Integrated Photon Upconversion Solar Cell via Molecular Self-Assembled Bilayers Sean P. Hill, Tristan Dilbeck, Enric Baduell, and Kenneth Hanson* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *
ABSTRACT: Molecular photon upconversion, by way of triplet−triplet annihilation (TTA-UC), is an intriguing strategy to increase solar cell efficiencies beyond the Shockley−Queisser limit. Here we introduce self-assembled bilayers of acceptor and sensitizer molecules on high surface area electrodes as a means of generating an integrated TTA-UC dye-sensitized solar cell. Intensity dependence and IPCE measurements indicate that bilayer films effectively generate photocurrent by two different mechanisms: (1) direct excitation and electron injection from the acceptor molecule and (2) low-energy light absorption by the sensitizer molecule followed by TTA-UC and electron injection from the upconverted state. The power conversion efficiency from the upconverted photons is the highest yet reported for an integrated TTA-UC solar cell. Energy transfer and photocurrent generation efficiency of the bilayer device is also directly compared to the previously reported heterogeneous UC scheme.
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Herein, we introduce self-assembled bilayers as an alternative strategy to generate an integrated TTA-UC dye-sensitized solar cell (DSSC). This self-assembled motif, via metal−ion linkages,19,20 provides a simple method for assembling sensitizer and acceptor molecules directly on a metal oxide surface (Figure 1a).21 As we demonstrate below, the bilayer exhibits efficient sensitizer-to-acceptor energy transfer, and the photon-to-current efficiency is the highest yet achieved for an integrated TTA-UC solar cell. Using similar dyes, we also compare the device performance of the bilayer TTA-UC DSSCs (Figure 1a) with the heterogeneous system employed by Morandeira and Nattestad (Figure 1b).15−18 The bilayer film (TiO2−DPPA−Zn−PtTCPP) is composed of nanocrystalline TiO2, 4,4′-(anthracene-9,10-diyl)bis(4,1phenylene)diphosphonic acid (DPPA), Zn2+ ions, and Pt(II)tetrakis(4-carboxyphenyl)porphyrin (PtTCPP) (Figure 1a) and was prepared by stepwise soaking according to our previously published procedure.21 Each step of the loading procedure was monitored using UV−vis (Figure 1c) and ATR-IR spectroscopy. DSSCs were then prepared in a standard sandwich cell architecture with TiO2−DPPA−Zn−PtTCPP films as the photoanode, a platinum counter electrode, and a Co(bpy)3(PF6)2-based redox mediator in nitrogen-deaerated MeCN as the electrolyte solution. The relative energetics within the device are summarized in Figure 2. The Co(bpy)32+/3+ redox mediator was selected because of its favorable
eveloping novel solar cell design strategies that can circumvent the Shockley−Queisser limit (∼34%)1 continues to grow in interest because of their potential to increase efficiencies and decrease solar module costs (cost W−1). Photon upconversion (UC), combining two lowerenergy photons to generate a higher-energy excited state, can be used to harness sub-band gap photons and reach maximum theoretical solar cell efficiencies of >40%.2,3 Molecular photon UC, by way of triplet−triplet annihilation (TTA-UC),4−7 is particularly appealing because UC is achievable even under lowintensity, noncoherent, solar irradiation.8,9 A particularly notable example is the TTA-UC metal−organic framework recently introduced by Kimizuka and co-workers, which can achieve maximum efficiency (>1%) at light intensities up to 30 times lower than solar irradiance.10 A majority of the previous efforts to harness solar energy via TTA-UC have been dedicated to generating UC-assisted solar cells with an UC solution or polymer film as a filter or reflector working in conjunction with a conventional solar cell.11−14 This strategy can suffer from self-absorption/inner filtering losses and requires additional design components that may increase complexity and manufacturing cost.9 Recently, Simpson et al. demonstrated the first example of an integrated TTA-UC solar cell.15 Their device, based on the heterogeneous TTA-UC scheme introduced by Morandeira and co-workers,16−18 is composed of an acceptor molecule bound to a TiO2 surface with a sensitizer dissolved in the electrolyte solution.15 While effective at harnessing TTA-UC, solution sensitization is diffusion-limited and suffers from additional loss pathways such as nonproductive sensitizer− sensitizer TTA, as noted by Morandeira and co-workers.16 © XXXX American Chemical Society
Received: March 1, 2016 Accepted: April 11, 2016
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DOI: 10.1021/acsenergylett.6b00001 ACS Energy Lett. 2016, 1, 3−8
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http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
Figure 1. Schematic representation of the (a) self-assembled bilayer (TiO2−DPPA−Zn−PtTCPP) and (b) heterogeneous (TiO2-DPPA with PtTPP in solution) TTA-UC architectures. (c) Absorption spectra for TiO2, TiO2−DPPA, TiO2−PtTCPP, and TiO2−DPPA−Zn−PtTCPP. The transmission windows for 375 and 495 nm long-pass filters are noted in blue and green, respectively.
