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

Abstract

The nanomorphologies of the bulk heterojunction (BHJ) layer of polymer solar cells are extremely sensitive to the electrode materials and thermal annealing conditions. In this work, the correlations of electrode materials, thermal annealing sequences, and resultant BHJ nanomorphological details of P3HT:PCBM BHJ polymer solar cell are studied by a series of large-scale, coarse-grained (CG) molecular simulations of system comprised of PEDOT:PSS/ P3HT:PCBM/Al layers. Simulations are performed for various configurations of electrode materials as well as processing temperature. The complex CG molecular data are characterized using a novel extension of our graph-based framework to quantify morphology and establish a link between morphology and processing conditions. Our analysis indicates that vertical phase segregation of P3HT:PCBM blend strongly depends on the electrode material and thermal annealing schedule. A thin P3HT-rich film is formed on the top, regardless of bottom electrode material, when the BHJ layer is exposed to the free surface during thermal annealing. In addition, preferential segregation of P3HT chains and PCBM molecules toward PEDOT:PSS and Al electrodes, respectively, is observed. Detailed morphology analysis indicated that, surprisingly, vertical phase segregation does not affect the connectivity of donor/acceptor domains with respective electrodes. However, the formation of P3HT/PCBM depletion zones next to the P3HT/PCBM-rich zones can be a potential bottleneck for electron/hole transport due to increase in transport pathway length. Analysis in terms of fraction of intraand interchain charge transports revealed that processing schedule affects the average vertical orientation of polymer chains, which may be crucial for enhanced charge transport, nongeminate recombination, and charge collection. The present study establishes a more detailed link between processing and morphology by combining multiscale molecular simulation framework with an extensive morphology feature analysis, providing a quantitative means for process optimization.

Conjugated‐Polymer Blends for Organic Photovoltaics: Rational Control of Vertical Stratification for High Performance

“…The simulation results indicated that the vertical stratification does not affect the connectivity of P3HT and PCBM domains with respective electrodes. However, the depletion region attached to the enriched domains may extend the transport length, which may cause recommbination …”

Using graphs to quantify energetic and structural order in semicrystalline oligothiophene thin films

“…Recent studies providing insights into the morphology of organic thin films have used both experimental measurements (e.g. tomography TEM 20 ) and computational techniques (including molecular dynamics 15 , phase field 21,22 , kinetic Monte Carlo [23][24][25][26] , and quantum chemical calculations 6,10,27,28 ). These techniques provide structural information ranging from electronic structure on the molecular scale 6,10,27,28 , up to the device scale 21,[29][30][31] .…”

“…This is the first attempt to combine data on the electronic-structure with the atomistic-level data on morphology to provide computational framework to establish quantitative structure-property relationships in organic thin films. Although morphology has been previously represented as graphs, the quantification has been focused on either meso scale 13,14 or atomistic level 15 . The work reported in this paper in an interesting step towards convergence of representation in multiscale modeling, as graphs have been mostly used to represent molecular structure [16][17][18] .…”

Morphology and ion diffusion in PEDOT:Tos. A coarse grained molecular dynamics simulation

“…A simplified CG model of PEDOT was used in ref. 34, where the entire EDOT monomer was treated as one bead. This model is however too crude and simplified and is not in a position to reproduce the most important morphological aspects of PEDOT on a subnanometer scale, such as its crystallization.…”