Letter pubs.acs.org/JPCL
Photon Upconversion and Photocurrent Generation via SelfAssembly at Organic−Inorganic Interfaces Sean P. Hill, Tanmay Banerjee, Tristan Dilbeck, and Kenneth Hanson* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *
ABSTRACT: Molecular photon upconversion via triplet−triplet annihilation (TTA-UC), combining two or more low energy photons to generate a higher energy excited state, is an intriguing strategy to surpass the maximum efficiency for a single junction solar cell (50% (with solar concentrators) can be achieved if UC is utilized to harness sub-bandgap portions of the solar spectrum.3,4 UC by way of small molecule sensitizer and acceptor pairs is particularly appealing because UC efficiencies >30% have been demonstrated,3,5 and UC can be achieved even under noncoherent, solar irradiation.6 In molecular UC, a sensitizer molecule absorbs low energy light (1 in Figure 1A) and undergoes intersystem crossing (2) to generate a triplet excited state. Subsequent sensitizer-to-acceptor triplet energy transfer (3) yields acceptor molecules in the triplet excited state. Two triplet acceptor molecules in proximity can then undergo triplet−triplet annihilation (TTA), generating one excited singlet acceptor and one ground state acceptor (4). Emission from the resulting singlet excited state (5) is hypsochromically shifted relative to the excitation light and thus the photon energy is upconverted during the process (TTA-UC). © 2015 American Chemical Society
Received: September 23, 2015 Accepted: October 22, 2015 Published: October 22, 2015 4510
DOI: 10.1021/acs.jpclett.5b02120 J. Phys. Chem. Lett. 2015, 6, 4510−4517
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The Journal of Physical Chemistry Letters
first was coprecipitation of the UC pair on the surface of nanocrystalline ZrO2.11 Though effective, the UC emission efficiency was relatively low, which can be partially attributed to the dilution of acceptor−acceptor interactions by the sensitizer molecule, resulting in inhibited cross-surface energy transfer. Additionally, given the limited surface area, codeposition effectively decreases the concentration and absorption of each species. The second strategy attempts to mitigate these issues by binding the acceptor molecule on the surface with the sensitizer dissolved in an external solution.12,13 Recently, Nattestad et al. applied the latter architecture with TiO2 as a charge separation interface and demonstrated the first and only intermediate band dye-sensitized solar cell.18 This strategy is effective at increasing acceptor−acceptor interactions but the sensitizer to acceptor energy transfer is still diffusion-limited and must compete with nonradiative decay. In this report, we utilize an inorganic−organic interface but with a new scaffold, self-assembled bilayers of a TTA molecular pair on metal oxide surfaces, to facilitate TTA-UC and charge separation of the upconverted state. This self-assembled motif, via metal ion linkages,19,20 is depicted in Figure 1b and provides a simple and modular method for multimolecular assembly on a metal oxide surface.21,22 This strategy is not diffusion limited and offers unprecedented geometric and spatial control of the donor−acceptor interactions at an interface. The close proximity of sensitizer and acceptor molecules facilitates TTA-UC emission, and we demonstrate that direct photocurrent generation can be achieved at the bilayer-semiconductor interface. The bilayer film depicted in Figure 1b is prepared by stepwise soaking of nanocrystalline TiO2 or ZrO2 films in three separate solutions of 4,4′-(anthracene-9,10-diyl)bis(4,1phenylene)diphosphonic acid (DPPA), Zn(CH3COO)2, and Pt(II)tetrakis(4-carboxyphenyl)porphyrin (PtTCPP). Anthracene and platinum porphyrin derivatives were selected as the TTA-UC pair because they are known to exhibit efficient UC emission (Φem > 20%).7,15 Each step of the surface modification procedure (Figure 2a) was monitored by UV−vis or attenuated total reflectance infrared (ATR-IR) spectroscopy. For ZrO2-DPPA the maximum surface coverage (Γ = 1.04 × 10−7 mol cm−2 for a 4 μm thick film; see Experimental Methods) was achieved by soaking
Figure 1. TTA-UC energy level diagram and the self-assembled bilayer schematic. (a) Energetic scheme of the TTA-UC process consisting of (1) absorption, (2) intersystem crossing, (3) triplet energy transfer, (4) triplet−triplet annihilation, and (5) emission. (b) Chemical structure of DPPA and PtTCPP and a schematic representation of the bilayer film on a metal oxide surface (MO2-DPPA-Zn-PtTCPP).
