Role of Spin-Coupled Polaron Pairs in the Recombination of Charges

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C: Physical Processes in Nanomaterials and Nanostructures

Role of Spin-Coupled Polaron Pairs in the Recombination of Charges in Electroluminescent Conjugated Polymers Rajarshi Chakraborty, Yongli Lu, and Lewis J. Rothberg J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01058 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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The Journal of Physical Chemistry

Role of Spin-Coupled Polaron Pairs in the Recombination of Charges in Electroluminescent Conjugated Polymers Rajarshi Chakraborty1, Yongli Lu2 and Lewis J. Rothberg*1,2 Materials Science Program1, University of Rochester, Rochester, NY 14627, USA. Department of Chemistry2, University of Rochester, Rochester, NY 14627, USA.

ABSTRACT: We study the delayed fluorescence following geminate recombination of photogenerated polaron pairs in films of a model conjugated polymer F8BT. Doping with tiny gold nanoparticles also enables us to observe phosphorescence from triplet states that arise via intersystem crossing from the directly photoexcited singlet state. Small magnetic fields ~ 5 mT have large effects on the delayed fluorescence by causing demixing of the hyperfine coupled singlet and triplet polaron pair configurations, implying polaron pair spin equilibration times much shorter than 1 µs. Nevertheless, the magnetic field effect on the fluorescence persists to tens of microseconds and we argue that this apparent inconsistency implies that photogenerated polarons reside primarily as uncoupled spin ½ particles with long spin memory times. We consider the implications for whether recombination of charges in organic light-emitting diodes is likely to be governed by quantum spin statistics.

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INTRODUCTION

Conjugated polymers remain interesting for OLED technology as their processability may yet enable economically valuable changes in display manufacturing. In addition, there are fundamental reasons to believe they may not be subject to the 3:1 recombination branching ratio of triplets to singlets presumably dictated by spin statistics1,2. While the results reamin controversial, several groups have inferred internal quantum efficiencies in fluorescent OLEDs based on conjugated polymers that exceed the presumed 25% limit3-5 and there is theoretical work that suggests this is possible6-8. As has been pointed out, both the energy of charge pairs and the delocalization of the polaronic wavefunctions are better matched to those of the singlet exciton than to those of the low energy, highly localized triplet state4,7. There is also experimental evidence from absorption detected magnetic resonance suggesting that the recombination branching ratio varies with conjugation length6 and that the singlet formation yield may be much larger than 25%, although the interpretation of those experimental data relies heavily on complex modeling. In contrast, the results from other types of experiments involving application of electric fields to delayed photoluminescence (PL) that arises from polaron pair (PP) recombination have been interpreted to mean that spin relaxation times are very long and therefore enforce a 3:1 recombination ratio9. A potentially related observation that is poorly understood is the dependence of photoluminescence and electroluminescence (EL) efficiencies on relatively small magnetic fields (several mT) that has been documented for over 20 years10. In the case of EL, it remains controversial whether these phenomena derive from magnetotransport effects11, effects of hyperfine mixing on the recombination of polaron pairs12-14, or both.


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In the present paper, we report data on the spectra and dynamics of delayed PL due to PP recombination in films of the model conjugated polymer F8BT (Poly[(9,9-di-n-octylfluorenyl2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)])

that have been doped with small gold

nanoparticles. The primary effect of incorporating the nanoparticles is to introduce spin-orbit coupling that makes the phosphorescence sufficiently allowed that we can monitor triplet state populations directly. We reasoned that it might be feasible to estimate recombination branching ratios directly if we could properly calibrate the amounts of fluorescence and phosphorescence in terms of the number of singlets and triplets produced by recombination. Moreover, if there are magnetic field effects on PL, it stands to reason that they appear entirely in the delayed PL and would therefore be much easier to study than using techniques that record steady state PL10,12. Furthermore, we would not expect to observe effects of magnetotransport on delayed PL so that we can confirm that some or all of the magnetic field effects in OLEDs arise from changing the recombination branching ratio.

