Author: Puja Agarwala

Publisher:

Published: 2024

Total Pages: 0

ISBN-13:

Conjugated polymer blend morphology dictates performance of many organic electronic devices, including electrochemical transistors, light-emitting diodes, and solar cells. In organic photovoltaics (OPVs), electronically active layer morphology of polymer and oligomer bulk-heterojunction influences charge transport and exciton dissociation properties and governs device performance. Yet a faithful representation of the blend interface and local morphology is lacking. In principle, molecular dynamics simulation can represent these blends. However, semiconducting polymers with aromatic rings are large, stiff, and slowly relaxing, which makes equilibration challenging. We develop a new coarse-graining (CG) method, which improves simulation efficiency ten-fold by representing aromatic rings as rigidly bonded moieties, in which we represent several atoms as virtual sites. P3HT simulations with virtual site coarse graining show that the polymer persistence length and the melt density agrees with experimental results. An agreement between scattering extracted from P3HT simulations and wide-angle X-ray scattering experiment validates the simulation local morphology. In the amorphous phase, the scattering results in two wide peaks: the low q peak originates from interchain backbone correlations, and the high q peak originates from interchain side group correlations. We use the virtual site method to characterize the morphology of a typical OPV blend: P3HT (donor) and O-IDTBR (acceptor) and their pure phases. The blend morphology shows that moieties with solubilizing side-groups have fewer electronic contacts because of steric hindrance. On slow cooling, the fast simulation method enables us to observe crystallization, which occurs more readily in pure P3HT than in the blend. Simulations of a low molecular weight P3HT with O-IDTBR represent the local structures of small mixed regions. To describe a de-mixed blend interface, we need the Flory-Huggins [chi] parameter. We develop a "push-pull" technique to measure [chi], which applies robustly to polymer blends of any architecture. The method applies equal and opposite potentials to polymers in a blend to induce a concentration gradient, which is more pronounced for polymers with repulsive interactions ([chi]>0). Chain flexibility plays an important role as stiffer polymers require more energy to induce concentration gradient. We validate the method by blends of bead-spring chains with varying flexibility and PE/PEO blend. The [chi] evaluated from "push-pull'' methods are comparable to the results from previously developed "morphing'' method. We obtain a comprehensive view of the OPV blend morphology by combining local structures from our CG representation and the [chi] parameter from the "push-pull" technique. The [chi] calculated for a blend of P3HT and O-IDTBR shows that the blend follows an upper critical solution temperature behavior and predicts the critical molecular weight of P3HT for phase separation. An amorphous blend of P3HT and O-IDTBR forms an interface of a few nanometers. In contrast, the presence of a crystal acceptor crystallizes the donor polymer on its surface, forming a sharp interface. Crystallization reduces overall contact between donor and acceptor but increases face-on contact, which is important for exciton dissociation. O-IDTBR solubilized in P3HT may also aid in exciton dissociation; however, the polarons formed can not percolate to the acceptor rich region with only 15% solubility and may result in recombination losses. Much higher solubility is required for charge percolation to occur. However, increasing the acceptor solubility in the donor phase may cause crystal structure disruption. A polaron formed by exciton dissociation hops from one chain to another, and the polaron hopping rate depends on the electronic coupling between neighboring molecules governed by their local structures. Electronic coupling of a few thousand P3HT monomer pairs from an amorphous melt shows that strong contacts with high electronic coupling are rare. Feature selection in machine learning helps identify the most important feature for strong contact. The key geometric features closely relate to coherent overlap between HOMO wavefunctions on nearby moieties for hole transport. We develop a machine learning model to evaluate electronic coupling distribution with morphological changes. Slow cooling induces crystallization in P3HT and increases the number of strong contacts. Furthermore, we provide a future direction to understand the high performing organic photovoltaic blend morphology and relate the morphology to their electronic properties. The structure-property relationship will aid in developing rational design of conjugated polymers for efficient organic photovoltaic application.