Letter pubs.acs.org/NanoLett
Broadband Up-Conversion at Subsolar Irradiance: Triplet−Triplet Annihilation Boosted by Fluorescent Semiconductor Nanocrystals A. Monguzzi,†,§ D. Braga,‡,§ M. Gandini,† V. C. Holmberg,‡ D. K. Kim,‡ A. Sahu,‡ D. J. Norris,*,‡ and F. Meinardi*,† †
Dipartimento di Scienza dei Materiali, Università degli Studi di Milano Bicocca, 20125 Milano, Italy Optical Materials Engineering Laboratory, ETH Zurich, 8092 Zurich, Switzerland
‡
S Supporting Information *
ABSTRACT: Conventional solar cells exhibit limited efficiencies in part due to their inability to absorb the entire solar spectrum. Sub-band-gap photons are typically lost but could be captured if a material that performs up-conversion, which shifts photon energies higher, is coupled to the device. Recently, molecular chromophores that undergo triplet−triplet annihilation (TTA) have shown promise for efficient up-conversion at low irradiance, suitable for some types of solar cells. However, the molecular systems that have shown the highest upconversion efficiency to date are ill suited to broadband light harvesting, reducing their applicability. Here we overcome this limitation by combining an organic TTA system with highly fluorescent CdSe semiconductor nanocrystals. Because of their broadband absorption and spectrally narrow, size-tunable fluorescence, the nanocrystals absorb the radiation lost by the TTA chromophores, returning this energy to the up-converter. The resulting nanocrystal-boosted system shows a doubled lightharvesting ability, which allows a green-to-blue conversion efficiency of ∼12.5% under 0.5 suns of incoherent excitation. This record efficiency at subsolar irradiance demonstrates that boosting the TTA by light-emitting nanocrystals can potentially provide a general route for up-conversion for different photovoltaic and photocatalytic applications. KEYWORDS: Up-conversion, semiconductor nanocrystals, colloidal quantum dots, triplet−triplet annihilation, photocatalysis, photovoltaics
T
traditional up-conversion methods rely on multiphoton processes (e.g., second-harmonic generation in nonlinear crystals or two-photon absorption in materials doped with rare-earth elements).12,14−16 These require high light intensities (MW to GW per cm2) that are well outside the working range of conventional solar cells.6 For low-power up-conversion, pairs of organic chromophores have recently been investigated (Figures 1a,b).17−20 Briefly, a light-harvesting molecule, called the sensitizer, absorbs a lowenergy photon and transfers its energy to the metastable triplet state of another molecule, called the emitter. Through a process known as triplet−triplet annihilation, two excited emitters then combine their energy yielding one emitter in a high-lying singlet state. When this molecule fluoresces, a high-energy photon is produced. The overall process, known as sensitized triplet−triplet annihilation up-conversion (sTTA-UC), has led to quantum yields as high as 20% at irradiances of only a few suns and can work under incoherent excitation.21−26 It has also been used to enhance the light-harvesting efficiency of photovoltaic and photocatalytic cells.25,27−33 In this case, the sTTA-UC layer is placed after the sunlight passes through the
he efficiency of photovoltaic and photocatalytic devices is limited by the electronic properties of the light-harvesting layer. For example, in semiconductor solar cells, the band gap leads to fundamental losses when photons with energies below the band gap cannot be absorbed.1 For single-crystal silicon solar cells, this means that infrared (IR) photons with wavelengths longer than ∼1000 nm are not harvested. In organic and dye-sensitized solar cells, which typically have wider band gaps, a bigger fraction of the solar spectrum, for example, starting in the near-IR (700−1000 nm), cannot be utilized.2,3 Other solar-conversion devices, such as photocatalytic cells for splitting water into hydrogen, exhibit even higher losses. Typically in those devices, the active material absorbs mainly in the ultraviolet (UV) and only a few percent of the incoming solar radiation is utilized.4,5 To reduce such light-harvesting losses, different strategies have been pursued. Multijunction solar cells, which incorporate several semiconductors with different band gaps, are the most advanced, but they also suffer from high fabrication costs.6 Another strategy is to utilize materials that shift the energy of the incoming photons higher, a process known as upconversion (UC).7−11 This permits the recovery of low-energy sub-band-gap photons that would otherwise not be absorbed by the light-harvesting layer. Up-conversion can, in principle, lead to performance improvements of 50% and 100% for photovoltaic and photocatalytic devices, respectively.7,9,12,13 However, © 2014 American Chemical Society
