Slow Singlet Fission Observed in a Polycrystalline Perylenediimide

Letter pubs.acs.org/JPCL

Slow Singlet Fission Observed in a Polycrystalline Perylenediimide Thin Film Aaron K. Le,† Jon A. Bender,† and Sean T. Roberts* Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712-1224, United States S Supporting Information *

ABSTRACT: Singlet exciton fission (SF) is a process wherein an exciton in an organic semiconductor divides its energy to form two excitations. This process can offset thermalization losses in light harvesting technologies, but requires photostable materials with high SF efficiency. We report ultrafast kinetics of polycrystalline films of N−N′dioctyl-3,4,9,10-perylenedicarboximide (C8-PDI), a chromophore predicted to undergo SF on picosecond time scales. While transient absorption measurements display picosecond dynamics, such kinetics are absent from low-fluence time-resolved emission experiments, indicating they result from singlet−singlet exciton annihilation. A model that accounts for annihilation can reproduce both measurements and highlights that care must be taken when extracting SF rates from time-resolved data. Our model also reveals SF proceeds in C8-PDI over 3.8 ns. Despite this slow rate, SF occurs in high yield (51%) due to a lack of competing singlet deactivation pathways. Our results show perylenediimides are a promising class of SF materials that merit further study.

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limited and has led to difficulty in the design of efficient SFbased photovoltaic cells.20−23 Among known SF chromophores, the family of linear acenes, namely tetracene,24−27 pentacene,19,28−30 and their functional derivatives,17,31−37 have by far received the most research attention due to their nearly quantitative SF yields. However, while the acenes undergo fast and efficient fission, their poor photostability38−40 and modest absorption coefficients41,42 make them nonideal for many photovoltaic applications. In contrast, perylenediimides (PDIs) are commonly employed as industrial pigments43 due to their high photostability and molar extinction (∼5−10 × 104 M−1 cm−1).18,44,45 PDIs also satisfy the energetic requirement for SF, possessing a low lying triplet energy of 1.1−1.2 eV that is roughly half that of its corresponding singlet state.18,44,46 Calculations have predicted that SF in PDI dimers arranged into appropriate slip-stacked geometries can display high fission yields47 with subpicosecond rates,48,49 and experiments have reported SF occurring in PDI thin films on a 180 ps time scale.18 In this Letter, we describe femtosecond transient absorption (TA) and time-resolved fluorescence measurements of one PDI derivative, N−N′-dioctyl-3,4,9,10-perylenedicarboximide (C8PDI), predicted to undergo SF with a rate on the order of 1 ps.48 While TA spectra show picosecond dynamics suggestive of triplet formation, time-resolved fluorescence data indicate SF occurs on a nanosecond time scale in polycrystalline C8-PDI films. Rather, the rapid dynamics present in TA measurements arise from singlet exciton annihilation. This process causes

ommercial photovoltaic cells have experienced a large reduction in module price since their inception, but their high cost relative to other energy technologies still hinders their widespread deployment. Improving solar module efficiency can allow for a reduction in their size and have a marked impact on their installation and production costs.1 A primary energy loss pathway that plagues many solar technologies is charge carrier thermalization, which accounts for ∼50% of the energy lost by silicon photovoltaics.2 Thermalization occurs when a photovoltaic cell absorbs a photon with energy in excess of its bandgap, creating “hot” carriers that rapidly cool to the cell’s band edge and release their excess energy as heat. One promising method to mitigate thermalization losses is to integrate materials into photovoltaic cells that use high energy photons to excite multiple charge carriers rather than allowing the excess energy of these photons to be lost as heat.3 Materials that undergo singlet exciton fission (SF) offer a path toward realizing this goal.3−5 SF occurs in select organic semiconductors with strong electron exchange interactions that lead to a large energy splitting between their spin-singlet and spin-triplet exciton states. In materials where the lowest excited triplet exciton state is half that of its corresponding singlet exciton state, E(S1) ≈ 2E(T1), it is energetically permissible for singlet excitons to evolve into a pair of triplet excitations.6−9 While singlet-to-triplet interconversion is typically slow in organic systems due to weak spin−orbit coupling, SF can occur on femtosecond-to-picosecond time scales as the triplet pair produced by SF is spin-correlated with no net spin.8,10−12 SF’s rapid nature allows it to effectively compete with other singlet exciton deactivation pathways and many systems with triplet yields over 100% have been identified.13−19 While work over the past decade has expanded the library of compounds known to undergo SF, this set of materials is still © XXXX American Chemical Society

