Article pubs.acs.org/JPCC
Quantifying the Relationship between the Maximum Achievable Voltage and Current Levels in Low-Bandgap Polymer Photovoltaics Deanna B. Rodovsky,†,‡ Jeff Peet,† Nan Shao,§ Jason D. Azoulay,∥ Guillermo C. Bazan,∥ Nicolas Drolet,† Qin Wu,§ and Matthew Y. Sfeir*,§ †
Konarka Technologies, 116 John St., Suite 12, Lowell, Massachusetts 01852, United States National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States § Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States ∥ Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California 93106, United States ‡
S Supporting Information *
ABSTRACT: A critical problem in the design of materials for organic photovoltaics is quantifying the driving force needed for efficient charge separation without losses associated with a large overpotential. Here, we directly measured the effect of the molecular driving force on the charge transfer rate in films of low-bandgap push−pull type polymers mixed with a series of fullerene-based molecular acceptors using broadband near-infrared transient absorption spectroscopy. By systematically tuning the absolute energy levels of the donor and acceptor, as well as the relative offset between them, we determine the minimum voltage loss required to achieve a high short circuit current. A molecular donor−acceptor framework provides a quantitative description of the charge transfer rate constants in our system and describes the scaling of the photogenerated current with S1−LUMO energy offset. These results point to potential efficiency gains for high performing polymer devices through recovery of additional voltage without sacrificing current output.
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While some overpotential, approximated by the offset between the lowest unoccupied molecular energy (LUMO) levels of the donor and acceptor materials, relative to the exciton binding energy is required for a high IQE, the energetic requirements that maximize cell efficiency, i.e., the product of the achieved current and voltage, have not been experimentally quantified. While an empirical value of ∼0.3 eV is commonly cited in the literature,3 the exact scaling of their dissociation efficiency with driving force has not been established. This is a major limiter to efficient devices, since in practice, the offset present in typical devices greatly exceeds the energy required to fully separate charges. This concept can be clearly illustrated in the well-studied blend of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) system, which demonstrates nearly 100% internal quantum efficiency on resonance.9 Typical losses are on the order of 1.4 V, well in excess of what is needed to sustain the high internal quantum efficiencies and limits maximum efficiencies to ∼4%. Here, using broadband infrared transient absorption methods to quantify the max IQE and minimum voltage loss in a “contactless” manner, we report the quantitative characterization of the charge generation rate as a function of energetic offset in polymer-based photovoltaic systems and elucidate the importance of “molecular-like” interactions. These results are reinforced by recent experiments that look at the effect of
rganic photovoltaics (OPV) consisting of mixed donor and acceptor materials are promising for low-cost solar applications due to the high internal quantum efficiencies (IQE), use of earth abundant materials, and rapid energy payback.1 Much of the recent progress on organic photovoltaics has focused on reducing the bandgap of the polymer material to better match the solar spectrum, i.e., with the goal of increasing the amount of photons that can be harvested.2,3 Of the lowbandgap polymer materials, the most common strategy uses the “push−pull” motif, in which electron-rich and electron-deficient units are alternated.2,4,5 Beyond modification of the optical absorption spectra of the polymer, there remain significant opportunities for improving the overall solar power conversion efficiency of these promising materials by optimizing the device open circuit voltage. Designing organic solar cells to minimize the voltage losses is challenging because, unlike typical bulk inorganic systems, these materials exhibit complex interactions on the molecular scale whereby photogenerated quasiparticles cascade through a series of bound states before carrier collection. For example, photogenerated carriers are excitonic, bound by Coulomb attraction with binding energies significantly exceeding room temperature. Furthermore, photogenerated excitons decay into intermediate “charge-transfer” or polaron states with binding energies and relative amplitudes that depend strongly on the physical and electronic structure of the materials.6−8 The requirement for dissociation of these bound states for charge collection reduces the total achievable open circuit voltage. © 2013 American Chemical Society
Received: October 15, 2013 Revised: November 1, 2013 Published: November 7, 2013 25955
dx.doi.org/10.1021/jp410234u | J. Phys. Chem. C 2013, 117, 25955−25960
The Journal of Physical Chemistry C
Article
transfer occurs not from the donor LUMO level, but from the exciton level, referred to in this article as S1, referenced by its ionization energy (4.0 eV). More information on the polymer structure and acceptor materials used in this study can be found in the Supporting Information. Since it has been shown that even small changes to the polymer chemical structure can lead to dramatic changes in both the interchain ordering14 and the photophysics,8 we choose to vary the acceptor structure to better isolate differences due to energetic factors alone. Excitation of the polymer S1 state is accomplished using a pulse wavelength of 700 nm, such that direct absorption of the fullerenes and energy transfer interactions can be neglected. Charge transfer occurs between the photoexcited S1 state and the LUMO of the fullerene acceptor molecule. In order to measure the exciton lifetime in each of the blended donor:acceptor samples, it is necessary to establish a means of identifying the spectral transients corresponding to the exciton state. We have accomplished this using a neat JA11 film, which allows us to clearly isolate the spectral distribution of the exciton decay process. Figure 2a,b shows the raw transient absorption data and the results of the singular value decomposition and global fitting analysis for the JA11 neat film. There are two significant short time scales, approximately 6 and 23 ps, and one long time scale component that exceeds the dynamic range of our experiment (3.3 ns) and cannot be accurately determined within the constraints of our experiment. We can immediately see that the two short times scale components have nearly identical spectral distributions peaked around 1420 nm and, as such, are likely correspond to transitions from the same configuration. Using