One intriguing upconversion strategy, introduced by Morandeira and co-workers, is the use of ZrO2 inorganic surface as a scaffold to facilitate sensitizer and acceptor interactions. This was achieved using two architectures. The
Figure 2. (a) Assembly procedure for the monolayer and bilayer films on TiO2 or ZrO2 (gray rectangle) where A is DPPA (blue sphere), Zn is Zn2+ ions (green diamond) and S is PtTCPP (red sphere). (b) Absorption spectra for ZrO2, ZrO2-DPPA, ZrO2-PtTCPP and ZrO2-DPPA after soaking in PtTCPP with (ZrO2-DPPA-Zn-PtTCPP) and without (ZrO2-DPPA + PtTCPP) Zn(CH3COO)2 pretreatment. The DPPA to PtTCPP ratio in ZrO2-DPPA-Zn-PtTCPP is ∼10:1. 4511
DOI: 10.1021/acs.jpclett.5b02120 J. Phys. Chem. Lett. 2015, 6, 4510−4517
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The Journal of Physical Chemistry Letters ZrO2 in a 200 μM DPPA solution of DMSO for 48 h or for 4 h with N2 bubbling (Supporting Information Figure S4). Presumably, bubbling aids in the percolation of the dye solution through the porous film. ATR-IR spectra of ZrO2DPPA after soaking in a methanol solution of 400 μM Zn(CH3COO)2 shows an increased absorption from 1000 to 1150 cm−1 (Supporting Information Figure S5). This increase, which is complete in 10 suns.28 It is important to acknowledge that these are only transient photocurrent measurements. Efforts are currently underway to generate sustained photocurrent by incorporating a redox mediator into the bilayer TTA-UC electrochemical cell. In summary, we have introduced a new scaffold, selfassembled bilayers, which can be prepared by stepwise assembly of sensitizer and acceptor molecules on a metal oxide surface. The proposed architecture is supported by UV− vis and ATR-IR measurements. This unique architecture maximizes dye loading and provides unprecedented geometric and spatial control of the donor−acceptor interactions at an interface. Both steady-state and excitation intensity dependent measurements indicate that the bilayer on ZrO2 successfully facilitates photon upconversion via triplet−triplet annihilation, as well as definitive evidence that we observe charge separation of the upconverted state. A 3-fold enhancement in transient photocurrent is achieved, relative to the sum of its parts, at light intensities as low as two equivalent suns for the bilayer on TiO2. A symptomatic quadratic to linear-like behavior is also
monolayers and bilayers on TiO2 and ZrO2 (Figure 6). Upon excitation of MO2-DPPA at 400 nm, intense blue emission is observed from the ZrO2 film but is completely quenched on TiO2 (Figure 6a). This result is consistent with the energetic scheme in Figure 3 where the singlet excited state potential for DPPA (E1/2 = −1.65 V vs NHE) is sufficiently negative for emission quenching by electron transfer to TiO2. Due to the lack of absorption, excitation at energies less than 495 nm does not generate singlet excited DPPA (Figure 4a) and nominal emission/photocurrent is observed (Figure 5a). In contrast, for MO2-PtTCPP, strong Q-band absorption below 495 nm generates a triplet excited state but it has insufficient energy (E1/2 = −0.86 V vs NHE) to undergo electron transfer and thus minimal emission quenching is observed for PtTCPP on TiO2 relative to ZrO2 (Figure 6b). The small decrease (∼10%) in integrated emission intensity from ZrO2-PtTCPP to TiO2PtTCPP is presumably responsible for the photocurrent observed in Figure 5a and may be due to at least some “hot” electron transfer from the singlet excited state of PtTCPP (E1/2 = −1.15 V vs NHE). Given that the DPPA triplet excited state potential (E1/2 = −0.45 V vs NHE) is less than that of PtTCPP we do not expect electron