Received: October 7, 2016 Accepted: November 14, 2016 Published: November 14, 2016 4922

DOI: 10.1021/acs.jpclett.6b02320 J. Phys. Chem. Lett. 2016, 7, 4922−4928

Letter

The Journal of Physical Chemistry Letters

while its absorption onset is decreased in energy (568 nm). AFM images of our C8-PDI films indicate that they are polycrystalline (see Supporting Information, Figure S2). Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements (see Supporting Information, Figure S3) indicate molecules within these grains adopt slip-stacked arrangements similar to C8-PDI single crystals,50 wherein neighboring C8-PDI molecules align cofacially with an interplane separation of 3.3 Å. While long-range dipole−dipole coupling between molecules that comprise each stack will likely have H-aggregate character,51,52 the close proximity of neighboring molecules within these stacks causes them to experience significant orbital overlap. These short-range chargeresonance interactions can be either H-like or J-like depending on the degree of slip between neighboring molecules.53 When included in a spectroscopic Hamiltonian, the presence of both short-range and long-range electronic coupling can account for the appearance of both H- and J-type features in slip-stacked molecular crystals.54 With regards to SF, the existence of intermolecular chargeresonance couplings is key, as these interactions can provide a path for accessing the spin-correlated triplet pair state, 1 (TT).47,48,55−58 As the degree of slip between neighboring PDIs in molecular crystals can be altered through chemical functionalization at the PDI’s imide position,59−61 Grozema and co-workers have used DFT to predict how coupling elements that lead to SF change with the degree of slip between cofacial PDI molecules.48,49 These calculations predict a maximum SF rate of ∼1/25 fs−1 for molecules slipped by 2.84 Å along their long axis with no slip along their short axis.48 Our GIWAXS data suggest that C8-PDI differs from this ideal arrangement, displaying slips of 3.07 and 1.20 Å along its long and short axes, respectively (see Supporting Information, Figure S3). Nevertheless, C8-PDI is predicted to display a SF rate of 1.15 ps, which should allow SF to effectively outcompete other singlet deactivation pathways. TA spectra of our C8-PDI films were collected using a 400 nm excitation source with 155 fs resolution to determine their SF rate and yield. At short time delays, we observe three prominent negative bands at 460, 530, and 570 nm that can each be assigned to ground state bleaching of C8-PDI. Superimposed on this bleach is a broad photoinduced absorption that extends beyond the red-edge of our spectral probe window (∼750 nm). As these spectral features arise within our time resolution, we assign them to C8-PDI’s lowest excited singlet state, S1. As we increase the time delay between our pump and probe, over ∼10−40 ps we see recovery of the S1 photoinduced absorption to baseline (blue arrow), indicating a decay of this state. In concert with these changes, we observe a decrease of each ground state bleaching feature (black arrows), but not a full recovery, suggesting that the photoexcited S1 state evolves to a new excited state rather than returning to the ground state. To highlight how the absorption line shape of this new state differs from that of C8-PDI’s S1 state, Figure 1C compares transient spectra measured for a C8-PDI film averaged over time windows at short (400−500 fs, red trace) and long (1.7− 1.8 ns, blue trace) time delays. While the S1 state displays a prominent photoinduced absorption to the red of the C8-PDI ground state bleach, the absorption features of the state born from the S1 state’s decay solely overlap with C8-PDI’s ground state bleach. This leads to superimposed negative and positive features throughout the region where C8-PDI absorbs. Prior

energy to dissipate as heat, leading to a nonequilibrium temperature distribution within C8-PDI films that alters their absorption spectrum. Here, we separate contributions from triplet production and photoinduced heating to TA spectra using a combination of triplet sensitization and temperaturedependent absorption measurements. Including photoinduced heating in a kinetic rate model changes the interpretation of the TA spectra, revealing that heating artifacts dominate the data. Although we find SF is slow in C8-PDI, our results indicate it occurs with just over 50% efficiency due to a lack of competing singlet decay pathways. As such, we conclude PDIs are a promising class of SF materials that merit further investigation. Figure 1A shows absorption and emission spectra of a C8PDI film thermally deposited on SiO2. In dichlormethane, C8-

