Article Cite This: J. Phys. Chem. C 2018, 122, 7013−7019
pubs.acs.org/JPCC
Role of Spin-Coupled Polaron Pairs in the Recombination of Charges in Electroluminescent Conjugated Polymers Rajarshi Chakraborty,† Yongli Lu,‡ and Lewis J. Rothberg†,‡,* †
Materials Science Program and ‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: We study the delayed fluorescence following geminate recombination of photogenerated polaron pairs (PPs) 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 PP configurations, implying PP 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 half 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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effects,11 effects of hyperfine mixing on the recombination of PPs,12−14 or both. 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-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]) that have been doped with small gold nanoparticles (Au-np). 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 the 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 PL.10,12 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.
INTRODUCTION
Conjugated polymers remain interesting for organic lightemitting diode (OLED) technology, as their processability may yet enable economically valuable changes in display manufacturing. In addition, there are fundamental reasons to believe that they may not be subject to the 3:1 recombination branching ratio of triplets to singlets presumably dictated by spin statistics.1,2 While the results remain controversial, several groups have inferred internal quantum efficiencies in fluorescent OLEDs based on conjugated polymers that exceed the presumed 25% limit,3−5 and there is theoretical work that suggests that this is possible.6−8 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 state.4,7 There is also experimental evidence from absorption-detected magnetic resonance suggesting that the recombination branching ratio varies with the 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. By contrast, the results from other types of experiments involving the 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 ratio.9 A potentially related observation that is poorly understood is the dependence of PL and electroluminescence (EL) efficiencies on relatively small magnetic fields (several mT) that has been documented for over 20 years.10 In the case of EL, it remains controversial whether these phenomena derive from magnetotransport © 2018 American Chemical Society
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EXPERIMENTAL SECTION Sample Preparation. Poly[(9,9-di-n-octylfluorenyl-2,7diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT) was obtained from Sigma-Aldrich in powder form, having an average molecular weight of 23 000 g/mol and polydispersity of ≤3. A
Received: January 30, 2018 Revised: March 7, 2018 Published: March 7, 2018 7013
DOI: 10.1021/acs.jpcc.8b01058 J. Phys. Chem. C 2018, 122, 7013−7019
Article
The Journal of Physical Chemistry C
Figure 1. (a) Normalized prompt (t = 0) and delayed (integrated from t = 100 ns to t = 1 ms) PL spectra of F8BT films with and without incorporated Au-np at 20 K. The delayed PL traces are multiplied by 590 (without Au) and 1900 (with Au) to facilitate the 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 PL spectra vs pump intensity (I1 < I2 < I3 < I4) where the pump fluence values are as labeled in Figure S2.
was collected around 20° 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 the 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 the gate width at 10% of the delay. To plot proper dynamics, the signals are therefore scaled for the appropriate gate width prior to presentation.
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 Au-np. The Au-np (2 nm diameter from nanoComposix, surface modified with C18 alkanethiol chains) were incorporated by mixing small amounts of Au-np solutions into the stock solution. The Au-np solutions were formed by dissolving 5 mg of Au-np in 10 mL of chloroform and stirring for 2 h. The Au-np solution (2 μL) 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 disks of 2 cm diameter and left to dry slowly in the dark prior to use. This corresponds to approximately one Au-np 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 a HORIBA Yvon Fluoromax 3 fluorimeter with the film tilted around 45° from the incident beam and collection direction. Delayed Luminescence. For delayed luminescence, a Qswitched pulsed Nd:YAG laser (Spectra-Physics-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 was 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, with excitation being done through the back of the substrate. The luminescence was collected with a 2 in. diameter and 2 in. focal length collimating lens followed by an imaging lens chosen to f-match a 0.3 m Oriel Instruments spectrometer fitted with an Andor time-gated intensified charge-coupled device (CCD) at its exit port. The emission from the sample
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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 PPs are relatively short (
