Semiconductor Nanocrystal Light Absorbers for Photon Upconversion

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Semiconductor Nanocrystal Light Absorbers for Photon Upconversion Zhiyuan Huang, and Ming Lee Tang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02154 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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The Journal of Physical Chemistry Letters

Semiconductor Nanocrystal Light Absorbers for Photon Upconversion Zhiyuan Huang, Ming Lee Tang* Department of Chemistry, University of California, Riverside, Riverside, CA, 92521, United States. Abstract Semiconductor nanocrystals (NCs) can initiate energy and charge transfer in multiple applications with their unique optical and electronic properties. In particular, NCs are excellent light absorbers for initiating triplet energy transfer (TET) to organic molecules, a key step in triplet-fusion based photon upconversion. Triplet energy transfer across this inorganic-organic interface is one of the bottlenecks that currently limits the overall photon upconversion quantum yield. In this Perspective, we summarize the progress made in the past three years on this hybrid photon upconversion platform. We discuss the effects of NC size, composition and surface states on TET. Nanocrystal surface engineering may address the loss mechanisms arising from defect states and exciton-phonon coupling. Alternative materials for NC triplet photosensitizers that do not contain toxic heavy metals will be especially useful for various biological applications. TOC Graphic

Semiconductor nanocrystals (NCs) are a class of materials that combine features found solely in the bulk or only in organic molecules. As a result of this unique combination, i.e., the high density of states compared to organics and quantum-confined wavefunctions; excitons, or tightly bound excited states can be created in NCs. In terms of energy conversion, this is particularly interesting, as it makes NCs amenable to multi-excitonic processes that may be able to circumvent hot carrier relaxation or address sub-bandgap losses in solar cells. For example, in certain geometries and materials, a lower threshold excitation density for multiple exciton generation (MEG) is observed at the nanoscale.1-3 Multi-excitonic process that up- or downconvert photons can be used to surpass the Shockley-Queisser limit. It would be ideal to use molecules and semiconductor NCs for this purpose. As a practical matter, both these two classes of materials are generally earth-abundant, have high absorption coefficients and can potentially be deposited via a roll to roll printing process to fabricate inexpensive flexible optoelectronic devices. In order to promote exciton splitting and charge collection, it is perhaps strategic to focus on spin-triplet excitons. The microsecond lifetime in spin-triplet excitons may allow diffusion to compete with recombination, e.g. in bulk heterojunctions. Indeed, in the past four years, since the pioneering reports of Baldo and Bawendi, Rao and Friend, a number of reports have focused on spin triplet exciton transport from molecules to NCs4-5 and in the reverse direction.6-13 This Perspective focuses on designing semiconductor NCs as triplet photosensitizers for photon upconversion. Photon upconversion is defined as the conversion of two or more low energy photons to