Figure 1. (A) Absorption (solid blue) and emission (red dashed) spectra of a 90.5 nm thick vapor deposited C8-PDI film. (B) TA spectra of the film at select time delays following 400 nm excitation. Plotted data correspond to an initial excitation density of 3.6 × 1018 cm−3. (C) Comparison of a transient spectrum indicative of PDI triplets obtained by sensitization (green dashed) with PDI TA spectra averaged over short (400−500 fs, red) and long (1.7−1.8 ns, blue) time delays.

PDI displays a prominent vibrionic progression with a spacing of 1355 cm−1, wherein the strongest absorption feature is the 0−0 transition, which peaks at 524 nm (See Supporting Information, Figure S1). Relative to solution, the film’s absorption maximum is shifted to higher energy (488 nm), 4923


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exponential and unfolds over nanosecond time scales. One potential explanation for the slow TCSPC decay is that the triplet fusion rate in C8-PDI may be comparable to its SF rate, leading to equilibration between singlet and triplet excitons. The blue curve in Figure 2A shows a fit to the data using a kinetic model with this hypothesis as a basis. While the fit captures the TCSPC dynamics on short time scales, it is unable to reproduce the slow decay component. An alternative explanation for the slow TCSPC dynamics is that SF in C8PDI occurs on a nanosecond time scale rather than a picosecond one. Plotted in red in Figure 2A is a fit to the TCSPC data based on a model that allows SF and triplet fusion to occur on nanosecond time scales (3.8 and 6.1 ns, respectively). This fit better reproduces the kinetics due to triplet fusion on longer time scales, suggesting that SF unfolds slowly in C8-PDI. While our measured TA and TCSPC data for C8-PDI films display behavior consistent with triplet formation, the kinetics measured in these experiments markedly differ. However, these experiments are not performed under identical conditions as higher excitation densities are used in TA experiments. Large excitation densities can lead to singlet−singlet exciton annihilation (SSA), a process wherein an excited singlet exciton transfers its energy to a second singlet exciton, promoting it to a higher energy singlet state (S1 + S1 → S0 + Sn). In organic thin films, SSA can lead to accelerated decay kinetics,17,68−72 suggesting that the occurrence of SSA may explain discrepancies between TA and TCSPC measurements. To determine if SSA contributes to our TA data, decay kinetics were recorded as a function of excitation fluence. Figure 2B plots the results of these experiments, which display a marked acceleration of the ground state bleach recovery with increasing excitation density. These results indicate that SSA contributes to our TA spectra and must be accounted for in the modeling of these measurements. In addition to accelerated decay kinetics, SSA can also cause photoinduced heating of C8-PDI films. High energy singlet excitons produced by SSA rapidly return to the S1 state via internal conversion, releasing their excess energy as heat. Such heating can be problematic, as peaks that appear in the ground state absorption spectrum of C8-PDI films shift and broaden as the film is heated (Figure 3A). As heat released by SSA is transferred to ground-state C8-PDI molecules, a thermal contribution to the TA spectrum will grow that resembles the difference in the absorption line shape of a C8-PDI film at elevated temperature and room temperature. Such thermal contributions to TA spectra have been previously observed in thin films of semiconducting polymers73 and SF materials74 and can persist for tens of nanoseconds. Figure 3B plots difference spectra that highlight how the ground-state absorption spectrum of a C8-PDI film shifts upon heating from 295 to 340 K. As temperature increases, we observe band shifts that yield gains in intensity near 470, 510, and 560 nm, which each overlap with the position of peaks in C8-PDI’s sensitized triplet spectrum (Figure 1C). This is not surprising, as both sets of difference spectra contain an identical negative contribution due to C8-PDI ground state bleaching. More distinctive differences between these spectra can be observed when the relative intensity of their positive peaks are compared, which shows that these peaks increase in amplitude moving from low to high energy in the C8-PDI triplet spectrum, whereas the opposite is seen for temperature difference spectra. As such, photoinduced heating resulting

