determination of the PLQY of CsPbBr3 NCs in DCM, comparison was made with a solution of Rhodamine 6G in ethanol (PLQY = 0.94). 1 H NMR spectroscopy was performed on a Bruker Avance ІІІ 400 MHz instrument. Spectra were collected at 25 °C using a standard proton pulse (zg), 32 scans, and 30 second delay between scans to complete relaxation between pulses56. NC/ligand samples were dried under vacuum, resuspended in 700 μL CDCl3, and transferred to J. Young tubes. FTIR spectra were collected using the disuse reflectance attachment for a Bruker Alpha FTIR spectrometer inside of glovebox. Spectra were collected by averaging 24 scans at 2 cm−1 resolution. Time-resolved PL and transient absorption experiments. Time-resolved PL was measured using a supercontinuum fiber laser (Fianium, SC-450-PP) operating at 5 MHz as the excitation source. The excitation wavelength was chosen as 450 nm using an acousto-optic tunable filter and attenuated to a pulse energy of approximately 0.1 nJ. A streak camera for detection in the range of 400 – 900 nm (Hamamatsu C10910-04) was used to detect time-resolved spectra. The instrument response function for a time range of 0-20 ns is approximately 200 ps. The region near the peak sample emission was integrated to produce the emission decay curves. Decays were fitted using a biexponential function convoluted with an instrument response function, which was measured by scattering the excitation beam from the NC solution. Transient absorption spectra were collected using a Coherent Libra Ti:sapphire laser, with an output of 800 nm at 1kHz. The 800 nm beam was directed into a TOPAS optical parametric amplifier to generate pump pulse (~150 fs) and was modulated at 500 Hz through an optical chopper to block every other laser pulse. Femtosecond TA spectra were collected using Helios spectrometer (Ultrafast Systems). A small amount of 800 nm light was used to pump a sapphire crystal to create 450 – 750 nm probe light for Vis TA or a 1 cm thick sapphire crystal to generate 750 – 1500 nm probe light for NIR TA. Nanosecond TA spectra were acquired using EOS Systems. The probe beam is derived from EOS system and was electronically delayed respect to pump laser pulse. The probe beam generated was a broadband UV-vis (400 – 900 nm) or NIR spectrum (800 – 1600 nm). All samples were prepared in a quartz cuvette (width 0.2 cm) with ODs of ~ 0.5 at the exciton peak under N2 atmosphere.

Associated Content Supporting Information. NCs characterization (XRD, FT-IR, NMR, TEM), raw TA spectra and kinetics analysis, detailed calculation of triplet formation and singlet energy transfer quantum yield.

Notes


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The authors declare no competing financial interest.

Acknowledgement We gratefully acknowledge support for nanocrystal synthesis, ligand exchange, and characterization of energy transfer from the Center for Hybrid Organic Inorganic Semiconductors for Energy (CHOISE) an Energy Frontier Research Center funded by the Office of Basic Energy Sciences, Office of Science within the U.S. Department of Energy through contract number DE-AC36-08G028308. Work on spectroscopy and mechanisms of singlet fission was supported by the Solar Photochemistry Program funded by the Office of Basic Energy Sciences, Office of Science, Division of Chemical Sciences, Geosciences, and Biosciences. Organic Synthesis was supported by NSF (DMREF-1627428). The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.

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