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one high energy photon. Photon upconversion can remedy transmission losses in solar cells. For example, the overall power conversion efficiency can be boosted to 43% for a solar cell with a bandgap of 1.78 eV, surpassing the Shockley−Queisser limit of 34%.14 In biological imaging, an upconversion probe excited with near-infrared (NIR) light may allow for deep tissue imaging while minimizing tissue autofluorescence and damage. Compared to the heavily studied lanthanide based upconversion probes15-16 that have forbidden optical transitions, low absorption cross sections and thus need high excitation intensities, photon upconversion based on the fusion of molecular triplet states17-18 has been demonstrated with diffuse sources of light comparable to the solar flux. Photon upconversion based on triplet-triplet annihilation (TTA) typically requires orders of magnitude less intense excitation compared to that required for the lanthanides. In TTA-based photon upconversion, the energy contained in photons absorbed by the triplet photosensitizers is transferred to molecular annihilators. When two annihilator molecules in their triplet excited state collide in a spin-allowed, energy conserved manner to form one molecule in the ground state, and one molecule in its first excited singlet state, a high energy photon is produced from two original lower energy photons absorbed. Like organic chromophores, NCs are excellent light absorbers. In contrast to molecules, NCs can be tuned to absorb across the entire solar spectrum. Organic molecules that absorb strongly in the NIR typically suffer from fast internal conversion and high rates of non-radiative recombination as described by Jortner’s energy gap law.19 If spin-triplet excitons could be shuttled seamlessly between NCs and molecules, then semiconductor NCs can be paired with any molecule annihilator to harvest the NIR wavelengths (700- 2500 nm) which comprise 53% of the solar spectrum. Since 2015, building on reports of triplet excitons migrating from tetracene and pentacene to lead chalcogenide NCs after singlet fission, at least four different research groups reported the triplet energy transfer in the reverse direction, from semiconductor NCs to molecules. Figure 1 presents the first published example by Huang et al6 of greento-violet and NIR-to-yellow photon upconversion employing NC triplet photosensitizers in the place of precious metal porphyrins. Here, the CdSe and PbS NCs are excited by a 532 nm and 808 nm continuouswave laser. Note that in the absence of a transmitter ligand anchored on the NC surface, photon upconversion QYs are low (Fig. 1a, the picture on top shows the scatter of the green laser used for excitation and green PL from CdSe NCs with no visible violet light produced). However, when the 9anthracenecarboxylic acid (9-ACA, red) transmitter is bound on the NC surface, the photon upconversion QY increases by up to three orders of magnitude to 15% because of the enhanced orbital overlap between NC donor and molecular acceptor.6 This promotes Dexter transfer to the 9,10-diphenylanthracene (DPA, yellow) annihilator, where TTA results in the emission of a high energy violet photon from two incoming green photons. Similarly, in NIR upconversion sensitized by lead chalcogenide NCs (Fig. 1b), the surface bound tetracene transmitter 5-tetracenecarboxylic acid, 5-CT, enhances upconversion QYs by 81 and 11 times for PbS and PbSe NCs respectively.12 Wu and Baldo have successfully fabricated thin films where PbS NC light absorbers in combination with a rubrene triplet transporting layer converts NIR to visible light with a DBP emitter.7, 20 Transient absorption and temperature dependent experiments conducted by Mongin and coworkers have shown that the triplet excitons created on molecular acceptors by NC photosensitizers distribute themselves according to Boltzmann statistics depending on the excited state energy levels in the organic and inorganic components.21 Unfortunately, the fact strong absorption by semiconductor NCs in the visible and UV regions is a double edge sword. This parasitic reabsorption can be mitigated by optical engineering, e.g. applying resonant,22 or keeping the NC layer as thin as one or two monolayers to minimize the reabsorption losses.7, 20


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Figure 1. Energy diagram and photos of photon upconversion with nanocrystal light absorbers (a) CdSe nanocrystal (NC) sensitized green to violet upconversion. The 9-anthracenecarboxylic acid (9-ACA) transmitter and 9,10-diphenylanthracene (DPA) annihilator are represented by the red and yellow balls. When excited with green light (532 nm) in hexanes at RT, CdSe (top) and CdSe/9-ACA (bottom) have photon upconversion QYs of 0.03% and 15% respectively. (b) Near-infrared to yellow photon upconversion with PbS NCs as photosensitizer, 5-tetracenecarboxylic acid (5-CT, magenta balls) as transmitter, and rubrene (blue balls) as annihilator in toluene at RT. The picture shows the upconverted rubrene emission excited by 781 nm laser. Reproduced from ref. 6. Copyright 2015 American Chemical Society. Fundamental understanding of the electronic coupling required for triplet energy transfer between the organic transmitter and the inorganic NC is lacking. A marked asymmetry in rates of triplet transfer has been reported, with sub-picosecond rates in the direction from acene to NC23-24, and transfer rates of hundreds of nanoseconds for the reverse direction from NC to acene8, 21, 25-27 (Fig. 1), even for systems where the exchange is strongly exothermic, exceeding 0.5 eV. The primary steps that enable this energy transfer remain a topic of debate. The problem is exacerbated by the difficulty in characterizing the interface where the organic transmitter molecules are anchored on semiconductor NCs. A thorough understanding of triplet energy transfer across this interface is required to eliminate this bottleneck for efficient photon upconversion. In this Perspective, we will summarize recent work on spin-triplet exciton transfer between NCs and surface bound molecules, focusing on the inorganic semiconductor. The effect of NC size, composition, surface states and exciton-phonon coupling on triplet energy transfer will be discussed. We conclude with suggestions of alternative materials for NC triplet photosensitizers. Like our Perspective on designing molecular transmitters for photon upconversion in this hybrid system,28 we hope that this article will stimulate efforts addressing the organic-inorganic interface to both promote understanding and improve the yields of energy transfer.