Received: August 29, 2014 Revised: October 3, 2014 Published: October 16, 2014 6644
dx.doi.org/10.1021/nl503322a | Nano Lett. 2014, 14, 6644−6650
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Figure 1. Sensitized triplet−triplet-annihilation up-conversion (sTTA-UC) with and without nanocrystal boosting. (a) In sTTA-UC, molecular sensitizers are excited by incoming photons with energy hνin, yielding triplet states (ST). These molecules then transfer the absorbed energy to emitter triplets (ET). The annihilation of two emitter triplets (TTA) generates a ground-state emitter (E0) and a high-energy emitter singlet state (ES) from which up-converted fluorescence occurs with energy hνout. (b) Energy-level diagram for the sTTA-UC process. A sensitizer with absorptance αsens is excited into a singlet state (Ss) that efficiently undergoes intersystem crossing (ISC) into the triplet state (ST). Energy transfer (ET) then competes with back energy transfer (BET) to emitter triplets (ET). These emitter triplets can either spontaneously decay (with rate constant kSD) or undergo triplet−triplet annihilation (TTA) to an excited singlet state of the emitter (ES). (c) Schematic of a solar cell (SC) coupled to a sTTA-UC system to enhance light-harvesting. Red light is converted by the sTTA-UC to blue photons (UC PL), while sub-band-gap photons not absorbed by the sensitizer (yellow beam) are lost. (d) Schematic of a boosted sTTA-UC system. The light transmitted through both the SC device and the sTTA-UC system (yellow beam) is absorbed by a highly luminescent nanocrystal layer (BST) and recycled back to the sensitizer (BST PL). For clarity, only photons emitted toward the solar cell by both the sTTA-UC and the BST are depicted.
aid other solar cells including conventional photovoltaic devices.29−31 We begin by modeling the effect of a fluorescent lightharvesting booster on the sTTA-UC process. When a suitable pair of chromophores is used in sTTA-UC (see Figures 1a,b), emitters in a high-energy singlet (ES) are generated by the annihilation of two non-luminescent, optically dark emitter triplets (ET). The latter are populated through Dexter energy transfer from phosphorescent sensitizers in their triplet state (ST). Sensitizer triplets are created by intersystem crossing (ISC) from sensitizer singlets (SS), which are generated by the absorption of incoming photons. Because these initial absorbing singlet states in the sensitizer have lower energy than the final emitter singlet that fluoresces, the outgoing photons have higher energy than the incoming photons. This process can be evaluated in terms of its effective conversion efficiency ϕeff, that is, the ratio of the photons emitted by the up-converter to the number of photons incident upon it, which can be written as
device (Figure 1c) to recapture some of the sub-band-gap photons. However, the improvement offered by these low power up-converting systems has so far been limited by the narrow absorption bands (15−20 nm) of the employed sensitizers.34 We report a strategy to overcome this limitation by combining state-of-the-art sTTA-UC chromophores with CdSe nanocrystals. In general, colloidal nanocrystals have unique optoelectronic properties that have been explored for many applications. This includes the use of doped nanocrystals16,35−42 and nanocrystal heterostructures43 as up-converting chromophores. However, here we take a completely different approach. We demonstrate their utility as a “booster” for sTTAUC. Because semiconductor nanocrystals have broadband absorption as well as efficient, narrow-band fluorescence that is tunable with nanocrystal size,44 they can be engineered to absorb light lost by the sTTA-UC system and return the energy back to the sensitizer (Figure 1d). This enhances the population of excited emitters, allowing the maximum TTA yield to be reached at lower irradiance. The tunability of the optical properties of the nanocrystals also makes this technique extremely versatile. We demonstrate a two-fold improvement in the light-harvesting ability of two benchmark sTTA-UC systems, achieving a record green-to-blue conversion efficiency of 12.5% at 0.5 suns of incoherent excitation. The resulting systems can therefore provide efficient broadband upconversion at subsolar irradiances for photocatalytic solar cells, which typically absorb wavelengths below 500 nm.4 More generally, nanocrystal-boosted sTTA-UCs have the potential to