Figure 3. Photocurrent density−voltage characteristics for DSSCs with photoanodes composed of TiO2−DPPA, TiO2−PtTCPP, and TiO2−DPPA−Zn−PtTCPP and [Co(bpy)3]2+/3+ redox mediator under AM1.5 irradiation with a 495 nm long-pass filter.
Figure 2. Electronic transitions and energetics for TiO2, DPPA, PtTCPP, and [Co(bpy)3]2+/3+ (vs NHE) in the TTA-UC DSSC. (A = acceptor/DPPA, S = sensitizer/PtTCPP, ISC = intersystem crossing, ET = energy transfer, TTA = triplet−triplet annihilation.)
Amperometric i−t measurements of the monolayer and bilayer DSSCs under a 2 equiv AM1.5 solar spectrum passed through 375 and 495 nm long-pass filters can be seen in Figure 4. Light below 375 nm was blocked in order to prevent the UVinitiated dimerization reaction between π-stacked anthracene molecules, which is reversible upon heating (Figure S2).25 Unlike with our previous transient photocurrent measurements,21 the addition of [Co(bpy)3]2+/3+ resulted in sustained photocurrent from both the monolayer and bilayer films. With a 495 nm long-pass filter (Figure 4b), to selectively excite the Q-band of PtTCPP, the Jsc for TiO2−DPPA−Zn−PtTCPP (0.028 ± 0.002 mA/cm2) was more than 5 times greater than that of TiO2−PtTCPP (0.005 ± 0.002 mA/cm2) and TiO2− DPPA (0.002 ± 0.002 mA/cm2). Upon excitation with the entire visible spectrum (375 nm long-pass filter, Figure 4a), there was a more than 10-fold increase in Jsc for the monolayer and bilayer devices relative to the 495 nm measurements primarily due to direct photoexcitation of DPPA at λ < 425 nm followed by electron injection from the singlet excited state of DPPA. The ∼25% higher Jsc for TiO2−DPPA−Zn−PtTCPP (0.285 ± 0.002 mA/cm2) relative to the sum of the monolayer films (0.220 ± 0.002 mA/cm2) as well as incident photon to current efficiency (IPCE) measurements (Figure S3) suggests that both the higher-energy, direct excitation of DPPA, and the lower-energy, TTA-UC electron injection mechanisms contribute to photocurrent generation. Further support for a TTA-UC photocurrent generation mechanism can be seen in the plot of Jsc versus 532 nm excitation intensity for TiO2−DPPA−Zn−PtTCPP and TiO2−
oxidation potential relative to TiO2−DPPA−Zn−PtTCPP (Figure 2) and its successful application in a heterogeneous TTA-UC DSSC.15,22,23 Photocurrent density−voltage (J−V) measurements for DSSCs with both monolayer and bilayer films were measured under AM1.5 solar irradiation, passed through a 495 nm longpass filter, and the results are summarized in Figure 3 and Table S1. The filter was selected to isolate the contribution of lowenergy light absorption and TTA-UC to the device performance. Relative to TiO2−DPPA−Zn−PtTCPP, a lower photoresponse was observed from TiO2−DPPA and TiO2−PtTCPP presumably due to the lack of absorption above 495 nm (Figure 1c) and insufficient excited-state electron injection potential (Figure 2), respectively. However, despite having the same absorption intensity as the sum of TiO2−PtTCPP and TiO2− DPPA monolayer films, for TiO2−DPPA−Zn−PtTCPP, we see an almost 2-fold enhancement in Jsc and an almost 5-fold enhancement in Voc (Table S1) relative to the monolayer films. The devices exhibit a low fill factor and shunt resistance likely due to the TiO2(e−) to [Co(bpy)3]3+ back electron transfer being highly competitive with the low current photogenerated by the TTA-UC mechanism.24 Given the low efficiency of the device, herein we focus on comparing Jsc (photocurrent at V = 0) between devices with respect to the excitation wavelength and intensity. 4
DOI: 10.1021/acsenergylett.6b00001 ACS Energy Lett. 2016, 1, 3−8
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ACS Energy Letters
Figure 4. Amperometric i−t curves for DSSCs containing TiO2−DPPA, TiO2−PtTCPP, and TiO2−DPPA−Zn−PtTCPP photoanodes with [Co(bpy)3]2+/3+ redox mediator under 2 equiv of AM1.5 solar irradiation passing through (a) a 375 nm and (b) a 495 nm long-pass filter (V = 0; shutter open at 0 s, closed at 10 s).