Figure 2. The rate of triplet energy transfer (TET) is exponentially dependent on the distance between nanocrystals (NCs) and molecules. (a) The relationship between the time constant for TET, τTET, from a



monolayer of PbS NCs to rubrene in thin film versus the length of aliphatic carboxylic acid ligands on the NCs. (b) The rate of TET drops exponentially as the distance NC donor and acene acceptor is increased by (b) rigid phenylene bridges between CdSe and anthracene in hexanes and (c) a CdS shell between PbS and rubrene in toluene. All measurements were at RT. (a) (b) and (c) are reproduced from ref 29, 10, and 9 respectively with copyright 2017, 2016 and 2016 to American Chemical Society respectively. At least three distinct NC-molecule donor-acceptor systems display an exponential dependence on triplet energy transfer as function of distance, both in solution and thin film. Nienhaus et al.29 studied triplet energy transfer from a single monolayer of PbS NCs to rubrene. As the length of the aliphatic acid bound on the surface of PbS NCs increased, the rate of triplet energy transfer decreased exponentially when ligands with more than 10 carbons were used (Figure 2a). They showed that hexanoic acid ligands are short enough for a near-unity transfer efficiency, and further decreasing the ligand length did not improve TET. Li et al.10 studied TET from CdSe NCs to surface bound anthracene with intervening phenyl bridges. The distance between CdSe NCs and anthracene are controlled by the length of rigid phenylene bridges in between the CdSe donor and anthracene acceptor. The rate of triplet transfer was exponentially dependent on the length of phenylene bridges (Figure 2b). Mahboub et al.9 also observed that the rate of TET from PbS NCs to rubrene increased exponentially with a linear decrease of CdS shell thickness (Figure 2c). In this system, due to the fact that excitons are localized within the PbS core, the CdS shell serves as a barrier for TET. All these results are consistent with a Dexter-type mechanism describing triplet energy transfer from NCs to molecules. The quantum confined excitons in semiconductor NCs means that smaller NCs with a larger bandgap have a larger driving force for transferring spin-triplet excitons from the bandedge to molecular acceptors. Photon upconversion is enhanced using small NCs as triplet photosensitizers. Huang et al.30 synthesized CdSe NC light absorbers with diameters varying 2.7 to 5.1 nm (Figure 3a). With 9-ACA as the transmitter and DPA as the annihilator, the photon upconversion QYs dropped from 11.0% to 0.26% with an increase of NC size. A similar size dependence is observed in the PbS(Se) NC sensitized NIR upconversion system. As shown in Figure 3b, Mahboub et al.13 showed that upon decreasing the diameter of PbS NCs from 3.5 to 2.9 nm, and PbSe NC diameter from 3.2 to 2.5 nm, photon upconversion QYs were enhanced 700 and 250 times respectively. This is also true in thin film, where Wu et al.7 reported the photon upconversion QYs with PbS NC photosensitizers were the highest with the smallest NCs. PbS NCs with absorption maxima at 850, 960 and 1010 nm produced thin films with photon upconversion QYs of 1.2, 0.51 and 0.21% respectively.

Figure 3. Triplet energy transfer from nanocrystals (NCs) to molecules is size dependent. The relative upconversion QYs with (a) CdSe NC and (b) PbS(Se) NCs of different sizes. In (a), the black crosses and red squares represent excitation wavelengths of 488 and 532 nm respectively. (c) illustrates the thermally activated delayed photoluminescence processes in CdSe-pyrene system. (a) (b) and (c) are reproduced from ref 30, 13, 21 respectively. The spin-triplet exciton may delocalize between the NC donor and molecular acceptor when the energetics of the lowest triplet excited state in both the organic and inorganic are close. Mongin et al.21 showed this back transfer in their study on CdSe NCs with surface bound pyrene (Figure 3c). After


