Letter pubs.acs.org/JPCL
Quantitative Transient Absorption Measurements of Polaron Yield and Absorption Coefficient in Neat Conjugated Polymers Obadiah G. Reid† and Garry Rumbles*,†,‡ †
Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States ‡ Department of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder, Colorado 80309, United States S Supporting Information *
ABSTRACT: Transient absorption methods are crucial for probing photogenerated polaron dynamics in conjugated polymers but are usually limited to qualitative studies because the polaron absorption coefficient is unknown. Herein, we quantify polaron absorption coefficients by exploiting the parasitic exciton−polaron quenching process, which appears in transient absorption experiments as a decrease in polaron yield at high fluence. We modulate the charge density in neat polymer films and measure the exciton− polaron quenching rate constant and dopant density via time-resolved photoluminescence. Using these parameters, we fit relative yield−fluence curves obtained from transient absorption, quantifying the yield and absorption coefficient of the polarons. We use time-resolved microwave conductivity as the transient probe and present results for the GHz mobility and polaron yield in films of three common conjugated polymers that are consistent with previous reports where they exist. These experiments demonstrate a new, generally accessible spectroscopic method for quantitative study of polaron dynamics in conjugated polymers. SECTION: Spectroscopy, Photochemistry, and Excited States
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traditional CMS measurements are difficult due to the low absorption coefficient and total accumulation of the transient species. A more practical, widely accessible method is needed. Herein, we show that quenching of singlet excitons by polarons, previously considered only as an experimental complication and loss mechanism, can be used to ascertain the polaron absorption coefficient. Our method uses only commonly available techniques, and the experiments are straightforward to conduct. Ready quantitative interpretation of transient absorption data will lead to more efficient screening of potential organic photovoltaic materials as it provides an electrode-free way to determine the internal quantum efficiency in an organic semiconductor. It will also significantly enhance our ability to correctly interpret the charge carrier dynamics that we observe in these systems. Figure 1 shows the initial photoconductance signal from the three neat polymers studied here as a function of pump fluence: (symbols), poly[3-hexylthiophene] (P3HT), poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole (PCDTBT), and poly[2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT). The photoconductance signal is represented as the product of charge carrier yield and the sum of electron and hole mobilities, the so-called yield−mobility product, ϕ∑μ. These values were obtained
hotogenerated charges in conjugated polymers relax to form polarons as the polymer backbone geometry reorganizes to accommodate the changed bond order and dielectric polarization relative to the neutral molecule.1 These polarons may be detected in at least two ways, through their electronic absorption band(s) in the near-infrared2 or via the electrostatic interaction of the charge with an external electric field at THz or GHz frequencies.3,4 Both of these spectroscopic windows have been used extensively to probe the dynamics of photogenerated charge in conjugated polymers from picosecond to millisecond time scales.5−15 However, these transient studies are generally limited to qualitative dynamical information as the absorption coefficient of the polaron is quite difficult to measure.16 Pulse radiolysis TRMC4,17,18 has been used to obtain the GHz absorption coefficient (GHz mobility) of both positive and negative polarons in conjugated polymers, and both chemical doping19,20 and charge modulation spectroscopy (CMS)21−23 have been used to obtain the electronic absorption coefficient of same in the NIR. However, none of these techniques have become particularly widespread. While pulse radiolysis TRMC is a versatile way of introducing quantitative charge densities into organic semiconductors, capable of studying these materials both in solution4,24,25 and in the solid state,11,17,25,26 it is a heroic experiment, involving scarce electron accelerators, which severely limits its use. Chemical doping experiments on the other hand suffer from uncertainty in the doping efficiency when applied to polymers, and © 2013 American Chemical Society
Received: June 3, 2013 Accepted: June 28, 2013 Published: June 28, 2013 2348
dx.doi.org/10.1021/jz401142e | J. Phys. Chem. Lett. 2013, 4, 2348−2355
The Journal of Physical Chemistry Letters
Letter
Figure 1. Yield−mobility product (ϕ∑μ) as a function of pump fluence for three conjugated polymers, P3HT, PBTTT, and PCDTBT. The initial values of ϕ∑μ were extracted from TRMC transients via a double-exponential fit function (Supporting Information), which approximately accounts for carrier trapping and recombination within the first 10 ns. The excitation wavelengths used were 520 (P3HT), 550 (PBTTT), and 550 nm (PCDTBT).
