Tying Together Multiscale Calculations for Charge Transport in P3HT: Structural Descriptors, Morphology, and Tie-Chains

Abstract

Evaluating new, promising organic molecules to make next-generation organic optoelectronic devices necessitates the evaluation of charge carrier transport performance through the semi-conducting medium. In this work, we utilize quantum chemical calculations (QCC) and kinetic Monte Carlo (KMC) simulations to predict the zero-field hole mobilities of ∼100 morphologies of the benchmark polymer poly(3-hexylthiophene), with varying simulation volume, structural order, and chain-length polydispersity. Morphologies with monodisperse chains were generated previously using an optimized molecular dynamics force-field and represent a spectrum of nanostructured order. We discover that a combined consideration of backbone clustering and system-wide disorder arising from side-chain conformations are correlated with hole mobility. Furthermore, we show that strongly interconnected thiophene backbones are required for efficient charge transport. This definitively shows the role “tie-chains” play in enabling mobile charges in P3HT. By marrying QCC and KMC over multiple length- and time-scales, we demonstrate that it is now possible to routinely probe the relationship between molecular nanostructure and device performance.

Machine learning predictions of electronic couplings for charge transport calculations of P3HT

“…In prior work, we describe combining these steps into the MorphCT software pipeline, the details of said implementation, and applications to P3HT …”

“…Here we draw training set chromophore pairs from one “disordered,” one “semi‐crystalline,” and one “crystalline” morphology from prior work . The degree of crystallinity is reported using ψ′ as in Reference , with “disordered,” “semi‐crystalline,” and “crystalline” corresponding to ψ′ of 0.17, 0.25, and 0.33, respectively. ψ′ is a quantification of fraction of thiophene rings composed into “large” clusters and the deviations in the aliphatic bond lengths.…”

“…Time‐of‐flight measurements of hole mobility in P3HT experiments range from μ = 1 × 10 −5 to 1 × 10 −3 cm 2 /Vs . Computational work has helped to explain the role of thiophene ring orientation on charge transport, and kinetic Monte Carlo (KMC) simulations of charge transport have predicted mobilities ranging from μ = 1 × 10 −4 to 0.6 cm 2 /Vs, depending on the degree of ordering in of the P3HT morphologies. These experimental and computational predictions of mobility provide references for validation: Calculated hole mobilities in P3HT should fall between μ = 1 × 10 −4 to 0.6 cm 2 /Vs and increase with increasing P3HT crystallinity.…”

“…In our own prior work, we predict charge transport through P3HT by first predicting P3HT morphologies at ∼350 processing state points (Supporting Information Section 1), then calculating charge mobility through ∼100 of these structures using KMC simulations. Doing so requires hopping rates between P3HT chromophores, which we calculate with Marcus semi‐classical hopping theory using quantum chemical ZINDO/S calculations to obtain the electronic transfer integrals between chromophores (couplings, J i,j ), which describe the amount of frontier molecular orbital overlap between pairs chromophores.…”

The Polymer Physics of Multiscale Charge Transport in Conjugated Systems

“…Recent work by Jankowski et al . with more sophisticated theorectical treatments, in which they combined the large simulation volumes from optimized molecular dynamics (MD) simulations of P3HT with quantum chemical calculations (QCC)‐informed charge transport, further bolsters the critical role of tie‐chain connectivity in charge transport …”