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excitation of the CdSe NCs and then TET to surface bound pyrene, the triplet exciton can transfer back to the CdSe NCs when the mismatch between NC’s bandedge and pyrene’s first excited triplet state (2.00 eV)31 is less than 0.54 eV. This increases the nanosecond radiative lifetime of the CdSe NCs to milliseconds and even microseconds. La Rosa32 reported the delayed photoluminescence (PL) of CdSe NCs due to back transfer in the same system. This delayed PL was quenched in air, suggesting triplet excitons are involved. In related work, Kroupa et al.24 observed that when exciting PbS NCs, the triplet exciton is transferred back and forth between PbS NCs and surface-anchored TIPS-tetracene when the exciton energy of NCs (1.45 and 1.18 eV) are similar to or larger than the triplet energy of TIPS-tetracene (1.25 eV). This allows the triplet lifetimes of surface-anchored TIPS-tetracene to be tuned by both the surface coverage of this molecule and NC size. Surface defects can greatly affect TET from semiconductor NCs to surface anchored molecules. The role of these defects in TET is complicated. On one hand, surface trap states can induce charge transfer from semiconductor NCs to molecules. This competes with TET and thus reduces the efficiency of TET, ΦTET. Ultrafast hole transfer, a loss pathway for the NC’s photoexcited state, can be suppressed by growing CdS shells on PbS core. This passivates the NC’s surface traps and increases the photon upconversion QYs by a factor of 1.4.33

Figure 4. (a) As CdS is adsorbed on the PbS nanocrystals (NCs), the NC photosensitizer emission linewidth increases in tandem with the photon upconversion QY, suggesting adsorbate induced mid-gap states participate in triplet energy transfer (TET) (b) Energy diagram of triplet energy transfer from PbS NCs to surface bound TIPS-pentacene (2-CP). (c) A potential transition state in TET involving the surface of PbS NCs predicted by constrained DFT calculations. The spin density associated with the spin-triplet state is in green. (a) is reproduced from ref 34. Copyright 2018 American Chemical Society. (b) and (c) are reproduced from ref 35. Copyright 2018 American Chemical Society. The nature of the excitonic state(s) on the NC responsible for coupling with molecular triplet states has not been experimentally determined. In order to conserve spin, triplet transfer should arise from the dark excitonic states at the band edge that have triplet character. However, defect states, which stem from the non-stoichiometric composition of NCs and dangling bonds on the surface, may very well possess sufficient triplet character necessary for sensitizing spin un-paired molecular excitonic states. We have recently shown that PbS-ZnS and PbS-CdS core-adsorbate NCs with sub nanometer shell thicknesses have enhanced photon upconversion QYs by 700 and 325 times respectively despite the fact that both the radiative and non-radiative recombination rates decrease and NC PL remains about the same34. Significantly, as shown in Fig. 4a, the PL linewidths follow the trend in the photon upconversion QYs, where the core-adsorbate NCs with broader PL maxima compared to the original PbS core give the highest photon upconversion QY, up to 0.25%. While these submonolayer Cd and Zn shells may chemically passivate certain trap states, other defects introduced in their presence enhance triplet energy transfer and thus the photon upconversion QY upon irradiation with cw NIR light. Our work suggests that thermally accessible mid-gap states created by surface bound adsorbates contribute to molecular triplet sensitization by NCs34.



Separately, in collaboration with Bender, Raulerson and Roberts, we have observed that competition between charge trapping and triplet energy transfer yields the large difference in rates between acene-toNC and NC-to-acene energy transfer.35 Using ultrafast transient absorption experiments to investigate the excited state dynamics of PbS NCs chemically functionalized with TIPS-pentacene ligands (2-CP) (Fig. 4b), we find photoexcitation of PbS does not directly result in energy transfer to 2-CP. Rather, we find the formation of a chemical intermediate within a few tens of picoseconds that decays over hundreds of nanoseconds to yield 2-CP triplet excitons (Fig. 4b,c). Analysis of spectra belonging to this intermediate suggests it results from states that localize charge carriers at PbS surfaces. Importantly, we find that saturating these states results in a significant speedup in 2-CP triplet exciton production, indicating these states slow NC-to-acene energy transfer by trapping carriers at surface sites. This suggests that careful control over the surface structure of semiconductor NCs should improve the rate and yield of NC-toacene energy transfer.

