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Register a new userMulti-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors
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The optoelectronic properties of macromolecular semiconductors depend fundamentally on their solid-state microstructure. For example, the molecular-weight distribution influences polymeric- semiconductor properties via diverse microstructures; polymers of low weight-average molecular weight (Mw) form unconnected, extended-chain crystals, usually of a paraffinic structure. Because of the non-entangled nature of the relatively short-chain macromolecules, this leads to a polycrystalline, one-phase morphology. In contrast, with high-Mw materials, where average chain lengths are longer than the length between entanglements, two-phase morphologies, comprised of crystalline moieties embedded in largely unordered (amorphous) regions, are obtained. We investigate charge photogeneration processes in neat regioregular poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination of long-lived charge pairs decays over microsecond timescales. Both the amplitude and decay rate distribution depend strongly on Mw. In films with dominant one-phase chain-extended microstructures, the delayed PL is suppressed as a result of a diminished yield of photoinduced charges, and its decay is significantly faster than in two-phase microstructures. However, independent of Mw, charge recombination regenerates singlet excitons in torsionally disordered chains forming more strongly coupled photophysical aggregates than those in the steady-state ensemble, with delayed PL lineshape reminiscent of that in paraffinic morphologies at steady state. We conclude that highly delocalized excitons in disordered regions between crystalline and amorphous phases dissociate extrinsically with yield and spatial distribution that depend intimately upon microstructure.
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We measure the homogeneous excitation linewidth of regioregular poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by means of two-dimensional coherent photoluminescence excitation spectroscopy. At a temperature of 8,K, we find a linewidth that is always $gtrsim 110$,meV full-width-at-half-maximum, which is a significant fraction of the total linewidth. It displays a spectral dependence and is minimum near the 0--0 origin peak. We interpret this spectral dependence of the homogeneous excitation linewidth within the context of a weakly coupled aggregate model.
We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunnelling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character.
Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Greens function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton delocalization controls these interactions, with thermally activated crystal phonons predominantly coupling to delocalized states, and molecular quantum fluctuations predominantly coupling to localized states. Based on this picture, we quantitatively predict and interpret the temperature and pressure dependence of excitonic peaks in the acene series of organic semiconductors, which we confirm experimentally, and we develop a simple experimental protocol for probing exciton delocalization. Overall, we provide a unified picture of exciton delocalization and vibrational effects in organic semiconductors, reconciling the complementary views of finite molecular clusters and periodic molecular solids.
We theoretically demonstrate a strong dependence of the annihilation rate between (singlet) excitons on the sign of dipole-dipole couplings between molecules. For molecular H-aggregates, where this sign is positive, the phase relation of the delocalized two-exciton wavefunctions causes a destructive interference in the annihilation probability. For J-aggregates, where this sign is negative, the interference is constructive instead, as a result of which no such coherent suppression of the annihilation rate occurs. As a consequence, room temperature annihilation rates of typical H- and J-aggregates differ by a factor of ~3, while an order of magnitude difference is found for low-temperature aggregates with a low degree of disorder. These findings, which explain experimental observations, reveal a fundamental principle underlying exciton-exciton annihilation, with major implications for technological devices and experimental studies involving high excitation densities.
As device miniaturization approaches the atomic limit, it becomes highly desirable to exploit novel paradigms for tailoring electronic structures and carrier dynamics in materials. Elastic strain can in principle be applied to achieve reversible and fast control of such properties, but it remains a great challenge to create and utilize precisely controlled inhomogeneous deformation in semiconductors. Here, we take a combined experimental and theoretical approach to demonstrate that elastic strain-gradient can be created controllably and reversibly in ZnO micro/nanowires. In particular, we show that the inhomogeneous strain distribution creates an effective field that fundamentally alters the dynamics of the neutral excitons. As the basic principles behind these results are quite generic and applicable to most semiconductors, this work points to a novel route to a wide range of applications in electronics, optoelectronics, and photochemistry.
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