ϕeff = αsensϕETQYuc
(1)
where αsens is the fraction of incident photons that are absorbed by the sensitizer (i.e., the wavelength-integrated absorptance) (see eq S1 in the Supporting Information), ϕET is the energytransfer efficiency from sensitizer triplets to emitter triplets, and QYuc is the quantum yield of up-converted photons, which is QYuc = 6645
1 fϕ 2 TTA
(2) dx.doi.org/10.1021/nl503322a | Nano Lett. 2014, 14, 6644−6650
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Here ϕTTA is the yield of the triplet−triplet annihilation, and f is the statistical probability to obtain an excited singlet state in the emitter after annihilation of two triplets, which is fixed for a given sTTA-UC system. 45,46 The quantum yields for intersystem crossing (ISC) in the sensitizer and fluorescence in the emitter are assumed to be 1. The maximum value for QYuc is 0.5 since two low-energy photons are needed to obtain one high-energy photon. Therefore, the prefactor of 1/2 is included in eq 2. The annihilation yield, ϕTTA, is not an intrinsic property of the system but depends on the excitation intensity. From the depopulation rates of the emitter triplet through spontaneous decay, RSD, and annihilation, RTTA, the annihilation yield can be written as ϕTTA = RTTA/(RSD + RTTA). The rates RSD and RTTA are equal to kSDCEm,T and γTTAC2Em,T, respectively, where kSD and γTTA are rate constants and CEm,T is the concentration of emitter triplets. CEm,T is directly related to αsens, which establishes the initial triplet concentration in the system.21,45 The relationship between the annihilation rate (RTTA) and the concentration of emitter triplets (CEm,T) allows for a deeper analysis of the sTTA-UC process. Specifically, we can consider a special threshold concentration, ρth = kSD/γTTA, for CEm,T. When CEm,T = ρth, the rate of spontaneous decay equals the rate of annihilation. In this case, ϕTTA is 0.5 and QYuc is half its maximum value according to eq 2. When CEm,T > ρth, triplet− triplet annihilation becomes the primary decay channel for the emitter triplets, and ϕTTA approaches 1. A detailed analysis24,34 shows that for a sample with optical path d, ρth is reached at an excitation intensity of
Ith =
ρth k SDd ϕETαsens
(3)
Figure 2. Model for sTTA-UC with and without nanocrystal boosting. (a) Sensitizer absorption spectrum (dotted red line) and emitter fluorescence spectrum (solid blue line) for a typical sTTA-UC system. The dashed vertical line represents the absorption edge of a model solar cell (SC Abs). The horizontal arrow represents the up-conversion of incoming photons. (b) Spectra for the same sTTA-UC system with nanocrystal boosting. Typical absorption (dashed green line) and fluorescence (solid red line) spectra of the nanocrystals are shown. (c) Calculated ϕBST eff as a function of the absorption edge of the booster and the absorption bandwidth of the sensitizer. The other parameters in eq 5 are taken from the model system in (b) and kept constant. The circle indicates a booster that absorbs all photons between 500 and 600 nm coupled to a sensitizer with narrow absorption ( 1: (i) the booster should have broadband absorption to capture all photons that pass through the sTTA-UC system, (ii) the booster and the sTTA-UC system should be properly coupled (β ∼ 1 and ξ ∼ 1), and (iii) the booster fluorescence yield should be high (ϕBST ∼ 1). Figure 2b shows a model for the components of a possible boosted-sTTA-UC system. Assuming that such a system is irradiated with the standard AM 1.5 solar spectrum, eq 5 can predict the dependence of ϕBST on the position of the eff absorption edge of the booster and the absorption bandwidth of the sensitizer. We modeled the absorption of the solar cell as a sigmoidal function with an edge at 500 nm, very similar to that of a photocatalytic water splitting cell.4 QYuc and ϕBST were kept constant, as well as ξ, which is given by the overlap between the sensitizer absorption and booster fluorescence peaks, and β, which is set by the geometry of the overall device. Figure 2c shows the dependence of ϕBST eff on the absorption of the booster and the sensitizer. The circle indicates the best case in which a booster that fully absorbs the photons between 500 and 600 nm (i.e., between the solar-cell absorption edge and the booster fluorescence) is coupled to a sensitizer with a narrow absorption width (
