Dark Carriers, Trapping, and Activation Control of Carrier

ARTICLE pubs.acs.org/JPCC

Dark Carriers, Trapping, and Activation Control of Carrier Recombination in Neat P3HT and P3HT:PCBM Blends Andrew J. Ferguson,† Nikos Kopidakis,† Sean E. Shaheen,‡ and Garry Rumbles*,†,§ †

Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States Department of Physics and Astronomy, University of Denver, Denver, Colorado 80208, United States § Department of Chemistry and Biochemistry, UCB 215, University of Colorado, Boulder, Colorado 80309, United States ‡

ABSTRACT: Using flash photolysis, time-resolved microwave conductivity we report the sub-200 ns photoconductivity transients for neat poly(3-hexylthiophene), P3HT, and four associated blends containing 1%, 5%, 20%, and 50%, by weight, of the soluble fullerene, [6,6]-phenyl-c61-butyric acid methyl ester, PCBM. We propose a detailed kinetic scheme that when solved numerically is consistent with all the data recorded as a function of excitation density and that describes the fate of mobile and trapped carriers in the system. In the neat polymer, mobile holes are the only contributor to the photoconductance transients, which decay according to first-order kinetics at all light intensities due to the presence of a large concentration of dark carriers present in the polymer. The signal decays with a characteristic rate constant (∼1  107 s
1) that describes the re-equilibration of trapped and mobile holes. In all four blends, the microwave absorption contains a significant contribution due to electrons in the PCBM clusters, even at the lowest blend ratio of 1%. The magnitude of the second-order rate coefficient, γb, for carrier recombination in all four blends (3.25  10
12 cm3 s
1 < γb < 10  10
12 cm3 s
1), and also that identified for the neat polymer, corresponds to a slow process that is not limited by diffusion but is activation controlled.

1. INTRODUCTION Progress in the photovoltaic performance of excitonic solar cells based on bulk heterojunctions comprised of a blend of a conjugated polymer and a fullerene has been impressive over the past decade, culminating in certified power conversion efficiencies that have reached over 8%.1 Despite these significant advances, fundamental investigations aimed at understanding the intricacies of photoinduced carrier generation and decay in binary donor
acceptor (D
A) composites are still critical to driving the development of novel photoactive materials and solar photoconversion architectures. One of the most studied binary polymer:fullerene systems consists of a blend of regioregular poly(3-hexylthiophene) (P3HT) and the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). P3HT is often considered a “model system” in conjugated polymer research; however, we will show that in some respects it can exhibit behavior not always typical of many conjugated polymers. While the rate of exciton dissociation in P3HT:PCBM blends is known to occur on an ultrafast time scale (∼tens to hundreds of femtoseconds),2
5 it is the significantly slower (hundreds of nanoseconds or longer) recombination process that ultimately determines device performance.6,7 It was initially assumed that oppositely charged electron
hole pairs are created with a short separation in a low dielectric medium, presumably occupying a charge-transfer (CT), or bound radical pair (BRP), state on neighboring donor and acceptor molecules at the interface.8
11 The need to overcome the binding energy of this interface state to generate uncorrelated r 2011 American Chemical Society

carriers implies that rapid primary geminate recombination12,13 would be the dominant decay pathway for the carriers, which is contrary to the observation of high external quantum efficiencies observed for P3HT:PCBM solar cells.6,14 It is this process that is typically observed under low pump fluence conditions in ultrafast optical pump
probe3 or transient terahertz spectroscopy2 measurements, where the ultrafast formation and decay of charged species (that may or may not be the mobile carriers probed in this study) in binary D
A systems are probed on time scales of nanoseconds or shorter, although it has been noted that the interpretation of transient data can be difficult due to the introduction of nonlinear processes at higher pump fluences.5 In the case where the carriers avoid primary geminate recombination and are able to diffuse away from the D
A interface, their recombination obeys second-order kinetics, as observed by a recent opticalpump terahertz-probe spectroscopic study of P3HT:PCBM films.15 The second-order carrier recombination process observed on a postmicrosecond time scale is dominated by trapping and detrapping processes that manifest themselves as a reduction in the measured bulk carrier mobility.16,17 It is a convolution of these effects that are probed by transient absorption spectroscopy on a microsecond
millisecond time scale,18
20 time-resolved photocurrent extraction methods,21
25 transient photovoltage experiments,20,26,27 and device modeling.28 It should be noted Received: August 19, 2011 Revised: September 26, 2011 Published: November 02, 2011 23134

