The quest for organic-based solar energy devices with high (≥10%) power conversion efficiencies has fueled an enormous international research effort in organic photovoltaics over the past several years.1–6 Amenable to solution-processing techniques (e.g. roll-to-roll or ink-jet printing processing),7, 8 as well as an enormous available structural parameter space, semiconducting polymers and polymer blends hold the potential for realizing this goal. However, unlike their “hard” semiconductor analogs, the “soft” nature of organic semiconductors—referring specifically to the response of the nuclear framework upon exciton generation—conspires to significantly limit charge mobility in these materials, and hence overall device efficiency.9, 10 An issue of perhaps equal importance is the limitation on mobility imposed by morphological heterogeneities in polymer-blend thin films, which has prompted a great deal of recent research aimed at controlled morphologies for long-range transport of charges.11–14 In this mini-review, we discuss recent reports in the literature highlighting the connection between polymer architecture, morphology and supramolecular structure, and the fundamental photophysical processes behind charge generation and separation.
The basic architecture for solution-processed organic photovoltaic devices was first discussed in 1995 when Heeger and Wudl first reported a solar cell device based on a thin-film bulk heterojunction formation of a blend of n-type and p-type organic semiconductors.15, 16 Since then a great deal of research has focused on elementary electronic processes in polyalkyl thiophenes and the connection with local polymer chain packing structure. The body of evidence that has emerged from these studies points overwhelmingly to a molecular picture in which crystalline or aggregated polythiophene (needed for efficient charge separation) phase-separates from the electron-transporting material (usually soluble fullerenes like PCBM) to form isolated domains. This feature of annealed composite films also conspires to limit charge transport, and hence overall device efficiency in organic photovoltaic structures. Thus, a great deal of recent effort has focused on ways to integrate these materials at the molecular level to form nanostructured composites or assemblies without nanophase separation.
This mini-review is organized as follows: First, we briefly summarize the fundamental excitations and spectral features in absorption and emission of P3HT within the context of vibronic transitions in aggregated P3HT. We then discuss some of the recent literature on time-resolved photophysics in extended P3HT and P3HT blend films and the relationship with exciton diffusion and charge separation. We also highlight recent Raman imaging and near-field spectroscopies as local probes of polymer morphology in P3HT and composite films. Finally, we highlight some of the intriguing new work on the photophysics of nanostructured organic semiconductors (nanoparticles and nanofibers) as well as hybrid materials (P3HT with quantum dots or rods and carbon nanotubes). The aim is to capture the essence of a rapidly growing body of work that addresses questions on the fundamental photophysical processes behind charge generation and separation and the relation to morphology and nanostructure. Ultimately, the knowledge gained from these experiments will pave the way toward the goal of cheaply fabricated, high-efficiency solar devices from polymeric semiconductors.
Excitonic coupling in P3HT is usually discussed within the framework of Frenkel polaronic excitons in H- and J-type aggregates, as proposed by Spano.9, 17, 18 In this picture, electronic excitation is accompanied by a molecular elongation that gives rise to relatively simple vibronic progressions associated with 0, 1, or 2 (or higher) quanta of excitation in a symmetric stretching mode (e.g., CC stretch, ω0 ≈ 170 meV) coupled with the electronic excitation.19, 20 The ground state of P3HT has electron density localized in the heterocycle, giving rise to a preferred ground state configuration with a torsional angle of ≈50° between thiophene rings, while the excited state has a quinoidal structure, with a high degree of planarization. As we show in later sections, this gives rise to interesting solution phase exciton relaxation dynamics. However, in thin solid films, aggregated chains may pack in different ways that potentially hinder torsional relaxation and strongly affect intrachain and interchain exciton coupling.
