Boron Subphthalocyanines as Triplet Harvesting ... - ACS Publications

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The Journal of Physical Chemistry Letters

Boron Subphthalocyanines as Triplet Harvesting Materials Within Organic Photovoltaics. Jeffrey S. Castrucci,†,‡ David S. Josey, † Emmanuel Thibau, ‡ Zheng-Hong Lu,‡ and Timothy P. Bender*,†,‡,§ †

Department of Chemical Engineering & Applied Chemistry, University of Toronto, 200 College

Street, Toronto, Ontario M5S 3E5, Canada ‡

Department of Materials Science and Engineering, University of Toronto, 184 College Street,

Toronto, Ontario M5S 3E4, Canada §

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S

3H6, Canada *To whom correspondences should be addressed. Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Singlet fission, the generation of two excited triplet states from a single absorbed photon, is currently an area of significant interest to photovoltaic researchers. In this letter we outline how a polychlorinated boron subphthalocyanine, previously hypothesized to be an effective harvester of singlet fission derived triplets from pentacene, is relatively efficient at facilitating the process. As expected we found a major increase in photocurrent generation at the expense of device voltage. For a direct point of comparison we also have paired the same polychlorinated boron subphthalocyanine with α-sexithiophene to probe the alternative technique of complimentary absorption engineering. The sum of these efforts have led us present new guidelines for the molecular design of boron subphthalocyanine for organic photovoltaic applications.

TABLE OF CONTENTS GRAPHIC

KEYWORDS organic, photovoltaic, pentacene, triplet, singlet, fission, subphthalocyanine, boron


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Singlet fission, the process where one singlet excited state is converted into two triplet excited states,1 is an area of active research in the field of photovoltaics.2-8 The advantages of generating two charge carriers from a single absorbed photon in a photovoltaic device include achieving higher current densities and a higher thermodynamic limit to the power conversion efficiency9 compared to the single junction Shockley-Queisser limit.10 There are both a limited number of materials for which the singlet fission process has been shown to occur within,11 and a limited number of material pairings capable of converting triplet excited states to charge carriers in photovoltaic devices.12-15 External quantum efficiencies (i.e. number of charge carriers extracted per number of incident photons of a given energy) greater than 100 % have been achieved in a highly optimized triplet-harvesting system.15 Developments in the area have been focused on fundamental questions of excited state energy and kinetics, areas that are difficult to explore experimentally and are heavily reliant upon computational models and detailed species balances. Consequently, most singlet fission materials explored thus far are linear acenes and related derivatives, which typically serve as electron donors in traditional donor/acceptor configurations of photovoltaic devices. A limited number of electron accepting materials have been demonstrated to harvest the triplets from the acenes, which include fullerene derivatives,5-6, 14-15 perylene derivatives,14 and quantum dots.3-4, 8, 12-13 There is a general need for the development of molecular design rules to guide the synthesis of new materials to be paired with singlet fission materials so that singlet fission derived triplets can be successfully dissociated into charge carriers within an organic photovoltaic device. Our laboratory has recently demonstrated that two classes of electron accepting materials, silicon phthalocyanines (SiPcs)16 and boron subphthalocyanines (BsubPcs),17 are able to dissociate triplet excited states generated within a pentacene singlet fission electron donor layer.


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In general, BsubPcs hold significant promise as organic electronic materials,18 having been demonstrate as both electron donors,19 electron acceptors,20-21 and ambipolar interlayers22 in singlet fission free organic photovoltaic devices. BsubPcs have been used with the complimentary absorbers α-sexithophene and subnaphthalcyanine chloride (Cl-BsubNc) to construct a planar heterojunction photovoltaic cell that absorbed a broad portion of the visible spectrum that also featured an energy cascade (transferring excitons from Cl-BsubPc to ClBsubNc within the cell) leading to a power conversion efficiency of up to ~8 %.23 In our recent work with BsubPc derivatives as electron acceptors paired with pentacene electron donors,17 we noted that the amount of pentacene triplets converted to charge carriers could be observed by proxy through examination of the external quantum efficiency spectrum, as the pentacene absorbance peak near 670 nm is an area of the spectrum where BsubPcs do not absorb. This by proxy observation is a result of the known quantitative conversion of singlets to triplets on a shorter time scale than exciton dissociation within pentacene.24 So one can assume that if a new material enables an EQE contribution near 670 nm that it is able to harvest triplet energy from pentacene. From the results of that study, we had proposed "that a BsubPc derivative with a deeper LUMO energy [than Cl-Cl6BsubPc] would likely be an even more effective at harvesting Pent-derived triplets, resulting in enhanced photocurrent."17 We have recently synthesized a new polychlorinated BsubPc derivative, chloro dodecachloro boron subphthalocyanine (Cl-Cl12BsubPc, Figure 1),25 that meets this criteria. While our previous work on polyfluorinated BsubPcs shows that a comparable deepening of the LUMO energy can be achieved, these materials were found to be unstable as acceptors within OLEDs and thus we feel are not suitable for photovoltaic application.26 In this letter, we report on the application of this new triplet harvesting polychlorinated BsubPc in photovoltaic devices and contrast the results