Figure 5. The emission from the surface states of semiconductor nanocrystals can be explained by models invoking (a) deep traps, (c) classical electron transfer, and (e) semiclassical electron transfer. The simulated photoluminescence spectra at different temperatures from models (a), (c) and (e) are shown in (b), (d) and (f) respectively. Reproduced from ref 36. Different models have been used to describe the behavior of the surface states on NCs.36-37 The observation of broad PL red-shifted from the bandedge is typically assigned to NC surface state emission. Additionally, temperature dependent experiments reveal that the relative contribution of surface PL increases for both CdSe and CdS NCs when temperature decreases from 300 to 50-100 K.36, 38-39 The deep trap model (Fig. 5a) assigns the broad, red-shifted emission to mid-gap surface states with a broad energetic distribution. However, since the position of the surface PL relative to the core is usually an order of magnitude larger than thermal energy (kBT), no temperature dependence is expected (Fig. 5b), contradicting experiment. Jones and Scholes recently used classical Marcus electron transfer theory to


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explain the lifetime of CdSe/CdS/ZnS NCs at different temperatures.40 Basically, charge carrier trapping by surface defects is modelled as electron transfer, as shown in Fig. 5c. This model requires the energy of the surface states to be lower than but within kBT of the bandedge. While classical Marcus theory explains the temperature dependent surface PL intensity, this model cannot explain the broadening and redshift of the surface PL more than 10kBT from the core emission (Fig. 5d). In contrast, a semiclassical electron transfer model that includes coupling between core and surface states to phonons better describes the observation of surface PL than deep trap model and classical Marcus model. As shown in Figure 5e, f, the classical charge transfer mode (solid line) explains the temperature dependence of surface PL, and the quantized phonons (dashed line) explains the broadening and redshift of surface PL. The coupling of excitons with phonons within NCs dissipates the excited state and reduces the efficiency of triplet energy transfer. In the past, a phonon-bottleneck41-42 was hypothesized where electron-phonon coupling in nanocrystals is suppressed because the discrete electronic density of states may slow intraband relaxation. However, it has been shown that electron-phonon coupling in PbSe and CdSe NCs is strong and the rate of energy dissipation can be as high as 1 eV/ps.43-46 Therefore, it is important to understand phonons in nanomaterials in order to rationally design and synthesize materials for TET. Using inelastic neutron scattering and ab initio molecular dynamics simulation, Wood and coworkers47 showed that in PbS NCs, electron-phonon coupling is enhanced due to surface softness relative to the bulk. This work suggests that surface engineering may be able to tune the mechanical properties of NCs and control electron-phonon coupling. Cui et al.48 showed that the electron-phonon coupling in CdSe NCs can be controlled with shell growth. Employing single NC emission linewidth as a proxy of the degree of electron-phonon coupling, they showed that the growth of ZnS on CdSe NCs decreases electron-phonon coupling, while the opposite is observed with a CdS shell. With this idea, inorganic shells were grown on CdS,49 CdSe50 and PbS34 NC triplet photosensitizers for visible-to-ultraviolet, visible-to-visible and NIRto-visible photon upconversion respectively. After optimizing shell thickness and transmitter density on the NC surface, the ZnS shell was found to enhance the photon upconversion QYs by 10, 1.6 and 1.4 times for CdS, CdSe and PbS NC cores respectively. Semiconductor II-VI NCs composed of one material usually have photoluminescence quantum yields (PLQYs) less than one. Non-radiative decay pathways like electron-phonon coupling, exciton-phonon coupling and Auger recombination are the typical loss mechanisms. Within a single NC, Auger recombination can occur either when a biexciton is formed, or at trap sites. When one of the biexcitons recombines, energy can be transferred to the other, creating a hot charge carrier that may be ejected from the NC, thus turning the PL off. Alternatively, Auger recombination may occur when either the excited hole or electron comprising the exciton is trapped, with concurrent energy transfer to the opposite charge carrier. Since these traps likely result from dangling bonds or unpassivated sites at the interface, the field has focused on alleviating the defects at this interface for photovoltaic or light emission. Auger recombination can be decreased in PbS NCs with perovskite shells, and in CdSe with an alloyed shell. Due to the band alignment of methylammonium lead iodide (MAPbI3) perovskite with PbS, PbS/MAPbI3 NCs have a decreased rate of Auger recombination51 and a larger depletion depth of 120 nm as unintentional doping is removed.52 Alloying at the CdSe-CdS core-shell interface creates a graded transition that smooths the local confinement potential which suppresses both types of Auger decay arising from biexcitons or charged trap states.53-54 Alloying also controls the population of the two lowest exciton states in ZnSe- CdS core-shell NCs.55 Interestingly, CdSe-CdS core-shell particles show enhanced intraband PL in the mid-infrared with thiols, but not with inorganic ligands.56 In terms of synthetic advances, mixed carboxylate ligands were used to epitaxially deposit CdS on CdSe to control size, shape and PLQY57-58 to obtain non-blinking, optically monodisperse, high PLQY dots. To improve NC photovoltaics, various groups have employed chloride treatments on lead sulfide NCs with differing counter-cations. In general, the adsorbed chloride does increase the NC thin film solar



