Multi-phase microstructures drive exciton dissociation in neat semicrystalline polymeric semiconductors

Abstract

The optoelectronic properties of macromolecular semiconductors depend fundamentally on their solid-state microstructure and phase morphology. Hence, it is of central importance to manipulate -from the outset -the molecular arrangement and packing of this special class of polymers from the nano-to the micrometer scale when they are integrated in thin film devices such as photovoltaic cells, transistors or light-emitting diodes, for example. One effective strategy for this purpose is to vary their molecular weight. The reason for this is that materials of different weight-average molecular weight (Mw) lead to different microstructures. Polymers of low Mw form unconnected, extended-chain crystals because of their non-entangled nature. As a result, a polycrystalline, one-phase morphology is obtained. In contrast, high-Mw materials, in which average chain lengths are longer than the length between entanglements, form two-phase morphologies comprised of crystalline moieties embedded in largely un-ordered (amorphous) regions. Here, we discuss how changes in these structural features affect exciton dissociation processes. We utilise neat regioregular poly(3-hexylthiophene) (P3HT) of varying Mw as a model system and apply time-resolved photoluminescence (PL) spectroscopy to probe the electronic landscape in a range of P3HT thin-film architectures. We find that at 10 K, PL originating from recombination of long-lived charge pairs decays over microsecond timescales. Tellingly, 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. Its decay is significantly slower than in two-phase microstructures. We therefore conclude that excitons in disordered regions between crystalline and amorphous phases dissociate extrinsically with yield and spatial distribution that depend intimately upon microstructure, in agreement with previous work [Paquin et al. Phys. Rev. Lett., 2011, 106, 197401]. We note, however, that independent of Mw, the delayed-PL lineshape due to charge recombination is representative of that in low-Mw microstructures. We thus hypothesize that charge recombination at these low temperatures -and likely also charge generation -occur in torsionally disordered chains forming more strongly coupled photophysical aggregates than those in the steady-state ensemble, producing a delayed PL lineshape reminiscent of that in paraffinic morphologies at steady state. 2

Orientation mapping of semicrystalline polymers using scanning electron nanobeam diffraction

“…The nanoscale orientation of semicrystalline polymers directly affects their electronic properties (Chu et al, 2008;Coropceanu et al, 2007), including their ability to generate excitons (Paquin et al, 2015) or carry charge (McMahon et al, 2011). While they do not yet rival the efficiency of ceramic materials, organic polymers and blends are promising candidates for organic photovoltaic (OPV) applications (Mazzio and Luscombe, 2014) due to their portability, low cost and ease of manufacturing.…”

Gold‐Catalyzed Oxidative Alkyne Functionalization by N−O/S−O/C−O Bond Oxidants

“…[93] Interestingly under the typical catalytic condition, probable the oxidatively generated α-oxo gold carbene 103 experiences cyclopropanation with arene overcoming the traditional Friedel-Craft type arylation. This type of cyclopropanation has been reported [94,95,96] earlier with Rhcatalysts and diazo ketones/esters instead of alkynes whereas Zhang and coworkers successfully explored terminal alkynes for such kinds of transformation. In presence of a gold catalyst and external oxidant initially generated α-oxo gold carbene 103 converted to norcaradiene intermediate 104 and subsequent reversible electrocyclic ring-opening afforded tetrahydropyranone-fused cycloheptatriene 105 as a Buchner reaction product, which again isomerized to stable conjugated enone 106.…”

Surface modification of magnetic TiO2 core-shell with doped cerium for enhancement of photocatalytic performance

“…Besides in Fig. 2, b the peak pore size distribution of TSF based on Barret-Joyner Halenda (BJH) desorption isotherm shows shaper than 20CTSF, suggesting a homogenous pore size in the sample [36]. The greater SSA might facilitate the more surface contact area, which could be beneficial for accelerating the photocatalyst reaction for reducing dyes molecules.…”