EXPERIMENTAL SECTION

Sample preparation. Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8diyl)] (F8BT) was obtained from Sigma Aldrich in powder form having average molecular weight of 23,000 g/mol and polydispersity of ≤ 3. A stock solution of F8BT in chloroform (1 mg/mL) was produced by stirring the mixture overnight to completely dissolve the polymer. The stock solution was used as is to make drop cast films of “pristine” F8BT as described below. The stock solution was also modified to make a second set of F8BT films containing gold nanoparticles (Au-np). The Au-np (2 nm diameter from Nanocomposix, surface modified with


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C18 alkanethiol chains) were incorporated by mixing small amounts of gold nanoparticle solutions into the stock solution. The Au-np solutions were formed by dissolving 5 mg Au-np in 10 mL chloroform and stirring for 2 hours. Two µL of the Au-np solution was added to 20 µL of the F8BT stock solution for use to make the Au-np containing F8BT films. In each case, several drops of the stock solution were placed on clean 2 mm thick quartz discs of 2 cm diameter and left to dry slowly in the dark prior to use. This corresponds to approximately one gold nanoparticle for every 2-3 F8BT monomer units.

Steady state spectroscopy. Absorption measurements were carried out at room temperature using a Cary 60 UV-VIS spectrophotometer. Steady state emission spectra were collected using Horiba Yvon Fluoromax 3 fluorimeter with the film tilted around 45 degrees from the incident beam and collection direction.

Delayed luminescence. For delayed luminescence, a Q-switched pulsed Nd:YAG laser (SpectraPhysics-Quanta Ray) with ~ 5 ns duration pulses and a repetition rate of 10 Hz was frequency doubled with a KDP crystal to produce 532 nm radiation. The beam was attenuated and its energy at the sample measured with a pyroelectric detector (Laser Precision RjP-7200). Pulse to pulse variation was approximately 20% and the long term stability was better than 5% so that 1000 pulse averages of the PL produce variation of order 1%. The film was placed in a cold finger sample holder that was cooled with a temperature controllable liquid Helium closed cycle refrigerator (APD Cryogenics Inc DE-202) with optical access. The laser was weakly focused with a cylindrical lens to a rectangular spot that approximately matched the spectrometer slits, around 8 mm high and 2 mm wide on the sample, excitation being done through the back of


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the substrate. The luminescence was collected with a 2 inch diameter and 2 inch focal length collimating lens followed by an imaging lens chosen to f-match a 0.3 meter Oriel Instruments spectrometer fitted with an Andor time-gated intensified CCD at its exit port. The emission from the sample was collected around 20 degrees from the direction of propagation of the excitation beam. Along with a holographic notch filter at 532 nm in the collection path, this helped to minimize collection of light from the excitation pulse. The CCD gating pulse was timed relative to an electrical pulse from the laser that preceded the laser Q-switch by ~ 700 ns with a SRS-545 digital delay generator.

For experiments with an applied magnetic field, we placed permanent nickel-plated neodymium magnets outside the cryostat between 3 and 30 cm from the sample. The magnitude of the field at the sample position was precalibrated with a Gaussmeter. The direction of the field was perpendicular to the plane of the F8BT film for all of the measurements reported here but we found that in plane magnetic fields produced qualitatively similar results.

The data acquisition protocol involved interleaving collection at various time delays to minimize the effects of any systematic drift of the laser intensity on the dynamics. For delays shorter than 1 µs, we used 100 ns gate width and for delays greater than 1 µs, we used gate width 10% of the delay. In order to plot proper dynamics, the signals are therefore scaled for the appropriate gate width prior to presentation.


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RESULTS AND DISCUSSION

The results of our study are complex. First, we find that there are substantial magnetic field effects on the recombination but they depend strongly on gate delay. Our results clearly imply that spin relaxation times for polaron pairs are relatively short (< 1 µs). Second, we find that, in spite of spin equilibration times in bound polaron pairs, magnetic field effects on the delayed photoluminescence surprisingly persist to hundreds of microseconds as will be documented and explained below. Finally, we learn from intensity dependent studies that singlet-singlet annihilation can result in polaron pair formation and augment delayed luminescence at high excitation intensity.