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with those of photovoltaics designed with a traditional complimentary light absorption and energy level alignment approach. As such, we selected two electron donor materials to pair with Cl-Cl12BsubPc, pentacene for its singlet fission properties, and α-sexithophene (α6T) for its complimentary absorbance and lack of singlet fission processes. These new results are considered in light of our previously reported pentacene/BsubPc devices based on Cl-BsubPc and Cl-Cl6BsubPc.17 The compound structures and abbreviations are detailed in Figure 1a. The highest occupied molecular orbitals (HOMOs) for Cl-BsubPc and Cl-Cl6BsubPc have previously been determined to be at energies of 5.7 eV27 and 6.0 eV20 (Figure 1b). For this study, we characterized Cl-Cl12BsubPc by ultraviolet photoelectron spectroscopy (UPS) and the results are shown in Figure 1b and the Supporting Information Figure S1. We measured the work function to be 4.5 eV, the HOMO-Fermi gap to be 1.8 eV, which sum to yield a HOMO energy to be 6.3 eV. To estimate the lowest unoccupied molecular orbital (LUMO) we then subtract the optical band gaps (described below) and the exciton binding energy (~ 0.3 eV)28 to yield estimated LUMO energies of 3.3 eV, 3.6 eV, and 3.9 eV, for Cl-BsubPc, Cl-Cl6BsubPc and Cl-Cl12BsubPc respectively. When paired with pentacene's HOMO of 4.9 eV,29 under the assumption of vacuum energy alignment, this yields interfacial gap energies (Igap = HOMODonor LUMOAcceptor) of 1.6 eV, 1.3 eV and 1.0 eV, respectively. While difficult to measure due to its non-emissive nature, the triplet energy of pentacene is estimated to be between 0.85 eV and 1.0 eV.12 By taking the HOMO energy of pentacene as the ground state (S0) and subtracting the 1.0 eV triplet energy, the upper bound of the energy of electrons generated by dissociation of a triplet state in the pentacene is 3.9 eV relative to vacuum.30 For the set Cl-BsubPc, Cl-Cl6BsubPc, Cl-Cl12BsubPc, we note that by optical absorbance measurements of their solid state films the optical band gaps are identical at 2.1 eV with nearly


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indistinguishably shaped absorbance profiles. The optical band gap for pentacene was measured to be 1.9 eV. When the absorbance profiles are normalized per unit thickness of film (Figure 1c), we find that the Cl-Cl12BsubPc film absorbs significantly more strongly than the Cl-BsubPc and Cl-Cl6BsubPc films. This is surprising as, when one considers the solvent free single crystal X-ray diffraction determined structures of these materials, the trend in chromophore densities (Cl-BsubPc 3.62 kmol m-3,31 Cl-Cl6BsubPc 2.72 kmol m-3,17 Cl-Cl12BsubPc 2.25 kmol m-3),25 is the reverse of the absorbance trend. Photovoltaic devices were fabricated in the manner previously described,32 details of which are included in the Supporting Information accompanying this letter. Plots of current density vs. voltage for the photovoltaic devices are shown in Figure 1c. Given the similar absorbance profiles and progressively reducing interfacial band gaps formed by this set of BsubPcs, when using them as electron acceptors of identical layer thickness in photovoltaic devices we would expect, in the absence of the singlet fission triplet harvesting process, that the short circuit current (JSC) would remain constant (with perhaps a slight rise in JSC due to the higher extinction coefficient of Cl-Cl12BsubPc) and the open circuit voltage (VOC) would get progressively and proportionally lower. Instead, we observe a dramatically rising JSC (more than tripling of the JSC between the Cl-Cl6BsubPc and the Cl-Cl12BsubPc containing devices) and (as expected) an accompanying reduction in VOC (Figure 1d, Table 1). Each observation is in line the expected effects of the singlet fission process within pentacene and the harvesting of triplet energy by Cl-Cl12BsubPc. We would note that the presented pentacene/ClCl12BsubPc devices are not as optimized as the Cl-BsubPc and Cl-Cl6BsubPc devices. However, the device metrics, especially the FF, are well within those previously published for BsubPc based OPV (see Table S1-S3 and accompanying discussion).27 As a comparison to the use of C60


