The influence of solid‐state microstructure on the origin and yield of long‐lived photogenerated charge in neat semiconducting polymers

Abstract

The influence of solid-state microstructure on the optoelectronic properties of conjugated polymers is widely recognized, but still poorly understood. Here, we show how the microstructure of conjugated polymers controls the yield and decay dynamics of long-lived photogenerated charge in neat films. Poly(3-hexylthiophene) was used as a model system. By varying the molecular weight, we drive a transition in the polymer microstructure from nonentangled, chain-extended, paraffinic-like to entangled, semicrystalline (M W ¼ 5.5-347 kg/mol). The molecular weight range at which this transition occurs (M W ¼ 40-50 kg/mol) can be deduced from the drastic change in elongation at break found in tensile tests. Linear absorption measurements of free-exciton bandwidth and time-resolved microwave conductivity (TRMC) measurements of transient photoconductance track the concomitant evolution in optoelectronic properties of the polymer as a function of M W . TRMC measurements show that the yield of free photogenerated charge increases with increasing molecular weight in the paraffinic regime and saturates at the transition into the entangled, semicrystalline regime. This transition in carrier yield correlates with a sharp transition in free-exciton bandwidth and decay dynamics at a similar molecular weight. We propose that the transition in microstructure controls the yield and decay dynamics of long-lived photogenerated charge. The evolution of a semicrystalline structure with well-defined interfaces between amorphous and crystalline domains of the polymer is required for spatial separation of the electron and hole. This structural characteristic not only largely controls the yield of free charges, but also serves as a recombination center, where mobile holes encounter a bath of dark electrons resident in the amorphous phase and recombine with quasi first-order kinetics.

Structural correlations in the generation of polaron pairs in low-bandgap polymers for photovoltaics

“…Here, polaron pair formation is driven by exciton delocalization at sites with energetic disorder (typically > 50 meV) 42 . A recent work by Reid et al 43 shows that the polaron yield in P3HT depends on the degree of crystallinity because the charge separation might occur at the interface between the crystalline and amorphous regions. A less pronounced carrier localization in one of the two regions can lead to large intrachain mobilities 39 and a longer polaron pair lifetime.…”

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

“…36,37 Using a timeresolved 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.…”

Excitonic coupling dominates the homogeneous photoluminescence excitation linewidth in semicrystalline polymeric semiconductors

“…This PL excitation linewidth is roughly an order of magnitude broader than the lower limit in the single-chain PL spectra reported by Thiessen et al [6], who investigated P3HT with M w that is near the transition in polymer microstructure from a nonentangled, paraffinic to an entangled, semicrystalline, two-phase morphology in bulk films [5,12]. In that work, while the emission energy of individual P3HT chains varied substantially over much of the visible range, the PL spectral line shape was found to be nearly invariant over measurement of hundreds of different chains.…”

“…In this high M w regime, the microstructure of P3HT films solution-cast from good solvents consists of crystalline domains embedded in chain-entangled phases [12], with exciton coherence length of ∼5 thiophene monomer units along chains, and extending over ∼2 chain [11]. We used transform-limited, 13-fs pulses.…”