Figure 1, taken from recent work by Grey and coworkers, illustrates the essential structural difference between H- and J-aggregated forms of P3HT.20 In H-type packing arrangements (typical of aggregated P3HT in thin films),21, 22 the thiophene units are noncoplanar, while in J-type structures (seen in P3HT nanofibers and nanoparticles, and in annealed films) the P3HT chains adopt a high-degree of planarization. As a result, excitonic coupling in H-aggregates is primarily interchain, while in J-type systems the coupling is intrachain. Since H- and J-type excitonic coupling gives rise to different selection rules for absorption and emission, the nature of the coupling can be readily determined from qualititative differences in the absorption or emission spectra. In weakly coupled H-aggregate systems, the 00 transition in emission is only weakly allowed due to structural or thermal disorder, and most of the photoluminescence (PL) intensity is carried in the 0-1 vibronic transition. This gives rise to 0-0/0-1 intensity ratios in PL < 1 for H-aggregate systems. Conversely, a rigid linear (and coplanar) arrangement of thiophene units gives rise to dominant intrachain coupling manifested as J-aggregate behavior, where the 0-0 transition in emission is strongly allowed, thus giving rise to 0-0/0-1 intensity ratios in PL > 1.20, 23
Figure 2 shows a typical example of an absorption spectrum of aggregated P3HT in solution. Crystalline aggregation, often enabled by addition of “bad” solvents for P3HT, is manifested in absorption as a series of vibronic transitions labeled as 0-n (n = 0, 1, 2,…) where n is the number of vibrational quanta in the terminal excited state. The energy separation between vibronic peaks is ≈1450 cm−1 (170 meV) corresponding to the symmetric vinyl stretch. In a weak-coupling limit (considering only nearest neighbors coupling), the excitonic coupling constant, j0, can be estimated from the relative absorption strengths of the 0-0 and 0-1 transitions from eq 1,9
where ep is the energy (eV) of the vibrational transition coupled to the electronic transition, here taken to be 0.17 eV, and I0-0/I0-1 is the ratio of absorption intensities. For the spectrum shown in Figure 2, the I0-0/I0-1 ratio of ≈0.7 corresponds to a numerical value of j0 of +20 meV, implying a weakly coupled H-aggregate in solution. As shown by recent work by Moule and coworkers, the microstructure of aggregated P3HT can be controlled to a certain extent by careful solvent management to yield planar J-type species (implied by a negative value of j0) in solution-assembled nanofibers.24 An interesting question that has only recently received attention is correlation between the solvated or suspended aggregated state—as implied by absorption measurements—and the detailed microstructure and associated photoluminescence/conductance properties in the solid or dry state.24
Spectroscopic Probes of Structure and Dynamics in P3HT and P3HT/PCBM Films
Ultrafast spectroscopies have received a great deal of attention in recent years to explore primary photoexcitations and short-time carrier dynamics in P3HT and composite films.25–32 The recent detailed work by Banerji and Heeger on transient absorption (TA) and fluorescence up-conversion (FU) probes of P3HT and P3HT/PCBM blend films have yielded important insights into the ultrafast primary photoexcitation, diffusion, and depolarization processes.27, 33 In transient absorption, one looks at the femtosecond-resolved absorption spectrum yielding kinetic (ground-state recovery) transients at specific wavelengths. In fluorescence up-conversion, a pulse of fluorescence photons generated from a femtosecond excitation pulse is mixed externally in a nonlinear crystal with a reference pulse to generate an up-converted (in frequency) pulse whose intensity is determined by polarization and time-overlap in the crystal. Thus, FU provides exquisite time and polarization sensitivity in a short-time regime (<1 ns). The work of Banerji and Heeger addresses central questions related to mechanisms and timescales for exciton diffusion processes—thought to be rate-limiting for charge separation in photovoltaic devices—and has generated a qualitatively different picture for fundamental photo-induced processes in these materials.27, 34 By spectrally resolving the photoluminescence in wavelength, time, and polarization, they were able to show that exciton migration occurs on timescales too fast to be attributed to an interchain hopping (Forster) mechanism. Based on an exciton diffusion constant of 1.8 × 10−3 cm/s, the root mean square diffusion radius is about 1 Å on a 100-fs timescale, yet the TA/FU measurements suggest excited state access to distances of ≈10 nm on that same timescale. In the authors' words, “it's not the exciton which migrates, but it's delocalized precursor.” The important implication is that exciton diffusion need not be the “show-stopper” in photovoltaic device efficiency as has been widely considered.
Understanding of primary photoexcitation processes is critical to the design of efficient organic photovoltaic devices. Recent work by Busby and Moule' using pump-probe and pump-dump-probe femtosecond-resolved spectroscopies on amorphous P3HT in solution has revealed interesting new insights into the energy-time landscape of excitons in both ground and excited states.30 As shown in Figure 3, the excitation process drives a vertical electronic excitation from the ground state (mean torsional angle between thiophene units of ≈60°); the quinoidal excited state structure exerts a torque on the nuclear framework, resulting in planarization, which the authors argue is the primary mechanism for exciton self-trapping.