Delayed luminescence and the effect of gold nanoparticles. Figure S1 portrays the steady state absorption and emission spectra of the F8BT samples formed with and without gold nanoparticles. The nanoparticles are ~ 2 nm in diameter and loaded at the level of roughly one per three F8BT monomers. The gold nanoparticles are too small to exhibit plasmon resonance and their main effect on the steady state spectra is the introduction of a slight blue shift, presumably by hindering chromophore aggregation so that fewer pi-stacking interactions between chromophores are formed. Figure 1(a) presents the prompt and delayed PL spectra at low temperature (~ 20 K). Unlike polymers whose backbones can support active torsional modes where twist is coupled to the HOMO-LUMO gap15, the spectra do not shift greatly with temperature in F8BT. The effect of gold on the prompt PL (here defined as PL within the first 100 ns but actually dominated by that within the natural lifetime of F8BT which is less than 1 ns) spectrum is similar to that observed in Figure S1, consistent with the fact the well over 99% of


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the PL is prompt. For the samples depicted in Figure 1, the delayed fluorescence integrated from 100 ns after photoexcitation to 1 ms after photoexcitation is approximately 1000 times weaker than the prompt PL at 20 K but this ratio varies from sample to sample by a factor of 2 or 3. It seems reasonable that charge pair photogeneration would depend strongly on interchain contact and therefore on the details of the preparation and resulting morphology. In fact, we have deliberately used drop cast samples that dry slowly in order to increase the delayed PL.

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Figure 1. (a). Normalized prompt (t = 0) and delayed (integrated from t = 100 ns to t = 1 ms) photoluminescence spectra of F8BT films with and without incorporated gold nanoparticles at 20 K. The delayed photoluminescence traces are multiplied by 590 (without Au) and 1900 (with Au) to facilitate comparison of the band shapes. The slight shoulder on the blue edge for the t = 0 spectra is a small amount of leakage of the 532 nm pump light through the holographic filter. (b). Normalized delayed photoluminescence spectra versus pump intensity (I1 < I2 < I3 < I4) where the pump fluence values are as labeled in Fig. S2.


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In samples without gold nanoparticles, the delayed luminescence spectrum is essentially identical to that of the prompt luminescence but an additional red band appears in samples where gold particles are introduced. We assign that band to the phosphorescence of F8BT which becomes partially allowed due to the spin orbit coupling associated with the introduction of heavy atoms (gold nanoparticles) into the polymer film. The position of the band ~ 0.7 eV below the fluorescence is typical of singlet-triplet separations in conjugated polymers16 and the fact that the red band is not observed without gold or above 100 K also supports our interpretation. Note that we have assumed that the effect of the spin-orbit coupling on the phosphorescence is much greater than that on the intersystem crossing from the singlet exciton to form triplets which seems reasonable because the triplet state is nearly four orders of magnitude longer lived than the singlet state.

Origin of delayed fluorescence. The delayed luminescence spectra shown in Figure 1(a) depend on excitation fluence as illustrated in Figure 1(b). The dependences of the prompt fluorescence, delayed fluorescence and phosphorescence on excitation fluence are presented in Figure S2 and further support our interpretation of the emission bands. At low fluence (< 30 µJ/cm2 corresponding to 7 x 1018 excitations/cm3 per pulse), the relative amount of phosphorescence is dramatically larger and all of the PL signals are linear. Raising the excitation fluence causes the phosphorescence and prompt PL to rise in a sublinear fashion. The reason for that is due to annihilation of singlet excitons via exciton-exciton “reactions” at high intensity. Our estimate is consistent with previous densities at which substantial annihilation sets in for typical conjugated polymers17 when we account for the fact that our excitation pulse duration is


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around 10 times longer than the low fluence exciton lifetime so that the exciton densities created are about an order of magnitude lower than the estimate above. As seen in Figure S2, the magnitude of phosphorescence tracks the singlet population (prompt PL) closely. As we will show later, the reason for that is because the triplet state is formed by direct intersystem crossing from the singlet and not from polaron pair recombination. The delayed fluorescence, however, exhibits dramatically different behavior and rises superlinearly with pump fluence because increasing the singlet exciton annihilation at high density provides an additional pathway for polaron pair formation. To keep interpretation of our data relatively simple, all of the data recorded in Figure 1(a) and in subsequent figures were taken using pump fluence of I3 ~ 30 µJ/cm2 which remains in the nearly linear regime for all of the species.