Figure 5. Photocurrent density from (a) TiO2−DPPA−Zn−PtTCPP and TiO2−PtTCPP devices with respect to 532 nm excitation intensity and (b) TiO2−DPPA−Zn−PtTCPP and TiO2−DPPA devices with respect to 366 nm excitation intensity. (TiO2 thin film working electrode, platinum counter electrode, with [Co(bpy)3]2+/3+ electrolyte in MeCN at 0 V applied potential.)
PtTCPP (Figure 5a). The TiO2−PtTCPP device exhibits a linear dependence (slope = 1) on excitation intensity that is expected for a single-molecule photoexcitation−electron injection process that is observed with standard DSSCs.26 On the other hand, TiO2−DPPA−Zn−PtTCPP exhibits quadratic (slope = 2) to linear (slope = 1) intensity-dependent behavior that is indicative of a TTA-UC mechanism.27−29 A similar measurement under 366 nm light, for preferential excitation of DPPA, results in a slope of 1 for both TiO2−DPPA−Zn− PtTCPP and TiO2−DPPA, as shown in Figure 5b. These results indicate that the bilayer film is effectively harnessing light through two different mechanisms: (1) high-energy excitation and electron injection from the singlet excited state of the acceptor molecule (linear dependence) and (2) lowenergy excitation of the sensitizer, followed by energy transfer, TTA-UC, and electron injection from the upconverted state (quadratic to linear dependence). As far as we know, this is the first observation of a quadratic to linear photocurrent−excitation intensity dependence from an integrated TTA-UC solar cell. This is in contrast to the previous report of the heterogeneous TTA-UC DSSC where a superlinear relationship (slope = 1.46) was observed.15 The above results indicate that self-assembled bilayers are an effective strategy to directly harness TTA-UC in a DSSC. This is only the second example of an integrated TTA-UC solar cell. The first example, by Simpson et al.,15 is based on a heterogeneous TTA-UC scheme introduced by Morandeira and co-workers.16−18 In the interest of comparing the TTA-UC
schemes, we have generated two types of devices: (1) a bilayer device composed of a TiO2−DPPA−Zn−PtTCPP photoanode, Pt counter electrode, and [Co(bpy)3]2+/3+ in DMF and (2) a heterogeneous device composed of a TiO2−DPPA photoanode, platinum counter electrode, with Pt(II)5,10,15,20tetraphenylporphyrin (PtTPP) and [Co(bpy)3]2+/3+ in DMF. PtTCPP and PtTPP were selected as the sensitizers because they exhibit nearly identical photophysical and electrochemical properties (Table S2), and thus, differences in performance cannot be due to energetic variables. The influence of sensitizer concentration on photocurrent is shown in Figure 6. The samples were excited with a high-intensity (5.5 W/cm2) 532 nm laser light to ensure that both devices operated within the strong annihilation limit.27−29 Three notable observations can be made from the data in Figure 6. The first is that the sensitizer concentration in the bilayer device is more than 2 orders of magnitude greater than that of the heterogeneous device. By binding the dye to a high surface area TiO2 substrate, a significantly higher sensitizer concentration (15 mM) can be achieved, which reduces transmission losses in the bilayer device (92% transmittance, A = 0.038) by 4-fold relative to that of the heterogeneous device (98% transmittance, A = 0.0096) at 532 nm. The second observation is that for the heterogeneous system, the current density first increases until [PtTPP] = 0.062 mM and then decreases at higher concentrations. An analogous response curve, relative to sensitizer concentration, is common in solution-based TTA-UC emission experiments30 and is 5
DOI: 10.1021/acsenergylett.6b00001 ACS Energy Lett. 2016, 1, 3−8
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ACS Energy Letters
Figure 6. Photocurrent density at 0 V with respect to absorbance at 532 nm and concentration for (a) TiO2−DPPA−Zn−PtTCPP with [Co(bpy)3]2+/3+ in DMF and (b) TiO2−DPPA with PtTPP and [Co(bpy)3]2+/3+ in DMF under 532 nm laser excitation (5.5 W/cm2). Sensitizer concentrations were determined from Beer’s Law using the absorbance, sensitizer extinction coefficient, and film thickness (for a) or from the solution path length (for b). Exponential and Gaussian fits are included as a visual guide, but we attribute no scientific significance to them.