transfer from the DPPA triplet state to TiO2. Upconverted emission from DPPA-Zn-PtTCPP (λex = 532 nm) is completely quenched on TiO2 (Figure 6c). Although the DPPA singlet excited state was generated via TTA-UC, the emission quenching on TiO2 is expected given the excited state electron transfer from 1DPPA* to the conduction band of TiO2 upon excitation at 400 nm. Equivalent phosphorescent emission intensities from DPPA-Zn-PtTCPP on both TiO2 and ZrO2 (Figure 6d) indicate there is minimal direct electron transfer from PtTCPP to TiO2 and that the dominant 4514
DOI: 10.1021/acs.jpclett.5b02120 J. Phys. Chem. Lett. 2015, 6, 4510−4517
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area for electrode contacts. The cells were then transferred to a glovebox where dry and oxygen free solvent (MeCN for the ZrO2 samples and a solution of 0.3 M TBAClO4 in MeCN for the TiO2 samples) was injected using a Vac’n Fill Syringe (Solaronix) through the 1 mm hole to fill the interior of the cells. The cell was then sealed with a meltonix film and small piece of micro glass cover slide covering the hole used for solvent injection. Absorption Spectra. Data were recorded on an Agilent 8453 UV−visible photo diode array spectrophotometer. Extinction coefficients for PtTCPP and DPPA in DMSO were determined from the absorption spectra of solutions having a known concentration of chromophore in a 1 × 1 cm quartz cuvette. Thin film absorption spectra were obtained by placing dry, derivatized TiO2 and ZrO2 slides perpendicular to the detection beam path. Steady-State Emission. Data were collected at room temperature using an Edinburgh FLS980 fluorescence spectrometer. The samples were excited using the output from either a housed 450 W Xe lamp/single grating (1800 λ/mm, 250 nm blaze) Czerny−Turner monochromator or a Nd:YAG laser (Aixiz, AD-532-400T). The output from the Nd:YAG laser was passed through a variable neutral density filter (Edinburgh F-B01 laser mount), a 2 mm diameter iris (Newport ID-1.0) and then directed to the sample via a flip mirror. Emission from the sample was first passed through a 532 nm notch filter (Thorlabs Inc., NF533-17), then a single grating (1800 l/mm, 500 nm blaze) Czerny−Turner monochromator and finally detected by a Peltier-cooled Hamamatsu R928 photomultiplier tube. Laser intensities were measured using a power meter (Ophir Vega 7Z01560) with a high sensitivity power sensor (Ophir 3A-FS 7Z02628). Spectroelectrochemical Measurements. Amperometric i−t data were collected using a CH Instruments CHI630E electrochemical analyzer using a two electrode configuration (TiO2 working, Pt counter) held at 0 V applied potential. The samples were irradiated with either an AM1.5 solar simulator (Light Model 66181 oriel corrected with a standard air-mass filter) passing through a 495 nm long pass filter or with 532 nm from a Nd:YAG laser (Aixiz, AD-532-400T). The intensity of light from the solar simulator was manipulated by varying the distance between the source and sample. The laser light intensity was controlled by using a neutral density filter as described above for the photophysical measurements. A Model T132 Shutter Driver/Timer (UniBlitz) coupled to a mechanical shutter (Vincent Associates, VS25) was placed between the light source and sample to control 5 s light-dark intervals over a 60 s time period.
observed for the photocurrent versus the incident excitation intensity indicating that we are effectively extracting charge from the upconverted state. These results are a key stepping stone toward the generation of a self-assembled bilayer TTAUC solar cell that can circumvent the Shockley−Queisser limit.