The decay dynamics of the bands at two low temperatures are reported in Figure 2. These dynamics are identical at the three lowest intensities in Figure 1(b) (two of these shown in Figure S3) confirming our intuition that the recombination events are nearly entirely from geminate pairs as triplet-triplet annihilation processes would result in excitation intensity dependent dynamics. The delayed fluorescence follows a power law decay over the delay and temperature ranges studied and is thermally activated (larger at 77 K than 20 K). Its integrated intensity from 100 ns – 1 ms is typically 300 – 500 times lower than the corresponding prompt fluorescence at 20K, 100 – 200 times lower at 77 K and 30 – 60 times lower at 300 K. These values are sample dependent with more aggregated samples as judged by the spectroscopy generally exhibiting more delayed fluorescence. All of these observations plus the intensity dependence in Figure 1(b) are consistent with the origin of the delayed fluorescence being geminate recombination of rapidly photogenerated pairs of polarons.


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Figure 2. (a) Decay dynamics of delayed fluorescence and phosphorescence in F8BT films with gold nanoparticles at two temperatures (20 K and 77 K). Fits of the 77 K delayed fluorescence to a t-0.85 power law decay and of the 77 K phosphorescence to an exponential decay exp(-t/8µs) are shown for reference. Values for the fluorescence and phosphorescence are obtained by spectral integration of the bands as described in Fig. S2. (b) Analogous data for F8BT films with no gold nanoparticles.

These polaron pairs are almost certainly formed preferentially on separate chain segments in “well packed” (pi-stacked) regions of the polymer.

The thermal activation barrier to

recombination is probably associated with polaron binding energy, the fact that a lattice distortion is required to facilitate an interchain electron hop. Whether this hopping is associated with the actual charge recombination event or with diffusion to form special configurations of


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polaron pairs susceptible to hopping is not clear but we will argue below that the latter process is an important element of the recombination mechanism. The slope of the power law decay behavior is consistent with a 0.1 eV wide distribution of recombination barriers as is common in disordered semiconductors18 and as has been observed and modeled previously in conjugated polymer delayed luminescence19-21. The delayed fluorescence decay dynamics are very similar for the samples with and without gold nanoparticles, the undoped delayed emission being slightly higher perhaps because the gold nanoparticles disrupt interchain ordering and may therefore suppress some polaron pair photogeneration.

Origin of phosphorescence. The phosphorescence decay dynamics observable in the samples incorporating gold nanoparticles are quite distinct from the fluorescence decay dynamics. It is worth noting that the triplet appears immediately (in less than our time resolution of 100 ns), faster than our estimate of spin flip times (to be discussed below), and that the phosphorescence decay is nearly exponential as indicated by the fit to the data in Figure 2(a). These behaviors plus the fact that the excitation intensity dependence of the phosphorescence tracks that of the prompt fluorescence rather than that of the delayed fluorescence imply that the triplets producing the bulk of the phosphorescence originate via intersystem crossing from the initially photogenerated singlet state and not through polaron pair recombination. The magnitude of the phosphorescence implies a lower limit (assuming that the radiative efficiency of the triplet is at most the same as that for the singlet at 20 K) of about 0.1% triplet formation, a reasonable minimum expectation for typical hydrocarbons22.