this information combined with fluence-dependent measurements (see also the Supporting Information), we have assigned the faster 6 ps process to Auger recombination of multiexciton states and the slower 23 ps process to the total lifetime of the single exciton state. The small amplitude, long time scale component that extends beyond the dynamic range of the measurement has been assigned to the positive polaron. We justify this assignment based on the observation that the relative amplitude of this feature per incident photon increases with increasing laser fluence (Supporting Information), which is consistent with a hot carrier mechanism for polaron formation observed in other polymer systems.15 Furthermore, we observe identical spectral distributions (with larger amplitudes) in the blend films where polaron formation is expected to dominate. Once the spectral signatures of the exciton and polaron are established in the neat film, we can identify these features in the blended films. Through inspection of the rate constant distribution plots, it becomes straightforward to pick out the time scales that reflect the dissociation dynamics of the exciton. Figure 2c,d shows the rate constant distribution plots for two different JA11/acceptor blends, the small energy offset trisindene blend, and the large offset PCBM blend. In both cases, we see that there is a rate constant with large amplitude around 1420 nm that can be assigned to the photoexcited exciton. In fact, as expected, the small offset trisindene blend has dynamics that are very similar to the neat film with two components that reflect the decay of the exciton (orange and red lines) that have already been assigned to the multi- and monoexciton decay process. It is important to note that although there is a nearly identical spectral component for all three films, the rate constants differ considerably among them. We note that for blends with faster exciton decays there is a corresponding rise in the number of polarons. This can be seen
changing the physical structure of the donor (polymer) component that have suggested a molecular picture is crucial to capture all of the relevant photophysical processes.8 While this overall framework is consistent with recent time-resolved microwave conductivity experiments10 that examined the free carrier yield upon photoexcitation of the acceptor (fullerene) rather than the polymer, the energy level configurations are quite different such that the results and conclusions reached here are not necessarily implied by this previous work. Most importantly, in contrast to our study, Coffey et al. utilized a polymer with a bandgap larger than the fullerene and performed the experiment in such a way that the “donor” is the molecular fullerene component and the “acceptor” is the large band gap polymer. Although recent work has begun to show the importance of both processes to real devices, quantification of both approaches is necessary to understand their relative contributions and OPV design constraints.11 It is particularly important to characterize these processes in lowbandgap “push−pull” type polymers, which have excited state properties that are quite distinct from homopolymer, which may have a nontrivial effect on charge separation.12 Here, we directly measure charge transfer rate constants and study the direct process in which a low-bandgap polymer is employed for photon absorption.10 We have studied thin films of polymer mixed with a series of fullerene-based molecular acceptors to systematically vary the driving force for carrier separation (Figure 1). The acceptors
Figure 1. (a) Raw data from NIR transient absorption measurements on a neat JA11 polymer film. (b) Rate constant distribution spectra derived from singular value decomposition of the near-infrared transient absorption data depicting the dynamics corresponding to multiexciton states (orange), single excitons (red), and polarons (green). Dynamics of the (c) trisindene blend and (d) the PCBM blend.
include a trisindene adduct of C60 (LUMO = 4.05 eV), bisindene C60 (4.1 eV), bis-PCBM (4.15 eV), PCBM (4.3), and C60(CF3)3 (4.5 eV). The primary polymer material, referred to as JA11,13 is representative of the “push−pull”, low-bandgap structures with alternating electron-rich and electron-deficient units.2,4,5 This material was chosen as a model system due to its strategically positioned excited state energy level relative to our selected acceptors (Figure 1) and ideal match of the optical gap (1.5 eV) to the solar flux maximum. We stress that the electron 25956
dx.doi.org/10.1021/jp410234u | J. Phys. Chem. C 2013, 117, 25955−25960
The Journal of Physical Chemistry C
Article
Figure 2. (a) Raw data from NIR transient absorption measurements on a neat JA11 polymer film. (b) Rate constant distribution spectra derived from singular value decomposition of the near-infrared transient absorption data depicting the dynamics corresponding to multiexciton states (orange), single excitons (red), and polarons (green). Dynamics of the (c) trisindene blend and (d) the PCBM blend.
in the green curves in Figure 2 where a dramatically increased amplitude (peaked around 1200 nm) is observed for the PCBM film relative to the neat film and trisindene blend. This is an important fact to establish, since photoluminescence studies alone cannot provide this information. Charge generation in our polymer does not appear to involve an intermediate charge transfer state in any significant way in contrast to other material systems.8 Furthermore, our analysis shows that instantaneous exciton dissociation (as has been observed in P3HT-based systems) is not a significant contributor to the overall polaron dynamics.16 For the deepest acceptor levels, we observe some low-amplitude intermediate time scale (10−20 ps) dynamics. These features resemble a slower rise of the polaron itself rather than a distinct electron configuration and are likely to result from the inhomogeneity of the donor−acceptor interface rather than a CT state. We note that here we employ near-gap excitation energies to minimize excess kinetic energy of the photoexcited carriers. As such, our data are not inconsistent with recent reports of “hot” exciton dissociation processes that occur when the system is pumped with higher energy photons.17 We have examined each of our JA11-acceptor structures using multiwavelength global fitting methods and analyzing the resulting rate constant distribution plots for known spectral features. In all cases, we focus on the single exciton lifetime since this represents the intrinsic, low fluence limit. Table 1 lists these data for each of the material combinations, with the corresponding S1−LUMO energy offsets. We find a systematic variation of the exciton lifetime with increasing energy offset energy, with exciton lifetimes ranging from ∼30 ps in the trisindene fullerene derivative to
