New insights into the dynamics and morphology of P3HT:PCBM active layers in bulk heterojunctions

Abstract

Organic photovoltaics (OPVs) are a topic of extensive research because of their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the-art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on the structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with published small-angle neutron scattering experiments in the literature is presented and an excellent agreement is found.

Electrode Materials, Thermal Annealing Sequences, and Lateral/Vertical Phase Separation of Polymer Solar Cells from Multiscale Molecular Simulations

“…30 Finally, the structure factors computed from the P3HT:PCBM CG model employed in the present study were once again in good agreement with small-angle neutron diffraction experiments. 25,31 Hence, the CG model employed in the present study is capable of reproducing important morphological aspects also observed experimentally. In the following, we will briefly describe the CG models for the anode material (Al) and cathode material (PEDOT:PSS).…”

“…23 Recently, several CG models of P3HT:fullerene system have been developed for studying BHJ morphology evolution with system size compatible with experiments. 25,26 In addition to deterministic MD simulations, stochastic Monte Carlo (MC) is another powerful method to investigate the BHJ morphology at/close to equilibrium in solution, 27,28 which could hold the key toward studying longterm morphology evolution. However, all of these aforementioned computational studies focused on the BHJ morphologies in the bulk; that is, the top and bottom electrodes were not taken into account.…”

Computationally connecting organic photovoltaic performance to atomistic arrangements and bulk morphology

“…A striking example of the diminishing simulation timescales accessible in large simulations is apparent in the 2013 work of Carrillo et. al., which used the then-fastest GPU-accelerated supercomputer in the world (Titan) to simulate evolving morphology in P3HT:PCBM active layers [38]. Simulating nearly 4 million particles, this work accessed the 100nm length-scales needed to model the full height of an active layer and accessed 400ns of simulation time, a significant achievement in scientific computing.…”

“…Accurate coarse-grained force fields require the potentials of mean force to be thoughtfully calculated from more detailed simulations if the coarse models are to be predictive over broad regions of state space [50,[52][53][54]. Alternatively, qualitative coarse-grained potentials permit the properties of a material system to be probed in response to assumptions about the underlying physics [38][39][40][55][56][57].…”

Optimizing legacy molecular dynamics software with directive-based offload

“…An extension of a recent large-scale coarse-grained molecular dynamics of P3HT/PCBM blend [15] indicates that the addition of low molecular weight P3HT polymer chains, as a third component of the blend, can be used to alter the morphology. Intuitively it is expected that shorter chains should be more mobile hence the system kinetics should be accelerated and equilibrium morphology is attained at a faster rate.…”