cell’s fill factor, short circuit current, and power conversion efficiencies by removing electron trap states.59-60 However, in our hands, chloride terminated PbS nanocrystals61 do not result in any photon upconversion. Separately, a CdS layer on PbS NCs grown by cation exchange improves PCE by decreasing recombination and decreasing electron trap density.62-64 A thick shell is expected to create a Type I electronic structure, while a monolayer shell a quasi-type II band alignment for PbS-CdS coreshell materials.65 However, when the CdS is deposited colloidally at room temperature using a sulfide precursor, then the PbS core emission decreases, despite forming a coherent crystal structure.66 When the CdS shell is in the rock salt phase, the PbS core is the brightest.67 Similarly, for giant PbSe-CdSe coreshell structures where the first layer of CdSe is grown by cation exchange, only cubic NCs are photostable and bright.68 The Sargent group found that thionyl chloride69 was more effective that trimethylsilylchloride70 in creating a passivating chloride layer which improved QD mobility, reducing the turn-on voltage, resulting in a brightness of 460,00 cd/m2, the current record for solution processed light emitting diodes (LEDs). They have also shown that in thin inorganic shells useful for CdSe NC LEDs, the first thin CdS shell reduces undesired absorption, and the second thin ZnS shell induces both chemical passivation and quantum confinement.71 Silicon in the bulk is an indirect gap material with a low absorption cross-section. This is because the conduction band minima and valence band maxima reside at the X and Γ point of the Brillouin zone respectively. In nanosized crystallites, direct recombination at the Γ-point (3.32 eV) may be enabled by perturbations to the bulk band-edge electronic wave functions. Physical perturbations arising from surface curvature, broken translation symmetry, interfacial discontinuities, geometrical distortions, or hybridization with surface ligands will broaden possible k-space distributions. Compared to bulk silicon, silicon nanoparticles are strongly emissive, with PLQYs as high as 50% reported72-82. Experimentally, it has been shown that the PL of silicon nanoparticles is strongly affected by their surface chemistry, e.g. silicon nanoparticles terminated by amines and aliphatic carbons emit in the blue and red respectively83-85, regardless of size. This blue emission is attributed to charge transfer states on silicon induced by the high reduction potential of amines, an assignment supported by transient absorption measurements84-85. Oxygen related surface states red-shift silicon nanoparticles86-87. Despite the controversy in the literature about the physical origin of the fast and slow components comprising the excitonic states in silicon nanoparticles, it is clear that depending on the energy level of lowest molecular triplet states of anthracene, perylene and tetracene (1.83 eV, 1.51 eV and 1.25 eV respectively), either one or both components are able to serve as triplet photosensitizers. Silicon nanoparticles smaller than the 4.5 nm Bohr radius will have quantum confined states, and thus a larger driving force for the photosensitization of molecular triplet states. Like the II-VI semiconductor NCs, the role of silicon nanoparticles as light absorbers for triplet-fusion based photon upconversion is based on access to the photogenerated triplet exciton, that has been shown by theorists like A. Zunger88 and G. Galli89 to be the lowest lying excited state in silicon nanoparticles. Other than potential applications for photovoltaics90, if the efficiency of photon upconversion with silicon nanoparticles as light absorbers is high, then this hybrid system will have many biological applications. Unlike Cd and Pb, Si is not toxic and biocompatible. Efficient photon upconversion at low excitation densities with near-infrared light has applications in optogenetics, phototherapy, fiducial markers for skin cancers and bioimaging.