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We would like to add one small but important caveat to our assertion that the phosphorescence derives from intersystem crossing by singlet excitons which is that the phosphorescence observed at the longest delays (> 30 µs) in Figure 2(a) could derive from triplets formed by polaron pair recombination rather than direct intersystem crossing. As can be seen in Figure 2(a), the exponential fit to the phosphorescence decay predicts that it should be an order of magnitude weaker at 30 µs delay where there remains no doubt that we can still see phosphorescence with good signal to noise ratio (Figure S4). The reason that this subtlety is potentially very important is that it appears to leave us with circumstances where it remains possible in principle to compare the relative numbers of singlet and triplet states formed by recombination of polaron pairs with random spin orientation, the critical branching ratio underpinning OLED efficiency. We return to a discussion concerning determination of the branching ratio below after reporting on how magnetic fields affect the delayed luminescence.

Magnetic field effects on delayed luminescence. Magnetic field effects on steady state photoluminescence and electroluminescence in conjugated polymers have been reported before10,12-14,23,24 but, with the exception of the very recent report by Lupton and coworkers14, these studies have not been reported for the delayed luminescence where the effects of magnetic fields, if any, should be concentrated. Figure 3(a) reports these effects for the delayed fluorescence and phosphorescence in F8BT films containing gold nanoparticles and for the delayed fluorescence in F8BT films without nanoparticles. The effects on the delayed fluorescence are very large even for fields in only the mT range where the Zeeman energies are ~ 1 µeV, much less than kBT even at 20 K (~ 1.7 meV). The energy scale alone implies that the importance of the field must be in lifting the near degeneracy between singlet and triplet polaron


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pair configurations12,25-27 enough that they are no longer mixed by the hyperfine effect. Our results are consistent with estimates of the hyperfine interaction as ~ 1 µeV in other conjugated polymers24-28. The sublinear increase of the size of the magnetic field effect (Figure 3(b)) is consistent with the idea that the effect saturates when the Zeeman energies become larger than the hyperfine interaction. In our experiments, polaron pairs are initially generated in the singlet configuration due to spin selection rules on the photoexcitation of the singlet excitons from which the pairs are formed. Because application of the field demixes the singlet 1PP and ms = ± 1 triplet 3PP spin configurations of the polaron pairs by overwhelming the hyperfine coupling, the conversion of singlet charge pairs to the triplet polaron pair configuration is retarded by the field. Therefore, we observe more recombination of

1

PP to the singlet state (more delayed

fluorescence) because recombination competes more favorably with formation of the triplet polaron pair 3PP when the magnetic field is applied. In this scenario, one might expect reduced formation of the triplet state when the field is applied but we observe no commensurate field effect on the phosphorescence. The absence of a magnetic field effect on phosphorescence is consistent with our conclusion that the triplet excitons are predominantly formed via intersystem crossing from initially photogenerated singlet excitons during their lifetime of ~ 1 ns.


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1.10 1.05 1.00

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Figure 3. (a) Ratio of signal with and without a 93 G applied magnetic field versus delay in F8BT films with and without gold and at two different temperatures. For reasons discussed in the text, we subsequently accumulated much more accurate data for delays greater than 30 us at 20 K, concluding that the change in phosphorescence with field is within error of zero. For clarity, error bars are not displayed but are ~ ± 0.04. (b). Field dependence of the delayed fluorescence at several different time delays after photoexcitation.

In order for the field to have substantial effects on the several hundred nanosecond timescale, the interconversion between spin configurations for the polaron pairs in the absence of the field must be on the submicrosecond timescale. Since the charge pairs must begin in the singlet configuration, there would be no way for the magnetic field to increase the delayed fluorescence at short times (≤ 100 ns in Fig. 3(a)) unless spins were already decorrelated and the applied field could slow the decorrelation. The rapid spin equilibration we observe appears to be in contradiction with the conclusions of previous experiments by Reufer etal9 who conclude that spin interconversion times are long and therefore suggest that recombination branching ratios for