Figure 2. TRPL transients (solid colored lines) for three conjugated polymers, (a) P3HT, (b) PBTTT, and (c) PCDTBT. PL transients were collected on metal−insulator−semiconductor (MIS) capacitor structures with varying applied voltage on a hole-injecting gold contact, as indicated in the color legend. As the voltage is increased, more holes are injected into the polymer, and the luminescence is quenched, resulting in a shorter, more distributed exciton lifetime. The red dashed lines show global fits to the TRPL data using our spatially distributed kinetic model. The wavelength of excitation and emission (ex/em) for each polymer sample is as follows: P3HT (520/640 nm); PBTTT (550/770 nm); PCDTBT (550/680 nm).
from double-exponential fits to TRMC photoconductance transients (Supporting Information) where the signal is extrapolated to t = 0, the time of the laser pulse. Thus, any charge recombination or trapping that happens within the instrument response (∼10 ns) is approximately accounted for. A detailed explanation of this data reduction procedure can be found elsewhere27,28 and in our Supporting Information. The red dashed lines in Figure 1 are fits using our kinetic model, which will be described in detail. The curvature in the yield−mobility product as a function of photon fluence, evident for all three polymers in Figure 1, is caused by a parasitic second-order process that reduces the polaron yield as the excitation fluence increases. In several recent papers, we27 and others29,30 have attributed this reduction in charge−yield at high excitation fluence to efficient quenching of singlet excitons by polarons rather than exciton− exciton quenching, as was previously thought.10 Likewise, exciton−polaron quenching has been identified as the cause of reduced organic light-emitting diode efficiencies at high driving currents.31 This process appears to proceed via an energytransfer mechanism,29,31 which, as has been pointed out, is likely to be a general feature in conjugated polymers given the typically strong overlap between the polaron absorption
spectrum and the exciton emission spectrum.21,29 Thus, the shapes of the relative yield−fluence curves in Figure 1a−c are characteristic of the density of excitons and polarons present in the sample, as well as the rate constant of the quenching process. Given a suitable independent measure of the exciton− polaron quenching rate constant and native doping density, one may use the curvature of these plots to extract the yield of carriers and their absorption coefficient, in this case, the GHz mobility. Time-resolved photoluminescence (TRPL) measurements on capacitor structures provide a convenient method of modulating the charge density in a conjugated polymer while measuring the effect of these charges on exciton dynamics.32 We sandwich a polymer film between gold and ITO electrodes, with a thick (>60 nm) dielectric layer of LiF separating the polymer from the ITO. A positive voltage is applied to the gold electrode, and holes are injected into the polymer film, accumulating at the polymer/LiF interface. TRPL transients are obtained in situ through the transparent ITO and LiF while the device is under bias. In a typical experiment, we observe an immediate drop in the PL intensity after applying a positive voltage to the device, reaching a new steady-state intensity within a few seconds. A similarly rapid recovery occurs when 2349
dx.doi.org/10.1021/jz401142e | J. Phys. Chem. Lett. 2013, 4, 2348−2355
The Journal of Physical Chemistry Letters
Letter
Thus, every exciton is generated close enough to a polaron to ensure energy-transfer interactions without the need for any polaron diffusion at all. We believe that this large interaction radius serves to severely limit the influence of the polaron diffusion coefficient on the quenching rate constant. Figure 3 shows an outline of the kinetic model that we chose to fit the TRPL and TRMC data. Excitons are generated in two
the bias is removed. More detail on the device structures, measurement geometry, and their behavior is presented in our Experimental Section and Supporting Information. Figure 2 shows three representative sets of PL transients collected on our capacitor devices at different applied voltages for samples of (a) P3HT, (b) PBTTT, and (c) PCDTBT. Only two voltages are shown for figure clarity, but typical PL data sets consisted of 3−5 transients at different applied voltages. All of the transients in Figure 2 exhibit distributed decay kinetics that become more pronounced when a voltage is applied. Likewise, they all exhibit significant net quenching as more charge is injected; both the peak photoluminescence (PL) and the lifetime decrease monotonically with increasing bias. The small hump that appears near 10 ns in the PBTTT transients is an artifact of the instrument response function. As in Figure 1, the dashed red lines are fits to the data using our kinetic model, which we motivate and describe below. The PL transients in Figure 2 illustrate clearly the activity of injected polarons in quenching excitons. This effect cannot be explained by electric field quenching first because the field is small (