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The Journal of Physical Chemistry C that the trapping and detrapping processes that dominate the carrier kinetics at times greater than ∼500 ns are often not dealt with in carrier recombination models; instead, kinetic processes associated with the population/depopulation of trap sites, and subsequent recombination events, result in stretched exponential29 or power law30,31 decay kinetics or the invocation of a density-dependent rate “constant” that controls the secondorder recombination rate.17,26,27,32 Moreover, trapping and recombination of photoinduced carriers are processes that are often referred to, yet little is understood about the molecular nature of the species involved in trapping or the precise mechanism for recombination. A model for second-order carrier recombination in pristine, disordered, low-mobility materials was originally formulated by Langevin:33 this model assumes that the rate-limiting step is the process of the two carriers finding each other and not the actual process of carrier annihilation. However, in the case where the carriers are spatially separated into different phases (i.e., by exciton dissociation at the interface in a binary D
A system) recombination of uncorrelated carriers occurs across the D
A interface, thus modifying the precise mobility dependence of the recombination rate.7,8,33 Under such circumstances, the experimentally observed recombination rate can be reduced compared to that predicted by Langevin-based models,7,8,15,33
35 suggesting that the recombination is not determined by the simple encounter probability of the Langevin model. In fact, reduced recombination rates in organic semiconductor systems have been rationalized by the spatial average or minimum carrier mobility as a result of the confinement of carriers to separate phases,7,8 the restriction of carrier transport to two dimensions in lamellar polymer systems,36 the thermal activation of trapped carriers in a random potential distribution,37 image force effects as a result of blending two materials with different dielectric constants,38 and carrier concentration gradients25 and have been examined theoretically within the context of a unified model for geminate and bulk recombination.39 Another important unresolved issue in binary D
A systems in general, and in P3HT:PCBM in particular, is the unintentional doping of the materials and the subsequent presence of mobile and trapped carriers in the dark under equilibrium conditions. The presence of dark carriers in pristine P3HT has only recently been established,22,29,34,40
42 and their importance to the photophysics and device performance is only now being appreciated. However, it is yet unclear how the presence of these carriers influences the dynamics of the nonequilibrium carriers generated under illumination. A recent optical pump
probe spectroscopy study investigates the photophysical properties of P3HT:PCBM blends in the intermediate time regime, on a nanosecond to submicrosecond time scale.43 That study indicates that exciton quenching on a time scale less than 1 ps (depending on the extent of polymer ordering) produces two carrier populations, bound and free charges, and that only the latter can be effectively harvested in a photovoltaic cell and is the dominant population in blends of regioregular P3HT and PCBM.43 Howard et al.43 also observed sub-Langevin carrier recombination kinetics, albeit with a recombination order approaching 2.5 that was attributed to the confinement of hole transport to two dimensions in the lamellar P3HT domains. The photogeneration and extraction of carriers in P3HT:PCBM devices have recently been studied using biasdependent time-delayed collection field (TDCF) experiments, where the prebias-dependent integrated photocurrent in the collection regime, as a function of the time delay between the

ARTICLE

laser excitation and collection bias pulses, can only be modeled by including a second-order recombination rate coefficient, γ = 5.0  10
12 cm3/s.35 Another aspect that impacts the photophysics of binary D
A systems, and ultimately controls the photovoltaic performance of their devices, is the precise microstructure formed upon blending the two components. P3HT is considered a semicrystalline conjugated polymer, meaning that it forms films composed of both crystalline and amorphous domains,44 and it has been suggested that PCBM is miscible only in the amorphous polymer phase, where the spatial constraints imposed by the lamellar structure and high alkyl side chain density of P3HT is severely relaxed in comparison to the ordered polymer phase.45,46 A more recent neutron scattering study of P3HT:PCBM films as a function of PCBM loading has suggested that the miscibility limit lies close to 20% PCBM (by volume), after which pure PCBM domains begin to form.47 Yin and Dadmun suggest that the molecular organization in the amorphous mixed phase must still be sufficient to facilitate very efficient exciton dissociation and the transport of free carriers to the pure P3HT and PCBM phases,47 thereby rationalizing the efficient device performance of a P3HT:PCBM active layer.14 In this paper, we use a contactless local photoconductivity probe, time-resolved microwave conductivity (TRMC), to probe the dynamics of mobile photoinduced carriers in P3HT:PCBM films. Flash photolysis TRMC (FP-TRMC) is ideally suited to the investigation of carrier dynamics in the intermediate time regime (nanosecond to submicrosecond), where only a small amount of experimental data exist,35,43 since the application of an appropriate model allows us to distinguish between carrier trapping, primary geminate recombination, and second-order carrier recombination. As we will show, with the application of an appropriate kinetic scheme to describe the physical processes in the excited state, FP-TRMC is capable of unraveling information about carrier trapping and detrapping, as well as the effect of intrinsic, dark carriers on the carrier decay kinetics. We use independently published ultrafast optical pump
probe results5 for processes related to exciton dynamics to provide input to our kinetic scheme for photoinduced carrier generation and decay. In the FP-TRMC experiment, the rates of the processes (trapping, detrapping, recombination) pertaining to electrons and holes (in the dark and under illumination) are modified by varying the intensity of the excitation pulse by 4 orders of magnitude. We also use neat P3HT and blends of the polymer with PCBM at wt % loadings of 1%, 5%, 20%, and 50%—these loadings correspond to 3-hexylthiophene monomer:PCBM molecule ratios of ∼540:1, 105:1, 22:1, and 11:2, respectively—to influence the rates of a number of the excited-state processes. Through the observation of a change in the magnitude and time dependence of the photoconductance signal, we identify mobile holes in the polymer phase and mobile electrons in PCBM clusters. Our analysis methodology allows us to decouple the carrier trapping, detrapping, and recombination processes, and extract the high-frequency electron mobility as a function of PCBM loading. This approach allows us to answer the following questions: 1. How does the presence of traps and dark carriers influence recombination dynamics? 2. What is the second-order recombination rate coefficient, once the effects of trapping and dark carriers have been accounted for, and how does it compare with that predicted by Langevin-based models? 23135

