Abstract
Although better means to model the properties of bulk heterojunction molecular blends are much needed in the field of organic optoelectronics, only a small subset of methods based on molecular dynamics- and Monte Carlo-based approaches have been hitherto employed to guide or replace empirical characterization and testing. Here, we present the first use of the integral equation theory of molecular liquids in modelling the structural properties of blends of phenyl-C-butyric acid methyl ester (PCBM) with poly(3-hexylthiophene) (P3HT) and a carboxylated poly(3-butylthiophene) (P3BT), respectively. For this, we use the Reference Interaction Site Model (RISM) with the Universal Force Field (UFF) to compute the microscopic structure of blends and obtain insight into the miscibility of its components. Input parameters for RISM, such as optimized molecular geometries and charge distribution of interaction sites, are derived by the Density Functional Theory (DFT) methods. We also run Molecular Dynamics (MD) simulation to compare the diffusivity of the PCBM in binary blends with P3HT and P3BT, respectively. A remarkably good agreement with available experimental data and results of alternative modelling/simulation is observed for PCBM in the P3HT system. We interpret this as a step in the validation of the use of our approach for organic photovoltaics and support of its results for new systems that do not have reference data for comparison or calibration. In particular, for the less-studied P3BT, our results show that expectations about its performance in binary blends with PCBM may be overestimated, as it does not demonstrate the required level of miscibility and short-range structural organization. In addition, the simulated mobility of PCBM in P3BT is somewhat higher than what is expected for polymer blends and falls into a range typical for fluids. The significance of our predictive multi-scale modelling lies in the insights it offers into nanoscale morphology and charge transport behaviour in multi-component organic semiconductor blends.
Highlights
Organic Photovoltaics (OPVs) are considered as a promising generation alternative power source that relies on sunlight due to their light weight, thin size, compatibility with flexible substrates and relatively simple fabrication advantages
Solutions to Reference Interaction Site Model (RISM) equations for these concentrations cannot be used for analysis of the system structure, as they exist in the form of slowly-decaying oscillations, which corresponds to long-range fluctuations of unstable homogeneous fluid with negative compressibility
This result is in very good agreement with the experimental study reported previously [59] in which a 19% v/v equilibrium concentration of phenyl-C61-butyric acid methyl ester (PCBM) in P3HT at 140 °C is interpreted as a point on the binodal curve of the composition-temperature phase diagram [60]
Summary
Organic Photovoltaics (OPVs) are considered as a promising generation alternative power source that relies on sunlight due to their light weight, thin size, compatibility with flexible substrates and relatively simple fabrication advantages. Such interactions can strongly influence blend miscibility, blend morphology and crystallite sizes In this context, we use the integral equation theory of molecular liquids in the interaction site formalism, known as the reference interaction site model (RISM) [25,26], to elucidate RDFs in single-component films and blends with PCBM of carboxylated polythiophene derivative P3BT and compare them to the same in P3HT neat films and P3HT-PCBM blend films. RISM is a statistical mechanical theory to describe the equilibrium structure of molecular liquids in terms of the site-site pair correlation functions, from which all of the thermodynamic quantities can be derived It provides a detailed microscopic insight into the organization of solvent molecules in the solvation shell structure and their contribution to the solvation thermodynamics. 1D-RDFs for selected interaction sites are provided in the Supplementary Materials (SM)
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