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polaron pairs with random spin must be 25%. Moreover, the rapid spin interconversion also appears to be internally inconsistent with our own observation (Figure 3(a)) that magnetic field effects on delayed fluorescence persist to at least 100 µs. Understanding this anomaly is critical to understanding the mechanism of charge recombination, the reason for magnetic field effects and for evaluating whether quantum spin statistics should be expected to limit singlet yields in OLEDs. We believe that the simplest explanation for persistent magnetic field effects is that the vast majority of polaron pairs diffuse far enough away from each other that their spins are uncoupled so that they behave as separate (though Coulomb bound) spin ½ particles whose spin relaxation times are very long. This is consistent with tens of microsecond spin relaxation times measured for isolated polarons in MEH-PPV29 and with the large light-induced electron spin resonance signals observed in conjugated polymers30. In this separated configuration, the charges retain memory of their original spins and, when they encounter one another in configurations susceptible to coupling into a singlet or triplet polaron pair, the hyperfine coupling of singlet and triplet pairs described above can be disturbed by the applied magnetic field during that encounter. Thus, even rapidly separated pairs that do not encounter one another for microseconds nevertheless reform preferentially in a singlet pair configuration so that demixing of the singlet and triplet polaron pair configurations produces enhanced fluorescence even after tens of microseconds of separation.


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Figure 4. Proposed photophysical scheme to explain the observed F8BT delayed luminescence decay dynamics and its magnetic field dependence. The states and their roles are described further in the text. The intersystem crossing rate for singlet excitons (1S) to form triplet excitons (3T) is denoted kISC. The spin flip rate for converting singlet polaron pairs (1PP) to a triplet configuration (3PP) is given by the spin flip rate kSF which depends on magnetic field as described in the text. The rate for the reverse process 3PP 1PP should be three times lower as prescribed by detailed balance.

The overall scheme we are proposing is represented in Figure 4. Photoexcitation primarily leads to separated charges but some of these rapidly form (or are created in) a “spin-coupled polaron pair” state where their spins are mixed by the hyperfine interaction. The magnetic field can turn off the hyperfine coupling, effectively reducing spin interconversion rates. In the delayed PL experiment, this results in lower triplet pair formation and increased fluorescence. Polaron pairs whose spins are uncoupled remain bound by the Coulomb force but have much


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longer spin memory times and can later become spin coupled prior to recombination so that magnetic fields still enhance fluorescence. Presumably, the spin coupled pairs represent the subset that are much closer to one another in that our model requires they be the precursors to reformation of the exciton. At the same time we note that our model requires small exchange energy which suggests minimal wavefunction overlap and that they are not very close. The picture in Figure 4 has implications for prior delayed PL experiments9 where ratios of fluorescence to phosphorescence were found to be delay independent. This observation was interpreted to mean spin equilibration times are very long and therefore dictate 25% singlet formation from charge recombination. It is clear from our data, however, that the relevant spin equilibration is determined by that of the spin-coupled pair and not the very slow equilibration associated with uncoupled pairs. Therefore, we do not think that the conclusions of the previous work are valid or relevant to OLED recombination physics. It is worth considering whether the introduction of gold particles introduces spin orbit coupling that accelerates spin-orbit coupling and greatly changes the dynamics in Figure 4, rendering our conclusions suspect. Three pieces of evidence suggest that this is not the case. First, the relative amounts of delayed and prompt fluorescence (Figure 2) are essentially unchanged by the introduction of the gold particles. Along with the fact that the singlet lifetime is unaffected, these suggest that gold does not enhance intersystem crossing to the triplet very much. Similarly, the recombination dynamics seen in Figure 2 are unchanged by gold so it seems unlikely that the spin flip rates for polarons and polaron pairs can be dramatically affected. Finally, we note that the magnitude of the magnetic field enhancement of the delayed fluorescence is similar in samples with and without gold particles (Figure 3(a)). Together, these suggest that the gold


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doping is a minor perturbation on the scheme in Figure 4 and that we are properly probing the spin branching dynamics.

As an aside, it is also worthy of note that the scheme in Figure 4 resembles the mechanism proposed for magnetic field effects on radical recombination31,32 but with the distinct difference that the precursor to recombination is the radical pair itself and not an intermediate charge transfer exciton. While we cannot rule out the role of a CT state in the recombination and CT states of suitably high energy have been reported in conjugated polymers33, we think that the simplicity of our model remains attractive and is able to explain our observations.