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EXPERIMENTAL METHODS Materials. 1,4-Dibromobenzene, anthraquinone, n-butyl lithium, nickel bromide, triethylphosphite, trimethylsilyl bromide, zinc acetate dehydrate, H2PtCl6, and tetrabutylammonium perchlorate (Sigma-Aldrich) and Pt(II) meso-tetra(4carboxyphenyl)porphine (Frontier Scientific), were purchased from their respective suppliers, in parentheses, and used as received. All other reagents and solvents (analytical reagent grade) have been purchased and used without further purification from Alfa Aesar. Tetrahydrofuran and dichloromethane used in synthesis have been dried and degassed prior to use. Fluorine-doped tin oxide (FTO) coated glass (sheet resistance 15 Ω/□) was purchased from Hartford Glass Co. Meltonix film (1170-25), Ti-Nanoxide T solgel paste (11421) and Vac’n Fill Syringe (65209) were purchased from Solaronix. Micro glass cover slides (18 × 18 mm) were obtained from VWR. ZrO2 solgel paste was prepared following a previously reported procedure.29 Sample Preparation. 4,4′-(Anthracene-9,10-diyl)bis(4,1-phenylene) diphosphonic acid (DPPA) was prepared by following known procedures for related anthracene complexes (Figure S1 in Supporting Information). Briefly, lithiated dibromobenzene was added dropwise to anthraquinone which was followed by reduction with KI and NaPO2H2 to generate 9,10-bis(4bromophenyl)anthracene.30 The phosphonate ester, tetraethyl 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonate, was generated using an Arbuzov reaction31 and the product was then hydrolyzed with TMS-Br to yield DPPA.32 Photophysical and Electrochemical Cells were prepared following our previously published procedure with minor modification.22 Briefly, FTO glass was cut into 2.2 × 2.2 cm (for ZrO2) or 2 × 2.5 cm (for TiO2) pieces and an active area of 1 cm2 metal oxide was prepared by doctor blading TiO2 (solaronix) or ZrO2 sol gel paste (1 layer Scotch tape) and sintering following previously published procedures.33 Dye loading was performed as described in the manuscript. Surface coverages (Γ in mol cm−2) were estimated with the expression Γ = (A(λ)/ε(λ))/1000.34 In these analyses the molar extinction coefficients (ε) for the complexes in DMSO solution were used (Supporting Information Table S1), and A(λ) was the maximum absorbance of the sensitized slides. For ZrO2DPPA the maximum surface coverage of Γ = 1.04 × 10−7 mol cm−2 was achieved for a 4 μm thick film. Comparable surface coverage (Γ = 7.49 × 10−8 mol cm−2) was achieved loading N3 on the same ZrO2 films. The additional components of the cell are shown in Supporting Information Figure S2. A small hole (d = 1.1 mm) was drilled into the corner of the 2.2 × 2.2 cm (or 2 × 2.5 cm for TiO2 cells) glass slide that does not have metal oxide ((b) in Supporting Information Figure S2). For the electrochemical cells, with TiO2, the counter electrode was prepared by dropcasting 50 μL of a 5 mM H2PtCl6 solution in ethanol that was heat dried at 400 °C for 15 min. A 1.5 mm wide 2.2 × 2.2 cm (for ZrO2) or 2 mm wide 2 × 2 cm (for TiO2) Meltonix film was placed between the two glass slides and the entire ensemble is heated to ∼150 °C for 7 s. For the TiO2 samples, the two glass slides were offset by ∼5 mm to ensure sufficient
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b02120. DPPA synthesis, analytical methods, photophysical and electrochemical measurements, absorption/emission spectra, further bilayer characterization, photophysical properties, emission quenching, and transient photocurrent at 1 sun and stability. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 4515
DOI: 10.1021/acs.jpclett.5b02120 J. Phys. Chem. Lett. 2015, 6, 4510−4517
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The Journal of Physical Chemistry Letters Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the Florida State University’s Energy and Materials Initiative for facilitating aspects of this research.
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