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as a pentacene triplet acceptor, the VOC for the Cl-Cl12BsubPc device is approximately 0.03 V higher with only marginally lower JSC. Transient optical absorption spectroscopy has demonstrated that the kinetics of converting one singlet to two triplets happens in < 200 fs for pentacene, while charge dissociation happens on the relatively slower ns timescale resulting in the quantitative conversion of singlets to triplets prior to dissociation.24 Therefore the presence of a contribution from pentacene in the EQE spectrum of a photovoltaic is an indication of harvesting triplets. Turning to the measurements of the external quantum efficiency (EQE) for these devices (Figure 1e), we see the characteristic pentacene EQE peak at 670 nm is completely absent for Cl-BsubPc, is small but noticeable with a peak of ~10 % EQE for Cl-Cl6BsubPc, and is dramatically increased to >40 % EQE for ClCl12BsubPc. While the EQE for pentacene/Cl-BsubPc shows only contribution from the BsubPc absorbance, the EQE for pentacene/Cl-Cl12BsubPc shows contributions from both layers and is comparable to that of the OPV based on pentacene/C60. Notable, for the shorter wavelengths in the range ~525 nm to 650 nm, where we see almost no change in EQE spectrum between the ClBsubPc and Cl-Cl6BsubPc containing devices whereas there is an increase in EQE to >40 % EQE in the Cl-Cl12BsubPc device. Given that wavelength range is also within pentacene absorbance, this rise in EQE is a result of either the higher efficiency of triplet dissociation at the pentacene/Cl-Cl12BsubPc interface (established by the increased EQE near 670 nm) or the stronger absorbance of the Cl-Cl12BsubPc. Detailed optical and excited state modeling would resolve the contribution of each factor to the increase in EQE but is beyond the scope of this letter. These photovoltaic device metrics and results are more evidence that the energy of the Igap relative to the triplet energy can be a reasonable molecular design rule for predicting whether a


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particular organic/organic heterojunction will dissociate triplets generated within a singlet fission capable material, perhaps regardless of whether one is only considering BsubPcs. The triplet energy in pentacene is estimated to be between 0.85 eV and 1.0 eV,12 and we see a trend of increasingly efficient dissociation of triplets in the solar cells with Igap approaching that range and meeting 1.0 eV (Figure 1b). Thus, as guidance for future synthetic work, and in agreement with findings for quantum dot based triplet harvesting photovoltaics,4,12,14 we suggest that organic donor and acceptor material based photovoltaic cells leveraging singlet fission for increased photocurrent are hampered by a limited VOC, as an Igap less than or equal to the triplet energy is required to efficiently dissociate triplet excitons into charge carriers. When looking at Figure 1e it is notable that the pentacene/BsubPc pairing suffers from a drop in the EQE from ~380 nm to 450 nm, corresponding to a lack of light absorbance in this same wavelength range (Figure 1c), something that is not seen for the pentacene/C60 OPV due to the absorption profile of C60 being complimentary to that of Pent. The design principle of complimentary absorbance suggests greater current extraction from a BsubPc based photovoltaic device could be achieved if the photons in this wavelength range could also be absorbed and dissociated into charge carriers. From Figure 2a it is clear that the light absorbance of α6T compliments that of BsubPcs. α6T has been previously paired with Cl-BsubPc and Cl-BsubNc in an highly efficient planar heterojunction OPV device,23 and we have suggested it as a good material for pairing with BsubPc derivatives as a rapid screening technique for new BsubPc derivatives.32 Therefore as a point of contrast and comparison to the pentacene/Cl-Cl12BsubPc device, an α6T/Cl-Cl12BsubPc device was constructed whereby the singlet fission process would not be present. The resulting photovoltaic devices showed a slightly lower VOC and a significantly


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reduced JSC (Table 1) resulting in the overall power conversion efficiency of the α6T/ClCl12BsubPc devices being 0.58 ± 0.05 %. By comparison, the overall power conversion efficiency of the pentacene/Cl-Cl12BsubPc photovoltaics was 1.2 ± 0.05 %. As with the pentacene containing cell, the α6T/Cl-Cl12BsubPc cell is a relatively unoptimized cell. Consulting the EQE plot (Figure 2), we did see a major gain in absorbance in the 400 nm to 500 nm wavelength region where α6T absorbs, but a loss in the longer wavelength region beyond 650 nm where the pentacene/Cl-Cl12BsubPc device previously benefitted from pentacene absorbance and the singlet fission process. Given the comparison point between the pentacene/Cl-Cl12BsubPc and α6T/Cl-Cl12BsubPc device, we can draw some conclusions. The first is that when a BsubPc is synthetically modified to deepen its HOMO and LUMO (in this case by polychlorination) the BsubPc becomes an efficient triplet harvester when paired with pentacene with comparable OPV device metrics to the well-known triplet harvester C60 with a more than tripled JSC when compared with other BsubPc acceptors. Conversely, when Cl-Cl12BsubPc is paired with the singlet fission free material α6T in an OPV, it is found to yield devices with significantly lower efficiency when compared against Cl-BsubPc and Cl-Cl6BsubPc; a result of the expected drop in VOC due to the reduction in the interfacial gap energy (Igap) and an observed drop in JSC. At this point in time we cannot offer an explanation for the disproportionate drop in JSC for Cl-Cl12BsubPc when compared to Cl-BsubPc and Cl-Cl6BsubPc. These results suggest that future BsubPc synthetic targets for triplet harvesting can use the criteria of interfacial gap energy less than or equal to the triplet energy to ensure efficient triplet dissociation and that the method of polychlorination is a valid approach to achieve this property. However, if using the concept of complimentary absorption engineering one should instead focus