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Figure 6. Energy diagram of perovskite nanocrystal CsPbX3 (X=Br/I) sensitized triplet-triplet annihilation based photon upconversion with the excitation of 532 nm. Reproduced from ref 91. Copyright 2017 Royal Society of Chemistry. Metal halide perovskites have drawn much attention in the field of solar cells due to the widely tunable bandgaps and small exciton binding energy. Recently, Kimizuka and coworkers91 reported the TTA based green-to-violet photon upconversion sensitized by CsPbX3 (X=Br/I) NCs, as shown in Figure 6. Combined with surface anchored DPA as the transmitter and free DPA as the annihilator, they realized a photon upconversion QY of 1.3%, with the quadratic-to-linear threshold excitation intensity of 25 mW/cm2. Interestingly, Becker et al.92 has shown that in CsPbX3 NCs, the lowest lying exciton with triplet character is bright. The bright triplet exciton leads to a radiative rate about 20 (room temperature) and 1000 (5 K) times faster than any other semiconductor NCs. This is in contrast to observations in other semiconductor NCs such as CdSe93 where the triplet exciton is dark. Simulations with an effective-mass model and group theory explain their results and predict that the criteria for such materials to exist are: s and p symmetry for valence and conduction band edges respectively, strong spin-orbit coupling and a non-zero Rashba coefficient for electron and hole in the same sign. In Conclusion, this Perspective introduced the sensitization of molecular triplet states by semiconductor NCs. We summarized existing knowledge and suggested different avenues to improve TET and slow down competing processes from the vantage point of semiconductor NC light absorbers for photon upconversion. While it is trivial to synthesize small NCs that result in a large driving force for TET and thus a high photon upconversion QY, it is less straightforward to eliminate the non-radiative losses, e.g. surface trapping of excitons, exciton-phonon coupling, etc. As alternative photosensitizers, we proposed silicon and perovskite NCs should be further explored. The discussion of semiconductor NCs as triplet donors discussed here, combined with our previous Perspective28 on the design of molecular triplet acceptors, provides insight into how high-performing NC-molecule hybrid systems may be molecularly engineered for a variety of photovoltaic and biological applications. AUTHOR INFORMATION Corresponding Author * [email protected]



Notes Authors declare no competing financial interest. ACKNOWLEDGMENT M.L.T is grateful to the Alfred P. Sloan foundation and the National Science Foundation (CHE-1351663).

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Bios Ming Lee Tang is a 2018 Department of Energy Early Career Research Program participant, a 2017 Sloan Research Fellow, a 2014 NSF CAREER and ARO YIP awardee. Tang obtained her Ph.D. working with Zhenan Bao at Stanford University in 2009 and completed postdoctoral research with A. Paul Alivisatos and Jeffrey R. Long at the University of California, Berkeley from 2009-2012, before starting her independent career at the University of California, Riverside. The Tang group employs an interdisciplinary approach to create synergy between quantum confined materials and organic molecules. Focus is on the rational design of hybrid platforms that can harness multiexcitonic processes like singlet fission or upconversion, for photovoltaics, photocatalytic and biomedical applications. Zhiyuan Huang obtained his Ph.D. in Chemistry in 2017 working with Ming Lee Tang at UC Riverside. Prior to that, he obtained his B.S. in Chemistry from Jilin University in 2011. His expertise is in the rational design of tailor-made organic ligands to control, enhance or mediate charge or energy transfer from quantized nanomaterials, coupled with state of the art spectrosopic and current-voltage measurements.


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Semiconductor Nanocrystal Light Absorbers for Photon Upconversion

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