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The Journal of Physical Chemistry C We find that the high density of dark carriers in neat P3HT results in the formation of an equilibrium between trapped and mobile holes, consistent with similar observations in the literature22,29,40
42 and the observation of pseudo first-order recombination kinetics, where the rate is almost independent of the photogenerated carrier density over more than 3 orders of magnitude of the absorbed photon flux. In blends with PCBM, the presence of dark carriers results in a redistribution of electrons from the polymer-rich phase to the fullerene-rich domains—from this point forward, these will be referred to simply as the polymer and fullerene domains, respectively. In agreement with measurements of second-order recombination using time-resolved photocurrent extraction methods,21,24,25,35,48 we determine a rate coefficient for all P3HT:PCBM blends that is more than 3 orders of magnitude lower than is predicted by Langevin-based recombination models.8,33 However, the secondorder recombination rate coefficients extracted in this study are not influenced by carrier gradients25 and electric fields induced across the active layer in a solar cell, or trapping/detrapping processes that manifest themselves in the observation of traplimited carrier recombination by “slow” time-resolved spectroscopic and device-based measurements. For these reasons, we believe that the rate coefficients determined through this analysis are a more fundamental measure of the low probability of carrier recombination, and they indicate that the carrier recombination process is ultimately determined by an activation-controlled step and not the diffusion-limited encounter probability predicted by the Langevin model. The low recombination rate coefficient in this prototypical polymer:fullerene blend is one of the key elements of the success of organic photovoltaic devices based on a bulk heterojunction architecture.6 The significance of this finding as well as the correlation between recombination and trapping and the implications on device operation are discussed.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The fullerene derivative PCBM (NanoC) and regioregular P3HT (Merck; average molecular weight, Mw = 21.2 kg/mol; number average, Mn = 10.6 kg/mol; 93.7% regioregular) were used as received. Solutions of neat P3HT, 99:1, 95:5, 80:20, and 50:50 (by weight) P3HT:PCBM (with total active material concentrations of 7.5 mg/mL) in chloroform, were prepared and stirred overnight at 50
C under a nitrogen atmosphere. The solutions were drop-cast onto clean, O2-plasma-treated quartz substrates and subsequently dried in air—in all cases the solvent evaporated over the course of 10 min or more, meaning that the film preparation method is qualitatively similar to the “slow-drying” procedure used to prepare the active layer for the best-performing P3HT:PCBM excitonic solar cells. After drying, the samples were heated for 1 min at 50
C to remove any residual solvent from the active layer. The optical density (OD) of the films prepared for this study was greater than 3 at 500 nm, ensuring complete absorption of the incident excitation pulse, and the thickness was 60 ns), the trapped and mobile holes have reestablished an equilibrium, which is only slowly modified through interfacial carrier recombination. Although poly(3-hexylthiophene) is often referred to as the prototypical conjugated polymer, it is clear that this polymer exhibits a number of properties that appear to be quite atypical: (i) it contains a concentration of dark carriers large enough to significantly influence photoinduced carrier dynamics, (ii) it has a substantial yield of intrinsic, photogenerated carriers, and (iii) it may provide an environment that promotes the clustering of PCBM at low concentrations, and thus bulk heterojunction solar cell devices are optimized at a loading level far lower than most other polymer:PCBM systems (polymer:fullerene 1:1 vs 1:4 by weight). Finally, we find that the recombination of photogenerated charge carriers at the polymer
fullerene interface is a process that is inefficient since carrier recombination is ultimately determined by an activation-controlled step and not the diffusion-limited encounter probability predicted by the Langevin model. Although this feature does not appear to be unique to P3HT, having been observed in a number of conjugated polymer-acceptor systems,15,21,24,25,48 it may be a necessary prerequisite for the design and screening of efficient polymer-based solar photoconversion systems.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was funded by the Solar Photochemistry program of the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Chemical Sciences, Geosciences 23146

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The Journal of Physical Chemistry C and Biosciences, under Contract No. DE-AC36-08GO28308 to NREL. We thank Obadiah Reid (National Renewable Energy Laboratory, USA) for helpful discussions.

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