reports that have examined the lowest excited triplet state, T1, of various PDIs have noted that absorption features due to this state often overlap with the PDI ground state bleach,18,44,45,62−64 suggesting that C8-PDI’s S1 state may be evolving into its T1 state. To determine whether the transient spectrum we observe at long delays results from triplet formation, a sensitizer, copper phthalocyanine (CuPc), was doped into a C8-PDI film. CuPc possesses a subpicosecond intersystem crossing rate65,66 and its triplet energy (1.1 eV)21,67 is nearly isoergic with that of PDI (1.1−1.2 eV).44 These features allow CuPc to rapidly form a triplet exciton and pass it to C8-PDI upon selective photoexcitation of CuPc in the doped film. The full kinetics showing the evolution of TA spectra of a CuPc-doped C8-PDI film following CuPc excitation are shown in the Supporting Information (Figure S5). Figure 1C plots the sensitized C8-PDI triplet spectrum recovered from this data (green dashed) alongside TA spectra of a neat C8-PDI film measured at short and long time delays. Overall, the spectral position of both photoinduced absorption and ground state bleaching features agree well between the TA spectra of the sensitized C8-PDI triplet and the neat C8-PDI film at long time delays, indicating that photoexcitation of neat C8-PDI results in triplet formation. The TA experiments described above suggest that generation of C8-PDI triplet excitons should be reflected in time-resolved emission kinetics measured by time-correlated single photon counting (TCSPC). Figure 2A shows the TCSPC decay measured for a C8-PDI film. As this measurement reports the population of C8-PDI’s S1 state, we would expect to observe an instrument-limited decay, given our TCSPC time resolution of 670 ps. Rather, we see a long-lived decay that is not single

Figure 2. (A) TCSPC data for a C8-PDI film. Overlaid traces show kinetic fits assuming picosecond (blue) and nanosecond (red) SF. The nanosecond SF fit captures the delayed fluorescence tail, while the picosecond fit does not. (B) TA decay kinetics measured at λprobe = 460 nm for a PDI film. Annihilation contributes to the kinetics across the range of excitation fluences employed for TA. 4924


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Figure 3. (A) Absorption spectra of a C8-PDI film measured as a function of temperature. (B) Difference spectra highlighting changes in the absorbance of a C8-PDI film upon heating from 295 to 340 K. Figure 4. (A) Kinetic model for SF in C8-PDI thin films that can adequately reproduce our measured TA and TCSPC data. (B) Singlet and triplet exciton populations extracted from our kinetic model as a function of excitation density. Reported populations are in units of excitations/cm3. It is not until the initial excitation density is reduced 97% with lifetimes in the 4925


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kinetics observed in TA. A kinetic model was constructed to describe SF that accounts for both SSA and photoinduced heating of C8-PDI films. We find an upper limit for triplet formation at 102%, with fission occurring on a time scale of 3.8 ns. One important result of the kinetic model is that the annihilation free regime occurs at excitation densities less than 1015 cm−3, which is lower than typical fluences used in TA measurements of organic thin films. While SF in C8-PDI is slow, a yield just in excess of 100% despite its slow SF rate is promising. Our results suggest that with some degree of modification, PDIs may be made to undergo SF in high yield, perhaps by raising their singlet energy. Further study of this class of materials for SF applications is warranted.