In addition to the information on short-time dynamics afforded by ultrafast spectroscopies, valuable information on exciton dissociation and charge carrier diffusion is also contained in long-time photoluminescence. In recent work by Silva and coworkers (Figure 4), the question of extrinsic versus intrinsic driving forces for charge separation were examined in neat P3HT (annealed) films at different temperatures.22 At long times (>6 ns) following a picosecond excitation pulse, they observed a power law behavior in the PL intensity versus time, which could be modeled quantitatively in terms of a mechanistic model involving exciton dissociation followed by carrier diffusion and subsequent recombination via tunneling; in this model, the power law exponent μ is a mean electron-hole separation distance divided by a tunneling width parameter.16 Power law behavior in charge-carrier density in polythiophene films has been extensively studied using both optical and magnetic resonance approaches.35 Within this model, the power law exponent, μ, is related in a straightforward way to a mean separated electron-hole radius, and a tunneling parameter. The important conclusion from this work was that exciton dissociation is driven mainly extrinsically at the interface between amorphous and crystalline domains (grain boundaries) resulting from a large charge-transfer character in the excited state in these regions of the film.
More recently, detailed studies by Rumbles and coworkers, of the long-time photoconductance transients in P3HT films and blends with electron-accepting moieties have revealed additional insights into carrier diffusion, trapping (and activation) and subsequent recombination.36, 37 Using a time-resolved microwave conductivity (TRMC) approach, Rumbles and coworkers studied the effect of doping of solubilized fullerene in P3HT films on the photoinduced conductivity transients as a result of trapped and mobile holes in the film. The results of this study provided a somewhat different picture of carrier dynamics in the long-time regime emerged from this work relative to that of the power law behavior described by Silva, namely, that carrier trapping (and subsequent release)—as opposed to a single recombination mechanism via tunneling. Two central conclusions emerged from this work: film morphology heavily impacts hole mobility where clustering of PCBM leads to accumulation of electrons, which in turn affects hole mobility; the presence of “dark carriers” (immobile electrons or holes generated at grain boundary interfaces without photoexcitation) interferes with conductivity of photogenerated carriers, again related to morphology.
Using a resonance Raman imaging approach, Grey and coworkers have recently shown that aggregated versus unaggregated P3HT in blended films with PCBM can be readily distinguished (Figure 5).19, 38, 39 Using small shifts in the symmetric vinyl stretching energy (1450 vs. 1470 cm−1 for aggregated vs. unaggregated P3HT, respectively), they were able to generate spatially resolved images of crystalline regions of P3HT and map qualitative changes upon annealing. By examining the ratio of aggregated versus unaggregated symmetric vinyl stretching Raman intensities as function of position within the film, four distinct P3HT chain packing regions were identified within the film: (i) highly aggregated or crystalline, (ii) partially aggregated, (iii) interfacial (grain-boundary), and (iv) unaggregated/PCBM-rich. They were also able to probe enhanced planarity of P3HT chains in crystalline regions by measuring intensity ratios of symmetric vinyl to symmetric alkyl Raman transitions, which give rise to stronger intrachain electronic coupling.
In addition to wide-field spectroscopies, exciting new work on near-field scanning probes of local aggregated structure in P3HT films is yielding insight into chain-packing structure, orientation, and morphology.40–43 Using polarized near-field absorption probes, Kuehn and coworkers investigated local aggregated domains of thiophene oligomers by scanning a white-light NSOM probe over a semitransparent sample and collected the transmitted light in wide-field detected with an imaging spectrometer.43 Figure 6 shows the resulting NSOM image capturing the characteristic vibronic structure in the aggregate absorption with a spatial resolution of ≈75 nm. Additional polarized measurements provided insight into orientation of crystalline domains. Using a slightly different approach, our group has investigated near-field absorption properties of P3HT films and nanostructures, using confocal-detected Rayleigh scattering from near-field excitation at 532 nm. Here we detect the attenuation in the far-field scattering signal at the NSOM probe wavelength. As shown in Figure 7, because the absorption cross-section at 532 nm for amorphous P3HT is approximately an order of magnitude smaller than that of aggregated P3HT, confocal detected Rayleigh scattering gives a very high contrast between amorphous and crystalline domains, as well as very strong extinction for crystalline nanostructures.