suggests that while the IPCE is higher for the bilayer device, the absorbed photon to current efficiency (APCE) is lower than that in the heterogeneous system. Because both photon absorption and energy transfer are more efficient in the bilayer device, the efficiency losses must occur during the subsequent events (i.e., TTA-UC, electron injection, etc.). Given the similarity in DPPA surface loading/packing, we assume that there is a similar cross surface energy transfer rate and TTA efficiency for both the bilayer and the heterogeneous device. However, it is possible that, given their close proximity in the bilayer device, 1DPPA* to PtTCPP resonance energy transfer may be competitive with electron injection from 1DPPA* to TiO2. In our previous report, we demonstrated that emission from 1DPPA* is ∼95% quenched by way of energy transfer to PtTCPP in a ZrO2−DPPA−Zn−PtTCPP bilayer.21 If the 1 DPPA* to PtTCPP energy transfer rate is ∼3 times faster than the electron injection rate, then we would expect a 4-fold reduction in the injection yield. For the heterogeneous system, 1 DPPA* to PtTPP energy transfer is diffusion-limited and expected to be much slower than electron injection. This competitive energy transfer may account for the 4-fold discrepancy between the IPCE and APCE in the bilayer device compared to that in the heterogeneous system. The above results demonstrate that self-assembled bilayers are an effective strategy to generate an integrated TTA-UC solar cell. The efficiency (η = 1.6 × 10−5 %) and Jsc (0.009 ± 0.002 mA/cm2) are the highest yet achieved by directly extracting charge from a TTA upconverted state under 1 sun illumination. However, the efficiency must be significantly increased for an integrated TTA-UC solar cell to be a viable strategy to surpass the Shockley−Queisser limit. Necessary improvements include (1) shifting absorption of the UC pair further into the visible and near-IR regions,9 (2) broadening low-energy absorption through the use of multiple sensitizers with complementary absorption features,2,14,30 (3) increasing the 1DPPA* to TiO2 electron injection rate34 to mitigate losses via 1DPPA* to PtTCPP resonance energy transfer, and (4) designing/selecting the appropriate mediator to maximize dye regeneration but mitigate excited-state quenching (Figure S9). In closing, we have introduced self-assembled bilayers of sensitizer and acceptor molecules on nanocrystalline TiO2 as an effective strategy to incorporate TTA-UC directly into a DSSC. Both intensity dependence and IPCE measurements indicate that bilayer films effectively harness sunlight and generate
attributed to concentration-dependent, nonproductive interactions including aggregation/excimer formation, sensitizer− sensitizer TTA, and external heavy atom effect.15,16,31 These results suggest that while modifications of the sensitizer structure, or the solvent, could be used to increase sensitizer concentration, it may not increase device performance. In contrast, the bilayer device exhibits an exponential increase in photocurrent density with respect to sensitizer concentration. The highest sensitizer concentration achieved under these loading conditions (15 mM) equates to a surface coverage of 1.28 × 10−8 mol cm−2 or