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on fully hydrogentated BsubPcs such as Cl-BsubPc, and its derivatives, to maximize the Igap and VOC with, for example, α6T. Work is ongoing in our laboratory on the examination of axially modified derivatives of Cl-Cl12BsubPc, both the synthetic chemistry and application in OPVs, to fully scope the ability of the Cl12BsubPc moiety to harvest the energy from the singlet fission process within pentacene.

ASSOCIATED CONTENT Supporting Information. Experimental details related to photovoltaic device fabrication and testing. UPS measurement data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported through Canada's National Science and Engineering Research Council (NSERC) Discovery Grant Program (TPB).


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FIGURES Figure 1. (a) The chemical structures of the electron donor and electron acceptor materials used in this study. Depiction is consistent with previous literature discussion on how to simultaneously depict frontier molecular orbitals and excited states on the same energy scale.30; (b) The singlet and triplet energy levels of pentacene relative to the HOMO and LUMO energies for C60 and the three BsubPc derivatives: Cl-BsubPc; Cl-Cl6BsubPc; Cl-Cl12BsubPc.; (c) Thickness adjusted absorbance for Pent, C60, Cl-BsubPc, Cl-Cl6BsubPc and Cl-Cl12BsubPc.; (d) JV curves for three BsubPc derivative containing photovoltaic devices with varying degrees of triplet harvesting. Shaded regions in (d) and (e) show the 95% confidence interval; Dashed lines are previously reported data. (e) External quantum efficiency as a function of wavelength for the materials and devices in part (d). Note(s): The EQE plot uses the same legend as part (b); the Cl-BsubPc and Cl-Cl6BsubPc cells and metrics were originally reported in Reference 17.

(a)


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(b)

(c)


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(d)

(e)


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Figure 2. (a) Normalized absorbance for Pent, α6T and Cl-Cl12BsubPc.; (b) JV curves and (c) external quantum efficiency plots for pentacene/Cl-Cl12BsubPc and α6T/Cl-Cl12BsubPc photovoltaic devices. JV and EQE data for α6T/Cl-BsubPc and α6T/Cl-Cl6BsubPc from Reference 32. Shaded regions in (b) and (c) shown the 95 % confidence interval.

(a)

(b)


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(c)


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TABLES Table 1. Mean device parameter comparison of OPV devices used in this study.a Chemical structures can be found in Figure 1a. Donor/dD

Acceptor/dA nm

JSC (SD)/mA cm-2

VOC (SD)/V

FF (SD)

ηP (SD)/%

nm

No. of

Source

cells tested

Pentacene/60 Cl-BsubPc/25

Pentacene/60 Cl-Cl6BsubPc/25

Pentacene/60 Cl-Cl12BsubPc/25

Pentacene/60 C60/40

α6T/55

α6T/55 a

Device

Cl-Cl12BsubPc/25

C60/40

structure

is

ITO/MoOx(5

1.3

0.87

0.59

0.65

(0.21)

(0.10)

(0.08)

(0.22)

2.1

0.50

0.48

0.50

(0.30)

(0.02)

(0.07)

(0.10)

6.0

0.41

0.49

1.2

(0.34)

(0.003)

(0.02)

(0.05)

6.9

0.38

0.53

1.4

(0.48)

(0.07)

(0.08)

(0.35)

3.4

0.35

0.49

0.58

(0.17)

(0.01)

(0.02)

(0.05)

3.5

0.38

0.53

0.71

(0.17)

(0.007)

(0.03)

(0.06)

nm)/Al(100

nm)

nm)/Donor/Acceptor/BCP(10

71 Ref 17

39 Ref 17

15 This work

50 Ref 17

15 This work

15 Ref 32

for

first

four

devices

and

ITO/PEDOT:PSS/Donor/Acceptor/BCP(10 nm)/Ag(100 nm) for final two devices.


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Boron Subphthalocyanines as Triplet Harvesting ... - ACS Publications

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