Implications for recombination in OLEDs. It is widely asserted that the expectation for the recombination ratio of singlets to triplets in OLEDs is 1:3 due to the statistical degeneracy of these states. The underlying presumption is that spin equilibration rates are very long compared to the formation rates of singlet and triplet excitons from pairs that encounter one another. The mechanism above makes it clear that the relevant spin equilibration rate is that for spin-coupled polaron pairs and that this is probably two to three orders of magnitude faster than the simple spin equilibration rate for isolated polarons. It is also clear that the presumption above must be false. If spin equilibration rates for the spin-coupled polaron pair were long compared to recombination (i.e. exciton formation) rates, then there would be no magnetic field effects on recombination in our experiment. We think it follows that the relative exciton formation rates are relevant to spin branching in OLEDs and that it remains plausible that some conjugated polymers will have singlet formation yields well in excess of 25%. We observe similar magnetic field effects regardless of whether gold nanoparticles are present in the sample so our conclusions


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regarding rapid spin interconversion are independent of any enhancement of that process that may be induced by spin-orbit coupling associated with the gold nanoparticles.

If Figure 4 is a correct description of the photophysics responsible for delayed photoluminescence, the best hope for an approximately correct determination of the singlet triplet exciton branching ratio is at very long delays where even the spin 1/2 equilibration has occurred leaving a spin distribution most representative of OLED conditions. Of course, these conditions are those under which the signal to noise ratio is very poor and our best attempts to study these long delays (e.g. Figure S4) still show no effect of magnetic field on the phosphorescence. Our work to improve the signal to noise ratio and observe luminescence that is even more delayed continues.

CONCLUSIONS

We have studied delayed fluorescence due to geminate recombination of photogenerated polaron pairs in F8BT films. Singlet-singlet exciton annihilation leads to further formation of polaron pairs at high excitation intensity. By doping F8BT films with small gold nanoparticles, we can also observe phosphorescence but find that the dominant mechanism for triplet formation is direct intersystem crossing from the singlet state. The delayed fluorescence is strongly enhanced by a magnetic field because the Zeeman splitting of the polaron pair states removes the hyperfine coupling of singlet and triplet configurations such that triplet pair formation from singlet pairs is less competitive with recombination. In order for the demixing of spin


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configurations to be important, the spin flip times to equilibrate spin-coupled polaron pairs in the absence of field must be quite fast, of the order of several hundred nanoseconds. The fact that the field enhancement of the delayed luminescence persists to times much longer than that after photoexcitation indicates that the polaron pairs must spend the vast majority of their time in spin uncoupled (spin ½) configurations. Because our triplet exciton populations are not primarily the result of polaron pair recombination, we are unable to deduce spin recombination branching ratios as would be pertinent to OLEDs from our experiments. Nevertheless, our results confirm the conclusions of previous work that quantum statistics are not the only determinant of recombination branching between singlets and triplets and that there are substantial field effects on that branching so that magnetic field effects on OLED performance cannot be entirely due to magnetotransport.

SUPPORTING INFORMATION The following files are available free of charge: Figure S1. Normalized steady state absorption and photoluminescence spectra of F8BT films with and without incorporated gold nanoparticles. (PDF) Figure S2. Excitation intensity dependence of F8BT prompt and delayed luminescence. (PDF) Figure S3. F8BT delayed luminescence dynamics at two different excitation intensities. (PDF) Figure S4. Delayed luminescence spectrum of F8BT at very long delay (PDF).


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AUTHOR INFORMATION Corresponding Author *Lewis Rothberg, Department of Chemistry, University of Rochester, Rochester, NY 14627, USA. [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. ACKNOWLEDGMENTS We thank Al Marchetti, Samir Farid and Ralph Young for helpful discussions. The authors are grateful for support from the National Science Foundation (DMR-1609451) and YL is grateful for a fellowship from The Professor Richard F. Eisenberg and Harriet Rippey Eisenberg Fund.

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TOC Graphic


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Role of Spin-Coupled Polaron Pairs in the Recombination of Charges

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