heating by using a third basis spectrum constructed from the slope of the change in absorbance as a function of temperature at each wavelength (see Supporting Information, Figure S6). With these basis spectra in hand, we perform a simultaneous fit to both our TCSPC and TA spectra across a range of excitation densities. A detailed comparison of the resulting fit and our TA spectra can be found in the Supporting Information (Figures S7 and S8), while the fit to the TCSPC data appears in Figure 2A (red trace). Overall, our model reproduces well the transient kinetics we observe and allows us to extract the SF rate for our C8-PDI films, 1/kSF = 3.8 ns. In contrast to calculations that suggest that SF in C8-PDI should occur with a time scale of 1.15 ps,48,49 our results show that SF in this material occurs roughly 3 orders of magnitude slower, on a nanosecond time scale. This difference from theoretical predictions may reflect the large shift in energy of C8-PDI’s S1 state upon moving from solution to the solid state, which was not accounted for in calculations of PDI dimers.48,49 C8-PDI’s S1 energy can be estimated from the crossing point of its normalized absorption and emission spectra (Figure 1A), yielding a value of 2.09 eV. This is substantially lower than C8-PDI’s S1 energy in solution, 2.35 eV (see Supporting Information, Figure S1). Comparing C8-PDI’s singlet energy to that reported for PDI triplet excitons, (1.1−1.2 eV),44 suggests that SF in C8-PDI may involve an activation energy in the range of 110−310 meV. In this regard, SF in C8-PDI may be more analogous to SF in tetracene than it is in pentacene. While pentacene undergoes SF on an 80 fs time scale19 due to the absence of an activation barrier, SF is markedly slower in tetracene (∼100 ps)13 due to the need for thermal activation.82 A similar need for thermal activation could explain the slow rate of SF in C8-PDI. However, despite its slow rate, our model suggests that SF remains somewhat efficient in C8-PDI. Figure 4B plots the singlet and triplet populations extracted from our model for a range of initial excitation densities. Extrapolating our model to the annihilation free regime shows that on average 1.02 triplets are produced per absorbed photon (SF yield of 51%). This surprisingly high yield despite a slow SF rate reflects that competing processes which return C8-PDI singlet excitons to the ground state are slow (1/kSD = 4.1 ns). Importantly, our results also highlight the danger of neglecting the effects of SSA on TA spectra of SF materials. The inset in Figure 4B shows the triplet population predicted by our model normalized to their asymptotic values at long time delays for different initial excitation densities. As the initial excitation density increases, we see that the rate with which the triplet population reaches its asymptotic value increases due to SSA competing with SF. The inset exemplifies the inherent problem in using TA to estimate SF rates: the triplet production appears faster when SSA is present as only fission events that outcompete SSA will be observed.13 Examining the populations shown in Figure 4B, it is not until the excitation density is lowered to a value of 1015 cm−3 that SSA can be completely neglected, and the triplet growth accurately reflects C8-PDI’s SF rate. As TA measurements of organic thin films are commonly performed with excitation densities that can be many orders of magnitude higher than this value, SSA must be taken into account when reporting SF rates. In this Letter, we examined SF in a polycrystalline PDI film using ultrafast TA and time-resolved emission. Delayed fluorescence observed in TCSPC data can be explained if SF occurs on a nanosecond time scale, contrary to the picosecond

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02320. Details regarding sample preparation by vapor deposition and characterization using AFM and GIWAXS; description of the experimental layouts used for TA and TCSPC measurements; results obtained from triplet sensitization experiments; detailed description of the kinetic model used to fit TA and TCSPC data alongside fits to these data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sean T. Roberts: 0000-0002-3322-3687 Author Contributions †

These two authors contributed equally to this manuscript.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation (Grant F-1885), the American Chemical Society Petroleum Research Fund (Grant 55184-DNI6), and start-up funding provided by the University of Texas at Austin. A.K.L. would like to acknowledge additional support from a J. M. White Endowed Presidential Fellowship in Chemistry, and J.A.B. would like to acknowledge support from a Dorothy B. Banks Summer Fellowship. The authors would also like to thank Prof. Ferdinand C. Grozema for providing the results of calculations published in ref 48. This work was performed in part at the Center for Nano- and Molecular Science (CNM), a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation (Grant ECCS-1542202).

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REFERENCES

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Slow Singlet Fission Observed in a Polycrystalline Perylenediimide

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